Estimating crustal thickness in Belgium and surrounding regions from Moho-reflected waves

NASA Astrophysics Data System (ADS)

Sichien, E.; Henriet, J.-P.; Camelbeeck, T.; De Baets, B.

2012-08-01

The Moho depth underneath parts of Belgium and neighbouring regions was evaluated, by analysing more than 750 PmP and SmS arrival times recorded by 37 seismic stations for 209 earthquakes or explosions. First the reflection points for all the recorded seismic source-station couples were located in a grid covering the studied region. Then the Moho depth was evaluated using data corresponding to reflection points located in the same cell of 400 km2. The results show that the crustal thickness varies between 28 and 37 km, with a maximum uncertainty of 2.5 km. Underneath the Brabant Massif, the Moho has been determined for the first time, and is situated at a depth of around 31 km. Underneath the Roer Valley Graben, the Moho depth is also 31-32 km, showing no evidence of an uplift. A small Moho uplift (29 km) is evidenced underneath the Campine Basin and the Eifel Volcanic Province. The first should be confirmed by complementary measurements. The second is in agreement with previous investigations. An abrupt change in the Moho depth is determined in the southeast of the studied region. It is not clear whether this change represents a double Moho as observed on the ECORS profile or just a very steep Moho. Furthermore, two reflections are determined here: one at a depth of 17 to 24 km and the other one at 29 to 37 km. The deepest reflections correspond to the Moho, whereas the shallow reflections could correspond to reflections on a granitic magma chamber or on an old crust-mantle boundary that has been thrust into the middle crust during the Variscan orogeny. This study also demonstrates that the Moho reflected PmP wave is best visible in the low frequency domain (< 8 Hz). This can help to improve the determination of PmP-arrival times in comparable studies.

The deep structure of the Sichuan basin and adjacent orogenic zones revealed by the aggregated deep seismic profiling datum

NASA Astrophysics Data System (ADS)

Xiong, X.; Gao, R.; Li, Q.; Wang, H.

2012-12-01

The sedimentary basin and the orogenic belt are the basic two tectonic units of the continental lithosphere, and form the basin-mountain coupling system, The research of which is the key element to the oil and gas exploration, the global tectonic theory and models and the development of the geological theory. The Sichuan basin and adjacent orogenic belts is one of the most ideal sites to research the issues above, in particular by the recent deep seismic profiling datum. From the 1980s to now, there are 11 deep seismic sounding profiles and 6 deep seismic reflection profiles and massive seismic broadband observation stations deployed around and crossed the Sichuan basin, which provide us a big opportunity to research the deep structure and other forward issues in this region. Supported by the National Natural Science Foundation of China (Grant No. 41104056) and the Fundamental Research Funds of the Institute of Geological Sciences, CAGS (No. J1119), we sampled the Moho depth and low-velocity zone depth and the Pn velocity of these datum, then formed the contour map of the Moho depth and Pn velocity by the interpolation of the sampled datum. The result shows the Moho depth beneath Sichuan basin ranges from 40 to 44 km, the sharp Moho offset appears in the western margin of the Sichuan basin, and there is a subtle Moho depression in the central southern part of the Sichuan basin; the P wave velocity can be 6.0 km/s at ca. 10 km deep, and increases gradually deeper, the average P wave velocity in this region is ca. 6.3 km/s; the Pn velocity is ca. 8.0-8.02 km/s in Sichuan basin, and 7.70-7.76 km/s in Chuan-Dian region; the low velocity zone appears in the western margin of the Sichuan basin, which maybe cause the cause of the earthquake.

Towards the Moho depth and Moho density contrast along with their uncertainties from seismic and satellite gravity observations

NASA Astrophysics Data System (ADS)

Abrehdary, M.; Sjöberg, L. E.; Bagherbandi, M.; Sampietro, D.

2017-12-01

We present a combined method for estimating a new global Moho model named KTH15C, containing Moho depth and Moho density contrast (or shortly Moho parameters), from a combination of global models of gravity (GOCO05S), topography (DTM2006) and seismic information (CRUST1.0 and MDN07) to a resolution of 1° × 1° based on a solution of Vening Meinesz-Moritz' inverse problem of isostasy. This paper also aims modelling of the observation standard errors propagated from the Vening Meinesz-Moritz and CRUST1.0 models in estimating the uncertainty of the final Moho model. The numerical results yield Moho depths ranging from 6.5 to 70.3 km, and the estimated Moho density contrasts ranging from 21 to 650 kg/m3, respectively. Moreover, test computations display that in most areas estimated uncertainties in the parameters are less than 3 km and 50 kg/m3, respectively, but they reach to more significant values under Gulf of Mexico, Chile, Eastern Mediterranean, Timor sea and parts of polar regions. Comparing the Moho depths estimated by KTH15C and those derived by KTH11C, GEMMA2012C, CRUST1.0, KTH14C, CRUST14 and GEMMA1.0 models shows that KTH15C agree fairly well with CRUST1.0 but rather poor with other models. The Moho density contrasts estimated by KTH15C and those of the KTH11C, KTH14C and VMM model agree to 112, 31 and 61 kg/m3 in RMS. The regional numerical studies show that the RMS differences between KTH15C and Moho depths from seismic information yields fits of 2 to 4 km in South and North America, Africa, Europe, Asia, Australia and Antarctica, respectively.

The Lunar Crust: Global Structure and Signature of Major Basins

NASA Technical Reports Server (NTRS)

Neumann, Gregory A.; Zuber, Maria T.; Smith, David E.; Lemoine, Frank G.

1996-01-01

New lunar gravity and topography data from the Clementine Mission provide a global Bouguer anomaly map corrected for the gravitational attraction of mare fill in mascon basins. Most of the gravity signal remaining after corrections for the attraction of topography and mare fill can be attributed to variations in depth to the lunar Moho and therefore crustal thickness. The large range of global crustal thickness (approx. 20-120 km) is indicative of major spatial variations in melting of the lunar exterior and/or significant impact-related redistribution. The 6l-km average crustal thickness, constrained by a depth-to-Moho measured during the Apollo 12 and 14 missions, is preferentially distributed toward the farside, accounting for much of the offset in center-of-figure from the center-of-mass. While the average farside thickness is 12 km greater than the nearside, the distribution is nonuniform, with dramatic thinning beneath the farside, South Pole-Aitken basin. With the global crustal thickness map as a constraint, regional inversions of gravity and topography resolve the crustal structure of major mascon basins to half wavelengths of 150 km. In order to yield crustal thickness maps with the maximum horizontal resolution permitted by the data, the downward continuation of the Bouguer gravity is stabilized by a three- dimensional, minimum-slope and curvature algorithm. Both mare and non-mare basins are characterized by a central upwarped moho that is surrounded by rings of thickened crust lying mainly within the basin rims. The inferred relief at this density interface suggests a deep structural component to the surficial features of multiring lunar impact basins. For large (greater than 300 km diameter) basins, moho relief appears uncorrelated with diameter, but is negatively correlated with basin age. In several cases, it appears that the multiring structures were out of isostatic equilibrium prior to mare emplacement, suggesting that the lithosphere was strong enough to maintain their state of stress to the present.

Basement-involved faults and deep structures in the West Philippine Basin: constrains from gravity field

NASA Astrophysics Data System (ADS)

Wang, Gang; Jiang, Suhua; Li, Sanzhong; Zhang, Huixuan; Lei, Jianping; Gao, Song; Zhao, Feiyu

2017-06-01

To reveal the basement-involved faults and deep structures of the West Philippine Basin (WPB), the gravitational responses caused by these faults are observed and analyzed based on the latest spherical gravity model: WGM2012 Model. By mapping the free-air and Bouguer gravity anomalies, several main faults and some other linear structures are located and observed in the WPB. Then, by conducting a 2D discrete multi-scale wavelet decomposition, the Bouguer anomalies are decomposed into the first- to eighth-order detail and approximation fields (the first- to eighth-order Details and Approximations). The first- to third-order Details reflect detailed and localized geological information of the crust at different depths, and of which the higher-order reflects gravity field of the deeper depth. The first- to fourth-order Approximations represent the regional gravity fields at different depths of the crust, respectively. The fourth-order Approximation represents the regional gravity fluctuation caused by the density inhomogeneity of Moho interface. Therefore, taking the fourth-order Approximation as input, and adopting Parker-Oldenburg interactive inversion, We calculated the depth of Moho interface in the WPB. Results show that the Moho interface depth in the WPB ranges approximately from 8 to 12 km, indicating that there is typical oceanic crust in the basin. In the Urdaneta Plateau and the Benham Rise, the Moho interface depths are about 14 and 16 km, respectively, which provides a piece of evidence to support that the Banham Rise could be a transitional crust caused by a large igneous province. The second-order vertical derivative and the horizontal derivatives in direction 0° and 90° are computed based on the data of the third-order Detail, and most of the basement-involved faults and structures in the WPB, such as the Central Basin Fault Zone, the Gagua Ridge, the Luzon-Okinawa Fault Zone, and the Mindanao Fault Zone are interpreted by the gravity derivatives.

The global Moho depth map for continental crust

NASA Astrophysics Data System (ADS)

Baranov, Alexey; Morelli, Andrea

2014-05-01

Different tectonic units cover the continents: platform, orogens and depression structures. This structural variability is reflected both in thickness and physical properties of the crust. We present a new global Moho map for the continental crust, derived from geophysical data selected from the literature and regional crustal models. The Moho depth is represented with a resolution of 1x1 on a Cartesian grid. A large volume of new data has been analyzed: mostly active seismic experiments, as well as receiver functions and geological studies. We have used the following regional studies: for Europe and Greenland, models EPcrust [Molinari and Morelli, 2011]and EUNAseis [Artemieva and Thybo, 2103]; for North Asia, Moho models from [Cherepanova et al., 2013; Iwasaki et al., 2013; Pavlenkova, 1996]; for Central and Southern Asia, model AsCrust [Baranov, 2010] with updates for India [Reddy and Rao, 2013]; China [Teng et al., 2013];Arabian [Mechie et al., 2013]; for Africa, the model by [Pasyanos and Nyblade, 2007] as a framework and added many others regional studies; for South America, models by [Assumpção et al.,2013; Chulick et al.,2013; Lloyd et al., 2010]; for North America, the model by [Keller, 2013]; for Australia, the model by [Salmon et al., 2013]; for Antarctica, model ANTMoho [Baranov and Morelli, 2013] with update for West Antarctica (POLENET project, [Chaput et al., 2013]). For two orogens we have found the maximum depth at - 75 km (Tibet and Andes). In our model the average thickness of the continental crust is about 34 km (st. deviation 9 km) whereas in CRUST 2.0 model the average Moho for continental areas is about 38 km. The new Moho model for continents exhibits some remarkable disagreement at places with respect to global model CRUST 2.0. The difference in crustal thickness between these two models may amount up to 30 km, mainly due to improved resolution of our model's Moho boundary. There are significant changes in several regions: among them, Darfur, Africa (-22 km); Madagascar (-28 / +14 km), Andes (-30 km); Parana delta, South America (-20 km); California (-20 km); Gamburtsev Mountains, East Antarctica (+24 km). Such analysis remains in large part true also for a comparison with the Moho from the recent CRUST 1.0 model, except for a better agreement in the Americas. Our model provides a starting point for numerical modeling of deep mantle structures via a thorough revision of the crustal effects in the observed fields. This model will be used as a starting point in the gravity modeling of the lithosphere and upper mantle structures. Also it may be used for wave propagation modelling at continental scale, crustal correction in tomography and other seismological applications. The new model will be available for download in digital format. We plan to update the model in the near future by including new data, particularly in the most poorly covered regions.

Seismic Discontinuities beneath the Southwestern United States from S Receiver Functions

NASA Astrophysics Data System (ADS)

Akanbi, O. E.; Li, A.

2015-12-01

S- Receiver functions along the Colorado Plateau-Rio Grande Rift-Great Plains Transect known as La RISTRA in the southwestern United States have been utilized to map the Moho and lithosphere-asthenosphere boundary (LAB) beneath this tectonically active region. The receiver functions were stacked according to ray piercing points with moveout corrections in order to improve the signal-to-noise ratio of converted S-to-P phases. The Moho appears at 30-40 km beneath the Rio Grande Rift (RGR) and deepens to 35-45 km beneath the Great Plains (GP) and the Colorado Plateau (CP). A sharp discontinuity is observed along the profile with the average depth of 80 km beneath the RGR, 100 km beneath the GP, and 160 km beneath the CP. This discontinuity is consistent with the top of a low velocity zone in a shear wave model beneath the array and is interpreted as the LAB. Strong phases imaged at ~90 km beneath the CP and GP could be a combination of side-lobes of the Moho conversions and primary Sp phases from a mid-lithosphere discontinuity (MLD). The relatively shallow Moho and LAB beneath the Rio Grande Rift is indicative of lithosphere extension and asthenosphere upwarp. In addition, the LAB shows depth-step depressions at the RGR-CP and RGR-GP boundaries, providing evidence for mantle downwelling. The variation of the lithospheric depth across the RISTRA array supports that edge-driven, small-scale mantle convection is largely responsible for the recent extension and uplift in the Rio Grande Rift and the Colorado Plateau.

Crustal structure of an exhumed IntraCONtinental Sag (ICONS): the Mekele Basin in Northern Ethiopia.

NASA Astrophysics Data System (ADS)

Alemu, T. B.; Abdelsalam, M. G.

2017-12-01

The Mekele Sedimentary Basin (MSB) in Ethiopia is a Paleozoic-Mesozoic IntraCONtinental Sag (ICONS) exposed due to Cenozoic domal and rift flank uplift associated with the Afar mantle plume and Afar Depression (AD). ICONS are formed over stable lithosphere, and in contrast to rift and foreland basins, show circular-elliptical shape in map view, saucer shaped in cross section, and concentric gravity minima. Surface geological features of the MSB have been shown to exhibit geologic characteristics similar to those of other ICONS. We used the World Gravity Map (WGM 2012) data to investigate subsurface-crustal structure of the MSB. We also used 2D power spectrum analysis and inversion of the gravity field to estimate the Moho depth. Our results show the Bouguer anomalies of the WGM 2012 ranges between 130 mGal and - 110 mGal with the highest values within the AD. Despite the effect of the AD on the gravity anomalies, the MSB is characterized by the presence of gravity low anomaly that reaches in places -110 mGal, especially in its western part. The Moho depth estimates, from both spectral analysis and inversion of the gravity data, is between 36 and 40 km depth over most of the western and southern margins of the MSB. However, as the AD is approached, in the eastern margins of the MSB, crustal thickness estimates are highly affected by the anomalously thin and magmatic segment of the AD, and the Moho depth range between 30 and 25 km. Our results are consistent with that of seismic studies in areas far from the MSB, but within the Northwestern Ethiopian Plateau where the MSB is located. Those studies have reported an abrupt decrease in Moho depth from 40 km beneath the Northwestern plateau, to 20 km in the adjacent AD. Though the MSB is small (100 kmX100 km) compared to other ICONS, and affected by the neighboring AD, it is characterized by elliptical gravity minima and a relatively thicker crust that gradually thickens away from the rift. In addition, seismic imaging of faster shear wave velocity beneath the southwestern MSB at 80 km depth by previous studies mimic the surface and shallow subsurface features that we interpret as indicative of major characteristics of ICONS. Due to their location away from active plate boundaries, most ICONS are buried since the time of their formation. The MSB represents a rare example of a completely exhumed ICONS.

Moho Structure of the Central Sierra Nevada From an EarthScope Flex Array Deployment

NASA Astrophysics Data System (ADS)

Burdick, S.; Zandt, G.; Gilbert, H.; Jones, C.; Owens, T.

2005-12-01

Findings from the southern Sierra Nevada (south of 37 degrees north) show that the crustal thickness in the southern Sierra Nevada range does not obey an Airy isostasy model. Receiver function data show that the crustal thickness generally increases across the range from the high eastern peaks to the low western foothills, and the Moho discontinuity disappears beneath parts of the western foothills. This disappearance of the Moho has been attributed to the entrainment of the crust into the mantle by the convective removal of the southern batholithic root during the past 3-4 M yrs (Zandt et al., Nature, 2004). Other possible causes of Moho disappearance include a very gradational, or even inverted, impedance contrast due to lower crustal or upper mantle wavespeed anomalies. During the summer of 2005, the Sierra Nevada Earthscope Project (SNEP) has deployed an Earthscope flex array of over forty broadband seismometers with 25 km spacing, designed to constrain lithospheric structure of the central Sierra Nevada between the latitudes of approximately 37 to 38 degrees north. We will report on a receiver function study to better define the boundaries of the Moho "hole" to the north. Initial receiver functions from the first stations deployed mainly on the western and eastern flanks of the range show a northward continuation of both the "hole" under the western margin and a high amplitude Moho under the eastern flank of the range. This new observation suggests either the Moho disappearance is unrelated to the convective removal of the southern root or that root removal has affected the Sierra Nevada significantly farther north than suggested by presently available volcanic and xenolith evidence. Receiver functions collected from SNEP data will be processed into move-out corrected depth stacks in order to present a more complete map of Moho depth and amplitude beneath the region. To quantify the range of impedance contrasts capable of producing the observed variability in Moho amplitude, observed receiver function arrivals will be compared to synthetic examples calculated for a range of lower crustal and upper mantle wavespeeds. In conjunction with other studies these results should lead to a better understanding of the scale and processes associated with a young lithospheric foundering event.

Mapping the crustal thickness in Shillong-Mikir Hills Plateau and its adjoining region of northeastern India using Moho reflected waves

NASA Astrophysics Data System (ADS)

Bora, Dipok K.; Baruah, Saurabh

2012-04-01

In this study we have tried to detect and collect later phases associated with Moho discontinuity and used them to study the lateral variations of the crustal thickness in Shillong-Mikir Hills Plateau and its adjoining region of northeastern India. We use the inversion algorithm by Nakajima et al. (Nakajima, J., Matsuzawa, T., Hasegawa, A. 2002. Moho depth variation in the central part of northeastern Japan estimated from reflected and converted waves. Physics of the Earth and Planetary Interiors, 130, 31-47), having epicentral distance ranging from 60 km to 150 km. Taking the advantage of high quality broadband data now available in northeast India, we have detected 1607 Moho reflected phases (PmP and SmS) from 300 numbers of shallow earthquake events (depth ⩽ 25 km) in Shillong-Mikir Hills Plateau and its adjoining region. Notably for PmP phase, this could be identified within 0.5-2.3 s after the first P-arrival. In case of SmS phase, the arrival times are observed within 1.0-4.2 s after the first S-arrival. We estimated the crustal thickness in the study area using travel time difference between the later phases (PmP and SmS) and the first P and S arrivals. The results shows that the Moho is thinner beneath the Shillong Plateau about 35-38 km and is the deepest beneath the Brahmaputra valley to the north about 39-41 km, deeper by 4-5 km compared to the Shillong Plateau with simultaneous observation of thinnest crust (˜33 km) in the western part of the Shillong Plateau in the Garo Hills region.

Three-dimensional Crustal Structure beneath the Tibetan Plateau Revealed by Multi-scale Gravity Analysis

NASA Astrophysics Data System (ADS)

Xu, C.; Luo, Z.; Sun, R.; Li, Q.

2017-12-01

The Tibetan Plateau, the largest and highest plateau on Earth, was uplifted, shorten and thicken by the collision and continuous convergence of the Indian and Eurasian plates since 50 million years ago, the Eocene epoch. Fine three-dimensional crustal structure of the Tibetan Plateau is helpful in understanding the tectonic development. At present, the ordinary method used for revealing crustal structure is seismic method, which is inhibited by poor seismic station coverage, especially in the central and western plateau primarily due to the rugged terrain. Fortunately, with the implementation of satellite gravity missions, gravity field models have demonstrated unprecedented global-scale accuracy and spatial resolution, which can subsequently be employed to study the crustal structure of the entire Tibetan Plateau. This study inverts three-dimensional crustal density and Moho topography of the Tibetan Plateau from gravity data using multi-scale gravity analysis. The inverted results are in agreement with those provided by the previous works. Besides, they can reveal rich tectonic development of the Tibetan Plateau: (1) The low-density channel flow can be observed from the inverted crustal density; (2) The Moho depth in the west is deeper than that in the east, and the deepest Moho, which is approximately 77 km, is located beneath the western Qiangtang Block; (3) The Moho fold, the directions of which are in agreement with the results of surface movement velocities estimated from Global Positioning System, exists clearly on the Moho topography.This study is supported by the National Natural Science Foundation of China (Grant No. 41504015), the China Postdoctoral Science Foundation (Grant No. 2015M572146), and the Surveying and Mapping Basic Research Programme of the National Administration of Surveying, Mapping and Geoinformation (Grant No. 15-01-08).

Complex N-S variations in Moho depth and Vp/Vs ratio beneath the western Tibetan Plateau as revealed by receiver function analysis

NASA Astrophysics Data System (ADS)

Murodov, Davlatkhudzha; Zhao, Junmeng; Xu, Qiang; Liu, Hongbing; Pei, Shunping

2018-04-01

We present herein detailed images of the Moho depth and Vp/Vs ratio along ANTILOPE-1 profile beneath the western Tibetan Plateau derived from receiver function analysis. Along the ANTILOPE -1 profile, a rapidly northward dipping Moho extends from ˜50 km below the Himalaya to ˜80 km across the Indus-Yarlung suture (IYS), shallowing to ˜66 km under the central Lhasa terrane. The Moho depth shows a dramatic increase from ˜66 km north of the Bangong-Nujiang suture (BNS) to ˜93 km beneath central Qiangtang terrane where it reaches the maximum depth observed along this profile before steeply rising to ˜73 km. We interpret both the 15 km and 20 km offsets of Moho depth occurring beneath the central Lhasa and central Qiangtang terranes as being related to the northern frontiers of the decoupled underthrusting Indian lower crust and lithospheric mantle, respectively. The Moho remains at a depth of ˜70 km with a slight undulation beneath the northern Qiangtang and Songpan-Ganzi terranes, and then abruptly shallows to ˜45 km near the Altyn Tagh Fault. The ˜25 km Moho offset observed at the conjunction of the Tarim Basin and the Altyn Tagh mountain range suggests that the crustal shortening is achieved by pure shear thickening without much underthrusting. The average crustal Vp/Vs ratio changes from 1.66 to 1.80 beneath the Himalaya, the Lhasa terrane and the Tarim Basin indicating a felsic-to-intermediate composition. However, higher Vp/Vs ratios between 1.76 and 1.83 (except for a few outlying low values) are found beneath the Qiangtang and Songpan-Ganzi terranes, which could be attributed to the joint effects of the more mafic composition and partial melt within the crust. The Moho depth and Vp/Vs ratio exhibit complex N-S variations along this profile, which can be attributed to the joint effects of Indian lower crust underthrusting, the low velocity zone of the mid-upper crust, crustal shortening and thickening and other involved dynamic mechanisms.

Estimating crustal thickness using SsPmp in regions covered by low-velocity sediments: Imaging the Moho beneath the Southeastern Suture of the Appalachian Margin Experiment (SESAME) array, SE Atlantic Coastal Plain

NASA Astrophysics Data System (ADS)

Parker, E. Horry, Jr.; Hawman, Robert B.; Fischer, Karen M.; Wagner, Lara S.

2016-09-01

Deconvolved waveforms for two earthquakes (Mw: 6.0 and 5.8) show clear postcritical SsPmp arrivals for broadband stations deployed across the coastal plain of Georgia, allowing mapping of crustal thickness in spite of strong reverberations generated by low-velocity sediments. Precritical SsPmp arrivals are also identified. For a basement in which velocity increases linearly with depth, a bootstrapped grid search suggests an average basement velocity of 6.5 ± 0.1 km/s and basement thickness of 29.8 ± 2.0 km. Corresponding normal-incidence Moho two-way times (including sediments) are 10.6 ± 0.6 s, consistent with times for events interpreted as Moho reflections on coincident active-source reflection profiles. Modeling of an underplated mafic layer (Vp = 7.2-7.4 km/s) using travel time constraints from SsPmp data and vertical-incidence Moho reflection times yields a total basement thickness of 30-35 km and average basement velocity of 6.35-6.65 km/s for an underplate thickness of 0-15 km.

Image of the Moho across the continent-ocean transition, US east coast

DOE Office of Scientific and Technical Information (OSTI.GOV)

Holbrook, W.S.; Purdy, G.M.; Reiter, E.C.

1992-03-01

Strong wide-angle reflections from the Moho were recorded by ocean-bottom seismic instruments during the 1988 Carolina Trough multichannel seismic experiment, in an area where the Moho is difficult to detect with vertical-incidence seismic data. Prestack depth migration of these reflections has enabled the construction of a seismic image of the Moho across the continent-ocean transition of a sedimented passive margin. The Moho rises across the margin at a slope of 10{degree}-12{degree}, from a depth of about 33 km beneath the continental shelf to 20 km beneath the outer rise. This zone of crustal thinning defines a distinct, 60-70-km-wide continent-ocean transitionmore » zone. The authors interpret the Moho in the Carolina Trough as a Jurassic feature, formed by magmatic intrusion and underplating during the rifting of Pangea.« less

Fast nonlinear gravity inversion in spherical coordinates with application to the South American Moho

NASA Astrophysics Data System (ADS)

Uieda, Leonardo; Barbosa, Valéria C. F.

2017-01-01

Estimating the relief of the Moho from gravity data is a computationally intensive nonlinear inverse problem. What is more, the modelling must take the Earths curvature into account when the study area is of regional scale or greater. We present a regularized nonlinear gravity inversion method that has a low computational footprint and employs a spherical Earth approximation. To achieve this, we combine the highly efficient Bott's method with smoothness regularization and a discretization of the anomalous Moho into tesseroids (spherical prisms). The computational efficiency of our method is attained by harnessing the fact that all matrices involved are sparse. The inversion results are controlled by three hyperparameters: the regularization parameter, the anomalous Moho density-contrast, and the reference Moho depth. We estimate the regularization parameter using the method of hold-out cross-validation. Additionally, we estimate the density-contrast and the reference depth using knowledge of the Moho depth at certain points. We apply the proposed method to estimate the Moho depth for the South American continent using satellite gravity data and seismological data. The final Moho model is in accordance with previous gravity-derived models and seismological data. The misfit to the gravity and seismological data is worse in the Andes and best in oceanic areas, central Brazil and Patagonia, and along the Atlantic coast. Similarly to previous results, the model suggests a thinner crust of 30-35 km under the Andean foreland basins. Discrepancies with the seismological data are greatest in the Guyana Shield, the central Solimões and Amazonas Basins, the Paraná Basin, and the Borborema province. These differences suggest the existence of crustal or mantle density anomalies that were unaccounted for during gravity data processing.

Surface wave tomography of the European crust and upper mantle from ambient seismic noise

NASA Astrophysics Data System (ADS)

LU, Y.; Stehly, L.; Paul, A.

2017-12-01

We present a high-resolution 3-D Shear wave velocity model of the European crust and upper mantle derived from ambient seismic noise tomography. In this study, we collect 4 years of continuous vertical-component seismic recordings from 1293 broadband stations across Europe (10W-35E, 30N-75N). We analyze group velocity dispersion from 5s to 150s for cross-correlations of more than 0.8 million virtual source-receiver pairs. 2-D group velocity maps are estimated using adaptive parameterization to accommodate the strong heterogeneity of path coverage. 3-D velocity model is obtained by merging 1-D models inverted at each pixel through a two-step data-driven inversion algorithm: a non-linear Bayesian Monte Carlo inversion, followed by a linearized inversion. Resulting S-wave velocity model and Moho depth are compared with previous geophysical studies: 1) The crustal model and Moho depth show striking agreement with active seismic imaging results. Moreover, it even provides new valuable information such as a strong difference of the European Moho along two seismic profiles in the Western Alps (Cifalps and ECORS-CROP). 2) The upper mantle model displays strong similarities with published models even at 150km deep, which is usually imaged using earthquake records.

Lithospheric Structure of Northeastern Tibet Plateau from P and S Receiver Functions

NASA Astrophysics Data System (ADS)

Zhang, C.; Guo, Z.; Chen, Y. J.

2017-12-01

We obtain the lithospheric structure of the Northeast Tibet (NE Tibet) along an N-S trending profile using P- and S-wave receiver function recorded by ChinArray-Himalaya II project. Both P- and S-receiver function migration images show highly consistent lithospheric features. The Moho depth is estimated to be 50 km beneath the Songpan-ganzi (SPGZ) and Qaidam-Kunlun-West Qinling (QD) blocks with little or no fluctuation. However, at the northern boundary of QD, the crust abruptly uplifts to 40 km depth within a distance of 50 km. Meanwhile, at the southernmost of QD, the Moho is found at the depth of 60 km, which forms a double Moho conversion beneath the western Qinling fault (WQF). At the Qilian block, the first order feature of the PRF image is the northward crustal thinning from 60 km to 45 km. The strong Moho fluctuations beneath the Qilian block reflects the on-going mountain building processes. Further to the north, the Moho depth begins to deepen to 55 km and then gradually thins to 40 km at the Alxa block. We observe significant Moho variations at the Central Asian Orogenic belt (CAOB). Furthermore, Moho jumps and offsets are shown beneath major thrust and strike-slip faults zones, such as the a >5 km Moho uplift across the North Qilian Fault (NQF), implying that these faults cut through the crust and partly accommodate the continuous deformation/crustal shorting that is propagated from the India-Eurasia collision. Strong negative signals found in both P and S receiver functions at around 100-150 km depth can be interpreted as the lithosphere-asthenosphere boundary (LAB). The LAB deepens from 100 km at the northern to a maximum of 150 km at the southern end of the CAOB. A relatively flat LAB with the depth of 150 km is shown beneath the Alax block, and then it gradually thins to 100 km from the QD to SPGZ. Beneath the SPGZ, our results indicate a thin and flat lithosphere ( 100 km).

Combined Gravimetric-Seismic Crustal Model for Antarctica

NASA Astrophysics Data System (ADS)

Baranov, Alexey; Tenzer, Robert; Bagherbandi, Mohammad

2018-01-01

The latest seismic data and improved information about the subglacial bedrock relief are used in this study to estimate the sediment and crustal thickness under the Antarctic continent. Since large parts of Antarctica are not yet covered by seismic surveys, the gravity and crustal structure models are used to interpolate the Moho information where seismic data are missing. The gravity information is also extended offshore to detect the Moho under continental margins and neighboring oceanic crust. The processing strategy involves the solution to the Vening Meinesz-Moritz's inverse problem of isostasy constrained on seismic data. A comparison of our new results with existing studies indicates a substantial improvement in the sediment and crustal models. The seismic data analysis shows significant sediment accumulations in Antarctica, with broad sedimentary basins. According to our result, the maximum sediment thickness in Antarctica is about 15 km under Filchner-Ronne Ice Shelf. The Moho relief closely resembles major geological and tectonic features. A rather thick continental crust of East Antarctic Craton is separated from a complex geological/tectonic structure of West Antarctica by the Transantarctic Mountains. The average Moho depth of 34.1 km under the Antarctic continent slightly differs from previous estimates. A maximum Moho deepening of 58.2 km under the Gamburtsev Subglacial Mountains in East Antarctica confirmed the presence of deep and compact orogenic roots. Another large Moho depth in East Antarctica is detected under Dronning Maud Land with two orogenic roots under Wohlthat Massif (48-50 km) and the Kottas Mountains (48-50 km) that are separated by a relatively thin crust along Jutulstraumen Rift. The Moho depth under central parts of the Transantarctic Mountains reaches 46 km. The maximum Moho deepening (34-38 km) in West Antarctica is under the Antarctic Peninsula. The Moho depth minima in East Antarctica are found under the Lambert Trench (24-28 km), while in West Antarctica the Moho depth minima are along the West Antarctic Rift System under the Bentley depression (20-22 km) and Ross Sea Ice Shelf (16-24 km). The gravimetric result confirmed a maximum extension of the Antarctic continental margins under the Ross Sea Embayment and the Weddell Sea Embayment with an extremely thin continental crust (10-20 km).

Moho depth across the Trans-European Suture Zone from P- and S-receiver functions

NASA Astrophysics Data System (ADS)

Knapmeyer-Endrun, Brigitte; Krüger, Frank; Passeq Working Group

2014-05-01

The Mohorovičić discontinuity, Moho for short, which marks the boundary between crust and mantle, is the main first-order structure within the lithosphere. Geodynamics and tectonic evolution determine its depth level and properties. Here, we present a map of the Moho in central Europe across the Teisseyre-Tornquist Zone, a region for which a number of previous studies are available. Our results are based on homogeneous and consistent processing of P- and S-receiver functions for the largest passive seismological data set in this region yet, consisting of more than 40 000 receiver functions from almost 500 station. Besides, we also provide new results for the crustal vP/vS ratio for the whole area. Our results are in good agreement with previous, more localized receiver function studies, as well as with the interpretation of seismic profiles, while at the same time resolving a higher level of detail than previous maps covering the area, for example regarding the Eifel Plume region, Rhine Graben and northern Alps. The close correspondence with the seismic data regarding crustal structure also increases confidence in use of the data in crustal corrections and the imaging of deeper structure, for which no independent seismic information is available. In addition to the pronounced, stepwise transition from crustal thicknesses of 30 km in Phanerozoic Europe to more than 45 beneath the East European Craton, we can distinguish other terrane boundaries based on Moho depth as well as average crustal vP/vS ratio and Moho phase amplitudes. The terranes with distinct crustal properties span a wide range of ages, from Palaeoproterozoic in Lithuania to Cenozoic in the Alps, reflecting the complex tectonic history of Europe. Crustal thickness and properties in the study area are also markedly influenced by tectonic overprinting, for example the formation of the Central European Basin System, and the European Cenozoic Rift System. In the areas affected by Cenozoic rifting and volcanism, thinning of the crust corresponds to lithospheric updoming reported in recent surface wave and S-receiver function studies, as expected for thermally induced deformation. The same correlation applies for crustal thickening, not only across the Trans-European Suture Zone, but also within the southern part of the Bohemian Massif. A high Poisson's ratio of 0.27 is obtained for the craton, which is consistent with a thick mafic lower crust. In contrast, we typically find Poisson's ratios around 0.25 for Phanerozoic Europe outside of deep sedimentary basins. Mapping of the thickness of the shallowest crustal layer, that is low-velocity sediments or weathered rock, indicates values in excess of 6 km for the most pronounced basins in the study area, while thicknesses of less than 4 km are found within the craton, central Germany and most of the Czech Republic.

Lithospheric Structure and Seismotectonics of Central East Antarctica

NASA Astrophysics Data System (ADS)

Reading, A. M.

2006-12-01

The lithosphere of central East Antarctica, the sector of the continent between 30°E - 120°E, is investigated using seismic methods including receiver function and shear-wave splitting analysis. Data from the broadband stations of the temporary SSCUA deployment (in the continental interior) are used together with records from the permanent GSN stations (on the coast) to carry out the first studies of crustal depth and structure, and patterns of seismic anisotropy across this region. The depth of the Moho is found to be 42 km (+/- 2 km) beneath Mawson station with similar structures extending southward across the Rayner province as far south as Beaver Lake. The Fisher Terrane is characterised by a crustal shear wavespeed profile showing few discontinuties with the Moho at a similar depth to the Rayner. South of Fisher, the crust becomes much shallower, with the Moho at 32 km depth. This shallow crust extends across the Lambert glacier to the Prydz coast and the Lambert Terrane. The characteristic crustal wavespeed profiles provide baseline structure for mapping the extent of the terrance beneath the Antarctic Ice Sheet in future deployments. Observations of seismic anisotropy are less well- defined but, at a reconnaissance level, show fast directions parallel to the present day coastline. This may be controlled by rift-related influences on the lithosphere associated with the breakup of East Gondwana. The seismicity is confirmed to be extremely low. The only seismogenic forces on the Antarctic plate in this region are acting at the boundary between the continental and oceanic lithosphere west of 50°E and east of 100°E and represent a superposition of tectonic and glaciogenic controls. The Lambert Glacier region shows little or no seismotectonic activity in the continental interior or on the oceanic margin.

The Moho discontinuity beneath Taiwan orogenic zone inferred from receiver function analysis

NASA Astrophysics Data System (ADS)

Chang, H.; Chen, C.; Liang, W.

2013-12-01

We determine the depth variations of the Moho discontinuity beneath Taiwan from receiver function analysis. Taiwan is a young (~6.5 Ma) orogenic zone as a consequence of oblique collision between the Philippine Sea Plate and the Eurasian Plate. In northeastern Taiwan, the Philippine Sea Plate subducts northwestward under the Eurasian Plate along the Ryukyu Trench; in southern Taiwan, the Eurasian Plate subducts eastward beneath the Philippine Sea Plate along the Manila Trench. Recent tomographic models of Taiwan reveal P-wave velocity variations of the lithospheric structure that provide important constraints on the orogenic processes in this region. However, the depth variations of the Moho discontinuity, a key observation for better understanding crustal deformation, remain elusive. In this study, we aim to delineate the Moho depth variations by analyzing seismic converted phases indicative of the presence of discontinuity structure. We analyze waveform data from teleseismic events recorded at the Broadband Array in Taiwan for Seismology (BATS). Preliminary results of receiver functions beneath BATS stations in eastern Taiwan show that more than one converted phase (P-to-S) are likely present in crustal depths, suggesting possible multiple crustal layering, which may complicate the detection of the Moho. We further carry out synthetic experiments to explore possible crustal structures that reconcile our observations.

Structure of the Flat Slab in Southern Peru

NASA Astrophysics Data System (ADS)

Ma, Y.; Clayton, R. W.

2014-12-01

We investigate the detailed structure of the flat-subduction portion of the subduction zone in Southern Peru using converted phases recorded by the PeruSE seismic array. The migrated image along a profile above the flat subduction is shown in the figure, overlain by the receiver functions of one well-recorded event. We see that the slab descends to 100 km depth at a distance of about 100 km inland from the coast, and then it rises to 90 km depth and remains flat for the next 300 km distance before diving into the mantle. The Moho itself has about 10 km relief above the flat slab, which is anti-correlated with the surface topography indicating Airy compensation. Interestingly, the flat slab image is missing under this part of Moho. The mid-crust structure is also evident. In the west, it coincides with the Andean Low Velocity Zone (ALVZ) mapped in this region (Ma and Clayton, 2014). In the east, it is related with the underthrusting Brazilian Shield (Phillips and Clayton, 2014). In this paper, we further investigate the causes of the missing or weak flat slab signal, possibly due to anomalous attenuation of S waves in the mantle wedge (but not P wave, since Moho is well imaged). We will also extend our study to the flat-normal transition area beneath the array.

Thermal classification of lithospheric discontinuities beneath USArray

NASA Astrophysics Data System (ADS)

Hansen, Steven M.; Dueker, Ken; Schmandt, Brandon

2015-12-01

Broadband seismic data from the United States were processed into Ps and Sp receiver function image volumes for the purpose of constraining negative velocity gradients (NVG) at depths between the Moho and 200 km. Moho depth picks from the two independent datasets are in good agreement, however, large discrepancies in NVG picks occur and are attributed to free-surface multiples which obscure deep NVG arrivals in the Ps data. From the Sp data, shallow NVG are found west of the Rockies and in the central US while deep and sporadic NVG are observed beneath the Great Plains and northern Rockies. To aid the interpretation of the observed NVG arrivals, the mantle thermal field is estimated by mapping surface wave tomography velocities to temperature assuming an anelastic olivine model. The distribution of temperature versus NVG depth is bi-modal and displays two distinct thermal populations that are interpreted to represent both the lithosphere-asthenosphere boundary (LAB) and mid-lithosphere discontinuities (MLD). LAB arrivals occur in the western US at 60-85 km and 1200-1400 °C depth suggesting that they manifest partial melt near the base of the thermal plate. MLD arrivals primarily occur at 70-110 km depth and 700-900 °C and we hypothesize that these arrivals are caused by a low-velocity metasomatic layer containing phlogopite resulting from magma crystallization products that accumulate within long-lived thick lithosphere.

A Least Squares Collocation Approach with GOCE gravity gradients for regional Moho-estimation

NASA Astrophysics Data System (ADS)

Rieser, Daniel; Mayer-Guerr, Torsten

2014-05-01

The depth of the Moho discontinuity is commonly derived by either seismic observations, gravity measurements or combinations of both. In this study, we aim to use the gravity gradient measurements of the GOCE satellite mission in a Least Squares Collocation (LSC) approach for the estimation of the Moho depth on regional scale. Due to its mission configuration and measurement setup, GOCE is able to contribute valuable information in particular in the medium wavelengths of the gravity field spectrum, which is also of special interest for the crust-mantle boundary. In contrast to other studies we use the full information of the gradient tensor in all three dimensions. The problem outline is formulated as isostatically compensated topography according to the Airy-Heiskanen model. By using a topography model in spherical harmonics representation the topographic influences can be reduced from the gradient observations. Under the assumption of constant mantle and crustal densities, surface densities are directly derived by LSC on regional scale, which in turn are converted in Moho depths. First investigations proofed the ability of this method to resolve the gravity inversion problem already with a small amount of GOCE data and comparisons with other seismic and gravitmetric Moho models for the European region show promising results. With the recently reprocessed GOCE gradients, an improved data set shall be used for the derivation of the Moho depth. In this contribution the processing strategy will be introduced and the most recent developments and results using the currently available GOCE data shall be presented.

Moho Modeling Using FFT Technique

NASA Astrophysics Data System (ADS)

Chen, Wenjin; Tenzer, Robert

2017-04-01

To improve the numerical efficiency, the Fast Fourier Transform (FFT) technique was facilitated in Parker-Oldenburg's method for a regional gravimetric Moho recovery, which assumes the Earth's planar approximation. In this study, we extend this definition for global applications while assuming a spherical approximation of the Earth. In particular, we utilize the FFT technique for a global Moho recovery, which is practically realized in two numerical steps. The gravimetric forward modeling is first applied, based on methods for a spherical harmonic analysis and synthesis of the global gravity and lithospheric structure models, to compute the refined gravity field, which comprises mainly the gravitational signature of the Moho geometry. The gravimetric inverse problem is then solved iteratively in order to determine the Moho depth. The application of FFT technique to both numerical steps reduces the computation time to a fraction of that required without applying this fast algorithm. The developed numerical producers are used to estimate the Moho depth globally, and the gravimetric result is validated using the global (CRUST1.0) and regional (ESC) seismic Moho models. The comparison reveals a relatively good agreement between the gravimetric and seismic models, with the RMS of differences (of 4-5 km) at the level of expected uncertainties of used input datasets, while without the presence of significant systematic bias.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fielding, E.J.; Barazangi, M.; Isacks, B.L.

Topography and heterogeneous crustal structure have major effects on the propagation of regional seismic phases. We are collecting topographical, geological, and geophysical datasets for Eurasia into an information system that can be accessed via Internet connections. Now available are digital topography, satellite imagery, and data on sedimentary basins and crustal structure thicknesses. New datasets for Eurasia include maps of depth to Moho beneath Europe and Scandinavia. We have created regularly spaced grids of the crustal thickness values from these maps that can be used to create profiles of crustal structure. These profiles can be compared by an analyst or anmore » automatic program with the crustal seismic phases received along the propagation path to better understand and predict the path effects on phase amplitudes, a key to estimating magnitudes and yields, and for understanding variations in travel-time delays for phases such as Pn, important for improving regional event locations. The gridded data could also be used to model propagation of crustal phases in three dimensions. Digital elevation models, Satellite imagery, Geographic information systems, Lg Propagation, Moho, Geology, Crustal structure, Topographic relief.« less

Seismic crustal structure of the North China Craton and surrounding area: Synthesis and analysis

NASA Astrophysics Data System (ADS)

Xia, B.; Thybo, H.; Artemieva, I. M.

2017-07-01

We present a new digital model (NCcrust) of the seismic crustal structure of the Neoarchean North China Craton (NCC) and its surrounding Paleozoic-Mesozoic orogenic belts (30°-45°N, 100°-130°E). All available seismic profiles, complemented by receiver function interpretations of crustal thickness, are used to constrain a new comprehensive crustal model NCcrust. The model, presented on a 0.25° × 0.25°grid, includes the Moho depth and the internal structure (thickness and velocity) of the crust specified for four layers (the sedimentary cover, upper, middle, and lower crust) and the Pn velocity in the uppermost mantle. The crust is thin (30-32 km) in the east, while the Moho depth in the western part of the NCC is 38-44 km. The Moho depth of the Sulu-Dabie-Qinling-Qilian orogenic belt ranges from 31 km to 51 km, with a general westward increase in crustal thickness. The sedimentary cover is 2-5 km thick in most of the region, and typical thicknesses of the upper crust, middle crust, and lower crust are 16-24 km, 6-24 km, and 0-6 km, respectively. We document a general trend of westward increase in the thickness of all crustal layers of the crystalline basement and as a consequence, the depth of the Moho. There is no systematic regional pattern in the average crustal Vp velocity and the Pn velocity. We examine correlation between the Moho depth and topography for seven tectonic provinces in the North China Craton and speculate on mechanisms of isostatic compensation.

The Moho structure beneath the Yarlung Zangbo Suture and its implications: Evidence from 2000 kg large dynamite shots

NASA Astrophysics Data System (ADS)

Gao, R.; Li, H.; Li, W.; Lu, Z.; Guo, X.; WANG, Y.

2017-12-01

The YZS (Yarlung Zangbo Suture) is the collisional front between the Indian and Eurasian plates. The depth and geometry of the Moho thus provide first-order information for the restoration of complex geodynamic systems. Over the past three decades, numerous seismic experiments have been conducted across the YZS, including deep seismic reflection profiles, deep seismic soundings and broadband observation studies. However, there is strong disagreement concerning the character of the Moho along the YZS in Tibet. Hirn proposed an offset of more than 15 km along the Moho below the YZS according to wide-angle observations acquired by a Sino-French cooperative experiment. Jiang argued that the Moho exhibits a 20-km offset after analyzing multiple broadband seismic profiles across the YZS. Gao did not find any significant changes in the Moho depth using deep seismic reflection profile data across the western YZS. The above mentioned summary of previous findings shows that similar geophysical observations have yielded contrasting models. Due to the shortage of high-resolution geophysical data, the above controversial problems cannot currently be resolved effectively without improving the accuracy of available geophysical observations and consequently obtaining reliable evidence. The rapid development of the technology of deep seismic reflection profiling has provided an opportunity to resolve the above controversies. two deep seismic reflection profiles across the YZS(88°E) were deployed in 2015(Fig .1 -YZS-B). Four large dynamite shots with 2000 kg charges were employed to improve the signal-to-noise ratio (S/N) along the two transects(Fig .1 and Fig.2). The primary purpose of this experiment is to study images of the Moho both adjacent to and beneath the YZS using four large dynamite shots along two profiles. These four large shots were processed to combine two single-fold profiles. Our two single-fold profiles across the YZS clearly show the existence of a well-imaged Moho. The reflections from the Moho are clear with a narrow band of reflections that are typically <0.3 s between 21-25 s. The depth of the Moho is approximately 63-75 km across the entire profile (assuming an average crustal velocity of 6 km/s). A gap in the Moho is observed approximately 20 km north of the YZS, the amplitude of which is less than 6 km.

Continental and oceanic crustal structure of the Pampean flat slab region, western Argentina, using receiver function analysis: new high-resolution results

NASA Astrophysics Data System (ADS)

Gans, Christine R.; Beck, Susan L.; Zandt, George; Gilbert, Hersh; Alvarado, Patricia; Anderson, Megan; Linkimer, Lepolt

2011-07-01

The Pampean flat slab of central Chile and Argentina (30°-32°S) has strongly influenced Cenozoic tectonics in western Argentina, which contains both the thick-skinned, basement-cored uplifts of the Sierras Pampeanas and the thin-skinned Andean Precordillera fold and thrust belt. In this region of South America, the Nazca Plate is subducting nearly horizontally beneath the South American Plate at ˜100 km depth. To gain a better understanding of the deeper structure of this region, including the transition from flat to 'normal' subduction to the south, three IRIS-PASSCAL arrays of broad-band seismic stations have been deployed in central Argentina. Using the dense SIEMBRA array, combined with the broader CHARGE and ESP arrays, the flat slab is imaged for the first time in 3-D detail using receiver function (RF) analysis. A distinct pair of RF arrivals consisting of a negative pulse that marks the top of the oceanic crust, followed by a positive pulse, which indicates the base of the oceanic crust, can be used to map the slab's structure. Depths to Moho and oceanic crustal thicknesses estimated from RF results provide new, more detailed regional maps. An improved depth to continental Moho map shows depths of more than 70 km in the main Cordillera and ˜50 km in the western Sierras Pampeanas, that shallow to ˜35 km in the eastern Sierras Pampeanas. Depth to Moho contours roughly follow terrane boundaries. Offshore, the hotspot seamount chain of the Juan Fernández Ridge (JFR) is thought to create overthickened oceanic crust, providing a mechanism for flat slab subduction. By comparing synthetic RFs, based on various structures, to the observed RF signal we determine that the thickness of the oceanic crust at the top of the slab averages at least ˜13-19 km, supporting the idea of a moderately overthickened crust to provide the additional buoyancy for the slab to remain flat. The overthickened region is broader than the area directly aligned with the path of the JFR, however, and indicates, along with the slab earthquake locations, that the flat slab area is wider than the JFR volcanic chain observed in the offshore bathymetry. Further, RFs indicate that the subducted oceanic crust in the region directly along the path of the subducted ridge is broken by trench-parallel faults. One explanation for these faults is that they are older structures within the oceanic crust that were created when the slab subducted. Alternatively, it is possible that faults formed recently from tectonic underplating caused by increased interplate coupling in the flat slab region.

Robustness of Global Radial Anisotropy Models of the Upper Mantle

NASA Astrophysics Data System (ADS)

Xing, Z.; Beghein, C.; Yuan, K.

2014-12-01

Radial anisotropy provides important constraints on mantle deformation. While its presence is well accepted in the uppermost mantle, large discrepancies remain among existing models, even at depths well sampled by seismic data, and its presence at greater depths is highly uncertain. Surface wave phase velocity dispersion measurements are routinely used to constrain lateral variations in mantle S-wave velocity (dlnVS) and radial anisotropy (ξ=VSH2/VSV2). Here, we employed the fundamental and higher mode surface wave phase velocity maps of Visser et al. (2008) that have unprecedented sensitivity to structure down to 800-1000km depth, and we adopted a probabilistic forward modeling approach, the Neighbourhood Algorithm, to quantify posterior model uncertainties and parameter trade-offs. We investigated the effect of prior crustal corrections on 3-D ξ and dlnVS models. To avoid mapping crustal structure onto mantle heterogeneities, it is indeed important to accurately account for 3-D crustal anomalies and variations in Moho depth. One approach is to solve the non-linear problem and simultaneously constrain Moho depth and mantle anomalies (Visser et al., 2008). Another approach, taken here, is to calculate non-linear crustal corrections with an a priori crustal model, which are then applied to the phase velocity maps before inverting the remaining signal for mantle structure. In this work, we also determined laterally varying sensitivity kernels to account for lateral changes in the crust. We compare models obtained using CRUST2.0 (Bassin et al., 2000) and the new CRUST1.0 (Laske et al., 2012) models, which mostly differ under continents. Our preliminary results show strong differences (ΔdlnVS>2%) between the two models in continental dlnVS for the upper 150-200km, and strong changes in x amplitudes in the top 200km (Δξ>2%). Some of the differences in ξ persist down to the transition zone, in particular beneath central Asia and South America. Despite these discrepancies, inferences on the depth of continental roots (~200-250km) based on either the extent of the dlnVS>0 anomalies or the depth at which ξ changes sign remain independent of the crustal model employed. We also note that VSV>VSH dominates the deep upper mantle except in central Pacific, which is characterized by VSH>VSV down to the transition zone.

Peeling linear inversion of upper mantle velocity structure with receiver functions

NASA Astrophysics Data System (ADS)

Shen, Xuzhang; Zhou, Huilan

2012-02-01

A peeling linear inversion method is presented to study the upper mantle (from Moho to 800 km depth) velocity structures with receiver functions. The influences of the crustal and upper mantle velocity ratio error on the inversion results are analyzed, and three valid measures are taken for its reduction. This method is tested with the IASP91 and the PREM models, and the upper mantle structures beneath the stations GTA, LZH, and AXX in northwestern China are then inverted. The results indicate that this inversion method is feasible to quantify upper mantle discontinuities, besides the discontinuities between 3 h M ( h M denotes the depth of Moho) and 5 h M due to the interference of multiples from Moho. Smoothing is used to overcome possible false discontinuities from the multiples and ensure the stability of the inversion results, but the detailed information on the depth range between 3 h M and 5 h M is sacrificed.

The rheological structure of the lithosphere in the Eastern Marmara region, Turkey

NASA Astrophysics Data System (ADS)

Oruç, Bülent; Sönmez, Tuba

2017-05-01

The aim of this work is to propose the geometries of the crustal-lithospheric mantle boundary (Moho) and lithosphere-asthenosphere boundary (LAB) and the 1D thermal structure of the lithosphere, in order to establish a rheological model of the Eastern Marmara region. The average depths of Moho and LAB are respectively 35 km and 51 km from radially averaged amplitude spectra of EGM08 Bouguer anomalies. The geometries of Moho and LAB interfaces are estimated from the Parker-Oldenburg gravity inversion algorithm. Our results show the Moho depth varies from 31 km at the northern part of North Anatolian Fault Zone (NAFZ) to 39 km below the mountain belt in the southern part of the NAFZ. The depth to the LAB beneath the same parts of the region ranges from 45 km to 55 km. Having lithospheric strength and thermal boundary layer structure, we analyzed the conditions of development of lithosphere thinning. A two-dimensional strength profile has been estimated for rheology model of the study area. Thus we suggest that the rheological structure consists of a strong upper crust, a weak lower crust, and a partly molten upper lithospheric mantle.

Evidence for Moho-lower crustal transition depth diking and rifting of the Sierra Nevada microplate

NASA Astrophysics Data System (ADS)

Smith, Kenneth D.; Kent, Graham M.; Seggern, David P.; Driscoll, Neal W.; Eisses, Amy

2016-10-01

Lithospheric rifting most often initiates in continental extensional settings where "breaking of a plate" may or may not progress to sea floor spreading. Generally, the strength of the lithosphere is greater than the tectonic forces required for rupture (i.e., the "tectonic force paradox"), and it has been proposed that rifting requires basaltic magmatism (e.g., dike emplacement) to reduce the strength and cause failure, except for the case of a thin lithosphere (<30 km thick). Here we isolate two very similar and unprecedented observations of Moho-lower crustal transition dike or fluid injection earthquake swarms under southern Sierra Valley (SV: 2011-2012) and North Lake Tahoe (LT: 2003-2004), California. These planar distributions of seismicity can be interpreted to define the end points, and cover 25% of the length, of an implied 56 km long structure, each striking N45°W and dipping 50°NE. A single event at 30 km depth that locates on the implied dipping feature between the two swarms is further evidence for a single Moho-transition depth structure. We propose that basaltic or fluid emplacement at or near Moho depths weakens the upper mantle lid, facilitating lithospheric rupture of the Sierra Microplate. Similar to the LT sequence, the SV event is also associated with increased upper crustal seismicity. An 27 October 2011, Mw 4.7 earthquake occurred directly above the deep SV sequence at the base of the upper crustal seismogenic zone ( 15 km depth).

Lithospheric structure of the South China Sea and adjacent regions: Results from potential field modelling

NASA Astrophysics Data System (ADS)

Chen, Ming; Fang, Jian; Cui, Ronghua

2018-02-01

This work aims to investigate the crustal and lithospheric mantle thickness of the South China Sea (SCS) and adjacent regions. The crust-mantle interface, average crustal density, and lithospheric mantle base are calculated from free-air gravity anomaly and topographic data using an iterative inversion method. We construct a three-dimensional lithospheric model with different hierarchical layers. The satellite-derived gravity is used to invert the average crustal density and Moho (crust-mantle interface) undulations. The average crustal density and LAB (lithosphere-asthenosphere boundary) depths are further adjusted by topographic data under the assumption of local isostasy. The average difference in Moho depths between this study and the seismic measurement results is <1.5 km. The results show that in oceanic regions, the Moho depths are 7.5-30 km and the LAB depths are 65-120 km. The lithospheric thickness of the SCS basin and the adjacent regions increases from the sea basin to the continental margin with a large gradient in the ocean-continent transition zones. The Moho depths of conjugate plots during the opening of SCS, Zhongsha Islands and Reed Bank, reveal the asymmetric spreading pattern of SCS seafloor spreading. The lithospheric thinning pattern indicate two different spreading directions during seafloor spreading, which changed from N-S to NW-SE after the southward transition of the spreading axis. The lithosphere of the SCS basin and adjacent regions indicate that the SCS basin is a young basin with a stable interior lithosphere.

Shallow Moho with aseismic upper crust and deep Moho with seismic lower crust beneath the Japanese Islands obtained by seismic tomography using data from dense seismic network

NASA Astrophysics Data System (ADS)

Matsubara, Makoto; Obara, Kazushige

2015-04-01

P-wave seismic velocity is well known to be up to 7.0 km/s and over 7.5 km/s in the lower crust and in the mantle, respectively. A large velocity gradient is the definition of the Moho discontinuity between the crust and mantle. In this paper, we investigates the configuration of Moho discontinuity defined as an isovelocity plane with large velocity gradient derived from our fine-scale three-dimensional seismic velocity structure beneath Japanese Islands using data obtained by dense seismic network with the tomographic method (Matsubara and Obara, 2011). Japanese Islands are mainly on the Eurasian and North American plates. The Philippine Sea and Pacific plates are subducting beneath these continental plates. We focus on the Moho discontinuity at the continental side. We calculate the P-wave velocity gradients between the vertical grid nodes since the grid inversion as our tomographic method does not produce velocity discontinuity. The largest velocity gradient is 0.078 (km/s)/km at velocities of 7.2 and 7.3 km/s. We define the iso-velocity plane of 7.2 km/s as the Moho discontinuity. We discuss the Moho discontinuity above the upper boundary of the subducting oceanic plates with consideration of configuration of plate boundaries of prior studies (Shiomi et al., 2008; Kita et al., 2010; Hirata et al, 2012) since the Moho depth derived from the iso-velocity plane denotes the oceanic Moho at the contact zones of the overriding continental plates and the subducting oceanic plates. The Moho discontinuity shallower than 30 km depth is distributed within the tension region like northern Kyushu and coastal line of the Pacific Ocean in the northeastern Japan and the tension region at the Cretaceous as the northeastern Kanto district. These regions have low seismicity within the upper crust. Positive Bouguer anomaly beneath the northeastern Kanto district indicates the ductile material with large density in lower crust at the shallower portion and the aseismic upper crust. The Moho discontinuity deepens over 35 km in the collision zone like as Kanto Mountains, the volcanic underplating zone as the Tohoku backbone range, and non-tension region like as Chugoku Mountains. These regions associated with deep Moho are characterized by the crustal seismicity within the depth range from 20 to 30 km. The iso-depth contour of 35 km beneath the southwestern Japan is consistent with that derived from the receiver function method (Shiomi et al. 2006). There are nonvolcanic tremors and short-time slow slip events (SSE) beneath the southwestern Japan (eg. Obara, 2002). Matsubara et al. (2009) consider that the tremors and SSEs occur along the contact zone of Moho discontinuity beneath the Eurasian plate and the subducting Philippine Sea plate beneath southwestern Japan. Our Moho model is consistent with this since they exist along the southern edge of the Moho discontinuity of the continental Eurasian plate. Reference: Hirata, N., Sakai, S., Nakagawa, S., Ishikawa, M., Sato, H., Kasahara, K., Kimura, H. and Honda, R. (2012) A new tomographic image on the Philippine Sea Slab beneath Tokyo - Implication to seismic hazard in the Tokyo metropolitan region, EOS, Transactions, AGU, T11C-06. Kita, S., T. Okada, A. Hasegawa, J. Nakajima, and T. Matsuzawa (2010) Anomalous deepening of a seismic belt in the upper-plane of the double seismic zone in the Pacific slab beneath the Hokkaido corner: Possible evidence for thermal shielding caused by subducted forearc crust materials, Earth Planet. Science Lett., 290, 415-426. Matsubara, M. and K. Obara (2011) The 2011 Off the Pacific Coast of Tohoku earthquake related to a strong velocity gradient with the Pacific plate, Earth Planets Space, 63, 663-667. Matsubara, M., K. Obara, and K. Kasahara (2009) High-Vp/Vs zone accompanying non-volcanic tremors and slow-slip events beneath southwestern Japan, Tectonophysics, 472, 6-17, doi:10.1016/j.tecto.2008.06.013. Obara, K. (2002) Nonvolcanic deep tremor associated with subduction in southwest Japan. Science 296, 1679-1681. Shiomi, K., K. Obara, and H. Sato (2006) Moho depth variation beneath southwestern Japan revealed from the velocity structure based on receiver function inversion , Tectonophysics, 420, 205-221, doi:10.1016/j.tecto.2006.01.017. Shiomi, K., M. Matsubara, Y. Ito, and K. Obara (2008) Simple relationship between seismic activity along Philippine Sea slab and geometry of oceanic Moho beneath southwest Japan, Geophys. J. Int., 173, 1018-1029.

Pn-waves Travel-time Anomaly beneath Taiwan from Dense Seismic Array Observations and its Possible Tectonic Implications

NASA Astrophysics Data System (ADS)

Lin, Y. Y.; Huang, B. S.; Ma, K. F.; Hsieh, M. C.

2015-12-01

We investigated travel times of Pn waves, which are of great important for understanding the Moho structure in Taiwan region. Although several high quality tomographic studies had been carried out, observations of Pn waves are still the most comprehensive way to elucidate the Moho structure. Mapping the Moho structure of Taiwan had been a challenging due to the small spatial dimension of Taiwan island with two subduction systems. To decipher the tectonic structure and understanding of earthquake hazard, the island of Taiwan have been implemented by several high density seismic stations, including 71 short-period stations of Central Weather Bureau Seismic Network (CWBSN) and 42 broardband stations of Broadband Array in Taiwan for Seismology (BATS). High quality seismic records of these stations would be used to identify precise Pn-wave arrival times. After station-elevation correction, we measure the difference between the observed and theoretical Pn arrivals from the IASPI 91 model for each station. For correcting uncertainties of earthquake location and origin time, we estimate relative Pn anomaly, ΔtPn , between each station and a reference station. The pattern of ΔtPn reflects the depth anomaly of Moho beneath Taiwan. In general, Pn waves are commonly observed from shallow earthquake at epicentral distance larger than 120 km. We search the global catalog since 2005 and the criteria are M > 5.5, focal depth < 30 km and epicentral distance > 150 km. The 12 medium earthquakes from north Luzon are considered for analysis. We choose a station, TWKB, in the most southern point of Taiwan as the reference station due to that all events are from the south. The results indicate obvious different patterns of ΔtPn from different back-azimuths. The ΔtPn pattern of the events in the first group from the south south-east indicates that the Pn arrivals delay suddenly when the Pn waves pass through the Central Range, suggesting the Moho becomes deep rapidly. However, we cannot recognize the same pattern when the events from due south in the second group. The ΔtPn pattern in the second group has a clear slow gradient from the south to north through Taiwan island. It may be relative to a smooth dipping structure of the Moho. Both ΔtPn patterns reveal large delays in northern Taiwan which may be related to the north subduction structure.

Petrochemical and petrophysical characterization of the lower crust and the Moho beneath the West African Craton, based on Xenoliths from Kimberlites

NASA Technical Reports Server (NTRS)

Haggerty, Stephen E.; Toft, Paul B.

1988-01-01

Additional evidence to the composition of the lower crust and uppermost mantle was presented in the form of xenolith data. Xenoliths from the 2.7-Ga West African Craton indicate that the Moho beneath this shield is a chemically and physically gradational boundary, with intercalations of garnet granulite and garnet eclogite. Inclusions in diamonds indicate a depleted upper mantle source, and zenolith barometry and thermometry data suggest a high mantle geotherm with a kink near the Moho. Metallic iron in the xenoliths indicates that the uppermost mantle has a significant magnetization, and that the depth to the Curie isotherm, which is usually considered to be at or above the Moho, may be deeper than the Moho.

Shear Wave Structure of Mount St. Helens, WA Region From Ambient Noise, Earthquake Surface Waves and Receiver Functions

NASA Astrophysics Data System (ADS)

Crosbie, K.; Abers, G. A.; Mann, M. E.; Janiszewski, H. A.; Creager, K. C.; Kiser, E.; Ulberg, C. W.; Denlinger, R. P.; Moran, S. C.

2017-12-01

Mount St. Helens (MSH) lies 50 km trenchward of the main arc front in Cascadia. The imaging Magma Under St. Helens (iMUSH) experiment probes its magmatic plumbing system in the mid to lower crust to understand how magmas could be generated in this setting. A 70-element broadband array was deployed for 2 years with a 10 km station spacing and 100 km aperture. Ambient noise and earthquake surface waves provide fundamental-mode Rayleigh wave phase velocity maps of the region from 0.01 to 0.18 Hz. From these, shear velocity (Vs) is estimated from 0 to 80 km depth. Initial attempts at integrating ambient-noise and earthquake datasets have been complicated by the lower resolution of earthquake-derived phase velocities compared to ambient noise, and care is being taken to minimize this incompatibility. Joint inversions with receiver functions help resolve these ambiguities and velocity contrasts across interfaces. For depths of 0-5 km, fast Vs zones (3.3 km/s) are imaged that correspond well with mapped plutons (Spud Mountain and Spirit Lake). Crust at 10-30 km depth has higher Vs (>3.9 km/s) west of MSH than east and north of it (Vs < 3.7 km/s). Crustal temperature variations from a cold forearc to a hot volcanic crust could partly explain this crustal velocity pattern. However, the exceedingly high Vs west of MSH requires a strong change in crustal composition, most likely revealing the east edge of the mafic Siletzia terrane with a predicted Vs similar to that observed. Just below the Moho, Vs variations are much smaller. The resulting Vs contrast across the Moho from surface waves and receiver functions is weak in the forearc and strong beneath the arc volcanoes. This pattern was previously interpreted as due to a serpentinized cold nose of the mantle. However, the anomalously high crustal velocities we observe west of MSH contribute to this forearc Moho absence more than mantle velocity variations, indicating that crustal geology enhances or dominates an effect attributed to mantle hydration. These results confirm that MSH lies on the edge of a notably cold forearc, exactly where crustal composition varies markedly. This sharp crustal terrane boundary immediately west of MSH may help localize volcanism.

A Gradient in Cooling Rate Beneath the Moho at the Oman Ophiolite: Fresh Insights into Cooling Processes at Mid-Ocean Ridges from REE-Based Thermometry

NASA Astrophysics Data System (ADS)

Dygert, N. J.; Kelemen, P. B.; Liang, Y.

2015-12-01

The Wadi Tayin massif in the southern Oman ophiolite has a more than 10 km thick mantle section and is believed to have formed in a mid-ocean ridge like environment with an intermediate to fast spreading rate. Previously, [1] used major element geothermometers to investigate spatial variations in temperatures recorded in mantle peridotites and observed that samples near the paleo-Moho have higher closure temperatures than samples at the base of the mantle section. Motivated by these observations, we measured major and trace elements in orthopyroxene and clinopyroxene in peridotites from depths of ~1-8km beneath the Moho to determine closure temperatures of REE in the samples using the REE-in-two-pyroxene thermometer [2]. Clinopyroxene are depleted in LREE and have REE concentrations that vary depending on distance from the Moho. Samples nearer the Moho have lower REE concentrations than those deeper in the section (e.g., chondrite normalized Yb ranges from ~1.5 at the Moho to 4 at 8km depth), consistent with near fractional melting along a mantle adiabat. Orthopyroxene are highly depleted in LREE but measurements of middle to heavy REE have good reproducibility. We find that REE-in-two-pyroxene temperatures decrease with increasing distance from the Moho, ranging from 1325±10°C near the Moho to 1063±24°C near the base of the mantle section. Using methods from [3], we calculate cooling rates of >1000°C/Myr near the Moho, dropping to rates of <10°C/Myr at the bottom of the section. The faster cooling rate is inconsistent with conductive cooling models. Fast cooling of the mantle lithosphere could be facilitated by infiltration of seawater to or beneath the petrologic Moho. This can explain why abyssal peridotites from ultra-slow spreading centers (which lack a crustal section) have cooling rates comparable to those of Oman peridotites [3]. [1] Hanghøj et al. (2010), JPet 51(1-2), 201-227. [2] Liang et al. (2013), GCA 102, 246-260. [3] Dygert & Liang (2015), EPSL 420, 151-161.

Exploring Moho sharpness in Northeastern North China Craton with frequency-dependence analysis of Ps receiver function

NASA Astrophysics Data System (ADS)

Zhang, P.; Yao, H.; Chen, L.; WANG, X.; Fang, L.

2017-12-01

The North China Craton (NCC), one of the oldest cratons in the world, has attracted wide attention in Earth Science for decades because of the unusual Mesozoic destruction of its cratonic lithosphere. Understanding the deep processes and mechanism of this craton destruction demands detailed knowledge about the deep structure of this region. In this study, we calculate P-wave receiver functions (RFs) with two-year teleseismic records from the North China Seismic Array ( 200 stations) deployed in the northeastern NCC. We observe both diffused and concentered PpPs signals from the Moho in RF waveforms, which indicates heterogeneous Moho sharpness variations in the study region. Synthetic Ps phases generated from broad positive velocity gradients at the depth of the Moho (referred as Pms) show a clear frequency dependence nature, which in turn is required to constrain the sharpness of the velocity gradient. Practically, characterizing such a frequency dependence feature in real data is challenging, because of low signal-to-noise ratio, contaminations by multiples generated from shallow structure, distorted signal stacking especially in double-peak Pms signals, etc. We attempt to address these issues by, firstly, utilizing a high-resolution Moho depth model of this region to predict theoretical delay times of Pms that facilitate more accurate Pms identifications. The Moho depth model is derived by wave-equation based poststack depth migration on both Ps phase and surface-reflected multiples in RFs in our previous study (Zhang et al., submitted to JGR). Second, we select data from a major back azimuth range of 100° - 220° that includes 70% teleseismic events due to the uneven data coverage and to avoid azimuthal influence as well. Finally, we apply an adaptive cross-correlation stacking of Pms signals in RFs for each station within different frequency bands. High-quality Pms signals at different frequencies will be selected after careful visual inspection and adaptive cross-correlation stacking. At last, we will model the stacked Pms signals within different frequency bands to obtain the final sharpness of crust-mantle boundary, which may shed new lights on understanding the mechanism of cratonic reactivation and destruction in the NCC.

A Three - Dimensional Receiver Function Study of the Western United States

NASA Astrophysics Data System (ADS)

Lindsey, C.; Gurrola, H.

2008-12-01

The western United States has a complex geologic history and has been the focus of many regional scale PASSCAL seismic studies that investigate depth variations to the Moho, the 410 km discontinuity, and the 660 km discontinuities. Analysis of depth variations to the Moho in relation to topography is important in understanding the isostatic compensation depth, the thermal state of the upper mantle and boundaries between tectonic provinces. Analysis of the 410 and 660 km discontinuities allow us to determine variations in mantle temperature at these depths and facilitates comparison with tectonic boundaries. This abstract summarizes results from stacking Pds phases throughout the western US using data from all available previous PASSCAL studies in the western U.S. together with data from the EarthScope Transportable array. These data sets enable us to produce an image over the entire western US from the Pacific coast to the Rocky mountain front. Common conversion point stacking of Pds phases was performed by back projecting the data through a 3-D seismic velocity model (surface wave tomography model NA04 by Van der Lee). The images produced show large variations in Moho topography with an average depth of 39.6 kilometer over the western US with ± 7.2 km standard deviation in depth. As would be expected the Moho appears to be deepest beneath the Colorado Plateau and central Montana and shallowest throughout the Basin and Raange. The Moho also appears very shallow beneath eastern Washington. There is a band oof thick crust along the Yellowstone hot spot track. The 410 km discontinuity appears to have a mean depth of 427 km with a standard deviation in depth of ± 10.2 km. At this time the images are still very noisy but in a regional sense the 410 appears deepest beneath the southern part of the image and shallower to the north. Depths to the 660 km discontinuity appear to average 675 km with standard deviation of ± 9.8 km. The 660 does not appear to have a north-south change in depth but appears deepest to the Eastern part of the image and shallower to the west. This relationship may indicate that the thermal state of the 410 is controlled by high temperatures to the south associated with the Basin and Range and cooler to the north were subduction is present. The 660 may be controlled by the transition from warm oceanic and transitional lithosphere to the west and cooler continental lithosphere to the east.

New Lithospheric Model of Taiwan based on the Receiver Function Method

NASA Astrophysics Data System (ADS)

Wang, H.; Zhu, L.; Chen, H.

2008-12-01

Taiwan is situated on the junction area between two subduction systems. The complex orogeny was developed by collision between the Eurasian continental plate and Philippine Sea plate and is still active in the present. Therefore, Taiwan provides unique opportunities for geophysical imaging of the ongoing process underneath. The TAiwan Integrated GEodynamics Research (TAIGER) combined a field program of active and passive seismology, which will undoubtedly be a major step forward in understanding mountain building process. In 2006, we developed a new crustal model of Taiwan from teleseismic waveforms by the receiver function method. We determined lateral variation of Moho discontinuity, crustal thickness (H), and Vp/Vs ratios (Kappa) for each permanent broadband station using all the available teleseismic data collected by BATS (Broadband Array in Taiwan for Seismology) and CWB (Central Weather Bureau). All the broadband stations are distributed uniformly over the whole Taiwan area so that we could delineate the Moho depth contour map. Recently, we concentrated on the three linear temporary arrays of the TAIGER project and obtained three high-resolution images of crustal structure across Taiwan along west-to-east direction from north to south by using the CCP (common-conversion-point) stacking of teleseismic P-to-S converted waves. Sharp impedance contrasts in these images clearly show the relief of each of seismic discontinuities in the crust and upper mantle. The preliminary results show that the Moho depth, 40 to 50 km of central Taiwan is deeper than in other parts of the island, which suggests crustal thickening due to collision. In addition, shallow part of western foothill area show highly acoustic impedance which probably results from thick sediment.

Crustal and upper mantle structure of the north-east of Egypt and the Afro-Arabian plate boundary region from Rayleigh-wave analysis

NASA Astrophysics Data System (ADS)

Corchete, V.; Chourak, M.; Hussein, H. M.; Atiya, K.; Timoulali, Y.

2017-05-01

The crustal and mantle structure of the north-eastern part of Egypt and the surrounding area is shown by means of S-velocity maps for depths ranging from zero to 45 km, determined by the regionalization and inversion of Rayleigh-wave dispersion. This analysis shows several types of crust with an average S-velocity ranging from 2.5 to 3.9 km/s. The values of S-velocity range from 2.5 km/s at the surface to 3.4 km/s at 10 km depth for the Sinai Peninsula, Gulf of Aqaba, Gulf of Suez, Red Sea, Dead Sea, western part of Dead sea and Arabian Plate. In the lower crust, the values of the S-velocity reach 4.0 km/s. In the uppermost mantle, the S-velocities range from 4.4 to 4.7 km/s. The crustal thickness ranges from the oceanic thin crust (around 15-20 km of thickness), for Red Sea and the extended continental margins, to 35-45 km of thickness for the Arabian plate. A gradual increasing crustal thickness is observed from north-east to south-west. While the Moho is located at 30-35 km of depth under the Sinai Peninsula, Gulf of Aqaba, Dead Sea Fault (DSF) and Dead Sea, a thinner crust (20-25 km of thickness) is found at the east of DSF and under the northern and the southern part of the Gulf of Suez. The crustal thickness varies within Sinai from the southern edge to the north, which provided an evidence for the presence of an Early Mesozoic passive margin with thinned continental crust in the north of Sinai. The change of crustal structure between the Gulf of Aqaba and the Gulf of Suez is due to the different tectonic and geodynamic processes affecting Sinai. In general, our results are consistent with surface geology and the Moho depth inferred from reflection and refraction data, receiver function, surface-wave analysis and P-S tomography. The strong variations in the base of the Moho reflect the complex evolution of the African and Arabian plate boundary region.

Synthetic Lg Attenuation on Moho Structure and Group Velocity on Source Depth

NASA Astrophysics Data System (ADS)

Hui, H.; Sandvol, E. A.; Ku, W.

2016-12-01

The regional phase Lg has been the subject of many studies due to its ability to reliably estimate source magnitude and to characterize crustal attenuation, however, the relationship between effective Lg Q and the true intrinsic attenuation of the crust is not well understood. We are working to investigate this relationship by conducting a number of numerical experiments to better understand the nature of Lg scattering attenuation for different type of crustal models. We have partitioned our models by 8-nodes hexahedral meshes with SPECFEM3D-Cartesian which is based on the Spectral Element Method. We have used a time step of 0.01 s to make the simulation stable at high frequencies sufficient enough for our study (about 1.0 Hz). It takes about 50 hours for each model running with 324 processors to generate the waveforms. Then we calculate the effective Lg Q with a Two-Station Method. In order to test our method, we have calculated effective Lg Q tomography for a 2-D model with laterally varying intrinsic attenuation and layered velocity. The tomography result matches the input attenuation model very well. We studied effective Lg Q of models with different Moho structures (flat and with a step) and found that Moho step would lead to lower effective Lg Q than that of flat Moho. Furthermore, we have studied the effective Lg Q tomography of model with 3-D Moho structure and found that effective Lg Q is lower at the area of Moho depth changing than that of flat Moho. This is likely caused by scattering attenuation. We have also modeled the group velocity delay of high frequency (1.0 Hz) Lg, which appears to be dependent on source depth (6km, 10km, 15km and 30km). We have found that the Lg energy arrives later for shallow sources than that of deeper sources which is consistent with prior studies. In the future, we plan to conduct more 3-D attenuation models to investigate azimuthally dependent effective Lg Q.

Eastern US crustal thickness estimates from spectral analysis and inversion of onshore Bouguer gravity anaomalies

NASA Astrophysics Data System (ADS)

Dybus, W.; Benoit, M. H.; Ebinger, C. J.

2011-12-01

The crustal thickness beneath much of the eastern half of the US is largely unconstrained. Though there have been several controlled source seismic surveys of the region, many of these studies suffer from rays that turn in the crust above the Moho, resulting in somewhat ambiguous crustal thickness values. Furthermore, the broadband seismic station coverage east of the Mississippi has been limited, and most of the region remains largely understudied. In this study, we estimated the depth to the Moho using both spectral analysis and inversion of Bouguer gravity anomalies. We systematically estimated depths to lithospheric density contrasts from radial power spectra of Bouguer gravity within 100 km X 100 km windows eastward from the Mississippi River to the Atlantic Coast, and northward from North Carolina to Maine. The slopes and slope breaks in the radial power spectra were computed using an automated algorithm. The slope values for each window were visually inspected and then used to estimate the depth to the Moho and other lithospheric density contrasts beneath each windowed region. Additionally, we performed a standard Oldenburg-Parker inversion for lithospheric density contrasts using various reference depths and density contrasts that are realistic for the different physiographic provinces in the Eastern US. Our preliminary results suggest that the gravity-derived Moho depths are similar to those found using seismic data, and that the crust is relatively thinner (~28-33 km) than expected in beneath the Piedmont region (~35-40 km). Given the relative paucity of seismic data in the eastern US, analysis of onshore gravity data is a valuable tool for interpolating between seismic stations.

The frequency-domain approach for apparent density mapping

NASA Astrophysics Data System (ADS)

Tong, T.; Guo, L.

2017-12-01

Apparent density mapping is a technique to estimate density distribution in the subsurface layer from the observed gravity data. It has been widely applied for geologic mapping, tectonic study and mineral exploration for decades. Apparent density mapping usually models the density layer as a collection of vertical, juxtaposed prisms in both horizontal directions, whose top and bottom surfaces are assumed to be horizontal or variable-depth, and then inverts or deconvolves the gravity anomalies to determine the density of each prism. Conventionally, the frequency-domain approach, which assumes that both top and bottom surfaces of the layer are horizontal, is usually utilized for fast density mapping. However, such assumption is not always valid in the real world, since either the top surface or the bottom surface may be variable-depth. Here, we presented a frequency-domain approach for apparent density mapping, which permits both the top and bottom surfaces of the layer to be variable-depth. We first derived the formula for forward calculation of gravity anomalies caused by the density layer, whose top and bottom surfaces are variable-depth, and the formula for inversion of gravity anomalies for the density distribution. Then we proposed the procedure for density mapping based on both the formulas of inversion and forward calculation. We tested the approach on the synthetic data, which verified its effectiveness. We also tested the approach on the real Bouguer gravity anomalies data from the central South China. The top surface was assumed to be flat and was on the sea level, and the bottom surface was considered as the Moho surface. The result presented the crustal density distribution, which was coinciding well with the basic tectonic features in the study area.

Imaging Ruptured Lithosphere Beneath the Arabian Peninsula Using S-wave Receiver Functions

NASA Astrophysics Data System (ADS)

Hansen, S. E.; Rodgers, A. J.; Schwartz, S. Y.; Al-Amri, A. M.

2006-12-01

The lithospheric thickness beneath the Arabian Peninsula has important implications for understanding the tectonic processes associated with continental rifting along the Red Sea. However, estimates of the lithospheric thickness are limited by the lack of high-resolution seismic observations sampling the lithosphere- asthenosphere boundary (LAB). The S-wave receiver function technique allows point determinations of Moho and LAB depths by identifying S-to-P conversions from these discontinuities beneath individual seismic stations. This method is superior to P-wave receiver functions for identifying the LAB because P-to-S multiple reverberations from shallower discontinuities (such as the Moho) often mask the direct conversion from the LAB while S-to-P boundary conversions arrive earlier than the direct S phase and all multiples arrive later. We interpret crustal and lithospheric structure across the entire Arabian Peninsula from S-wave receiver functions computed at 29 stations from four different seismic networks. Generally, both the Moho and the LAB are shallowest near the Red Sea and become deeper towards the Arabian interior. Near the coast, the Moho increases from about 12 to 35 km, with a few exceptions showing a deeper Moho beneath stations that are situated on higher topography in the Asir Province. The crustal thickening continues until an average depth of about 40-45 km is reached over both the central Arabian Shield and Platform. The LAB near the coast is at a depth of about 50 km, increases rapidly, and reaches an average maximum depth of about 120 km beneath the Arabian Shield. At the Shield-Platform boundary, a distinct step is observed in the lithospheric thickness where the LAB depth increases to about 160 km. This step may reflect remnant lithospheric thickening associated with the Shield's accretion onto the Platform and has an important role in guiding asthenospheric flow beneath the eastern margin of the Red Sea. This work was performed in part under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under contract W-7405-Eng-48.

Global Mapping of Oceanic and Continental Shelf Crustal Thickness and Ocean-Continent Transition Structure

NASA Astrophysics Data System (ADS)

Kusznir, Nick; Alvey, Andy; Roberts, Alan

2017-04-01

The 3D mapping of crustal thickness for continental shelves and oceanic crust, and the determination of ocean-continent transition (OCT) structure and continent-ocean boundary (COB) location, represents a substantial challenge. Geophysical inversion of satellite derived free-air gravity anomaly data incorporating a lithosphere thermal anomaly correction (Chappell & Kusznir, 2008) now provides a useful and reliable methodology for mapping crustal thickness in the marine domain. Using this we have produced the first comprehensive maps of global crustal thickness for oceanic and continental shelf regions. Maps of crustal thickness and continental lithosphere thinning factor from gravity inversion may be used to determine the distribution of oceanic lithosphere, micro-continents and oceanic plateaux including for the inaccessible polar regions (e.g. Arctic Ocean, Alvey et al.,2008). The gravity inversion method provides a prediction of continent-ocean boundary location which is independent of ocean magnetic anomaly and isochron interpretation. Using crustal thickness and continental lithosphere thinning factor maps with superimposed shaded-relief free-air gravity anomaly, we can improve the determination of pre-breakup rifted margin conjugacy and sea-floor spreading trajectory during ocean basin formation. By restoring crustal thickness & continental lithosphere thinning to their initial post-breakup configuration we show the geometry and segmentation of the rifted continental margins at their time of breakup, together with the location of highly-stretched failed breakup basins and rifted micro-continents. For detailed analysis to constrain OCT structure, margin type (i.e. magma poor, "normal" or magma rich) and COB location, a suite of quantitative analytical methods may be used which include: (i) Crustal cross-sections showing Moho depth and crustal basement thickness from gravity inversion. (ii) Residual depth anomaly (RDA) analysis which is used to investigate OCT bathymetric anomalies with respect to expected oceanic values. This includes flexural backstripping to produce bathymetry corrected for sediment loading. (iii) Subsidence analysis which is used to determine the distribution of continental lithosphere thinning. (iv) Joint inversion of time-domain deep seismic reflection and gravity anomaly data which is used to determine lateral variations in crustal basement density and velocity across the OCT, and to validate deep seismic reflection interpretations of Moho depth. The combined interpretation of these independent quantitative measurements is used to determine crustal thickness and composition across the ocean-continent-transition. This integrated approach has been validated on the Iberian margin where ODP drilling provides ground-truth of ocean-continent-transition crustal structure, continent-ocean-boundary location and magmatic type.

Probing magnetic bottom and crustal temperature variations along the Red Sea margin of Egypt

USGS Publications Warehouse

Ravat, D.; Salem, A.; Abdelaziz, A.M.S.; Elawadi, E.; Morgan, P.

2011-01-01

Over 50 magnetic bottom depths derived from spectra of magnetic anomalies in Eastern Egypt along the Red Sea margin show variable magnetic bottoms ranging from 10 to 34. km. The deep magnetic bottoms correspond more closely to the Moho depth in the region, and not the depth of 580??C, which lies significantly deeper on the steady state geotherms. These results support the idea of Wasilewski and coworkers that the Moho is a magnetic boundary in continental regions. Reduced-to-pole magnetic highs correspond to areas of Younger Granites that were emplaced toward the end of the Precambrian. Other crystalline Precambrian units formed earlier during the closure of ocean basins are not strongly magnetic. In the north, magnetic bottoms are shallow (10-15. km) in regions with a high proportion of these Younger Granites. In the south, the shoaling of the magnetic bottom associated with the Younger Granites appears to be restricted to the Aswan and Ras Banas regions. Complexity in the variation of magnetic bottom depths may arise due to a combination of factors: i) regions of Younger (Precambrian) Granites with high magnetite content in the upper crust, leaving behind low Curie temperature titanomagnetite components in the middle and lower crust, ii) rise in the depth of 580??C isotherm where the crust may have been heated due to initiation of intense magmatism at the time of the Red Sea rifting (~. 20. Ma), and iii) the contrast of the above two factors with respect to the neighboring regions where the Moho and/or Curie temperature truncates lithospheric ferromagnetism. Estimates of fractal and centroid magnetic bottoms in the oceanic regions of the Red Sea are significantly below the Moho in places suggesting that oceanic uppermost mantle may be serpentinized to the depth of 15-30 km in those regions. ?? 2011 Elsevier B.V.

Sharp at any Age: Moho boundary thickness estimates along a trans-sect through 2 Ga of tectonic history.

NASA Astrophysics Data System (ADS)

Servali, A.; Levin, V. L.; VanTongeren, J. A.

2015-12-01

In this study we evaluate crustal thickness and Moho sharpness beneath seismic stations in three different tectonic units of the North American continent: the Archean Superior Province, the Proterozoic Grenville Province, and the Paleozoic Appalachian Orogen. Our analysis involves two steps. First, for each site, we produce P-to-S receiver functions (RFs) organized by backazimuth and epicentral distance, and use them to identify the phase most likely representing a conversion from the Moho. Second, we construct averaged RFs for groups of telesismic events located in a similar geographic region, which we employ to examine shapes of Moho P-to-S converted phases in time series with maximum frequencies increasing from 0.25Hz to 2-3 Hz. At some sites we observe a progressive narrowing of a simple Moho converted phase with an increase in frequency, typical of a vertically instantaneous boundary, while at others the converted phase becomes progressively more complex, typical of a diffuse Moho. Thus, we adopt this difference in converted wave shape dependence on increasing frequency as a measure of Moho thickness. Our estimates of Moho thickness range from less than 300 m to over 2 km, with some locations showing evidence for multiple converting boundaries in the 35-50 km depth range. In this study we define "sharp" Moho at those sites where its vertical thickness is less than 1 km. Our results show that sharp Moho is universal in the Archean terranes regardless of surface lithology, likely due to higher Moho temperatures facilitating wide-spread delamination of dense lower crustal rocks. While a sharp Moho is not unique to the Superior Province, all Grenville and Appalachians sites where we find sharp Moho are in regions of granitic plutonism, suggesting a possible general association with reworking and density sorting of the crustal material (e.g. volcanic arc).

Constraining the crustal root geometry beneath Northern Morocco

NASA Astrophysics Data System (ADS)

Díaz, J.; Gil, A.; Carbonell, R.; Gallart, J.; Harnafi, M.

2016-10-01

Consistent constraints of an over-thickened crust beneath the Rif Cordillera (N. Morocco) are inferred from analyses of recently acquired seismic datasets including controlled source wide-angle reflections and receiver functions from teleseismic events. Offline arrivals of Moho-reflected phases recorded in RIFSIS project provide estimations of the crustal thicknesses in 3D. Additional constraints on the onshore-offshore transition are inferred from shots in a coeval experiment in the Alboran Sea recorded at land stations in northern Morocco. A regional crustal thickness map is computed from all these results. In parallel, we use natural seismicity data collected throughout TopoIberia and PICASSO experiments, and from a new RIFSIS deployment, to obtain receiver functions and explore the crustal thickness variations with a H-κ grid-search approach. This larger dataset provides better resolution constraints and reveals a number of abrupt crustal changes. A gridded surface is built up by interpolating the Moho depths inferred for each seismic station, then compared with the map from controlled source experiments. A remarkably consistent image is observed in both maps, derived from completely independent data and methods. Both approaches document a large crustal root, exceeding 50 km depth in the central part of the Rif, in contrast with the rather small topographic elevations. This large crustal thickness, consistent with the available Bouguer anomaly data, favors models proposing that the high velocity slab imaged by seismic tomography beneath the Alboran Sea is still attached to the lithosphere beneath the Rif, hence pulling down the lithosphere and thickening the crust. The thickened area corresponds to a quiet seismic zone located between the western Morocco arcuate seismic zone, the deep seismicity area beneath western Alboran Sea and the superficial seismicity in Alhoceima area. Therefore, the presence of a crustal root seems to play also a major role in the seismicity distribution in northern Morocco.

Constraining the crustal root geometry beneath the Rif Cordillera (North Morocco)

NASA Astrophysics Data System (ADS)

Diaz, Jordi; Gil, Alba; Carbonell, Ramon; Gallart, Josep; Harnafi, Mimoun

2016-04-01

The analyses of wide-angle reflections of controlled source experiments and receiver functions calculated from teleseismic events provide consistent constraints of an over-thickened crust beneath the Rif Cordillera (North Morocco). Regarding active source data, we investigate now offline arrivals of Moho-reflected phases recorded in RIFSIS project to get new estimations of 3D crustal thickness variations beneath North Morocco. Additional constrains on the onshore-offshore transition are derived from onland recording of marine airgun shots from the coeval Gassis-Topomed profiles. A regional crustal thickness map is computed from all these results. In parallel, we use natural seismicity data collected throughout TopoIberia and PICASSO experiments, and from a new RIFSIS deployment, to obtain teleseismic receiver functions and explore the crustal thickness variations with a H-κ grid-search approach. The use of a larger dataset including new stations covering the complex areas beneath the Rif Cordillera allow us to improve the resolution of previous contributions, revealing abrupt crustal changes beneath the region. A gridded surface is built up by interpolating the Moho depths inferred for each seismic station, then compared with the map from controlled source experiments. A remarkably consistent image is observed in both maps, derived from completely independent data and methods. Both approaches document a large modest root, exceeding 50 km depth in the central part of the Rif, in contrast with the rather small topographic elevations. This large crustal thickness, consistent with the available Bouguer anomaly data, favor models proposing that the high velocity slab imaged by seismic tomography beneath the Alboran Sea is still attached to the lithosphere beneath the Rif, hence pulling down the lithosphere and thickening the crust. The thickened area corresponds to a quiet seismic zone located between the western Morocco arcuate seismic zone, the deep seismicity area beneath western Alboran Sea and the superficial seismicity in Alhoceima area. Therefore, the presence of a crustal root seems to play also a major role in the seismicity distribution in northern Morocco.

Depth to the bottom of magnetic sources (DBMS) from aeromagnetic data of Central India using modified centroid method for fractal distribution of sources

NASA Astrophysics Data System (ADS)

Bansal, A. R.; Anand, S.; Rajaram, M.; Rao, V.; Dimri, V. P.

2012-12-01

The depth to the bottom of the magnetic sources (DBMS) may be used as an estimate of the Curie - point depth. The DBMSs can also be interpreted in term of thermal structure of the crust. The thermal structure of the crust is a sensitive parameter and depends on the many properties of crust e.g. modes of deformation, depths of brittle and ductile deformation zones, regional heat flow variations, seismicity, subsidence/uplift patterns and maturity of organic matter in sedimentary basins. The conventional centroid method of DBMS estimation assumes random uniform uncorrelated distribution of sources and to overcome this limitation a modified centroid method based on fractal distribution has been proposed. We applied this modified centroid method to the aeromagnetic data of the central Indian region and selected 29 half overlapping blocks of dimension 200 km x 200 km covering different parts of the central India. Shallower values of the DBMS are found for the western and southern portion of Indian shield. The DBMSs values are found as low as close to middle crust in the south west Deccan trap and probably deeper than Moho in the Chhatisgarh basin. In few places DBMS are close to the Moho depth found from the seismic study and others places shallower than the Moho. The DBMS indicate complex nature of the Indian crust.

Magmatism and underplating, a broadband seismic perspective on the Proterozoic tectonics of the Great Falls and Snowbird Tectonic Zones

NASA Astrophysics Data System (ADS)

Chen, Y.; Gu, Y. J.; Dokht, R.; Wang, R.

2017-12-01

The crustal and lithospheric structures beneath the Western Canada Sedimentary Basin (WCSB) and northern Montana contain vital records of the Precambrian tectonic development of Laurentia. In this study, we analyze the broadband seismic data recorded by the USArray and the most complete set of regional seismic networks to date near the WCSB. We adopt an integrated approach to investigate crustal structure and history, based primarily on P-to-S receiver functions but incorporate results from noise correlation functions, finite-frequency tomography and potential field measurements. In comparison with existing regional and global models, our stacked receiver functions show considerable improvements in the resolution of both Moho depth and Vp/Vs ratio. We identify major variations in Moho depth from the WCSB to the adjacent Cordillera. The Moho deepens steeply from 40 km in the Alberta basin to 50 km beneath the foothills, following Airy isostasy, but thermal buoyancy may be responsible for a flat, shallow ( 35 km) Moho to the west of the Rocky Mountain Trench. The Moho depth also increases sharply near the Snowbird Tectonic Zone (STZ), which is consistent with earlier findings from active-source data. Multiple lower crustal phases, a high velocity shallow mantle and elevated Vp/Vs ratios along the westernmost STZ jointly suggest major Proterozoic subduction and magmatism along this collisional boundary. In northern Montana, the Moho deepens along the Great Falls Tectonic Zone (GFTZ), a proposed Proterozoic suture between the Medicine Hat Block and Wyoming craton. This transition occurs near the Little Belt Mountain, which is located south of the Great Falls Shear Zone, an extensive northeast striking fault system characterized by strong potential field gradients. Similar to the STZ, our receiver functions offer new evidence for Proterozoic underplating in the vicinity of the GFTZ. In view of similar rock ages near the collisional boundaries in all parts of northern Montana and the WCSB basement, we conjecture that the Rae, Hearn, Medicine Hat and Wyoming cratons were all active during the Paleoproterozoic era and their interactions, particularly coeval subductions and collisions, are largely responsible for the basement geology beneath western Laurentia.

An approach to Moho discontinuity recovery from on-orbit GOCE data with application over Indo-Pak region

NASA Astrophysics Data System (ADS)

Eshagh, Mehdi; Hussain, Matloob

2016-10-01

In this research, a modified form of Vening Meinesz-Moritz (VMM) theory of isostasy for the second-order radial derivative of gravitational potential, measured from the Gravity field and steady-state Ocean Circulation Explorer (GOCE), is developed for local Moho depth recovery. An integral equation is organised for inverting the GOCE data to compute a Moho model in combination with topographic/bathymetric heights of SRTM30, sediment and consolidated crystalline basement and the laterally-varying density contrast model of CRUST1.0. A Moho model from EGM2008 to degree and order 180 is also computed based on the same principle for the purpose of comparison. In addition, we compare both of them with the 3 available seismic Moho models; two global and one regional over the Indo-Pak region. Numerical results show that our GOCE-based Moho model is closer to the all seismic models than that of EGM2008. The model is closest to the regional one with a standard deviation of 5.5 km and a root mean squares error of 7.8 km, which is 2.3 km smaller than the corresponding one based on EGM2008.

Lithospheric thickness variations across the North Anatolian Fault Zone

NASA Astrophysics Data System (ADS)

Thompson, D. A.; Rost, S.; Cornwell, D. G.; Houseman, G.; Turkelli, N.; Teoman, U.; Altuncu Poyraz, S.; Kahraman, M.; Gulen, L.; Utkucu, M.; Williams, J. R.

2017-12-01

The North Anatolian Fault Zone (NAFZ) is a major continental strike-slip fault zone, similar in size and scale to the San Andreas system, that extends 1200km across Turkey. These type of faults may broaden significantly with depth or penetrate as narrow features all the way to the lithosphere-asthenosphere boundary (LAB), potentially providing pathways for fluids and magma to shallower levels. The Dense Array for North Anatolia (DANA) was a 73 station broadband seismic network arranged in a rectangular grid (7km station spacing) deployed to image the deep structure of the fault zone. We present here new S-receiver function images that map out both the depth to the Moho and to negative velocity gradients commonly ascribed to the LAB, with preliminary results suggesting lithospheric thicknesses on the order of 80-100km for the region.

3D crustal model of the US and Canada East Coast rifted margin

NASA Astrophysics Data System (ADS)

Dowla, N.; Bird, D. E.; Murphy, M. A.

2017-12-01

We integrate seismic reflection and refraction data with gravity and magnetic data to generate a continent-scale 3D crustal model of the US and Canada East Coast, extending north from the Straits of Florida to Newfoundland, and east from the Appalachian Mountains to the Central Atlantic Ocean. The model includes five layers separated by four horizons: sea surface, topography, crystalline basement, and Moho. We tested magnetic depth-to-source techniques to improve the basement morphology, from published sources, beneath the continental Triassic rift basins and outboard to the Jurassic ocean floor. A laterally varying density grid was then produced for the resultant sedimentary rock layer thickness based on an exponential decay function that approximates sedimentary compaction. Using constant density values for the remaining layers, we calculated an isostatically compensated Moho. The following structural inversion results of the Moho, controlled by seismic refraction depths, advances our understanding of rift-to-drift crustal geometries, and provides a regional context for additional studies.

Crustal structure between Lake Mead, Nevada, and Mono Lake, California

USGS Publications Warehouse

Johnson, Lane R.

1964-01-01

Interpretation of a reversed seismic-refraction profile between Lake Mead, Nevada, and Mono Lake, California, indicates velocities of 6.15 km/sec for the upper layer of the crust, 7.10 km/sec for an intermediate layer, and 7.80 km/sec for the uppermost mantle. Phases interpreted to be reflections from the top of the intermediate layer and the Mohorovicic discontinuity were used with the refraction data to calculate depths. The depth to the Moho increases from about 30 km near Lake Mead to about 40 km near Mono Lake. Variations in arrival times provide evidence for fairly sharp flexures in the Moho. Offsets in the Moho of 4 km at one point and 2 1/2 km at another correspond to large faults at the surface, and it is suggested that fracture zones in the upper crust may displace the Moho and extend into the upper mantle. The phase P appears to be an extension of the reflection from the top of the intermediate layer beyond the critical angle. Bouguer gravity, computed for the seismic model of the crust, is in good agreement with the measured Bouguer gravity. Thus a model of the crustal structure is presented which is consistent with three semi-independent sources of geophysical data: seismic-refraction, seismic-reflection, and gravity.

Imaging the Lower Crust and Moho Beneath Long Beach, CA Using Autocorrelations

NASA Astrophysics Data System (ADS)

Clayton, R. W.

2017-12-01

Three-dimensional images of the lower crust and Moho in a 10x10 km region beneath Long Beach, CA are constructed from autocorrelations of ambient noise. The results show the Moho at a depth of 15 km at the coast and dipping at 45 degrees inland to a depth of 25 km. The shape of the Moho interface is irregular in both the coast perpendicular and parallel directions. The lower crust appears as a zone of enhanced reflectivity with numerous small-scale structures. The autocorrelations are constructed from virtual source gathers that were computed from the dense Long Beach array that were used in the Lin et al (2013) study. All near zero-offset traces within a 200 m disk are stacked to produce a single autocorrelation at that point. The stack typically is over 50-60 traces. To convert the auto correlation to reflectivity as in Claerbout (1968), the noise source autocorrelation, which is estimated as the average of all autocorrelations is subtracted from each trace. The subsurface image is then constructed with a 0.1-2 Hz filter and AGC scaling. The main features of the image are confirmed with broadband receiver functions from the LASSIE survey (Ma et al, 2016). The use of stacked autocorrelations extends ambient noise into the lower crust.

Gravity modelling of the Hellenic subduction zone — a regional study

NASA Astrophysics Data System (ADS)

Casten, U.; Snopek, K.

2006-05-01

The Hellenic subduction zone is clearly expressed in the arc-shaped distribution of earthquake epicenters and gravity anomalies, which connect the Peloponnesos with Crete and Anatolia. In this region, oceanic crust of the African plate collides northward with continental crust of the Aegean microplate, which itself is pushed apart to the south-west by the Anatolian plate and, at the same time, is characterised by crustal extension. The result is an overall collision rate of up to 4 cm/year and a retreating subduction process. Recent passive and active seismic studies on and around Crete gave first, but not in all details consistent, structural results useful for supporting gravity modelling. This was undertaken with the aim of presenting the first 3D density structure of the entire subduction zone. Gravity interpretation was based on a Bouguer map, newly compiled using data from land, marine and satellite sources. The anomalies range from + 170 mGal (Cretan Sea) to - 10 mGal (Mediterranean Ridge). 3D gravity modelling was done applying the modelling software IGMAS. The computed Bouguer map fits the low frequency part of the observed one, which is controlled by variations in Moho depth (less than 20 km below the Cretan Sea and extending 30 km below Crete) and the extremely thick sedimentary cover (partly up to 18 km) of the Mediterranean Ridge. The southernmost edge of the Eurasian plate, with its more triangular-shaped backstop area, was traced south off Crete. Only 50 to 100 km further to the south, the edge of the African continent was traced as well. In between these boundaries there is African oceanic crust, which has a clear arc-shaped detachment line situated at the Eurasian continental edge. The subduction arc is open towards the north, its slab separates hotter mantle material (lower density) below the updoming Moho of the Cretan Sea from colder one (higher density) in the south. Subjacent to the upper continental crust of Crete is a thickened layer of lower crust followed by the subducted oceanic crust with some mantle material as intermediate layer. The depth of the oceanic Moho below Crete is 50 km. The presence and structure of subducted or underplated sediments remains uncertain.

Moho depth and equivalent elastic thickness of the lithosphere over the Vema Channel: A new evidence of an aborted ridge

NASA Astrophysics Data System (ADS)

Constantino, Renata Regina; Costa, Iago Sousa Lima; Hackspacher, Peter Christian; de Souza, Iata Anderson

2018-03-01

We investigate the Vema Channel in terms of spatial variations of the elastic thickness (Te) in the frame of the thin plate flexure model using the convolutive method. The modeling of the Moho in terms of the thin plate flexure model is done by a least squares approximation of the Moho obtained from gravity inversion. The flexure is calculated by the convolution of the crustal load with the point-load flexure response curves. The RMS difference between the gravity and flexure Moho surfaces is minimized by varying the Te by inverse modeling. The result is a solution of the flexed crust that is in best agreement with the long-wavelength component of the gravity field. The flexure Moho depths vary between 12 and 18 km and agree well with those obtained from gravity inversion. The spatial variations of Te range from 2 to 30 km and have a good correlation with the geological interpretation for an aborted ridge near Vema Channel, called in this paper as the Vema Aborted Ridge (VAR). The occurring of seamounts appears to be correlated to a weak and deformed region. Attempts of crustal breakup are marked by high Te values (30 km) while lower values (3-12 km) are found for the suggested aborted ridge. The VAR is on Isochron of 93 Ma and shows symmetrical older along both sides of its axis. Asymmetric magnetic anomalies are found over the ridge and may be related to upper-extended continental crust broken by the Vema.

Surface wave tomography of Europe from ambient seismic noise

NASA Astrophysics Data System (ADS)

Lu, Yang; Stehly, Laurent; Paul, Anne

2017-04-01

We present a European scale high-resolution 3-D shear wave velocity model derived from ambient seismic noise tomography. In this study, we collect 4 years of continuous seismic recordings from 1293 stations across much of the European region (10˚W-35˚E, 30˚N-75˚N), which yields more than 0.8 million virtual station pairs. This data set compiles records from 67 seismic networks, both permanent and temporary from the EIDA (European Integrated Data Archive). Rayleigh wave group velocity are measured at each station pair using the multiple-filter analysis technique. Group velocity maps are estimated through a linearized tomographic inversion algorithm at period from 5s to 100s. Adaptive parameterization is used to accommodate heterogeneity in data coverage. We then apply a two-step data-driven inversion method to obtain the shear wave velocity model. The two steps refer to a Monte Carlo inversion to build the starting model, followed by a linearized inversion for further improvement. Finally, Moho depth (and its uncertainty) are determined over most of our study region by identifying and analysing sharp velocity discontinuities (and sharpness). The resulting velocity model shows good agreement with main geological features and previous geophyical studies. Moho depth coincides well with that obtained from active seismic experiments. A focus on the Greater Alpine region (covered by the AlpArray seismic network) displays a clear crustal thinning that follows the arcuate shape of the Alps from the southern French Massif Central to southern Germany.

Magmatic arc structure around Mount Rainier, WA, from the joint inversion of receiver functions and surface wave dispersion

NASA Astrophysics Data System (ADS)

Obrebski, Mathias; Abers, Geoffrey A.; Foster, Anna

2015-01-01

The deep magmatic processes in volcanic arcs are often poorly understood. We analyze the shear wave velocity (VS) distribution in the crust and uppermost mantle below Mount Rainier, in the Cascades arc, resolving the main velocity contrasts based on converted phases within P coda via source normalization or receiver function (RF) analysis. To alleviate the trade-off between depth and velocity, we use long period phase velocities (25-100 s) obtained from earthquake surface waves, and at shorter period (7-21 s) we use seismic noise cross correlograms. We use a transdimensional Bayesian scheme to explore the model space (VS in each layer, number of interfaces and their respective depths, level of noise on data). We apply this tool to 15 broadband stations from permanent and Earthscope temporary stations. Most results fall into two groups with distinctive properties. Stations east of the arc (Group I) have comparatively slower middle-to-lower crust (VS = 3.4-3.8 km/s at 25 km depth), a sharp Moho and faster uppermost mantle (VS = 4.2-4.4 km/s). Stations in the arc (Group II) have a faster lower crust (VS = 3.7-4 km/s) overlying a slower uppermost mantle (VS = 4.0-4.3 km/s), yielding a weak Moho. Lower crustal velocities east of the arc (Group I) most likely represent ancient subduction mélanges mapped nearby. The lower crust for Group II ranges from intermediate to felsic. We propose that intermediate-felsic to felsic rocks represent the prearc basement, while intermediate composition indicates the mushy andesitic crustal magmatic system plus solidified intrusion along the volcanic conduits. We interpret the slow upper mantle as partial melt.

Subduction of lower continental crust beneath the Pamir imaged by receiver functions from the seismological TIPAGE network

NASA Astrophysics Data System (ADS)

Schneider, F. M.; Yuan, X.; Schurr, B.; Mechie, J.; Sippl, C.; Kufner, S.; Haberland, C. A.; Minaev, V.; Oimahmadov, I.; Gadoev, M.; Abdybachaev, U.; Orunbaev, S.

2013-12-01

As the northwestern promontory of the Tibetan Plateau, the Pamir forms an outstanding part of the India-Asia convergence zone. The Pamir plateau has an average elevation of more than 4000 m surrounded by peaks exceeding 7000 m at its northern, eastern and southern borders. The Pamir is thought to consist of the same collage of continental terranes as Tibet. However, in this region the Indian-Asian continental collision presents an extreme situation since, compared to Tibet, in the Pamir a similar amount of north-south convergence has been accommodated within a much smaller distance. The Pamir hosts a zone of intermediate depth earthquakes being the seismic imprint of Earth's most spectacular active intra-continental subduction zone. We present receiver function (RF) images from the TIPAGE seismic profile giving evidence that the intermediate depth seismicity is situated within a subducted layer of lower continental crust: We observe a southerly dipping 10-15 km thick low-velocity zone (LVZ), that starts from the base of the crust and extends to a depth of more than 150 km enveloping the intermediate depth earthquakes that have been located with high precision from our local network records. In a second northwest to southeast cross section we observe that towards the western Pamir the dip direction of the LVZ bends to the southeast following the geometry of the intermediate depth seismic zone. Our observations imply that the complete arcuate intermediate depth seismic zone beneath the Pamir traces a slab of subducting Eurasian continental lower crust. These observations provide important implications for the geodynamics of continental collision: First, it shows that under extreme conditions lower crust can be brought to mantle depths despite its buoyancy, a fact that is also testified by the exhumation of ultra-high pressure metamorphic rocks. Recent results from teleseismic tomography show a signal of Asian mantle lithosphere down to 600 km depth, implying a great amount of mantle lithosphere to be involved in the subduction, which possibly transmits pull forces to the lower crust to overcome its buoyancy. Secondly, the observation that earthquakes occur within the subducted crust implies that similar to oceanic subduction, metamorphic processes within the lower continental crust can cause or enable earthquakes at depths, where the high pressure and temperature conditions would normally not allow brittle failure of rocks. For imaging of the dipping LVZ, cross sections of Q- and T-component RFs are generated using a migration technique that accounts for the inclination of the conversion layers. Furthermore we present a Moho map of the Pamir, showing crustal thickness in most places of the Pamir ranging between 65 km and 75 km, while the greatest Moho depths of around 80 km are observed at the upper end of the LVZ. The surrounding areas namely the Tajik Depression, and the Ferghana and Tarim Basins show Moho depths of around 40 to 45 km giving an estimate of the pre-collisional crustal thickness of the former Basin area that was overthrust by the Pamir.

Gravity field and nature of continent-continent collision along the Himalaya

NASA Astrophysics Data System (ADS)

Verma, R. K.

Gravity field (Bouguer) in the Himalaya is characterised by large negative-values ranging from nearly -180 mGal to over -450 mGal in Naga-Parbat/Haramosh massif which go up to -550 mGal in the Karakoram region. The observed Bouguer anomaly in NW Himalaya has been interpreted along a profile passing from Gujranwala (located at the edge of the Indian shield) to the Haramosh massif in terms of Moho depth and density contrast between the crust and the mantle. The Moho depth is interpreted to increase from nearly 35 km near the edge of Indian shield to 75 km (below sea level) underneath the Haramosh massif. A similar model is applicable to a profile passing to the west of Nanga Parbat massif, from Gujranwala to Ghizar, through the Kohistan region. However, along this profile high density lower crustal rocks appear to have been emplaced in the upper part along the Main Mantle thrust. The gravityanomalies in the Nepal-Tibet region hasbeen interpreted in terms of a northward sloping Moho which down faulted by about 15 km to attain a depth of 65 km around Tingri which corresponds to explosion seismology data. The nature of isostatic compensation prevailing underneath the Himalaya has been discussed.

It's Still Downhill From Tonopah to Las Vegas, but the Crust Doesn't Ride for Free

NASA Astrophysics Data System (ADS)

Pettit, M. M.; Schulte-Pelkum, V.; Sheehan, A.

2008-12-01

We investigate the crustal thickness in the central Basin and Range province of the western US. There is a gravity anomaly at 37 degrees N latitude at which the gravity increases ~100mgal from North to South over a distance of ~100 km. The majority of recent publications ascribe the gravity signal to a mantle influence based on observations of near constant crustal thickness in the area. However, Moho depth estimates are sparse in the area, and therefore higher gravity due to a thinner crust in the south is still a possible explanation to date. In order to determine Moho depths, we examined teleseismic receiver functions from broadband and short-period stations from 1993 to 2008 located within the region, including stations from the recent Earthscope Transportable Array deployment. We used a total of 11,751 high-quality receiver functions at 80 stations and picked arrival times of the Moho converted phase from backazimuthal and moveout stacks. Moho depths were determined from these arrival times using a fixed velocity model, as well as from forward modeling of moveout curves of the direct conversion as well as multiples. Our results confirm the presence of thinner crust south of 37N latitude. Assuming an average crustal velocity of 6.3 km/s and a Vp/Vs ratio of 1.732, we found an average crustal thickness between 33 and 34 km north of 37N, and roughly 27 km south of 37N. We also found an interesting pattern of thin crust trending NE from the southern tip of Nevada to approximately 38N, 245E. The findings indicate that a least part of the gravity signal is of crustal origin.

Lithospheric structure of the southern French Alps inferred from broadband analysis

NASA Astrophysics Data System (ADS)

Bertrand, E.; Deschamps, A.

2000-11-01

Broadband receiver functions analysis is commonly used to evaluate the fine-scale S-velocity structure of the lithosphere. We analyse teleseismic P-waves and their coda from 30 selected teleseismic events recorded at three seismological stations of to the French TGRS network in the Alpes Maritimes. Receiver functions are computed in the time domain using an SVD matrix inversion method. Dipping Moho and lateral heterogeneities beneath the array are inferred from the amplitude, arrival time and polarity of locally-generated PS phases. We propose that the Moho dips 11° towards 25°±10°N below station CALF, in the outer part of the Alpine belt. At this station, we determine a Moho depth of about 20±2 km; the same depth is suggested below SAOF station also located in the fold-trust belt. Beneath station STET located in the inner part of the Alpine belt, the Moho depth increases to 30 km and dips towards the N-NW. Moreover, 1D-modelling of summed receiver function from STET station constrains a crustal structure significantly different from that observed at stations located in the outer part of the Alps. Indeed, beneath CALF and SAOF stations we need a 2 km thick shallow low velocity layer to fit best the observed receiver functions whereas this layer seems not to be present beneath STET station. Because recent P-coda studies have shown that near-receiver scattering can dominate teleseismic P-wave recordings in tectonically complicated areas, we account for effect of scattering energy in our records from array measurements. As the array aperture is wide relative to the heterogeneity scale length in the area, the array analysis produces only smooth imaging of scatterers beneath the stations.

Passive seismic experiment and receiver functions analysis to determine crustal structure at the contact of the northern Dinarides and southwestern Pannonian Basin

NASA Astrophysics Data System (ADS)

Šumanovac, Franjo; Hegedűs, Endre; Orešković, Jasna; Kolar, Saša; Kovács, Attila C.; Dudjak, Darko; Kovács, István J.

2016-06-01

Passive seismic experiment was carried out at the SW contact of the Dinarides and Pannonian basin to determine the crustal structure and velocity discontinuities. The aim of the experiment was to define the relationship between the Adriatic microplate and the Pannonian segment as a part of the European plate. Most of the temporary seismic stations were deployed in Croatia along the Alp07 profile-a part of the active-source ALP 2002 project. About 300-km-long profile stretches from Istra peninsula to the Drava river, in a WSW-ESE direction. Teleseismic events recorded on 13 temporary seismic stations along the profile were analysed by P-receiver function method. Two types of characteristic receiver functions (RF) have been identified, belonging to Dinaridic and Pannonian crusts as defined on the Alp07 profile, while in transitional zone there are both types. Three major crustal discontinuities can be identified for the Dinaridic type: sedimentary basement, intracrustal discontinuity and Mohorovičić discontinuity, whereas the Pannonian type revealed only two discontinuities. The intracrustal discontinuity was not observed in the Pannonian type, thus pointing to a single-layered crust in the Pannonian basin. Two interpretation methods were applied: forward modelling of the receiver functions and H-κ stacking method, and the results were compared with the active-source seismic data at deep refraction profile Alp07. The receiver function modelling has given reliable results of the Moho depths that are in accordance with the seismic refraction results at the end of the Alp07 profile, that is in the area of Pannonian crust characterized by simple crustal structure and low seismic velocities (Vp between 5.9 and 6.2 km s-1). In the Dinarides and its peripheral parts, receiver function modelling regularly gives greater Moho depths, up to +15 per cent, due to more complex crustal structure. The depths of the Moho calculated by the H-κ stacking method vary within wide limits (±13 km), due to band limited data of short-period stations. The results at five stations have to be rejected because of huge deviations in comparison with all previous results, while at the other seven stations the Moho depths vary within ±15 per cent around the Moho discontinuity of the Alp07 profile.

The Lunar Crustal Thickness from Analysis of the Lunar Prospector Gravity and Clementine Topography Datasets

NASA Technical Reports Server (NTRS)

Asmar, S.; Schubert, G.; Konopliv, A.; Moore, W.

1999-01-01

The Lunar Prospector spacecraft has mapped the gravity field of the Moon to a level of resolution never achieved before, and a spherical harmonic representation to degree and order 100 is available. When combined with the topography dataset produced by the Clementine mission, the resulting Bouguer anomaly map is interpreted to model the thickness of the lunar crust. Such models are crucial to understanding the lunar thermal history and the formation of geological features such as mascon basins, several more of which have been newly discovered from this dataset. A two-layer planetary model was used to compute the variations of the depth to the lunar Moho. The thickness values ranged from near 0 to 120 km. There is significant agreement with previous work using the Clementine gravitational field data with differences in specific locations such as South Pole-Aitken Basin, for example.

Lithospheric Structure of the Zagros and Alborz Mountain Belts (Iran) from Seismic Imaging

NASA Astrophysics Data System (ADS)

Paul, A.; Hatzfeld, D.; Kaviani, A.; Tatar, M.

2008-12-01

We present a synthesis of the results of two dense temporary passive seismic experiments installed for a few months across Central Zagros for the first one, and from North-western Zagros to Alborz for the second one. On both transects, the receiver function analysis shows that the crust has an average thickness of ~ 43 km beneath the Zagros fold-and-thrust belt and the Iranian plateau. The crust is thicker in the back side of the Main Zagros Reverse Fault (MZRF), with a larger maximum Moho depth in Central Zagros (69 ± 2 km) than in North-western Zagros (56 ± 2 km). To reconcile Bouguer anomaly data and Moho depth profile of Central Zagros, we proposed that the thickening is related to overthrusting of the Arabian margin by Central Iran on the MZRF considered as a major thrust fault rooted at Moho depth. The better-quality receiver functions of NW Zagros display clear conversions on a low-velocity channel which cross-cuts the whole crust from the surface trace of the MZRF to the Moho on 250-km length. Waveform modeling shows that the crustal LVZ is ~ 10-km thick with a S-wave velocity 8-30 % smaller than the average crustal velocity. We interpret the low-velocity channel as the trace of the thrust fault and the suture between the Arabian and the Iranian lithospheres. We favour the hypothesis of the LVZ being due to sediments of the Arabian margin dragged to depth during the subduction of the Neotethyan Ocean. At upper mantle depth, we find shield-like shear-wave velocities in the Arabian upper-mantle, and lower velocities in the Iranian shallow mantle (50-150 km) which are likely due to higher temperature. The lack of a high-velocity anomaly in the mantle northeast of the MZRF suture suggests that the Neotethian oceanic lithosphere is now detached from the Arabian margin. The crust of the Alborz mountain range is not thickened in relation with its high elevations, but its upper mantle has low P-wave velocities.

Moho depth across the Trans-European Suture Zone from ambient vibration autocorrelations

NASA Astrophysics Data System (ADS)

Becker, Gesa; Knapmeyer-Endrun, Brigitte

2017-04-01

In 2018 the InSight mission to Mars will deploy a seismic station on the planet. This seismic station will consist of a three-component very broadband seismic sensor and a collocated three-component short period seismometer. Single station methods are therefore needed to extract information from the data and learn more about the interior structure of Mars. One potential method is the extraction of reflected phases from autocorrelations. Here autocorrelations are derived from ambient seismic noise to make the most of the data expected, as seismicity on Mars is likely less abundant than on Earth. These autocorrelations are calculated using a phase autocorrelation algorithm and time-frequency domain phase-weighted stacking as the main processing steps in addition to smoothing the spectrum of the data with a short term-long term average algorithm. Afterward the obtained results are filtered and analyzed in the frequency range of 1-2 Hz. The developed processing scheme is applied to data from permanent seismic stations located in different geological provinces across Europe, i.e. the Upper Rhine Graben, Central European Platform, Bohemian Massif, Northern German and Polish Basin, and the East European Craton, with varying Moho depths between 25-50 km. These crustal thicknesses are comparable to various estimates for Mars, therefore providing a good reference and indication of resolvability for Moho depths that might be encountered at the landing site. Changes in reflectivity can be observed in the calculated autocorrelations. The lag times of these changes are converted into depths with the help of available velocity information (EPcrust and local models for Poland and the Czech Republic, respectively) and the results are compared to existing information on Moho depths, which show good agreement. The results are temporarily stable, but show a clear correlation with the existence of cultural noise. Based on the closely located broadband and short period stations of the GERESS-array, it is shown that the processing scheme is also applicable to short period stations. Subsequently it is applied to the mainly short period and temporary stations of the PASSEQ network along the seismic profile POLONAISE P4, running from Eastern Germany to Lithuania crossing the Trans-European Suture Zone.

The South Tibetan Tadpole Zone: Ongoing density sorting at the Moho beneath the Indus-Tsangpo suture zone (and beneath volcanic arcs?)

NASA Astrophysics Data System (ADS)

Kelemen, Peter; Hacker, Bradley

2016-04-01

Some Himalayan cross-sections show Indian crust thrust beneath Tibetan crust, with no intervening mantle wedge (e.g., Powell & Conaghan 73), others indicate thickening of both crustal sections, juxtaposed along a steep suture (e.g., Dewey & Burke 73), and many combine features of both end-members (e.g., Argand 24). To understand crustal scale structure and related phenomena, we focus on data from an area in southern Tibet at 28-30°N, 84-91°E. 21st century observations in this area show a horizontal Moho at ca 80 km depth, extending from thickened Indian crust, across a region where Tibetan crust is interpreted to overlie Indian crust, into thickened Tibetan crust (Zhao et al 01; Monsalve et al 08; Wittlinger et al 09; Nabelek et al 09; Kind et al 02; Schulte-Pelkum et al 05; Shi et al 15). About half the subducted Indian crustal volume is present, whereas the other half is missing (Replumaz et al 10). Vp/Vs indicates the alpha-beta quartz transition is at ca 50 km depth (Sheehan et al 13). Miocene lavas include primitive andesites probably formed by interaction of crustal material with mantle peridotite at > 1000°C (Turner et al 93; Williams et al 01, 04; Chung et al 05). Thermobarometry of xenoliths in a 12.7 Ma dike records ~ 1100°C at 2.2-2.6 GPa and 920°C at 1.7 GPa (Chan et al 09). Biotite-rich pyroxenites among the xenoliths, similar to those in central Tibet (Hacker et al 00) and the Pamirs (Hacker et al 05), may form via reaction of hot crustal lithologies and mantle peridotite (e.g., Sekine & Wyllie 82, 83). These data, taken together, indicate Miocene to present day temperatures exceeding 800°C from ca 50 km depth to the Moho, unlike thermal models with a hot mid-crust and cold Moho (McKenzie & Priestley 08, Craig et al 12, Wang et al 13; Nabelek & Nabelek 14), and despite the observation of numerous, near-Moho earthquakes (Chen & Molnar 83; Chen & Yang 04; Monsalve et al 06; Priestley et al 08; Craig et al 12) interpreted by many as brittle failure at less than 700°C (e.g. Jackson 02). We build on earlier studies (LePichon et al 92, 97; Schulte-Pelkum et al 05; Monsalve et al 08) to develop the hypothesis that there is rapid growth of garnet at 80 km and 1000°C within subducting Indian crust, causing increased rock densities. Dense eclogites founder into the mantle, while relatively buoyant lithologies accumulate in thickening lower crust. Mantle return flow plus radioactive heating in thick, felsic crust maintains high temperature, facilitating formation of hybrid magmas and pyroxenites. The crustal volume grows at 760 cubic m/yr/m of strike length. Moho-depth earthquakes may be due to localized deformation and thermal runaway in weak layers and along the margins of dense, foundering diapirs (e.g., Larsen & Yuen 97; Braeck & Podladchikov 07; Kelemen & Hirth 07; Lister et al 08; Kufner et al 16). A similar process may take place at some convergent margins, where forearc crust is thrust beneath hot, magmatic arc crust, leading to extensive, Moho-depth density sorting and hybrid crust-mantle magmatism in Arc Tadpole Zones.

Curie Point Depth of the Iberian Peninsula and Surrounding Margins. A Thermal and Tectonic Perspective of its Evolution

NASA Astrophysics Data System (ADS)

Andrés, J.; Marzán, I.; Ayarza, P.; Martí, D.; Palomeras, I.; Torné, M.; Campbell, S.; Carbonell, R.

2018-03-01

In this work the thermal structure of the Iberian Peninsula is derived from magnetic data by calculating the bottom of the magnetization, assumed to be the Curie-point depth (CPD) isotherm, which accounts for the depth at which magnetite becomes paramagnetic (580°C). Comparison of the CPD with crustal thickness maps along with a heat flow map derived from the CPD provides new insights on the lithospheric thermal regime. Within Iberia, the CPD isotherm has thickness in the range of 17 to 29 km. This isotherm is shallow (<18 km) offshore, where the lithosphere is thinner. In continental Iberia, the NW Variscan domain presents a magnetic response that is most probably linked to thickening and later extension processes during the late Variscan Orogeny, which resulted in widespread crustal melting and emplacement of granites (in the Central Iberian Arc). The signature of the CPD at the Gibraltar Arc reveals a geometry consistent with the slab roll-back geodynamic model that shaped the western Mediterranean. In offshore areas, a broad extension of magnetized upper mantle is found. Serpentinization of the upper mantle, probably triggered in an extensional context, is proposed to account for the magnetic signal. The Atlantic margin presents up to 8 km of serpentinites, which, according to the identification of exhumed mantle, correlates with a hyperextended margin. The Mediterranean also presents generalized serpentinization up to 6 km in the Algerian Basin. Furthermore, a heat flow map and a Moho temperature map derived from the CPD are presented.

Array-Based Receiver Function Analysis of the Subducting Juan de Fuca Plate Beneath the Mount St. Helens Region and its Implications for Subduction Geometry and Metamorphism

NASA Astrophysics Data System (ADS)

Mann, M. E.; Abers, G. A.; Creager, K. C.; Ulberg, C. W.; Crosbie, K.

2017-12-01

Mount St. Helens (MSH) is unusual as a prolific arc volcano located 50 km towards the forearc of the main Cascade arc. The iMUSH (imaging Magma Under mount St. Helens) broadband deployment featured 70 seismometers at 10-km spacing in a 50-km radius around MSH, spanning a sufficient width for testing along-strike variation in subsurface geometry as well as deep controls on volcanism in the Cascade arc. Previous estimates of the geometry of the subducting Juan de Fuca (JdF) slab are extrapolated to MSH from several hundred km to the north and south. We analyze both P-to-S receiver functions and 2-D Born migrations of the full data set to locate the upper plate Moho and the dip and depth of the subducting slab. The strongest coherent phase off the subducting slab is the primary reverberation (Ppxs; topside P-to-S reflection) from the Moho of the subducting JdF plate, as indicated by its polarity and spatial pattern. Migration images show a dipping low velocity layer at depths less than 50 km that we interpret as the subducting JdF crust. Its disappearance beyond 50 km depth may indicate dehydration of subducting crust or disruption of high fluid pressures along the megathrust. The lower boundary of the low velocity zone, the JdF Moho, persists in the migration image to depths of at least 90 km and is imaged at 74 km beneath MSH, dipping 23 degrees. The slab surface is 68 km beneath MSH and 85 km beneath Mount Adams volcano to the east. The JdF Moho exhibits 10% velocity contrasts as deep as 85 km, an observation difficult to reconcile with simple models of crustal eclogitization. The geometry and thickness of the JdF crust and upper plate Moho is consistent with similar transects of Cascadia and does not vary along strike beneath iMUSH, indicating a continuous slab with no major disruption. The upper plate Moho is clear on the east side of the array but it disappears west of MSH, a feature we interpret as a result of both serpentinization of the mantle wedge and a westward increase in wavespeed of the continental crust. The seismically-imaged surface of the subducting JdF slab at 68 km beneath MSH is the shallowest yet documented beneath an arc volcano. Combined with the inference of serpentinization in the mantle wedge, this geometry presents a problem in that vertical mantle melt migration seems unfeasible, yet mantle melts contribute to erupted MSH magmas.

Seismic anisotropy in central North Anatolian Fault Zone and its implications on crustal deformation

NASA Astrophysics Data System (ADS)

Licciardi, A.; Eken, T.; Taymaz, T.; Piana Agostinetti, N.; Yolsal-Çevikbilen, S.

2018-04-01

We investigate the crustal seismic structure and anisotropy around the central portion of the North Anatolian Fault Zone, a major plate boundary, using receiver function analysis. The characterization of crustal seismic anisotropy plays a key role in our understanding of present and past deformation processes at plate boundaries. The development of seismic anisotropy in the crust arises from the response of the rocks to complicated deformation regimes induced by plate interaction. Through the analysis of azimuthally-varying signals of teleseismic receiver functions, we map the anisotropic properties of the crust as a function of depth, by employing the harmonic decomposition technique. Although the Moho is located at a depth of about 40 km, with no major offset across the area, our results show a clear asymmetric distribution of crustal properties between the northern and southern blocks, divided by the North Anatolian Fault Zone. Heterogeneous and strongly anisotropic crust is present in the southern block, where complex intra-crustal signals are the results of strong deformation. In the north, a simpler and weakly anisotropic crust is typically observed. The strongest anisotropic signal is located in the first 15 km of the crust and is widespread in the southern block. Stations located on top of the main active faults in the area indicate the highest amplitudes, together with fault-parallel strikes of the fast plane of anisotropy. We interpret the origin of this signal as due to structure-induced anisotropy, and roughly determine its depth extent up to 15-20 km for these stations. Away from the faults, we suggest the contribution of previously documented uplifted basement blocks to explain the observed anisotropy at upper and middle crustal depths. Finally, we interpret coherent NE-SW orientations below the Moho as a result of frozen-in anisotropy in the upper mantle, as suggested by previous studies.

Receiver Functions Imaging of the Moho and LAB in the Southern Caribbean plate boundary and Venezuela

NASA Astrophysics Data System (ADS)

Masy, J.; Levander, A.; Niu, F.

2011-12-01

We have made teleseismic Ps and Sp receiver functions from data recorded from 2003 to 2009 by the permanent national seismic network of Venezuela, the BOLIVAR (Broadband Onshore-offshore Lithospheric Investigation of Venezuela and the Antilles arc Region) and WAVE (Western Array for Venezuela) experiments. The receiver functions show rapid variations in Moho and lithosphere-asthenosphere boundary (LAB) depths both across and along the southern Caribbean plate boundary region. We used a total of 69 events with Mw > 6 occurring at epicentral distances from 30° to 90° for the Ps receiver functions, and 43 events with Mw > 5.7 from 55° to 85° to make Sp receiver functions. For CCP stacking we constructed a 3D velocity model from numerous active source profiles (Schmitz et al., 2001; Bezada et al., 2007; Clark et al., 2008; Guedez, 2008; Magnani et al., 2009), from finite-frequency P wave upper mantle tomography model of Bezada et al., (2010) and the Rayleigh wave tomography model of Miller et al., (2009). The Moho ranges in depth from ~25 km beneath the Caribbean Large Igneous Provinces to ~55 km beneath the Mérida Andes in western Venezuela. These results are consistent with previous receiver functions studies (Niu et al., 2007) and the available active source profiles. Beneath the Maracaibo Block in northwestern Venezuela, we observe a strong positive signal at 40 to 60 km depth dipping ~6° towards the continent. We interpret this as the Moho of the Caribbean slab subducting beneath northernmost South America from the west. Beneath northern Colombia and northwestern Venezuela the top of this slab has been previously inferred from intermediate depth seismicity (Malavé and Suarez, 1995), which indicates a slab dipping between 20° - 30° beneath Lake Maracaibo. Our results could indicate that the slab is tearing beneath Lake Maracaibo as suggested previously by Masy et al. (2011). The deeper (> 100 km depth) part of the slab has been imaged using P-wave tomography (Bezada et al, 2010). Like others we attribute the uplift of the Mérida Andes to flat Caribbean slab subduction (for example Kellogg and Bonini, 1982). In central Venezuela beneath the Cordillera de la Costa we observe a positive signal shallower than the Moho at <30 km depth beneath the entire range. We interpret this as a detachment surface beneath Caribbean & arc terranes thrust onto the SA margin (Bezada et al., 2010). The lithosphere-asthenosphere boundary (LAB) beneath the Mérida Andes is shallow, ~65km depth, and parallels the range. In the plate boundary region under the Cordillera de la Costa the lithosphere is also thin, ~65km, beneath the Cariaco basin the lithosphere thickens to 85 km. In the far east under Serranía del Interior the lithosphere is ~75 km. Cratonic lithosphere thickness varies from 85 to 100 km.

Crustal P-wave velocity structure from Altyn Tagh to Longmen mountains along the Taiwan-Altay geoscience transect

USGS Publications Warehouse

Wang, Y.-X.; Mooney, W.D.; Han, G.-H.; Yuan, X.-C.; Jiang, M.

2005-01-01

Based upon the seismic experiments along Geoscience Transect from the Altyn Tagh to the Longmen Mountains, the crustal P-wave velocity structure was derived to outline the characteristics of the crustal structure. The section shows a few significant features. The crustal thickness varies dramatically, and is consistent with tectonic settings. The Moho boundary abruptly drops to 73km depth beneath the southern Altyn Tagh from 50km below the Tarim basin, then rises again to about 58km depth beneath the Qaidam basin. Finally, the Moho drops again to about 70km underneath the Songpan-Garze Terrane and rises to 60km near the Longmen Mountains with a step-shape. Further southeast, the crust thins to 52km beneath the Sichuan basin in the southeast of the Longmen Mountains. In the north of the Kunlun fault, a low-velocity zone, which may be a layer of melted rocks due to high temperature and pressure at depth, exists in the the bottom of the middle crust. The two depressions of the Moho correlate with the Qilian and Songpan-Garze terranes, implying that these two mountains have thick roots. According to our results, it is deduced that the thick crust of the northeastern Tibetan Plateau probably is a result of east-west and northwest-southeast crustal shortening since Mesozoic time during the collision between the Asian and Indian plates.

Tomographic imaging of the transition from asthenospheric to lithospheric melt migration processes: 3-D structure of the topmost mantle and crust at the Endeavour Segment, Juan de Fuca Ridge

NASA Astrophysics Data System (ADS)

Arnoux, G. M.; Toomey, D. R.; Hooft, E. E. E.; Wilcock, W. S. D.

2017-12-01

We present tomographic images of the intermediate-spreading Endeavour Segment that constrain the nature of an axial magmatic system as it transitions from asthenospheric- to lithospheric-dominated rheologies. We use seismic energy from 5500 air gun shots refracted through the crust (Pg), reflected off the Moho (PmP), and refracted below the Moho (Pn)—as recorded by 64 OBSs from the Endeavour tomography experiment—to image the isotropic and anisotropic P-wave velocity structure of the topmost mantle and crust, as well as crustal thickness. At crustal depths, results reveal a low-velocity zone (LVZ)—inferred to be the axial magmatic system—that: (i) is continuous along the entire Endeavour Segment at depths of 2-3 km below seafloor and closely follows the axis of spreading, (ii) broadens and becomes more discontinuous at lower crustal depths, and (iii) has its largest amplitude from the mid- to lower-crust at the segment center. The ridge-tracking trend of the mid-crustal LVZ is in contrast to the crustal thickness pattern; in particular, a swath of thin crust is skewed with respect to both the ridge axis and the mid-crustal magmatic system and connects two overlapping spreading centers bounding the segment. The trend of thinner crust, however, is aligned with the mantle LVZ, which constrains the thermal structure and distribution of melt within the topmost mantle. The systematic depth variation of the map-view orientation and structure of the magmatic system indicates a distinct transition from a broad, cross-axis regime in the topmost asthenosphere governed by a regional, north-south trending thermal structure, to a narrow, cross-axis regime in the mid- to upper-crust governed by lithospheric rifting, magma injection, and hydrothermal processes. The lower-crustal magmatic system connects these two regimes. We also postulate that accumulation and differentiation of magma immediately beneath the crust-mantle boundary increases temperatures and suppresses plagioclase crystallization, thereby reducing the depth of lower crustal accretion and resulting in the observed north-south trending swath of thinner crust.

Effect of the lithospheric thermal state on the Moho interface: A case study in South America

NASA Astrophysics Data System (ADS)

Bagherbandi, Mohammad; Bai, Yongliang; Sjöberg, Lars E.; Tenzer, Robert; Abrehdary, Majid; Miranda, Silvia; Alcacer Sanchez, Juan M.

2017-07-01

Gravimetric methods applied for Moho recovery in areas with sparse and irregular distribution of seismic data often assume only a constant crustal density. Results of latest studies, however, indicate that corrections for crustal density heterogeneities could improve the gravimetric result, especially in regions with a complex geologic/tectonic structure. Moreover, the isostatic mass balance reflects also the density structure within the lithosphere. The gravimetric methods should therefore incorporate an additional correction for the lithospheric mantle as well as deeper mantle density heterogeneities. Following this principle, we solve the Vening Meinesz-Moritz (VMM) inverse problem of isostasy constrained by seismic data to determine the Moho depth of the South American tectonic plate including surrounding oceans, while taking into consideration the crustal and mantle density heterogeneities. Our numerical result confirms that contribution of sediments significantly modifies the estimation of the Moho geometry especially along the continental margins with large sediment deposits. To account for the mantle density heterogeneities we develop and apply a method in order to correct the Moho geometry for the contribution of the lithospheric thermal state (i.e., the lithospheric thermal-pressure correction). In addition, the misfit between the isostatic and seismic Moho models, attributed mainly to deep mantle density heterogeneities and other geophysical phenomena, is corrected for by applying the non-isostatic correction. The results reveal that the application of the lithospheric thermal-pressure correction improves the RMS fit of the VMM gravimetric Moho solution to the CRUST1.0 (improves ∼ 1.9 km) and GEMMA (∼1.1 km) models and the point-wise seismic data (∼0.7 km) in South America.

Lateral variations in the crustal structure of the Indo-Eurasian collision zone

NASA Astrophysics Data System (ADS)

Gilligan, Amy; Priestley, Keith

2018-05-01

The processes involved in continental collisions remain contested, yet knowledge of these processes is crucial to improving our understanding of how some of the most dramatic features on Earth have formed. As the largest and highest orogenic plateau on Earth today, Tibet is an excellent natural laboratory for investigating collisional processes. To understand the development of the Tibetan Plateau we need to understand the crustal structure beneath both Tibet and the Indian Plate. Building on previous work, we measure new group velocity dispersion curves using data from regional earthquakes (4424 paths) and ambient noise data (5696 paths), and use these to obtain new fundamental mode Rayleigh Wave group velocity maps for periods from 5-70 s for a region including Tibet, Pakistan and India. The dense path coverage at the shortest periods, due to the inclusion of ambient noise measurements, allows features of up to 100 km scale to be resolved in some areas of the collision zone, providing one of the highest resolution models of the crust and uppermost mantle across this region. We invert the Rayleigh wave group velocity maps for shear wave velocity structure to 120 km depth and construct a 3D velocity model for the crust and uppermost mantle of the Indo-Eurasian collision zone. We use this 3D model to map the lateral variations in the crust and in the nature of the crust-mantle transition (Moho) across the Indo-Eurasian collision zone. The Moho occurs at lower shear velocities below north eastern Tibet than it does beneath western and southern Tibet and below India. The east-west difference across Tibet is particularly apparent in the elevated velocities observed west of 84° E at depths exceeding 90 km. This suggests that Indian lithosphere underlies the whole of the Plateau in the west, but possibly not in the east. At depths of 20-40 km our crustal model shows the existence of a pervasive mid-crustal low velocity layer (˜10% decrease in velocity, Vs <3.4 km/s) throughout all of Tibet, as well as beneath the Pamirs, but not below India. The thickness of this layer, the lowest velocity in the layer and the degree of velocity reduction vary across the region. Combining our Rayleigh wave observations with previously published Love wave dispersion measurements (Acton et al., 2010), we find that the low velocity layer has a radial anisotropic signature with Vsh > Vsv. The characteristics of the low velocity layer are supportive of deformation occurring through ductile flow in the mid-crust.

Topography of the lithosphere-asthenosphere boundary below the Upper Rhine Graben Rift and the volcanic Eifel region, Central Europe

NASA Astrophysics Data System (ADS)

Seiberlich, C. K. A.; Ritter, J. R. R.; Wawerzinek, B.

2013-09-01

We study the crust-mantle and lithosphere-asthenosphere boundaries (Moho and LAB) in Central Europe, specifically below the Upper Rhine Graben (URG) rift, the Eifel volcanic region and their surrounding areas. Teleseismic recordings at permanent and mobile stations are analysed to search for shear (S) wave to compressional (P) wave converted phases. After a special processing these phases are identified in shear wave receiver functions (S-RFs). Conversions from the Moho at 2.9-3.3 s arrival time are the clearest signals in the S-RFs and indicate a relatively flat Moho at 27-30 km depth. A negative polarity conversion signal at 7-9 s arrival time can be explained with a low shear wave velocity zone (LVsZ) in the upper mantle. We use forward S-RF waveform modelling and Monte-Carlo techniques to determine shear wave velocity (vs)-depth (z) profiles which explain the observed S-RF and which outline variations of the lithospheric thickness in the study region. Across the URG rift and its surrounding mountain ranges (Black Forest, Odenwald etc.) the LAB is at a depth of about 60 ± 5 km. This depth is found for the rift itself as well as for the rift shoulders. Southeast and southwest of the URG, in the regions of the Swabian Alb and Vosges Mountains, the LAB dips to about 78 ± 5 km depth. In the volcanic Eifel region the LAB is at a much shallower depth of just 41 ± 5 km. There an upwelling mantle plume thermally eroded the lower lithosphere. The reduction of vs is about 2%-4% in the upper asthenosphere compared to the lower lithosphere. This vs contrast may be explained with a low portion of partial melt or hydrous minerals in the asthenosphere.

CCP Receiver-Function Imaging of the Moho beneath Volcanic Fields in Western Saudi Arabia

NASA Astrophysics Data System (ADS)

Blanchette, A. R.; Mooney, W. D.; Klemperer, S. L.; Zahran, H. M.; El-Hadidy, S. Y.

2015-12-01

We are searching for structural complexity in the crust and upper mantle beneath the Neogene volcanic fields ('harrats') of western Saudi Arabia. We determined P-wave seismic receiver functions for 50 broadband seismographic stations located within or adjacent to three volcanic fields: Harrats Lunayyir, Rahat, and Khaybar. There are 18 seismographic stations within Lunayyir, 11 in Khaybar, and 15 in Rahat with average interstation spacing of 10 km, 30km, and 50 km. For each station we calculated 300 to 600 receiver functions with an iterative time-domain deconvolution; noisy receiver functions (outliers) were rejected by cross correlating each receiver function with a station stack; we only accepted those with a cross correlation coefficient ≥ 0.6. We used these receiver functions to create a common-conversion point (CCP) image of the crust and upper mantle. The Moho and lithosphere-asthenosphere boundary (LAB) are clearly imaged, particularly beneath Lunayyir, and have average depths of about 38 km and 60 km. We do not find any evidence for structural disruption of the Moho within our ~70 km x 70 km image of the Moho beneath Lunayyir. We image a clear crust-mantle boundary beneath Rahat and Khaybar also at ~38 km, 2-3 km deeper than anticipated from prior receiver function results outside of the harrats. Mid-crustal low velocity zones seen locally beneath all three harrats, most commonly at 10-15 km or 15-20 km in depth, may more likely represent silicic Precambrian basement than accumulations of magma. Estimates of up to ~0.5 km3 of magma erupted during each eruptive episode are consistent with the lack of a disrupted Moho. However, the total erupted volume of magma, e.g. > 1000 km3 at Rahat, together with associated intrusions from the mantle, is consistent with crustal thickening of ~2 km beneath the harrats.

A Moho ramp imaged beneath the High Himalaya in Garhwal, India

NASA Astrophysics Data System (ADS)

Caldwell, W. B.; Klemperer, S. L.; Lawrence, J.; Rai, S. S.; Ashish, A.

2011-12-01

In this study we image the Moho beneath the Himalaya of Garhwal, India (at approximately 79°E) using common conversion point (CCP) stacking of receiver functions (RFs). We calculate RFs using iterative time-domain deconvolution on a catalog of 450 events recorded on a linear array of 21 broadband seismometers operated for 21 months in 2005-2006 by India's National Geophysical Research Institute (NGRI). Our images show a horizontal Moho beneath the Lesser Himalaya and an abrupt increase of ≥ 5 km in Moho depth beneath the High Himalaya, implying a local dip of 20±5°. A steeply-dipping Moho beneath the High Himalaya has been proposed by some workers on the basis of gravity modeling, and is observed in some seismic images elsewhere in the range, but is not a widely-recognized feature of the Himalaya. Geophysical profiles across the Himalaya are not numerous enough to say whether the steep Moho is a local feature only, or is widespread but has not yet been consistently observed. A steeply-dipping Moho implies a flexure in the downgoing India plate, which we propose may play a role in the formation of the topographic front of the Himalaya. Recent studies have proposed that a ramp in the Main Himalayan Thrust-the basal décollement into which the Himalayan thrust faults root-may focus rock uplift, leading to an abrupt steepening of topography and the observed physiographic transition between the Lesser and Higher Himalaya. The mechanism of rock uplift may be out-of-sequence thrusting on the MCT-I, or stacking of imbricate thrust sheets which form as a result of underplating at the ramp. A flexure of the India plate, implied by the steep Moho dip that we observe, is a likely mechanism for controlling the formation and location of this décollement ramp, and thereby the initiation of high topography. Geophysical profiles across the Himalaya are not yet numerous enough to constrain along-strike variations in this steeply-dipping Moho, so its relationship to the formation of the topographic front of the Himalaya throughout the rest of the range remains a topic for further study.

Inhomogeneous Crustal Structure of the Rifting in the Okinawa Trough, a Backarc Basin West of Kyushu, Japan, Deduced from Seismic Reflection and Refraction Data

NASA Astrophysics Data System (ADS)

Nishizawa, A.; Kaneda, K.; Oikawa, M.; Horiuchi, D.; Fujioka, Y.; Okada, C.

2017-12-01

Several depressions found under the thick sediments in the East China Sea shelf have been considered as failed rift basins. Their formation age becomes progressively younger from NW to SE and the youngest rift basin is the Okinawa Trough, an active backarc basin of the Ryukyu (Nansei-Shoto) arc-trench system, to the southwest of Kyusyu, Japan. Its rifting is in progress and related hydrothermal activity is present in the trough. The knowledge of the crustal structure of the trough is fundamental to understand the current active tectonics and predict the future of the trough. We, Japan Coast Guard, have conducted extensive seismic reflection and refraction surveys in the Ryukyu region since 2008 and compiled the seismic structures of the Okinawa Trough. We will show the crustal structures along seven along-trough and ten across-trough seismic survey lines. The P-wave velocity models beneath the Okinawa Trough generally show a thinned continental/island arc crust consisting of upper, middle, and lower crusts. Moho depths below the trough were estimated mainly from Moho reflection (PmP) travel times. The crustal thickness of the trough is thinner than those of the East China Sea shelf and of the Ryukyu Islands. The depth of the Moho below the trough decreases from over 30 km in the north to about 13 km in the south, indicating a difference in degree of the rifting process. The position of the shallowest Moho along the across-trough lines in the northern trough does not necessarily correspond to the center of the trough defined as the deepest water depth, but it corresponds to the transition area between the East China Sea shelf and the Okinawa Trough. An M7.1 earthquake occurred at the transition area on Nov. 14, 2015 (JST) and many aftershocks were observed along the transition. This seismic activity demonstrated that the area is under rifting tectonics in the present.

Crustal Structure and Evidence for a Hales Discontinuity Beneath the Seychelles Microcontinent

NASA Astrophysics Data System (ADS)

Hammond, J.; Kendall, J.; Collier, J.; Rumpker, G.; Pilidou, S.; Stuart, G.

2005-12-01

It is well known that the Seychelles Plateau consists of a sliver of continental crust cast adrift during the formation of the Indian ocean. However the extent of the continental crust beneath the microcontinent and the cause of its isolation is poorly understood. Here we use receiver functions, interstation phase velocities obtained from surface waves, and wide angle reflections from controlled-source seismic data to investigate the lithospheric structure of the region. The H-κ method is used to calculate depths and Poison's ratio at 26 temporary stations distributed across the plateau and Mascarene basin. The Vp/V_s ratios and depths at stations on the plateau are typical of continental crust. To explain the major features of the RFs a simple two layer crust is proposed for the island of Mahé. The islands of Silhouette and Nord display a more complex crust consistent with the islands volcanic history. Praslin and its satellite islands display a simpler crust but display signs of a deeper discontinuity (~40 km) beneath the Moho which is possible evidence for underplating associated with Deccan age volcanism. Bird Island (Moho~18 km) and Desroche (Moho~23 km) show signs of being situated on islands above the transition from continental to oceanic crust. Alphonse, Coetivy and Platte all show receiver functions expected for oceanic crust, with Moho depths ~10 km. Inter-station phase velocity inversions from surface waves support these results with paths sampling the plateau region showing dispersion curves expected for continental crust, and those travelling between stations off the plateau showing evidence for oceanic crust. A deeper arrival is observed on the plateau stations at ~7 s or ~65 km. This feature is also seen in wide-angle controlled source work and the inter-station phase velocity inversions. Candidate interpretion for this Hales discontinuity include a Precambrian suture assoicated with shallow subduction or a shear-zone assoicated with deformation during breakup. Either feature may have influenced plume-related breakup in the region.

Moho Depth and Geometry in the Illinois Basin Region Based on Gravity and Seismic Data from an EarthScope FlexArray Experiment

NASA Astrophysics Data System (ADS)

Curcio, D. D.; Pavlis, G. L.; Yang, X.; Hamburger, M. W.; Zhang, H.; Ravat, D.

2017-12-01

We present results from a combined analysis of seismic and gravity in the Illinois Basin region that demonstrate the presence of an unusually deep and highly variable Moho discontinuity. We construct a new, high-resolution image of the Earth's crust beneath the Illinois Basin using teleseismic P-wave receiver functions from the EarthScope OIINK (Ozarks, Illinois, INdiana, Kentucky) Flexible Array and the USArray Transportable Array. Our seismic analyses involved data from 143 OIINK stations and 80 USArray stations, using 3D plane-wave migration and common conversion point (CCP) stacking of P-to-S conversion data. Seismic interpretation has been done using the seismic exploration software package Petrel. One of the most surprising results is the anomalous depth of the Moho in this area, ranging from 41 to 63 km, with an average depth of 50 km. This thickened crust is unexpected in the Illinois Basin area, which has not been subject to convergence and mountain building processes in the last 900 Ma. This anomalously thick crust in combination with the minimal topography requires abnormally dense lower crust or unusually light upper mantle in order to retain gravitational equilibrium. Combining gravity modeling with the seismically identified Moho and a ubiquitous lower crustal boundary, we solve for the density variation of the middle and lower crust. We test the hypothesis that the anomalously thick crust and its high lower crustal layer observed in most of the central and southeastern Illinois Basin predates the formation and development of the current Illinois Basin. Post-formation tectonic activity, such as late Precambrian rifting or underplating are inferred to have modified the crustal thickness as well. The combination of high-resolution seismic data analysis and gravity modeling promises to provide additional insight into the geometry and composition of the lower crust in the Illinois Basin area.

Contrasts in Lower Crustal Structure and Evolution Between the Northern and Southern Rocky Mountains From Xenoliths and Seismic Data

NASA Astrophysics Data System (ADS)

Schulte-Pelkum, V.; Mahan, K. H.; Shen, W.; Stachnik, J. C.

2016-12-01

We compare and contrast crustal structure and composition along a transect from the Southern to Northern Rocky Mountains, with a focus on the lower crust. Evolution of the crust can include processes of emplacement, differentiation, and thermal changes that may generate lower crust with high seismic wavespeeds. The high seismic velocities can be due to mafic composition, the presence of garnet, or both. We seek to find seismic signatures preserved from such processes and compare xenolith samples and present-day seismic appearance between regions with varying tectonic histories. We review recent seismic results from the EarthScope Transportable Array from receiver functions and surface waves, compilations of active source studies, and xenolith studies to compare lower crustal structure along transects through the Northern and Southern Rocky Mountains traversing Montana, Wyoming, Colorado, Utah, and New Mexico. Xenoliths from an unusually thick lower crustal layer with high seismic velocities in Montana record magmatic emplacement processes dating back to the Archean. The lower crustal layer possesses internal velocity contrasts that lead to conflicting interpretations of Moho depth depending on the method used, with xenoliths and a refraction study placing the Moho at 55 km depth, while studies using surface waves and receiver functions identify the largest contrast at 40-45 km depth as the Moho. An additional confounding factor is the presence of metasomatized uppermost mantle with low seismic velocities, which may further diminish the seismic signature of the petrological Moho. To the south, the high-velocity layer diminishes, and seismic velocities in the deep crust under southern Wyoming, Colorado, and New Mexico are lower. In the literature, north-south gradients in lower crustal velocity in this area and observed differences in garnet content have variously been ascribed to thermal dehydration of Archean-age hydrous crust or Laramide-age hydration of previously garnet-rich crust.

Architecture of the crust and uppermost mantle in the northern Canadian Cordillera from receiver functions

NASA Astrophysics Data System (ADS)

Tarayoun, Alizia; Audet, Pascal; Mazzotti, Stéphane; Ashoori, Azadeh

2017-07-01

The northern Canadian Cordillera (NCC) is an active orogenic belt in northwestern Canada characterized by deformed autochtonous and allochtonous structures that were emplaced in successive episodes of convergence since the Late Cretaceous. Seismicity and crustal deformation are concentrated along corridors located far (>200 to 800 km) from the convergent plate margin. Proposed geodynamic models require information on crust and mantle structure and strain history, which are poorly constrained. We calculate receiver functions using 66 broadband seismic stations within and around the NCC and process them to estimate Moho depth and P-to-S velocity ratio (Vp/Vs) of the Cordilleran crust. We also perform a harmonic decomposition to determine the anisotropy of the subsurface layers. From these results, we construct simple seismic velocity models at selected stations and simulate receiver function data to constrain crust and uppermost mantle structure and anisotropy. Our results indicate a relatively flat and sharp Moho at 32 ± 2 km depth and crustal Vp/Vs of 1.75 ± 0.05. Seismic anisotropy is pervasive in the upper crust and within a thin ( 10-15 km thick) sub-Moho layer. The modeled plunging slow axis of hexagonal symmetry of the upper crustal anisotropic layer may reflect the presence of fractures or mica-rich mylonites. The subhorizontal fast axis of hexagonal anisotropy within the sub-Moho layer is generally consistent with the SE-NW orientation of large-scale tectonic structures. These results allow us to revise the geodynamic models proposed to explain active deformation within the NCC.

Thinned crustal structure and tectonic boundary of the Nansha Block, southern South China Sea

NASA Astrophysics Data System (ADS)

Dong, Miao; Wu, Shi-Guo; Zhang, Jian

2016-12-01

The southern South China Sea margin consists of the thinned crustal Nansha Block and a compressional collision zone. The Nansha Block's deep structure and tectonic evolution contains critical information about the South China Sea's rifting. Multiple geophysical data sets, including regional magnetic, gravity and reflection seismic data, reveal the deep structure and rifting processes. Curie point depth (CPD), estimated from magnetic anomalies using a windowed wavenumber-domain algorithm, enables us to image thermal structures. To derive a 3D Moho topography and crustal thickness model, we apply Oldenburg algorithm to the gravity anomaly, which was extracted from the observed free air gravity anomaly data after removing the gravity effect of density variations of sediments, and temperature and pressure variations of the lithospheric mantle. We found that the Moho depth (20 km) is shallower than the CPD (24 km) in the Northwest Borneo Trough, possibly caused by thinned crust, low heat flow and a low vertical geothermal gradient. The Nansha Block's northern boundary is a narrow continent-ocean transition zone constrained by magnetic anomalies, reflection seismic data, gravity anomalies and an interpretation of Moho depth (about 13 km). The block extends southward beneath a gravity-driven deformed sediment wedge caused by uplift on land after a collision, with a contribution from deep crustal flow. Its southwestern boundary is close to the Lupar Line defined by a significant negative reduction to the pole (RTP) of magnetic anomaly and short-length-scale variation in crustal thickness, increasing from 18 to 26 km.

Shear velocity profiles in the crust and lithospheric mantle across Tibet

NASA Astrophysics Data System (ADS)

Agius, M. R.; Lebedev, S.

2010-12-01

We constrain variations in the crustal and lithospheric structure across Tibet, using phase velocities of seismic surface waves. The data are seismograms recorded by broadband instruments of permanent and temporary networks within and around the plateau. Phase-velocity measurements are performed in broad period ranges using an elaborate recent implementation of the 2-station method. A combination of the cross-correlation and multimode-waveform-inversion measurements using tens to hundreds of seismograms per station pair produces robust, accurate phase-velocity curves for Rayleigh and Love waves. We use our new measurements to infer phase-velocity variations and to constrain S-velocity profiles in different parts of the plateau, including radial anisotropy and depths of lithospheric discontinuities. We observe a mid-crustal low-velocity zone (LVZ) in the 20-45 km depth range across the plateau, with S-velocities within a 3.2-3.5 km/s range. This LVZ coincides with a low-resistivity layer inferred from magnetotelluric studies, interpreted as evidence for partial melting in the middle crust. Surface-wave data are also consistent with radial anisotropy in this layer, indicative of horizontal flow. At the north-eastern boundary of the plateau, past the Kunlun Fault, the mid-crustal LVZ, in the sense of an S-velocity decrease with depth in the 15-25 km depth range, is not required by the surface-wave data although the velocity is still relatively low. The mantle-lithosphere structure shows a pronounced contrast between the south-western and central-northern parts of the plateau. The south-west is underlain by a thick, high-velocity, craton-like lithospheric mantle. Below central Lhasa the uppermost mantle appears to be close to global average with an increase in velocity between 150 - 250 km depth. Beneath central and northern Tibet, the average S velocity between the Moho and 200 km depth is close to the global continental average (4.5 km/s). In order to investigate the finer detail of the lithosphere in the North we perform an extensive series of test inversions. We find that surface-wave dispersion measurements alone are consistent both with models that have low S velocity just beneath the Moho, increasing with depth below, and with models that display a thin high-velocity mantle lid underlain by a low-velocity zone (asthenosphere). To resolve this non-uniqueness from the inversion model, we combine our surface-wave measurements in the Qiangtang Block with receiver-function constraints on the Moho depth, and Sn constraints on the uppermost mantle S velocities. We show that the data is matched significantly better with models that contain a thin, high-velocity lithosphere (up to 90 km thick) underlain by a low-velocity zone than by models with no wave-speed decrease between the Moho and ~100 km depth. In the deeper upper mantle (below ~150 km depth), S velocity increases and is likely to exceed the global average value.

Crustal structure beneath the Blue Mountains terranes and cratonic North America, eastern Oregon, and Idaho, from teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Christian Stanciu, A.; Russo, Raymond M.; Mocanu, Victor I.; Bremner, Paul M.; Hongsresawat, Sutatcha; Torpey, Megan E.; VanDecar, John C.; Foster, David A.; Hole, John A.

2016-07-01

We present new images of lithospheric structure obtained from P-to-S conversions defined by receiver functions at the 85 broadband seismic stations of the EarthScope IDaho-ORegon experiment. We resolve the crustal thickness beneath the Blue Mountains province and the former western margin of cratonic North America, the geometry of the western Idaho shear zone (WISZ), and the boundary between the Grouse Creek and Farmington provinces. We calculated P-to-S receiver functions using the iterative time domain deconvolution method, and we used the H-k grid search method and common conversion point stacking to image the lithospheric structure. Moho depths beneath the Blue Mountains terranes range from 24 to 34 km, whereas the crust is 32-40 km thick beneath the Idaho batholith and the regions of extended crust of east-central Idaho. The Blue Mountains group Olds Ferry terrane is characterized by the thinnest crust in the study area, 24 km thick. There is a clear break in the continuity of the Moho across the WISZ, with depths increasing from 28 km west of the shear zone to 36 km just east of its surface expression. The presence of a strong midcrustal converting interface at 18 km depth beneath the Idaho batholith extending 20 km east of the WISZ indicates tectonic wedging in this region. A north striking 7 km offset in Moho depth, thinning to the east, is present beneath the Lost River Range and Pahsimeroi Valley; we identify this sharp offset as the boundary that juxtaposes the Archean Grouse Creek block with the Paleoproterozoic Farmington zone.

Variation in crustal structure in Iran and the surrounding region

NASA Astrophysics Data System (ADS)

Rham, D.; Tatar, M.; Ashtiany, M.; Mokhtari, M.; Priestley, K.; Paul, A.

2007-12-01

We present a model for the topography of the Moho discontinuity for Iran and its surrounding regions. This is produced using data from field deployments within Iran by the University of Cambridge (UK) and the Universite Joseph-Fourier (FRA) in conjunction with International Institute of Earthquake Engineering and Seismology (Iran), in addition to data from IRIS and Geofone. We determine tomographic group velocity maps for periods between 10 and 60 s from multiple filter analysis of ~5500 seismograms. Because of the dense path coverage, these images have substantially higher lateral resolution for this region than is currently available from global and regional group velocity studies. Joint inversion of receiver functions and Rayleigh wave dispersion give accurate crustal velocity structures at 96 sites within Iran These provide a constraint for the less sharp crustal velocity profile produced by inverting the Rayleigh wave dispersion curve across all of Iran. We observe variations in the crustal thickness across the region, consistent with the surface topography. The thickest crust (55-60 km) is found beneath the central Zagros mountains, with the crust in the remainder of Iran having a thicknesses of 40-50 km. No significant increase in Moho depth is seen beneath the Alborz or Kopet Dagh mountains. The structure of the South Caspian Basin is presented with a different structure to that found in previous studies, with a crustal thickness of 50 km in the west, and beneath the Caucasus and Talesh mountains, in the middle part of the basin, over the course of the ~100km, this decreases to 40km, and continues to 35 km beneath the Turkmen Platform. Comparisons are also made between the joint inversion results, and accurate hypocentre depths for regional earthquakes. This shows most events occur in the upper crystalline crust (~10-20km depth), with few in the lowest velocity layer. Almost no events are located in the lower crust, and only in the Makran and Aspheron- Balkhan Sill do earthquakes appear in the Upper Mantle.

Intrastab Earthquakes: Dehydration of the Cascadia Slab

USGS Publications Warehouse

Preston, L.A.; Creager, K.C.; Crosson, R.S.; Brocher, T.M.; Trehu, A.M.

2003-01-01

We simultaneously invert travel times of refracted and wide-angle reflected waves for three-dimensional compressional-wave velocity structure, earthquake locations, and reflector geometry in northwest Washington state. The reflector, interpreted to be the crust-mantle boundary (Moho) of the subducting Juan de Fuca plate, separates intrastab earthquakes into two groups, permitting a new understanding of the origins of intrastab earthquakes in Cascadia. Earthquakes up-dip of the Moho's 45-kilometer depth contour occur below the reflector, in the subducted oceanic mantle, consistent with serpentinite dehydration; earthquakes located down-dip occur primarily within the subducted crust, consistent with the basalt-to-eclogite transformation.

Shear velocity structure of central Eurasia from inversion of surface wave velocities

NASA Astrophysics Data System (ADS)

Villaseñor, A.; Ritzwoller, M. H.; Levshin, A. L.; Barmin, M. P.; Engdahl, E. R.; Spakman, W.; Trampert, J.

2001-04-01

We present a shear velocity model of the crust and upper mantle beneath central Eurasia by simultaneous inversion of broadband group and phase velocity maps of fundamental-mode Love and Rayleigh waves. The model is parameterized in terms of velocity depth profiles on a discrete 2°×2° grid. The model is isotropic for the crust and for the upper mantle below 220 km but, to fit simultaneously long period Love and Rayleigh waves, the model is transversely isotropic in the uppermost mantle, from the Moho discontinuity to 220 km depth. We have used newly available a priori models for the crust and sedimentary cover as starting models for the inversion. Therefore, the crustal part of the estimated model shows good correlation with known surface features such as sedimentary basins and mountain ranges. The velocity anomalies in the upper mantle are related to differences between tectonic and stable regions. Old, stable regions such as the East European, Siberian, and Indian cratons are characterized by high upper-mantle shear velocities. Other large high velocity anomalies occur beneath the Persian Gulf and the Tarim block. Slow shear velocity anomalies are related to regions of current extension (Red Sea and Andaman ridges) and are also found beneath the Tibetan and Turkish-Iranian Plateaus, structures originated by continent-continent collision. A large low velocity anomaly beneath western Mongolia corresponds to the location of a hypothesized mantle plume. A clear low velocity zone in vSH between Moho and 220 km exists across most of Eurasia, but is absent for vSV. The character and magnitude of anisotropy in the model is on average similar to PREM, with the most prominent anisotropic region occurring beneath the Tibetan Plateau.

Project WILAS: Seismic imaging of crustal and upper mantle structures beneath the western Iberian Peninsula by means of the receiver-function technique

NASA Astrophysics Data System (ADS)

Dündar, Süleyman; Dias, Nuno A.; Silveira, Graça; Vinnik, Lev; Haberland, Christian

2013-04-01

An accurate knowledge of the structure of the earth's interior is of great importance to our understanding of tectonic processes. The WILAS-project (REF: PTDC/CTE-GIX/097946/2008) is a three-year collaborative project developed to study the subsurface structure of the western Iberian Peninsula, putting the main emphases on the lithosphere-asthenosphere system beneath the mainland of Portugal. The tectonic evolution of the target area has been driven by major plate-tectonic processes such as the historical opening of the Central Atlantic and the subsequent African-Eurasian convergence. Still, very little is known about the spatial structure of the continental collision. Within the framework of this research, a temporary network of 30 broadband three-component digital stations was operated between 2010 and 2012 in the target area. To carry out a large-scale structural analysis and facilitate a dense station-coverage for the area under investigation, the permanent Global Seismic Network stations, and temporary broadband stations deployed within the scope of the several seismic experiments (e.g. Doctar Network, Portuguese National Seismic Network), were included in the research analysis. In doing so, an unprecedented volume of high-quality data of a ca. 60X60 km density along with a combined network of 65 temporary and permanent broadband seismic stations are currently available for research purposes. One of the tasks of the WILAS research project has been a study of seismic velocity discontinuities beneath the western Iberian Peninsula region, up to a depth range of 700 km, utilizing the P- and S-receiver function techniques (PRF, SRF). Both techniques are based mainly on mode conversion of the elastic body-waves at an interface dividing the layers with different elastic properties. In the first phase of the project, PRF analysis was conducted in order to image the crust-mantle interface (Moho) and the mantle-transition-zone discontinuities at a depth of 410 km and 660 km beneath the area under investigation. While applying the common data processing steps (e.g., rotation, deconvolution and moveout-correction) to the selected data-set, we were able to create approximately 4.500 PRFs. The signals from the Moho, 410-km and 660-km discontinuities are clearly visible in many PRF stacks. The Moho depth range is from 26 to 34 km, with an average value of 29 km. No significant lateral variations in the depths of the "410-km" and "660-km" discontinuities have been identified so far. In the second phase of this project, the S-receiver-function technique will be applied in order to map the thickness of the underlying mantle lithosphere. Additionally, joint inversion of PRFs and waveforms of SKS will be used to investigate depth-localized azimuthal anisotropy and the related past and present mantle flows.

Imaging the Moho beneath Sedimentary Basins: A Comparative Study of Virtual Deep Seismic Sounding (VDSS) and P Wave Receiver Functions (PRF)

NASA Astrophysics Data System (ADS)

Liu, T.; Klemperer, S. L.; Yu, C.; Ning, J.

2017-12-01

In the past decades, P wave receiver functions (PRF) have been routinely used to image the Moho, although it is well known that PRFs are susceptible to contamination from sedimentary multiples. Recently, Virtual Deep Seismic Sounding (VDSS) emerged as a novel method to image the Moho. However, despite successful applications of VDSS on multiple datasets from different areas, how sedimentary basins affect the waveforms of post-critical SsPmp, the Moho reflection phase used in VDSS, is not widely understood. Here, motivated by a dataset collected in the Ordos plateau, which shows distinct effects of sedimentary basins on SsPmp and Pms waveforms, we use synthetic seismograms to study the effects of sedimentary basins on SsPmp and Pms, the phases used in VDSS and PRF respectively. The results show that when the sedimentary thickness is on the same order of magnitude as the dominant wavelength of the incident S wave, SsPmp amplitude decreases significantly with S velocity of the sedimentary layer, whereas increasing sedimentary thickness has little effect in SsPmp amplitude. Our explanation is that the low S velocity layer at the virtual source reduces the incident angle of S wave at the free surface, thus decreases the S-to-P reflection coefficient at the virtual source. In addition, transmission loss associated with the bottom of sedimentary basins also contributes to reducing SsPmp amplitude. This explains not only our observations from the Ordos plateau, but also observations from other areas where post-critical SsPmp is expected to be observable, but instead is too weak to be identified. As for Pms, we observe that increasing sedimentary thickness and decreasing sedimentary velocities both can cause interference between sedimentary multiples and Pms, rendering the Moho depths inferred from Pms arrival times unreliable. The reason is that although Pms amplitude does not vary with sedimentary thickness or velocities, as sedimentary velocities decrease and thickness grows, the sedimentary multiples will become stronger and arrive later, and will eventually interfere with Pms. In summary, although both VDSS and PRF are subject to sedimentary effects, when the sedimentary velocity is relatively high, we can still expect VDSS to give reasonable estimations of Moho depths, whereas PRF in such cases might be too noisy to use.

Apulian crust: Top to bottom

NASA Astrophysics Data System (ADS)

Amato, Alessandro; Bianchi, Irene; Agostinetti, Nicola Piana

2014-12-01

We investigate the crustal seismic structure of the Adria plate using teleseismic receiver functions (RF) recorded at 12 broadband seismic stations in the Apulia region. Detailed models of the Apulian crust, e.g. the structure of the Apulian Multi-layer Platform (AMP), are crucial for assessing the presence of potential décollements at different depth levels that may play a role in the evolution of the Apenninic orogen. We reconstruct S-wave velocity profiles applying a trans-dimensional Monte Carlo method for the inversion of RF data. Using this method, the resolution at the different depth level is completely dictated by the data and we avoid introducing artifacts in the crustal structure. We focus our study on three different key-elements: the Moho depth, the lower crust S-velocity, and the fine-structure of the AMP. We find a well defined and relatively flat Moho discontinuity below the region at 28-32 km depth, possibly indicating that the original Moho is still preserved in the area. The lower crust appears as a generally low velocity layer (average Vs = 3.7 km/s in the 15-26 km depth interval), likely suggestive of a felsic composition, with no significant velocity discontinuities except for its upper and lower boundaries where we find layering. Finally, for the shallow structure, the comparison of RF results with deep well stratigraphic and sonic log data allowed us to constrain the structure of the AMP and the presence of underlying Permo-Triassic (P-T) sediments. We find that the AMP structure displays small-scale heterogeneities in the region, with a thickness of the carbonates layers varying between 4 and 12 km, and is underlain by a thin, discontinuous layer of P-T terrigenous sediments, that are lacking in some areas. This fact may be due to the roughness in the original topography of the continental margins or to heterogeneities in its shallow structure due to the rifting process.

Moho Depth and Bulk Crustal Properties in Northern Quebec and Labrador

NASA Astrophysics Data System (ADS)

Vervaet, F.; Darbyshire, F. A.

2016-12-01

Northern Quebec and Labrador lie at the heart of the Laurentian landmass and preserve over 3 billion years of continental evolution. In this region the Archean Superior and Nain cratons are surrounded by Paleoproterozoic orogens such as New-Quebec, Trans-Hudson and Torngat, as well as the younger Grenville orogen to the SE. Study of crustal structure in this region provides valuable information on the assembly of the North American continent. We use data from 8 seismic stations installed in summer 2011 as part of the QUiLLE (Quebec-Labrador Lithospheric Experiment) project to investigate crustal structure, using receiver function analysis. The data set covers 5 years (2011-2016) for most of the stations, comprising several hundred events of magnitude ≥5 and epicentral distance 30-90°. After initial data processing and quality control, several tens of events per station were used in an H-κ stacking analysis to estimate Moho depth and bulk crustal properties. Some stations show significant complexity in their receiver functions, leading to inconclusive H-κ results, but the majority show a consistent Moho signal from which crustal parameters are successfully extracted. Crustal thickness varies from 33 to 49 km, with the thickest crust associated with the Trans-Hudson orogen in the Ungava region of northernmost Quebec and the thinnest beneath the central Labrador coast. Vp/Vs ratios (κ) lie in the range 1.71-1.86, with the majority of values consistent with granite-gneiss-tonalite bulk crustal compositions. The receiver functions are combined with surface-wave group velocity data to model the crustal structures in more detail beneath each station, allowing us to investigate crustal layering, Moho complexity and lateral heterogeneity.

Receiver Function Study of the Crustal Structure Beneath the Northern Andes (colombia)

NASA Astrophysics Data System (ADS)

Poveda, E.; Monsalve, G.; Vargas-Jimenez, C. A.

2013-05-01

We have investigated crustal thickness beneath the Northern Andes with the teleseismic receiver function technique. We used teleseismic data recorded by an array of 18 broadband stations deployed by the Colombian Seismological Network, and operated by the Colombian Geological Survey. We used the primary P-to-S conversion and crustal reverberations to estimate crustal thickness and average Vp/Vs ratio; using Wadati diagrams, we also calculated the mean crustal Vp/Vs ratio around stations to further constrain the crustal thickness estimation. In northern Colombia, near the Caribbean coast, the estimated crustal thickness ranges from 25 to 30 km; in the Middle Magdalena Valley, crustal thickness is around 40 km; beneath the northern Central Cordillera, the Moho depth is nearly 40 km; at the Ecuador-Colombia border, beneath the western flank of the Andes, the estimated thickness is about 46 km. Receiver functions at a station at the craton in South East Colombia, near the foothills of the Eastern Cordillera, clearly indicate the presence of the Moho discontinuity at a depth near 36 km. The greatest values of crustal thickness occur beneath a plateau (Altiplano Cundiboyacense) on the Eastern Cordillera, near the location of Bogota, with values around 58 km. Receiver functions in the volcanic areas of the south-western Colombian Andes do not show a systematic signal from the Moho, indicating abrupt changes in Moho geometry. Signals at stations on the Eastern Cordillera near Bogota reveal a highly complex crustal structure, with a combination of sedimentary layers up to 9 km thick, dipping interfaces, low velocity layers, anisotropy and/or lateral heterogeneity that still remain to be evaluated. This complexity obeys to the location of these stations at a region of a highly deformed fold and thrust belt.

Crustal properties of the northern Scandinavian mountains and Fennoscandian shield from analysis of teleseismic Receiver Functions

NASA Astrophysics Data System (ADS)

Mansour, Walid Ben; England, Richard W.; Fishwick, Stewart; Moorkamp, Max

2018-04-01

The presence of high mountains along passive margins is not unusual, as shown by their presence in several regions (Scandinavia, Greenland, East US, SW Africa, Brazil, West India and SE Australia). However, the origin of this topography is not well understood. The mountain range between the Scandinavian passive margin and the Fennoscandian shield is a good example. A simple Airy isostatic model would predict a compensating root beneath the mountains but existing seismic measurements of variations in crustal thickness do not provide evidence of a root of sufficient size to produce the necessary compensation. In order to better constrain the physical properties of the crust in northern Scandinavia two broadband seismic networks were deployed between 2007 and 2009 and between 2013 and 2014. A new map of crustal thickness has been produced from P-receiver function analysis of teleseismic data recorded at 31 seismic stations. The map shows an increase in crustal thickness from the Atlantic coast (38.7 +/- 1.8 km) to the Gulf of Bothnia (43.5 +/- 2.4 km). This gradient in thickness demonstrates that the Moho topography does not mirror the variation in surface topography in this region. Thus, classical Airy isostatic models cannot explain how the surface topography is supported. New maps showing variation in Poisson's ratio and Moho sharpness together with forward and inverse modelling provide new information about the contrasting properties of the Fennoscandian shield and crust reworked by the Caledonian orogeny. A sharp Moho transition (R > 1) and low value of Vs (3.5 +/- 0.2 km.s-1) are observed beneath the orogen. The shield is characterised by a gradual transition across the Moho (R < 1) and Vs of (3.8 +/- 0.1 km.s-1) which is more typical of average continental crust. These observations are explained by a Fennoscandian shield underplated with a thick layer of high velocity, high density material. It is proposed that this layer has been removed or reworked beneath the orogen.

Modelling the Crust beneath the Kashmir valley in Northwestern Himalaya

NASA Astrophysics Data System (ADS)

Mir, R. R.; Parvez, I. A.; Gaur, V. K.; A.; Chandra, R.; Romshoo, S. A.

2015-12-01

We investigate the crustal structure beneath five broadband seismic stations in the NW-SE trendingoval shaped Kashmir valley sandwiched between the Zanskar and the Pir Panjal ranges of thenorthwestern Himalaya. Three of these sites were located along the southwestern edge of the valley andthe other two adjoined the southeastern. Receiver Functions (RFs) at these sites were calculated usingthe iterative time domain deconvolution method and jointly inverted with surface wave dispersiondata to estimate the shear wave velocity structure beneath each station. To further test the results ofinversion, we applied forward modelling by dividing the crust beneath each station into 4-6homogeneous, isotropic layers. Moho depths were separately calculated at different piercing pointsfrom the inversion of only a few stacked receiver functions of high quality around each piercing point.These uncertainties were further reduced to ±2 km by trial forward modelling as Moho depths werevaried over a range of ±6 km in steps of 2 km and the synthetic receiver functions matched with theinverted ones. The final values were also found to be close to those independently estimated using theH-K stacks. The Moho depths on the eastern edge of the valley and at piercing points in itssouthwestern half are close to 55 km, but increase to about 58 km on the eastern edge, suggesting thathere, as in the central and Nepal Himalaya, the Indian plate dips northeastwards beneath the Himalaya.We also calculated the Vp/Vs ratio beneath these 5 stations which were found to lie between 1.7 and1.76, yielding a Poisson's ratio of ~0.25 which is characteristic of a felsic composition.

Interpretation of the 'Trans European Suture Zone' by a multiscale aeromagnetic dataset

NASA Astrophysics Data System (ADS)

Milano, Maurizio; Fedi, Maurizio

2015-04-01

One of the main goals in crustal geomagnetic prospecting is to obtain information about the sources of magnetic anomalies in order to model the geological structure of the Earth's crust. A "multiscale approach" is very useful to analyze, concurrently, the effects of sources placed at different depths, observing the potential field at various altitudes from the Earth's surface. The aim of this work is the study of the main geological structure of Central Europe, the "Trans European Suture Zone", using high-resolution aeromagnetic data. The 'TESZ' is the most prominent geological boundary in Europe, oriented NW-SE from the North Sea to the Black Sea and separating The Paleozoic platform in the south and west from the Precambrian East European craton. At high altitudes the European magnetic field is characterized by a large and extended magnetic low, which is related to the deep TESZ structure. The study of this anomaly field began by detecting the position of the anomaly sources using the properties of the Analytical Signal modulus (AS). The AS map presents anomalies in which the dipolar behavior of the magnetic anomaly field is substantially removed and the maxima are placed directly above the anomaly sources. The multiridge method has been applied to the Analytical Signal modulus in order to have information about the sources' depths in the TESZ region. Many profiles were tracked transversely to the fault line in order to map at depth the main magnetic discontinuities. Cause of the low heat flow of the Central Europe, we were able to get information also in the lower crust and to map the deep Moho discontinuity. Available geological sections based on seismic data show consistent results with our interpretation.

Crustal and Moho Reflections Beneath Mount St. Helens from the iMUSH Experiment

NASA Astrophysics Data System (ADS)

Levander, A.; Kiser, E.; Schmandt, B.; Hansen, S. M.; Creager, K.

2017-12-01

The multi-disciplinary iMUSH project (imaging Magma Under St. Helens) was designed to illuminate the magmatic system beneath Mount St Helens (MSH) from the subducting Juan de Fuca slab to the surface using seismic, magnetotelluric, and petrologic data. The iMUSH active source experiment consisted of 23 large shots and 6000 seismograph stations. Included in the active-source seismic experiment were 2 dense linear profiles striking NW-SE and NE-SW, each with over 1000 receivers ( 150 m spacing) and 8 shots. Using averaged 1D velocity models around each shotpoint taken from the 2D velocity models of Kiser et al., 2016 (Geology), we have made CMP stacked sections of the two profiles. We made images using several types of signal preconditioning and enhancement methods, including analytic signal and STA/LTA envelopes. Reflection time corrections were determined using standard NMO, long-offset NMO, p-tau, and 2D travel time analyses. Bright reflection events in the CMP sections show remarkably close correspondence to abrupt velocity changes in the mid to lower crust and at the Moho in the 2D velocity models: Reflections appear at 20-25 km depth at the tops of two lower crustal high velocity (Vp > 7.5 km/s) bodies. One of these high velocity bodies is directly beneath MSH. The other is 40 km SE of MSH, under the 9ka Indian Heaven basaltic volcanic field. We interpret the high Vp bodies as cumulates from Quaternary or Tertiary volcanism. Separating the two high Vp bodies is a lower velocity column (Vp ≤ 6.5 km/s) dipping to the SE from the midcrust to the Moho. In the CMP section, the Moho reflection is bright under the region of low velocity and dims beneath both of the high velocity lower crustal bodies. The CMP images of the Moho are consistent with the PmP reflection amplitude analysis of Hansen et al, 2016 (Nature Communications). The 1980 eruption seismicity extended from the MSH summit to 20 km depth, stopping just above the bright reflection at the top of the MSH high Vp body. Deep long period events under MSH, often associated with motion of magmatic fluids, cluster at 20-30 km depth along the southeastern edge of the same reflection. We suggest that lower crustal magmas migrate from the southeast at the boundary of the MSH high velocity body, and then laterally across its top to continue vertical ascent to the magma storage zone under the summit.

Comparison of publically available Moho depth and crustal thickness grids with newly derived grids by 3D gravity inversion for the High Arctic region.

NASA Astrophysics Data System (ADS)

Lebedeva-Ivanova, Nina; Gaina, Carmen; Minakov, Alexander; Kashubin, Sergey

2016-04-01

We derived Moho depth and crustal thickness for the High Arctic region by 3D forward and inverse gravity modelling method in the spectral domain (Minakov et al. 2012) using lithosphere thermal gravity anomaly correction (Alvey et al., 2008); a vertical density variation for the sedimentary layer and lateral crustal variation density. Recently updated grids of bathymetry (Jakobsson et al., 2012), gravity anomaly (Gaina et al, 2011) and dynamic topography (Spasojevic & Gurnis, 2012) were used as input data for the algorithm. TeMAr sedimentary thickness grid (Petrov et al., 2013) was modified according to the most recently published seismic data, and was re-gridded and utilized as input data. Other input parameters for the algorithm were calibrated using seismic crustal scale profiles. The results are numerically compared with publically available grids of the Moho depth and crustal thickness for the High Arctic region (CRUST 1 and GEMMA global grids; the deep Arctic Ocean grids by Glebovsky et al., 2013) and seismic crustal scale profiles. The global grids provide coarser resolution of 0.5-1.0 geographic degrees and not focused on the High Arctic region. Our grids better capture all main features of the region and show smaller error in relation to the seismic crustal profiles compare to CRUST 1 and GEMMA grids. Results of 3D gravity modelling by Glebovsky et al. (2013) with separated geostructures approach show also good fit with seismic profiles; however these grids cover the deep part of the Arctic Ocean only. Alvey A, Gaina C, Kusznir NJ, Torsvik TH (2008). Integrated crustal thickness mapping and plate recon-structions for the high Arctic. Earth Planet Sci Lett 274:310-321. Gaina C, Werner SC, Saltus R, Maus S (2011). Circum-Arctic mapping project: new magnetic and gravity anomaly maps of the Arctic. Geol Soc Lond Mem 35, 39-48. Glebovsky V.Yu., Astafurova E.G., Chernykh A.A., Korneva M.A., Kaminsky V.D., Poselov V.A. (2013). Thickness of the Earth's crust in the deep Arctic Ocean: results of a 3D gravity modeling Russian Geology and Geophysics 54, 247-262. Jakobsson M, Mayer L, Coakley B, Dowdeswell JA, Forbes S, Fridman B, Hodnesdal H, Noormets R, Pedersen R, Rebesco M, Schenke HW, Zarayskaya Y, Accettella D, Armstrong A, Anderson RM, Bienhoff P, Camerlenghi A, Church I, Edwards M, Gardner JV, Hall JK, Hell B, Hestvik O, Krist-offersen Y, Marcussen C, Mohammad R, Mosher D, Nghiem SV, Pedrosa MT, Travaglini PG, Weatherall P (2012). The international bathymetric chart of the Arctic Ocean (IBCAO) version 3.0. Geophys Res Lett 39, L12609. Laske, G., Masters., G., Ma, Z. and Pasyanos, M. (2013). Update on CRUST1.0 - A 1-degree Global Model of Earth's Crust, Geophys. Res. Abstracts, 15, Abstract EGU2013-2658, 2013. Minakov A, Faleide JI, Glebovsky VY, Mjelde R (2012) Structure and evolution of the northern Barents-Kara Sea continental margin from integrated analysis of potential fields, bathymetry and sparse seismic data. Geophys J Int 188, 79-102. Petrov O., Smelror M., Shokalsky S., Morozov A., Kashubin S., Grikurov G., Sobolev N., Petrov E., (2013). A new international tectonic map of the Arctic (TeMAr) at 1:5 M scale and geodynamic evolution in the Arctic region. EGU2013-13481. Reguzzoni, M., & Sampietro, D. (2014). GEMMA: An Earth crustal model based on GOCE satellite data. International Journal of Applied Earth Observation and Geoinformation Spasojevic S. & Gurnis M., (2012). Sea level and vertical motion of continents from dynamic earth models since the late Cretaceous. American Association of Petroleum Geologists Bulletin, 96, pp. 2037-2064.

The Moho as a magnetic boundary. [Earth crust-mantle boundary

NASA Technical Reports Server (NTRS)

Wasilewski, P. J.; Thomas, H. H.; Mayhew, M. A.

1979-01-01

Magnetism in the crust and the upper mantle and magnetic results indicating that the seismic Moho is a magnetic boundary are considered. Mantle derived rocks - peridotites from St. Pauls rocks, dunite xenoliths from the Kaupulehu flow, and peridotite, dunite, and eclogite xenoliths from Roberts Victor and San Carlos diatremes - are weakly magnetic with saturation magnetization values from 0.013 emu/gm to less than 0.001 emu/gm which is equivalent to 0.01 to 0.001 wt% Fe304. Literature on the minerals in mantle xenoliths shows that metals and primary Fe304 are absent, and that complex Cr, Mg, Al, and Fe spinels are dominant. These spinels are non-magnetic at mantle temperatures, and the crust/mantle boundary can be specified as a magnetic mineralogy discontinuity. The new magnetic results indicate that the seismic Moho is a magnetic boundary, the source of magnetization is in the crust, and the maximum Curie isotherm depends on magnetic mineralogy and is located at depths which vary with the regional geothermal gradient.

Gravity data inversion for 3D topography of the Moho discontinuity by separation of sources in Taiwan region

NASA Astrophysics Data System (ADS)

Lo, Y. T.; Yen, H. Y.

2012-04-01

Taiwan is located at a complex juncture between the Eurasian and Philippine Sea plates. The mountains in Taiwan are very young, formed as a result of the collision between an island arc system and the Asian continental margin. To separate sources of gravity field in depth, a method is suggested, based on upward and downward continuation. Both new methods are applied to isolate the contribution of the Moho interface to the total field and to find its 3D topography. At the first stage, we separate near surface and deeper sources. At the next stage, we isolate the effect of very deep sources. After subtracting this field from the total effect of deeper sources, we obtain the contribution of the Moho interface. We make inversion separately for the area. In this study, we use the detail gravity data around this area to investigate the reliable subsurface density structure. First, we combine with land and marine gravity data to obtain gravity anomaly. Second, considering the geology, tomography and other constrains, we simulate the 3D density structure. The main goal of our study is to understand the Moho topography and sediment-crustal boundary in Taiwan area. We expect that our result can consistent with previous studies.

Relationship between Curie isotherm surface and Moho discontinuity in the Arabian shield, Saudi Arabia

NASA Astrophysics Data System (ADS)

Aboud, Essam; Alotaibi, Abdulrahman M.; Saud, Ramzi

2016-10-01

The Arabian shield is a Precambrian complex of igneous and metamorphic rocks located approximately one-third of the way across the western Arabian Peninsula, with uncommon exposures along the Red Sea coast. We used aeromagnetic data acquired by others over the past several decades to estimate the depth to the Curie temperature isotherm throughout this region. Our goal was to further understand the lithospheric structure, thermal activity, and seismicity to assist in geothermal exploration. We also compared the Curie temperature isotherm with the crustal thickness to investigate the possibility that mantle rocks are magnetic in some parts of the Arabian shield. Depths to the Curie isotherm were estimated by dividing the regional aeromagnetic grid into 26 overlapping windows. Each window was then used to estimate the shape of the power spectrum. The windows had dimensions of 250 × 250 km to allow investigation of depths as deep as 50 km. The results show the presence of a Curie isotherm at a depth of 10-20 km near the Red Sea, increasing to 35-45 km in the interior of the Arabian shield. The Curie isotherm generally lies above the Moho in this region but deepens into the mantle in some locations, notably beneath the Asir Terrane.

Passive Seismic Experiment to understand the basement and crustal structure, Northern Red Sea

NASA Astrophysics Data System (ADS)

Sinadinovski, Cvetan; Aldamegh, Khalid; Ball, Philip; Janoubi, Emad; Afifi, AbdulKader; Ion, Dumitru; Nayak, Goutam; Borsato, Ron

2017-04-01

In 2011, air gun seismic surveys were performed in the Red Sea in conjunction with an offshore survey where portable seismic stations were deployed onshore up to 250 km inland from the shoreline. In total, 30 temporary broadband stations were deployed in the northern Red Sea. The recorded shot data were analyzed in conjunction with earthquake records that occurred during the three-month deployment period. The receiver function data were modeled using an advanced 3D modeling software. Gravity data were modeled as well on five regional profiles to provide additional constraints for the depth-to-basement and depth-to-Moho discontinuity. The passive (earthquakes) and active (air gun) data for both areas were modeled separately and then in a joint scheme. This experiment was unique, where no previous deployment at this scale had been attempted before in Saudi Arabia. The tomography results provide for the first time a detailed insight of the deeper crustal structure in the Red Sea margin. The results reveal a complex geology with a heterogeneous crust and upper mantle. The crustal-mantle discontinuity was picked assuming a Vp velocity of around 8.0 km/s. The Moho discontinuity offshore appears to vary in depth from 17 km to 27 km, increasing to 22 km to 35 km onshore. The average crustal thickness inland is 28 km, whereas the average thickness offshore is 22 km. These 3D images of the Moho show that thinning of the crust was not just coast-parallel as proposed from previous 2D or 1D studies. Such findings can help in better understanding of the rift related processes in the Red Sea

The crustal thickness and lithospheric structure of active and inactive volcanic arc terrains in Fiji and Tonga

NASA Astrophysics Data System (ADS)

Chen, J.; Wiens, D.; Wei, S. S.; Zha, Y.; Julià, J.; Cai, C.; Chen, Y. J.

2015-12-01

In order to investigate the crustal thickness and lithospheric structure beneath active and inactive volcanic arcs in Fiji and Tonga, we analyzed receiver functions from teleseismic P waves as well as Rayleigh waves from teleseismic earthquakes and ambient noise. The data were recorded by stations from three previous temporary seismic arrays deployed on the islands during 1993-1995, 2001-2002, and 2009-2010. Receiver functions were calculated with an iterative deconvolution in the time domain. We used an H-k stacking method to get preliminary Moho depth estimates under the island arcs, after assuming constant seismic average crustal P velocity. We also determined the shear wave velocity structure beneath each station from a 1-D combined inversion of receiver functions and Rayleigh wave phase velocity dispersion curves from ambient noise cross correlation at 8s - 20s and teleseismic surface waves at 20s-90s. The joint inversion models reveal that the Moho beneath the main islands of the Fiji plateau is 26-31 km deep, whereas the crust under the outer islands - including the Lau Ridge - is generally thinner, with Moho depths of 21-23.5 km. The thinnest crust (16 km) is found beneath Moala Island located between the Fiji Platform and the Lau Ridge. Crustal thickness beneath several Tonga islands is about 18-20 km. A relatively high velocity lithosphere (Vs of 4.4 - 4.5 km/s) extends to only about 60 km depth beneath the outer Fiji Islands and Lau Ridge, but to depths of 90 km underneath the main islands of the Fiji Plateau. The much thicker crust and lithosphere of the Fiji plateau relative to the Lau Ridge and Tonga Arc reflects its much longer geological history of arc crust building, going back to the early Miocene.

Thermal structure of the crust in Inner East Anatolia from aeromagnetic and gravity data

NASA Astrophysics Data System (ADS)

Bektaş, Özcan

2013-08-01

Inner East Anatolia has many hot spring outcomes. In this study, the relationship between the thermal structure and hot spring outcomes is investigated. The residual aeromagnetic and gravity anomalies of the Inner East Anatolia, surveyed by the Mineral Research and Exploration (MTA) of Turkey, show complexities. The magnetic data were analyzed to produce Curie point depth estimates. The depth of magnetic dipole was calculated by azimuthally averaged power spectrum method for the whole area. The Curie point depth (CPD) map covering the Inner East Anatolia has been produced. The Curie point depths of the region between Sivas and Malatya vary from 16.5 to 18.7 km. Values of heat flow were calculated according to continental geotherm from the model. The heat flow values vary between 89 and 99 mW m-2. Heat flow values are incorporated with surface heat flow values. Gravity anomalies were modeled by means of a three-dimensional method. The deepest part of the basin (12-14 km), determined from the 3D model, are located below the settlement of Hafik and to the south of Zara towns. Two-dimensional cross sections produced from the basin depths, Curie values and MOHO depths. Based on the analysis of magnetic, gravity anomalies, thermal structures and geology, it seems likely that the hot springs are not related to rising asthenosphere, in the regions of shallow CPDs (∼16.5 km), and mostly hot springs are related to faulting systems in Inner East Anatolia.

Locating the depth of magma supply for volcanic eruptions, insights from Mt. Cameroon

PubMed Central

Geiger, Harri; Barker, Abigail K.; Troll, Valentin R.

2016-01-01

Mt. Cameroon is one of the most active volcanoes in Africa and poses a possible threat to about half a million people in the area, yet knowledge of the volcano’s underlying magma supply system is sparse. To characterize Mt. Cameroon’s magma plumbing system, we employed mineral-melt equilibrium thermobarometry on the products of the volcano’s two most recent eruptions of 1999 and 2000. Our results suggest pre-eruptive magma storage between 20 and 39 km beneath Mt. Cameroon, which corresponds to the Moho level and below. Additionally, the 1999 eruption products reveal several shallow magma pockets between 3 and 12 km depth, which are not detected in the 2000 lavas. This implies that small-volume magma batches actively migrate through the plumbing system during repose intervals. Evolving and migrating magma parcels potentially cause temporary unrest and short-lived explosive outbursts, and may be remobilized during major eruptions that are fed from sub-Moho magma reservoirs. PMID:27713494

Locating the depth of magma supply for volcanic eruptions, insights from Mt. Cameroon.

PubMed

Geiger, Harri; Barker, Abigail K; Troll, Valentin R

2016-10-07

Mt. Cameroon is one of the most active volcanoes in Africa and poses a possible threat to about half a million people in the area, yet knowledge of the volcano's underlying magma supply system is sparse. To characterize Mt. Cameroon's magma plumbing system, we employed mineral-melt equilibrium thermobarometry on the products of the volcano's two most recent eruptions of 1999 and 2000. Our results suggest pre-eruptive magma storage between 20 and 39 km beneath Mt. Cameroon, which corresponds to the Moho level and below. Additionally, the 1999 eruption products reveal several shallow magma pockets between 3 and 12 km depth, which are not detected in the 2000 lavas. This implies that small-volume magma batches actively migrate through the plumbing system during repose intervals. Evolving and migrating magma parcels potentially cause temporary unrest and short-lived explosive outbursts, and may be remobilized during major eruptions that are fed from sub-Moho magma reservoirs.

Calculation of the 3D density model of the Earth

NASA Astrophysics Data System (ADS)

Piskarev, A.; Butsenko, V.; Poselov, V.; Savin, V.

2009-04-01

The study of the Earth's crust is a part of investigation aimed at extension of the Russian Federation continental shelf in the Sea of Okhotsk Gathered data allow to consider the Sea of Okhotsk' area located outside the exclusive economic zone of the Russian Federation as the natural continuation of Russian territory. The Sea of Okhotsk is an Epi-Mesozoic platform with Pre-Cenozoic heterogeneous folded basement of polycyclic development and sediment cover mainly composed of Paleocene - Neocene - Quaternary deposits. Results of processing and complex interpretation of seismic, gravity, and aeromagnetic data along profile 2-DV-M, as well as analysis of available geological and geophysical information on the Sea of Okhotsk region, allowed to calculate of the Earth crust model. 4 layers stand out (bottom-up) in structure of the Earth crust: granulite-basic (density 2.90 g/cm3), granite-gneiss (limits of density 2.60-2.76 g/cm3), volcanogenic-sedimentary (2.45 g/cm3) and sedimentary (density 2.10 g/cm3). The last one is absent on the continent; it is observed only on the water area. Density of the upper mantle is taken as 3.30 g/cm3. The observed gravity anomalies are mostly related to the surface relief of the above mentioned layers or to the density variations of the granite-metamorphic basement. So outlining of the basement blocks of different constitution preceded to the modeling. This operation is executed after Double Fourier Spectrum analysis of the gravity and magnetic anomalies and following compilation of the synthetic anomaly maps, related to the basement density and magnetic heterogeneity. According to bathymetry data, the Sea of Okhotsk can be subdivided at three mega-blocks. Taking in consideration that central Sea of Okhotsk area is aseismatic, i.e. isostatic compensated, it is obvious that Earth crust structure of these three blocks is different. The South-Okhotsk depression is characteristics by 3200-3300 m of sea depths. Moho surface in this area is at the depth 15-17 km and intracrustal Konrad surface - at the depth 8-9 km. Thickness of sediment cover in the South-Okhotsk depression is up to 4-6 km. Type of the Earth's crust in the South-Okhotsk depression has to be defined as suboceanic. Steep slope with the more than 1.5 km depth difference separates South-Okhotsk depression from mega-block including Academy of Sciences and Institute of Oceanology Uplifts and Central Okhotsk and Deryugin Depression. Sea depths in this area are alterating mostly between 1000 and 1500 m. Moho surface in this mega-block is at the depth 23-25 km and intracrustal Konrad surface - at the depth 13-14 km with the exception of Deryugin Depression (10 km). Thickness of sediment cover varies up to 2-4 km and only in the Deryugin Depression reaches 9 km. Type of the Earth's crust in this mega-block has to be defined as continental. Northern and north-eastern parts of the Sea of Okhotsk is characteristics by 200-300 m sea depths (with the exclusion of the Tinro depression). Moho surface in this area is at the depth 28-32 km and intracrustal Konrad surface - at the depth 13-18 km with the exception of Tinro Depression (10 km). Thickness of sediment cover is minimal and only in the Tinro Depression reaches 8-9 km. Complete similarity of the potential field anomaly distribution in this region and in the western Kamchatka region is remarkable. The distribution of the potential field anomalies and features of the constructed Earth's crust density model give us evidences of Pre-Late Cretaceous consolidation of basement in northern and north-eastern parts of the Sea of Okhotsk as well as in the Russian territory of western Kamchatka peninsula.

Crustal-scale shear zones and heterogeneous structure beneath the North Anatolian Fault Zone, Turkey, revealed by a high-density seismometer array

NASA Astrophysics Data System (ADS)

Kahraman, Metin; Cornwell, David G.; Thompson, David A.; Rost, Sebastian; Houseman, Gregory A.; Türkelli, Niyazi; Teoman, Uğur; Altuncu Poyraz, Selda; Utkucu, Murat; Gülen, Levent

2015-11-01

Continental scale deformation is often localised along strike-slip faults constituting considerable seismic hazard in many locations. Nonetheless, the depth extent and precise geometry of such faults, key factors in how strain is accumulated in the earthquake cycle and the assessment of seismic hazard, are poorly constrained in the mid to lower crust. Using a dense broadband network of 71 seismic stations with a nominal station spacing of 7 km in the vicinity of the 1999 Izmit earthquake we map previously unknown small-scale structure in the crust and upper mantle along this part of the North Anatolian Fault Zone (NAFZ). We show that lithological and structural variations exist in the upper, mid and lower crust on length scales of less than 10 km and less than 20 km in the upper mantle. The surface expression of the NAFZ in this region comprises two major branches; both are shown to continue at depth with differences in dip, depth extent and (possibly) width. We interpret a <10 km wide northern branch that passes downward into a shear zone that traverses the entire crust and penetrates the upper mantle to a depth of at least 50 km. The dip of this structure appears to decrease west-east from ∼90° to ∼65° to the north over a distance of 30 to 40 km. Deformation along the southern branch may be accommodated over a wider (>10 km) zone in the crust with a similar variation of dip but there is no clear evidence that this shear zone penetrates the Moho. Layers of anomalously low velocity in the mid crust (20-25 km depth) and high velocity in the lower crust (extending from depths of 28-30 km to the Moho) are best developed in the Armutlu-Almacik block between the two shear zones. A mafic lower crust, possibly resulting from ophiolitic obduction or magmatic intrusion, can best explain the coherent lower crustal structure of this block. Our images show that strain has developed in the lower crust beneath both northern and southern strands of the North Anatolian Fault. Our new high resolution images provide new insights into the structure and evolution of the NAFZ and show that a small and dense passive seismic network is able to image previously undetectable crust and upper mantle heterogeneity on lateral length scales of less than 10 km.

Magnetic mapping of (carbonated) oceanic crust-mantle boundary: New insights from Linnajavri, northern Norway

NASA Astrophysics Data System (ADS)

Tominaga, M.; Beinlich, A.; Tivey, M.; Andrade Lima, E.; Weiss, B. P.

2012-12-01

The contribution of lower oceanic crust and upper mantle to marine magnetic anomalies has long been recognized, but the detailed magnetic character of this non-volcanic source layer remains to be fully defined. Here, we report preliminary results of a magnetic survey and source characterization of a "carbonated" oceanic Moho (petrological "Mohorovicic discontinuity") sequence observed at the Linnajavri Serpentinite Complex (LSC), northern Norway. The LSC is located at 67° 36'N and 16° 24'E within the upper Allochthon of the Norwegian Caledonides and represents a dismembered ophiolite. Particularly in the southern ("Ridoalggicohkka") area of the LSC, gabbro, serpentinite and its talc-carbonate (soapstone) and quartz-carbonate (listvenite) altered equivalents are extraordinarily well-exposed [1]. An intact oceanic Moho is exposed here, despite its complex tectonic setting. The small degree of arctic rock weathering (≤ 2 mm weathering surface) allowed for detailed regional-scale surface magnetic mapping across alteration fronts (serpentinite-soapstone; soapstone-listvenite) and lithological contacts (soapstone-gabbro). Magnetic mapping was conducted using a handheld 3-axis magnetometer, surface-towed resistivity meter and Teka surface magnetic susceptometer with sample spacing of 1 m. Geophysical field mapping was combined with petrological observations and scanning SQUID microscopy (SM) mapping conducted on thin sections from rock samples that were drilled along the survey lines. Regional scale magnetic mapping indicates that the total magnetic field across both the "carbonated" Moho and the soapstone-serpentinite interfaces show higher frequency changes in their magnetic anomaly character and amplitudes than the surface-towed resistivity data. SQUID microscopy mapping of both natural remanence magnetization (NRM) and anhysteretic remanence magnetization (ARM) on gabbro, serpentinite, soapstone, and listvenite samples, with a sensor-sample separation of ˜190 μm, show that the distribution of microscopically measurable ferromagnetic and possibly sulfide minerals produces a different bulk intensity for each of the rock types. SM vector magnetic field maps of these samples also reveal that the magnetization associated with these grains (observed as dipole-like fields in SM maps) is variable in direction from grain to grain, which may result from different alteration histories for each grain. These complex magnetization patterns acquired through thermal and chemical alteration history may explain the short wavelength magnetic anomalies observed along our traverse lines. [1] Beinlich, A., Plümper, O., Hövelmann, J., Austrheim, H. and Jamtveit, B. (2012), Terra Nova, in press.

Geological structure analysis in Central Java using travel time tomography technique of S waves

NASA Astrophysics Data System (ADS)

Palupi, I. R.; Raharjo, W.; Nurdian, S. W.; Giamboro, W. S.; Santoso, A.

2016-11-01

Java is one of the islands in Indonesia that is prone to the earthquakes, in south of Java, there is the Australian Plate move to the Java island and press with perpendicular direction. This plate movement formed subduction zone and cause earthquakes. The earthquake is the release of energy due to the sudden movement of the plates. When an earthquake occurs, the energy is released and record by seismometers in the waveform. The first wave recorded is called the P waves (primary) and the next wave is called S waves (secondary). Both of these waves have different characteristics in terms of propagation and direction of movement. S wave is composed of waves of Rayleigh and Love waves, with each direction of movement of the vertical and horizontal, subsurface imaging by using S wave tomography technique can describe the type of the S wave through the medium. The variation of wave velocity under Central Java (esearch area) is ranging from -10% to 10% at the depth of 20, 30 and 40 km, the velocity decrease with the depth increase. Moho discontinuity is lies in the depth of 32 km under the crust, it is indicates there is strong heterogenity in Moho.

The lithospheric-scale 3D structural configuration of the North Alpine Foreland Basin constrained by gravity modelling and the calculation of the 3D load distribution

NASA Astrophysics Data System (ADS)

Przybycin, Anna M.; Scheck-Wenderoth, Magdalena; Schneider, Michael

2014-05-01

The North Alpine Foreland Basin is situated in the northern front of the European Alps and extends over parts of France, Switzerland, Germany and Austria. It formed as a wedge shaped depression since the Tertiary in consequence of the Euro - Adriatic continental collision and the Alpine orogeny. The basin is filled with clastic sediments, the Molasse, originating from erosional processes of the Alps and underlain by Mesozoic sedimentary successions and a Paleozoic crystalline crust. For our study we have focused on the German part of the basin. To investigate the deep structure, the isostatic state and the load distribution of this region we have constructed a 3D structural model of the basin and the Alpine area using available depth and thickness maps, regional scale 3D structural models as well as seismic and well data for the sedimentary part. The crust (from the top Paleozoic down to the Moho (Grad et al. 2008)) has been considered as two-parted with a lighter upper crust and a denser lower crust; the partition has been calculated following the approach of isostatic equilibrium of Pratt (1855). By implementing a seismic Lithosphere-Asthenosphere-Boundary (LAB) (Tesauro 2009) the crustal scale model has been extended to the lithospheric-scale. The layer geometry and the assigned bulk densities of this starting model have been constrained by means of 3D gravity modelling (BGI, 2012). Afterwards the 3D load distribution has been calculated using a 3D finite element method. Our results show that the North Alpine Foreland Basin is not isostatically balanced and that the configuration of the crystalline crust strongly controls the gravity field in this area. Furthermore, our results show that the basin area is influenced by varying lateral load differences down to a depth of more than 150 km what allows a first order statement of the required compensating horizontal stress needed to prevent gravitational collapse of the system. BGI (2012). The International Gravimetric Bureau. IAG Geodesist's Handbook, 2012 - Journal of Geodesy, 86(10) Springer Grad, M., Tiira, T. and ESC Working Group (2009). The Moho depth map of 1 the European Plate. Geophysical Journal International 176(1): 279-292. Tesauro, M. (2009). An integrated study of the structure and thermomechanical properties of the European lithosphere. Department of Tectonics Faculty of Earth & Life Sciences. Amsterdam, Vrije Universiteit, Dissertation

Crust and Upper Mantle Structure beneath Isparta Angle in SW Turkey from P and S Receiver Functions

NASA Astrophysics Data System (ADS)

Kahraman, M.; Turkelli, N.; Özacar, A.; Sandvol, E. A.; Teoman, U.

2015-12-01

Isparta Angle (IA) constitutes a triangular shape elevated tectonic domain in SW Turkey which contains units stacked with opposing thrust vergences during Late Cretaceous to Miocene. The region which is located at the junction between Aegean and Cyprus arcs separated by a slab tear is now bounded by Fethiye-Burdur Fault Zone (FBFZ) in the west and Akşehir-Afyon Fault Zones (AAFZ) in the east. In the area, seismicity displays ongoing extension along active grabens oriented at different directions. In the past, many competing geodynamic scenarios had been proposed to explain the complex tectonic evolution of the area. In this study, we used both P and S receiver functions (RFs) to present high resolution crustal and upper mantle images down to 200 km. Moho and upper crustal discontinuities were well resolved by P Rfs; however S RFs were utilized to image lithospheric-asthenospheric boundaries having the benefit of being free of multiple conversions. RFs were calculated from 916 teleseismic earthquakes (Mw ≥ 5.5) recorded by 42 permanent and temporary broadband stations BU-KOERI/NEMC, DEMP/ERD and Isparta Angle Seismic Experiment deployed by collaboration of BU-KOERI and University of Missouri. Totally, 4501 P and 946 S RFs with the cut-off frequencies of ~1.0 Hz and ~0.5 Hz, respectively, were obtained by applying iterative-time domain deconvolution. Crustal thickness and Vp/Vs ratios were calculated by grid search of maximum amplitude of P RFs(Ps,PpPs and PsPs+PpSs) in depth and Vp/Vs domain. Then, we created 2-D P and S migrated cross-sections to observe crustal and lithospheric-asthenospheric discontinuities beneath the region. P RFs indicates that, average crustal thickness and Vp/Vs ratio is ~36 km and 1.78 in the region with small changing values close to the edges. Migrated P RFs cross-sections revealed a sharp change in Moho (Moho offset) on the western boundary that spatially correlates with the FBFZ. We also found a relatively flat Moho in the center and what appears to be imaged northern tip of slab at ~45 km depth. Finally, ~30km crustal thickness released in southeast beneath the Cyprus. On the other hand; preliminary results of S RFs cross-sections present the LAB boundary between ~60 to ~90 km depth range, observed almost beneath all profiles and clear positive phase arrivals right below the LAB depths.

Variation in Crustal Structure of the Lesser Caucasus Region from Teleseismic Receiver Functions

NASA Astrophysics Data System (ADS)

Lin, C. M.; Tseng, T. L.; Huang, B. S.; Legendre, C. P.; Karakhanian, A.

2016-12-01

The Caucasus, including the mountains of Greater and Lesser Caucasus, is formed by the continental collision between Arabia and Eurasia. The crustal thickness for this region was mostly constrained by joint analysis of receiver functions and surface waves. Although the thickest value of 52 km was reported under the Lesser Caucasus, the resolution of earlier studies were often limited by sparse array. Large gradient across Moho also makes the definition of Moho difficult. Moreover, higher value of the Vp/Vs ratio is commonly reported in the northeastern Turkey but no estimates had been made for the Caucasus. To further investigate the detail structure around the Lesser Caucasus, we constructed a new seismic network in Georgia and Armenia. We also include other broadband stations to enhance the coverage. The average interval in the Lesser Caucasus is roughly 30 km, much denser than any previous experiments. We selected P-waveforms from teleseismic earthquakes during the operation (January 2012 - June 2016) to calculate receiver functions and then estimate the crustal thickness (H) and Vp/Vs ratio (k) with the H-k stacking technique. Our preliminary results show that Moho depth increases from 40 km under the northeastern Turkey to 50 km beneath northern Georgia, no station with Moho deeper than 50 km under the Lesser Caucasus. The Vp/Vs ratios in the northeastern Anatolian plateau are around 1.8, which is slightly higher than the average of global continents but consistent with the previous estimates. Further to the east, some stations show anomalously higher Vp/Vs ratio in central & southern Armenia that may be associated with Holocene volcanism. In the future, we plan to join locally measured dispersion curves to invert the velocity model without velocity-depth trade-off. We expect to resolve the velocity variations of the crust beneath this region in small scale that may be tied to the continental collision and surface volcanism. Keywords: Caucasus, receiver function, continental collision, volcanic plateau, crustal structure

Subsurface signature of North Anatolian Fault Zone and its relation with old sutures: New insight from receiver function analysis.

NASA Astrophysics Data System (ADS)

Özacar, Arda A.; Abgarmi, Bizhan

2017-04-01

The North Anatolian Fault Zone (NAFZ) is an active continental transform plate boundary that accommodates the westward extrusion of the Anatolian plate. The central segment of NAFZ displays northward convex surface trace which coincides partly with the Paleo-Tethyan suture formed during the early Cenozoic. The depth extent and detailed structure of the actively deforming crust along the NAF is still under much debate and processes responsible from rapid uplift are enigmatic. In this study, over five thousand high quality P receiver functions are computed using teleseismic earthquakes recorded by permanent stations of national agencies and temporary North Anatolian Fault Passive Seismic experiment (2005-2008). In order to map the crustal thickness and Vp/Vs variations accurately, the study area is divided into grids with 20 km spacing and along each grid line Moho phase and its multiples are picked through constructed common conversion point (CCP) profiles. According to our results, nature of discontinuities and crustal thickness display sharp changes across the main strand of NAFZ supporting a lithospheric scale faulting that offsets Moho discontinuity. In the southern block, crust is relatively thin in the west ( 35 km) and becomes thicker gradually towards east ( 40 km). In contrast, the northern block displays a strong lateral change in crustal thickness reaching up to 10 km across a narrow roughly N-S oriented zone which is interpreted as the subsurface signature of the ambiguous boundary between Istanbul Block and Pontides located further west at the surface.

Imaging of the Main Himalayan Thrust and Moho beneath Satluj Valley, Northwest Himalaya

NASA Astrophysics Data System (ADS)

Wadhawan, M.; Hazarika, D.; Paul, A.; Kumar, N.

2016-12-01

The ongoing continental collision between India and Eurasia gave rise to the formation of the great Himalayan fold-thrust belt. Satluj valley is found to be well exposed from foreland to Higher Himalayan Crystalline series along the Satluj River. Receiver function method has been utilized to image crustal features using Common Conversion Point (CCP) stacking beneath Satluj valley recorded by a seismological array of 18 broadband seismometers. The seismological stations cover the geotectonic units starting from the Himalayan Frontal Thrust (HFT) in the south to the Tethyan Himalaya (TH) to the north. The study inferred gentle northward dipping nature of the Main Himalayan Thrust (MHT) between Sub Himalaya (SH) and Higher Himalaya (HH) in the study area rather than flat-ramp-flat geometry as reported in Nepal Himalaya and Garhwal Himalaya. The depth of the MHT obtained from CCP image and inversion of receiver functions shows that it varies from 16 km in the SH to 27 km near the STD which further increases up to 38 km beneath the TH. The absence of both large and moderate magnitude earthquakes in the Himalayan Seismic Belt (HSB) straddling northern Lesser Himalaya and southern Higher Himalaya in Satluj valley is correlated with absence of ramp structure in this part of HSB. The CCP image has mapped the Moho discontinuity at 44 km depth near the HFT which has increased to 62 km beneath the TH. An extremely low shear wave velocity ranging between 0.8 and 1.8 km s-1 is estimated at stations near the HFT, in the upper most 3-4 km of the crust which indicates the effect of sedimentary column of Indo-Gangetic plains. An intra crustal low velocity layer (IC-LVL) is observed beneath the study profile and inferred as partial melt and/or aqueous fluid at mid-crustal depth beneath the TH. The H-K stacking is applied and average Poisson's ratio is observed to be higher in the TH as compared to the stations to the south of STD.

Tectonics and crustal structure of the Saurashtra peninsula: based on Gravity and Magnetic data

NASA Astrophysics Data System (ADS)

Mishra, A. K.; Singh, A.; Singh, U. K.

2016-12-01

The Saurashtra peninsula is located at the North Western margin of the Indian shield which occurs as a horst block between the rifts namely as Kachchh, Cambay and Narmada. It is important because of occurrence of moderate earthquake and presence of mesozoic sediments below the Deccan trap. The maps of bouguer gravity anomaly and the total intensity magnetic anomalies of Saurashtra have delineated six circular gravity highs of magnitudes 40-60 mGal and 800-1000 nT respectively. In order to understand the location, structure and depth of the source body, methods like continuous wavelet transform (CWT), Euler deconvolution and power spectrum analysis have been implemented in the potential field data. The CWT and Euler deconvolution give 16-18 km average depth of volcanic plug in Junagadh and Rajula region. From the power spectrum analysis, it is found that average Moho depth in the Saurashtra is about 36-38 km. Keeping the constraints obtained from geophysical studies like borehole, deep seismic survey, receiver function analysis and geological information, combined gravity and magnetic modeling have been performed. Detailed crustal structure of the Saurashtra region has been delineated along two profiles which pass from prominent geological features Junagadh and Rajula volcanic plugs respectively.

The structure, isostasy and gravity field of the Levant continental margin and the southeast Mediterranean area

NASA Astrophysics Data System (ADS)

Segev, Amit; Rybakov, Michael; Lyakhovsky, Vladimir; Hofstetter, Avraham; Tibor, Gidon; Goldshmidt, Vladimir; Ben Avraham, Zvi

2006-10-01

A 3-D layered structure of the Levant and the southeastern Mediterranean lithospheric plates was constructed using interpretations of seismic measurements and borehole data. Structural maps of three principal interfaces, elevation, top basement and the Moho, were constructed for the area studied. This area includes the African, Sinai and Arabian plates, the Herodotus and the Levant marine basins and the Nile sedimentary cone. In addition, an isopach map of the Pliocene sediments, as well as the contemporaneous amount of denuded rock units, was prepared to enable setting up the structural map of the base Pliocene sediment. Variable density distributions are suggested for the sedimentary succession in accord with its composition and compaction. The spatial density distribution in the crystalline crust was calculated by weighting the thicknesses of the lower mafic and the upper felsic crustal layers, with densities of 2.9 g/cm 3 and 2.77 g/cm 3, respectively. Results of the local (Airy) isostatic modeling with compensation on the Moho interface show significant deviations from the local isostasy and require variable density distribution in the upper mantle. Moving the compensation level to the base of the lithosphere (˜ 100 km depth) and adopting density variations in the mantle lithosphere yielded isostatic compensation (± 200 m) over most of the area studied. The spatial pattern obtained of a density distribution with a range of ± 0.05 g/cm 3 is supported by a regional heat flux. Simulations of the flexure (Vening Meinesz) isostasy related to the Pliocene to Recent sedimentary loading and unloading revealed concentric oscillatory negative and positive anomalies mostly related to the Nile sedimentary cone. Such anomalies may explain the rapid subsidence in the Levant Basin and the arching in central Israel, northern Sinai and Egypt during Pliocene-Recent times. Comparison between the observed (Bouguer) gravity and the calculated gravity for the constructed 3-D lithospheric structure, which has variable density distributions, provided a good match and an independent constraint for the large-scale structure suggested and confirmed an oceanic nature for the Levant Basin lithosphere.

High-resolution surface wave tomography of the European crust and uppermost mantle from ambient seismic noise

NASA Astrophysics Data System (ADS)

Lu, Yang; Stehly, Laurent; Paul, Anne; AlpArray Working Group

2018-05-01

Taking advantage of the large number of seismic stations installed in Europe, in particular in the greater Alpine region with the AlpArray experiment, we derive a new high-resolution 3-D shear-wave velocity model of the European crust and uppermost mantle from ambient noise tomography. The correlation of up to four years of continuous vertical-component seismic recordings from 1293 broadband stations (10° W-35° E, 30° N-75° N) provides Rayleigh wave group velocity dispersion data in the period band 5-150 s at more than 0.8 million virtual source-receiver pairs. Two-dimensional Rayleigh wave group velocity maps are estimated using adaptive parameterization to accommodate the strong heterogeneity of path coverage. A probabilistic 3-D shear-wave velocity model, including probability densities for the depth of layer boundaries and S-wave velocity values, is obtained by non-linear Bayesian inversion. A weighted average of the probabilistic model is then used as starting model for the linear inversion step, providing the final Vs model. The resulting S-wave velocity model and Moho depth are validated by comparison with previous geophysical studies. Although surface-wave tomography is weakly sensitive to layer boundaries, vertical cross-sections through our Vs model and the associated probability of presence of interfaces display striking similarities with reference controlled-source (CSS) and receiver-function sections across the Alpine belt. Our model even provides new structural information such as a ˜8 km Moho jump along the CSS ECORS-CROP profile that was not imaged by reflection data due to poor penetration across a heterogeneous upper crust. Our probabilistic and final shear wave velocity models have the potential to become new reference models of the European crust, both for crustal structure probing and geophysical studies including waveform modeling or full waveform inversion.

Imaging the lithosphere-asthenosphere boundary across the transition from Phanerozoic Europe to the East-European Craton with S-receiver functions

NASA Astrophysics Data System (ADS)

Knapmeyer-Endrun, Brigitte; Krüger, Frank

2013-04-01

Cratons are characterized by their thick lithospheric roots. In the case of the Eastern European Craton, high seismic velocities have been imaged tomographically to more than 200 km depth. However, the exact depth extent of the cratonic lithosphere and especially the properties of the transition to a much thinner lithosphere beneath Phanerozoic central Europe still remain under discussion. Whereas a number of recent seismic campaigns has significantly increased the knowledge about crustal structure and Moho topography in central Europe, comparably detailed, 3-D information on upper mantle structure, e.g. the lithosphere-asthenosphere boundary (LAB), is yet missing. The international PASSEQ experiment, which was conducted from 2006 to 2008, strived to fill this gap with the deployment of 196 seismological stations, roughly a quarter of which were equipped with broad-band sensors, between eastern Germany and Lithuania. With a mean inter-station distance of 60 km, reduced to about 20 km along the central profile, PASSEQ offers the densest coverage for a passive experiment in this region yet. Here, we present first S-receiver function results for this data set, complemented by additional data from national and regional networks and other temporary deployments. This increases the number of available broad-band stations to almost 300, though mostly located to the west of the Trans-European Suture Zone (TESZ). Besides, we also process data from short-period (1 s and 5 s) sensors. The visibility of mantle-transition zone phases, even in single-station data, provides confidence in the quality of the obtained S-receiver functions. Moho conversions can be confidently identified for all stations. In case of a low-velocity sedimentary cover, as found for example in the Polish Basin, the S-receiver functions even provide clearer information on Moho depth than the P-receiver functions, which are heavily disturbed by shallow reverberations. For stations west of the TESZ, a clear negative conversion, indicative of a velocity decrease with depth and identified as the LAB, is detected from an average depth of 90 km. This is in good agreement with estimates of the lithospheric thickness beneath Phanerozoic Europe from surface waves. Highest amplitudes of this conversion are obtained when the data are low-passed around 3 s. This indicates that the corresponding interface is less sharp than the Moho, which shows highest amplitudes for a 1 s-lowpass, but still limited in width to about 15 km. For stations located on the East European Craton, we likewise observe a negative conversion caused by a velocity reduction at about 100 km depth. However, in this case, the cause cannot be the tomographically imaged LAB. We rather explain the observation as mid-lithospheric discontinuity, which has also been found in S-receiver functions from other cratonic areas worldwide. At some of the cratonic stations, we observe a negative conversion of similar size that could be related to a velocity decrease at 190 km to 230 km depth, in agreement with depth estimates for the cratonic LAB. The lack of this observation for the other cratonic stations might imply spatial variations in the sharpness of the corresponding velocity change.

Evaluation of Regional Travel-Time and Location Improvement Along the Tethyan Margin Using a New Three-Dimensional Velocity Model

DTIC Science & Technology

2008-09-01

part of the Atlantic Ocean for reference. The Moho depth result is broadly consistent with CRUST2.0, except in mid-northern Africa, where the crust...plate boundaries is shown by the pink line in Figure 1. The interaction of these five major tectonic plates with each other and with several microplates ...acquired from literatures. Artificial point constraints of 10 km depth are placed to the Atlantic and Indian Oceans where measurements are absent

Seismic anisotropy of 70 Ma Pacific-plate upper mantle

NASA Astrophysics Data System (ADS)

Mark, H. F.; Lizarralde, D.; Collins, J. A.; Miller, N. C.; Hirth, G.; Gaherty, J. B.; Evans, R. L.

2017-12-01

We present a new measurement of seismic anisotropy and velocity gradients in the Pacific-plate upper mantle based on data from the NoMelt experiment. The seismic velocity structure of oceanic lithosphere reflects the processes involved in its formation at mid-ocean ridges and subsequent evolution off-axis. Increasing mantle depletion with depth due to melt extraction predicts negative velocity gradients, as does cooling with age. Alignment of olivine by corner flow predicts azimuthal anisotropy. Some models predict the strength of anisotropy should decrease with depth. Measurements of uppermost mantle velocities have not fully verified these predictions. Observations of direct Pn phases demonstrate that positive velocity gradients exist; and anisotropy measurements, while consistent with strain-induced olivine alignment, vary widely and generally suggest weaker fabric development than is observed in ophiolite samples. These discrepancies raise questions about the extent to which mantle structure evolves through time due to processes such as cracking and alteration, and hinder the use of seismic measurements to make more detailed inferences on aspects of lithospheric formation processes. We have measured anisotropy and vertical velocity gradients to 10 km below the Moho on 70 Ma lithosphere between the Clarion and Clipperton fracture zones. The lithosphere at the study site has not been obviously affected by tectonic or magmatic events since its formation. We find 6.2% anisotropy at the Moho with a mean velocity of 8.14 km/s and the fast direction parallel to paleospreading. Velocity gradients are estimated at 0.02 km/s/km in the fast direction and near 0 km/s/km in the slow direction. The gradient estimates can be explained by aligned microcracks oriented perpendicular to spreading that close with depth. Cracks are expected to close by 10 km below the Moho. At that depth the strength of anisotropy increases to 9%, close to the strength estimated from ophiolite fabrics. These results are consistent with observed olivine fabrics and the predicted effects of lithospheric formation processes, and suggest that lithospheric evolution is modest even at 70 Ma, involving microcracks oriented by a stress field consistent with thermal contraction.

Arc Crustal Structure around Mount Rainier Constrained by Receiver Functions and Seismic Noise

NASA Astrophysics Data System (ADS)

Obrebski, M. J.; Abers, G. A.; Foster, A. E.

2013-12-01

Volcanic arcs along subduction zones are thought to be loci for continental growth. Nevertheless, the amount of material transferred from the mantle to crust and the associated magmatic plumbing are poorly understood. While partial melting of mantle peridotite produces basaltic melt, the average composition of continental crust is andesitic. Several models of magma production, migration and differentiation have been proposed to explain the average crust composition in volcanic arcs. The formation of mafic cumulate and restite during fractional crystallization and partial melting has potential to alter the structure of the crust-mantle interface (Moho). The computed composition and distribution of crust and mantle rocks based on these different models convert into distinctive vertical velocity profiles, which seismic imaging methods can unravel . With a view to put more constraints on magmatic processes in volcanic arc, we analyze the shear wave velocity (Vs) distribution in the crust and uppermost mantle below Mount Rainier, WA, in the Cascadia arc. We resolve the depth of the main velocity contrasts based on converted phases, for which detection in the P coda is facilitated by source normalization or receiver function (RF) analysis. To alleviate the trade-off between depth and velocity intrinsic to RF analysis, we jointly invert RF with frequency-dependent surface wave velocities. We analyze earthquake surface waves to constrain long period dispersion curves (20-100 s). For shorter period (5-20s), we use seismic noise cross-correlograms and Aki's spectral formulation, which allows longer periods for given path. We use a transdimensional Bayesian scheme to explore the model space (shear velocity in each layer, number of interfaces and their respective depths). This approach tends to minimize the number of layers required to fit the observations given their noise level. We apply this tool to a set of broad-band stations from permanent and EarthScope temporary stations, all within 35 km of Mt Rainier. The receiver functions significantly differ from one station to another, indicating short wavelength lateral contrast in the lithospheric structure. Below arc stations offset from Mount Rainier, preliminary models show a rather clear Moho transition around 40km, separating lower crust with 3.6-3.9 km/s shear velocity, from a ~ 20 km thick mantle lid with Vs ~ 4.2 km/s. In contrast, at station PANH located 9 km east of Mount Rainier, the exact location of the Moho is not clear. Shear velocity ranges from 3.3 to 3.9 km/s from the surface down to 55 km depth, with the exception of a fast layer imaged between 25 and 32 km depth with Vs ~ 4.2 km/s. It seems likely that partial melt in the mantle, combined with high-velocity underplated or differentiated lower crust, are acting in various ways to create a complicated structure around the Moho.

Moho depth model for the Central Asian Orogenic Belt from satellite gravity gradients

NASA Astrophysics Data System (ADS)

Guy, Alexandra; Holzrichter, Nils; Ebbing, Jörg

2017-09-01

The main purpose of this study is to construct a new 3-D model of the Central Asian Orogenic Belt (CAOB) crust, which can be used as a starting point for future lithospheric studies. The CAOB is a Paleozoic accretionary orogen surrounded by the Siberian Craton to the north and the North China and Tarim Cratons to the south. This area is of great interest due to its enigmatic and still not completely understood geodynamic evolution. First, we estimate an initial crustal thickness by inversion of the vertical gravity component of the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and DTU10 models. Second, 3-D forward modeling of the GOCE gravity gradients is performed, which determines the topography of the Moho, the geometry, and the density distribution of the deeper parts of the CAOB and its surroundings, taking into account the lateral and vertical density variations of the crust. The model is constrained by seismic refraction, reflection, and receiver function studies and geological studies. In addition, we discuss the isostatic implications of the differences between the seismic Moho and the resulting 3-D gravity Moho, complemented by the analysis of the lithostatic load distribution at the upper mantle level. Finally, the correlation between the contrasting tectonic domains and the thickness of the crust reveals the inheritance of Paleozoic and Mesozoic geodynamics, particularly the magmatic provinces and the orocline which preserve their crustal features.

Seismic wide-angle constraints on the crust of the southern Urals

NASA Astrophysics Data System (ADS)

Carbonell, R.; Gallart, J.; PéRez-Estaún, A.; Diaz, J.; Kashubin, S.; Mechie, J.; Wenzel, F.; Knapp, J.

2000-06-01

A wide-angle seismic reflection/refraction data set was acquired during spring 1995 across the southern Urals to characterize the lithosphere beneath this Paleozoic orogen. The wide-angle reflectivity features a strong frequency dependence. While the lower crustal reflectivity is in the range of 6-15 Hz, the PmP is characterized by frequencies below 6 Hz. After detailed frequency filtering, the seismic phases constrain a new average P wave velocity crustal model that consists of an upper layer of 5.0-6.0 km/s, which correlates with the surface geology; 5-7 km depths at which the velocities increase to 6.2-6.3 km/s; 10-30 km depths at which, on average, the crust is characterized by velocities of 6.6 km/s; and finally, the lower crust, from 30-35 km down to the Moho, which has velocities ranging from 6.8 to 7.4 km/s. Two different S wave velocity models, one for the N-S and one for the E-W, were derived from the analysis of the horizontal component recordings. Crustal sections of Poisson's ratio and anisotropy were calculated from the velocity models. The Poisson's ratio increases in the lower crust at both sides of the root zone. A localized 2-3% anisotropy zone is imaged within the lower crust beneath the terranes east of the root. This feature is supported by time differences in the SmS phase and by the particle motion diagrams, which reveal two polarized directions of motion. Velocities are higher in the central part of the orogen than for the Siberian and eastern plates. These seismic recordings support a 50-56 km crustal thickness beneath the central part of the orogen in contrast to Moho depths of ≈ 45 km documented at the edges of the transect. The lateral variation of the PmP phase in frequency content and in waveform can be taken as evidence of different genetic origins of the Moho in the southern Urals.

Seismic character of the crust and upper mantle beneath the Sierra Nevada

NASA Astrophysics Data System (ADS)

Frassetto, A.; Gilbert, H.; Zandt, G.; Owens, T. J.; Jones, C.

2008-12-01

Recent geophysical studies of the Southern Sierra Nevada suggest that the removal of a gravitationally unstable, eclogitic residue links to recent volcanism and uplift in the Eastern Sierra. The Sierra Nevada EarthScope Project (SNEP) investigates the extent of this process beneath Central and Northern Sierra Nevada. We present receiver functions, which provide estimates of crustal thickness and Vp/Vs and image the response of the crust and upper mantle to lithospheric removal. For completeness this study combines data from the 2005-2007 SNEP broadband experiment, EarthScope's BigFoot Array, regional backbone stations, and earlier PASSCAL deployments. We analyze transects of teleseismic receiver functions generated using a common-conversion-point stacking algorithm. These identify a narrow, "bright" conversion from the Moho at depths of ~25-35 km along the crest of the Eastern Sierra and adjacent Basin and Range northward to the Cascade Arc. Trade-off analysis using the primary conversion and reverberations shows a high Vp/Vs (~1.9) throughout the Eastern Sierra, which may relate to partial melt present in the lower crust. To the west the crust-mantle boundary vanishes beneath the western foothills. However, low frequency receiver functions do image the crust-mantle boundary exceeding 50 km depth along the foothills to the west and south of Yosemite National Park. Unusually deep, intraplate earthquakes (Ryan et al., this session) occur in the center of this region. The frequency dependence of the Moho conversion implies a gradational increase from crust to mantle wavespeeds over a significant depth interval. The transition from a sharp to gradational Moho probably relates to the change from a delaminated granitic crust to crust with an intact, dense, eclogitic residue. The spatial correlation and focal mechanisms of the deep earthquakes suggest that a segment of this still intact residue is currently delaminating.

New insights on regional tectonics and crustal composition of the eastern Sierras Pampeanas in the Andean back arc region, Argentina (31-32ºS)

NASA Astrophysics Data System (ADS)

Ammirati, J. B.; Venerdini, A. L.; Alvarado, P. M.; Gilbert, H. J.

2017-12-01

Within the flat slab region of the south central Andes, the eastern Sierras Pampeanas (ESP) are the easternmost expression of a series of foreland uplifts affecting the Argentine back arc region ( 31-32ºS). This important crustal deformation has been related to the subduction of the Juan Fernández Ridge (JFR) under the South American plate. Geological observations suggest that the regional crustal structure is inherited from the accretion of different terranes during the Ordovician and later reactivated since the Miocene during the Andean compression. Geophysical experiments allowed to image how the structure observed at the surface behave in depth as décollement levels that accommodate regional crustal shortening. In order to get new insights on the mechanisms that control crustal regional tectonics, we computed teleseismic receiver functions (RF) and jointly invert them with Rayleigh-wave phase velocity dispersion curves. RFs allow resolving crustal thickness and intra crustal velocity variations with a good vertical resolution whereas surface wave information helps to constrain absolute seismic wave velocities. Our results show how the crustal thickness is increasing to the west with an important step in Moho depth. We observe that this step presents a NW-SE orientation, parallel to the trace at the surface of the Valle Fértil - La Huerta (SVF-LH) fault which suggest that this Moho step marks the transition in depth between the Pampia terrane (east) and the Cuyania terrane (west). Our images also reveal the presence of a high wave velocity lower crust west of this Moho step, beneath the eastern Sierras Pampeanas. This observation suggests that the SVF-LH fault is underthrusting the Cuyania lower crust under the Pampia terrane. Finally, our seismic images show very localized low velocity zones located at 10 km beneath late Cenozoic volcanic fields. We believe that these low velocity zones correspond to old magma chambers associated to the recent flat slab-related volcanism in the ESP.

Determining OCT structure and COB Location of the Omani Gulf of Aden Continental Margin from Gravity Inversion, Residual Depth Anomaly and Subsidence Analysis.

NASA Astrophysics Data System (ADS)

Cowie, Leanne; Kusznir, Nick; Leroy, Sylvie; Manatshal, Gianreto

2013-04-01

Knowledge and understanding of the ocean-continent transition (OCT) structure and continent-ocean boundary (COB) location, the distribution of thinned continental crust and lithosphere, its distal extent and the start of unequivocal oceanic crust are of critical importance in evaluating rifted continental margin formation and evolution. In order to determine the OCT structure and COB location for the eastern Gulf of Aden, along the Oman margin, we use a combination of gravity inversion, subsidence analysis and residual depth anomaly (RDA) analysis. Gravity inversion has been used to determine Moho depth, crustal basement thickness and continental lithosphere thinning; subsidence analysis has been used to determine the distribution of continental lithosphere thinning; and RDAs have been used to investigate the OCT bathymetric anomalies with respect to expected oceanic bathymetries at rifted margins. The gravity inversion method, which is carried out in the 3D spectral domain, incorporates a lithosphere thermal gravity anomaly and includes a correction for volcanic addition due to decompression melting. Reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. RDAs have been calculated by comparing observed and age predicted oceanic bathymetries, using the thermal plate model predictions from Crosby and McKenzie (2009). RDAs have been computed along profiles and have been corrected for sediment loading using flexural back-stripping and decompaction. In addition, gravity inversion crustal basement thicknesses together with Airy isostasy have been used to predict a synthetic RDA. The RDA results show a change in RDA signature and may be used to estimate the distal extent of thinned continental crust and where oceanic crust begins. Continental lithosphere thinning has been determined using flexural back-stripping and subsidence analysis assuming the classical rift model of McKenzie (1978) with a correction for volcanic addition due to decompression melting based on White & McKenzie (1989). Gravity inversion and the "synthetic" gravity derived RDA both show generally normal thickness oceanic crust, with some localised thin oceanic crust. Continental lithosphere thinning factors determined from gravity inversion and subsidence analysis are in good agreement and have been used to constrain COB location along the profile lines. These techniques show that the OCT in the eastern Gulf of Aden, is relatively narrow, with the distance between the COB and the margin hinge measuring less than 100km.

Detailed Configuration of the Underthrusting Indian Lithosphere Beneath Western Tibet Revealed by Receiver Function Images

NASA Astrophysics Data System (ADS)

Xu, Qiang; Zhao, Junmeng; Yuan, Xiaohui; Liu, Hongbing; Pei, Shunping

2017-10-01

We analyze the teleseismic waveform data recorded by 42 temporary stations from the Y2 and ANTILOPE-1 arrays using the P and S receiver function techniques to investigate the lithospheric structure beneath western Tibet. The Moho is reliably identified as a prominent feature at depths of 55-82 km in the stacked traces and in depth migrated images. It has a concave shape and reaches the deepest location at about 80 km north of the Indus-Yarlung suture (IYS). An intracrustal discontinuity is observed at 55 km depth below the southern Lhasa terrane, which could represent the upper border of the eclogitized underthrusting Indian lower crust. Underthrusting of the Indian crust has been widely observed beneath the Lhasa terrane and correlates well with the Bouguer gravity low, suggesting that the gravity anomalies in the Lhasa terrane are induced by topography of the Moho. At 20 km depth, a midcrustal low-velocity zone (LVZ) is observed beneath the Tethyan Himalaya and southern Lhasa terrane, suggesting a layer of partial melts that decouples the thrust/fold deformation of the upper crust from the shortening and underthrusting in the lower crust. The Sp conversions at the lithosphere-asthenosphere boundary (LAB) can be recognized at depths of 130-200 km, showing that the Indian lithospheric mantle is underthrusting with a ramp-flat shape beneath southern Tibet and probably is detached from the lower crust immediately under the IYS. Our observations reconstruct the configuration of the underthrusting Indian lithosphere and indicate significant along strike variations.

Crustal structure of north Peru from analysis of teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Condori, Cristobal; França, George S.; Tavera, Hernando J.; Albuquerque, Diogo F.; Bishop, Brandon T.; Beck, Susan L.

2017-07-01

In this study, we present results from teleseismic receiver functions, in order to investigate the crustal thickness and Vp/Vs ratio beneath northern Peru. A total number of 981 receiver functions were analyzed, from data recorded by 28 broadband seismic stations from the Peruvian permanent seismic network, the regional temporary SisNort network and one CTBTO station. The Moho depth and average crustal Vp/Vs ratio were determined at each station using the H-k stacking technique to identify the arrival times of primary P to S conversion and crustal reverberations (PpPms, PpSs + PsPms). The results show that the Moho depth correlates well with the surface topography and varies significantly from west to east, showing a shallow depth of around 25 km near the coast, a maximum depth of 55-60 km beneath the Andean Cordillera, and a depth of 35-40 km further to the east in the Amazonian Basin. The bulk crustal Vp/Vs ratio ranges between 1.60 and 1.88 with the mean of 1.75. Higher values between 1.75 and 1.88 are found beneath the Eastern and Western Cordilleras, consistent with a mafic composition in the lower crust. In contrast values vary from 1.60 to 1.75 in the extreme flanks of the Eastern and Western Cordillera indicating a felsic composition. We find a positive relationship between crustal thickness, Vp/Vs ratio, the Bouguer anomaly, and topography. These results are consistent with previous studies in other parts of Peru (central and southern regions) and provide the first crustal thickness estimates for the high cordillera in northern Peru.

South China Sea crustal thickness and lithosphere thinning from satellite gravity inversion incorporating a lithospheric thermal gravity anomaly correction

NASA Astrophysics Data System (ADS)

Kusznir, Nick; Gozzard, Simon; Alvey, Andy

2016-04-01

The distribution of ocean crust and lithosphere within the South China Sea (SCS) are controversial. Sea-floor spreading re-orientation and ridge jumps during the Oligocene-Miocene formation of the South China Sea led to the present complex distribution of oceanic crust, thinned continental crust, micro-continents and volcanic ridges. We determine Moho depth, crustal thickness and continental lithosphere thinning (1- 1/beta) for the South China Sea using a gravity inversion method which incorporates a lithosphere thermal gravity anomaly correction (Chappell & Kusznir, 2008). The gravity inversion method provides a prediction of ocean-continent transition structure and continent-ocean boundary location which is independent of ocean isochron information. A correction is required for the lithosphere thermal gravity anomaly in order to determine Moho depth accurately from gravity inversion; the elevated lithosphere geotherm of the young oceanic and rifted continental margin lithosphere of the South China Sea produces a large lithosphere thermal gravity anomaly which in places exceeds -150 mGal. The gravity anomaly inversion is carried out in the 3D spectral domain (using Parker 1972) to determine 3D Moho geometry and invokes Smith's uniqueness theorem. The gravity anomaly contribution from sediments assumes a compaction controlled sediment density increase with depth. The gravity inversion includes a parameterization of the decompression melting model of White & McKenzie (1999) to predict volcanic addition generated during continental breakup lithosphere thinning and seafloor spreading. Public domain free air gravity anomaly, bathymetry and sediment thickness data are used in this gravity inversion. Using crustal thickness and continental lithosphere thinning factor maps with superimposed shaded-relief free-air gravity anomaly, we improve the determination of pre-breakup rifted margin conjugacy, rift orientation and sea-floor spreading trajectory. SCS conjugate margins are highly asymmetric and have several striking features such as the Macclesfield Bank, Xisha Trough, Reed Bank and Dangerous Grounds. Thin continental crust is predicted extending westwards from thin oceanic crust north of Macclesfield Bank into the Quiondongnan (QDN) basin and is interpreted as being generated ahead of westward propagating sea-floor spreading most in the Oligocene. Further south, highly thinned continental crust or possibly serpentinised exhumed mantle is predicted in the Phu Khanh Basin. Ahead of the failed propagating tip of seafloor spreading, offshore southern Vietnam, thinned continental crust is predicted for the Cuu Long and Nam Con Son Basins. Crustal thicknesses from gravity inversion confirms that the southern margin of the SCS consists of fragmented blocks of thinned continental crust separated by thinner regions of continental crust that have undergone higher degrees of stretching and thinning. The Reed Bank is predicted to have a crustal thickness of 20 to 25km, similar to that of Macclesfield Bank. The Dangerous Grounds, west of the Reed Bank, are also predicted to consist of continental crust. This region has been thinned to a higher degree than the Reed Bank, with continental crustal thickness ranging between 10 and 20km thick.

Lithospheric flexural strength and effective elastic thicknesses of the Eastern Anatolia (Turkey) and surrounding region

NASA Astrophysics Data System (ADS)

Oruç, Bülent; Gomez-Ortiz, David; Petit, Carole

2017-12-01

The Lithospheric structure of Eastern Anatolia and the surrounding region, including the northern part of the Arabian platform is investigated via the analysis and modeling of Bouguer anomalies from the Earth Gravitational Model EGM08. The effective elastic thickness of the lithosphere (EET) that corresponds to the mechanical cores of the crust and lithospheric mantle is determined from the spectral coherence between Bouguer anomalies and surface elevation data. Its average value is 18.7 km. From the logarithmic amplitude spectra of Bouguer anomalies, average depths of the lithosphere-asthenosphere boundary (LAB), Moho, Conrad and basement in the study area are constrained at 84 km, 39 km, 16 km and 7 km, respectively. The geometries of the LAB and Moho are then estimated using the Parker-Oldenburg inversion algorithm. We also present a lithospheric strength map obtained from the spatial variations of EET determined by Yield Stress Envelopes (YSE). The EET varies in the range of 12-23 km, which is in good agreement with the average value obtained from spectral analysis. Low EET values are interpreted as resulting from thermal and flexural lithospheric weakening. According to the lithospheric strength of the Eastern Anatolian region, the rheology model consists of a strong but brittle upper crust, a weak and ductile lower crust, and a weak lower part of the lithosphere. On the other hand, lithosphere strength corresponds to weak and ductile lower crust, a strong upper crust and a strong uppermost lithospheric mantle for the northern part of the Arabian platform.

Receiver function analysis applied to refraction survey data

NASA Astrophysics Data System (ADS)

Subaru, T.; Kyosuke, O.; Hitoshi, M.

2008-12-01

For the estimation of the thickness of oceanic crust or petrophysical investigation of subsurface material, refraction or reflection seismic exploration is one of the methods frequently practiced. These explorations use four-component (x,y,z component of acceleration and pressure) seismometer, but only compressional wave or vertical component of seismometers tends to be used in the analyses. Hence, it is needed to use shear wave or lateral component of seismograms for more precise investigation to estimate the thickness of oceanic crust. Receiver function is a function at a place that can be used to estimate the depth of velocity interfaces by receiving waves from teleseismic signal including shear wave. Receiver function analysis uses both vertical and horizontal components of seismograms and deconvolves the horizontal with the vertical to estimate the spectral difference of P-S converted waves arriving after the direct P wave. Once the phase information of the receiver function is obtained, then one can estimate the depth of the velocity interface. This analysis has advantage in the estimation of the depth of velocity interface including Mohorovicic discontinuity using two components of seismograms when P-to-S converted waves are generated at the interface. Our study presents results of the preliminary study using synthetic seismograms. First, we use three types of geological models that are composed of a single sediment layer, a crust layer, and a sloped Moho, respectively, for underground sources. The receiver function can estimate the depth and shape of Moho interface precisely for the three models. Second, We applied this method to synthetic refraction survey data generated not by earthquakes but by artificial sources on the ground or sea surface. Compressional seismic waves propagate under the velocity interface and radiate converted shear waves as well as at the other deep underground layer interfaces. However, the receiver function analysis applied to the second model cannot clearly estimate the velocity interface behind S-P converted wave or multi-reflected waves in a sediment layer. One of the causes is that the incidence angles of upcoming waves are too large compared to the underground source model due to the slanted interface. As a result, incident converted shear waves have non-negligible energy contaminating the vertical component of seismometers. Therefore, recorded refraction waves need to be transformed from depth-lateral coordinate into radial-tangential coordinate, and then Ps converted waves can be observed clearly. Finally, we applied the receiver function analysis to a more realistic model. This model has not only similar sloping Mohorovicic discontinuity and surface source locations as second model but the surface water layer. Receivers are aligned on the sea bottom (OBS; Ocean Bottom Seismometer survey case) Due to intricately bounced reflections, simulated seismic section becomes more complex than the other previously-mentioned models. In spite of the complexity in the seismic records, we could pick up the refraction waves from Moho interface, after stacking more than 20 receiver functions independently produced from each shot gather. After these processing, the receiver function analysis is justified as a method to estimate the depths of velocity interfaces and would be the applicable method for refraction wave analysis. The further study will be conducted for more realistic model that contain inhomogeneous sediment model, for example, and finally used in the inversion of the depth of velocity interfaces like Moho.

Lateral Moho variations and the geometry of Main Himalayan Thrust beneath Nepal Himalayan orogen revealed by teleseismic receiver functions

NASA Astrophysics Data System (ADS)

He, Ping; Lei, Jianshe; Yuan, Xiaohui; Xu, Xiwei; Xu, Qiang; Liu, Zhikun; Mi, Qi; Zhou, Lianqing

2018-05-01

The lateral Moho variations and the geometry of the Main Himalayan Thrust under the Nepal Himalayan orogen are investigated to determine a new crustal model using a large number of high-quality receiver functions recorded by the HIMNT and HiCLIMB portable seismic networks. Our new model shows an evident and complicated lateral Moho depth variation of 8-16 km in the east-west direction, which is related to the surface tectonic features. These results suggest a non-uniformed crustal deformation, resulted from the splitting and/or tearing of the Indian plate during the northward subduction. Our migrated receiver function images illustrate a discernible ramp structure of the Main Himalayan Thrust with an abrupt downward bending close to the hypocenter of the 2015 Gorkha Mw 7.8 earthquake. The distribution of the aftershocks coincides with the present decollement structure. Integrating previous magnetotelluric soundings and tomographic results, our results suggest that the ramp-shaped structure within the Main Himalayan Thrust could enhance stress concentration leading to the nucleation of the large earthquake. Our new crustal model provides new clues to the formation of the Himalayan orogen.

Deep continental margin reflectors

USGS Publications Warehouse

Ewing, J.; Heirtzler, J.; Purdy, M.; Klitgord, Kim D.

1985-01-01

In contrast to the rarity of such observations a decade ago, seismic reflecting and refracting horizons are now being observed to Moho depths under continental shelves in a number of places. These observations provide knowledge of the entire crustal thickness from the shoreline to the oceanic crust on passive margins and supplement Consortium for Continental Reflection Profiling (COCORP)-type measurements on land.

Seismological Structure of the 1.8Ga Trans-Hudson Orogen of North America and its affinity to present-day Tibet

NASA Astrophysics Data System (ADS)

Gilligan, A.; Bastow, I. D.; Darbyshire, F. A.

2015-12-01

How tectonic processes operated and changed through the Precambrian is debated: what was the nature and scale of orogenic events and were they different on the younger, hotter, more ductile Earth? The geology of northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior plates: the 1.8Ga Trans-Hudson Orogeny (THO) and is thus an ideal study locale to address this issue. It has been suggested, primarily on the strength of traditional field geology, that the THO was comparable in scale and style to the present-day Himalayan-Karakoram-Tibet Orogen (HKTO). However, understanding of the deep crustal architecture of the THO, and how it compares to the evolving HKTO is presently lacking. Through joint inversion of teleseismic receiver functions and surface wave data, we obtain new Moho depth estimates and shear velocity models for the crust and upper mantle. Archean crust in the Rae, Hearne and Churchill domains is thin and structurally simple, with a sharp Moho; upper crustal wavespeed variations are readily attributed to post-formation events. However, the Paleoproterozoic Quebec-Baffin segment of the THO has a deeper Moho and more complex crustal structure. Our observations are strikingly similar to recent models, computed using the same methods, of the HKTO lithosphere, where deformation also extends >400km beyond the collision front. On the strength of Moho character, present-day crustal thickness, and metamorphic grade, we thus propose that southern Baffin experienced uplift of a similar magnitude and spatial extent to the Himalayas during the Paleoproterozoic Trans-Hudson Orogeny.

Seismological structure of the 1.8 Ga Trans-Hudson Orogen of North America

NASA Astrophysics Data System (ADS)

Gilligan, Amy; Bastow, Ian D.; Darbyshire, Fiona A.

2016-06-01

Precambrian tectonic processes are debated: what was the nature and scale of orogenic events on the younger, hotter, and more ductile Earth? Northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior plates—the ˜1.8 Ga Trans-Hudson Orogeny (THO)—and is an ideal locality to study Precambrian tectonic structure. Integrated field, geochronological, and thermobarometric studies suggest that the THO was comparable to the present-day Himalayan-Karakoram-Tibet Orogen (HKTO). However, detailed understanding of the deep crustal architecture of the THO, and how it compares to that of the evolving HKTO, is lacking. The joint inversion of receiver functions and surface wave data provides new Moho depth estimates and shear velocity models for the crust and uppermost mantle of the THO. Most of the Archean crust is relatively thin (˜39 km) and structurally simple, with a sharp Moho; upper-crustal wave speed variations are attributed to postformation events. However, the Quebec-Baffin segment of the THO has a deeper Moho (˜45 km) and a more complex crustal structure. Observations show some similarity to recent models, computed using the same methods, of the HKTO crust. Based on Moho character, present-day crustal thickness, and metamorphic grade, we support the view that southern Baffin Island experienced thickening during the THO of a similar magnitude and width to present-day Tibet. Fast seismic velocities at >10 km below southern Baffin Island may be the result of partial eclogitization of the lower crust during the THO, as is currently thought to be happening in Tibet.

Evidence for Depth-Dependent Metasomatism in Cratonic Lithosphere

NASA Astrophysics Data System (ADS)

Eeken, T.; Goes, S. D. B.; Pedersen, H.; Arndt, N. T.; Bouilhol, P.

2017-12-01

The long-term stability of the cratonic cores of continents has been attributed to low temperatures and depletion in iron and water. However, a long-standing enigma is that steady-state thermal models based on heat flow measurements and xenoliths systematically overpredict the seismic velocities in Archean lithospheric mantle. We perform a Monte-Carlo inversion for thermal parameters and water content (leading to metasomatism) to fit 1-D geotherms to average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To satisfactorily match the seismic profiles, we need a significant amount of hydrous and/or carbonated minerals starting between the Moho and 70 km depth and extending down to at least 100-150 km depth (if distributed over this depth range, this requires 0.5 and 1 wt% water for amphiboles, or 0.2 wt% water plus sufficient potassium to form phlogopites or 5 wt% CO2 and sufficient Ca to make carbonate, or a combination thereof). Lithospheric temperatures that lead to a good fit of the seismic constraints are commonly lower than those inferred from xenoliths, but consistent with heat flow constraints. The dispersion data also require differences in Moho heatflux between regions and 100-200°C lower sublithospheric mantle temperatures below Yilgarn, Slave and Finland than below Kaapvaal, consistent with regional tectonic settings inferred from global tomography. Thus, significant upward-increasing metasomatism by water and CO2-rich fluids is a plausible mechanism to explain the average seismic structure of cratonic lithosphere. Such metasomatism would also contribute to the positive chemical buoyancy of cratonic roots.

Seismic evidence for depth-dependent metasomatism in cratons

NASA Astrophysics Data System (ADS)

Eeken, Thomas; Goes, Saskia; Pedersen, Helle A.; Arndt, Nicholas T.; Bouilhol, Pierre

2018-06-01

The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100-150 km. The metasomatic component can consist of 0.5-1 wt% water bound in amphibole, antigorite and chlorite, ∼0.2 wt% water plus potassium to form phlogopite, or ∼5 wt% CO2 plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100-200 °C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots.

Structure, mechanical properties and evolution of the lithosphere below the northwest continental margin of India

NASA Astrophysics Data System (ADS)

Rao, G. Srinivasa; Kumar, Manish; Radhakrishna, M.

2018-02-01

The continental breakup history at the northwest continental margin of India remained conjectural due to lack of clearly discernable magnetic anomaly identifications and the presence of several enigmatic structural/basement features whose structure was partly obscured by the Late Cretaceous Deccan magmatic event. In this study, a detailed analysis of the existing seismic and seismological data covering both onshore and offshore areas of the northwest Indian margin along with 3-D/2-D constrained potential field (gravity, magnetic and geoid) modeling has been carried out. The crustal structure and lithosphere-asthenosphere boundary (LAB) delineated across the margin provided valuable insights on the mechanism of continental extension. An analysis of the residual geoid anomaly (degree-10) map and the modeled LAB below Deccan volcanic province (DVP) revealed significant variation in upper mantle characteristics between the northwest (NW) and south central (SC) parts of DVP having thinner lithosphere in the NW part. The depth to LAB ranges 80-130 km at the margin with gradual thinning towards the western offshore having sharp gradient in the south (SC part of DVP) and gentle gradient in the north (NW part of DVP). The Moho configuration obtained from seismically constrained 3-D gravity inversion reveals that Moho depths vary 34-42 km below DVP and gradually thins to 16-20 km in the western offshore. The effective elastic thickness (Te) map computed through 3-D flexural modeling indicates that the Te values are in general lower in the region and range 12-25 km. Such lower Te values could be ascribed to the combined effect of the lithosphere stretching during Gondwana fragmentation in the Mesozoic and subsequent thermal influence of the Reunion plume. Based on the crustal stretching factors (β), Te estimates and the modeled lithosphere geometry at the margin in this study, we propose that the lithosphere below Laxmi-Gop basin region (β > 3.0) had undergone continuous stretching since India-Madagascar rifting ( 88 Ma) /much prior to this event. However, this continuous stretching did not lead to breakup. Due to syn-rift cooling, the developed necking zone (brittle-ductile deformation) got ceased and led to the development of a new necking zone between Seychelles and Laxmi Ridge. Subsequent stretching between Seychelles and the Laxmi Ridge contemporaneous with the Deccan flood basalts eruption led to the seafloor spreading in the Western Basin (anomaly C28n). Thus, the Laxmi Ridge became a continental sliver.

Seismic images of the Brooks Range fold and thrust belt, Arctic Alaska, from an integrated seismic reflection/refraction experiment

USGS Publications Warehouse

Levander, A.; Fuis, G.S.; Wissinger, E.S.; Lutter, W.J.; Oldow, J.S.; Moore, Thomas E.

1994-01-01

We describe results of an integrated seismic reflection/refraction experiment across the Brooks Range and flanking geologic provinces in Arctic Alaska. The seismic acquisition was unusual in that reflection and refraction data were collected simultaneously with a 700 channel seismograph system deployed numerous times along a 315 km profile. Shot records show continuous Moho reflections from 0-180 km offset, as well as numerous upper- and mid-crustal wide-angle events. Single and low-fold near-vertical incidence common midpoint (CMP) reflection images show complex upper- and middle-crustal structure across the range from the unmetamorphosed Endicott Mountains allochthon (EMA) in the north, to the metamorphic belts in the south. Lower-crustal and Moho reflections are visible across the entire reflection profile. Travel-time inversion of PmP arrivals shows that the Moho, at 33 km depth beneath the North Slope foothills, deepens abruptly beneath the EMA to a maximum of 46 km, and then shallows southward to 35 km at the southern edge of the range. Two zones of upper- and middle-crustal reflections underlie the northern Brooks Range above ~ 12-15 km depth. The upper zone, interpreted as the base of the EMA, lies at a maximum depth of 6 km and extends over 50 km from the range front to the north central Brooks Range where the base of the EMA outcrops above the metasedimentary rocks exposed in the Doonerak window. We interpret the base of the lower zone, at ~ 12 km depth, to be from carbonate rocks above the master detachment upon which the Brooks Range formed. The seismic data suggest that the master detachment is connected to the faults in the EMA by several ramps. In the highly metamorphosed terranes south of the Doonerak window, the CMP section shows numerous south-dipping events which we interpret as a crustal scale duplex involving the Doonerak window rocks. The basal detachment reflections can be traced approximately 100 km, and dip southward from about 10-12 km near the range front, to 14-18 km beneath the Doonerak window, to 26-28 km beneath the metamorphic belts in the central Brooks Range. The section documents middle- and lower-crustal involvement in the formation of the Brooks Range. ?? 1994.

GLIMPCE Seismic reflection evidence of deep-crustal and upper-mantle intrusions and magmatic underplating associated with the Midcontinent Rift system of North America

USGS Publications Warehouse

Behrendt, John C.; Hutchinson, D.R.; Lee, M.; Thornber, C.R.; Tréhu, A.; Cannon, W.; Green, A.

1990-01-01

Deep-crustal and Moho reflections, recorded on vertical incidence and wide angle ocean bottom Seismometer (OBS) data in the 1986 GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) experiment, provide evidence for magmatic underplating and intrusions within the lower crust and upper mantle contemporaneous with crustal extension in the Midcontinent Rift system at 1100 Ma. The rift fill consists of 20-30 km (7-10 s) of basalt flows, secondary syn-rift volcaniclastic and post-basalt sedimentary rock. Moho reflections recorded in Lake Superior over the Midcontinent Rift system have times from 14-18 s (about 46 km to as great as 58 km) in contrast to times of about 11-13 s (about 36-42 km crustal thickness) beneath the surrounding Great Lakes. The Seismically complex deep-crust to mantle transition zone (30-60 km) in north-central Lake Superior, which is 100 km wider than the rift half-graben, reflects the complicated products of tectonic and magmatic interaction of lower-crustal and mantle components during evolution or shutdown of the aborted Midcontinent Rift. In effect, mantle was changed into crust by lowering Seismic velocity (through intrusion of lower density magmatic rocks) and increasing Moho (about 8.1 km s-1 depth. 

Seismicity Structure of the Downgoing Nazca Slab in Northern Chile

NASA Astrophysics Data System (ADS)

Sippl, C.; Schurr, B.

2017-12-01

We applied an automatized earthquake detection and location algorithm to 8 years of continuous seismic data from the IPOC network in Northern Chile, located in the forearc between about 18.5°S and 24°S. The resulting seismicity catalog contains more than 113k double-difference relocated earthquake hypocenters and features a completeness magnitude around 2.8. Despite the occurrence of two megathrust earthquakes with vigorous aftershock seismicity in the studied time period (the 2007 Tocopilla and the 2014 Iquique earthquakes), >60% of the retrieved seismicity is located in a highly active band of intermediate-depth earthquakes (80-120 km deep) within the downgoing Nazca slab.We obtain a triple seismic zone in the updip part of the slab, with the three parallel dipping planes corresponding to the plate interface, the oceanic Moho (ca. 8 km below the interface) and a third band in the mantle lithosphere 26-28 km beneath the slab top. The plate interface seismicity terminates abruptly at a depth of 55 km. At about 80-90 km depth, the remaining two planes of seismicity then merge into the single, 20 km thick cluster of vigorous seismicity mentioned above, which terminates at 120 km depth. This cluster is located directly beneath the volcanic arc and shows a pronounced kink in the slab dipping angle. Intra-slab seismicity is most likely related to metamorphic dehydration reactions, hence our high-resolution earthquake distribution can be considered a map of metamorphic reactions (although a possibly incomplete one, since not all reactions necessarily invoke seismicity). By correlating this distribution with isotherms from thermal models as well as geophysical imaging results from previous studies, we attempt to get a glimpse at the processes that produce the different patches of intraslab seismicity at intermediate depths.

The system CaO-MgO-SiO2-CO2 at 1 GPa, metasomatic wehrlites, and primary carbonatite magmas

NASA Astrophysics Data System (ADS)

Lee, W. J.; Wyllie, P. J.

New experimental data in CaO-MgO-SiO2-CO2 at 1GPa define the vapor-saturated silicate-carbonate liquidus field boundary involving primary minerals calcite, forsterite and diopside. The eutectic reaction for melting of model calcite (1% MC)-wehrlite at 1GPa is at 1100°C, with liquid composition (by weight) 72% CaCO3 (CC), 9% MgCO3 (MC), and 18% CaMgSi2O6 (Di). These data combined with previous results permit construction of the isotherm-contoured vapor-saturated liquidus surface for the calcite/dolomite field, and part of the adjacent forsterite and diopside fields. Nearly pure calcite crystals in mantle xenoliths cannot represent equilibrium liquids. We recently determined the complete vapor-saturated liquidus surface between carbonates and model peridotites at 2.7GPa the peritectic reaction for dolomite (25% MC)-wehrlite at 2.7GPa occurs at 1300°C, with liquid composition 60% CC, 29% MC, and 11% Di. The liquidus field boundaries on these two surfaces provide the road-map for interpretation of magmatic processes in various peridotite-CO2 systems at depths between the Moho and about 100km. Relationships among kimberlites, melilitites, carbonatites and the liquidus phase boundaries are discussed. Experimental data for carbonatite liquid protected by metasomatic wehrlite have been reported. The liquid trends directly from dolomitic towards CaCO3 with decreasing pressure. The 1.5GPa liquid contains 87% CC and 4% Di, much lower in silicate components than our phase boundary. However, the liquids contain approximately the same CaCO3 (90+/- 1wt%) in terms of only carbonate components. For CO2-bearing mantle, all magmas at depth must pass through initial dolomitic compositions. Rising dolomitic carbonatite melt will vesiculate and may erupt as primary magmas through cracks from about 70km. If it percolates through metasomatic wehrlite from 70km toward the Moho at 35-40km, primary calcic siliceous carbonatite magma can be generated with silicate content at least 11-18% (70-40km) on the silicate-carbonate boundary.

Crustal structure and evolution of the Arctic Caledonides: Results from controlled-source seismology

NASA Astrophysics Data System (ADS)

Aarseth, Iselin; Mjelde, Rolf; Breivik, Asbjørn Johan; Minakov, Alexander; Faleide, Jan Inge; Flueh, Ernst; Huismans, Ritske S.

2017-10-01

The continuation of the Caledonides into the Barents Sea has long been a subject of discussion, and two major orientations of the Caledonian deformation fronts have been suggested: NNW-SSE striking and NE-SW striking. A regional NW-SE oriented ocean bottom seismic profile across the western Barents Sea was acquired in 2014. In this paper we map the crust and upper mantle structure along this profile in order to discriminate between different interpretations of Caledonian structural trends and orientation of rift basins in the western Barents Sea. Modeling of P-wave travel times has been done using a ray-tracing method, and combined with gravity modeling. The results show high P-wave velocities (4 km/s) close to the seafloor, as well as localized sub-horizontal high velocity zones (6.0 km/s and 6.9 km/s) at shallow depths which are interpreted as magmatic sills. Refractions from the top of the crystalline basement together with reflections from the Moho give basement velocities from 6.0 km/s at the top to 6.7 km/s at the base of the crust. P-wave travel time modeling of the OBS profile indicate an eastwards increase in velocities from 6.4 km/s to 6.7 km/s at the base of the crystalline crust, and the western part of the profile is characterized by a higher seismic reflectivity than the eastern part. This change in seismic character is consistent with observations from vintage reflection seismic data and is interpreted as a Caledonian suture extending through the Barents Sea, separating Barentsia and Baltica. Local deepening of Moho (from 27 km to 33 km depth) creates ;root structures; that can be linked to the Caledonian compressional deformation or a suture zone imprinted in the lower crust. Our model supports a separate NE-SW Caledonian trend extending into the central Barents Sea, branching off from the northerly trending Svalbard Caledonides, implying the existence of Barentsia as an independent microcontinent between Laurentia and Baltica.

Barents Sea Paleozoic basement and basin configurations: Crustal structure from deep seismic and potential field data

NASA Astrophysics Data System (ADS)

Aarseth, Iselin; Mjelde, Rolf; Breivik, Asbjørn Johan; Huismans, Ritske; Faleide, Jan Inge

2016-04-01

The Barents Sea is underlain by at least two different basement domains; the Caledonian in the west and the Timanian in the east. The transition between these two domains is not well constrained and contrasting interpretations have been published recently. Interpretations of new high-quality magnetic data covering most of the SW Barents Sea has challenged the Late Paleozoic basin configurations in the western and central Barents Sea as outlined in previous studies. Two regional ocean bottom seismic (OBS) profiles were acquired in 2014. This new dataset crosses the two major directions of Caledonian deformation proposed by different authors: N-S direction and SW-NE direction. Of particular importance are the high velocity anomalies related to Caledonian eclogites, revealing the location of Caledonian suture zones in the northern Barents Sea. One of the main objectives with this project is to locate the main Caledonian suture in the western Barents Sea, as well as the possible Barentsia-Baltica suture postulated further eastwards. The collapse of the Caledonian mountain range predominantly along these suture zones is expected to be tightly linked to the deposition of large thicknesses of Devonian erosional products, and later rifting is expected to be influenced by inheritance of Caledonian trends. The P-wave travel-time modelling is done by use of a combined ray-tracing and inversion scheme, and gravity- and magnetic modelling will be used to augment the seismic model. The preliminary results indicate high P-wave velocities (mostly over 4 km/s) close to the seafloor as well as high velocity (around 6 km/s) zones at shallow depths which are interpreted as volcanic sills. The crustal transects reveal areas of complex geology and velocity inversions. A low seismic impedance contrast between the sedimentary section and top crystalline basement makes identification of this interface uncertain. Depth to Moho mostly lies around 30 km, except in an area of rapid change in Moho depth, from about 27 km to 32 km. This drop in Moho could indicate a relict subduction zone related to the Caledonian suture zone in the western Barents Sea

Detecting Moho Boundary under Taiwan with Wide-angle Data by Ray-tracing Method - The TAIGER Project

NASA Astrophysics Data System (ADS)

Kuo, Y. N.; Wang, C.; Okaya, D. A.

2009-12-01

Taiwan is located at the converging boundary of the Eurasian plate and the Philippine Sea plate, and is one of the most rapidly uplifting orogeny in the world. The geological structure is relatively complicated. There exist several models of tectonic collisions from the thin-skinned thrust, the lithospheric collision, to uplifting by buoyancy. The shape of Moho should be a key factor to evaluate these models. In this study, we try to detect the Moho beneath Taiwan using the newly collected wide-angle data from the Taiwan Integrated Geodynamic Research (TAIGER) project. The results could be of help to set up some constrains for the Taiwan tectonics. The TAIGER project is a collaboration between America and Taiwan. The land stations collected two parts of data (land and marine) generated by active sources. The land part was carried out in 2008/2~3, which created 6 kinds of data from explosion sources including: 1) 3 E-W wide-angle reflections of Texans arrays; 2) 2 N-S seismometer arrays; 3) the seismic networks of Central Weather Bureau(CWB) and Institute of Earth Science(IES) over the island; 4) a short array of RT130; 5) 2 short period OBS arrays in the Taiwan Strait; 6) 2 temporary seismic arrays in Fujan, mainland China. The marine part was carried out in 2009/4~6, which provided 4 kinds of data from air-gun sources including: 1) 4 wide-angle refractions of E-W RT130 arrays; 2) 2 N-S seismometer arrays; 3) the CWB network; 4) the broad band array in Taiwan for Seismology(BATS). In this study, we focus on analyzing the wide-angle data, which contain land explosion data, onshore-offshore data, OBS data and mainland data, especially concentrate on the line in the southern Taiwan (Transect T4). We make a summary of the TAIGER project and show several plots of real data and arrivals. A 2D E-W velocity model was constructed from the mainland side to the ocean side about 600 km long using the ray-tracing method with layer-striping technique. The preliminary results are: 1) the distribution of Moho depth is basically getting deeper from the west to the east, but becoming shallower rapidly in the area of Coast Range; 2) the crust thickens to the range of 40 km in the mountain area; 3) the Moho depth is shallower than 30 km in the Peikang High and deeper than 32 km at the coast line of Fujan, no crust bulge in the Taiwan Strait; 4) the structures derived from PmP phase and Pn phase from land explosions and onshore-offshore air-gun shots are highly consistent.

Crustal Thickness Mapping of the Rifted Margin Ocean-Continent Transition using Satellite Gravity Inversion Incorporating a Lithosphere Thermal Correction

NASA Astrophysics Data System (ADS)

Hurst, N. W.; Kusznir, N. J.

2005-05-01

A new method of inverting satellite gravity at rifted continental margins to give crustal thickness, incorporating a lithosphere thermal correction, has been developed which does not use a priori information about the location of the ocean-continent transition (OCT) and provides an independent prediction of OCT location. Satellite derived gravity anomaly data (Sandwell and Smith 1997) and bathymetry data (Gebco 2003) are used to derive the mantle residual gravity anomaly which is inverted in 3D in the spectral domain to give Moho depth. Oceanic lithosphere and stretched continental margin lithosphere produce a large negative residual thermal gravity anomaly (up to -380 mgal), which must be corrected for in order to determine Moho depth. This thermal gravity correction may be determined for oceanic lithosphere using oceanic isochron data, and for the thinned continental margin lithosphere using margin rift age and beta stretching estimates iteratively derived from crustal basement thickness determined from the gravity inversion. The gravity inversion using the thermal gravity correction predicts oceanic crustal thicknesses consistent with seismic observations, while that without the thermal correction predicts much too great oceanic crustal thicknesses. Predicted Moho depth and crustal thinning across the Hatton and Faroes rifted margins, using the gravity inversion with embedded thermal correction, compare well with those produced by wide-angle seismology. A new gravity inversion method has been developed in which no isochrons are used to define the thermal gravity correction. The new method assumes all lithosphere to be initially continental and a uniform lithosphere stretching age is used corresponding to the time of continental breakup. The thinning factor produced by the gravity inversion is used to predict the thickness of oceanic crust. This new modified form of gravity inversion with embedded thermal correction provides an improved estimate of rifted continental margin crustal thinning and an improved (and isochron independent) prediction of OCT location. The new method uses an empirical relationship to predict the thickness of oceanic crust as a function of lithosphere thinning factor controlled by two input parameters: a critical thinning factor for the start of ocean crust production and the maximum oceanic crustal thickness produced when the thinning factor = 1, corresponding to infinite lithosphere stretching. The disadvantage of using a uniform stretching age corresponding to the age of continental breakup is that the inversion fails to predict increasing thermal gravity correction towards the ocean ridge and incorrectly predicts thickening of oceanic crust with decreasing oceanic age. The new gravity inversion method has been applied to N. Atlantic rifted margins. This work forms part of the NERC Margins iSIMM project. iSIMM investigators are from Liverpool and Cambridge Universities, Badley Geoscience & Schlumberger Cambridge Research supported by the NERC, the DTI, Agip UK, BP, Amerada Hess Ltd, Anadarko, ConocoPhillips, Shell, Statoil and WesternGeco. The iSIMM team comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, A Chappell, J Eccles, R Fletcher, D Healy, N Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, V Tymms & R Spitzer.

100 years of seismic research on the Moho

NASA Astrophysics Data System (ADS)

Prodehl, Claus; Kennett, Brian; Artemieva, Irina M.; Thybo, Hans

2013-12-01

The detection of a seismic boundary, the “Moho”, between the outermost shell of the Earth, the Earth's crust, and the Earth's mantle by A. Mohorovičić was the consequence of increased insight into the propagation of seismic waves caused by earthquakes. This short history of seismic research on the Moho is primarily based on the comprehensive overview of the worldwide history of seismological studies of the Earth's crust using controlled sources from 1850 to 2005, by Prodehl and Mooney (2012). Though the art of applying explosions, so-called “artificial events”, as energy sources for studies of the uppermost crustal layers began in the early 1900s, its effective use for studying the entire crust only began at the end of World War II. From 1945 onwards, controlled-source seismology has been the major approach to study details of the crust and underlying crust-mantle boundary, the Moho. The subsequent description of history of controlled-source crustal seismology and its seminal results is subdivided into separate chapters for each decade, highlighting the major advances achieved during that decade in terms of data acquisition, processing technology, and interpretation methods. Since the late 1980s, passive seismology using distant earthquakes has played an increasingly important role in studies of crustal structure. The receiver function technique exploiting conversions between P and SV waves at discontinuities in seismic wavespeed below a seismic station has been extensively applied to the increasing numbers of permanent and portable broad-band seismic stations across the globe. Receiver function studies supplement controlled source work with improved geographic coverage and now make a significant contribution to knowledge of the nature of the crust and the depth to Moho.

Trans-Alaska Crustal Transect and continental evolution involving subduction underplating and synchronous foreland thrusting

USGS Publications Warehouse

Fuis, G.S.; Moore, Thomas E.; Plafker, G.; Brocher, T.M.; Fisher, M.A.; Mooney, W.D.; Nokleberg, W.J.; Page, R.A.; Beaudoin, B.C.; Christensen, N.I.; Levander, A.R.; Lutter, W.J.; Saltus, R.W.; Ruppert, N.A.

2008-01-01

We investigate the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic underplating. In the 1980s and early 1990s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-long corridor from the Aleutian Trench to the Arctic coast. The most distinctive crustal structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, we infer a stack of tectonically underplated oceanic layers interpreted as remnants of the extinct Kula (or Resurrection) plate. Continental Moho just north of this underplated stack is more than 55 km deep. Near the Arctic margin, the Brooks Range is underlain by large-scale duplex structures that overlie a tectonic wedge of North Slope crust and mantle. There, the Moho has been depressed to nearly 50 km depth. In contrast, the Moho of central Alaska is on average 32 km deep. In the Paleogene, tectonic underplating of Kula (or Resurrection) plate fragments overlapped in time with duplexing in the Brooks Range. Possible tectonic models linking these two regions include flat-slab subduction and an orogenic-float model. In the Neogene, the tectonics of the accreting Yakutat terrane have differed across a newly interpreted tear in the subducting Pacific oceanic lithosphere. East of the tear, Pacific oceanic lithosphere subducts steeply and alone beneath the Wrangell volcanoes, because the overlying Yakutat terrane has been left behind as underplated rocks beneath the rising St. Elias Range, in the coastal region. West of the tear, the Yakutat terrane and Pacific oceanic lithosphere subduct together at a gentle angle, and this thickened package inhibits volcanism. ?? 2008 The Geological Society of America.

The GEORIFT 2013 wide-angle seismic profile, along Pripyat-Dnieper-Donets Basin

NASA Astrophysics Data System (ADS)

Starostenko, Vitaliy; Janik, Tomasz; Yegorova, Tamara; Czuba, Wojciech; Sroda, Piotr; Lysynchuk, Dmytro; Aizberg, Roman; Garetsky, Radim; Karataev, German; Gribik, Yaroslav; Farfuliak, Lliudmyla; Kolomiyets, Katerina; Omelchenko, Victor; Gryn, Dmytro; Guterch, Aleksander; Komminaho, Kari; Legostaeva, Olga; Thybo, Hans; Tiira, Timo; Tolkunov, Anatoly

2017-04-01

The GEORIFT 2013 deep seismic sounding (DSS) experiment was carried in August 2013 on territory of Belarus and Ukraine in wide international co-operation. The aim of the work is to study basin architecture and the deep structure of the Pripyat-Dnieper-Donets Basin (PDDB), which is the deepest and best studied Palaeozoic rift basin in Europe. The PDDB locates in the southern part of the East European Craton (EEC) and crosses in NW direction the Sarmatia, the southernmost of three major segments forming the EEC. The long PDDB was formed by Late Devonian rifting in the arch of the ancient Sarmatian shield. During the Late Devonian, rifting, associated with domal basement uplift and magmatism, was widespread in the EEC from the PDDB rift basin in the south to Eastern Barents Sea in the north. The GEORIFT 2013 runs in NW-SE direction along the PDDB and crosses the Pripyat Trough and Dnieper Graben separated by Bragin uplift of the basement. The total profile length was 675 km: 315 km on the Belarusian territory and 360 km in Ukraine. The field acquisition included 14 shot points (charge 600-1000 kg of TNT), and 309 recording stations every 2.2 km. The data quality of the data was good, with visible first arrivals even up to 670 km. We present final model of the structure to the depth of 80 km. Ray-tracing forward modelling (SEIS83 package) was used for the modelling of the seismic data. The thickness of the sedimentary layer (Vp < 6.0 km/s) changes along the profile from 1-4 km in the NW, through 5 km in the central part, to 10-13 km in the SE part of the profile. In 330-530 km distance range, an updoming of the lower crust (with Vp of 7.1 km/s) to 25 km depth is observed. Large variations in the internal structure of the crust and the Moho topography were detected. The depth of the Moho varies from 47 km in the northwestern part of the model, to 40 km in central part, and to 38 km in the southeastern part of the profile. The sub-Moho velocities are 8.25 km/s. Second, near-horizontal mantle discontinuity was found in the northwestern part of the profile at the depth of 50-47 km. It dips to the depth of 60 km at distances of 360-405 km, similarly as on crossing EUROBRIDGE'97 profile (Thybo et al., 2003). In the central part of the profile (distances 180-330 km and 300-480 km) two reflectors were found in the lower lithosphere at depths of about 62 km and 75 km, respectively.

The Effect of Finite Thickness Extent on Estimating Depth to Basement from Aeromagnetic Data

NASA Astrophysics Data System (ADS)

Blakely, R. J.; Salem, A.; Green, C. M.; Fairhead, D.; Ravat, D.

2014-12-01

Depth to basement estimation methods using various components of the spectral content of magnetic anomalies are in common use by geophysicists. Examples of these are the Tilt-Depth and SPI methods. These methods use simple models having the base of the magnetic body at infinity. Recent publications have shown that this 'infinite depth' assumption causes underestimation of the depth to the top of sources, especially in areas where the bottom of the magnetic layer is shallow, as would occur in high heat-flow regions. This error has been demonstrated in both model studies and using real data with seismic or well control. To overcome the limitation of infinite depth this contribution presents the mathematics for a finite depth contact body in the Tilt depth and SPI methods and applies it to the central Red Sea where the Curie isotherm and Moho are shallow. The difference in the depth estimation between the infinite and finite contacts is such a case is significant and can exceed 200%.

Acerca del moho

EPA Pesticide Factsheets

El moho forma parte del medio ambiente natural. Afuera del hogar, el moho juega un papel en la naturaleza al desintegrar materias organicas tales como las hojas que se han caido o los arboles muertos. El moho puede crecer adentro del hogar cuando las espor

Basement structures over Rio Grande Rise from gravity inversion

NASA Astrophysics Data System (ADS)

Constantino, Renata Regina; Hackspacher, Peter Christian; de Souza, Iata Anderson; Lima Costa, Iago Sousa

2017-04-01

The basement depth in the Rio Grande Rise (RGR), South Atlantic, is estimated from combining gravity data obtained from satellite altimetry, marine surveys, bathymetry, sediment thickness and crustal thickness information. We formulate a crustal model of the region by inverse gravity modeling. The effect of the sediment layer is evaluated using the global sediment thickness model of National Oceanic and Atmospheric Administration (NOAA) and fitting the sediment compaction model to observed density values from Deep Sea Drilling Project (DSDP) reports. The Global Relief Model ETOPO1 and constraining data from seismic interpretation on crustal thickness are integrated in the inversion process. The modeled Moho depth values vary between 6 and 27 km over the area, being thicker under the RGR and also in the direction of São Paulo Plateau. The inversion for the gravity-equivalent basement topography is applied to gravity residual data, which is free from the gravity effect of sediments and from the gravity effect of the estimated Moho interface. We find several short-wavelengths structures not present in the bathymetry data. Our model shows a rift crossing the entire Rio Grande Rise deeper than previously presented in literature, with depths up to 5 km in the East Rio Grande Rise (ERGR) and deeper in the West Rio Grande Rise (WRGR), reaching 6.4 km. An interesting NS structure that goes from 34°S and extends through de São Paulo Ridge may be related to the South Atlantic Opening and could reveal an extinct spreading center.

The spatial distribution of earthquake stress rotations following large subduction zone earthquakes

USGS Publications Warehouse

Hardebeck, Jeanne L.

2017-01-01

Rotations of the principal stress axes due to great subduction zone earthquakes have been used to infer low differential stress and near-complete stress drop. The spatial distribution of coseismic and postseismic stress rotation as a function of depth and along-strike distance is explored for three recent M ≥ 8.8 subduction megathrust earthquakes. In the down-dip direction, the largest coseismic stress rotations are found just above the Moho depth of the overriding plate. This zone has been identified as hosting large patches of large slip in great earthquakes, based on the lack of high-frequency radiated energy. The large continuous slip patches may facilitate near-complete stress drop. There is seismological evidence for high fluid pressures in the subducted slab around the Moho depth of the overriding plate, suggesting low differential stress levels in this zone due to high fluid pressure, also facilitating stress rotations. The coseismic stress rotations have similar along-strike extent as the mainshock rupture. Postseismic stress rotations tend to occur in the same locations as the coseismic stress rotations, probably due to the very low remaining differential stress following the near-complete coseismic stress drop. The spatial complexity of the observed stress changes suggests that an analytical solution for finding the differential stress from the coseismic stress rotation may be overly simplistic, and that modeling of the full spatial distribution of the mainshock static stress changes is necessary.

A Method to Retrieve the Multi-Receiver Moho Reflection Response from SH-Wave Scattering Coda in the Radiative Transfer Regime

NASA Astrophysics Data System (ADS)

Hartstra, I.; Wapenaar, C. P. A.

2015-12-01

We discuss a method to retrieve the multi-receiver Moho reflection response by interferometry from SH-wave coda in the 0.5-3 Hz frequency range. An image derived from a reflection response with a well defined virtual source would provide deterministic impedance contrasts, which can complement transmission tomography. For an accurate retrieval, cross-correlation interferometry requires the coda wave field to sample the imaging target and isotropically illuminate the receiver array. When these illumination requirements are not or only partially met, the stationary phase cannot be fully captured and artifacts will contaminate the retrieved reflection response. Here we conduct numerical scalar 2D finite difference simulations to investigate the challenging situation in which only shallow crustal earthquake sources illuminate the Moho and the response is recorded by a 2D linear array. We quantify to what extent the prevalence of scatterers in the crust can improve the illumination conditions and thus the retrieval of the Moho reflection. The accuracy of the retrieved reflection is evaluated for two physically different scattering regimes: the Rayleigh and Mie regime. We only use the earlier part of the scattering coda, because we have found that the later diffusive part does not significantly improve the retrieval. The density of the spherical scatterers is varied in order to change the scattering mean free path. This characteristic length scale is calculated for each model with the 2D radiative transfer equation, which is the governing equation in the earlier part of the scattering coda. The experiment is repeated for models of different geological settings derived from existing S-wave tomographies, which vary in Moho depth and reflectivity. The scattering mean free path can be approximated for real data if intrinsic attenuation is known, because the wavenumber-dependent scattering attenuation of the coherent wave amplitude is dependent on the scattering mean free path. This link makes it possible to determine in which spatial and temporal bandwidth retrieval is most optimal for a specific geological setting.

Arctic and N Atlantic Crustal Thickness and Oceanic Lithosphere Distribution from Gravity Inversion

NASA Astrophysics Data System (ADS)

Kusznir, Nick; Alvey, Andy

2014-05-01

The ocean basins of the Arctic and N. Atlantic formed during the Mesozoic and Cenozoic as a series of distinct ocean basins, both small and large, leading to a complex distribution of oceanic crust, thinned continental crust and rifted continental margins. The plate tectonic framework of this region was demonstrated by the pioneering work of Peter Ziegler in AAPG Memoir 43 " Evolution of the Arctic-North Atlantic and the Western Tethys" published in 1988. The spatial evolution of Arctic Ocean and N Atlantic ocean basin geometry and bathymetry are critical not only for hydrocarbon exploration but also for understanding regional palaeo-oceanography and ocean gateway connectivity, and its influence on global climate. Mapping crustal thickness and oceanic lithosphere distribution represents a substantial challenge for the Polar Regions. Using gravity anomaly inversion we have produced comprehensive maps of crustal thickness and oceanic lithosphere distribution for the Arctic and N Atlantic region, We determine Moho depth, crustal basement thickness, continental lithosphere thinning and ocean-continent transition location using a 3D spectral domain gravity inversion method, which incorporates a lithosphere thermal gravity anomaly correction (Chappell & Kusznir 2008). Gravity anomaly and bathymetry data used in the gravity inversion are from the NGA (U) Arctic Gravity Project and IBCAO respectively; sediment thickness is from a new regional compilation. The resulting maps of crustal thickness and continental lithosphere thinning factor are used to determine continent-ocean boundary location and the distribution of oceanic lithosphere. Crustal cross-sections using Moho depth from the gravity inversion allow continent-ocean transition structure to be determined and magmatic type (magma poor, "normal" or magma rich). Our gravity inversion predicts thin crust and high continental lithosphere thinning factors in the Eurasia, Canada, Makarov, Podvodnikov and Baffin Basins consistent with these basins being oceanic. Larger crustal thicknesses, in the range 20 - 30 km, are predicted for the Lomonosov, Alpha and Mendeleev Ridges. Crustal basement thicknesses of 10-15 km are predicted under the Laptev Sea which is interpreted as highly thinned continental crust formed at the eastward continuation of Eurasia Basin sea-floor spreading. Thin continental or oceanic crust of thickness 7 km or less is predicted under the North Chukchi Basin and has major implications for understanding the Mesozoic and Cenozoic plate tectonic history of the Siberian and Chukchi Amerasia Basin margins. Restoration of crustal thickness and continent-ocean boundary location from gravity inversion may be used to test and refine plate tectonic reconstructions. Using crustal thickness and continental lithosphere thinning factor maps with superimposed shaded-relief free-air gravity anomaly, we improve the determination of pre-breakup rifted margin conjugacy and sea-floor spreading trajectory within the Arctic and N Atlantic basins. By restoring crustal thickness & continental lithosphere thinning maps of the Eurasia Basin & NE Atlantic to their initial post-breakup configuration we show the geometry and segmentation of the rifted continental margins at their time of breakup, together with the location of highly-stretched failed breakup basins and rifted micro-continents. We interpret gravity inversion crustal thicknesses underneath Morris Jessop Rise & Yermak Plateau as continental crust which provided a barrier to the tectonic and palaeo-oceanic linkage between the Arctic & North Atlantic until the Oligocene. Before this time, we link the seafloor spreading within the Eurasia Basin to that in Baffin Bay.

Crust and upper mantle shear wave structure of Northeast Algeria from Rayleigh wave dispersion analysis

NASA Astrophysics Data System (ADS)

Radi, Zohir; Yelles-Chaouche, Abdelkrim; Corchete, Victor; Guettouche, Salim

2017-09-01

We resolve the crust and upper mantle structure beneath Northeast Algeria at depths of 0-400 km, using inversion of fundamental mode Rayleigh wave. Our data set consists of 490 earthquakes recorded between 2007 and 2014 by five permanent broadband seismic stations in the study area. Applying a combination of different filtering technics and inversion method shear wave velocities structure were determined as functions of depth. The resolved changes in Vs at 50 km depth are in perfect agreement with crustal thickness estimates, which reflect the study area's orogenic setting, partly overlying the collision zone between the African and Eurasian plates. The inferred Moho discontinuity depths are close to those estimated for other convergent areas. In addition, there is good agreement between our results and variations in orientations of regional seismic anisotropy. At depths of 80-180 km, negative Vs anomalies at station CBBR suggest the existence of a failed subduction slab.

Complex local Moho topography in the Western Carpathians: Indication of the ALCAPA and the European Plate contact

NASA Astrophysics Data System (ADS)

Hrubcová, Pavla; Środa, Piotr

2015-01-01

Seismic data from deep refraction and wide-angle reflection profiles intersecting the Western Carpathians show distinct upper-mantle Pn phases with anomalous apparent velocities identified in the first and later arrivals. Their systematic analysis indicates that such phases are present in numerous seismic sections both for in-line and off-line shots. They are observed in data from profiles intersecting the Carpathians in the west at the contact with the Bohemian Massif; similar feature was also found in data at the northern edge of the Carpathians at the contact with the North European Platform. Modelling of these anomalous Pn phases shows that they originate due to local structural anomalies of the Moho discontinuity detected in several places along the Western Carpathian arc. Such anomalies are located in close lateral proximity of the Pieniny Klippen Belt representing the contact between the stable European Plate in the north and the ALCAPA (Alpine-Carpathian-Pannonian) microplate in the south. Thus, the complex local Moho topography modelled from the Pn phases suggests tectonic relation to the formation of the Carpathian orogen. The result is supported by correlation with the large-scale Carpathian conductivity anomaly modelled in the Carpathians at a mid-crustal level. Relative lateral position of these two structures together with the Pieniny Klippen Belt at the surface delineates a zone affected by deformations at various depths along the whole Western Carpathian arc.

Moho topography, ranges and folds of Tibet by analysis of global gravity models and GOCE data

PubMed Central

Shin, Young Hong; Shum, C.K.; Braitenberg, Carla; Lee, Sang Mook; Na, Sung -Ho; Choi, Kwang Sun; Hsu, Houtse; Park, Young-Sue; Lim, Mutaek

2015-01-01

The determination of the crustal structure is essential in geophysics, as it gives insight into the geohistory, tectonic environment, geohazard mitigation, etc. Here we present the latest advance on three-dimensional modeling representing the Tibetan Mohorovičić discontinuity (topography and ranges) and its deformation (fold), revealed by analyzing gravity data from GOCE mission. Our study shows noticeable advances in estimated Tibetan Moho model which is superior to the results using the earlier gravity models prior to GOCE. The higher quality gravity field of GOCE is reflected in the Moho solution: we find that the Moho is deeper than 65 km, which is twice the normal continental crust beneath most of the Qinghai-Tibetan plateau, while the deepest Moho, up to 82 km, is located in western Tibet. The amplitude of the Moho fold is estimated to be ranging from −9 km to 9 km with a standard deviation of ~2 km. The improved GOCE gravity derived Moho signals reveal a clear directionality of the Moho ranges and Moho fold structure, orthogonal to deformation rates observed by GPS. This geophysical feature, clearly more evident than the ones estimated using earlier gravity models, reveals that it is the result of the large compressional tectonic process. PMID:26114224

Crust and lithosphere structure in the eastern segment of the Xing-Meng orogenic belt from S-receiver function

NASA Astrophysics Data System (ADS)

Zhang, R.; Wu, Q.

2013-12-01

From 2009 to 2011, a 60 station broadband seismic array extending over 1200km was deployed in northeast China (NEC) by the Institute of geophysics, China Earthquake Administration (CEA). The recently linear deployment of seismic array in Northeast China (NEC) facilitated collection of more high-quality broadband data, thus provide us an opportunity to use S-wave receiver functions to investigate its crustal and mantle lithosphere structure with high resolution. Two distinct signals with large amplitude can be identified in our imaged S receiver functions. The strong positive one from the Moho can be observed continuously at depths from 40 km beneath Great Xing'an Range to less than 30 km beneath the Songliao Basin. The imaged Moho agrees with previous estimate of crustal thickness, and the lateral variations correlate to its surface tomography. The deep negative Sp phase interpreted as from the lithosphere-asthenosphere boundary (LAB) is as shallow as ~100km in the Songliao basin, down to 140-160km in the westward of Xingmeng block. The boundary is less prominent east of the Songliao Basin. The imaged Moho and LAB structure indicate the crust and lithosphere thinning in the Songliao Basin, and the vertical thinning of LAB is more obvious, evidence in a depth variation up to 50 km. The Songliao Basin is a continental rifting where a large amount of extension occurs, and the coupling of thinning between in the crust and underlying lithosphere indicated that the lithosphere stretching may be involved to the crustal rifting. The stretching can be more explained by the pure shear regime proposed in extensional tectonics. Acknowledgments. Seismic data were collected by the by the Institute of Geophysics, China Earthquake Administration. This work was supported by the NSF of China (grants 40974061, 90814013), the Chinese government's executive program (SinoProbe-02-03) and the international cooperation project of the Ministry of Science and Technology of China (2011DFB20120).

Insights on the lithospheric structure of the Zagros mountain belt from seismological data analysis

NASA Astrophysics Data System (ADS)

Paul, A.; Kaviani, A.; Vergne, J.; Hatzfeld, D.; Mokhtari, M.

2003-04-01

As part of a French-Iranian collaboration, we installed a temporary seismological network across the Zagros for 4.5 months in 2000-2001 to investigate the lithospheric structure of the mountain belt. The network included 65 stations located along a 600-km long line (average spacing of ˜10 km) from the coast of the Persian Gulf to the stable block of Central Iran. A migrated depth cross-section computed from radial receiver functions displays clear P-to-S conversions at the Moho beneath most of the profile. The average Moho depth is 45 to 50 km beneath the folded belt. It deepens rather abruptly beneath the suture zone of the MZT (Main Zagros Thrust) and the Sanandaj-Sirjan (SS) metamorphic zone. The maximum crustal thickness of ˜65 km is reached 50 km NE of the surface trace of the MZT. The region of over-thickened crust is shifted to the NE with respect to the areas of highest elevations and the strongest negative Bouguer anomaly. To the NE, the crust of the block of Central Iran is 40-km thick on average. Two patches of Ps converted energy can be seen below the Moho in the northern half of the transect that cannot be attributed to multiple reflections. Teleseismic P residual travel time curves display lateral variations as large as 1.5 s with both long (faster arrivals in the SW than in the NE) and short-scale variations (in the MZT region). They were inverted for variations of P wave velocity with the ACH technique. The crustal layer exhibits rather strong lateral variations of Vp with lower velocities under the MZT and the Urumieh-Dokhtar magmatic assemblage, and faster velocities under the SS zone. In the mantle, a clear difference appears between the faster P wave velocities of the Arabian craton and the relatively lower velocities of the mantle of Central Iran.

Transition from continental to oceanic crust on the Wilkes-Adelie margin of Antarctica

NASA Astrophysics Data System (ADS)

Eittreim, Stephen L.

1994-12-01

The Wilkes-Adelie margin of East Antarctica, a passive margin rifted in the Early Cretaceous, has an unusually reflective Moho which can be traced seismically across the continent-ocean transition. Velocity models and depth sections were constructed from a combined set of U.S. and French multichannel seismic reflection lines to investigate the transition from continental to oceanic crust. These data show that the boundary between oldest oceanic crust and transitional continental crust is marked by a minimum in subsediment crustal thickness and, in places, by a shoaling of Moho. The Moho reflection is continuous across the edge of oceanic crust, and gradually deepens landward under the continental edge. A marginal rift basin, some tens of kilometers in width, lies in the transition between continental and oceanic crust, contains an average of about 4 km of synrift sediment that is prograded in places, and has characteristics of a former rift valley, now subsided to about 10 km. Three types of reflections in the seismic data are interpreted as volcanic deposits: (1) high-amplitude reflections that floor the marginal rift basin, (2) irregularly seaward dipping sequences that comprise an anomalously thick edge of oceanic crust, and (3) highly irregular and diffractive reflections from oceanic crustal basins that cap a normal-thickness ocean crust. The present depth to the prefit surface of continental crust is compatible with passive margin subsidence since 95 Ma, corrected for its load of synrift and postrift sediment and mechanically stretched by factors of beta = 1.8 or higher. Comparison of seismic crustal thickness measurements with inferred crustal thinning from subsidence analysis shows agreement for areas where beta less than 4. In areas where beta greater than 4, measured thickness is greater than that inferred from subsidence analysis, a result that could be explained by underplating the crust beneath the marginal rift basin.

Investigation of the lithosphere of the Texas Gulf Coast using phase-specific Ps receiver functions produced by wavefield iterative deconvolution

NASA Astrophysics Data System (ADS)

Gurrola, H.; Berdine, A.; Pulliam, J.

2017-12-01

Interference between Ps phases and reverberations (PPs, PSs phases and reverberations thereof) make it difficult to use Ps receiver functions (RF) in regions with thick sediments. Crustal reverberations typically interfere with Ps phases from the lithosphere-asthenosphere boundary (LAB). We have developed a method to separate Ps phases from reverberations by deconvolution of all the data recorded at a seismic station by removing phases from a single wavefront at each iteration of the deconvolution (wavefield iterative deconvolution or WID). We applied WID to data collected in the Gulf Coast and Llano Front regions of Texas by the EarthScope Transportable array and by a temporary deployment of 23 broadband seismometers (deployed by Texas Tech and Baylor Universities). The 23 station temporary deployment was 300 km long; crossing from Matagorda Island onto the Llano uplift. 3-D imaging using these data shows that the deepest part of the sedimentary basin may be inboard of the coastline. The Moho beneath the Gulf Coast plain does not appear in many of the images. This could be due to interference from reverberations from shallower layers or it may indicate the lack of a strong velocity contrast at the Moho perhaps due to serpentinization of the uppermost mantle. The Moho appears to be flat, at 40 km) beneath most of the Llano uplift but may thicken to the south and thin beneath the Coastal plain. After application of WID, we were able to identify a negatively polarized Ps phase consistent with LAB depths identified in Sp RF images. The LAB appears to be 80-100 km deep beneath most of the coast but is 100 to 120 km deep beneath the Llano uplift. There are other negatively polarized phases between 160 and 200 km depths beneath the Gulf Coast and the Llano Uplift. These deeper phases may indicate that, in this region, the LAB is transitional in nature and rather than a discrete boundary.

Crustal Structure and Subsidence of the Williston Basin: Evidence from Receiver Function Stacking and Gravity Modeling

NASA Astrophysics Data System (ADS)

Song, J.; Liu, K. H.; Yu, Y.; Mickus, K. L.; Gao, S. S.

2017-12-01

The Williston Basin of the northcentral United States and southern Canada is a typical intracratonic sag basin, with nearly continuous subsidence from the Cambrian to the Jurassic. A number of contrasting models on the subsidence mechanism of this approximately circular basin have been proposed. While in principle 3D variations of crustal thickness, layering, and Poisson's ratio can provide essential constraints on the models, thick layers of Phanerozoic sediment with up to 4.5 km thickness prevented reliable determinations of those crustal properties using active or passive source seismic techniques. Specifically, the strong reverberations of teleseismic P-to-S converted waves (a.k.a. receiver functions or RFs) from the Moho and intracrustal interfaces in the loose sedimentary layer can severely contaminate the RFs. Here we use RFs recorded by about 200 USArray and other stations in the Williston Basin and adjacent areas to obtain spatial distributions of the crustal properties. We have found that virtually all of the RFs recorded by stations in the Basin contain strong reverberations, which are effectively removed using a recently developed deconvolution-based filter (Yu et al., 2015, DOI: 10.1002/2014JB011610). A "double Moho" structure is clearly imaged beneath the Basin. The top interface has a depth of about 40 km beneath the Basin, and shallows gradually toward the east from the depocenter. It joins with the Moho beneath the western margin of the Superior Craton, where the crust is about 30 km thick. The bottom interface has a depth of 55 km beneath the Wyoming Craton, and deepens to about 70 km beneath the depocenter. Based on preliminary results of H-k stacking and gravity modeling, we interpret the layer between the two interfaces as a high density, probably eclogized layer. Continuous eclogitization from the Cambrian to the Jurassic resulted in the previously observed rates of subsidence being nearly linear rather than exponential.

Various Approaches to Forward and Inverse Wide-Angle Seismic Modelling Tested on Data from DOBRE-4 Experiment

NASA Astrophysics Data System (ADS)

Janik, Tomasz; Środa, Piotr; Czuba, Wojciech; Lysynchuk, Dmytro

2016-12-01

The interpretation of seismic refraction and wide angle reflection data usually involves the creation of a velocity model based on an inverse or forward modelling of the travel times of crustal and mantle phases using the ray theory approach. The modelling codes differ in terms of model parameterization, data used for modelling, regularization of the result, etc. It is helpful to know the capabilities, advantages and limitations of the code used compared to others. This work compares some popular 2D seismic modelling codes using the dataset collected along the seismic wide-angle profile DOBRE-4, where quite peculiar/uncommon reflected phases were observed in the wavefield. The 505 km long profile was realized in southern Ukraine in 2009, using 13 shot points and 230 recording stations. Double PMP phases with a different reduced time (7.5-11 s) and a different apparent velocity, intersecting each other, are observed in the seismic wavefield. This is the most striking feature of the data. They are interpreted as reflections from strongly dipping Moho segments with an opposite dip. Two steps were used for the modelling. In the previous work by Starostenko et al. (2013), the trial-and-error forward model based on refracted and reflected phases (SEIS83 code) was published. The interesting feature is the high-amplitude (8-17 km) variability of the Moho depth in the form of downward and upward bends. This model is compared with results from other seismic inversion methods: the first arrivals tomography package FAST based on first arrivals; the JIVE3D code, which can also use later refracted arrivals and reflections; and the forward and inversion code RAYINVR using both refracted and reflected phases. Modelling with all the codes tested showed substantial variability of the Moho depth along the DOBRE-4 profile. However, SEIS83 and RAYINVR packages seem to give the most coincident results.

Lunar Gravity Studies from the Lunar Prospector Line-of-Sight Acceleration Data: Isostatic Compensation of Medium Sized Craters

NASA Astrophysics Data System (ADS)

Sugano, T.; Heki, K.

2002-12-01

Direct estimation of mass distribution on the lunar nearside surface using the Lunar Prospector (LP) line-of-sight (LOS) acceleration data has several merits over conventional methods to estimate Stokes' coefficients of the lunar gravity field, such as (1) high resolution gravity anomaly recovery without introducing Kaula's constraint, (2) fast inversion calculation by stepwise estimation of parameter sets enabled by small correlation between parameters sets. Resolution of the lunar free-air gravity anomaly map obtained here, is as high as a gravity model complete to degree/order 225, and yet less noisy than the recent models. Next we performed terrain correction for the raw LOS acceleration data using lunar topography model from the Clementine laser altimetry data and the average crustal density of 2.9 g/cm3. By conducting the same inversion for the data after the correction, we obtained the map of Bouguer gravity anomaly that mainly reflects the MOHO topography. By comparing maps we notice that signatures of medium-sized (80-300 km in diameter) craters visible as topographic depression and negative free air anomaly, disappear in the Bouguer anomaly. The absence of mass deficits in the Bouguer anomaly suggests that the MOHO beneath them is flat. Generally speaking, longer wavelength topographic features have to be supported by MOHO topography (Airy isostatic compensation) while small scale topographic features are supported by lithospheric strength. The boundary between these two modes constrains the lithosphere thickness, and hence thermal structure near the surface. Larger craters are known to have become Mascons; mantle plugs and high-density mare basalts cause positive gravity anomalies there. The smallest Mascon has diameters a little larger than 300 km (e.g. Schiller-Zuccius), and the boundary between the two compensation status seems to lie around 300 km. Thermal evolution history of the Moon suggests temporally increasing thickness of lithosphere over its entire history, and the lithosphere as thick as 50-100 km around 4.0 Ga. This is consistent with the isostatic compensation status of the craters studied here, and a model describing the degree of lithospheric supports for various wavelength topographies.

Crustal thickness of Antarctica estimated using data from gravimetric satellites

NASA Astrophysics Data System (ADS)

Llubes, Muriel; Seoane, Lucia; Bruinsma, Sean; Rémy, Frédérique

2018-04-01

Computing a better crustal thickness model is still a necessary improvement in Antarctica. In this remote continent where almost all the bedrock is covered by the ice sheet, seismic investigations do not reach a sufficient spatial resolution for geological and geophysical purposes. Here, we present a global map of Antarctic crustal thickness computed from space gravity observations. The DIR5 gravity field model, built from GOCE and GRACE gravimetric data, is inverted with the Parker-Oldenburg iterative algorithm. The BEDMAP products are used to estimate the gravity effect of the ice and the rocky surface. Our result is compared to crustal thickness calculated from seismological studies and the CRUST1.0 and AN1 models. Although the CRUST1.0 model shows a very good agreement with ours, its spatial resolution is larger than the one we obtain with gravimetric data. Finally, we compute a model in which the crust-mantle density contrast is adjusted to fit the Moho depth from the CRUST1.0 model. In East Antarctica, the resulting density contrast clearly shows higher values than in West Antarctica.

Imaging the Juan de Fuca subduction plate using 3D Kirchoff Prestack Depth Migration

NASA Astrophysics Data System (ADS)

Cheng, C.; Bodin, T.; Allen, R. M.; Tauzin, B.

2014-12-01

We propose a new Receiver Function migration method to image the subducting plate in the western US that utilizes the US array and regional network data. While the well-developed CCP (common conversion point) poststack migration is commonly used for such imaging; our method applies a 3D prestack depth migration approach. The traditional CCP and post-stack depth mapping approaches implement the ray tracing and moveout correction for the incoming teleseismic plane wave based on a 1D earth reference model and the assumption of horizontal discontinuities. Although this works well in mapping the reflection position of relatively flat discontinuities (such as the Moho or the LAB), CCP is known to give poor results in the presence of lateral volumetric velocity variations and dipping layers. Instead of making the flat layer assumption and 1D moveout correction, seismic rays are traced in a 3D tomographic model with the Fast Marching Method. With travel time information stored, our Kirchoff migration is done where the amplitude of the receiver function at a given time is distributed over all possible conversion points (i.e. along a semi-elipse) on the output migrated depth section. The migrated reflectors will appear where the semicircles constructively interfere, whereas destructive interference will cancel out noise. Synthetic tests show that in the case of a horizontal discontinuity, the prestack Kirchoff migration gives similar results to CCP, but without spurious multiples as this energy is stacked destructively and cancels out. For 45 degree and 60 degree dipping discontinuities, it also performs better in terms of imaging at the right boundary and dip angle. This is especially useful in the Western US case, beneath which the Juan de Fuca plate subducted to ~450km with a dipping angle that may exceed 50 degree. While the traditional CCP method will underestimate the dipping angle, our proposed imaging method will provide an accurate 3D subducting plate image without heavy computation. This will provide further thoughts for geodynamic research on the evolution of western US.

Thinning Factors and Crustal Thicknesses at the Propagating Tip of Sea-floor Spreading in the Woodlark Basin

NASA Astrophysics Data System (ADS)

Gozzard, S. P.; Kusznir, N.; Goodliffe, A.; Manatschal, G.

2007-12-01

Understanding how the continental crust and lithosphere thins at the propagating tip of sea-floor spreading is the key to understanding the continental breakup process. The Woodlark Basin, a young ocean basin located in the Western Pacific to the east of Papua New Guinea, commenced formation at approximately 8.4Ma and is propagating westwards at a rate of approximately 140km/Myr. Immediately to the west of the most recent segment of sea-floor spreading propagation, in the vicinity of the Moresby Seamount, evidence from bathymetry, subsidence and seismic Moho depth suggests that continental lithosphere is being thinned. In this study we have determined lithosphere thinning in the vicinity of the Moresby Seamount at the level of the whole lithosphere, the whole crust and the upper crust. Whole lithosphere thinning factors have been determined from subsidence analysis; whole continental crustal thinning factors have been determined from gravity inversions and upper crustal thinning factors have been determined from fault analysis. Three 2D seismic profiles surrounding the Moresby Seamount have been flexurally backstripped to the base of the syn-rift sediments to determine the water loaded subsidence. Using the McKenzie lithosphere extension model, modified to include volcanic addition at high thinning factors, whole thinning factors for the lithosphere have been determined from the water loaded subsidence. Results show that thermal subsidence alone cannot account for the observed subsidence, and that an additional initial subsidence is needed. Whole lithosphere thinning factors increase from an average of 0.5 to 0.8 across the Moresby Seamount eastwards towards the propagating tip. A satellite gravity inversion incorporating a lithosphere thermal gravity anomaly correction has been used to determine Moho depth, crustal thickness and thinning factors for the propagating tip in the Woodlark Basin. Moho depths are consistent with depths obtained from receiver function analysis (Ferris et al. 2006). Crustal thickness estimates do not include a correction for sediment thickness and are upper bounds. Crustal thinning factors in the vicinity of the Moresby Seamount are similar to those observed for the whole lithosphere. Fault analysis of the three 2D profiles have been used to determine upper crustal thinning factors. Upper crustal thinning factors between 0.1 to 0.2 are observed for the vicinity of the Moresby Seamount, substantially lower than thinning factors predicted for the whole lithosphere and continental crust, suggesting depth-dependent lithosphere thinning. Crustal thicknesses predicted from gravity inversion immediately to the east of the Moresby Seamount are substantially greater than would be expected for oceanic lithosphere in this region, while highly thinned, has not completely ruptured.

The Subduction of an Exhumed and Serpentinized Magma-Poor Basement Beneath the Northern Lesser Antilles Reveals the Early Tectonic Fabric at Slow-Spreading Mid-Oceanic Ridges

NASA Astrophysics Data System (ADS)

Marcaillou, B.; Klingelhoefer, F.; Laurencin, M.; Biari, Y.; Graindorge, D.; Jean-Frederic, L.; Laigle, M.; Lallemand, S.

2017-12-01

Multichannel and wide-angle seismic data as well as heat-flow measurements (ANTITHESIS cruise, 2016) reveal a 200x200km patch of magma-poor oceanic basement in the trench and beneath the outer fore-arc offshore of Antigua to Saint Martin in the Northern Lesser Antilles. These data highlight an oceanic basement with the following features: 1/ Absence of any reflection at typical Moho depth and layer2/layer3 limit depths. 2/ High Velocity Vp at the top (>5.5 km/s), low velocity gradient with depth (<0.3 s-1) and no significant velocity change at theoretical Moho depth. 3/ Anomalously low heat-flow (40±15mW.m-2) compared to the central Antilles and to theoretical values for an 80 Myr-old oceanic plate suggesting the influence of deep hydrothermal circulation. 4/ Two sets of reflections dipping toward the paleo mid-Atlantic ridge and toward the Vidal Transform Fault Zone respectively. These highly reflective planes sometimes fracture the top of the basement, deforming the interplate contact and extend downward to 20km depth with a 20° angle. We thus propose that a large patch of mantle rocks, exhumed and serpentinized at the slow-spreading mid-Atlantic Ridge 80 Myr ago, is currently subducting beneath the Northern Lesser Antilles. During the exhumation, early extension triggers penetrative shear zones sub-parallel to the ridge and to the transform fault. Eventually, this early extension generates sliding along the so-called detachment fault, while the other proto-detachment abort. Approaching the trench, the plate bending reactivates these weak zones in normal faults and fluid pathways promoting deep serpentinisation and localizing tectonic deformation at the plate interface. These subducting fluid-rich mechanically weak mantle rocks rise questions about their relation to the faster slab deepening, the lower seismic activity and the pervasive tectonic partitioning in this margin segment.

Flexural bending of the Zagros foreland basin

NASA Astrophysics Data System (ADS)

Pirouz, Mortaza; Avouac, Jean-Philippe; Gualandi, Adriano; Hassanzadeh, Jamshid; Sternai, Pietro

2017-09-01

We constrain and model the geometry of the Zagros foreland to assess the equivalent elastic thickness of the northern edge of the Arabian plate and the loads that have originated due to the Arabia-Eurasia collision. The Oligo-Miocene Asmari formation, and its equivalents in Iraq and Syria, is used to estimate the post-collisional subsidence as they separate passive margin sediments from the younger foreland deposits. The depth to these formations is obtained by synthesizing a large database of well logs, seismic profiles and structural sections from the Mesopotamian basin and the Persian Gulf. The foreland depth varies along strike of the Zagros wedge between 1 and 6 km. The foreland is deepest beneath the Dezful embayment, in southwest Iran, and becomes shallower towards both ends. We investigate how the geometry of the foreland relates to the range topography loading based on simple flexural models. Deflection of the Arabian plate is modelled using point load distribution and convolution technique. The results show that the foreland depth is well predicted with a flexural model which assumes loading by the basin sedimentary fill, and thickened crust of the Zagros. The model also predicts a Moho depth consistent with Free-Air anomalies over the foreland and Zagros wedge. The equivalent elastic thickness of the flexed Arabian lithosphere is estimated to be ca. 50 km. We conclude that other sources of loading of the lithosphere, either related to the density variations (e.g. due to a possible lithospheric root) or dynamic origin (e.g. due to sublithospheric mantle flow or lithospheric buckling) have a negligible influence on the foreland geometry, Moho depth and topography of the Zagros. We calculate the shortening across the Zagros assuming conservation of crustal mass during deformation, trapping of all the sediments eroded from the range in the foreland, and an initial crustal thickness of 38 km. This calculation implies a minimum of 126 ± 18 km of crustal shortening due to ophiolite obduction and post-collisional shortening.

Stress field models from Maxwell stress functions: southern California

NASA Astrophysics Data System (ADS)

Bird, Peter

2017-08-01

The lithospheric stress field is formally divided into three components: a standard pressure which is a function of elevation (only), a topographic stress anomaly (3-D tensor field) and a tectonic stress anomaly (3-D tensor field). The boundary between topographic and tectonic stress anomalies is somewhat arbitrary, and here is based on the modeling tools available. The topographic stress anomaly is computed by numerical convolution of density anomalies with three tensor Green's functions provided by Boussinesq, Cerruti and Mindlin. By assuming either a seismically estimated or isostatic Moho depth, and by using Poisson ratio of either 0.25 or 0.5, I obtain four alternative topographic stress models. The tectonic stress field, which satisfies the homogeneous quasi-static momentum equation, is obtained from particular second derivatives of Maxwell vector potential fields which are weighted sums of basis functions representing constant tectonic stress components, linearly varying tectonic stress components and tectonic stress components that vary harmonically in one, two and three dimensions. Boundary conditions include zero traction due to tectonic stress anomaly at sea level, and zero traction due to the total stress anomaly on model boundaries at depths within the asthenosphere. The total stress anomaly is fit by least squares to both World Stress Map data and to a previous faulted-lithosphere, realistic-rheology dynamic model of the region computed with finite-element program Shells. No conflict is seen between the two target data sets, and the best-fitting model (using an isostatic Moho and Poisson ratio 0.5) gives minimum directional misfits relative to both targets. Constraints of computer memory, execution time and ill-conditioning of the linear system (which requires damping) limit harmonically varying tectonic stress to no more than six cycles along each axis of the model. The primary limitation on close fitting is that the Shells model predicts very sharp shallow stress maxima and discontinuous horizontal compression at the Moho, which the new model can only approximate. The new model also lacks the spatial resolution to portray the localized stress states that may occur near the central surfaces of weak faults; instead, the model portrays the regional or background stress field which provides boundary conditions for weak faults. Peak shear stresses in one registered model and one alternate model are 120 and 150 MPa, respectively, while peak vertically integrated shear stresses are 2.9 × 1012 and 4.1 × 1012 N m-1. Channeling of deviatoric stress along the strong Great Valley and the western slope of the Peninsular Ranges is evident. In the neotectonics of southern California, it appears that deviatoric stress and long-term strain rate have a negative correlation, because regions of low heat flow are strong and act as stress guides, while undergoing very little internal deformation. In contrast, active faults lie preferentially in areas with higher heat flow, and their low strength keeps deviatoric stresses locally modest.

Seismic anisotropy of the Slave craton, NW Canada, from joint interpretation of SKS and Rayleigh waves

NASA Astrophysics Data System (ADS)

Snyder, David; Bruneton, Marianne

2007-04-01

Teleseismic events recorded at a 25-element array in NW Canada between 2001 and 2006 provided sufficient distribution in back azimuth to demonstrate birefringence in SKS and SKKS waves as well as directional dependence of Rayleigh-wave phase velocities. Typical delays between orthogonally polarized SKS waves are 0.8-1.2 s, and modelling of azimuthal dependence indicates two nearly horizontal layers of anisotropy within the mantle. Anisotropy of Rayleigh waves is generally consistent with models of layered Vs anisotropies that increase with depth from 1 per cent at the Moho to 9 per cent at 200 km but vary between subarrays. Consistency between the SKS and Rayleigh wave anisotropies in one subarray suggests that the assumption of symmetry about a horizontal axis is valid there but is not fully valid in other parts of the craton. The upper layer of anisotropy occupies approximately the uppermost 120 km in which the fast polarization direction strikes generally north-south, coinciding with regional-scale fold axes mapped at the surface. The fast polarization direction of the deeper layer aligns with current North America plate motion, but its correlation with trends of coeval kimberlite eruptions within the Lac de Gras field suggests it can be at least partly attributed to structural preferred orientation of vertical dykes inferred to exist to depths of 200 km.

Structural variation of the oceanic Moho in the Pacific plate revealed by active-source seismic data

NASA Astrophysics Data System (ADS)

Ohira, Akane; Kodaira, Shuichi; Nakamura, Yasuyuki; Fujie, Gou; Arai, Ryuta; Miura, Seiichi

2017-10-01

The characteristics of the oceanic Moho are known to depend on various factors, such as seafloor spreading rate, crustal age, and accretionary processes at a ridge. However, the effect of local magmatic activities on the seismic signature of the Moho is poorly understood. Here an active-source reflection and refraction survey is used to investigate crustal structure and Moho characteristics along a >1000-km-long profile southeast of the Shatsky Rise in a Pacific Ocean basin formed from the Late Jurassic to Early Cretaceous and spanning the onset of Shatsky Rise volcanism. Although the seismic velocity structure estimated from the refraction data showed typical characteristics of the oceanic crust of the old Pacific plate, the appearance of the Moho reflections was spatially variable. We observed clear Moho reflections such as those to be expected where the spreading rate is fast to intermediate only at the southwestern end of the profile, whereas Moho reflections were diffuse, weak, or absent along other parts of the profile. The poor Moho reflections can be explained by the presence of a thick crust-mantle transition layer, which is temporally coincident with the formation of the Shatsky Rise. We inferred that the crust-mantle transition layer was formed by changes in on-axis accretion process or modification of the primary Moho by off-axis magmatism, induced by magmatic activity of the Shatsky Rise.

Crustal structure of central Lake Baikal: Insights into intracontinental rifting

USGS Publications Warehouse

ten Brink, Uri S.; Taylor, M.H.

2002-01-01

The Cenozoic rift system of Baikal, located in the interior of the largest continental mass on Earth, is thought to represent a potential analog of the early stage of breakup of supercontinents. We present a detailed P wave velocity structure of the crust and sediments beneath the Central Basin, the deepest basin in the Baikal rift system. The structure is characterized by a Moho depth of 39-42.5 km; an 8-km-thick, laterally continuous high-velocity (7.05-7.4 km/s) lower crust, normal upper mantle velocity (8 km/s), a sedimentary section reaching maximum depths of 9 km, and a gradual increase of sediment velocity with depth. We interpret the high-velocity lower crust to be part of the Siberian Platform that was not thinned or altered significantly during rifting. In comparison to published results from the Siberian Platform, Moho under the basin is elevated by <3 km. On the basis of these results we propose that the basin was formed by upper crustal extension, possibly reactivating structures in an ancient fold-and-thrust belt. The extent and location of upper mantle extension are not revealed by our data, and it may be offset from the rift. We believe that the Baikal rift structure is similar in many respects to the Mesozoic Atlantic rift system, the precursor to the formation of the North Atlantic Ocean. We also propose that the Central Baikal rift evolved by episodic fault propagation and basin enlargement, rather than by two-stage rift evolution as is commonly assumed.

Segmentation of plate coupling, fate of subduction fluids, and modes of arc magmatism in Cascadia, inferred from magnetotelluric resistivity

USGS Publications Warehouse

Wannamaker, Philip E.; Evans, Rob L.; Bedrosian, Paul A.; Unsworth, Martyn J.; Maris, Virginie; McGary, R. Shane

2014-01-01

Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2-D and 3-D nonlinear inversion to yield electrical resistivity cross sections to depths of ∼200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid- and sediment-deficient conditions. In contrast, the north-central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low-resistivity fluidized zones develop at slab depths of 35–40 km starting ∼100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low-frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and nearby backarc. The low- to high-temperature mantle wedge transition lies slightly trenchward of the arc.

Exploring Sources of Uncertainties in Global Radial Anisotropy Models

NASA Astrophysics Data System (ADS)

Xing, Z.; Beghein, C.; Yuan, K.

2013-12-01

We investigate sources of uncertainties in radial anisotropy models with a focus on the transition zone (TZ). Radial anisotropy describes the velocity difference between horizontally (SH) and vertically (SV) polarized shear waves. Its presence in the top 200 km of the mantle is well documented and thought of as an indicator of deformation by dislocation creep due to mantle shear. There is however no consensus regarding its presence at larger depths, which affects our understanding of deep upper mantle deformation. Several of the models that display radial anisotropy in the TZ are characterized by SH waves faster than SV waves (VSH>VSV) at these depths. Model VTLK08 (Visser et al., 2008) is however characterized by VSV>VSH in the TZ. The first part of this study aims at determining the origin of this discrepancy and the robustness of the VSV>VSH TZ signal in VTLK08. We used the global phase velocity maps of Visser et al (2008) for fundamental and higher mode Love and Rayleigh waves, which provide sensitivity to structure well below the TZ. We first tested the effect of imposing prior crustal corrections instead of inverting for the Moho depth as in VTLK08. We applied non-linear crustal corrections to the data on a 5 by 5 degree grid using CRUST2.0, and calculated laterally varying sensitivity kernels to account for the effect of the crust on the partial derivatives. We employed a depth parametrization in terms of cubic splines of varying depth spacing defined between the local Moho and 1400 km depth. We applied similar prior relationships between P- and S-wave elastic parameters as in VTLK08, and solved the problem using both a traditional inversion method and the same Neighbourhod Algorithm (NA) forward modeling approach as in VTLK08. The first stage of the NA enables us to randomly sample the model space, including the null space. The second stage describes each model parameter with probability density functions, thereby providing quantitative model uncertainties. Our preliminary results show that the TZ signal in VTLK08 is not strongly dependent on crustal corrections or on the inversion method employed. In both cases, we obtained average anisotropy and velocity profiles consistent with VTLK08, with 2% VSV>VSH anisotropy in the TZ for the best fitting model obtained with NA. The 3-D anisotropy anomalies are in agreement with VTLK08 at most depths. However the models differ in the TZ under the central Pacific where we found a positive anisotropy signal that does not appear in VTLK08. With the model uncertainties provided by the second stage of the NA, we will be able to determine whether the significance of this positive signal. The next phase of our work will consist in analyzing the effect of assuming that P- and S-wave anisotropies are proportional, an assumption often made to deal with model non-uniqueness. These prior constraints do not strongly affect the most likely model in the uppermost 200km of the mantle (Beghein, 2010) but they may affect mantle models at greater depths. The use of the NA will enable us to determine whether the introduction of such prior significantly affects the range of models compatible with the data, which in turn will enable us to determine whether the TZ signal of VTLK08 is well constrained by the data.

The Mohorovičić discontinuity beneath the continental crust: An overview of seismic constraints

NASA Astrophysics Data System (ADS)

Carbonell, Ramon; Levander, Alan; Kind, Rainer

2013-12-01

The seismic signature of the Moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image. Seismic data of different types, passive source (earthquake) broad-band recordings, and controlled source seismic refraction, densely recorded wide-angle deep seismic reflection, and normal incidence reflection (using VibroseisTM, explosives, or airguns), have contributed to the description of the Moho as a relatively complex transition zone. Of critical importance for the quality and resolution of the seismic image are the acquisition parameters, used in the imaging experiments. A variety of signatures have been obtained for the Moho at different scales generally dependent upon bandwidth of the seismic source. This variety prevents the development of a single universally applicable interpretation. In this way source frequency content, and source and sensor spacing determine the vertical and lateral resolution of the images, respectively. In most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the Moho, and its relationship to a petrologic crust-mantle transition. In regional seismic studies carried out using passive source recordings the Moho is a relatively well defined structure with marked lateral continuity. The characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area. Refraction and wide-angle studies suggest the Moho to be often a relatively sharp velocity contrast, whereas the Moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity. The origin of the Moho and its relation to the crust-mantle boundary is probably better constrained by careful analysis of its internal details, which can be complex and geographically varied. Unlike the oceanic Moho which is formed in a relatively simple, well understood process, the continental Moho can be subject to an extensive variety of tectonic processes, making overarching conclusions about the continental Moho difficult. Speaking very broadly: 1) In orogenic belts still undergoing compression and active continental volcanic arcs, the Moho evolves with the mountain belt, 2) In collapsed Phanerozoic orogenic belts the Moho under the collapse structure was formed during the collapse, often by a combination of processes. 3) In regions having experienced widespread basaltic volcanism, the Moho can result from underplated basalt and basaltic residuum. In Precambrian terranes the Moho may be as ancient as the formation of the crust, in others Precambrian tectonic and magmatic processes have reset it. We note that seismic reflection data in Phanerosoic orogens as well as from Precambrian cratonic terranes often show thrust type structures extending as deep as the Moho, and suggest that even where crust and mantle xenoliths provide similar age of formation dates, the crust may be semi-allochothonous.

Seismic reflection profiling in the Boulder batholith, Montana

NASA Astrophysics Data System (ADS)

Vejmelek, Libor; Smithson, Scott B.

1995-09-01

Seismic reflection profiling combined with gravity data allows more exact determination of the geometry of the controversial Boulder batholith of Montana, reveals laminated structure of the lower crust beneath the batholith, and identifies the Moho at a depth of 38 km. The batholith has inward-dipping contacts, the dip being about 50° on the west side, on the basis of seismic data; and the depth to the batholith floor is constrained between 12 and 18 km, indicating a great volume for the batholith. The Boulder batholith was emplaced between 80 and 70 Ma during an eastward thrusting in the fold-and-thrust belt. A presumed basal decollement of the thrust system might coincide with the batholith floor and may correspond to the top of the lower-crustal layering at a depth of 18 km.

Imaging hydration and dehydration across the Cascadia subduction zone (Invited)

NASA Astrophysics Data System (ADS)

Abers, G. A.; Van Keken, P. E.; Hacker, B. R.; Mann, M. E.; Crosbie, K.; Creager, K.

2017-12-01

Arc volcanoes and exhumed forearc metamorphic rocks show clear evidence for upward transport of slab-derived fluids, but geophysical measurements rarely image features that could constrain the mode of this fluid transport. The hottest subduction zones such as Cascadia pose a particular challenge, as the depths where hydrous minerals are stable seaward of trenches is limited, and much of the water is expected to depart the slab before reaching sub-arc depths. Here we improve our understanding of this problem by developing a new thermal model for central Cascadia, leveraging new results several onshore and offshore geophysical investigations, notably the iMUSH project (Imaging Magma Under mount St. Helens), to evaluate constraints on the fluid flux. Offshore onshore heat flow measurements require a cold forearc and preclude detectable shear heating. Several puzzles emerge. The first is that Mount St. Helens overlies a continuous subducting plate which has an upper surface only 65-70 km deep beneath the volcano, imaged by migrated scattered P coda. This location, together with heat flow observations and inferences from the strength of the upper plate Moho, place the volcano over a cold forearc mantle wedge that is substantially hydrated. It is unclear how the wide range of magmas at Mount St. Helens could emerge in this setting since many have mantle origin. A second puzzle is that a large velocity step, about 10% in Vs, is seen along the slab Moho to depths exceeding 90 km where thermal models predict the subducting crust is in eclogite facies; eclogite and peridotite should have nearly indistinguishable Vs. Possibly a gabbroic oceanic crust persists metastably well below the arc, or perhaps the interface represents a deeper hydration front rather than petrologic Moho. A third puzzle is the persistent indication of H2O in arc magmas here despite almost certain dehydration of subducting sediments and upper oceanic crust. This indicates substantial H2O delivered by hydrated mantle lithosphere despite seismic evidence offshore for very little hydration. Perhaps the subducting lower crust carries more H2O than previously thought, or H2O transports structurally downward into the slab after subduction commences. Overall, substantial evidence exists for lateral transport of hydrous fluids in their path from slab to surface.

Crustal and upper-mantle structure beneath ice-covered regions in Antarctica from S-wave receiver functions and implications for heat flow

NASA Astrophysics Data System (ADS)

Ramirez, C.; Nyblade, A.; Hansen, S. E.; Wiens, D. A.; Anandakrishnan, S.; Aster, R. C.; Huerta, A. D.; Shore, P.; Wilson, T.

2016-03-01

S-wave receiver functions (SRFs) are used to investigate crustal and upper-mantle structure beneath several ice-covered areas of Antarctica. Moho S-to-P (Sp) arrivals are observed at ˜6-8 s in SRF stacks for stations in the Gamburtsev Mountains (GAM) and Vostok Highlands (VHIG), ˜5-6 s for stations in the Transantarctic Mountains (TAM) and the Wilkes Basin (WILK), and ˜3-4 s for stations in the West Antarctic Rift System (WARS) and the Marie Byrd Land Dome (MBLD). A grid search is used to model the Moho Sp conversion time with Rayleigh wave phase velocities from 18 to 30 s period to estimate crustal thickness and mean crustal shear wave velocity. The Moho depths obtained are between 43 and 58 km for GAM, 36 and 47 km for VHIG, 39 and 46 km for WILK, 39 and 45 km for TAM, 19 and 29 km for WARS and 20 and 35 km for MBLD. SRF stacks for GAM, VHIG, WILK and TAM show little evidence of Sp arrivals coming from upper-mantle depths. SRF stacks for WARS and MBLD show Sp energy arriving from upper-mantle depths but arrival amplitudes do not rise above bootstrapped uncertainty bounds. The age and thickness of the crust is used as a heat flow proxy through comparison with other similar terrains where heat flow has been measured. Crustal structure in GAM, VHIG and WILK is similar to Precambrian terrains in other continents where heat flow ranges from ˜41 to 58 mW m-2, suggesting that heat flow across those areas of East Antarctica is not elevated. For the WARS, we use the Cretaceous Newfoundland-Iberia rifted margins and the Mesozoic-Tertiary North Sea rift as tectonic analogues. The low-to-moderate heat flow reported for the Newfoundland-Iberia margins (40-65 mW m-2) and North Sea rift (60-85 mW m-2) suggest that heat flow across the WARS also may not be elevated. However, the possibility of high heat flow associated with localized Cenozoic extension or Cenozoic-recent magmatic activity in some parts of the WARS cannot be ruled out.

Density Sorting During the Evolution of Continental Crust

NASA Astrophysics Data System (ADS)

Kelemen, P. B.; Behn, M. D.; Hacker, B. R.

2015-12-01

We consider two settings - in addition to "delamination" of arc lower crust - in which dense, mafic eclogites founder into the convecting mantle while buoyant, felsic lithologies accumulate at the base of evolving continental crust. Arc processes play a central role in generating continental crust, but it remains uncertain how basaltic arc crust is transformed to andesitic continental crust. Dense, SiO2-poor products of fractionation may founder from the base of arc crust by "delamination", but lower arc crust after delamination has significantly different trace elements compared to lower continental crust (LCC). In an alternative model, buoyant magmatic rocks generated at arcs are first subducted, mainly via subduction erosion. Upon heating, these buoyant lithologies ascend through the mantle wedge or along a subduction channel, and are "relaminated" at
the base of overlying crust (e.g., Hacker et al EPSL 11, AREPS 15). Average buoyant lavas and plutons
for the Aleutians, Izu-Bonin-Marianas, Kohistan and Talkeetna arcs fall within the range of estimated LCC major and trace elements. Relamination is more efficient in generating continental crust than delamination. Himalayan cross-sections show Indian crust thrust beneath Tibetan crust, with no intervening mantle. There is a horizontal Moho at ca 80 km depth, extending from thickened Indian crust, across the region where Tibetan crust overlies Indian crust, into thickened Tibetan crust. About half the subducted Indian crust is present, whereas the other half is missing. Data (Vp/Vs; Miocene lavas formed by interaction of continental crust with mantle; xenolith thermometry) indicate 1000°C or more from ca 50 km depth to the Moho since the Miocene. We build on earlier studies (LePichon et al Tectonics 92, T'phys 97; Schulte-Pelkum et al Nature 05; Monsalve et al JGR 08) to advance the hypothesis that rapid growth of garnet occurs at 70-80 km and 1000°C within subducting Indian crust. Dense eclogites founder, while buoyant lithologies accumulate in thickening crust. Mantle return flow and radioactive heating in thick, felsic crust maintains high T. The crustal volume in this region grows at 760 m3/yr/m of strike length. Moho-depth earthquakes may be due to localized deformation and thermal runaway in weak layers and diapir margins.

Upper mantle and crustal structure of southwestern Scandinavia: Results of the TopoScandiaDeep project

NASA Astrophysics Data System (ADS)

Köhler, A.; Balling, N.; Ebbing, J.; England, R.; Frassetto, A.; Gradmann, S.; Jacobsen, B. H.; Kvarven, T.; Maupin, V.; Medhus, A. Bondo; Mjelde, R.; Ritter, J.; Schweizer, J.; Stratford, W.; Thybo, H.; Wawerzinek, B.; Weidle, C.

2012-04-01

The origin of the Scandinavian mountains, located far away from any presently active plate margin, is still not well understood. In particular, it is not clear if the mountains are sustained isostatically either by crustal thickening or by light upper mantle material. Within the TopoScandiaDeep project (a collaborative research project within the ESF TOPO-EUROPE programme), we therefore analyse recently collected passive seismological and active seismic data in the southern Scandes and surrounding regions. We infer crustal and upper mantle (velocity) structures and relate them to results of gravity and temperature-composition modelling. The Moho under the high topography of southern Norway appears from controlled source seismic refraction and Receiver Functions as relatively shallow (<= 45 km) compared to the deeper conversion (>55 km) imaged beneath the low topography in Sweden and elsewhere in the Baltic Shield area outside Norway. The Receiver Function modeling as well as the active seismic results suggest that the differences in the observed Moho response may represent the transition between tectonically reworked Moho under southern Norway and an intact, cratonic crust-mantle boundary beneath the Baltic Shield. Furthermore, the 410km-discontinuity and the LAB is imaged, the latter one suggesting a lithospheric thickening in NE direction. Upper mantle P-wave and S-wave velocities in southern Sweden and southern Norway east of the Oslo Graben are correspondingly relatively high while lower velocities are observed in the southwestern part of Norway and northern Denmark. The lateral velocity gradient, interpreted as the southwestern boundary of thick Baltic Shield lithosphere, is remarkably sharp. Differences in upper mantle velocities are found at depths of 100-400 km and amount to ± 2-3%. S-to-P wave conversions, interpreted to originate from the lithosphere-asthenosphere boundary, are preliminary estimated to 90-120 km depth. Inversion of Rayleigh and Love surface wave phase velocity dispersion curves from observations of ambient noise and earthquakes yield another independent model of the crust and upper mantle structure below southern Norway. Inverted crustal velocities and Moho depths are consistent with the results of seismic refraction and receiver functions. Additionally, indications for radial crustal anisotropy of up to about 3% are found. The inferred upper mantle S-wave velocities show that the lithosphere under southern Norway has characteristics usually found under continental platforms and changes towards a cratonic-like velocity structure in the East, in agreement with the body wave tomography. All in all, these separate investigations give a very consistent and stable picture of the crust and upper mantle configuration. Integrated geophysical modeling of the results shows that a lateral transition from thinner, warmer lithosphere under southern Norway towards thicker, colder lithosphere under Sweden results in a density distribution that significantly adds to the isostatic support of Norway's high topography.

Gravity anomaly and crustal density structure in Jilantai rift zone and its adjacent region

NASA Astrophysics Data System (ADS)

Wu, Guiju; Shen, Chongyang; Tan, Hongbo; Yang, Guangliang

2016-08-01

This paper deals with the interpretation of Bouguer gravity anomalies measured along a 250 km long Suhaitu-Etuokeqi gravity profile located at the transitional zone of the Alxa and Ordos blocks where geophysical characteristics are very complex. The analysis is carried out in terms of the ratio of elevation and Bouguer gravity anomaly, the normalized full gradient of a section of the Bouguer gravity anomaly ( G h ) and the crustal density structure reveal that (1) the ratio of highs and lows of elevation and Bouguer gravity anomaly is large between Zhengyiguan fault (F4) and Helandonglu fault (F6), which can be explained due to crustal inhomogeneities related to the uplift of the Qinghai-Tibet block in the northeast; (2) the main active faults correspond to the G h contour strip or cut the local region, and generally show strong deformation characteristics, for example the Bayanwulashan mountain front fault ( F1) or the southeast boundary of Alxa block is in accord with the western change belt of G h , a belt about 10 km wide that extends to about 30 km; (3) Yinchuan-Pingluo fault ( F8) is the seismogenic structure of the Pingluo M earthquake, and its focal depth is about 15 km; (4) the Moho depth trend and Bouguer gravity anomaly variation indicates that the regional gravity field is strongly correlated with the Moho discontinuity.

Complex local Moho topography in the Western Carpathians: Indication of the ALCAPA and the European Plate contact

NASA Astrophysics Data System (ADS)

Hrubcová, Pavla; Środa, Piotr

2015-04-01

Seismic data from deep refraction and wide-angle reflection profiles intersecting the Western Carpathians show distinct upper-mantle Pn phases with anomalous apparent velocities identified in the first and later arrivals. Their systematic analysis indicates that such phases are present in numerous seismic sections both for in-line and off-line shots. They are observed in data from profiles intersecting the Carpathians in the west at the contact with the Bohemian Massif; similar feature was also found in data at the northern edge of the Carpathians at the contact with the North European Platform. Modelling of these anomalous Pn phases shows that they originate due to local structural anomalies of the Moho discontinuity detected in several places along the Western Carpathian arc. Such anomalies are located in close lateral proximity of the Pieniny Klippen Belt representing the contact between the stable European Plate in the north and the ALCAPA (Alpine-Carpathian-Pannonian) microplate in the south. Thus, the complex local Moho topography modelled from the Pn phases suggests tectonic relation to the formation of the Carpathian orogen. The result is supported by correlation with the large-scale Carpathian conductivity anomaly modelled in the Carpathians at a mid-crustal level. Relative lateral position of these two structures together with the Pieniny Klippen Belt at the surface delineates a zone affected by deformations at various depths along the whole Western Carpathian arc. Tectonically, such course of the anomalous zone suggests that its origin is connected with the lithospheric deformations occurring near the contact of the European Plate and the ALCAPA microplate during the Carpathian orogeny, i.e., it is related to the collisional/transpressional processes during and after the Tertiary. Reference: Hrubcová, P., and Środa, P., Complex local Moho topography in the Western Carpathians: Indication of the ALCAPA and the European Plate contact. Tectonophysics, 2014, doi: 10.1016/j.tecto.2014.10.013.

Rayleigh wave tomography of the British Isles from ambient seismic noise

NASA Astrophysics Data System (ADS)

Nicolson, Heather; Curtis, Andrew; Baptie, Brian

2014-08-01

We present the first Rayleigh wave group speed maps of the British Isles constructed from ambient seismic noise. The maps also constitute the first surface wave tomography study of the crust under the British Isles at a relatively high resolution. We computed interferometric, interstation Rayleigh waves from vertical component records of ambient seismic noise recorded on 63 broad-band and short-period stations across the UK and Ireland. Group velocity measurements were made from the resulting surface wave dispersion curves between 5 and 25 s using a multiple phase-matched filter method. Uncertainties in the group velocities were computed by calculating the standard deviation of four dispersion curves constructed by stacking a random selection of daily cross-correlations. Where an uncertainty could not be obtained for a ray path using this method, we estimated it as a function of the interreceiver distance. Group velocity maps were computed for 5-25-s period using the Fast Marching forward solution of the eikonal equation and iterative, linearized inversion. At short and intermediate periods, the maps show remarkable agreement with the major geological features of the British Isles including: terrane boundaries in Scotland; regions of late Palaeozoic basement uplift; areas of exposed late Proterozoic/early Palaeozoic rocks in southwest Scotland, northern England and northwest Wales and, sedimentary basins formed during the Mesozoic such as the Irish Sea Basin, the Chester Basin, the Worcester Graben and the Wessex Basin. The maps also show a consistent low-velocity anomaly in the region of the Midlands Platform, a Proterozoic crustal block in the English Midlands. At longer periods, which are sensitive velocities in the lower crustal/upper mantle, the maps suggest that the depth of Moho beneath the British Isles decreases towards the north and west. Areas of fast velocity in the lower crust also coincide with areas thought to be associated with underplating of the lower crust such as Northern Ireland, the eastern Irish Sea and northwest Wales.

Depth to the Moho in Southern New England and Eastern New York State from Seismic Receiver Functions

NASA Astrophysics Data System (ADS)

Cipar, J. J.; Ebel, J.

2016-12-01

The thickness of the Earth's crust is a fundamental parameter of geophysics and geology. The eastern New York/southern New England area encompasses the suture between the Paleozoic Appalachian orogen and the Proterozoic Laurentian craton. The recent installation of the IRIS Traveling Array (TA) in 2013-2014 coupled with stations operated by Boston College, Lamont-Doherty, and the US National Seismic Network provide an unprecedented source of data for seismic studies of crustal structure. We use the receiver functions complied by the EarthScope Automated Receiver Survey (EARS) to measure crustal thickness. Our procedure is to stack receiver functions (RFs) at each station using the correct moveout for the P-to-S conversion at the Moho (Ps phase). The time difference between the Ps and direct P arrivals (Ps-P time) is dependent on crustal thickness (H) and crustal S-wave velocity (Vs). To get an estimate of H, we assume that the mean P-wave velocity (Vp) in the crust is 6.5 km/s, and determine the range of Vs for a range of Poisson's ratio (0.23-0.27). We then solve for H using the P-Ps times measured from the RF stacks (at Δ=60°) and our estimates for Vp and Vs. The uncertainty in S-wave velocity translates to approximately ±2 km uncertainty in crustal thickness. Our crustal thickness map shows the well-known general progression from shallow crust near the Atlantic coast line ( 30 km) to deeper crust (45+ km) in the Laurentian craton. However, some detailed features become evident on our map. Most notably, thin crust ( 30 km) extends inland from the coast to the Connecticut River valley in eastern-central Massachusetts and southeastern New Hampshire. The Berkshire Hills of western Massachusetts have thick crust (43 km), reaching as deep as 46 km in extreme northwestern Massachusetts. Thus, there is a 13-15 km increase in crustal thickness over a distance of about 60 km. Currently, no stations are located in that zone. We find that the eastern Adirondacks have very thick crust, generally in excess of 45 km. Overall, our crustal thickness measurements are in excellent agreement with those from the 1988 Ontario-New York-New England refraction experiment (USGS) and from a local receiver function study conducted using closely-spaced stations (John Schuh, Boston College).

Joint Inversion for 3-Dimensional S-Velocity Mantle Structure Along the Tethyan Margin

DTIC Science & Technology

2007-09-01

Hindu Kush and encompasses northeastern Africa, the Arabian peninsula, the Middle East, and part of the Atlantic Ocean for reference. We have fitted...several microplates within an area of one quarter of the Earth’s circumference yields this region rich with tectonic complexity. The three...assigned the largest errors. For the oceans we use a constraint of 10 km for Moho depth, but only for points also covered by data from our other data sets

Structure of the Korean Peninsula from Waveform Travel-Time Analysis

DTIC Science & Technology

2008-09-01

Bondár’s criteria (Bondár et al., 2004) to the database of 230 KMA events with depth locations requiring that each potential GT5 event is located...hypocenter database . They are well located within the dense network of KMA stations as required by Bondár’s criteria. Estimation of 3-D Moho...However, not all of these phase picks can be utilized during the velocity inversion as the implemented ray tracing is based on the eikonal solver

Seismic Discontinuities within the Crust and Mantle Beneath Indonesia as Inferred from P Receiver Functions

NASA Astrophysics Data System (ADS)

Woelbern, I.; Rumpker, G.

2015-12-01

Indonesia is situated at the southern margin of SE Asia, which comprises an assemblage of Gondwana-derived continental terranes, suture zones and volcanic arcs. The formation of SE Asia is believed to have started in Early Devonian. Its complex history involves the opening and closure of three distinct Tethys oceans, each accompanied by the rifting of continental fragments. We apply the receiver function technique to data of the temporary MERAMEX network operated in Central Java from May to October 2004 by the GeoForschungsZentrum Potsdam. The network consisted of 112 mobile stations with a spacing of about 10 km covering the full width of the island between the southern and northern coast lines. The tectonic history is reflected in a complex crustal structure of Central Java exhibiting strong topography of the Moho discontinuity related to different tectonic units. A discontinuity of negative impedance contrast is observed throughout the mid-crust interpreted as the top of a low-velocity layer which shows no depth correlation with the Moho interface. Converted phases generated at greater depth beneath Indonesia indicate the existence of multiple seismic discontinuities within the upper mantle and even below. The strongest signal originates from the base of the mantle transition zone, i.e. the 660 km discontinuity. The phase related to the 410 km discontinuity is less pronounced, but clearly identifiable as well. The derived thickness of the mantle-transition zone is in good agreement with the IASP91 velocity model. Additional phases are observed at roughly 33 s and 90 s relative to the P onset, corresponding to about 300 km and 920 km, respectively. A signal of reversed polarity indicates the top of a low velocity layer at about 370 km depth overlying the mantle transition zone.

Seismic velocity structure of the incoming Pacific Plate subducting into the central part of the Japan Trench revealed by traveltime tomography using OBS data

NASA Astrophysics Data System (ADS)

Obana, K.; Fujie, G.; Kodaira, S.; Takahashi, T.; Yamamoto, Y.; Miura, S.; Shinohara, M.

2016-12-01

Subduction of oceanic plates plays an important role in the water transportation from the earth surface into the deep mantle. Recent active seismic survey studies succeed to image that the seismic velocities within the oceanic crust and the uppermost mantle in the outer rise region decreases toward the trench axis. These velocity changes are considered as an indication of the hydration and alteration of the incoming oceanic plates prior to the subduction. However, the area with sufficient resolution of the active seismic studies is often limited at depths corresponding to the oceanic crust and several km beneath the oceanic Moho. In this study, we have examined the seismic velocity structure of the incoming/subducting Pacific Plate beneath the trench axis and outer trench-slope of the central part of the Japan Trench. The seismicity in the Pacific Plate, including several M7-class intra-plate earthquakes, has been active since the 2011 Tohoku-Oki earthquake in the study area. These activities were observed by the ocean bottom seismographs (OBS) deployed repeatedly. The data obtained from these OBS observations allow us to resolve the seismic velocity structures at greater depths compared to the active seismic surveys. We conducted 3-D traveltime tomography by using double-difference tomography method (Zhang and Thurber, 2003). The results show that the seismic velocities within the oceanic mantle decreased toward the trench axis. The velocity reduction begins at about 80 km seaward of the trench axis and extended to a depth of at least 30 km beneath the trench axis area. If the observed P-wave velocity reduction from 8.4 km/s to 7.7 km/s at a depth of 15 km below the oceanic Moho is caused by the serpentinization of the oceanic mantle (Carlson and Miller, 2003), roughly 2.5 weight per cent of water is expected in the low velocity anomalies in the oceanic mantle.

USArray Receiver Function Imaging of Multiple-Layer Crustal Structure of the Contiguous United States

NASA Astrophysics Data System (ADS)

Ma, X.; Lowry, A. R.; Ravat, D.

2014-12-01

Thickness andseismic velocity of crustal layers are useful for understanding the history and evolution of continental lithosphere. Lowry and Pérez-Gussinyé (2011) observed that low bulk crustal seismic velocity ratio, Vp/Vs, strongly correlates with high geothermal gradient and active deformation, indicating quartz (to which Vp/Vs is most sensitive) plays a role in these processes. The lower crust (where ductile flow occurs which might explain the relationship) is commonly thought to be quartz-poor. However, layering of the crust may represent changes in either lithology or the phase of quartz. Laboratory strain-stress experiments on quartz indicate that near the a- to b-quartz phase transition, both Vp and Vp/Vs initially drop dramatically but then increase relative to the a-quartz regime because Young's modulus initially decreases by 30% before increasing by a net ~20%. Shear modulus varies only ~3% across the transition. Crustal structure is commonly represented by an upper, mid- and lower layer (e.g., Crust1.0) and conceptualized as primarily reflecting a change to more mafic lithology at greater depth, but estimates of Moho temperature indicate a quartz phase transition should be present in much of the western and central U.S. We have imaged multiple layering of the contiguous U.S. by applying a new cross-correlation and stacking method to USArray receiver functions. Synthetic models of a multiple layer crust indicate 'splitting' of converted-phase arrivals would be expected if a quartz phase transition were responsible. Preliminary imaging using cross-correlation of observed receiver functions with multiple layer synthetics demonstrates a marked improvement in correlation coefficients relative to a single-layer crust. In this presentation we will examine observational evidence for possible a- to b- phase transition layering (indicating quartz at depth) and compare with depths predicted for the quartz phase transition based on Pn-derived Moho temperatures and estimates of magnetic Curie depths.

Velocity model of the crust and upper mantle at the southern margin of the East European Craton (Azov Sea-Crimea-Black Sea area), DOBRE-2 & DOBRE'99 transect

NASA Astrophysics Data System (ADS)

Starostenko, Vitaly; Janik, Tomasz; Stephenson, Randell; Gryn, Dmytro; Tolkunov, Anatoliy; Czuba, Wojciech; Środa, Piotr; Sydorenko, Grigoriy; Lysynchuk, Dmytro; Omelchenko, Victor; Grad, Marek; Guterch, Aleksander; Kolomiyets, Katerina; Thybo, Hans; Dannowski, Anke; Flűh, Ernst R.; Legostaeva, Olga

2013-04-01

The southern part of the eastern European continental landmass consists mainly of a thick platform of Vendian and younger sediments overlying Precambrian basement, part of the East European Craton (EEC). The Scythian Platform (SP) lies between the EEC and the (mainly Alpine) deformed belt running from Dobrudja (Romania) to Crimea (Ukraine) and the Greater Caucasus (Russia), along the northern margin of the Black Sea. Hard constraints on the Palaeozoic history on the SP are very sparse and little is known of its crustal structure in this area. The poster presents the seismic results of a multidisciplinary project that fills some of this gap. The project is called DOBRE-2 (as it forms a prolongation of the successful DOBRE project executed in 1999-2001). The main objectives of DOBRE-2 were to elucidate the deep-seated structure of the lithosphere and geodynamic setting of the shelf zones of the Azov and Black seas and the Crimean peninsula and to study the deep controls on the structure of basement and sedimentary cover. DOBRE-2 traverses a number of major faults and suture zones separating the EEC from the SP, the Crimean Mountains, and the Black Sea depression. Significant hydrocarbon reserves occur in the basins traversed by DOBRE-2. Deep seismic reflection profiling (30 second, Vibroseis) has been completed on a 100-km segment of the profile on the Azov massif (part of the Ukrainian Shield) as well as a 47-km segment in Crimea. These are complemented by refraction profiling on the shelf zones of the Azov (~53 km) and Black (~160 km) seas and coincident near-vertical (CDP) in the Black Sea, using a combination of onshore seismograph stations, ocean-bottom seismometers, onshore explosive energy sources (6 shot points), as well as ship-borne seismic acquisition. We present a 2-D seismic velocity model (Vp in the crust, depth to the Moho and depth to the intracrustal reflectors) along (~780 km) the DOBRE-2 & DOBRE'99 transect. Our model extends the model published for the DOBRE'99 profile (The DOBREfraction'99 Working Group, 2003) to the southwest. The Moho dips in this direction, from a depth of 40 km below the Azov Sea to ~47 km, below Crimea. A short segment of a reflector interpreted to represent Moho was detected at a depth of ~37 km in the Black Sea part of the profile. We also present a comparison of the DOBRE-2 velocity model with an interpretation of a coincident CDP profile.

A receiver function investigation of the Lithosphere beneath Southern California using Wavefield Iterative Deconvolution(WID)

NASA Astrophysics Data System (ADS)

Ainiwaer, A.; Gurrola, H.

2017-12-01

In traditional Ps receiver functions (RFs) imaging, PPs and PSs phases from the shallow layers (near surface and crust) can be miss stacked as Ps phases or interfere with deeper Ps phases. To overcome interference between phases, we developed a method to produce phase specific Ps, PPs and PSs receiver functions (wavefield iterative deconvolution or WID). Rather than preforming a separate deconvolution of each seismogram recorded at a station, WID processes all the seismograms from a seismic station in a single run. Each iteration of WID identifies the most prominent phase remaining in the data set, based on the shape of its wavefield (or moveout curve), and then places this phase on the appropriate phase specific RF. As a result, we produce PsRFs that are free of PPs and PSs phase; and reverberations thereof. We also produce phase specific PPsRFs and PSsRFs but moveout curves for these phases and their higher order reverberations are not as distinct from one another. So the PPsRFs and the PSsRFs are not as clean as the PsRFs. These phase specific RFs can be stacked to image 2-D or 3-D Earth structure using common conversion point (CCP) stacking or migration. We applied WID to 524 Southern California seismic stations to construct 3-D PsRF image of lithosphere beneath southern California. These CCP images exhibit a Ps phases from the Moho and the lithosphere asthenosphere boundary (LAB) that are free of interference from the crustal reverberations. The Moho and LAB were found to be deepest beneath the Sierra Nevada, Tansverse Range and Peninsular Range. Shallow Moho and Lab is apparent beneath the Inner Borderland and Salton Trough. The LAB depth that we estimate is in close agreement to recent published results that used Sp imaging (Lekic et al., 2011). We also found complicated structure beneath Mojave Block where mid crustal features are apparent and anomalous Ps phases at 60 km depth are observed beneath Western Mojave dessert.

Crustal and uppermost mantle structures of the South China from joint analysis of receiver functions and Rayleigh wave dispersions

NASA Astrophysics Data System (ADS)

Guo, Zhi; Gao, Xing; Li, Tong; Wang, Wei

2018-05-01

We use P-wave receiver function H-k stacking and joint inversion of receiver functions and Rayleigh wave dispersions to investigate crustal and uppermost mantle structure beneath the South China. The obtained results reveal prominent crustal structure variations in the study area, Moho depth increases from ∼30 km in the Cathaysia Block to more than ∼60 km in the eastern Tibetan Plateau. A Moho undulation and Vp/Vs ratio variations can be observed from the Cathaysia Block to Yangtze Craton. These observations consistent with the crustal structures predict by the flat slab subduction model. We interpret these lateral crustal structure variations reflect the tectonic evolution of the Yangtze Craton and Cathaysia Block prior the Mesozoic and the post-orogenic magmatism due to the breaking up of the subducted flat slab and subsequent slab rollback in the South China. The observed variations of the crustal structures not only reveal the lateral crustal inhomogeneity, but also provide constraints on the geodynamic evolution of the South China.

Receiver Function Imaging of Crustal and Lithospheric Structure Beneath the Jalisco Block and Western Michoacan, Mexico.

NASA Astrophysics Data System (ADS)

Reyes Alfaro, G.; Cruz-Atienza, V. M.; Perez-Campos, X.; Reyes Dávila, G. A.

2014-12-01

We used a receiver function technique for imaging western Mexico, a unique area with several active seismic and volcanic zones like the triple junction of Rivera, Cocos and North American plates and the Colima volcano complex (CVC), the most active in Mexico. Clear images of the distribution of the crust and the lithosphere-asthenosphere boundary are obtained using P-to-S receiver functions (RF) from around ~80 broadband stations recorded by the Mapping the Rivera Subduction Zone (MARS), the Colima Volcano Deep Seismic Experiment (CODEX) and a local network (RESCO) that allowed us to considerably increase the teleseismic database used in the project. For imaging, we constructed several 2-D profiles of depth transformed RFs to delineate the seismic discontinuities of the region. Low seismic velocities associated with the Michoacan-Guanajuato and the Mascota-Ayutla-Tapalpa volcanic fields are also observed. Most impressive, a large and well delineated magma body 100 km underneath CVC is recognized along a surely related depression of the moho discontinuity just above it. We bring more tools for a better understanding of the deep processes that ultimately control eruptive behavior in the region.

A tentative 2D thermal model of central India across the Narmada-Son Lineament (NSL)

NASA Astrophysics Data System (ADS)

Rai, S. N.; Thiagarajan, S.

2006-12-01

This work deals with 2D thermal modeling in order to delineate the crustal thermal structure of central India along two Deep Seismic Sounding (DSS) profiles, namely Khajuriakalan-Pulgaon and Ujjan-Mahan, traversing the Narmada-Son-Lineament (NSL) in an almost north-south direction. Knowledge of the crustal structure and P-wave velocity distribution up to the Moho, obtained from DSS studies, has been used for the development of the thermal model. Numerical results reveal that the Moho temperature in this region of central India varies between 500 and 580 °C. The estimated heat flow density value is found to vary between 46 and 49 mW/m 2. The Curie depth varies between 40 and 42 km and is in close agreement with the Curie depth (40±4 km) estimated from the analysis of MAGSAT data. Based on the present work and previous work, it is suggested that the major part of peninsular India consisting of the Wardha-Pranhita Godavari graben/basin, Bastar craton and the adjoining region of the Narmada Son Lineament between profiles I and III towards the north and northwest of the Bastar craton are characterized with a similar mantle heat flow density value equal to ˜23 mW/m 2. Variation in surface heat flow density values in these regions are caused by variation in the radioactive heat production and fluid circulation in the upper crustal layer.

Integrating shear velocity observations of the Hudson Bay

NASA Astrophysics Data System (ADS)

Porritt, R. W.; Miller, M. S.; Darbyshire, F. A.

2013-12-01

Hudson Bay is the core of the Laurentia craton of North America. This region contains some of the thickest lithosphere globally, reaching 250-300 km depth. Previous studies have shown that much of this region is composed of amalgamated proto-continents including the Western Churchill and Superior provinces and that much of the structure of these constituents has been retained since the Trans-Hudson Orogen at 1.8 Ga. Using the Hudson Bay Lithospheric Experiment (HuBLE) and other permanent and POLARIS broadband seismic data, we image the region with S to P receiver functions, joint inversion of P to S receiver functions with surface waves, and teleseismic S and P wave travel-times. The receiver function imaging reveals a persistent mid-lithospheric layer at ~80 km depth under all stations, but a variable lithospheric thickness. The teleseismic S delay times show a pattern of early arrivals around the center of the network, beneath Hudson Bay where the lithosphere is thickest, while the P delay times are early in the Superior province relative to the Western Churchill province. This suggests higher Vp/Vs ratios in the Superior province, which is evidence that stacked oceanic plates formed this province. The relatively flat Moho imaged by earlier receiver function studies and the lower mantle Vp/Vs of the Western Churchill province provides evidence of formation by plume head extraction. The joint inversion shows an LAB that is typically a broad discontinuity spanning ~20-30 km at ~220 km depth suggesting a primarily thermal boundary zone. The mid-lithospheric layer is composed of increasing velocity from the ~40 km depth Moho defined by H-k stacking of PRFs to a broad, constant velocity lithospheric lid spanning 80-200 km depth. We suggest this mid-lithospheric layer represents the mantle lithosphere of the proto-continents prior to collision and the lid formed due to post-collisional cooling. The integration of these seismic datasets furthers our understanding of plate tectonic and non-tectonic processes during the Archean formation of Laurentia craton.

Seismic Tomography Of The Caucasus Region

NASA Astrophysics Data System (ADS)

Javakhishvili, Z.; Godoladze, T.; Gok, R.; Elashvili, M.

2007-12-01

The Caucasus is one of the most active segments of the Alpine-Himalayan collision belt. We used the catalog data of Georgian Seismic Network to calculate the reference 1-D and 3-D P-velocity model of the Caucasus region. The analog recording period in Georgia was quite long and 17,000 events reported in the catalog between 1956 and 1990. We carefully eliminated some arrivals due to ambiguities for analog type data picking and station time corrections. We choose arrivals with comparably low residuals between observed and calculated travel times (<1 sec). We also limited our data to minimum 10 P-arrivals and maximum azimuthal gap of 180 degrees. Finally,475 events were selected with magnitude greater than 1.5 recorded by 84 stations. We obtained good resolution down to 70 km. First, we used 1-D coupled inversion algorithm (VELEST) to calculate the velocity model and the relocations. The same model convergence is observed for the mid and lower crust. The upper layer (0-10km) is observed to be sensitive to the starting model. We used vertical seismic prospecting data from boreholes in Georgia to fix upper layer velocities. We relocated all events in the region using the new reference 1- D velocity model. The 3-D coupled inversion algorithm (SIMULPS14) was applied using the 1-D reference model as a starting model. We observed very large amount of shift at horizontal directions (up to 50 km). We observed clustered events where they are well correlated with query blasts from Tkibuli mining area. We applied the resolution test to estimate the spatial resolution of the tomographic images. The results of the test indicate that the initial model is well reconstructed for all depth slices, though it is badly reconstructed for the shallowest layer (with depth = 5km). The Moho geometry beneath Caucasus has been determined reliably by the previous geophysical studies. It has a relatively large depth variation in this region from 28 to 61 km depth, according to those studies and our tomography result for the uppermost mantle (50 km) reflects this depth variation of the Moho discontinuity.

Probabilistic surface reconstruction from multiple data sets: An example for the Australian Moho

NASA Astrophysics Data System (ADS)

Bodin, T.; Salmon, M.; Kennett, B. L. N.; Sambridge, M.

2012-10-01

Interpolation of spatial data is a widely used technique across the Earth sciences. For example, the thickness of the crust can be estimated by different active and passive seismic source surveys, and seismologists reconstruct the topography of the Moho by interpolating these different estimates. Although much research has been done on improving the quantity and quality of observations, the interpolation algorithms utilized often remain standard linear regression schemes, with three main weaknesses: (1) the level of structure in the surface, or smoothness, has to be predefined by the user; (2) different classes of measurements with varying and often poorly constrained uncertainties are used together, and hence it is difficult to give appropriate weight to different data types with standard algorithms; (3) there is typically no simple way to propagate uncertainties in the data to uncertainty in the estimated surface. Hence the situation can be expressed by Mackenzie (2004): "We use fantastic telescopes, the best physical models, and the best computers. The weak link in this chain is interpreting our data using 100 year old mathematics". Here we use recent developments made in Bayesian statistics and apply them to the problem of surface reconstruction. We show how the reversible jump Markov chain Monte Carlo (rj-McMC) algorithm can be used to let the degree of structure in the surface be directly determined by the data. The solution is described in probabilistic terms, allowing uncertainties to be fully accounted for. The method is illustrated with an application to Moho depth reconstruction in Australia.

Crustal Thickness and Structure in Southern Chile: Patagonia plate assembly structures and continental arc modifications

NASA Astrophysics Data System (ADS)

Rodriguez, E. E.; Russo, R. M.

2016-12-01

Crustal structure is the product of the processes that operated during a region's tectonic history. For Patagonia, these tectonic processes include its early Paleozoic assembly and accretion to the South America portion of Gondwana, Triassic rifting of Gondwana, and a long history as the upper plate during oceanic subduction since the Mesozoic. To assess the crustal structure and glean insight into how these tectonic processes affected the region, we combined data from two seismic networks, the Chile Ridge Subduction Project and Seismic Experiment of Aisen Chile - yielding a total of 77 broadband seismic stations - deployed from 2004 to 2007. The stations were concentrated 300 km inboard of the Chile trench, above structures unlikely to have been affected by ongoing Chile Ridge subduction. Events suitable for receiver function (RF) analyses (M > 5.9, of various backazimuths, epicentral distances of 30 - 90°) yielded 995 radial RFs, constructed using iterative time deconvolution (Ligorria and Ammon, 1999). We estimated crustal thicknesses and compressional to shear wave velocity ratios (Vp/Vs) using the H-k grid search method (Zhu and Kanamori, 2000); common conversion point (CCP) stacking (Zhu, et al., 2006) allowed imaging of crustal structure. Results limit crustal thicknesses to between 30 and 45 km. The crust varies smoothly from 30 km at the N margin of our study area ( 43°S) to a max depth of 45 km at 44.75°S, shallowing to 30 km at 49°S. On E-W CCP sections north of 46°S, the Moho dips westward, from a depth of 35 at 71°W to 45 km at its deepest near 72.75°W. Beneath the active Southern Volcanic Zone, which is bounded to the west by the Liquiñe-Ofqui fault, the Moho is ambiguous, producing unclear Ps phases possibly reflecting a lack of sharp impedance contrast or poor conversion efficiency at the base of the crust, perhaps due to deep-seated volcanic arc processes. The proximity of the Liquiñe-Ofqui strike-slip fault may also complicate the expected velocity discontinuity at the Moho by juxtaposing crustal blocks of different thicknesses. We also observe an extensive, undulating mid-crustal converter between 12-20 km depth. Peaks and troughs of this surface strike E-W, implying that the surface may have formed during N-S crustal shortening. If so, this surface likely formed during Paleozoic assembly of Patagonia.

Depth to the bottom of magnetic sources (DBMS) from aeromagnetic data of Central India using modified centroid method for fractal distribution of sources

NASA Astrophysics Data System (ADS)

Bansal, A. R.; Anand, S. P.; Rajaram, Mita; Rao, V. K.; Dimri, V. P.

2013-09-01

The depth to the bottom of the magnetic sources (DBMS) has been estimated from the aeromagnetic data of Central India. The conventional centroid method of DBMS estimation assumes random uniform uncorrelated distribution of sources and to overcome this limitation a modified centroid method based on scaling distribution has been proposed. Shallower values of the DBMS are found for the south western region. The DBMS values are found as low as 22 km in the south west Deccan trap covered regions and as deep as 43 km in the Chhattisgarh Basin. In most of the places DBMS are much shallower than the Moho depth, earlier found from the seismic study and may be representing the thermal/compositional/petrological boundaries. The large variation in the DBMS indicates the complex nature of the Indian crust.

Crustal Models Assessment in Western Part of Romania Employing Active Seismic and Seismologic Methods

NASA Astrophysics Data System (ADS)

Bala, Andrei; Toma-Danila, Dragos; Tataru, Dragos; Grecu, Bogdan

2017-12-01

In the years 1999 - 2000 two regional seismic refraction lines were performed within a close cooperation with German partners from University of Karlsruhe. One of these lines is Vrancea 2001, with 420 km in length, almost half of them recorded in Transylvanian Basin. The structure of the crust along the seismic line revealed a very complicated crustal structure beginning with Eastern Carpathians and continuing in the Transylvanian Basin until Medias. As a result of the development of the National Seismic Network in the last ten years, more than 100 permanent broadband stations are now continuously operating in Romania. Complementary to this national dataset, maintained and developed in the National Institute for Earth Physics, new data emerged from the temporary seismologic networks established during the joint projects with European partners in the last decades. The data gathered so far is valuable both for seismology purposes and crustal structure studies, especially for the western part of the country, where this kind of data were sparse until now. Between 2009 and 2011, a new reference model for the Earth’s crust and mantle of the European Plate was defined through the NERIES project from existing data and models. The database gathered from different kind of measurements in Transylvanian Basin and eastern Pannonian Basin were included in this NERIES model and an improved and upgraded model of the Earth crust emerged for western part of Romania. Although the dataset has its origins in several periods over the last 50 years, the results are homogeneous and they improve and strengthen our image about the depth of the principal boundaries in the crust. In the last chapter two maps regarding these boundaries are constructed, one for mid-crustal boundary and one for Moho. They were build considering all the punctual information available from different sources in active seismic and seismology which are introduced in the general maps from the NERIES project for Romania. The depths maps in the study region are presented with all their regional peculiarities as they appear, projected on the local tectonic structure for the area under examination.

Continental Evolution Involving Subduction Underplating and Synchronous Foreland Thrusting: Evidence from the Trans-Alaska Crustal Transect

NASA Astrophysics Data System (ADS)

Fuis, G. S.; Moore, T. E.; Plafker, G.; Brocher, T. M.; Fisher, M. A.; Mooney, W. D.; Nokleberg, W. J.; Page, R. A.; Beaudoin, B. C.; Christensen, N. I.; Levander, A.; Lutter, W. J.; Saltus, R. W.; Ruppert, N. A.

2010-12-01

We investigated the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic underplating. In the 1980’s and early 1990’s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-long corridor from the Aleutian Trench to the Arctic coast. The most distinctive crustal structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, we infer a stack of tectonically underplated oceanic layers interpreted to be remnants of the extinct Kula (or Resurrection) Plate. Continental Moho just north of this underplated stack is more than 55 km deep. Near the Arctic margin, the Brooks Range is underlain by north-vergent, crustal-scale duplexes that overlie a ramp on autochthonous North Slope crust. There, Moho has been depressed to nearly 50-km depth. In contrast, the Moho of central Alaska is on average 32 km deep. In the Paleogene, tectonic underplating of Kula- (or Resurrection-) Plate fragments overlapped in time with duplexing in the Brooks Range. Possible tectonic models linking these two widely separated regions include “flat-slab” subduction and an “orogenic-float” model. In the Neogene, the collision of the Yakutat terrane (YAK), in southern Alaska, correlates with renewed compression in northeast Alaska and northwest Canada, in a fashion somewhat similar to the tectonics in the Paleogene. The Yakutat terrane, riding atop the subducting Pacific oceanic lithosphere (POL), spans a newly interpreted tear in the POL. East of the tear, POL is interpreted to subduct steeply and alone beneath the Wrangell arc volcanoes because the overlying YAK has been left behind as tectonically underplated rocks beneath the rising St. Elias Range in the coastal region. West of the tear, the YAK and POL are interpreted to subduct together at a gentle angle (a few degrees from 0 to 400 km from the trench), and this thickened package inhibits arc volcanism.

Variations in the Crust-Mantle Transition Beneath the Andean Cordillera and Implications for Orogenic Processes.

NASA Astrophysics Data System (ADS)

Koch, C.; Isaacs, D.; Delph, J. R.; Beck, S. L.

2017-12-01

The South American Andes, generated along an active oceanic-continental convergent margin between the Nazca and South American plates, make up the world's longest arc and encompass the second highest orogenic plateau on Earth. Along-strike variations in shortening, slab subduction angle, and volcanism, along with other tectonic processes, have created extraordinarily complex topography, crustal thickness, and compositional variations reflected in the seismic characteristics of the region. Ps receiver functions (PRFs) have been widely used to investigate the Andes, and these studies provide a wealth of information regarding the structure of the Andean crust and the continental Moho beneath the orogen. However, these studies have focused largely on individual networks or latitudinal segments of the Andes, and a regional-scale model that combines all available data has yet to be analyzed, hence it is hard to compare the amplitudes of conversions at the major discontinuities. This study compiles and analyzes all available data from permanent and temporary seismic networks from (1989-2017) to create a continuous, high spatial resolution common conversion point (CCP) volume for the Andes. In total, receiver functions were calculated for over 1500 seismic stations in the Andes, enabling us to obtain high-resolution, regional-scale CCP images of the continental Moho beneath the Andes from Colombia to southern Chile. The resulting CCP volume shows strong lateral variations in P-to-S conversion amplitudes at the base of the crust, indicating a complex and variable crust-mantle transition. In some places, the back-arc of the central Andes is characterized by relatively thick crust (60 - 75 km) and a broad, low amplitude Moho conversion indicative of a gradational Moho possibly due to the eclogitization of the lower crust. Combined with other geophysical data, this may suggest these are sites of ongoing delamination in the central Andes. Additionally, in the central Andes, beneath the interior plateau, parts of the active arc and backarc, we image a pervasive, relatively shallow (15-25 km depth), large-amplitude negative P-to-S converter that exhibits variations in amplitude and structure along strike, likely corresponding to the top of the Andean low velocity zone.

A probabilistic seismic model for the European Arctic

NASA Astrophysics Data System (ADS)

Hauser, Juerg; Dyer, Kathleen M.; Pasyanos, Michael E.; Bungum, Hilmar; Faleide, Jan I.; Clark, Stephen A.; Schweitzer, Johannes

2011-01-01

The development of three-dimensional seismic models for the crust and upper mantle has traditionally focused on finding one model that provides the best fit to the data while observing some regularization constraints. In contrast to this, the inversion employed here fits the data in a probabilistic sense and thus provides a quantitative measure of model uncertainty. Our probabilistic model is based on two sources of information: (1) prior information, which is independent from the data, and (2) different geophysical data sets, including thickness constraints, velocity profiles, gravity data, surface wave group velocities, and regional body wave traveltimes. We use a Markov chain Monte Carlo (MCMC) algorithm to sample models from the prior distribution, the set of plausible models, and test them against the data to generate the posterior distribution, the ensemble of models that fit the data with assigned uncertainties. While being computationally more expensive, such a probabilistic inversion provides a more complete picture of solution space and allows us to combine various data sets. The complex geology of the European Arctic, encompassing oceanic crust, continental shelf regions, rift basins and old cratonic crust, as well as the nonuniform coverage of the region by data with varying degrees of uncertainty, makes it a challenging setting for any imaging technique and, therefore, an ideal environment for demonstrating the practical advantages of a probabilistic approach. Maps of depth to basement and depth to Moho derived from the posterior distribution are in good agreement with previously published maps and interpretations of the regional tectonic setting. The predicted uncertainties, which are as important as the absolute values, correlate well with the variations in data coverage and quality in the region. A practical advantage of our probabilistic model is that it can provide estimates for the uncertainties of observables due to model uncertainties. We will demonstrate how this can be used for the formulation of earthquake location algorithms that take model uncertainties into account when estimating location uncertainties.

Crustal structure of the Kaapvaal craton and its significance for early crustal evolution

NASA Astrophysics Data System (ADS)

James, David E.; Niu, Fenglin; Rokosky, Juliana

2003-12-01

High-quality seismic data obtained from a dense broadband array near Kimberley, South Africa, exhibit crustal reverberations of remarkable clarity that provide well-resolved constraints on the structure of the lowermost crust and Moho. Receiver function analysis of Moho conversions and crustal multiples beneath the Kimberley array shows that the crust is 35 km thick with an average Poisson's ratio of 0.25. The density contrast across the Moho is ˜15%, indicating a crustal density about 2.86 gm/cc just above the Moho, appropriate for felsic to intermediate rock compositions. Analysis of waveform broadening of the crustal reverberation phases suggests that the Moho transition can be no more than 0.5 km thick and the total variation in crustal thickness over the 2400 km 2 footprint of the array no more than 1 km. Waveform and travel time analysis of a large earthquake triggered by deep gold mining operations (the Welkom mine event) some 200 km away from the array yield an average crustal thickness of 35 km along the propagation path between the Kimberley array and the event. P- and S-wave velocities for the lowermost crust are modeled to be 6.75 and 3.90 km/s, respectively, with uppermost mantle velocities of 8.2 and 4.79 km/s, respectively. Seismograms from the Welkom event exhibit theoretically predicted but rarely observed crustal reverberation phases that involve reflection or conversion at the Moho. Correlation between observed and synthetic waveforms and phase amplitudes of the Moho reverberations suggests that the crust along the propagation path between source and receiver is highly uniform in both thickness and average seismic velocity and that the Moho transition zone is everywhere less than about 2 km thick. While the extremely flat Moho, sharp transition zone and low crustal densities beneath the region of study may date from the time of crustal formation, a more geologically plausible interpretation involves extensive crustal melting and ductile flow during the major craton-wide Ventersdorp tectonomagmatic event near the end of Archean time.

Färoe-Iceland Ridge Experiment: 1. Crustal structure of northeastern Iceland

USGS Publications Warehouse

Staples, Robert K.; White, Robert S.; Brandsdottir, Bryndis; Menke, William; Maguire, Peter K.H.; McBride, John H.

1997-01-01

Results from the Färoe-Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s−1). A Poisson's ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poisson's ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (<800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.

Investigating Segmentation in Cascadia: Anisotropic Crustal Structure and Mantle Wedge Serpentinization from Receiver Functions

NASA Astrophysics Data System (ADS)

Krueger, Hannah E.; Wirth, Erin A.

2017-10-01

The Cascadia subduction zone exhibits along-strike segmentation in structure, processes, and seismogenic behavior. While characterization of seismic anisotropy can constrain deformation processes at depth, the character of seismic anisotropy in Cascadia remains poorly understood. This is primarily due to a lack of seismicity in the subducting Juan de Fuca slab, which limits shear wave splitting and other seismological analyses that interrogate the fine-scale anisotropic structure of the crust and mantle wedge. We investigate lower crustal anisotropy and mantle wedge structure by computing P-to-S receiver functions at 12 broadband seismic stations along the Cascadia subduction zone. We observe P-to-SV converted energy consistent with previously estimated Moho depths. Several stations exhibit evidence of an "inverted Moho" (i.e., a downward velocity decrease across the crust-mantle boundary), indicative of a serpentinized mantle wedge. Stations with an underlying hydrated mantle wedge appear prevalent from northern Washington to central Oregon, but sparse in southern Oregon and northern California. Transverse component receiver functions are complex, suggesting anisotropic and/or dipping crustal structure. To constrain the orientation of crustal anisotropy we compute synthetic receiver functions using manual forward modeling. We determine that the lower crust shows variable orientations of anisotropy along-strike, with highly complex anisotropy in northern Cascadia, and generally NW-SE and NE-SW orientations of slow-axis anisotropy in central and southern Cascadia, respectively. The orientations of anisotropy from this work generally agree with those inferred from shear wave splitting of tremor studies at similar locations, lending confidence to this relatively new method of inferring seismic anisotropy from slow earthquakes.

Pn tomography with Moho depth correction from eastern Europe to western China

NASA Astrophysics Data System (ADS)

Lü, Yan; Ni, Sidao; Chen, Ling; Chen, Qi-Fu

2017-02-01

We proposed a modified Pn velocity and anisotropy tomography method by considering the Moho depth variations using the Crust 1.0 model and obtained high-resolution images of the uppermost mantle Pn velocity and anisotropy structure from eastern Europe to western China. The tomography results indicate that the average Pn velocities are approximately 8.0 and 8.1 km/s under the western and eastern parts of the study area, respectively, with maximum velocity perturbations of 3%-4%. We observed high Pn velocities under the Adriatic Sea, Black Sea, Caspian Sea, Arabian Plate, Indian Plate, and in the Tarim and Sichuan Basins but low Pn velocities under the Apennine Peninsula, Dead Sea fault zone, Anatolia, Caucasus, Iranian Plateau, Hindu Kush, and in the Yunnan and Myanmar regions. Generally, regions with stable structures and low lithospheric temperatures exhibit high Pn velocities. Low Pn velocities provide evidence for the upwelling of hot material, which is associated with plate subduction and continental collision processes. Our Pn velocity and anisotropy imaging results indicate that the Adriatic microplate dives to the east and west, the hot material upwelling caused by subduction beneath the Tibetan Plateau is not as significant as that in the Caucasus and Myanmar regions, the lithosphere exhibits coupled rotational movement around the Eastern Himalayan syntaxes, and the areas to the north and south of 26°N in the Yunnan region are affected by different geodynamic processes. Our newly captured images of the uppermost mantle velocity and anisotropy structure provide further information about continental collision processes and associated dynamic mechanisms.

An inverted continental Moho and serpentinization of the forearc mantle.

PubMed

Bostock, M G; Hyndman, R D; Rondenay, S; Peacock, S M

2002-05-30

Volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge, resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere. Asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back-arc region, but the forearc mantle (in the corner of the wedge) is thought to be significantly cooler. Here we explore the structure of the mantle wedge in the southern Cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers. We find very low shear-wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an 'inverted' continental Moho, which reverts to normal polarity seaward of the Cascade arc. This observation provides compelling evidence for a highly hydrated and serpentinized forearc region, consistent with thermal and petrological models of the forearc mantle wedge. This serpentinized material is thought to have low strength and may therefore control the down-dip rupture limit of great thrust earthquakes, as well as the nature of large-scale flow in the mantle wedge.

The lithospheric Structure of the Sahara Metacraton From Joint Analysis of Satellite Gravity Gradients and Seismological Data

NASA Astrophysics Data System (ADS)

Sobh, M.; Ebbing, J.; Goetze, H. J.; Abdelsalam, M. G.

2016-12-01

For the Saharan Metacraton in northern Africa only a few geophysical results exists, which can be used to characterize its deep structure. We combine recent seismological models with satellite gravity gradients to build a 3D lithospheric density model of the metacraton and its surrounding regions. Due to the sparse distribution of seismic data, we estimate the Moho boundary by non-linear gravity inversion in spherical coordinates. The model is constrained by some wide angle refraction seismic profiles and receiver function Moho depths. Despite the high topography of the Darfur and Tibisti Cenozoic volcanic provinces, we estimate thin crust which indicates an upper mantle contribution to the isostatic balance. In combination with seismic tomography models, we found that the lithospheric thickness in the western part of the Metacraton is thicker than in the eastern part. This indicates that the western resembles the remnants of the pre-Neoproterozoic Sahara craton (e.g. the Marzuk craton which escaped the metacratonization process). In order to explain the partial loss of the expected cratonic root beneath the Metacraton, we present different petrological-geophysical scenario testing for different upper mantle compositions.

Ophiolitic basement to the Great Valley forearc basin, California, from seismic and gravity data: Implications for crustal growth at the North American continental margin

USGS Publications Warehouse

Godfrey, N.J.; Beaudoin, B.C.; Klemperer, S.L.; Levander, A.; Luetgert, J.; Meltzer, A.; Mooney, W.; Tréhu, A.

1997-01-01

The nature of the Great Valley basement, whether oceanic or continental, has long been a source of controversy. A velocity model (derived from a 200-km-long east-west reflection-refraction profile collected south of the Mendocino triple junction, northern California, in 1993), further constrained by density and magnetic models, reveals an ophiolite underlying the Great Valley (Great Valley ophiolite), which in turn is underlain by a westward extension of lower-density continental crust (Sierran affinity material). We used an integrated modeling philosophy, first modeling the seismic-refraction data to obtain a final velocity model, and then modeling the long-wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model. The crustal section of Great Valley ophiolite is 7-8 km thick, and the Great Valley ophiolite relict oceanic Moho is at 11-16 km depth. The Great Valley ophiolite does not extend west beneath the Coast Ranges, but only as far as the western margin of the Great Valley, where the 5-7-km-thick Great Valley ophiolite mantle section dips west into the present-day mantle. There are 16-18 km of lower-density Sierran affinity material beneath the Great Valley ophiolite mantle section, such that a second, deeper, "present-day" continental Moho is at about 34 km depth. At mid-crustal depths, the boundary between the eastern extent of the Great Valley ophiolite and the western extent of Sierran affinity material is a near-vertical velocity and density discontinuity about 80 km east of the western margin of the Great Valley. Our model has important implications for crustal growth at the North American continental margin. We suggest that a thick ophiolite sequence was obducted onto continental material, probably during the Jurassic Nevadan orogeny, so that the Great Valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle.

Mid-Ocean Ridge Melt Supply and Glacial Cycles: A 3D EPR Study of Crustal Thickness, Layer 2A, and Bathymetry

NASA Astrophysics Data System (ADS)

Boulahanis, B.; Aghaei, O.; Carbotte, S. M.; Huybers, P. J.; Langmuir, C. H.; Nedimovic, M. R.; Carton, H. D.; Canales, J. P.

2017-12-01

Recent studies suggest that eustatic sea level fluctuations induced by glacial cycles in the Pleistocene may influence mantle-melting and volcanic eruptions at mid-ocean ridges (MOR), with models predicting variation in oceanic crustal thickness linked to sea level change. Previous analyses of seafloor bathymetry as a proxy for crustal thickness show significant spectral energy at frequencies linked to Milankovitch cycles of 1/23, 1/41, and 1/100 ky-1, however the effects of faulting in seafloor relief and its spectral characteristics are difficult to separate from climatic signals. Here we investigate the hypothesis of climate driven periodicity in MOR magmatism through spectral analysis, time series comparisons, and statistical characterization of bathymetry data, seismic layer 2A thickness (as a proxy for extrusive volcanism), and seafloor-to-Moho thickness (as a proxy for total magma production). We utilize information from a three-dimensional multichannel seismic study of the East Pacific Rise and its flanks from 9°36`N to 9°57`N. We compare these datasets to the paleoclimate "LR04" benthic δ18O stack. The seismic dataset covers 770 km2 and provides resolution of Moho for 92% of the imaged region. This is the only existing high-resolution 3-D image across oceanic crust, making it ideal for assessing the possibility that glacial cycles modulate magma supply at fast spreading MORs. The layer 2A grid extends 9 km (170 ky) from the ridge axis, while Moho imaging extends to a maximum of 16 km (310 ky). Initial results from the East Pacific Rise show a relationship between sea level and both crustal thickness and sea floor depth, consistent with the hypothesis that magma supply to MORs may be modulated by glacial cycles. Analysis of crustal thickness and bathymetry data reveals spectral peaks at Milankovitch frequencies of 1/100 ky-1 and 1/41 ky-1 where datasets extend sufficiently far from the ridge. The layer 2A grid does not extend sufficiently far from the ridge to be conclusive. Correlations between sea level and crustal thickness suggest a lag of 65 ky between sea level forcing and crustal thickness response. A further lag of 25 ky is observed between crustal thickness variations and seafloor depth change, which we attribute to the finite width of the crustal formation zone.

Crustal and mantle structure beneath the Terre Adélie Craton, East Antarctica: insights from receiver function and seismic anisotropy measurements

NASA Astrophysics Data System (ADS)

Lamarque, Gaëlle; Barruol, Guilhem; Fontaine, Fabrice R.; Bascou, Jérôme; Ménot, René-Pierre

2015-02-01

The Terre Adélie and George V Land (East Antarctica) represent key areas for understanding tectonic relationships between terranes forming the Neoarchean-Palaeoproterozoic Terre Adélie Craton (TAC) and the neighbouring lithospheric blocks, together with the nature of its boundary. This region that represents the eastern border of the TAC is limited on its eastern side by the Mertz shear zone (MSZ) separating more recent Palaeozoic units from the craton. The MSZ, that recorded dextral strike-slip movement at 1.7 and 1.5 Ga, is likely correlated with the Kalinjala or Coorong shear zone in South Australia, east of the Gawler Craton and may therefore represent a frozen lithospheric-scale structure. In order to investigate the lithospheric structure of the TAC and the MSZ, we deployed from 2009 October to 2011 October four temporary seismic stations, which sampled the various lithospheric units of the TAC and of the neighbouring Palaeozoic block, together with the MSZ. We used receiver function method to deduce Moho depths and seismic anisotropy technique to infer the upper mantle deformation. Results from receiver functions analysis reveal Moho at 40-44 km depth beneath the TAC, at 36 km under the MSZ and at 28 km beneath the eastern Palaeozoic domain. The MSZ therefore delimits two crustal blocks of different thicknesses with a vertical offset of the Moho of 12 km. Seismic anisotropy deduced from SKS splitting at stations on the TAC shows fast polarisation directions (Φ) trending E-W, that is, parallel to the continental margin, and delay times (δt) ranging from 0.8 to 1.6 s. These results are similar to the splitting parameters observed at the permanent GEOSCOPE Dumont D'Urville station (DRV: Φ 95°N, δt 1.1 s) located in the Palaeoproterozoic domain of TAC. On the MSZ, the small number of good quality measurements limits the investigation of the deep signature of the shear zone. However, the station in the Palaeozoic domain shows Φ trending N60°E, which is significantly different to the Φ trending measurements from stations on the TAC, suggesting that the MSZ may also represent a major frontier between the Neoarchean-Palaeoproterozoic and Palaeozoic terranes.

Crustal and upper mantle velocity model along the DOBRE-4 profile from North Dobruja to the central region of the Ukrainian Shield: 1. seismic data

NASA Astrophysics Data System (ADS)

Starostenko, V. I.; Janik, T.; Gintov, O. B.; Lysynchuk, D. V.; Środa, P.; Czuba, W.; Kolomiyets, E. V.; Aleksandrowski, P.; Omelchenko, V. D.; Komminaho, K.; Guterch, A.; Tiira, T.; Gryn, D. N.; Legostaeva, O. V.; Thybo, G.; Tolkunov, A. V.

2017-03-01

For studying the structure of the lithosphere in southern Ukraine, wide-angle seismic studies that recorded the reflected and refracted waves were carried out under the DOBRE-4 project. The field works were conducted in October 2009. Thirteen chemical shot points spaced 35-50 km apart from each other were implemented with a charge weight varying from 600 to 1000 kg. Overall 230 recording stations with an interval of 2.5 km between them were used. The high quality of the obtained data allowed us to model the velocity section along the profile for P- and S-waves. Seismic modeling was carried out by two methods. Initially, trial-and-error ray tracing using the arrival times of the main reflected and refracted P- and S-phases was conducted. Next, the amplitudes of the recorded phases were analyzed by the finite-difference full waveform method. The resulting velocity model demonstrates a fairly homogeneous structure from the middle to lower crust both in the vertical and horizontal directions. A drastically different situation is observed in the upper crust, where the V p velocities decrease upwards along the section from 6.35 km/s at a depth of 15-20 km to 5.9-5.8 km/s on the surface of the crystalline basement; in the Neoproterozoic and Paleozoic deposits, it diminishes from 5.15 to 3.80 km/s, and in the Mesozoic layers, it decreases from 2.70 to 2.30 km/s. The subcrustal V p gradually increases downwards from 6.50 to 6.7-6.8 km/s at the crustal base, which complicates the problem of separating the middle and lower crust. The V p velocities above 6.80 km/s have not been revealed even in the lowermost part of the crust, in contrast to the similar profiles in the East European Platform. The Moho is clearly delineated by the velocity contrast of 1.3-1.7 km/s. The alternating pattern of the changes in the Moho depths corresponding to Moho undulations with a wavelength of about 150 km and the amplitude reaching 8 to 17 km is a peculiarity of the velocity model.

The boundary between the Indian and Asian tectonic plates below Tibet

PubMed Central

Zhao, Junmeng; Yuan, Xiaohui; Liu, Hongbing; Kumar, Prakash; Pei, Shunping; Kind, Rainer; Zhang, Zhongjie; Teng, Jiwen; Ding, Lin; Gao, Xing; Xu, Qiang; Wang, Wei

2010-01-01

The fate of the colliding Indian and Asian tectonic plates below the Tibetan high plateau may be visualized by, in addition to seismic tomography, mapping the deep seismic discontinuities, like the crust-mantle boundary (Moho), the lithosphere-asthenosphere boundary (LAB), or the discontinuities at 410 and 660 km depth. We herein present observations of seismic discontinuities with the P and S receiver function techniques beneath central and western Tibet along two new profiles and discuss the results in connection with results from earlier profiles, which did observe the LAB. The LAB of the Indian and Asian plates is well-imaged by several profiles and suggests a changing mode of India-Asia collision in the east-west direction. From eastern Himalayan syntaxis to the western edge of the Tarim Basin, the Indian lithosphere is underthrusting Tibet at an increasingly shallower angle and reaching progressively further to the north. A particular lithospheric region was formed in northern and eastern Tibet as a crush zone between the two colliding plates, the existence of which is marked by high temperature, low mantle seismic wavespeed (correlating with late arriving signals from the 410 discontinuity), poor Sn propagation, east and southeast oriented global positioning system displacements, and strikingly larger seismic (SKS) anisotropy. PMID:20534567

The boundary between the Indian and Asian tectonic plates below Tibet.

PubMed

Zhao, Junmeng; Yuan, Xiaohui; Liu, Hongbing; Kumar, Prakash; Pei, Shunping; Kind, Rainer; Zhang, Zhongjie; Teng, Jiwen; Ding, Lin; Gao, Xing; Xu, Qiang; Wang, Wei

2010-06-22

The fate of the colliding Indian and Asian tectonic plates below the Tibetan high plateau may be visualized by, in addition to seismic tomography, mapping the deep seismic discontinuities, like the crust-mantle boundary (Moho), the lithosphere-asthenosphere boundary (LAB), or the discontinuities at 410 and 660 km depth. We herein present observations of seismic discontinuities with the P and S receiver function techniques beneath central and western Tibet along two new profiles and discuss the results in connection with results from earlier profiles, which did observe the LAB. The LAB of the Indian and Asian plates is well-imaged by several profiles and suggests a changing mode of India-Asia collision in the east-west direction. From eastern Himalayan syntaxis to the western edge of the Tarim Basin, the Indian lithosphere is underthrusting Tibet at an increasingly shallower angle and reaching progressively further to the north. A particular lithospheric region was formed in northern and eastern Tibet as a crush zone between the two colliding plates, the existence of which is marked by high temperature, low mantle seismic wavespeed (correlating with late arriving signals from the 410 discontinuity), poor Sn propagation, east and southeast oriented global positioning system displacements, and strikingly larger seismic (SKS) anisotropy.

Australian Seismological Reference Model (AuSREM): crustal component

NASA Astrophysics Data System (ADS)

Salmon, M.; Kennett, B. L. N.; Saygin, E.

2013-01-01

Although Australia has been the subject of a wide range of seismological studies, these have concentrated on specific features of the continent at crustal scales and on the broad scale features in the mantle. The Australian Seismological Reference Model (AuSREM) is designed to bring together the existing information, and provide a synthesis in the form of a 3-D model that can provide the basis for future refinement from more detailed studies. Extensive studies in the last few decades provide good coverage for much of the continent, and the crustal model builds on the various data sources to produce a representative model that captures the major features of the continental structure and provides a basis for a broad range of further studies. The model is grid based with a 0.5° sampling in latitude and longitude, and is designed to be fully interpolable, so that properties can be extracted at any point. The crustal structure is built from five-layer representations of refraction and receiver function studies and tomographic information. The AuSREM crustal model is available at 1 km intervals. The crustal component makes use of prior compilations of sediment thicknesses, with cross checks against recent reflection profiling, and provides P and S wavespeed distributions through the crust. The primary information for P wavespeed comes from refraction profiles, for S wavespeed from receiver function studies. We are also able to use the results of ambient noise tomography to link the point observations into national coverage. Density values are derived using results from gravity interpretations with an empirical relation between P wavespeed and density. AuSREM is able to build on a new map of depth to Moho, which has been created using all available information including Moho picks from over 12 000 km of full crustal profiling across the continent. The crustal component of AuSREM provides a representative model that should be useful for modelling of seismic wave propagation and calculation of crustal corrections for tomography. Other applications include gravity studies and dynamic topography at the continental scale.

Lithospheric studies along seismic profile KOKKY, between Gulf of Bothnia and Gulf of Finland, Baltic Shield

NASA Astrophysics Data System (ADS)

Tiira, Timo; Skrzynik, Tymon; Janik, Tomasz; Komminaho, Kari; Väkevä, Sakari; Korja, Annakaisa

2017-04-01

Controlled source seismology is one of the main tools used in Earth imaging, especially when aiming towards the middle and lower crust structures, Moho shape and upper mantle. Data for such studies are acquired during wide-angle reflection and refraction (WARR) profiles, which are hundreds of kilometers long and require strong explosive sources like e.g. TNT. Given the cost of such experiments, difficult logistics, and the strict regulation on experiments involving explosives in the ground, an attempt was made to register quarry blasts along the set profile. Quarries consume tons of explosive material per week and their utility in crustal studies was already tested during HUKKA experiment in 2007. Profile KOKKY begins on the coast of Bothnian Bay and runs south-east towards Sankt Petersburg in Russia. It crosses three main geological units of southern Finland, Pohjanmaa area, Central Finland Granitoid Complex, and Saimaa area, all represented by Late Proterozoic rocks. Two summer acquisition campaigns resulted in nearly 500 km long profile, crossing southern Finland. Numerous explosions from quarries were recorded, resulting in 63 usable seismic sections. Average distance between shots and the profile was 14 km. The velocity model has high P wave velocities across the entire profile. Upper crust, reaching depths of 22 km, is characterized by velocities from 5.9-6.2 km/s near surface down to 6.25-6.4 km/s. Middle crust is thinning from 30 km at NW, down to 14 km in the central part of the profile, thickening again to 20 km at SE, and has uniformed velocities 6.6-6.8 km/s. High, homogeneous velocities in lower crust, up to 7.4 km/s. Layer is thickening from 4 km in SE part of the profile, reaching 18 km in its central part corresponding to CFGC, and then thinning again to about 12 km in NW part. Moho depth varies from 54 km near the Gulf of Bothnia to 63 km in the middle of the profile, and up to 43 km in Saimaa area. Velocities below the crust are 8.2-8.25 km/s. Good quality of the data allowed to compute S velocity model and Vp/Vs ratio. This profile crosses two pre-existing WARR profiles, SVEKA and BALTIC. New model supports previous interpretations. Velocities, depth to the Moho, and other major boundaries are similar in profile crossing points. However, unlike in perpendicular profiles, no elongated and thin low velocity zones were distinguished along the entire profile. This study was a good lesson for future cost effective DSS profiles and points out key issues.

Thermomechanical Modeling of the Formation of a Multilevel, Crustal-Scale Magmatic System by the Yellowstone Plume

NASA Astrophysics Data System (ADS)

Colón, D. P.; Bindeman, I. N.; Gerya, T. V.

2018-05-01

Geophysical imaging of the Yellowstone supervolcano shows a broad zone of partial melt interrupted by an amagmatic gap at depths of 15-20 km. We reproduce this structure through a series of regional-scale magmatic-thermomechanical forward models which assume that magmatic dikes stall at rheologic discontinuities in the crust. We find that basaltic magmas accumulate at the Moho and at the brittle-ductile transition, which naturally forms at depths of 5-10 km. This leads to the development of a 10- to 15-km thick midcrustal sill complex with a top at a depth of approximately 10 km, consistent with geophysical observations of the pre-Yellowstone hot spot track. We show a linear relationship between melting rates in the mantle and rhyolite eruption rates along the hot spot track. Finally, melt production rates from our models suggest that the Yellowstone plume is 175°C hotter than the surrounding mantle and that the thickness of the overlying lithosphere is 80 km.

Climate variability and volcanic history of the Eastern Romanian Carpathians since early MIS 3 recorded in sediments from Mohoş crater

NASA Astrophysics Data System (ADS)

Bormann, M.; Veres, D.; Wulf, S.; Papadopoulou, M.; Panagiotopoulos, K.; Schaebitz, F.

2015-12-01

We present a 30m long sediment record covering the last ca. 50,000 years from the in-filled Mohoş crater (46°05' N; 25°55' E) located on Ciomadul volcano (Romania) that was retrieved in 2014. The record consists of bog and lacustrine sediments that are inter-bedded with tephra deposits. Ciomadul volcano, hosting the superimposed craters of Mohoş and Sf. Ana, is the youngest volcanic edifice in the Carpathian-Balkan region. Thus, tephra-analysis on the Mohoş sediments gives valuable insights into the volcanic history of that region, mainly arising from the younger crater of Sf Ana and several secondary domes. For investigations into the past climate history, the Mohoş sediment sequence has been analysed using a multi-proxy approach including geophysical, geochemical and sedimentological parameters. Multi-Sensor core logging and ITRAX X-ray fluorescence scanning have been performed at high-resolution, whereas grain size analysis, TOC and C/N ratios supplement the geophysical and geochemical data. Chronological control is based on radiocarbon and luminescence dating. We also present first results of the tephra-analysis on the Mohoş sediment record and their correlation to medium-distal pyroclastic deposits originating in this volcanic field. We further discuss responses of this mid-altitude site (1050 m a.s.l.) to past climate oscillations since early MIS 3. To date, the Mohoş core record provides the longest time series from the Carpathian region. This study is part of the Collaborative Research Centre 806 "Our Way To Europe; Culture-Environment Interaction and Human Mobility in the Late Quaternary" (www.sfb806.de); subproject B2.

Western US Seismic Observations Viewed Through Lead Isotope Maps

NASA Astrophysics Data System (ADS)

Bouchet, R. A.; Blichert-Toft, J.; Levander, A.; Reid, M. R.; Albarede, F.

2013-12-01

To shed light on the nature and history of the different geological units identified by the seismic models that have come from USArray in the western US, we compiled literature Pb isotope compositions of ores (n=1200), K-feldspars from granites (n=400), and felsic plutonic rocks (n=1300), data that for most part were not in the NAVDAT database. We complemented this compilation by analyzing the Pb isotope compositions of K-feldspars (76) and whole-rocks (6) of felsic xenoliths and felsic plutonic rocks from the Colorado Plateau (CP). The raw Pb isotope abundances for the complete data set were converted into three independent, geologically informative parameters in the form of the model age T (time of last U/Pb fractionation) and the two chemical ratios 238U/204Pb (μ) and 232Th/238U (κ). These parameters were then imaged on isotopic maps of the western US using 0.5°×0.5° grid-cell averaging for μ and κ and a 0.5°×0.5° grid-cell maximum after removing the 2.5% highest values (outliers) for T. Comparing these chemical maps to seismic maps of tomographic anomalies [1] and Moho and LAB depths determined from receiver functions [2] leads to the following observations: (i) Pb model ages: they match geological ages mostly where the continental mantle is cold and the Moho is deep. Elsewhere, Pb model ages are younger than geological ages. We interpret this feature as the chronological expression of a delayed cooling of deep crustal layers below the closure temperature (˜550-700°C) of Pb in K-feldspar, the major host of this element in the crust [3] or of age resetting by orogenic activity. (ii) While U/Pb (μ) does not vary systematically with other geochemical or seismic data, high Th/U (κ) values are usually observed where Vp/Vs is also high, as in the Snake River basin and central Colorado. High kappa values also form a 'ridge' trending south from northwestern Utah through the Basin and Range into the Mojave-Yavapai block. High-κ areas may reflect the presence of deep-seated rocks exhumed as a result of regional extension or collapse. They may also reveal the presence of channels of flowing crust originating either beneath the CP and spreading north, or along the track of the Yellowstone hotspot track and spreading south [4]. [1] Schmandt, B., and E. Humphreys (2010), Complex subduction and small-scale convection revealed by body-wave tomography of the western United States upper mantle, Earth Planet. Sci. Lett., 297, 435-445. [2] Levander, A., and M.S. Miller (2012), Evolutionary aspects of the lithosphere discontinuity structure in the western U.S., G-cubed, 13, 1-22. [3] Cherniak, D.J. (1995), Diffusion of lead in plagioclase and K-feldspar: an investigation using Rutherford Backscattering and Resonant Nuclear Reaction Analysis, Contrib. Mineral. Petrol., 120, 358-371. [4] Yuan, H., K. Dueker, and J. Stachnik (2010), Crustal structure and thickness along the Yellowstone hot spot track: evidence for lower crustal outflow from beneath the eastern Snake River Plain, G-cubed, 11, 1-14.

Deep seismic structure and tectonics of northern Alaska: Crustal-scale duplexing with deformation extending into the upper mantle

USGS Publications Warehouse

Fuis, G.S.; Murphy, J.M.; Lutter, W.J.; Moore, Thomas E.; Bird, K.J.; Christensen, N.I.

1997-01-01

Seismic reflection and refraction and laboratory velocity data collected along a transect of northern Alaska (including the east edge of the Koyukuk basin, the Brooks Range, and the North Slope) yield a composite picture of the crustal and upper mantle structure of this Mesozoic and Cenozoic compressional orogen. The following observations are made: (1) Northern Alaska is underlain by nested tectonic wedges, most with northward vergence (i.e., with their tips pointed north). (2) High reflectivity throughout the crust above a basal decollement, which deepens southward from about 10 km depth beneath the northern front of the Brooks Range to about 30 km depth beneath the southern Brooks Range, is interpreted as structural complexity due to the presence of these tectonic wedges, or duplexes. (3) Low reflectivity throughout the crust below the decollement is interpreted as minimal deformation, which appears to involve chiefly bending of a relatively rigid plate consisting of the parautochthonous North Slope crust and a 10- to 15-km-thick section of mantle material. (4) This plate is interpreted as a southward verging tectonic wedge, with its tip in the lower crust or at the Moho beneath the southern Brooks Range. In this interpretation the middle and upper crust, or all of the crust, is detached in the southern Brooks Range by the tectonic wedge, or indentor: as a result, crust is uplifted and deformed above the wedge, and mantle is depressed and underthrust beneath this wedge. (5) Underthrusting has juxtaposed mantle of two different origins (and seismic velocities), giving rise to a prominent sub-Moho reflector. Copyright 1997 by the American Geophysical Union.

Anomalous Subsidence at Rifted Continental Margins: Distinguishing Mantle Dynamic Topography from Anomalous Oceanic Crustal Thickness

NASA Astrophysics Data System (ADS)

Cowie, L.; Kusznir, N. J.

2012-12-01

It has been proposed that some continental rifted margins have anomalous subsidence histories and that at breakup they were elevated at shallower bathymetries than the isostatic response of classical rift models (McKenzie 1978) would predict. The existence of anomalous syn or post breakup subsidence of this form would have important implications for our understanding of the geodynamics of continental breakup and rifted continental margin formation, margin subsidence history and the evolution of syn and post breakup depositional systems. We have investigated three rifted continental margins; the Gulf of Aden, Galicia Bank and the Gulf of Lions, to determine whether the oceanic crust in the ocean-continent transition of these margins has present day anomalous subsidence and if so, whether it is caused by mantle dynamic topography or anomalous oceanic crustal thickness. Residual depth anomalies (RDA) corrected for sediment loading, using flexural backstripping and decompaction, have been calculated by comparing observed and age predicted oceanic bathymetries in order to identify anomalous oceanic bathymetry and subsidence at these margins. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions from Crosby & McKenzie (2009). Non-zero sediment corrected RDAs may result from anomalous oceanic crustal thickness with respect to the global average, or from mantle dynamic uplift. Positive RDAs may result from thicker than average oceanic crust or mantle dynamic uplift; negative RDAs may result from thinner than average oceanic crust or mantle dynamic subsidence. Gravity inversion incorporating a lithosphere thermal gravity anomaly correction and sediment thickness from 2D seismic data has been used to determine Moho depth and oceanic crustal basement thickness. The reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. The gravity inversion crustal basement thicknesses together with Airy isostasy have been used to predict a "synthetic" gravity derived RDA. Sediment corrected RDA for oceanic crust in the Gulf of Aden are positive (+750m) indicating anomalous uplift with respect to normal subsidence. Gravity inversion predicts normal thickness oceanic crust and a zero "synthetic" gravity derived RDA in the oceanic domain. The difference between the positive sediment corrected RDA and the zero "synthetic" gravity derived RDA, implies that the anomalous subsidence reported in the Gulf of Aden is the result of mantle dynamic uplift. For the oceanic crust outboard of Galicia Bank both the sediment corrected RDA and the "synthetic" gravity derived RDA are negative (-800m) and of similar magnitude, indicating anomalous subsidence, which is the result of anomalously thin oceanic crust, not mantle dynamic topography. We conclude that there is negligible mantle dynamic topography influencing the Galicia Bank region. In the Gulf of Lions, gravity inversion predicts thinner than average oceanic crust. Both sediment corrected RDA (-1km) and "synthetic" gravity derived RDA (-500m) are negative. The more negative sediment corrected RDA compared with the "synthetic" gravity derived RDA implies that the anomalous subsidence in the Gulf of Lions is the result of mantle dynamic subsidence as well as thinner than average oceanic crust.

Lithospheric structure along wide-angle seismic profile GEORIFT 2013 in Pripyat-Dnieper-Donets Basin (Belarus and Ukraine)

NASA Astrophysics Data System (ADS)

Starostenko, V.; Janik, T.; Yegorova, T.; Czuba, W.; Środa, P.; Lysynchuk, D.; Aizberg, R.; Garetsky, R.; Karataev, G.; Gribik, Y.; Farfuliak, L.; Kolomiyets, K.; Omelchenko, V.; Komminaho, K.; Tiira, T.; Gryn, D.; Guterch, A.; Legostaeva, O.; Thybo, H.; Tolkunov, A.

2018-03-01

The GEORIFT 2013 (GR'13) WARR (wide-angle reflection and refraction) experiment was carried out in 2013 in the territory of Belarus and Ukraine with broad international co-operation. The aim of the work is to study basin architecture and deep structure of the Pripyat-Dnieper-Donets Basin (PDDB), which is the deepest and best studied Palaeozoic rift basin in Europe. The PDDB is located in the southern part of the East European Craton (EEC) and crosses Sarmatia—one of the three segments of the EEC. The PDDB was formed by Late Devonian rifting associated with domal basement uplift and magmatism. The GR'13 extends in NW-SE direction along the PDDB strike and crosses the Pripyat Trough (PT) and Dnieper Graben (DG) separated by the Bragin Uplift (BU) of the basement. The field acquisition along the GR'13 (of 670 km total length) involved 14 shots and recorders deployed every ˜2.2 km for several shot points. The good quality of the data, with first arrivals visible up to 670 km for several shot points, allowed for construction of a velocity model extending to 80 km depth using ray-tracing modelling. The thickness of the sediments (Vp < 6.0 km s-1) varies from 1-4 km in the PT, to ˜5 km in the NW part of the DG, to 10-13 km in the SE part of the profile. Below the DG, at ˜330-530 km distance, we observed an upwarping of the lower crust (with Vp of ˜7.1 km s-1) to ˜25 km depth that represents a rift pillow or mantle underplate. The Moho shallows southeastwards from ˜47 km in the PT to 40-38 km in the DG with mantle velocities of 8.35 and ˜8.25 km s-1 in the PT and DG, respectively. A near-horizontal mantle discontinuity was found beneath BU (a transition zone from the PT to the DG) at the depth of 50-47 km. It dips to the depth of ˜60 km at distances of 360-405 km, similar to the intersecting EUROBRIDGE'97 profile. The crust and upper mantle structure on the GR'13 may reflect varying intensity of rifting in the PDDB from a passive stage in the PT to active rifting in the DG. The absence of Moho uplift and relatively thick crystalline crust under the PT is explained by its tectonic position as a closing unit of the PDDB, with a gradual attenuation of rifting from the southeast to the northwest. The most active stage of rifting is evidenced in the DG by a shallower Moho and by a presence of a rift pillow caused by mafic and ultramafic intrusions during the active phase. The junction of the PT and the DG (the BU) locates just at its intersection with the NS regional tectonic zone Odessa-Gomel. Most likely, the `blocking' effect of this zone did not allow for further propagation of active rifting to the NW.

Variations in Melt Generation and Migration along the Aleutian Arc (Invited)

NASA Astrophysics Data System (ADS)

Plank, T. A.; Van Keken, P. E.

2013-12-01

The generation and ascent of mantle melt beneath volcanic arcs sets the course for how magmas differentiate to form the continental crust and erupt explosively from volcanoes. Although the basic framework of melting at subduction zones is understood to involve the convective influx of hot mantle (Tp ≥ 1300°C) and advective transport of water-rich fluids from the subducting slab, the P-T paths that melts follow during melt generation and migration are still not well known. The Aleutian Arc provides an opportunity to explore the conditions of mantle melting in the context of volcanoes that span an unusually large range in the depth to the slab, from Seguam island, with among the shallowest depths to the slab worldwide (~65 km, [1]) to Bogoslof island, behind the main volcanic front and twice the depth to the slab (~130 km). Here we combine thermal models tuned to Aleutian subduction parameters [after 2] with petrological estimates of the T and P of mantle-melt equilibration, using a major element geothermometer [3] and estimates of H2O and fO2 from olivine-hosted melt inclusion measurements [4] for basaltic magmas from 6 volcanoes in the central Aleutians (Korovin, Seguam, Bogoslof, Pakushin, Akutan, Shishaldin). We find mantle-melt equilibration conditions to vary systematically as a function of the depth to the slab, from 30 km and 1220°C (for Seguam) to 60 km and 1300°C (for Bogoslof). Such shallow depths, which extend up to the Moho, define a region perched well above the hot core of the mantle wedge predicted from thermal models, even considering the shallow depths of slab-mantle coupling (< 60 km) required to supply hot mantle beneath Seguam. Thus, even though the greatest melt production will occur in the hot core of the wedge (50-100 km depth), melts apparently ascend and re-equilibrate in the shallowest mantle. Volcanoes that overlie the greatest depth to the slab, and lie furthest from the wedge corner, stall at greater depths (~60 km), at the base of the conductive upper plate (i.e., lithosphere). The conductive lid and isotherms shallow toward the wedge corner. This leads to shallower depths of melt equilibration at shallower depths to the slab. A second effect is infiltration of melt into the thinning lithosphere, likely due to the increase in strain-rate toward the wedge corner, which favors melt segregation, migration, and shallow equilibration [5]. Such a process is developed most beneath Seguam, where melts collect at the Moho (~ 30km), but are still > 1200°C. Such equilibration depths in the uppermost mantle (30-60 km) and temperatures typical of the base of the conductive lid appear to characterize most modeled primary arc magmas [6], and point to a final re-setting point in the mantle that controls the composition of bulk arc crust. [1] Syracuse & Abers, 2006, G3. [2] Syracuse, van Keken, Abers, (2010) PEPI. [3] Lee, Luffi, Plank, Dalton, Leeman (2009) EPSL. [4] Zimmer et al. (2010) J.Pet. [5] Holzman & Kendall (2010). [6] Ruscitto et al. (2012) G3.

Global maps of the magnetic thickness and magnetization of the Earth's lithosphere

NASA Astrophysics Data System (ADS)

Vervelidou, Foteini; Thébault, Erwan

2015-10-01

We have constructed global maps of the large-scale magnetic thickness and magnetization of Earth's lithosphere. Deriving such large-scale maps based on lithospheric magnetic field measurements faces the challenge of the masking effect of the core field. In this study, the maps were obtained through analyses in the spectral domain by means of a new regional spatial power spectrum based on the Revised Spherical Cap Harmonic Analysis (R-SCHA) formalism. A series of regional spectral analyses were conducted covering the entire Earth. The R-SCHA surface power spectrum for each region was estimated using the NGDC-720 spherical harmonic (SH) model of the lithospheric magnetic field, which is based on satellite, aeromagnetic, and marine measurements. These observational regional spectra were fitted to a recently proposed statistical expression of the power spectrum of Earth's lithospheric magnetic field, whose free parameters include the thickness and magnetization of the magnetic sources. The resulting global magnetic thickness map is compared to other crustal and magnetic thickness maps based upon different geophysical data. We conclude that the large-scale magnetic thickness of the lithosphere is on average confined to a layer that does not exceed the Moho.

The nature of crustal boundaries: combined interpretation of wide-angle and normal-incidence seismic data

NASA Astrophysics Data System (ADS)

Long, Roger E.; Matthews, Patricia A.; Graham, Daniel P.

1994-04-01

After a few seconds two-way traveltime, normal-incidence seismic reflection sections are composed mainly of assemblages of short reflections. Very rarely are seen continuous reflections that might correspond to the Moho or a mid-crustal discontinuity. The inferred continuity of these boundaries has traditionally come from refraction seismology. There is now a body of high quality, coincident wide-angle and normal-incidence seismic data that have been recorded with 50-100 m shot spacing and with high frequency sources (e.g. MOBIL, BABEL). The complexity and characteristics of the wide-angle arrivals seen on these data suggest that they do not originate from continuous boundaries. It is suggested that these arrivals are reflections from the same assemblage of short length reflectors that are responsible for normal-incidence reflections. Seismic velocities below the middle crust may (1) change corresponding to normal-incidence reflectivity, or (2) generally increase with depth with localised sills or lens structures of different velocity accounting for the observed reflections. Wide-angle arrivals that have traditionally been identified as reflections from crustal boundaries (e.g. the mid-crust and Moho) and which were considered indicative of a sharp velocity discontinuity from continuous boundaries, may instead result from a concentration of lamellae.

Imaging the deep structures of the convergent plates along the Ecuadorian subduction zone through receiver function analysis

NASA Astrophysics Data System (ADS)

Galve, A.; Charvis, P.; Regnier, M. M.; Font, Y.; Nocquet, J. M.; Segovia, M.

2017-12-01

The Ecuadorian subduction zone was affected by several large M>7.5 earthquakes. While we have low resolution on the 1942, 1958 earthquakes rupture zones extension, the 2016 Pedernales earthquake, that occurs at the same location than the 1942 earthquake, give strong constraints on the deep limit of the seismogenic zone. This downdip limit is caused by the onset of plasticity at a critical temperature (> 350-450 °C for crustal materials, or serpentinized mantle wedge, and eventually > 700 °C for dry mantle). However we still don't know exactly where is the upper plate Moho and therefore what controls the downdip limit of Ecuadorian large earthquakes seismogenic zone. For several years Géoazur and IG-EPN have maintained permanent and temporary networks (ADN and JUAN projects) along the margin to register the subduction zone seismological activity. Although Ecuador is not a good place to perform receiver function due to its position with respect to the worldwide teleseismic sources, the very long time deployment compensate this issue. We performed a frequency dependent receiver function analysis to derive (1) the thickness of the downgoing plate, (2) the interplate depth and (3) the upper plate Moho. These constraints give the frame to interpretation on the seismogenic zone of the 2016 Pedernales earthquake.

Crustal structure and mantle transition zone thickness beneath a hydrothermal vent at the ultra-slow spreading Southwest Indian Ridge (49°39'E): a supplementary study based on passive seismic receiver functions

NASA Astrophysics Data System (ADS)

Ruan, Aiguo; Hu, Hao; Li, Jiabiao; Niu, Xiongwei; Wei, Xiaodong; Zhang, Jie; Wang, Aoxing

2017-06-01

As a supplementary study, we used passive seismic data recorded by one ocean bottom seismometer (OBS) station (49°41.8'E) close to a hydrothermal vent (49°39'E) at the Southwest Indian Ridge to invert the crustal structure and mantle transition zone (MTZ) thickness by P-to-S receiver functions to investigate previous active seismic tomographic crustal models and determine the influence of the deep mantle thermal anomaly on seafloor hydrothermal venting at an ultra-slow spreading ridge. The new passive seismic S-wave model shows that the crust has a low velocity layer (2.6 km/s) from 4.0 to 6.0 km below the sea floor, which is interpreted as partial melting. We suggest that the Moho discontinuity at 9.0 km is the bottom of a layer (2-3 km thick); the Moho (at depth of 6-7 km), defined by active seismic P-wave models, is interpreted as a serpentinized front. The velocity spectrum stacking plot made from passive seismic data shows that the 410 discontinuity is depressed by 15 km, the 660 discontinuity is elevated by 18 km, and a positive thermal anomaly between 182 and 237 K is inferred.

Crustal reflectivity in the Skagerrak area

NASA Astrophysics Data System (ADS)

Larsson, F. R.; Husebye, E. S.

1991-04-01

Reflectors within the crystalline crust are often observed in deep seismic reflection profiling surveys. The lower crust in extensional areas is generally credited with an abundance of reflectors. The deep seismic reflection data (16 s TWT) from the M.V. Mobil Search cruise in Skagerrak show a reflective lower crust and a relatively transparent upper crust in most of the area. Reflectivity seems to be less inside the Oslo Rift, and also beneath the sediment-covered areas. Reflectivity maxima are found near the Moho and at depths of 10-20 km. The latter is taken to coincide with the transition between the brittle upper and ductile lower crust. The distribution of crustal reflectors in Skagerrak and their possible relationships with seismic velocities, earthquake depth distribution and major tectonic elements such as the Fennoscandian Border Zone, the Oslo Rift system and the shield environment are discussed. Hypotheses on the formation of the crustal reflectors are also briefly reviewed.

Comparision between crustal density and velocity variations in Southern California

USGS Publications Warehouse

Langenheim, V.E.; Hauksson, E.

2001-01-01

We predict gravity from a three-dimensional Vp model of the upper crust and compare it to the observed isostatic residual gravity field. In general this comparison shows that the isostatic residual gravity field reflects the density variations in the upper to middle crust. Both data sets show similar density variations for the upper crust in areas such as the Peninsular Ranges and the Los Angeles basin. Both show similar variations across major faults, such as the San Andreas and Garlock faults in the Mojave Desert. The difference between the two data sets in regions such as the Salton Trough, the Eastern California Shear Zone, and the eastern Ventura basin (where depth to Moho is <30 km), however, suggests high-density middle to lower crust beneath these regions. Hence the joint interpretation of these data sets improves the depth constraints of crustal density variations.

Shear wave velocity variation across the Taupo Volcanic Zone, New Zealand, from receiver function inversion

USGS Publications Warehouse

Bannister, S.; Bryan, C.J.; Bibby, H.M.

2004-01-01

The Taupo Volcanic Zone (TVZ), New Zealand is a region characterized by very high magma eruption rates and extremely high heat flow, which is manifest in high-temperature geothermal waters. The shear wave velocity structure across the region is inferred using non-linear inversion of receiver functions, which were derived from teleseismic earthquake data. Results from the non-linear inversion, and from forward synthetic modelling, indicate low S velocities at ???6- 16 km depth near the Rotorua and Reporoa calderas. We infer these low-velocity layers to represent the presence of high-level bodies of partial melt associated with the volcanism. Receiver functions at other stations are complicated by reverberations associated with near-surface sedimentary layers. The receiver function data also indicate that the Moho lies between 25 and 30 km, deeper than the 15 ?? 2 km depth previously inferred for the crust-mantle boundary beneath the TVZ. ?? 2004 RAS.

Leucosome distribution in migmatitic paragneisses and orthogneisses: A record of self-organized melt migration and entrapment in a heterogeneous partially-molten crust

NASA Astrophysics Data System (ADS)

Yakymchuk, C.; Brown, M.; Ivanic, T. J.; Korhonen, F. J.

2013-09-01

The depth to the bottom of the magnetic sources (DBMS) has been estimated from the aeromagnetic data of Central India. The conventional centroid method of DBMS estimation assumes random uniform uncorrelated distribution of sources and to overcome this limitation a modified centroid method based on scaling distribution has been proposed. Shallower values of the DBMS are found for the south western region. The DBMS values are found as low as 22 km in the south west Deccan trap covered regions and as deep as 43 km in the Chhattisgarh Basin. In most of the places DBMS are much shallower than the Moho depth, earlier found from the seismic study and may be representing the thermal/compositional/petrological boundaries. The large variation in the DBMS indicates the complex nature of the Indian crust.

Using thermodynamic data to reproduce main seismic features of transition zone

NASA Astrophysics Data System (ADS)

Fomin, Ilya; Saukko, Anna; Edwards, Paul; Schiffer, Christian

2016-04-01

Most of the seismic tomography studies nowadays are based on comprehensive models with optimization of lots of parameters. These models are able to resolve very subtle features of the Earth's mantle, but the influence of each specific parameter is not seen directly. In our research we try to minimize the number of processed parameters to produce simple synthetic cases. The main goals of our model are to see how water content influences the depth of the transition zone, and if melting at the transition zone is plausible. We also attempt to see how water content and the presence of melts influence the signal strength of the transition zone in receiver functions. Our MATLAB-code calculates phase assemblage according to specific temperature and pressure within 2D numerical domain (e.g. 300x700 km). Phase properties are calculated with database of Stixrude and Lithgow-Bertelloni [2011], with corrections for water impact on elastic constants according to Liu et al., [2012]. We use the mantle phase composition 55% garnet and 45% olivine-polymorph, soliduses by Ohtani et al. [2004] and melt properties by Sakamaki et al. [2006]. These data are used to calculate seismic velocities and, furthermore, receiver functions with standard routines (e.g.[Schiffer et al., 2012]). Model predicts Vs within 5 to 5.5 km/s and Vp around 9.5-10 km/s within transition zone (Vp/Vs = 1.84-1.87), which is close to standard values. The presence of water enlarges the wadsleyite region, but also dampens the peak of receiver functions down to background level. Increase in water content causes melting at much shallower depths. Using a normal thermal gradient, we can get up to 10% of melt at depths around 390 km with 80% of water saturation, shown by a negative anomaly on receiver functions. This result is similar to data obtained for Afar Plateau [Thompson et al., 2015]. With cratonic thermal gradient, the olivine-wadsleyite transition and corresponding melt layer appear at depths around 350 km. This is comparable to data by Vinnik and Farra [2007], who proposed the presence of melt-rich piles at 350 km under continental crust at several locations worldwide. Our model also shows that in case of Moho depths close to 35 km, the Moho itself produces a multiple of receiver functions close to the 410 discontinuity. This multiple peak can affect the interpretation of the position of the real olivine-wadsleyite transition depth. It may also explain why the 410 km peak is still observed in cases with low-depth melting [Thompson et al., 2015; Vinnik & Farra, 2007], which probably should be related to the beginning of transition zone.

Seismic Waveform Tomography of the Iranian Region

NASA Astrophysics Data System (ADS)

Maggi, A.; Priestley, K.; Jackson, J.

2001-05-01

Surprisingly little is known about the detailed velocity structure of Iran, despite the region's importance in the tectonics of the Middle East. Previous studies have concentrated mainly on fundamental mode surface wave dispersion measurements along isolated paths (e.g.~Asudeh, 1982; Cong & Mitchell, 1998; Ritzwoller et.~al, 1998), and the propagation characteristics of crust and upper mantle body waves (e.g. Hearn & Ni 1994; Rodgers et.~al 1997). We use the partitioned waveform inversion method of Nolet (1990) on several hundred regional waveforms crossing the Iranian region to produce a 3-D seismic velocity map for the crust and upper mantle of the area. The method consists of using long period seismograms from earthquakes with well determined focal mechanisms and depths to constrain 1-D path-averaged shear wave models along regional paths. The constraints imposed on the 1-D models by the seismograms are then combined with independent constraints from other methods (e.g.~Moho depths from reciever function analysis etc.), to solve for the 3-D seismic velocity structure of the region. A dense coverage of fundamental mode rayleigh waves at a period of 100~s ensures good resolution of lithospheric scale structure. We also use 20~s period fundamental mode rayleigh waves and some Pnl wavetrains to make estimates of crustal thickness variations and average crustal velocities. A few deeper events give us some coverage of higher mode rayleigh waves and mantle S waves, which sample to the base of the upper mantle. Our crustal thickness estimates range from 45~km in the southern Zagros mountains, to 40~km in central Iran and 35~km towards the north of the region. We also find inconsistencies between the 1-D models required to fit the vertical and the tranverse seismograms, indicating the presence of anisotropy.

Slowness based CCP stacking technique in suppressing crustal multiples

NASA Astrophysics Data System (ADS)

Guan, Z.; Niu, F.

2016-12-01

Common-conversion-point (CCP) stacking of receiver function is a widely used technique to image velocity discontinuities in the mantle, such as the lithosphere-asthenosphere boundary (LAB) in the upper mantle, the 410-km and the 660-km discontinuities in the mantle transition zone. In a layered medium, a teleseismic record can be considered as the summation of the direct arrival and a series of conversions and reflections at boundaries below the station. Receiver functions are an attempt to approximate a Green's function associated with structure beneath the receiver by deconvolving one component of a teleseismic signal from another to remove source signals from seismograms. The CCP technique assumes that receiver functions composed solely of P to S conversions at velocity boundaries, whose depths can be mapped out through their arrival times. The multiple reflections at shallow boundaries with large velocity contrasts, such as the base of unconsolidated sediments and the Moho, can pose significant challenges to the accuracy of CCP imaging. In principle, the P to S conversions and multiples originated from deep and shallow boundaries arrive at a seismic station with incident angles that are, respectively, smaller and larger than that of the direct P wave. Therefore the corresponding slowness can be used to isolate the conversions from multiples, allowing for minimizing multiple-induced artifacts. We developed a refined CCP stacking method that uses relative slowness as a weighting factor to suppress the multiples. We performed extensive numerical tests with synthetic data to seek the best weighting scheme and to verify the robustness of the images. We applied the refined technique to the NECESSArray data, and found that the complicated low velocity structures in the depth range of 200-400 km shown in the CCP images of previous studies are mostly artifacts resulted from crustal multiples.

Crustal Structure of Australia from Ambient Seismic Noise Tomography

NASA Astrophysics Data System (ADS)

Saygin, E.; Kennett, B. L.

2011-12-01

We create surface wave tomography for Australian crustal structure by using the group velocity measurements in the period range 1-32 s extracted from the stacked transfer functions of ambient noise between station pairs. Both Rayleigh wave and Love wave group velocity maps are constructed for each period using the vertical and transverse component of the Green's function estimates from the ambient noise. The all of the portable broadband deployments and permanent stations on the continent have been used with over 250 stations in all and up to 7500 paths. The permanent stations fill the gap between the various shorter-term portable deployments. At each period the group velocity maps are constructed with a fully nonlinear tomographic inversion exploiting a subspace technique and the Fast Marching Method for wavefront tracking. For Rayleigh waves the continental coverage is good enough to allow the construction of a 3D shear wavespeed model in a two stage approach. Local group dispersion information is collated for a distribution of points across the continent and inverted for a 1D SV wavespeed profile using a Neighbourhood Algorithm method with weak constraints on the sedimentary thickness and Moho depth. The resulting set of 1D models are then interpolated to produce the final 3D wavespeed model. The group velocity maps show the strong influence of thick sediments at shorter periods, and distinct fast zones associated with cratonic regions. Below the sediments the 3D shear wavespeed model displays significant heterogeneity with only moderate correlation with surface tectonic features. For example, there is no evident expression of the Tasman Line marking the eastern edge of Precambrian outcrop. The large number of available interstation paths extracted from the ambient noise analysis provide detailed shear wavespeed information for crustal structure across the Australian continent for the first time, including regions where there was no prior sampling because of difficult logistics.

Pyroxene megacrysts in Proterozoic anorthosites: Implications for tectonic setting, magma source and magmatic processes at the Moho

NASA Astrophysics Data System (ADS)

Bybee, G. M.; Ashwal, L. D.; Shirey, S. B.; Horan, M.; Mock, T.; Andersen, T. B.

2014-03-01

Proterozoic anorthosites from the 1630-1650 Ma Mealy Mountains Intrusive Suite (Grenville Province, Canada), the 1289-1363 Ma Nain Plutonic Suite (Nain-Churchill Provinces, Canada) and the 920-949 Ma Rogaland Anorthosite Province (Sveconorwegian Province, Norway), all entrain comagmatic, cumulate, high-alumina orthopyroxene megacrysts (HAOMs). The orthopyroxene megacrysts range in size from 0.2 to 1 m and all contain exsolution lamellae of plagioclase that indicate the incorporation of an excess Ca-Al component inherited from the host magma at pressures in excess of 10 kbar at or near Moho depths (>30-40 km). Suites of HAOMs from each intrusion display a large range in 147Sm/144Nd (0.10 to 0.34) making them amenable for precise age dating with the Sm-Nd system. Sm-Nd isochrons for HAOMs give ages of 1765±12 Ma (Mealy Mountains), 1041±17 Ma (Rogaland) and 1444±100 Ma (Nain), all of them older by about 80 to 120 m.y. than the respective 1630-1650, 920-949 and 1289-1363 Ma crystallization ages of their host anorthosites. Internal mineral Sm-Nd isochrons between plagioclase exsolution lamellae and the orthopyroxene host for HAOMs from the Rogaland and Nain complexes yield ages of 968±43 and 1347±6 Ma, respectively - identical within error to the ages of the anorthosites themselves. This age concordance establishes that decompression exsolution in the HAOM was coincident with magmatic emplacement of the anorthosites, ∼100 m.y. after HAOMs crystallization at the Moho. Correspondence of Pb isotope ages (206Pb/204Pb vs. 207Pb/204Pb) with Sm-Nd ages and other strong lines of evidence indicate that the older megacryst ages represent true crystallization ages and not the effects of time-integrated mixing processes in the magmas. Nd isotopic evolution curves, AFC/mixing calculations and the age relations between the HOAMs and their anorthosite hosts show that the HAOMs are much less contaminated with crustal components and are an older part of the same magmatic system from which the anorthosites are derived. Modeling of these anorthositic magmas with MELTS indicates that their ultramafic cumulates would have sunk in the magma and been sequestered at the Moho, where they may have sunk deeper into the mantle resulting in large-scale compositional differentiation. The HAOMs thus represent a rare example of part of a cumulate assemblage that was carried to the upper crust during anorthosite emplacement and, together with the anorthosites, illustrate the dramatic influence that magma ponding and differentiation at the Moho has on residual magmas traveling towards the surface. The new geochronologic and isotopic data indicate that the magmas were derived by melting of the mantle, forming magmatic systems that could have been long-lived (e.g. 80-100 m.y.). A geologic setting that would fit these temporal constraints is a long-lived Andean-type margin.

Dense array recordings in the San Bernardino Valley of landers-big bear aftershocks: Basin surface waves, Moho reflections, and three-dimensional simulations

USGS Publications Warehouse

Frankel, Arthur

1994-01-01

Fourteen GEOS seismic recorders were deployed in the San Bernardino Valley to study the propagation of short-period (T ≈ 1 to 3 sec) surface waves and Moho reflections. Three dense arrays were used to determine the direction and speed of propagation of arrivals in the seismograms. The seismograms for a shallow (d ≈ 1 km) M 4.9 aftershock of the Big Bear earthquake exhibit a very long duration (60 sec) of sustained shaking at periods of about 2 sec. Array analysis indicates that these late arrivals are dominated by surface waves traveling in various directions across the Valley. Some energy is arriving from a direction 180° from the epicenter and was apparently reflected from the edge of the Valley opposite the source. A close-in aftershock (Δ = 25 km, depth = 7 km) displays substantial short-period surface waves at deep-soil sites. A three-dimensional (3D) finite difference simulation produces synthetic seismograms with durations similar to those of the observed records for this event, indicating the importance of S-wave to surface-wave conversion near the edge of the basin. Flat-layered models severely underpredict the duration and spectral amplification of this deep-soil site. I show an example where the coda wave amplitude ratio at 1 to 2 Hz between a deep-soil and a rock site does not equal the S-wave amplitude ratio, because of the presence of surface waves in the coda of the deep-soil site. For one of the events studied (Δ ≈ 90 km), there are sizable phases that are critically reflected from the Moho (PmP and SmS). At one of the rock sites, the SmS phase has a more peaked spectrum that the direct S wave.

Modeling Wide-Angle Seismic Data from the Hi-CLIMB Experiment in Tibet

NASA Astrophysics Data System (ADS)

Nowack, R. L.; Griffin, J. D.; Tseng, T.; Chen, W.

2009-12-01

Using data from local and regional events recorded by the Hi-CLIMB array in Tibet, we utilize seismic attributes, including arrival times, Hilbert amplitudes and pulse frequencies, to constrain structures of seismic wave speed and attenuation in the crust and the upper mantle in western China. We construct more than 30 high-quality, regional seismic profiles, and select 14 of these, which show excellent crustal and Pn arrivals, for further analysis. Travel-times from events at regional distances constrain large-scale velocity structures, and four close-in events provide further details on crustal structure. We use the 3-D ray tracer, CRT, to model the travel-times. Initial results indicate that the Moho beneath the Lhasa terrane of southern Tibet is over 73 km deep with a high Pn speed of about 8.2 km/s. In contrast, the Qiangtang terrane farther north shows a thinner crust, by up to 10 km, and a low Pn speed of 7.8-7.9 km/s. Preliminary estimates of upper mantle velocity gradients are between .003 and .004 km/s per km, consistent with previous results by Phillips et al. (2007). We also use P to SV conversions from teleseismic earthquakes to independently constrain variations in speeds of Pn and depths of the Moho. For instance, amplitudes of the SsPmP phase, when its last reflection off the Moho is near-critical, are particularly sensitive to the contrast in seismic wave speeds across the crust-mantle interface; and results from these additional data are consistent with those from modeling of travel-times. Additional seismic attributes, extracted from wave-trains containing Pn and major crustal phases, are being compared with results of numerical modeling based on the spectral element method and asymptotic calculations in laterally varying media, where both lateral and vertical gradients in seismic wave speeds can strongly affect Pn amplitudes and pulse frequencies.

A new insight on magma generation environment beneath Jeju (Cheju) volcanic island

NASA Astrophysics Data System (ADS)

Shin, Y.; CHOI, K.; Koh, J.; Yun, S.; Nakamura, E.; Na, S.

2011-12-01

We present a Moho undulation model from gravity inversion that gives a new insight on the magma generation environment beneath Jeju (Cheju) volcanic island, Korea. The island is an intra-plate volcanic island located behind Ryukyu Trench, the collisional boundary between Eurasian plate and Philippine plate. Jeju island is a symmetrical shield volcano of oval shape (74 km by 32 km) whose peak is Hallasan (Mt. Halla: 1950m). The landform, which is closely related to the volcanism, can be divided topographically into the lava plateau, the shield-shaped Halla volcanic edifice and the monogenetic cinder cones, which numbers over 365. The basement rock mainly consists of Precambrian gneiss, Mesozoic granite and volcanic rocks. Unconsolidated sedimentary rock is found between basement rock and lava. The lava plateau is composed of voluminous basaltic lava flows, which extend to the coast region with a gentle slope. Based on volcanic stratigraphy, paleontology and geochronology, the Jeju basalts range from the early Pleistocene to Holocene in age. The mean density of the island is estimated to be very low, 2390 kg/cubic cm from gravity data analysis, which reflects the abundant unconsolidated pyroclastic sediments below the surface lava. The mean Moho depth is estimated to be 29.5 km from power spectral density of gravity anomaly, which means it has continental crust. It is noticeable that the gravity inversion indicates the island is developed above and along a swelled-up belt (ridge), several hundred meters higher than the surrounding area. The structure is also shows positive correlation with high magnetic anomaly distribution that could indicate existence of volcanic rocks. We interpret the Moho structure has a key to the magma generation: 1) the high gravity anomaly belt is formed by folding/buckling process under compressional environment, 2) it causes decrease of pressure beneath the lithosphere along the belt, and 3) it accelerates melting of basaltic magma in addition to the hot thermal structure widely distributed behind the collisional boundary.

Polymineralic inclusions in mantle chromitites from the Oman ophiolite indicate a highly magnesian parental melt

NASA Astrophysics Data System (ADS)

Rollinson, Hugh; Mameri, Lucan; Barry, Tiffany

2018-06-01

Polymineralic inclusions interpreted as melt inclusions in chromite from the dunitic Moho Transition Zone in the Maqsad area of the Oman ophiolite have been analysed and compositions integrated using a rastering technique on the scanning electron microscope. The inclusions now comprise a range of inter-grown hydrous phases including pargasite, aspidolite, phlogopite and chlorite, indicating that the parental melts were hydrous. Average inclusion compositions for seven samples contain between 23.1 and 26.8 wt% MgO and 1.7-3.6 wt% FeO. Compositions were corrected to allow for the low FeO concentrations using coexisting olivine compositions. These suggest that the primary melt has between 20 and 22 wt% MgO and 7-9.7 wt% FeO and has an affinity with boninitic melts, although the melts have a higher Ti content than most boninites. Average rare earth element concentrations suggest that the melts were derived from a REE depleted mantle source although fluid-mobile trace elements indicate a more enriched source. Given the hydrous nature of the inclusions this enrichment could be fluid driven. An estimate of the melt temperature can be made from the results of homogenisation experiments on these inclusions and suggests 1300 °C, which implies for a harzburgite solidus, relatively shallow melting at depths of <50 km and is consistent with a boninitic origin. The current "basaltic" nature of the chromite host to highly magnesian melt inclusions suggests that the dunitic Moho Transition Zone operated as a reaction filter in which magnesian melts were transformed into basalts by the removal of high magnesian olivines, particularly in areas where the Moho Transition Zone is unusually thick. We propose therefore that podiform mantle chromitites, even those with an apparent MORB-like chemical signature, have crystallised from a highly magnesian parental melt. The data presented here strongly support the view that this took place in a subduction initiation setting.

Numerical simulations of the mantle lithosphere delamination

NASA Astrophysics Data System (ADS)

Morency, C.; Doin, M.-P.

2004-03-01

Sudden uplift, extension, and increased igneous activity are often explained by rapid mechanical thinning of the lithospheric mantle. Two main thinning mechanisms have been proposed, convective removal of a thickened lithospheric root and delamination of the mantle lithosphere along the Moho. In the latter case, the whole mantle lithosphere peels away from the crust by the propagation of a localized shear zone and sinks into the mantle. To study this mechanism, we perform two-dimensional (2-D) numerical simulations of convection using a viscoplastic rheology with an effective viscosity depending strongly on temperature, depth, composition (crust/mantle), and stress. The simulations develop in four steps. (1) We first obtain "classical" sublithospheric convection for a long time period (˜300 Myr), yielding a slightly heterogeneous lithospheric temperature structure. (2) At some time, in some simulations, a strong thinning of the mantle occurs progressively in a small area (˜100 km wide). This process puts the asthenosphere in direct contact with the lower crust. (3) Large pieces of mantle lithosphere then quickly sink into the mantle by the horizontal propagation of a detachment level away from the "asthenospheric conduit" or by progressive erosion on the flanks of the delaminated area. (4) Delamination pauses or stops when the lithospheric mantle part detaches or when small-scale convection on the flanks of the delaminated area is counterbalanced by heat diffusion. We determine the parameters (crustal thicknesses, activation energies, and friction coefficients) leading to delamination initiation (step 2). We find that delamination initiates where the Moho temperature is the highest, as soon as the crust and mantle viscosities are sufficiently low. Delamination should occur on Earth when the Moho temperature exceeds ˜800°C. This condition can be reached by thermal relaxation in a thickened crust in orogenic setting or by corner flow lithospheric erosion in the overriding lithosphere of subduction zones.

Evidence for a Slow Spreading Ocean Ridge in the Southern Rockall Trough From Satellite Gravity Inversion and Seismic Data

NASA Astrophysics Data System (ADS)

Chappell, A. R.; Kusznir, N. J.

2005-12-01

The southern Rockall Trough, located to the west of Ireland and the UK in the NE Atlantic, has been interpreted as both a Mesozoic intra-continental rift basin (O'Reilly 1995) and a mid Cretaceous ocean basin (e.g. Roberts et al. 1980). The continental rift hypothesis (O'Reilly 1995) requires differential stretching of the upper and lower crust and syn-tectonic cooling to mechanically explain the formation of 5-6km thick continental crust and allow serpentinisation of the upper mantle. In this model serpentinisation of the upper mantle is needed to explain low upper mantle seismic velocities. The serpentinisation has also been required to fit gravity modelling of seismic transects to the observed gravity (e.g. Shannon 1999). We use satellite gravity inversion to map Moho depth and crustal thickness (Chappell & Kusznir 2005) for the Rockall Trough area. The satellite gravity inversion is a 3D spectral method incorporating a correction for the residual lithosphere thermal gravity anomaly present in continental rifted margin lithosphere and oceanic lithosphere. The gravity inversion predicts Moho depth and geometry in agreement with wide-angle seismic estimates without invoking the extensive serpentinisation of the upper-mantle needed by the intra-continental rift hypothesis (O'Reilly 1995). Recent seismic modelling (Morewood 2005) suggests that the thin crust in the southern Rockall Trough does not have the seismic layering associated with oceanic crust formed at intermediate or fast spreading rates. Also, wide-angle seismic data shows low upper mantle seismic velocities are present and spatially associated with the thin 5-6km crust (Shannon 1999). These observations are consistent with models and observations of oceanic crust formed at slow spreading ocean ridges (Cannat 1996, Jokat 2003). Such models are based on a proportion of melt being retained in the upper mantle, producing low seismic velocities, and a reduced supply of melt to the crust, resulting in thin seismic crust with some serpentinised mantle material included. We propose that the southern Rockall Trough was formed by continental break-up and a period of slow mid Cretaceous sea floor spreading rather than as an intra- continental rift basin. This work forms part of the NERC Margins iSIMM project. iSIMM investigators are from Liverpool and Cambridge Universities, Badley Geoscience & Schlumberger Cambridge Research supported by the NERC, the DTI, Agip UK, BP, Amerada Hess Ltd, Anadarko, Conoco-Phillips, Shell, Statoil and WesternGeco. The iSIMM team comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, AR Chappell, J Eccles, RJ Fletcher, D Healy, N Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, VJ Tymms & R Spitzer.

Vertical and Horizontal Analysis of Crustal Structure of Southeastern Mediterranean and the Egyptian Coastal Zone, from Bouguer and Satellite Mission Data

NASA Astrophysics Data System (ADS)

Saleh, Salah

2016-07-01

The present Tectonic system of Southeastern Mediterranean is driven by the collision of the African and Eurasian plates, the Arabian Eurasian convergence and the displacement of the Anatolian Aegean microplate, which generally represents the characteristic of lithospheric structure of the region. In the scope of this study, Bouguer and the satellite gravity (satellite altimetry) anomalies of southeastern Mediterranean and North Eastern part of Egypt were used for investigating the lithospheric structures. Second order trend analyses were applied firstly to Bouguer and satellite altimetry data for examining the characteristic of the anomaly. Later, the vertical and horizontal derivatives applications were applied to the same data. Generally, the purpose of the applying derivative methods is determining the vertical and horizontal borders of the structure. According to the results of derivatives maps, the study area could mainly divided into important four tectonic subzones depending on basement and Moho depth maps. These subzones are distributed from south to the north as: Nile delta-northern Sinai zone, north Egyptian coastal zone, Levantine basin zone and northern thrusting (Cyprus and its surroundings) zone. These zones are separated from each other by horizontal tectonic boundaries and/or near-vertical faults that display the block-faulting tectonic style of this belt. Finally, the gravity studies were evaluated together with the seismic activity of the region. Consequently, the geodynamical structure of the region is examined with the previous studies done in the region. Thus, the current study indicates that satellite gravity mission data is a valuable source of data in understanding the tectonic boundary behavior of the studied region and that satellite gravity data is an important modern source of data in the geodynamical studies.

Basement structures over Rio Grande Rise from gravity inversion

NASA Astrophysics Data System (ADS)

Constantino, Renata; Hackspacker, Peter Christian; Anderson de Souza, Iata; Sousa Lima Costa, Iago

2017-04-01

In this study, we show that from satellite-derived gravity field, bathymetry and sediment thicknesses, it is possible to give a 3-D model of the basement over oceanic areas, and for this purpose, we have chosen the Rio Grande Rise, in South Atlantic Ocean, to build a gravity-equivalent basement topography. The advantages of the method applied in this study are manifold: does not depend directly on reflection seismic data; can be applied quickly and with fewer costs for acquiring and interpreting the data; and as the main result, presents the physical surface below the sedimentary layer, which may be different from the acoustic basement. We evaluated the gravity effect of the sediments using the global sediment thickness model of NOAA, fitting a sediment compaction model to observed density values from Deep Sea Drilling Program (DSDP) reports. The Global Relief Model ETOPO1 and constraining data from seismic interpretation on crustal thickness are integrated in the gravity inversion procedure. The modeled Moho depth values vary between 6 to 27 km over the area, being thicker under the Rio Grande Rise and also in the direction of São Paulo Plateau. The inversion for the gravity-equivalent basement topography is applied for a gravity residual data, which is free from the gravity effect of sediments and from the gravity effect of the estimated Moho interface. A description of the basement depth over Rio Grande Rise area is unprecedented in the literature, however, our results could be compared to in situ data, provided by DSDP, and a small difference of only 9 m between our basement depth and leg 516 F was found. Our model shows a rift crossing the entire Rio Grande Rise deeper than previously presented in literature, with depths up to 5 km in the East Rio Grande Rise (ERGR) and deeper in the West Rio Grande Rise (WRGR), reaching 6.4 km. We find several short-wavelengths structures not present in the bathymetry data. Seamounts, guyots and fracture zones are much more clearly defined in the basement than in the bathymetric model. An interesting NS structure that goes from 34S and extends through de São Paulo Ridge is interpreted in the basement model, and we propose that this feature can be related to the South Atlantic opening, revealing an extinct spreading center.

Three-dimensional velocity models of the Mount St. Helens magmatic system using the iMUSH active-source data set

NASA Astrophysics Data System (ADS)

Kiser, E.; Levander, A.; Zelt, C. A.; Palomeras, I.; Creager, K.; Ulberg, C. W.; Schmandt, B.; Hansen, S. M.; Harder, S. H.; Abers, G. A.; Crosbie, K.

2017-12-01

Building upon previously published 2D results, this presentation will show the first 3D velocity models down to the Moho using the iMUSH (imaging Magma Under St. Helens) active-source seismic data set. Direct P and S wave travel times from 23 borehole shots recorded at approximately 6000 seismograph locations are used to model Vp, Vs, and Vp/Vs over an area extending approximately 75 km from the edifice of Mount St. Helens and down to approximately 15 km depth. At shallow depths, results show several high and low velocity anomalies that correspond well with known geological features. These include the Chehalis Basin northwest of Mount St. Helens, and the Silver Star Mountain, Spirit Lake, and Spud Mountain plutons. Starting at 4 km depth, low velocities and high Vp/Vs values are observed near Mount St. Helens, which may be associated with shallow magmatic fluids. High Vp/Vs values are also observed beneath the Indian Heaven Volcanic Field southeast of Mount St. Helens. At the regional scale, high amplitude north/south trending low and high velocity features extend from the western margin of the resolved models to approximately 30 km west of Mount St. Helens. In general these high and low velocity features also correspond to high and low Vp/Vs anomalies, respectively. These results are in agreement with previous studies that conclude that the accreted terrane Siletzia is composed of multiple igneous bodies interspersed with sedimentary units in this region. Another regional feature of interest is a broad low Vp/Vs area between Mount St. Helens, Mount Adams, and Mount Rainier that spatially correlates with the Southern Washington Cascades Conductor, indicating a non-magmatic origin to this body at shallow and mid-crustal depths. In addition to these shallow results, preliminary 3D velocity models of the entire crust will be presented that utilize both direct and reflected seismic phases from the Moho and other mid-crustal discontinuities. These models will constrain the lateral extents of lower-crustal high and low velocity features observed in previous 2D results. This information will be critical for understanding the distribution of cumulate bodies, magma reservoirs, and accreted terranes in the lower crust, and how these features have affected recent volcanic activity in this region.

The structure of 0- to 0.2-m.y.-old oceanic crust at 9°N on the East Pacific Rise from expanded spread profiles

NASA Astrophysics Data System (ADS)

Vera, E. E.; Mutter, J. C.; Buhl, P.; Orcutt, J. A.; Harding, A. J.; Kappus, M. E.; Detrick, R. S.; Brocher, T. M.

1990-09-01

We analyze four expanded spread profiles acquired at distances of 0, 2.1, 3.1, and 10 km (0-0.2 m.y.) from the axis of the East Pacific Rise between 9° and 10°N. Velocity-depth models for these profiles have been obtained by travel time inversion in the τ-p domain, and by x-t forward modeling using the WKBJ and the reflectivity methods. We observe refracted arrivals that allow us to determine directly the uppermost crustal velocity structure (layer 2A). At the seafloor we find very low Vp and VS/Vp values around 2.2 km/s and ≤ 0.43. In the topmost 100-200 m of the crust, Vp remains low (≤ 2.5 km/s) then rapidly increases to 5 km/s at ˜500 m below the seafloor. High attenuation values (Qp < 100) are suggested in the topmost ˜500 m of the crust. The layer 2-3 transition probably occurs within the dike unit, a few hundred meters above the dike-gabbro transition. This transition may mark the maximum depth of penetration by a cracking front and associated hydrothermal circulation in the axial region above the axial magma chamber (AMC). The on-axis profile shows arrivals that correspond to the bright AMC event seen in reflection lines within 2 km of the rise axis. The top of the AMC lies 1.6 km below the seafloor and consists of molten material where Vp ≈ 3 km/s and VS = 0. Immediately above the AMC, there is a zone of large negative velocity gradients where, on the average, Vp decreases from ˜6.3 to 3 km/s over a depth of approximately 250 m. Associated with the AMC there is a low velocity zone (LVZ) that extends to a distance no greater than 10 km away from the rise axis. At the top of the LVZ, sharp velocity contrasts are confined to within 2 km of the rise axis and are associated with molten material or material with a high percentage of melt which would be concentrated only in a thin zone at the apex of the LVZ, in the axial region where the AMC event is seen in reflection lines. Away from the axis, the transition to the LVZ is smoother, the top of the LVZ is deeper, and the LVZ is less pronounced. The bottom of the LVZ is probably located near the bottom of the crust and above the Moho. Moho arrivals are observed in the profiles at zero and at 10 km from the rise axis. Rather than a single discontinuity, these arrivals indicate an approximately 1-km-thick Moho transition zone.

Seismic structure of the lithosphere and upper mantle beneath the ocean islands near mid-oceanic ridges

NASA Astrophysics Data System (ADS)

Haldar, C.; Kumar, P.; Kumar, M. Ravi

2014-05-01

Deciphering the seismic character of the young lithosphere near mid-oceanic ridges (MORs) is a challenging endeavor. In this study, we determine the seismic structure of the oceanic plate near the MORs using the P-to-S conversions isolated from quality data recorded at five broadband seismological stations situated on ocean islands in their vicinity. Estimates of the crustal and lithospheric thickness values from waveform inversion of the P-receiver function stacks at individual stations reveal that the Moho depth varies between ~ 10 ± 1 km and ~ 20 ± 1 km with the depths of the lithosphere-asthenosphere boundary (LAB) varying between ~ 40 ± 4 and ~ 65 ± 7 km. We found evidence for an additional low-velocity layer below the expected LAB depths at stations on Ascension, São Jorge and Easter islands. The layer probably relates to the presence of a hot spot corresponding to a magma chamber. Further, thinning of the upper mantle transition zone suggests a hotter mantle transition zone due to the possible presence of plumes in the mantle beneath the stations.

Lithospheric velocity structure of the Anatolian plateau-Caucasus-Caspian region

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gök, R.; Mellors, R. J.; Sandvol, E.

The Anatolian plateau-Caucasus-Caspian region is an area of complex lithospheric structure accompanied by large variations in seismic wave velocities. Despite the complexity of the region, little is known about the detailed lithospheric structure. Using data from 31 new, permanent broadband seismic stations along with results from a previous 29 temporary seismic stations and 3 existing global seismic stations in the region, a 3-D velocity model is developed using joint inversion of teleseismic receiver functions and surface waves. Both group and phase dispersion curves (Love and Rayleigh) were derived from regional and teleseismic events. Additional Rayleigh wave group dispersion curves weremore » determined using ambient noise correlation. Receiver functions were calculated using P arrivals from 789 teleseismic (30°–90°) earthquakes. The stacked receiver functions and surface wave dispersion curves were jointly inverted to yield the absolute shear wave velocity to a depth of 100 km at each station. The depths of major discontinuities (sediment-basement, crust-mantle, and lithosphere-asthenosphere) were inferred from the velocity-depth profiles at the location of each station. Distinct spatial variations in crustal and upper mantle shear velocities were observed. The Kura basin showed slow (~2.7–2.9 km/s) upper crustal (0–11 km) velocities but elevated (~3.8–3.9 km/s) velocities in the lower crust. The Anatolian plateau varied from ~3.1–3.2 in the upper crust to ~3.5–3.7 in the lower crust, while velocities in the Arabian plate (south of the Bitlis suture) were slightly faster (upper crust between 3.3 and 3.4 km/s and lower crust between 3.8 and 3.9 km/s). The depth of the Moho, which was estimated from the shear velocity profiles, was 35 km in the Arabian plate and increased northward to 54 km at the southern edge of the Greater Caucasus. Moho depths in the Kura and at the edge of the Caspian showed more spatial variability but ranged between 35 and 45 km. Upper mantle velocities were slow under the Anatolian plateau but increased to the south under the Arabian plate and to the east (4.3–4.4 km/s) under the Kura basin and Greater Caucasus. The areas of slow mantle coincided with the locations of Holocene volcanoes. Differences between Rayleigh and Love dispersions at long wavelengths reveal a pronounced variation in anisotropy between the Anatolian plateau and the Kura basin.« less

Lithospheric velocity structure of the Anatolian plateau-Caucasus-Caspian region

DOE PAGES

Gök, R.; Mellors, R. J.; Sandvol, E.; ...

2011-05-07

The Anatolian plateau-Caucasus-Caspian region is an area of complex lithospheric structure accompanied by large variations in seismic wave velocities. Despite the complexity of the region, little is known about the detailed lithospheric structure. Using data from 31 new, permanent broadband seismic stations along with results from a previous 29 temporary seismic stations and 3 existing global seismic stations in the region, a 3-D velocity model is developed using joint inversion of teleseismic receiver functions and surface waves. Both group and phase dispersion curves (Love and Rayleigh) were derived from regional and teleseismic events. Additional Rayleigh wave group dispersion curves weremore » determined using ambient noise correlation. Receiver functions were calculated using P arrivals from 789 teleseismic (30°–90°) earthquakes. The stacked receiver functions and surface wave dispersion curves were jointly inverted to yield the absolute shear wave velocity to a depth of 100 km at each station. The depths of major discontinuities (sediment-basement, crust-mantle, and lithosphere-asthenosphere) were inferred from the velocity-depth profiles at the location of each station. Distinct spatial variations in crustal and upper mantle shear velocities were observed. The Kura basin showed slow (~2.7–2.9 km/s) upper crustal (0–11 km) velocities but elevated (~3.8–3.9 km/s) velocities in the lower crust. The Anatolian plateau varied from ~3.1–3.2 in the upper crust to ~3.5–3.7 in the lower crust, while velocities in the Arabian plate (south of the Bitlis suture) were slightly faster (upper crust between 3.3 and 3.4 km/s and lower crust between 3.8 and 3.9 km/s). The depth of the Moho, which was estimated from the shear velocity profiles, was 35 km in the Arabian plate and increased northward to 54 km at the southern edge of the Greater Caucasus. Moho depths in the Kura and at the edge of the Caspian showed more spatial variability but ranged between 35 and 45 km. Upper mantle velocities were slow under the Anatolian plateau but increased to the south under the Arabian plate and to the east (4.3–4.4 km/s) under the Kura basin and Greater Caucasus. The areas of slow mantle coincided with the locations of Holocene volcanoes. Differences between Rayleigh and Love dispersions at long wavelengths reveal a pronounced variation in anisotropy between the Anatolian plateau and the Kura basin.« less

Seismic Refraction & Wide-angle Reflection Experiment on the Northern Margin of North China Craton -Data Acquisition and Preliminary Processing Result

NASA Astrophysics Data System (ADS)

Li, W.; Gao, R.; Keller, G. R.; Hou, H.; Li, Q.; Cox, C. M.; Chang, J. C.; Zhang, J.; Guan, Y.

2010-12-01

The evolution history of Central Asian Orogen Belt (CAOB) is still the main tectonic problems in northeastern Asia. The Siberia Craton (NC), North China Craton (NCC) and several blocks collided, and the resulting tectonic collage formed as the Paleo-Asian Ocean disappeared. Concerning the northern margin of North China Craton, many different geological questions remain unanswered, such as: the intracontinental orogenic process in the Yanshan orogen and the nature and location of the suture between the southern NC and the northern NCC. In Dec 2009, a 400 km long seismic refraction and wide-angle reflection profile was completed jointly by Institute of Geology, CAGS and University of Oklahoma. The survey line extended from the west end of the Yanshan orogen, across a granitoid belt to the Solonker suture zone. The recording of seismic waves from 8 explosions (500~1500 kg each) was conducted in four deployments of 300 Reftek125 (Texan) seismic recorders, with an average spacing of 1 km. For the calculations, we used the Rayinvr, Vmed and Zplot programs for ray tracing, model modification and phase picking. The initial result show that: 1)the depth of low velocity sediment cover ranges from 0.6 to 2.7 km (velocity: 2.8~5.6 km/s); 2)the depth of basement is 5.6~10 km (the depth of basement under the granitoid belt deepens to 10 km and velocity increases to 6.2 km/s); 3)the upper crust extends to a depth of 15.5~21 km and has the P-wave velocities between 5.6 and 6.4 km/s; 4)the thickness of the lower crust ranges from 22~28 km(velocity: 6.4~6.9 km/s); and 5)the depth of Moho varies from 39.5 km under the granitoid belt to 49 km under the Yanshan orogen. Based on these results, we can preliminarily deduce that: 1) the concave depression of the Moho observed represents the root of the Yanshan orogen, and it may prove that the orogen is dominated by thick-skinned tectonics; 2) the shape of velocity variations under the granitoid belt is suggestive of a magma conduit. It may be connected with subduction-collision magmatism between the southern NC and the northern NCC along the Solonker suture zone. Supported by Sinoprobe-02 and US NSF PIRE grant (0730154)

Along-arc variation in water distribution in the upper mantle beneath Kyushu, Japan, as derived from receiver function analyses

NASA Astrophysics Data System (ADS)

Abe, Y.; Ohkura, T.; Hirahara, K.; Shibutani, T.

2013-12-01

The Kyushu district, Japan, under which the Philippine Sea (PHS) plate is subducting in a WNW direction, has several active volcanoes. On the volcanic front in Kyushu, a 110 km long gap in volcanism exists in the central part of Kyushu and volcanic rocks with various degrees of contamination by slab-derived fluid are distributed. To reveal the causes of the gap in volcanism and the chemical properties of volcanic rocks and to understand the process of magma genesis and water transportation, we should reveal along-arc variation in water distribution beneath Kyushu. We investigated the seismic velocity discontinuities in the upper mantle beneath Kyushu, with seismic waveform data from 65 stations of Hi-net, which are established by National Research Institute for Earth Science and Disaster Prevention, and 55 stations of the J-array, which are established by Japan Meteorological Agency, Kyushu University, Kagoshima University and Kyoto University. We used receiver function analyses developed especially for discontinuities with high dipping angles (Abe et al., 2011, GJI). We obtained the geometry and velocity contrasts of the continental Moho, the oceanic Moho, and the upper boundary of the PHS slab. From the geometry of these discontinuities and contrast in S wave velocities, we interpreted that the oceanic crust of the PHS slab has a low S wave velocity and is hydrated to a depth of 70 km beneath south Kyushu, to a depth of 80-90 km beneath central Kyushu, and to a depth of no more than 50 km beneath north Kyushu. We also interpreted that the fore-arc mantle beneath central Kyushu has a low velocity region (Vs < 3.2 km/s) that can contain hydrated materials and free aqueous fluid. Such a low velocity fore-arc mantle does not exist beneath north and south Kyushu. Beneath north Kyushu, the oceanic crust does not appear to convey much water in the mantle wedge. Beneath south Kyushu, water dehydrated from the slab could move to the back-arc side and cause arc volcanism, while it could move to the fore-arc side and cause a gap in volcanism and hydration of the fore-arc mantle materials.

An integrated geological and geophysical study of the Parnaíba cratonic basin, North-East Brazil

NASA Astrophysics Data System (ADS)

Tozer, B.; Watts, A. B.; Daly, M.

2015-12-01

Cratonic basins are characterized by their sub-circular shape, long-lived (>100 Myr) subsidence, shallow marine/terrestrial sediments that young towards the center of the basin and exhibit little internal deformation, and thick seismic lithosphere. Despite the recognition of >30 world-wide, the paucity of geological and geophysical data over these basins means their origin remains enigmatic. In order to address this problem, we have used a recently acquired 1400 km long seismic reflection profile recorded to 20 s TWTT, field observations and well logs, gravity and magnetic data acquired at 1 km intervals, and five wide-angle refection/refraction receiver gathers recorded at offsets up to 100 km, to constrain the origin of the Parnaíba basin, North-East Brazil. We find a depth to pre-Paleozoic basement and Moho of ~ 3.5 and ~ 40 - 42 km respectively beneath the basin center. A prominent mid-crustal reflection (MCR) can be tracked laterally for ~ 300 km at depths between 17 - 25 km and a low-fold wide-angle receiver gather stack shows that the crust below the MCR is characterized by a ~ 4 s TWTT package of anastomosing reflections. Gravity modelling suggests that the MCR represents the upper surface of a high density (+0.14 kg m-3) lower crustal body, which is probably of magmatic origin. Backstripping of biostratigraphic data from wells in the center of the basin show an exponentially decreasing subsidence. We show that although cooling of a thick (180 km) lithosphere following prolonged rifting (~ 65 Myr) can provide a good fit to the tectonic subsidence curves, process-oriented gravity and flexure modelling suggest that other processes must be important, as rifting does not account for the observed gravity anomaly and predicts too thin a crust (~ 34 km). The thicker than expected crust suggests warping due, for example, to far-field stresses or basal tractions. Another possibility, which is compatible with existing geophysical data, is a dense magmatic intrusion in the lower crust that has loaded and flexed the pre-existing Moho downwards to greater depths than beneath flanking Archaen and Proterozoic terranes.

3D Integrated geophysical-petrological modelling of the Iranian lithosphere

NASA Astrophysics Data System (ADS)

Mousavi, Naeim; Ardestani, Vahid E.; Ebbing, Jörg; Fullea, Javier

2016-04-01

The present-day Iranian Plateau is the result of complex tectonic processes associated with the Arabia-Eurasia Plate convergence at a lithospheric scale. In spite of previous mostly 2D geophysical studies, fundamental questions regarding the deep lithospheric and sub-lithospheric structure beneath Iran remain open. A robust 3D model of the thermochemical lithospheric structure in Iran is an important step toward a better understanding of the geological history and tectonic events in the area. Here, we apply a combined geophysical-petrological methodology (LitMod3D) to investigate the present-day thermal and compositional structure in the crust and upper mantle beneath the Arabia-Eurasia collision zone using a comprehensive variety of constraining data: elevation, surface heat flow, gravity potential fields, satellite gravity gradients, xenoliths and seismic tomography. Different mantle compositions were tested in our model based on local xenolith samples and global data base averages for different tectonothermal ages. A uniform mantle composition fails to explain the observed gravity field, gravity gradients and surface topography. A tectonically regionalized lithospheric mantle compositional model is able to explain all data sets including seismic tomography models. Our preliminary thermochemical lithospheric study constrains the depth to Moho discontinuity and intra crustal geometries including depth to sediments. We also determine the depth to Curie isotherm which is known as the base of magnetized crustal/uppermost mantle bodies. Discrepancies with respect to previous studies include mantle composition and the geometry of Moho and Lithosphere-Asthenosphere Boundary (LAB). Synthetic seismic Vs and Vp velocities match existing seismic tomography models in the area. In this study, depleted mantle compositions are modelled beneath cold and thick lithosphere in Arabian and Turan platforms. A more fertile mantle composition is found in collision zones. Based on our 3D thermochemical model we propose a new scenario to interpret the geodynamical history of area. In this context the present-day central Iran block would be as remain of the older and larger Iranian block present before the onset of Turan platform subduction beneath the Iranian Plateau. Further analysis of sub-lithospheric density anomalies (e.g., subducted slabs) is required to fully understand the geodynamics of the area.

Crustal and Upper Mantle S-velocity Structure From Receiver Functions Analisys Around Terra Nova Bay Base, Antartica

NASA Astrophysics Data System (ADS)

Piana Agostinetti, N.; Amato, A.; Cattaneo, M.; de Gori, P.; di Bona, M.

In the framework of the italian PNRA (Progetto Nazionale di Ricerche in Antartide), we have started to re-analize teleseismic waveforms recorded, using three-components seismometers (equipped with 5 seconds sensors, Lennartz 3D-5s), during five summer campaings, from 1993 to 2000. Seismic stations were deployed around Terra Nova Bay (TNB) italian base, from the sea to reach the interior of the Transantartic Moun- tains (TAM), the most striking example of nocontractional mountain belt. During the last campaingn (1999-2000) seismic stations were deployed deep into Northern Vic- toria Land to reach Rennik and Lillie Glaciers Area and George V coast region, the northest part of TAM. Our main goals were: to compute, using frequency-domanin deconvolution method by Di Bona [1998], Receiver Functions covering all the area around TNB italian antartic base; to map of Moho-depth and intercrustal S-waves ve- locity discontinuity from 1-D velocity model computed using Sambridge's inversion scheme [Sambridge,1999]; to analize new teleseimic waveforms recorded near TNB base: continuos recording, from 1999 to present, permits more accurate modelling S-velocity crustal structure in this critical area situated at the edge of the ipothetic rift [Stern and ten Brik, 1989; Stump and Fitzgerald, 1992; ten Brik et al., 1997]; to present final results from BACKTAM expedition.

Imaging the midcontinent rift beneath Lake Superior using large aperture seismic data

USGS Publications Warehouse

Tréhu, Anne M.; Morel-a-l'Huissier, Patrick; Meyer, R.; Hajnal, Z.; Karl, J.; Mereu, R.F.; Sexton, John L.; Shay, J.; Chan, W. K.; Epili, D.; Jefferson, T.; Shih, X. R.; Wendling, S.; Milkereit, B.; Green, A.; Hutchinson, Deborah R.

1991-01-01

We present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore-offshore large-aperture seismic and gravity data. High velocities obtained within a highly reflective half-graben that was imaged on coincident seismic reflection data demonstrate the dominantly mafic composition of the graben fill and constrain its total thickness to be at least 30km. Strong wide-angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (55–60km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high-velocity lower crust interpreted to result from syn-rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half-graben. A second region of thick lower crust is found approximately 100km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre-rift Moho is probably within or above the high-velocity lower crust, whereas the post-rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.

Rifting-to-drifting transition of the South China Sea: Moho reflection characteristics in continental-ocean transition zone

NASA Astrophysics Data System (ADS)

Wen, Y.; Li, C.

2017-12-01

Dispute remains on the process of continental rifting to subsequent seafloor spreading in the South China Sea (SCS). Several crust-scale multi-channel seismic reflection profiles acquired in the continent-ocean transition zone (COT) of the SCS provide a detailed overview of Moho and deep crustal reflectors and give key information on rifting-to-drifting transition of the area. Moho has strong but discontinuous seismic reflection in COT. These discontinuities are mainly located in the landward side of continent-ocean boundary (COB), and may own to upwelling of lower crustal materials during initial continental extension, leading to numerous volcanic edifices and volcanic ridges. The continental crust in COT shows discontinuous Moho reflections at 11-8.5 s in two-way travel time (twtt), and thins from 18-20.5 km under the uppermost slope to 6-7 km under the lower slope, assuming an average crustal velocity of 6.0 km/s. The oceanic crust has Moho reflections of moderate to high continuity mostly at 1.8-2.2 s twtt below the top of the igneous basement, which means that the crustal thickness excluding sediment layer in COT is 5.4-6.6 km. Subhorizontal Moho reflections are often abruptly interrupted by large seaward dipping normal faults in southern COT but are more continuous compared with the fluctuant and very discontinuous Moho reflections in northern COT. The thickness of thinned continental crust (4.2-4.8 km) is smaller than that of oceanic crust (5.4-6.0 km) near southern COB, indicating that the continental crust has experienced a long period of rifting before seafloor spreading started. The smaller width of northern COT (0-40 km) than in southern COT (0-60 km), and thinner continental crust in southern COT, all indicate that the continental margin rifting and extension was asymmetric. The COT width in the SCS is narrower than that found in other magma-poor continental margins, indicating a swift transition from the final stage of rifting to the inception of normal seafloor spreading.

Mantle wedge anisotropy beneath the Western Alps: insights from Receiver Function analysis

NASA Astrophysics Data System (ADS)

Piana Agostinetti, Nicola; Salimbeni, Simone; Pondrelli, Silvia; Malusa', Marco; Zhao, Liang; Eva, Elena; Solarino, Stefano; Paul, Anne; Guillot, Stéphane; Schwartz, Stéphane; Dumont, Thierry; Aubert, Coralie; Wang, Qingchen; Zhu, Rixiang

2017-04-01

Orogens and subductions zones are the locus where crustal materials are recycled into the upper mantle. Such rocks undergo to several metamorphic reactions during which their seismic properties vary due to the changes in P-T conditions. Metamorphic reactions can imply: (a) the formation of schist-like materials, and (b) a pronounced water flux from the subducted crust. Both these processes should generate highly anisotropic volumes at upper mantle depths. Thus, unveiling the presence of seismic anisotropy at such depth level can put constraints on the metamorphic reactions and the P-T conditions of the subducted materials. The Alpine orogen is composed of three main regions where different geodynamic processes shaped a highly heterogeneous mountain chain. Beneath the Alps, a high velocity body has been imaged sinking in the upper mantle, indicating the presence of a relict of subduction. Such subduction process has been probably terminated with the closure of the Piemont-Liguria Ocean, but evidence of continental subduction has been found beneath the Western Alps. Seismic anisotropy is likely to develop both in the subducted materials and in the mantle wedge, where serpentinized materials could be found due to the low T conditions. We analysed P receiver function (RF) from 46 seismic stations deployed along a linear array crossing the Western Alps, where previous studies revealed the presence of the subducted European lower crust to 80 km depth. RF is a widely used tool for reconstructing subsurface seismic structures, based on the recognition of P-to-S converted phases in teleseismic P-wave coda. The RF data-set is migrated at depth and decomposed into azimuthal harmonics. Computing the first, k=0, and the second, k=1, harmonics allows to separate the "isotropic" contribution, due to the change of the isotropic properties of the sampled materials (recorded on the k=0 harmonics), from the "anisotropic" contribution, where the energy is related to the propagation of the P-wave through anisotropic materials (recorded on the k=1 harmonics). Preliminary results show the presence of a Ps phase on the k=0 harmonics along the western portion of the profile, with increasing time-delay toward East. This phase is interpreted as the European Moho Ps, confirming the geometry of the European Moho beneath the Western Alps. Beneath the internal portion of the orogen, the k=1 harmonics display energetic pulses between 3-7 s, indicating the development of anisotropy within a broad volume of rocks, at lower crustal and upper mantle depths. The presence of anisotropic materials is jointly interpreted with the depicted geometry of the main seismic discontinuities and the location of the intermediate-depth seismicity recorded in the region.

Seismic signature of the Alpine indentation, evidence from the Eastern Alps

PubMed Central

Bianchi, I.; Bokelmann, G.

2014-01-01

The type of collision between the European and the Adriatic plates in the easternmost Alps is one of the most interesting questions regarding the Alpine evolution. Tectonic processes such as compression, escape and uplift are interconnected and shape this area. We can understand these ongoing processes better, if we look for signs of the deformation within the Earth's deep crust of the region. By collecting records from permanent and temporary seismic networks, we assemble a receiver function dataset, and analyze it with the aim of giving new insights on the structure of the lower crust and of the shallow portion of the upper mantle, which are inaccessible to direct observation. Imaging is accomplished by performing common conversion depth stacks along three profiles that crosscut the Eastern Alpine orogen, and allow isolating features consistently persistent in the area. The study shows a moderately flat Moho underlying a seismically anisotropic middle-lower crust from the Southern Alps to the Austroalpine nappes. The spatial progression of anisotropic axes reflects the orientation of the relative motion and of the stress field detected at the surface. These observations suggest that distributed deformation is due to the effect of the Alpine indentation. In the shallow upper mantle right below the Moho interface, a further anisotropic layer is recognized, extended from the Bohemian Massif to the Northern Calcareous Alps. PMID:26525181

Cordilleran Longevity, Elevation and Heat Driven by Lithospheric Mantle Removal

NASA Astrophysics Data System (ADS)

Mackay-Hill, A.; Currie, C. A.; Audet, P.; Schaeffer, A. J.

2017-12-01

Cordilleran evolution is controlled by subduction zone back-arc processes that generate and maintain high topography due to elevated uppermost mantle temperatures. In the northern Canadian Cordillera (NCC), the persisting high mean elevation long after subduction has stopped (>50 Ma) requires a sustained source of heat either from small-scale mantle convection or lithospheric mantle removal; however direct structural constraints of these processes are sparse. We image the crust and uppermost mantle beneath the NCC using scattered teleseismic waves recorded on an array of broadband seismograph stations. We resolve two sharp and flat seismic discontinuities: a downward velocity increase at 35 km that we interpret as the Moho; and a deeper discontinuity with opposite velocity contrast at 50 km depth. Based on petrologic estimates, we interpret the deeper interface as the lithosphere-asthenosphere boundary (LAB), which implies an extremely thin ( 15 km) lithospheric mantle. We calculate the temperature at the Moho and the LAB in the range 800-900C and 1200-1300C, respectively. Below the LAB, we find west-dipping features far below the LAB beneath the eastern NCC that we associate with laminar downwelling of Cordilleran lithosphere. Whether these structures are fossilized or active, they suggest that lithospheric mantle removal near the Cordillera-Craton boundary may have provided the source of heat and elevation and therefore played a role in the longevity and stability of the Cordillera.

Seismic signature of the Alpine indentation, evidence from the Eastern Alps.

PubMed

Bianchi, I; Bokelmann, G

2014-12-01

The type of collision between the European and the Adriatic plates in the easternmost Alps is one of the most interesting questions regarding the Alpine evolution. Tectonic processes such as compression, escape and uplift are interconnected and shape this area. We can understand these ongoing processes better, if we look for signs of the deformation within the Earth's deep crust of the region. By collecting records from permanent and temporary seismic networks, we assemble a receiver function dataset, and analyze it with the aim of giving new insights on the structure of the lower crust and of the shallow portion of the upper mantle, which are inaccessible to direct observation. Imaging is accomplished by performing common conversion depth stacks along three profiles that crosscut the Eastern Alpine orogen, and allow isolating features consistently persistent in the area. The study shows a moderately flat Moho underlying a seismically anisotropic middle-lower crust from the Southern Alps to the Austroalpine nappes. The spatial progression of anisotropic axes reflects the orientation of the relative motion and of the stress field detected at the surface. These observations suggest that distributed deformation is due to the effect of the Alpine indentation. In the shallow upper mantle right below the Moho interface, a further anisotropic layer is recognized, extended from the Bohemian Massif to the Northern Calcareous Alps.

Exploring the Earth's crust: history and results of controlled-source seismology

USGS Publications Warehouse

Prodehl, Claus; Mooney, Walter D.

2012-01-01

This volume contains a comprehensive, worldwide history of seismological studies of the Earth’s crust using controlled sources from 1850 to 2005. Essentially all major seismic projects on land and the most important oceanic projects are covered. The time period 1850 to 1939 is presented as a general synthesis, and from 1940 onward the history and results are presented in separate chapters for each decade, with the material organized by geographical region. Each chapter highlights the major advances achieved during that decade in terms of data acquisition, processing technology, and interpretation methods. For all major seismic projects, the authors provide specific details on field observations, interpreted crustal cross sections, and key references. They conclude with global and continental-scale maps of all field measurements and interpreted Moho contours. An accompanying DVD contains important out-of-print publications and an extensive collection of controlled-source data, location maps, and crustal cross sections.

Restoration of the Cretaceous uplift of the Harz Mountains, North Germany: Evidence for thick-skinned thrusting

NASA Astrophysics Data System (ADS)

Tanner, David C.; Krawczyk, Charlotte M.

2017-04-01

Fault prediction and kinematic restoration are useful tools to firstly determine the likely geometry of a fault at depth and secondly restore the pre-deformation state to discover, for instance, paleogeometry. The inclined-shear method with constant slip uses the known geometry of the surface position and dip of the fault and the geometries of the hanging and footwall beds to predict the probable shape of the fault at depth, down to a detachment level. We use this method to determine the geometry of the Northern Harz Boundary Fault in northern Germany that was responsible for the uplift of the Harz Mountains during Late Cretaceous inversion. A shear angle of 30° was most likely in this case, as indicated by geological and geophysical data. This suggests that the detachment level is at a depth of ca. 25 km. Kinematic restoration of the Harz Mountains using this fault geometry does not produce a flat horizon, rather it results in a 3.5 km depression. Restoration also causes a rotation of fabrics within the Harz Mountains of approximately 11° clockwise. Airy-Heiskanen isostatic equilibrium adjustment reduces the depression to ca. 1 km depth, as well as raising the Moho from 41 to 36 km depth. We show that this model geometry is also a very good fit to the interpreted DEKORP BASIN 9601 deep seismic profile.

Structure of the crust and upper mantle beneath the Balearic Islands (Western Mediterranean)

NASA Astrophysics Data System (ADS)

Banda, E.; Ansorge, J.; Boloix, M.; Córdoba, D.

1980-09-01

Data are presented from deep seismic sounding along the strike of the Balearic Islands carried out in 1976. The interpretation of the data gives the following results: A sedimentary cover of 4 km around Ibiza to 7 km under Mallorca overlies the crystalline basement. This basement with a P-wave velocity of 6.0 km/s at the top reaches a depth of at least 15 km under Ibiza and 17 km under Mallorca with an increase to 6.1 km/s at these depths. The crust-mantle boundary lies at a depth of 20 km and 25 km, respectively. A well documented upper-mantle velocity of 7.7 km/s is found along the entire profile. The Moho rises to a depth of 20 km about 30 km north of Mallorca and probably continues rising towards the center of the North Balearic Sea. The newly deduced crustal structure together with previously determined velocity-depth sections in the North Balearic Sea as well as heat flow and aeromagnetic data can be interpreted as an extended rift structure caused by large-scale tensional processes in the upper mantle. The available data suggest that the entire zone from the eastern Alboran Sea to the area north of the Balearic Islands represents the southeastern flank of this rift system. In this model the provinces of Spain along the east coast would represent the northwestern rift flank.

Reflection imaging of the Moon's interior using deep-moonquake seismic interferometry

NASA Astrophysics Data System (ADS)

Nishitsuji, Yohei; Rowe, C. A.; Wapenaar, Kees; Draganov, Deyan

2016-04-01

The internal structure of the Moon has been investigated over many years using a variety of seismic methods, such as travel time analysis, receiver functions, and tomography. Here we propose to apply body-wave seismic interferometry to deep moonquakes in order to retrieve zero-offset reflection responses (and thus images) beneath the Apollo stations on the nearside of the Moon from virtual sources colocated with the stations. This method is called deep-moonquake seismic interferometry (DMSI). Our results show a laterally coherent acoustic boundary around 50 km depth beneath all four Apollo stations. We interpret this boundary as the lunar seismic Moho. This depth agrees with Japan Aerospace Exploration Agency's (JAXA) SELenological and Engineering Explorer (SELENE) result and previous travel time analysis at the Apollo 12/14 sites. The deeper part of the image we obtain from DMSI shows laterally incoherent structures. Such lateral inhomogeneity we interpret as representing a zone characterized by strong scattering and constant apparent seismic velocity at our resolution scale (0.2-2.0 Hz).

Mantle Earthquakes in Thinned Proterozoic Lithosphere: Harrat Lunayyir, Saudi Arabia

NASA Astrophysics Data System (ADS)

Blanchette, A. R.; Klemperer, S. L.; Mooney, W. D.; Zahran, H. M.

2017-12-01

Harrat Lunayyir is an active volcanic field located in the western Arabian Shield 100 km outside of the Red Sea rift margin. We use common conversion point (CCP) stacking of P-wave receiver functions (PRFs) to show that the Moho is at 38 km depth, close to the 40 km crustal thickness measured in the center of the craton, whereas the lithosphere-asthenosphere boundary (LAB) is at 60 km, far shallower than the 150 km furthest in the craton. We locate 67 high-frequency earthquakes with mL ≤ 2.5 at depths of 40-50 km below the surface, located clearly within the mantle lid. The occurrence of earthquakes within the lithospheric mantle requires a geothermal temperature profile that is below equilibrium. The lithosphere cannot have thinned to its present thickness earlier than 15 Ma, either during an extended period of rifting possibly beginning 24 Ma or, more likely, as part of the second stage of rifting following collision between Arabia and Eurasia.

Deep Structure of Northern Apennines Subduction Orogen (Italy) as Revealed by a Joint Interpretation of Passive and Active Seismic Data

NASA Astrophysics Data System (ADS)

Piana Agostinetti, Nicola; Faccenna, Claudio

2018-05-01

The Apennines is a well-studied orogeny formed by the accretion of continental slivers during the subduction of the Adriatic plate, but its deep structure is still a topic of controversy. Here we illuminated the deep structure of the Northern Apennines belt by combining results from the analysis of active seismic (CROP03) and receiver function data. The result from combining these two approaches provides a new robust view of the structure of the deep crust/upper mantle, from the back-arc region to the Adriatic subduction zone. Our analysis confirms the shallow Moho depth beneath the back-arc region and defines the top of the downgoing plate, showing that the two plates separate at depth about 40 km closer to the trench than reported in previous reconstructions. This spatial relationship has profound implications for the geometry of the shallow subduction zone and of the mantle wedge, by the amount of crustal material consumed at trench.

Moho geometry along a north-south passive seismic transect through Central Australia

NASA Astrophysics Data System (ADS)

Sippl, Christian

2016-04-01

Receiver functions from a temporary deployment of 25 broadband stations along a north-south transect through Central Australia are used to retrieve crustal and uppermost mantle structural constraints from a combination of different methods. Using H-K stacking as well as receiver function inversion, overall thick crust with significant thickness variation along the profile (40 to ≥ 55 km) is found. Bulk crustal vp/vs values are largely in the felsic to intermediate range, with the southernmost stations on the Gawler Craton exhibiting higher values in excess of 1.8. A common conversion point (CCP) stacking profile shows three major discontinuities of the crust-mantle boundary: (1) a two-sided Moho downwarp beneath the Musgrave Province, which has previously been associated with the Neoproterozoic to early Cambrian Petermann Orogeny, (2) a Moho offset along the Redbank Shear Zone further north attributed to the Middle to Late Paleozoic Alice Springs Orogeny, and (3) another Moho offset further north, located at the boundary between the Davenport and Warramunga Provinces, which has not been imaged before. In all cases, the difference in crustal thickness between the two sides of the offset is > 8-10 km. Unlike the two southern Moho offsets, the northernmost one does not coincide with a prominent gravity anomaly. Its location and the absence of known reactivation events in the region make it likely that it belongs to a Proterozoic suture zone that marks a previously unknown block boundary within the North Australian Craton.

Imaging Ancient Sutures with EarthScope Transportable Array Magnetotelluric Data

NASA Astrophysics Data System (ADS)

Egbert, G. D.

2014-12-01

Magnetotellurics (MT) provides a powerful geophysical tool for imaging of ancient suture zones, which are frequently marked by elongated zones of very low resistivity. These conductive anomalies, which can extend to great depths and have apparently persisted for several billion years, most likely result from graphite and sulfides deeply emplaced and remobilized, through subduction, accretion and orogenesis. The Earthscope MT transportable array provides a unique broad-scale view of sutures in the continental US. In the northwestern US subvertical conductive features bound all of the major cratonic blocks. These can be identified with the Cheyenne Belt between the Wyoming Craton (WC) and Yavapai Terranes (YT), the Great Falls Tectonic Zone between WC and the Medicine Hat Block (MHB), and the Vulcan Structure of southern Alberta between MHB and the Hearne Craton. In all cases the conductive anomalies extend well into the mantle lithosphere. The more recent MT TA footprint in the north-central US (surrounding the Mid-Continent Rift (MCR)) also reveals conductive signatures of ancient sutures. The most prominent lies south of Lake Superior, just north of the Niagara Fault (NF), and can be associated with the Penokean Orogeny (~1.85 Ga). A second, further south beneath Iowa and western Wisconsin, just south of the Spirit Lake tectonic zone (SLtz), can be identified with YT accretion (~1.75 Ga). Both of these sutures are cleanly cut by the MCR. The break in the anomalies is narrow (comparable to the surface expression of the MCR) indicating that rifting impacts on the entire crustal section were highly localized. The south-dipping NF conductive anomaly extends from surface outcrop to at least the Moho. The SLtz anomaly is north-dipping, extending from mid-crust through the Moho. In both cases there is some evidence for a modestly conductive layer (likely carbon) thrust to mid-lithospheric depths within the overriding terrane.

Estimating gravity changes caused by crustal strain: application to the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Yin, Zhi; Xu, Caijun

2017-08-01

Temporal gravimetry is an efficient tool for monitoring mass transfers, but distinguishing the contribution of each process to the measured signals is challenging. Few effective methods have been developed to estimate the changes in gravity caused by crustal strain for large-scale geophysical problems. To fill this research gap, we proposed a formula that describes a negative linear correlation between changes in gravity and crustal dilatational strain. Surface observations of gravity changes and dilatational strains were simulated using PSGRN/PSCMP, which is a numerical code used to calculate the surface response to fault dislocations, and the accuracy of the formula was quantitatively verified. Four parameters are required for this formula: the crustal dilatational strain, the crustal density, the Moho depth, and a coefficient that characterizes the degree of crust-mantle coupling. To illustrate the application of this new method to a natural case study, including specifying the values of the necessary parameters, the crustal strain-caused gravity changes (CSGCs) were calculated at 1° × 1° grid nodes over the Tibetan Plateau (TP). The CSGC model shows that most of the crust of the TP is undergoing extension, which generates negative gravity signals. The magnitude of the Tibetan CSGC model is approximately 0.2 μGal yr-1, which is similar to the results obtained from numerical modelling of the crustal tectonics of the Taiwanese Orogen. To evaluate the reliability of the Tibetan CSGC model, the uncertainties in the crustal dilatational strain, crustal density, Moho depth, and crust-mantle coupling factor were evaluated and then used to estimate the CSGC uncertainty by applying the error propagation law. The CSGC model was used to analyse the mass transfers of the TP. The results suggest that a significant mass accumulation process may be occurring beneath the crust of the northern TP.

No Evidence for Connectivity between the Bushveld Igneous Complex and the Molopo Farms Complex from Forward Modeling of Receiver Functions

NASA Astrophysics Data System (ADS)

Skryzalin, P. A.; Ramirez, C.; Durrheim, R. J.; Raveloson, A.; Nyblade, A.; Feineman, M. D.

2016-12-01

The Bushveld Igneous Complex contains one of the most studied and economically important layered mafic intrusions in the world. The Rustenburg Layered Suite outcrops in northern South Africa over an area of 65,000 km2, and has a volume of up to 1,000,000 km3. Both the Bushveld Igneous Complex and the Molopo Farms Complex in Botswana intruded the crust at 2.05 Ga. Despite being extensively exploited by the mining industry, many questions still exist regarding the structure of the Bushveld Igneous Complex, specifically the total size and connectivity of the different outcrops. In this study, we used receiver function analysis, a technique for determining the seismic velocity structure of the crust and upper mantle, to search for evidence of the Bushveld at station LBTB, which lies in Botswana, between the Far Western Limb of the Bushveld and the Molopo Farms Complex. The goal of our study was to determine whether a fast, high-density mafic body can be seen in the crust beneath this region using receiver functions. Observation of a high density layer would argue in favor of connectivity of the Bushveld between The Far Western Limb and the Molopo Farms Complex. We forward modeled stacks of receiver functions as well as sub-stacks that were split into azimuthal groups which share similar characteristics. We found that there was no evidence for a high velocity zone in the crust, and that the Moho in this region is located at a depth of 38 ± 3 km, about 8-9 km shallower than Moho depths determined beneath the Bushveld Complex. These two lines of evidence give no reason to assume connectivity between the Bushveld Igneous Complex and the Molopo Farms Complex, and rather suggest two separate intrusive suites.

Velocity Structures underneath NRIL seismic station, Russia: Imaging the difference between the Siberian Craton and the West Siberian Basin

NASA Astrophysics Data System (ADS)

Youssof, M.; Thybo, H.; Artemieva, I. M.; Vinnik, L. P.

2016-12-01

This study discusses analysis of seismic P- and S-wave receiver functions (RFs). We construct RFs using records from NRIL seismic station, which is located between the northwestern edge of the Siberian Craton and the northeastern corner of the West Siberian Basin. We select 511 P- and S-RFs during a span of 7 years of recordings. Analysis of these records is very important as it might reveal the structural difference between these two tectonic settings at the crustal and lithospheric mantle scales as we split the analysis of arrivals from the east and west. The crust seems on average felsic, as concluded by Poisson's rations of 0.25 - 0.27, from the surface to the shallow Moho signal at 32 km. A prominent doublet Moho-like signal is seen in the observation as well as the inversion results. This might be a suggestive of a zone of underplating signature. The inversion evidences some lithospheric mantle stratifications within a frequency of 50 km thickness, in the range from 80 to 240 km depth. These layers observed beneath the region at depths of 85, 130 and 180 and 230 km. The observation images the deep discontinuities within the transition zone (410, 520 and 660 km). We find these converted phases are relatively shallower than the standard timing with earlier signals by 1.5 and 4.5 s, for P and S relative residuals respectively. We present here an extra interpretation based on dividing the directionality of rays into two clusters. This division simulates the present topographic/tectonic division exposed on the surface of Norilsk, due to the major NNW-SSE (150° azimuth from North) main divide. From this analysis, we find that the western division (West Basin mantle) has more distinct mantle discontinuities than the case in the eastern side (Cratonic mantle).

High-resolution 3D seismic model of the crustal and uppermost mantle structure in Poland

NASA Astrophysics Data System (ADS)

Grad, Marek; Polkowski, Marcin; Ostaficzuk, Stanisław R.

2016-01-01

In the area of Poland a contact between the Precambrian and Phanerozoic Europe and the Carpathians has a complicated structure and a complex P-wave velocity of the sedimentary cover, crystalline crust, Moho depth and the uppermost mantle. The geometry of the uppermost several kilometers of sediments is relatively well recognized from over 100,000 boreholes. The vertical seismic profiling (VSP) from 1188 boreholes provided detailed velocity data for regional tectonic units and for stratigraphic successions from Permian to the Tertiary and Quaternary deposits. These data, however, do not provide information about the velocity and basement depth in the central part of the Trans-European suture zone (TESZ) and in the Carpathians. So, the data set is supplemented by 2D velocity models from 32 deep seismic sounding refraction profiles which also provide information about the crust and uppermost mantle. Together with the results of other methods: vertical seismic profiling, magnetotelluric, allow for the creation of a detailed, high-resolution 3D model for the entire Earth's crust and the uppermost mantle down to a depth of 60 km. The thinnest sedimentary cover in the Mazury-Belarus anteclise is only 0.3 to 1 km thick, which increases to 7 to 8 km along the East European Craton (EEC) margin, and 9 to 12 km in the TESZ. The Variscan domain is characterized by a 1-4 km thick sedimentary cover, while the Carpathians are characterized by very thick sedimentary layers, up to about 20 km. The crystalline crust is differentiated and has a layered structure. The crust beneath the West European Platform (WEP; Variscan domain) is characterized by P-wave velocities of 5.8-6.6 km/s. The upper and middle crusts beneath the EEC are characterized by velocities of 6.1-6.6 km/s, and are underlain by a high velocity lower crust with a velocity of about 7 km/s. A general decrease in velocity is observed from the older to the younger tectonic domains. The TESZ is associated with a steep dip in the Moho depth, from 30-35 km in the Paleozoic Platform to 42-52 km in the Precambrian craton. The new model confirms the Moho depth derived from previous compilations. In the TESZ the lower crust has a very high seismic velocity (> 7.0 km/s) which correlates to the high P-wave velocity (about 8.4 km/s) in the uppermost mantle beneath the Polish Basin. The Cratonic area is generally characterized by high P-wave velocities (> 8.2 km/s), while the Phanerozoic area is characterized by velocities of ~ 8.0 km/s. In the TESZ very high velocities of 8.3-8.4 km/s are observed, and the southwestern limitation of this area coincides with a high velocity lower crust, and could be continued to the NW toward the Elbe line. The influence of the structure for teleseismic tomography time residuals of seismic waves traveling through the 3D seismic model was analyzed. Lithological candidates for the crust and uppermost mantle of the EEC and WEP were suggested by comparison to laboratory data. The presented 3D seismic model may make more reliable studies on global dynamics, and geotectonic correlations, particularly for sedimentary basins in the Polish Lowlands, the napped flysch sediment series in the Carpathians, the basement shape, the southwestern edge of the EEC, a high-velocity lower crust and the high-velocity uppermost mantle in the TESZ. Finally, the new 3D velocity model of the crust shows a heterogeneous structure and offers a starting point for the numerical modeling of deeper structures by allowing for a correction of the crustal effects in studies of the mantle heterogeneities.

Action recognition using multi-scale histograms of oriented gradients based depth motion trail Images

NASA Astrophysics Data System (ADS)

Wang, Guanxi; Tie, Yun; Qi, Lin

2017-07-01

In this paper, we propose a novel approach based on Depth Maps and compute Multi-Scale Histograms of Oriented Gradient (MSHOG) from sequences of depth maps to recognize actions. Each depth frame in a depth video sequence is projected onto three orthogonal Cartesian planes. Under each projection view, the absolute difference between two consecutive projected maps is accumulated through a depth video sequence to form a Depth Map, which is called Depth Motion Trail Images (DMTI). The MSHOG is then computed from the Depth Maps for the representation of an action. In addition, we apply L2-Regularized Collaborative Representation (L2-CRC) to classify actions. We evaluate the proposed approach on MSR Action3D dataset and MSRGesture3D dataset. Promising experimental result demonstrates the effectiveness of our proposed method.

Elastic thickness determination based on Vening Meinesz-Moritz and flexural theories of isostasy

NASA Astrophysics Data System (ADS)

Eshagh, Mehdi

2018-06-01

Elastic thickness (Te) is one of mechanical properties of the Earth's lithosphere. The lithosphere is assumed to be a thin elastic shell, which is bended under the topographic, bathymetric and sediment loads on. The flexure of this elastic shell depends on its thickness or Te. Those shells having larger Te flex less. In this paper, a forward computational method is presented based on the Vening Meinesz-Moritz (VMM) and flexural theories of isostasy. Two Moho flexure models are determined using these theories, considering effects of surface and subsurface loads. Different values are selected for Te in the flexural method to see by which one, the closest Moho flexure to that of the VMM is achieved. The effects of topographic/bathymetric, sediments and crustal crystalline masses, and laterally variable upper mantle density, Young's modulus and Poisson's ratio are considered in whole computational process. Our mathematical derivations are based on spherical harmonics, which can be used to estimate Te at any single point, meaning that there is no edge effect in the method. However, the Te map needs to be filtered to remove noise at some points. A median filter with a window size of 5° × 5° and overlap of 4° works well for this purpose. The method is applied to estimate Te over South America using the data of CRUST1.0 and a global gravity model.

Crustal magmatism and lithospheric geothermal state of western North America and their implications for a magnetic mantle

NASA Astrophysics Data System (ADS)

Wang, Jian; Li, Chun-Feng

2015-01-01

The western North American lithosphere experienced extensive magmatism and large-scale crustal deformation due to the interactions between the Farallon and North American plates. To further understand such subduction-related dynamic processes, we characterize crustal structure, magmatism and lithospheric thermal state of western North America based on various data processing and interpretation of gravimetric, magnetic and surface heat flow data. A fractal exponent of 2.5 for the 3D magnetization model is used in the Curie-point depth inversion. Curie depths are mostly small to the north of the Yellowstone-Snake River Plain hotspot track, including the Steens Mountain and McDermitt caldera that are the incipient eruption locations of the Columbia River Basalts and Yellowstone hotspot track. To the south of the Yellowstone hotspot track, larger Curie depths are found in the Great Basin. The distinct Curie depths across the Yellowstone-Snake River Plain hotspot track can be attributed to subduction-related magmatism induced by edge flow around fractured slabs. Curie depths confirm that the Great Valley ophiolite is underlain by the Sierra Nevada batholith, which can extend further west to the California Coast Range. The Curie depths, thermal lithospheric thickness and surface heat flow together define the western edge of the North American craton near the Roberts Mountains Thrust (RMT). To the east of the RMT, large Curie depths, large thermal lithospheric thickness, and low thermal gradient are found. From the differences between Curie-point and Moho depth, we argue that the uppermost mantle in the oceanic region is serpentinized. The low temperature gradients beneath the eastern Great Basin, Montana and Wyoming permit magnetic uppermost mantle, either by serpentinization/metasomatism or in-situ magnetization, which can contribute to long-wavelength and low-amplitude magnetic anomalies and thereby large Curie-point depths.

Testing The Magmatic Underplating Hypothesis: An Example From The Uk and Ireland

NASA Astrophysics Data System (ADS)

Al-Kindi, S.; White, N.; Sinha, M.; England, R.

Magmatic underplating associated with mantle plume activity is an important mech- anism for driving regional surface uplift and denudation of large portion of the continents. Here we present quantitative and predictive models linking the surface- measured uplift and denudation with deep crustal structure across the British Isles. The crustal model was derived from re-interpreting the 1982 wide-angle Caledonian Suture Seismic Project and it's Irish extension (CSSP&ICSSP) data sets. A joint CSSP/ICSSP velocity model was obtained for the first time by inverting for six main travel-time phases comprising more than 3000 picks having picking uncertainty ranges of 50-100 ms and average of 82 ms with best picks for first breaks at close offsets. Two indepen- dent tomographic codes namely RAYINVR (Zelt and Smith, 1992) and Jive3D (Ho- bro, 2000) were used to model the picked travel-times adopting 'interpreter-guided' and 'pure tomographic' approaches, respectively. The codes represent natural end- member approaches to travel-time tomography where the former seeks an irregular grid, minimum-parameter velocity model, whereas the later seeks minimum-structure velocity model. The final outcome of the two methods are remarkably similar which has greatly boosted confidence in the interpretation. Complementary resolution and uncertainty tests were preformed. The most striking feature of the outcome of the inversion processes is the emergence of a discrete high-velocity (7.0-7.5 km/s) intermediate layer above the Moho. The top interface of this layer is sampled by lower crustal reflections, whereas the layer velocity is sampled by refracted rays. The base of the layer is bounded by the Moho interface roughly at 33 km, constrained by upper mantle diving rays. Some Moho reflections were observed on some record sections, but the majority are believed to be masked by the early arriving, highly-reflective coda generated by resonance of seismic waves within the intermediate layer. The layer has maximum thickness of 8 + 1.6 km roughly half-way across the East Irish Sea and thins out towards the edges. The minimum width of this layer is well constrained by the strong lower crustal reflections to be approximately 550 km. The maximum width, could extend outside the ray coverage of this experiment with maximum layer thickness of 2 km at the edge of the model. This is roughly of the thickness of the smallest resolvable structure 1 at a depth of 30 km using a 5 Hz signal. Filtered gravity data was used to model a density model derived by converting the final preferred velocity model using an appropriate P-wave velocity-to-density conversion. The profile is characterised by a high positive gravity anomaly of about 30 mGal over the East Irish Sea. An excess of denser material in the lower crust (+0.20 Mgm-3) was essential to account for this gravity high, which is consistent with the wide-angle velocity model. Synthetic denudation values were calculated along the 2D crustal model assuming Airly isostasy for different elastic thickness, and were compared to real estimates pro- vided by Rowely, 1998. The two data sets show good correlation within the uncertain- ties of the estimates, which has encouraged futher analysis. An attempt at extending the two-dimensional results into the third dimension was carried out based on a sta- tistical correlation of 800 pairs of modelled underplating thickness values and 150 km high-pass gravity samples along the 2D line. The analysis has shown a high posi- tive correlation with R2=0.72 with a significant linear regression at the 95% confident level. This relationship was then computed to predict underplate thickness from the filtered gravity map and then compared with the available denudation maps. This anal- ysis has highlighted specific areas where underplating is postulated to derive surface uplift. 2

RGB-D depth-map restoration using smooth depth neighborhood supports

NASA Astrophysics Data System (ADS)

Liu, Wei; Xue, Haoyang; Yu, Zhongjie; Wu, Qiang; Yang, Jie

2015-05-01

A method to restore the depth map of an RGB-D image using smooth depth neighborhood (SDN) supports is presented. The SDN supports are computed based on the corresponding color image of the depth map. Compared with the most widely used square supports, the proposed SDN supports can well-capture the local structure of the object. Only pixels with similar depth values are allowed to be included in the support. We combine our SDN supports with the joint bilateral filter (JBF) to form the SDN-JBF and use it to restore depth maps. Experimental results show that our SDN-JBF can not only rectify the misaligned depth pixels but also preserve sharp depth discontinuities.

Rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones

USGS Publications Warehouse

Kirby, S.H.

1985-01-01

Emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures, conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones. Much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock-mechanics-based yield criteria for the inelastic deformations at the top and base. At mid-plate depths, stresses are likely to be supported elastically because bending strains and elastic stresses are low. The collisional tectonic regime, however, is far more complex because very large permanent strains are sustained at mid-plate depths and this requires us to include the broad transition between brittle and ductile flow. Moreover, important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid-plate depths. Two specific contributions of laboratory rock rheology research are considered in this paper. First, the high-temperature steady-state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks. Rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere. The thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm Tc above which (at geologic strain rates) the high-temperature ductile strength falls below some arbitrary strength isobar (e.g., 100 MPa). For olivine Tc is about 700??-800??C but for other crustal silicates, Tc may be as low as 400??-600??C, suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the Moho, as suggested by a number of workers on independent grounds. Put another way, the Moho is a rheological discontinuity. A second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones. Ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant-strain-rate tests or as an accelerating-creep-rate stage at constant differential stress. A number of physical mechanisms have been identified that may be responsible for ductile faulting, including the onset of dynamic recrystallization, phase changes, hydrothermal alteration and hydrolytic weakening. Microscopic evidence for these processes as well as larger-scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere. ?? 1985.

Is the Isabella anomaly a fossil slab or the foundered lithospheric root of the Sierra Nevada batholith?

NASA Astrophysics Data System (ADS)

Hoots, C. R.; Schmandt, B.; Clayton, R. W.; Hansen, S. M.; Dougherty, S. L.

2015-12-01

The Isabella Anomaly is a volume of relatively high seismic velocity upper mantle beneath the southern Great Valley in California. We deployed ~45 broadband seismometers in central California to test two main hypotheses for the origin of the Isabella Anomaly. One suggests that the Isabella Anomaly is the foundered lithospheric root of the southern Sierra Nevada batholith, which delaminated on account of eclogite-rich composition and translated westward as it began to sink into the asthenosphere. The other hypothesis suggests that the Isabella Anomaly is a fossil slab fragment attached to the Monterey microplate that lies offshore of central California and thus it is mechanically coupled to the Pacific plate. Prior seismic imaging with ~70 km station spacing cannot resolve the landward termination of Monterey microplate lithosphere beneath coastal California or where/if the Isabella Anomaly is attached to North America lithosphere beneath the Great Valley. The new temporary broadband array consists of 40 broadband seismometers with ~7 km spacing extending from the central California coast to the western Sierra Nevada batholith, plus some outliers to fill gaps in the regional network coverage. The temporary array was initially deployed in early 2014 and will continue to record until October 2015 so the complete data are not yet available. Preliminary Ps scattered wave images show an abrupt ~6 km increase in Moho depth eastward across the San Andreas fault, a strong positive impedance contrast that dips westward from ~7-25 km beneath Great Valley, and a sharp Moho with a slight westward dip beneath the western edge of the Sierra Nevada batholith. Apparently low impedance contrast characterizes the Moho beneath the eastern Great Valley and foothills, consistent with near mantle velocities in the lower crust. Processing of the cumulative data that will be available in October 2015 and incorporation of new tomography models into scattered wave imaging are needed before assessing the significance of potential uppermost mantle interfaces that may represent edges of the Isabella Anomaly. Results from Ps and Sp scattered wave imaging, ambient noise surface wave tomography, teleseismic body-wave tomography, and teleseismic shear wave splitting will be presented.

Crustal anisotropy along the North Anatolian Fault Zone from receiver functions

NASA Astrophysics Data System (ADS)

Licciardi, Andrea; Eken, Tuna; Taymaz, Tuncay; Piana Agostinetti, Nicola; Yolsal-Çevikbilen, Seda; Tilmann, Frederik

2016-04-01

The North Anatolian Fault Zone (NAFZ) that is considered to be one of the largest plate-bounding transform faults separates the Anatolian Plate to the south from the Eurasian Plate to the north. A proper estimation of the crustal anisotropy in the area is a key point to understand the present and past tectonic processes associated with the plate boundary as well as for assessing its strength and stability. In this work we used data from the North Anatolian Fault (NAF) passive seismic experiment in order to retrieve the anisotropic properties of the crust by means of the receiver function (RF) method. This approach provides robust constraints on the location at depth of anisotropic bodies compared to other seismological tools like S-waves splitting observations where anisotropic parameters are obtained through a path-integrated measurement process over depth. We computed RFs from teleseismic events, for 39 stations with a recording period of nearly 2 years, providing an excellent azimuthal coverage. The observed azimuthal variations in amplitudes and delay times on the Radial and Transverse RF indicate the presence of anisotropy in the crust. Isotropic and anisotropic effects on the RFs are analyzed separately after harmonic decomposition of the RF dataset (Bianchi et al. 2010). Pseudo 2D profiles are built to observe both the seismic isotropic structure and the depth-dependent lateral variations of crustal anisotropy in the area, including orientation of the symmetry axis. Preliminary results show that the isotropic structure is characterized by a complex crustal setting above a nearly flat Moho at a depth of ~40 km in the central portion of the studied area. Strong anisotropy is present in the upper crust along some portions of the NAFZ and the Ezinepazari-Sungurlu Fault (ESF), with a strong correlation between the orientation of the symmetry axis of anisotropy and the strike of the main geological structures. More complex patterns of anisotropy are present in the middle and lower crust as well as in the upper mantle. Bianchi, I., J. Park, N. Piana Agostinetti, and V. Levin (2010), Mapping seismic anisotropy using harmonic decomposition of receiver functions: An application to Northern Apennines, Italy, J. Geophys. Res., 115, B12317, doi:10.1029/2009JB007061.

The shallow seismic structure of the Larderello geothermal field (Italy) as seen from Receiver Function analysis

NASA Astrophysics Data System (ADS)

Piana Agostinetti, Nicola; Licciardi, Andrea; Piccinini, Davide; Mazzarini, Francesco; Musumeci, Giovanni; Saccorotti, Gilberto

2017-04-01

The Larderello field (Tuscany, Italy) is the oldest example in the world of geothermal energy exploitation for industrial purposes. Despite its century long history of exploration and exploitation, the deep structure (4-8km depth) of the Larderello field is still poorly known, due to (a) the lack of resolution of the applied exploration techniques and (b) the lack of interest in the investigation of deep geothermal reservoirs, given the abundant amount of energy extracted from the shallow reservoirs. Recently, the increasing demand of green-energy promoted a renewed interest in the geothermal industrial sector, which translated into new exploration efforts, especially to obtain a detailed characterization of deep geothermal sources. We investigate the seismic structure of the Larderello geothermal field using Receiver Function (RF) analysis. Crustal seismic structures are routinely investigated using the RF methodology, where teleseismic P-wave are analysed to extract P-to-S converted phases that can be related to the propagation of the P-wave across a seismic discontinuity. We compute RF from 26 seismic stations, belonging to both temporary and permanent networks: the GAPSS and RETREAT experiments and the Italian Seismic Network. The RF data-set is migrated at depth and decomposed into azimuthal harmonics. Computing the first, k=0, and the second, k=1, harmonics allows to separate the "isotropic" contribution, due to the change of the isotropic properties of the sampled materials (recorded on the k=0 harmonics), from the "anisotropic" contribution, where the energy is related to the propagation of the P-wave through anisotropic materials (recorded on the k=1 harmonics). Preliminary results allow us: (1) to infer the position of the main S-wave velocity discontinuities in the study area, mainly a shallow Tyrrhenian Moho and a very-low S-wave velocity body in the center of the Larderello dome, at about 5-15km depth; and (2) to map the presence of anisotropic materials at depth beneath the central part of the geothermal field. Our finding are discussed in relation to the distribution of local microseismicity recorded during the GAPSS experiment and to the geometry of the main seismic interfaces inferred from the analysis of active seismic data.

Evidencing a prominent Moho topography beneath the Iberian-Western Mediterranean Region, compiled from controlled-source and natural seismic surveys

NASA Astrophysics Data System (ADS)

Diaz, Jordi; Gallart, Josep; Carbonell, Ramon

2016-04-01

The complex tectonic interaction processes between the European and African plates at the Western Mediterranean since Mesozoic times have left marked imprints in the present-day crustal architecture of this area, particularly as regarding the lateral variations in crustal and lithospheric thicknesses. The detailed mapping of such variations is essential to understand the regional geodynamics, as it provides major constraints for different seismological, geophysical and geodynamic modeling methods both at lithospheric and asthenospheric scales. Since the 1970s, the lithospheric structure beneath the Iberian Peninsula and its continental margins has been extensively investigated using deep multichannel seismic reflection and refraction/wide-angle reflection profiling experiments. Diaz and Gallart (2009) presented a compilation of the results then available beneath the Iberian Peninsula. In order to improve the picture of the whole region, we have now extended the geographical area to include northern Morocco and surrounding waters. We have also included in the compilation the results arising from all the seismic surveys performed in the area and documented in the last few years. The availability of broad-band sensors and data-loggers equipped with large storage capabilities has allowed in the last decade to boost the investigations on crustal and lithospheric structure using natural seismicity, providing a spatial resolution never achieved before. The TopoIberia-Iberarray network, deployed over Iberia and northern Morocco, has provided a good example of those new generation seismic experiments. The data base holds ~300 sites, including the permanent networks in the area and hence forming a unique seismic database in Europe. In this contribution, we retrieve the results on crustal thickness presented by Mancilla and Diaz (2015) using data from the TopoIberia and associated experiments and we complement them with additional estimations beneath the Rif Cordillera arising from more recent deployments. We have now included also the sparse results in the region previously published, with the aim of checking the consistency of the results, hence giving more strength to the retained features. Combining the Moho depth values coming from controlled source and natural seismicity experiments has finally allowed us to build up a high quality grid of the region at crustal scale, which is completed in the non-sampled areas by the wide-scale CRUST1.0 model. The final picture evidences the geodynamic diversity of the area, including crustal imbrication in the Pyrenean range, a large and relatively undisturbed Variscan Massif in the center of Iberia and a probable delamination process beneath the Gibraltar Arc. Crustal thicknesses range from values around 15 km in continental margins (Cantabrian margin and Valencia Trough) to depths exceeding 50 km beneath the Pyrenees and the Rif Cordillera. A new 3D model of those variations is presented here to illustrate and summarize such large variations

Ubiquitous Creation of Bas-Relief Surfaces with Depth-of-Field Effects Using Smartphones.

PubMed

Sohn, Bong-Soo

2017-03-11

This paper describes a new method to automatically generate digital bas-reliefs with depth-of-field effects from general scenes. Most previous methods for bas-relief generation take input in the form of 3D models. However, obtaining 3D models of real scenes or objects is often difficult, inaccurate, and time-consuming. From this motivation, we developed a method that takes as input a set of photographs that can be quickly and ubiquitously captured by ordinary smartphone cameras. A depth map is computed from the input photographs. The value range of the depth map is compressed and used as a base map representing the overall shape of the bas-relief. However, the resulting base map contains little information on details of the scene. Thus, we construct a detail map using pixel values of the input image to express the details. The base and detail maps are blended to generate a new depth map that reflects both overall depth and scene detail information. This map is selectively blurred to simulate the depth-of-field effects. The final depth map is converted to a bas-relief surface mesh. Experimental results show that our method generates a realistic bas-relief surface of general scenes with no expensive manual processing.

Ubiquitous Creation of Bas-Relief Surfaces with Depth-of-Field Effects Using Smartphones

PubMed Central

Sohn, Bong-Soo

2017-01-01

This paper describes a new method to automatically generate digital bas-reliefs with depth-of-field effects from general scenes. Most previous methods for bas-relief generation take input in the form of 3D models. However, obtaining 3D models of real scenes or objects is often difficult, inaccurate, and time-consuming. From this motivation, we developed a method that takes as input a set of photographs that can be quickly and ubiquitously captured by ordinary smartphone cameras. A depth map is computed from the input photographs. The value range of the depth map is compressed and used as a base map representing the overall shape of the bas-relief. However, the resulting base map contains little information on details of the scene. Thus, we construct a detail map using pixel values of the input image to express the details. The base and detail maps are blended to generate a new depth map that reflects both overall depth and scene detail information. This map is selectively blurred to simulate the depth-of-field effects. The final depth map is converted to a bas-relief surface mesh. Experimental results show that our method generates a realistic bas-relief surface of general scenes with no expensive manual processing. PMID:28287487

Structure and Deformation in the Transpressive Zone of Southern California Inferred from Seismicity, Velocity, and Qp Models

NASA Astrophysics Data System (ADS)

Hauksson, E.; Shearer, P.

2004-12-01

We synthesize relocated regional seismicity and 3D velocity and Qp models to infer structure and deformation in the transpressive zone of southern California. These models provide a comprehensive synthesis of the tectonic fabric of the upper to middle crust, and the brittle ductile transition zone that in some cases extends into the lower crust. The regional seismicity patterns in southern California are brought into focus when the hypocenters are relocated using the double difference method. In detail, often the spatial correlation between background seismicity and late Quaternary faults is improved as the hypocenters become more clustered, and the spatial patterns are more sharply defined. Along some of the strike-slip faults the seismicity clusters decrease in width and form alignments implying that in many cases the clusters are associated with a single fault. In contrast, the Los Angeles Basin seismicity remains mostly scattered, reflecting a 3D distribution of the tectonic compression. We present the results of relocating 327,000 southern California earthquakes that occurred between 1984 and 2002. In particular, the depth distribution is improved and less affected by layer boundaries in velocity models or other similar artifacts, and thus improves the definition of the brittle ductile transition zone. The 3D VP and VP/VS models confirm existing tectonic interpretations and provide new insights into the configuration of the geological structures in southern California. The models extend from the US-Mexico border in the south to the Coast Ranges and Sierra Nevada in the north, and have 15 km horizontal grid spacing and an average vertical grid spacing of 4 km, down to 22 km depth. The heterogeneity of the crustal structure as imaged in both the VP and VP/VS models is larger within the Pacific than the North America plate, reflecting regional asymmetric variations in the crustal composition and past tectonic processes. Similarly, the relocated seismicity is deeper and shows a more complex 3D distribution in areas exhibiting compressional tectonics within the Pacific plate. The VP values are 0.2 to 0.4 km/s too high to support an abundant occurrence of schist beneath the Mojave Desert and the San Gabriel Mountains. The models reflect mapped changes, from east to west, in the lithology of the Peninsular Ranges. The interface between the shallow Moho of the Continental Borderland and the deep Moho of the continent forms a broad zone to the north beneath the western Transverse Ranges, Ventura basin and the Los Angles Basin and a narrow zone to the south, along the Peninsular Ranges. Similarly, the 3D Qp model includes several features that correspond to regional tectonic features and possibly the thermal structure of the southern California crust. A clear low Qp zone extends from the San Bernardino Basin, across the Chino Basin, San Gabriel Valley, into the Los Angeles Basin. This zone is consistent with the geology and decreases with depth from east to west. The Peninsular Ranges have a high Qp zone consistent with the high velocities in the 3D VP model. There are also zones of high Qp in the southern Mojave and southern Sierras. Several clear transition zones of rapidly varying Qp, extend across major late Quaternary faults and connect regions of high and low Qp. The strongest low Qp zone coincides with the Salton Trough where near-surface low Qp is associated with the sediments and the deeper low Qp may be associated with elevated mid-crustal temperatures.

Earthquake focal parameters and lithospheric structure of the anatolian plateau from complete regional waveform modeling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rodgers, A

2000-12-28

This is an informal report on preliminary efforts to investigate earthquake focal mechanisms and earth structure in the Anatolian (Turkish) Plateau. Seismic velocity structure of the crust and upper mantle and earthquake focal parameters for event in the Anatolian Plateau are estimated from complete regional waveforms. Focal mechanisms, depths and seismic moments of moderately large crustal events are inferred from long-period (40-100 seconds) waveforms and compared with focal parameters derived from global teleseismic data. Using shorter periods (10-100 seconds) we estimate the shear and compressional velocity structure of the crust and uppermost mantle. Results are broadly consistent with previous studiesmore » and imply relatively little crustal thickening beneath the central Anatolian Plateau. Crustal thickness is about 35 km in western Anatolia and greater than 40 km in eastern Anatolia, however the long regional paths require considerable averaging and limit resolution. Crustal velocities are lower than typical continental averages, and even lower than typical active orogens. The mantle P-wave velocity was fixed to 7.9 km/s, in accord with tomographic models. A high sub-Moho Poisson's Ratio of 0.29 was required to fit the Sn-Pn differential times. This is suggestive of high sub-Moho temperatures, high shear wave attenuation and possibly partial melt. The combination of relatively thin crust in a region of high topography and high mantle temperatures suggests that the mantle plays a substantial role in maintaining the elevation.« less

Improved images of crustal structures in the Bergslagen, central Sweden, through seismic reprocessing of BABEL lines 1, 6 and 7

NASA Astrophysics Data System (ADS)

Buntin, Sebastian; Malehmir, Alireza; Malinowski, Michał; Högdahl, Karin; Juhlin, Christopher; Buske, Stefan

2017-04-01

In a joint effort through the BABEL project, geoscientists from five countries acquired marine seismic data in the Baltic Sea with a total length of 2268 km in the year 1989. These consisted of near-vertical reflection and wide-angle refraction seismic data, providing insights into the subsurface down to the Moho and suggesting the existence of plate tectonics already during the Paleoproterozoic. The seismic data were acquired using a receiver group interval of 50 m and a total cable length of 3 km. In total, 60 groups of 64 hydrophones at 15 m depth were used. An airgun array consisting of six equal subarrays towed at 7.5 m depth was used to generate the seismic signal. The shot interval and the corresponding record lengths were different among the lines. A record length of 25 s and 75 m shot spacing for lines 1 and 7, respectively and 23 s and 62.5 m for line 6, respectively was used. The sampling rate was 4 ms for all three profiles. Lines 1, 6 and 7 are located at the boundary to the world-class and historical Bergslagen mineral district, and are being revisited in this study. Improved images can be used to refine previous interpretations, particularly at shallower depths (< 5 km). About 27 years after the acquisition, these data have been processed again in our study. Aside from the original processing steps, like spherical divergence correction, deconvolution and NMO corrections, additional processing steps such as DMO corrections or pre- and post-stack deconvolutions and coherency enhancements were applied. The reprocessing revealed reflections in the shallow part of the profiles, likely from major deformation (multi-phase) zones extending down to the lower crust, which were not present in the previous images. Also the images of the reflections in the deeper parts are remarkably improved. This also includes a few sub-Moho reflections. The three reprocessed profiles help constrain the nature of the northern boundary of Bergslagen and associated crustal structures. Furthermore they should assist in the planning of an onshore refraction and reflection profile, to be acquired in 2017, crossing the northern boundary of the Bergslagen district. Acknowledgments: This work is supported by the Swedish Research Council (VR) grant number 2015-05177 for which we are grateful. S. Buntin's PhD work is supported by the grant.

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

NASA Astrophysics Data System (ADS)

Plasman, M.; Tiberi, C.; Ebinger, C.; Gautier, S.; Albaric, J.; Peyrat, S.; Déverchère, J.; Le Gall, B.; Tarits, P.; Roecker, S.; Wambura, F.; Muzuka, A.; Mulibo, G.; Mtelela, K.; Msabi, M.; Kianji, G.; Hautot, S.; Perrot, J.; Gama, R.

2017-07-01

Rifting in a cratonic lithosphere is strongly controlled by several interacting processes including crust/mantle rheology, magmatism, inherited structure and stress regime. In order to better understand how these physical parameters interact, a 2 yr long seismological experiment has been carried out in the North Tanzanian Divergence (NTD), at the southern tip of the eastern magmatic branch of the East African rift, where the southward-propagating continental rift is at its earliest stage. We analyse teleseismic data from 38 broad-band stations ca. 25 km spaced and present here results from their receiver function (RF) analysis. The crustal thickness and Vp/Vs ratio are retrieved over a ca. 200 × 200 km2 area encompassing the South Kenya magmatic rift, the NTD and the Ngorongoro-Kilimanjaro transverse volcanic chain. Cratonic nature of the lithosphere is clearly evinced through thick (up to ca. 40 km) homogeneous crust beneath the rift shoulders. Where rifting is present, Moho rises up to 27 km depth and the crust is strongly layered with clear velocity contrasts in the RF signal. The Vp/Vs ratio reaches its highest values (ca. 1.9) beneath volcanic edifices location and thinner crust, advocating for melting within the crust. We also clearly identify two major low-velocity zones (LVZs) within the NTD, one in the lower crust and the second in the upper part of the mantle. The first one starts at 15-18 km depth and correlates well with recent tomographic models. This LVZ does not always coexist with high Vp/Vs ratio, pleading for a supplementary source of velocity decrease, such as temperature or composition. At a greater depth of ca. 60 km, a mid-lithospheric discontinuity roughly mimics the step-like and symmetrically outward-dipping geometry of the Moho but with a more slanting direction (NE-SW) compared to the NS rift. By comparison with synthetic RF, we estimate the associated velocity reduction to be 8-9 per cent. We relate this interface to melt ponding, possibly favouring here deformation process such as grain-boundary sliding (EAGBS) due to lithospheric strain. Its geometry might have been controlled by inherited lithospheric fabrics and heterogeneous upper mantle structure. We evidence that crustal and mantle magmatic processes represent first order mechanisms to ease and locate the deformation during the first stage of a cratonic lithospheric breakup.

Deep seismic reflection evidence for ancient subduction and collision zones within the continental lithosphere of northwestern Europe

NASA Astrophysics Data System (ADS)

Balling, N.

2000-12-01

Deep seismic profiling experiments in the region of NW Europe (including BABEL in the Gulf of Bothnia and the Baltic Sea, Mobil Search in the Skagerrak and MONA LISA in the North Sea) have demonstrated the existence of seismic reflectors in the mantle lithosphere beneath the Baltic Shield, the Tornquist Zone and the North Sea basins. Different sets of reflectors are observed, notably dipping and sub-horizontal. Dipping, distinct reflectivity, which may be followed from Moho/Moho offsets into the deeper parts of the continental lithosphere, is of special interest because of its tectonic and geodynamic significance. Such reflectivity, observed in several places, dipping 15-35° and covering a depth range of 30-90 km, constrained by surface geological information and radiometric age data, is interpreted to represent fossil, ancient subduction and collison zones. Subduction slabs with remnant oceanic basaltic crust transformed into eclogite is assumed, in particular, to generate deep seismic reflectivity. Deep seismic evidence is presented for subduction, crustal accretion and collision processes with inferred ages from 1.9 to 1.1 Ga from the main structural provinces within the Baltic Shield including Svecofennian, Transscandinavian Igneous Belt, Gothian and Sveconorwegian. Along the southwestern border of Baltica (in the southeastern North Sea) south-dipping crustal and sub-crustal reflectivity is observed down to a depth of about 90 km, close to the lithosphere-asthenosphere boundary. These structures are interpreted to reveal a lithosphere-scale Caledonian (ca. 440 Ma) suture zone resulting from the closure of the Tornquist Sea/Thor Ocean and the amalgamation of Baltica and Eastern Avalonia. These results demonstrate that deep structures within the continental lithosphere, originating from early crust-forming plate tectonic processes, may survive for a very long time and form seismic marker reflectivity of great value in geotectonic interpretation and reconstructions. Furthermore, the depth of dipping reflectivity from ancient structures, such as subduction slabs, significantly contributes information about the thickness of the coherent lithosphere. The seismic observations and our interpretations support plate tectonic and structural models, suggesting crustal growth and amalgamation of tectonic units in the Baltic Shield and along its southwestern margin generally from the northeast (in present-day orientation) towards the southwest and west, likely to result in regional deep structural and tectonic age zonations.

Crustal structure of the Transantarctic Mountains, Ellsworth Mountains and Marie Byrd Land, Antarctica: constraints on shear wave velocities, Poisson's ratios and Moho depths

NASA Astrophysics Data System (ADS)

Ramirez, C.; Nyblade, A.; Emry, E. L.; Julià, J.; Sun, X.; Anandakrishnan, S.; Wiens, D. A.; Aster, R. C.; Huerta, A. D.; Winberry, P.; Wilson, T.

2017-12-01

A uniform set of crustal parameters for seismic stations deployed on rock in West Antarctica and the Transantarctic Mountains (TAM) has been obtained to help elucidate similarities and differences in crustal structure within and between several tectonic blocks that make up these regions. P-wave receiver functions have been analysed using the H-κ stacking method to develop estimates of thickness and bulk Poisson's ratio for the crust, and jointly inverted with surface wave dispersion measurements to obtain depth-dependent shear wave velocity models for the crust and uppermost mantle. The results from 33 stations are reported, including three stations for which no previous results were available. The average crustal thickness is 30 ± 5 km along the TAM front, and 38 ± 2 km in the interior of the mountain range. The average Poisson's ratios for these two regions are 0.25 ± 0.03 and 0.26 ± 0.02, respectively, and they have similar average crustal Vs of 3.7 ± 0.1 km s-1. At multiple stations within the TAM, we observe evidence for mafic layering within or at the base of the crust, which may have resulted from the Ferrar magmatic event. The Ellsworth Mountains have an average crustal thickness of 37 ± 2 km, a Poisson's ratio of 0.27, and average crustal Vs of 3.7 ± 0.1 km s-1, similar to the TAM. This similarity is consistent with interpretations of the Ellsworth Mountains as a tectonically rotated TAM block. The Ross Island region has an average Moho depth of 25 ± 1 km, an average crustal Vs of 3.6 ± 0.1 km s-1 and Poisson's ratio of 0.30, consistent with the mafic Cenozoic volcanism found there and its proximity to the Terror Rift. Marie Byrd Land has an average crustal thickness of 30 ± 2 km, Poisson's ratio of 0.25 ± 0.04 and crustal Vs of 3.7 ± 0.1 km s-1. One station (SILY) in Marie Byrd Land is near an area of recent volcanism and deep (25-40 km) seismicity, and has a high Poisson's ratio, consistent with the presence of partial melt in the crust.

Determination of Magma Ascent Rates From D/H Fractionation in Olivine-Hosted Melt Inclusions

NASA Astrophysics Data System (ADS)

Gaetani, G. A.; Bucholz, C. E.; Le Roux, V.; Klein, F.; Ghiorso, M. S.; Wallace, P. J.; Sims, K. W. W.

2016-12-01

The depths at which magmas are stored and the rates at which they ascend to Earth's surface are important controls on the dynamics of volcanic eruptions. Eruptive style is influenced by the rate at which magma ascends from the reservoir to the surface through its effect on vapor bubble nucleation, growth, and coalescence. However, ascent rates are difficult to quantify because few accurate geospeedometers are appropriate for a process occurring on such short timescales. We developed a new approach to determining ascent rates on the basis of D/H fraction associated with diffusive H2O loss from olivine-hosted melt inclusions. The utility of this approach was demonstrated on olivine-hosted melt inclusions in a hyaloclastite recovered from within Dry Valley Drilling Project core 3 from Hut Point Peninsula, Antarctica. All of the melt inclusions are glassy and contain vapor bubbles. The volumes of melt inclusions and vapor bubbles were determined by X-ray microtomography, and the density of CO2 within each bubble was determined using Raman spectroscopy. Olivines were then polished to expose individual inclusions and analyzed for volatiles and dDVSMOW by secondary ion mass spectrometry. Total CO2 was reconstructed by summing CO2 in the included glass and vapor bubble. Entrapment pressures calculated on the basis of reconstructed CO2 and maximum H2O concentrations using the MagmaSat solubility model [1] indicate a depth of origin of 24 km - in good agreement with the seismically determined depth to the Moho beneath Ross Island [2]. Magma ascent rates were determined using a finite difference model for melt inclusion dehydration during magma ascent. The positive correlation between H2O and CO2 is consistent with diffusive loss during ascent, but does not provide direct information on magma ascent rate. In contrast, the slope of the negative correlation between H2O and dDVSMOW is a reflection of transport time and, therefore, ascent rate. If it is assumed that magmas did not stall between the Moho and the surface, our results indicate an ascent rate of 0.1 m/s. Our new approach has broad applicability to determining magma ascent rates for both active and extinct volcanic centers in all tectonic environments. References: [1] Ghiorso and Gualda (2015) Cont Miner Pet 169; [2] Finotello et al. (2011) Geophys J Int 185:85-92.

Viscous relaxation of the Moho under large lunar basins

NASA Technical Reports Server (NTRS)

Brown, C. David; Grimm, Robert E.

1993-01-01

Viscously relaxed topography on the Moon is evidence of a period in lunar history of higher internal temperatures and greater surface activity. Previous work has demonstrated the viscous relaxation of the Tranquilitatis basin surface. Profiles of the lunar Moho under nine basins were constructed from an inversion of lunar gravity data. These profiles show a pattern of increasingly subdued relief with age, for which two explanations have been proposed. First, ancient basins may have initially had extreme Moho relief like that of younger basins like Orientale, but, due to higher internal temperatures in early lunar history, this relief viscously relaxed to that observed today. Second, ductile flow in the crust immediately after basin formation resulted in an initially shallow basin and subdued mantle uplift. The intent is to test the first hypothesis.

The design and implementation of postprocessing for depth map on real-time extraction system.

PubMed

Tang, Zhiwei; Li, Bin; Li, Huosheng; Xu, Zheng

2014-01-01

Depth estimation becomes the key technology to resolve the communications of the stereo vision. We can get the real-time depth map based on hardware, which cannot implement complicated algorithm as software, because there are some restrictions in the hardware structure. Eventually, some wrong stereo matching will inevitably exist in the process of depth estimation by hardware, such as FPGA. In order to solve the problem a postprocessing function is designed in this paper. After matching cost unique test, the both left-right and right-left consistency check solutions are implemented, respectively; then, the cavities in depth maps can be filled by right depth values on the basis of right-left consistency check solution. The results in the experiments have shown that the depth map extraction and postprocessing function can be implemented in real time in the same system; what is more, the quality of the depth maps is satisfactory.

Slab rupture and delamination under the Betics and Rif constrained from receiver functions

NASA Astrophysics Data System (ADS)

Mancilla, Flor de Lis; Booth-Rea, Guillermo; Stich, Daniel; Pérez-Peña, José Vicente; Morales, José; Azañón, José Miguel; Martin, Rosa; Giaconia, Flavio

2015-11-01

We map the lithospheric structure under the westernmost Mediterranean convergent setting interpreting P-receiver functions obtained from a dense seismic network. No orogenic root occurs under the eastern and great part of the central Betics. However, the subducted South Iberian continental lithosphere is found beneath the western Betics where the Iberian Moho reaches depths of approximately 65 km, dipping gently towards the SE. Meanwhile, at the Rif, strong crustal and lithospheric thickness contrasts occur across the Nekor NW-SE sinistral fault that overlies the region of present slab tearing. East of the Nekor fault there is no orogenic root and the crust has been thinned to approximately 22 km, whilst to the west the crust reaches 55 km thickness and the Maghrebian continental lithosphere is attached to the lithospheric slab imaged by tomography under the Alboran basin. These data suggest that subduction rollback under the Alboran and Algerian basins, together with continental slab tearing or detachment producing edge delamination under the Betics and Rif have been the main tectonic mechanisms driving extension, magmatism and regional uplift in the westernmost Mediterranean since the Late Miocene until present. The surface expression of edge-delamination and slab tearing is marked by regional uplift, denudation of HP rocks in elongated core-complex type domes, late Miocene volcanism in the Eastern Betics and Rif, and by large NE-SW strike-slip transfer faults like the Alpujarras, Crevillente, Torcal or Nekor faults that accommodate strong gradients in crustal displacements. The Iberian slab is still attached to the oceanic slab imaged under the Alboran basin at the western Betics where intermediate depth seismicity, recent dextral strike-slip faulting and folding, could reflect slab tearing. Meanwhile, active faulting and differential GPS-measured displacements would mark slab tearing beneath the Rif coinciding with the trace of the sinistral Nekor fault.

Probing the Cypriot Lithosphere: Insights from Broadband Seismology

NASA Astrophysics Data System (ADS)

Ogden, C. S.; Bastow, I. D.; Pilidou, S.; Dimitriadis, I.; Iosif, P.; Constantinou, C.; Kounoudis, R.

2017-12-01

Cyprus, an island in the eastern Mediterranean Sea, is an ideal study locale for understanding both the final stages of subduction, and the internal structure of so-called `ophiolites' - rare, on-land exposures of oceanic crust. The Troodos ophiolite offers an excellent opportunity to interrogate a complete ophiolite sequence from mantle rocks to pillow lavas. However, determining its internal architecture, and that of the subducting African plate deep below it, cannot be easily achieved using traditional field geology. To address this issue, we have built a new network of five broadband seismograph stations across the island. These, along with existing permanent stations, record both local and teleseismic earthquakes that we are now using to image Cyprus' crust and mantle seismic structure. Receiver functions are time series, computed from three-component seismograms, which contain information about lithospheric seismic discontinuities. When a P-wave strikes a velocity discontinuity such as the Moho, energy is converted to S-waves (direct Ps phase). The widely-used H-K Stacking technique utilises this arrival, and subsequent crustal reverberations (PpPs and PsPs+PpSs), to calculate crustal thickness (H) and bulk-crustal Vp/Vs ratio (K). Central to the method is the assumption that the Moho produces the largest amplitude conversions, after the direct P-arrival, which is valid where the Moho is sharp. Where the Moho is gradational or upper crustal discontinuities are present, the Moho signals are weakened and masked by shallow crustal conversions, potentially rendering the H-K stacking method unreliable. Using a combination of synthetic and observed seismograms, we explore Cyprus' crustal structure and, specifically, the reliability of the H-K method in constraining it. Data quality is excellent across the island, but the receiver function Ps phase amplitude is low, and crustal reverberations are almost non-existent. Therefore, a simple, abrupt wavespeed jump at the Moho is lacking (perhaps due to the subducting African plate), and/or evidence for it is obscured by complex structure associated with the Troodos ophiolite. On-going analyses also include joint inversion of receiver functions and surface wave data, which together, are capable of resolving complex lithospheric seismic structure.

International Tectonic Map of the Circumpolar Arctic and its Significance for Geodynamic Interpretations

NASA Astrophysics Data System (ADS)

Petrov, O. V.; Morozov, A.; Shokalsky, S.; Leonov, Y.; Grikurov, G.; Poselov, V.; Pospelov, I.; Kashubin, S.

2011-12-01

In 2003 geological surveys of circum-arctic states initiated the international project "Atlas of Geological Maps of Circumpolar Arctic at 1:5 000000 scale". The project received active support of the UNESCO Commission for the Geological Map of the World (CGMW) and engaged a number of scientists from national academies of sciences and universities. Magnetic and gravity maps were prepared and printed by the Norwegian Geological Survey, and geological map was produced by the Geological Survey of Canada. Completion of these maps made possible compilation of a new Tectonic Map of the Arctic (TeMAr), and this work is now in progress with Russian Geological Research Institute (VSEGEI) in the lead of joint international activities. The map area (north of 60o N) includes three distinct roughly concentric zones. The outer onshore rim is composed of predominantly mature continental crust whose structure and history are illustrated on the map by the age of consolidation of craton basements and orogenic belts. The zone of offshore shelf basins is unique in dimensions with respect to other continental margins of the world. Its deep structure can in most cases be positively related to thinning and rifting of consolidated crust, sometimes to the extent of disruption of its upper layer, whereas the pre-rift evolution can be inferred from geophysical data and extrapolation of geological evidence from the mainland and island archipelagoes. The central Arctic core is occupied by abyssal deeps and intervening bathymetric highs. The Eurasia basin is commonly recognized as a typical oceanic opening separating the Barents-Kara and Lomonosov Ridge passive margins, but geodynamic evolution of Amerasia basin are subject to much controversy, despite significant intensification of earth science researchin the recent years. A growing support to the concept of predominance in the Amerasia basin of continental crust, particularly in the area concealed under High Arctic Large Igneous Province, is based on two lines of evidence: (1) seismic studies and gravity modeling of deep structure of the Earth's crust suggesting a continuity of its main layers from Central Arctic bathymetric highs to the adjoining shelves, and (2) geochrolology and isotope geochemistry of bottom rocks in the central Arctic Ocean indicating the likely occurrence here of Paleozoic supracrustal bedrock possibly resting on a Precambrian basement. In the process of compilation activities all possible effort will be made to reflect in the new international tectonic map our current understanding of present-day distribution of crust types in the Arctic. It will be illustrated by smaller-scale insets depicting, along with the crust types, additional information used for their recognition (e.g. depth to Moho, total sediment thickness, geotransects, etc. This will help to integrate geological history of Central Arctic Ocean with its continental rim and provide a sound basis for testing various paleogeodynamic models.

Seismic receiver function interpretation: Ps splitting or anisotropic underplating?

NASA Astrophysics Data System (ADS)

Liu, Z.; Park, J. J.

2016-12-01

Crustal anisotropy is crucial to understanding the evolutionary history of Earth's lithosphere. Shear-wave splitting of Moho P-to-s converted phases in receiver functions has often been used to infer crustal anisotropy. In addition to estimating birefringence directly, the harmonic variations of Moho Ps phases in delay times can be used to infer splitting parameters of averaged anisotropy in the crust. However, crustal anisotropy may localize at various levels within the crust due to complex deformational processes. Layered anisotropy requires careful investigation of the distribution of anisotropy before interpreting Moho Ps splitting. In this study, we show results from stations ARU in Russia, KIP in Hawaiian Islands and LSA in Tibetan Plateau, where layered anisotropy is well constrained by intra-crust Ps conversions at high frequencies using harmonic decomposition of multiple-taper correlation receiver functions. Anisotropic velocity models are inferred by forward-modeling decomposed RF waveforms. Our results of ARU and KIP show that the harmonic behavior of Moho Ps phases can be explained by a uniformly anisotropic crust model at lower cut-off frequencies, but higher-resolution RF-signals reveal a thin, highly anisotropic layer at the base of the crust. Station LSA tells a similar story with a twist: a modest Ps birefringence is revealed at high frequencies to stem from multiple thin (5-10-km) layers of localized anisotropy within the middle crust, but no strongly-sheared basal layer is inferred. We suggest that the harmonic variation of Moho Ps phases should always be investigated as a result of anisotropic layering using RFs with frequency content above 1Hz, rather than simply reporting averaged anisotropy of the whole crust.

Density contrast across the Moho beneath the Indian shield: Implications for isostasy

NASA Astrophysics Data System (ADS)

Paul, Himangshu; Mangalampally, Ravi Kumar; Tiwari, Virendra Mani; Singh, Arun; Chadha, Rajender Kumar; Davuluri, Srinagesh

2018-04-01

Knowledge of isostasy provides insights into how excess (or deficit) of mass on and within the lithosphere is maintained over different time scales, and also helps decipher the vertical dynamics. In continental regions, isostasy is primarily manifested as a crustal root, the extent of which is defined by the lithospheric strength and the density contrast at the Moho. In this study, we briefly review the methodology for extracting the density contrast across the Moho using the amplitudes of the P-to-s converted and free-surface reverberating phases in a receiver function (RF). We test the efficacy of this technique by applying it on synthetic and real data from 10 broadband seismic stations sited on diverse tectonic provinces in the Indian shield. We determine the density contrast after parameterizing the shear-wave velocity structure beneath the stations using the nearest neighbourhood algorithm. We find considerable variation in the density contrast across the Moho beneath the stations (0.4-0.65 gm/cc). This is explained in terms of isostatic compensation, incorporating the existing estimates of lithospheric strength (Te). Crustal roots computed using the estimated Te and the deduced density contrast substantiate the crustal thickness values inferred through RF analysis, and vice versa. This illustrates isostasy as a combination of variation in density contrast and Te. The density contrasts and crustal thicknesses inferred from RF analysis explain well the isostatic compensation mechanism in different regions. However, unusually large density contrasts (∼0.6 gm/cc) corresponding to elevated regions are intriguing and warrant further investigations. Our observation of varied density contrasts at the Moho in a Precambrian continental setting is interesting and raises a question about the existence of such situations in other parts of the world.

Depth profile measurement with lenslet images of the plenoptic camera

NASA Astrophysics Data System (ADS)

Yang, Peng; Wang, Zhaomin; Zhang, Wei; Zhao, Hongying; Qu, Weijuan; Zhao, Haimeng; Asundi, Anand; Yan, Lei

2018-03-01

An approach for carrying out depth profile measurement of an object with the plenoptic camera is proposed. A single plenoptic image consists of multiple lenslet images. To begin with, these images are processed directly with a refocusing technique to obtain the depth map, which does not need to align and decode the plenoptic image. Then, a linear depth calibration is applied based on the optical structure of the plenoptic camera for depth profile reconstruction. One significant improvement of the proposed method concerns the resolution of the depth map. Unlike the traditional method, our resolution is not limited by the number of microlenses inside the camera, and the depth map can be globally optimized. We validated the method with experiments on depth map reconstruction, depth calibration, and depth profile measurement, with the results indicating that the proposed approach is both efficient and accurate.

Depth inpainting by tensor voting.

PubMed

Kulkarni, Mandar; Rajagopalan, Ambasamudram N

2013-06-01

Depth maps captured by range scanning devices or by using optical cameras often suffer from missing regions due to occlusions, reflectivity, limited scanning area, sensor imperfections, etc. In this paper, we propose a fast and reliable algorithm for depth map inpainting using the tensor voting (TV) framework. For less complex missing regions, local edge and depth information is utilized for synthesizing missing values. The depth variations are modeled by local planes using 3D TV, and missing values are estimated using plane equations. For large and complex missing regions, we collect and evaluate depth estimates from self-similar (training) datasets. We align the depth maps of the training set with the target (defective) depth map and evaluate the goodness of depth estimates among candidate values using 3D TV. We demonstrate the effectiveness of the proposed approaches on real as well as synthetic data.

Rheological decoupling at the Moho and implication to Venusian tectonics.

PubMed

Azuma, Shintaro; Katayama, Ikuo; Nakakuki, Tomoeki

2014-03-18

Plate tectonics is largely responsible for material and heat circulation in Earth, but for unknown reasons it does not exist on Venus. The strength of planetary materials is a key control on plate tectonics because physical properties, such as temperature, pressure, stress, and chemical composition, result in strong rheological layering and convection in planetary interiors. Our deformation experiments show that crustal plagioclase is much weaker than mantle olivine at conditions corresponding to the Moho in Venus. Consequently, this strength contrast may produce a mechanical decoupling between the Venusian crust and interior mantle convection. One-dimensional numerical modeling using our experimental data confirms that this large strength contrast at the Moho impedes the surface motion of the Venusian crust and, as such, is an important factor in explaining the absence of plate tectonics on Venus.

Rheological decoupling at the Moho and implication to Venusian tectonics

PubMed Central

Azuma, Shintaro; Katayama, Ikuo; Nakakuki, Tomoeki

2014-01-01

Plate tectonics is largely responsible for material and heat circulation in Earth, but for unknown reasons it does not exist on Venus. The strength of planetary materials is a key control on plate tectonics because physical properties, such as temperature, pressure, stress, and chemical composition, result in strong rheological layering and convection in planetary interiors. Our deformation experiments show that crustal plagioclase is much weaker than mantle olivine at conditions corresponding to the Moho in Venus. Consequently, this strength contrast may produce a mechanical decoupling between the Venusian crust and interior mantle convection. One-dimensional numerical modeling using our experimental data confirms that this large strength contrast at the Moho impedes the surface motion of the Venusian crust and, as such, is an important factor in explaining the absence of plate tectonics on Venus. PMID:24638113

Comparison of Antarctic Crustal Thickness from Gravity Inversion and Seismology: Evidence for Mantle Dynamic Uplift under Marie Byrd Land

NASA Astrophysics Data System (ADS)

Ferraccioli, F.; Kusznir, N. J.; Jordan, T. A.

2017-12-01

Using gravity anomaly inversion, we produce comprehensive regional maps of crustal thickness and oceanic lithosphere distribution for Antarctica and the Southern Ocean. Antarctic crustal thicknesses derived from gravity inversion are compared with seismic estimates from Baranov (2011) and An et al. (2015). We determine Moho depth, crustal basement thickness, continental lithosphere thinning (1-1/) and ocean-continent transition location using a 3D spectral domain gravity inversion method, which incorporates a lithosphere thermal gravity anomaly correction (Chappell & Kusznir 2008). Data used in the gravity inversion are elevation and bathymetry, free-air gravity anomaly, the Bedmap 2 ice thickness and bedrock topography compilation south of 60 degrees south and relatively sparse constraints on sediment thickness. Our gravity inversion study predicts thick crust (> 45 km) under interior East Antarctica, which is penetrated by narrow continental rifts featuring relatively thinner crust. The largest crustal thicknesses predicted from gravity inversion lie in the region of the Gamburtsev Subglacial Mountains, and are consistent with seismic estimates. The East Antarctic Rift System (EARS), a major Permian to Cretaceous age rift system, is imaged by our inversion and appears to extend from the continental margin at the Lambert Rift (LR) to the South Pole region, a distance of 2500 km. Thin crust is predicted under the Ross Sea and beneath the West Antarctic Ice Sheet and delineates the regional extent of the broad West Antarctic Rift System (WARS). Substantial regional uplift is required under Marie Byrd Land to reconcile gravity and seismic estimates. A mantle dynamic uplift origin of the uplift is preferred to a thermal anomaly from a very young rift. The new crustal thickness map produced by this gravity inversion study support the hypothesis that one branch of the WARS links through to the De Gerlache sea-mounts (DG) and Peter I Island (PI) in the Bellingshausen Sea region, while another branch may link to the George V Sound Rift in the Antarctic Peninsula region.

Analysis of obsidian from moho cay, belize: new evidence on classic maya trade routes.

PubMed

Healy, P F; McKillop, H I; Walsh, B

1984-07-27

Trace element analysis of obsidian artifacts from Moho Cay, Belize, reveals that the obsidian derives primarily from the El Chayal outcrop in highland Guatemala and not from the Ixtepeque source. This is contrary to the widely accepted obsidian trade route model for Classic Maya civilization and suggests that Classic Maya obsidian trade was a more complex economic phenomenon than has been recognized.

Evolution of deep crustal magma structures beneath Mount Baekdu volcano (MBV) intraplate volcano in northeast Asia

NASA Astrophysics Data System (ADS)

Rhie, J.; Kim, S.; Tkalcic, H.; Baag, S. Y.

2017-12-01

Heterogeneous features of magmatic structures beneath intraplate volcanoes are attributed to interactions between the ascending magma and lithospheric structures. Here, we investigate the evolution of crustal magmatic stuructures beneath Mount Baekdu volcano (MBV), which is one of the largest continental intraplate volcanoes in northeast Asia. The result of our seismic imaging shows that the deeper Moho depth ( 40 km) and relatively higher shear wave velocities (>3.8 km/s) at middle-to-lower crustal depths beneath the volcano. In addition, the pattern at the bottom of our model shows that the lithosphere beneath the MBV is shallower (< 100 km) compared to surrounding regions. Togather with previous P-wave velocity models, we interpret the observations as a compositional double layering of mafic underplating and a overlying cooled felsic structure due to fractional crystallization of asthenosphere origin magma. To achieve enhanced vertical and horizontal model coverage, we apply two approaches in this work, including (1) a grid-search based phase velocity measurement using real-coherency of ambient noise data and (2) a transdimensional Bayesian joint inversion using multiple ambient noise dispersion data.

Fusion of Kinect depth data with trifocal disparity estimation for near real-time high quality depth maps generation

NASA Astrophysics Data System (ADS)

Boisson, Guillaume; Kerbiriou, Paul; Drazic, Valter; Bureller, Olivier; Sabater, Neus; Schubert, Arno

2014-03-01

Generating depth maps along with video streams is valuable for Cinema and Television production. Thanks to the improvements of depth acquisition systems, the challenge of fusion between depth sensing and disparity estimation is widely investigated in computer vision. This paper presents a new framework for generating depth maps from a rig made of a professional camera with two satellite cameras and a Kinect device. A new disparity-based calibration method is proposed so that registered Kinect depth samples become perfectly consistent with disparities estimated between rectified views. Also, a new hierarchical fusion approach is proposed for combining on the flow depth sensing and disparity estimation in order to circumvent their respective weaknesses. Depth is determined by minimizing a global energy criterion that takes into account the matching reliability and the consistency with the Kinect input. Thus generated depth maps are relevant both in uniform and textured areas, without holes due to occlusions or structured light shadows. Our GPU implementation reaches 20fps for generating quarter-pel accurate HD720p depth maps along with main view, which is close to real-time performances for video applications. The estimated depth is high quality and suitable for 3D reconstruction or virtual view synthesis.

Depth map occlusion filling and scene reconstruction using modified exemplar-based inpainting

NASA Astrophysics Data System (ADS)

Voronin, V. V.; Marchuk, V. I.; Fisunov, A. V.; Tokareva, S. V.; Egiazarian, K. O.

2015-03-01

RGB-D sensors are relatively inexpensive and are commercially available off-the-shelf. However, owing to their low complexity, there are several artifacts that one encounters in the depth map like holes, mis-alignment between the depth and color image and lack of sharp object boundaries in the depth map. Depth map generated by Kinect cameras also contain a significant amount of missing pixels and strong noise, limiting their usability in many computer vision applications. In this paper, we present an efficient hole filling and damaged region restoration method that improves the quality of the depth maps obtained with the Microsoft Kinect device. The proposed approach is based on a modified exemplar-based inpainting and LPA-ICI filtering by exploiting the correlation between color and depth values in local image neighborhoods. As a result, edges of the objects are sharpened and aligned with the objects in the color image. Several examples considered in this paper show the effectiveness of the proposed approach for large holes removal as well as recovery of small regions on several test images of depth maps. We perform a comparative study and show that statistically, the proposed algorithm delivers superior quality results compared to existing algorithms.

Restoration of the Cretaceous uplift of the Harz Mountains, North Germany: evidence for the geometry of a thick-skinned thrust

NASA Astrophysics Data System (ADS)

Tanner, David C.; Krawczyk, Charlotte M.

2017-04-01

Reverse movement on the Harz Northern Boundary Fault was responsible for the Late Cretaceous uplift of the Harz Mountains in northern Germany. Using the known geometry of the surface position and dip of the fault, and a published cross section of the Base Permian horizon, we show that it is possible to predict the probable shape of the fault at depth, down to a detachment level. We use the `inclined-shear' method with constant heave and argue that a shear angle of 30° was most likely. In this case, the detachment level is at a depth of ca. 25 km. Kinematic restoration of the Harz Mountains using this fault geometry does not produce a flat horizon, rather it results in a ca. 4 km depression. Airy-Heiskanen isostatic equilibrium adjustment of the Harz Mountains restores the Base Permian horizon to the horizontal, as well as raising the Moho to a depth of 32 km, a typical value for northern Germany. Restoration also causes a rotation of tectonic fabrics within the Harz Mountains of about 11° clockwise. We show that this model geometry is very good fit to the interpreted DEKORP BASIN 9601 deep seismic profile.

Seismic Evidence for Widespread Serpentinized Forearc Mantle Along the Mariana Convergence Margin

NASA Astrophysics Data System (ADS)

Tibi, R.; Wiens, D. A.

2007-12-01

We use P-to-S converted phases from teleseisms recorded at broadband stations in the Mariana Islands to image the forearc and arc regions of the Mariana convergence margin. The Moho in the subducting Pacific plate is observed at depths between 75 and 110 km beneath the region extending from Rota to Saipan. The S-wave velocity in the subducting crust is inferred to be ~10% slower than the surrounding mantle. This demonstrates that the crust has not yet undergone conversion to eclogite at these depths, in agreement with observations made for other arcs. A low velocity zone (LVZ), approximately 10--25 km thick, whose upper boundary is imaged at about 40--55 km depth, is detected in the forearc region of the mantle wedge along the entire margin. The anomaly is located too shallow to represent subducted oceanic crust. We interpret the LVZ as a serpentinized region in the forearc mantle, resulting from hydration by slab-expelled water. The occurrence of the serpentinized zone along the entire margin suggests that serpentinization of the forearc mantle is a widespread phenomenon in the Mariana arc. The inferred S wave velocity in the LVZ of as low as ~3.6 km/s represents a level of serpentinization of 30--50%, corresponding to a water content of about 4--6 wt%.

Mapping Antarctic Crustal Thickness using Gravity Inversion and Comparison with Seismic Estimates

NASA Astrophysics Data System (ADS)

Kusznir, Nick; Ferraccioli, Fausto; Jordan, Tom

2017-04-01

Using gravity anomaly inversion, we produce comprehensive regional maps of crustal thickness and oceanic lithosphere distribution for Antarctica and the Southern Ocean. Crustal thicknesses derived from gravity inversion are consistent with seismic estimates. We determine Moho depth, crustal basement thickness, continental lithosphere thinning (1-1/β) and ocean-continent transition location using a 3D spectral domain gravity inversion method, which incorporates a lithosphere thermal gravity anomaly correction (Chappell & Kusznir 2008). The gravity anomaly contribution from ice thickness is included in the gravity inversion, as is the contribution from sediments which assumes a compaction controlled sediment density increase with depth. Data used in the gravity inversion are elevation and bathymetry, free-air gravity anomaly, the Bedmap 2 ice thickness and bedrock topography compilation south of 60 degrees south and relatively sparse constraints on sediment thickness. Ocean isochrons are used to define the cooling age of oceanic lithosphere. Crustal thicknesses from gravity inversion are compared with independent seismic estimates, which are still relatively sparse over Antarctica. Our gravity inversion study predicts thick crust (> 45 km) under interior East Antarctica, which is penetrated by narrow continental rifts featuring relatively thinner crust. The largest crustal thicknesses predicted from gravity inversion lie in the region of the Gamburtsev Subglacial Mountains, and are consistent with seismic estimates. The East Antarctic Rift System (EARS), a major Permian to Cretaceous age rift system, is imaged by our inversion and appears to extend from the continental margin at the Lambert Rift to the South Pole region, a distance of 2500 km. Offshore an extensive region of either thick oceanic crust or highly thinned continental crust lies adjacent to Oates Land and north Victoria Land, and also off West Antarctica around the Amundsen Ridges. Thin crust is predicted under the Ross Sea and beneath the West Antarctic Ice Sheet and delineates the regional extent of the broad West Antarctic Rift System (WARS). Substantial regional uplift is required under Marie Byrd Land to reconcile gravity and seismic estimates. A mantle dynamic uplift origin of the uplift is preferred to a thermal anomaly from a very young rift. The new maps produced by this study support the hypothesis that one branch of the WARS links through to the De Gerlache sea-mounts and Peter I Island in the Bellingshausen Sea region, while another branch may link to the George V Sound Rift in the Antarctic Peninsula region. Crustal thickness and lithosphere thinning derived from gravity inversion also allows the determination of circum-Antarctic ocean-continent transition structure and the mapping of continent-ocean boundary location. Superposition of illuminated satellite gravity data onto crustal thickness maps from gravity inversion provides improved determination of Southern Ocean rift orientation, pre-breakup rifted margin conjugacy and continental breakup trajectory. The continental lithosphere thinning distribution, used to define the initial thermal model temperature perturbation, is derived from the gravity inversion and uses no a priori isochron information; as a consequence the gravity inversion method provides a prediction of ocean-continent transition location, which is independent of ocean isochron information.

Project Hi-CLIMB: A Synoptic View of the Himalayan Collision Zone and Southern Tibet

NASA Astrophysics Data System (ADS)

Nábělek, J. L.; Vergne, J.; Hetenyi, G.

2005-12-01

Project Hi-CLIMB is a broadband seismic experiment whose goal is to produce a high-resolution continuous profile across the Himalaya and southern Tibet. The centerpiece of the project is a closely spaced, linear array of broadband seismographs, extending from the Ganga lowland, across the Himalayas, and onto the central Tibetan plateau. A complementary array of sparsely spaced stations flanks the linear array. Over 270 sites were occupied during the experiment. The principal institutions involved in the field operations were the Oregon State U. and U. of Illinois (USA), Dept. of Mines and Geology (Nepal), Chinese Academy of Geol. Sci. and Peking U. (China) and the Inst. of Earth Sci. (Taiwan). The major funding for this project was provided by the NSF, Continental Dynamics program. We focus on the receiver function images from the main profile. We observe clear Moho and the upper-mantle discontinuities. The Moho, which in southern Nepal is at 45 km depth (relative to sea level), dips at a gentle angle under the Himalaya. Crossing the Himalaya, its depth rapidly increases, reaching the of 70 km near the Yarlung River. We have succeeded in imagining the Main Himalayan Trust (MHT) as it descends northward at a shallow depth from its surface expression, the Main Frontal Thrust in southern Nepal. In Nepal along the profile, MHT is expressed by a pronounced seismic low velocity zone, which we believe indicates a presence of trapped aqueous fluids in the fault zone, thus lowering the strength of the megathrust. The low velocity associated with the MHT disappears for a short distance north but reappears again as the MHT increases its dip under S. Tibet. We believe the characteristics of the low velocity associated with the MHT in S. Tibet indicate a presence of partial melt due to an increase in depth and frictional heating. A low-velocity wedge above the MHT suggests an accumulation of the melt. This could be an ongoing process of generation of the Himalayan granites. The Tibetan data reveal Indian crust tucking under the "Asian" crust and sliding under it all the way to the Banggong-Nujiang suture where its lower portion peals off and subducts steeply under the Qiangtang terrene. Under the Lhasa terrene, where we observe fully doubled-up Indian and Asian crust, the relative motion appears to be taken up along a midcrustal low-viscosity/low-velocity zone. The lower crust is high velocity, dense and strong, thus enabling its subduction north of the BNS. The strength of the lower crust seems to be inherited from the lower Indian crust, which is high velocity already under Nepal and undergoes further densification by eclogitisation as it slides to greater depths under Tibet. North of the BNS the high-velocity lower crust is absent. The lower crust north of the BNS may be formed by a northward transfer of unsubducted viscous quartz-rich midcrustal material from the Lhasa terrene.

Crust and upper-mantle structure of Wanganui Basin and southern Hikurangi margin, North Island, New Zealand as revealed by active source seismic data

NASA Astrophysics Data System (ADS)

Tozer, B.; Stern, T. A.; Lamb, S. L.; Henrys, S. A.

2017-11-01

Wide-angle reflection and refraction data recorded during the Seismic Array HiKurangi Experiment (SAHKE) are used to constrain the crustal P-wave velocity (Vp) structure along two profiles spanning the length and width of Wanganui Basin, located landwards of the southern Hikurangi subduction margin, New Zealand. These models provide high-resolution constraints on the structure and crustal thickness of the overlying Australian and subducted Pacific plates and plate interface geometry. Wide-angle reflections are modelled to show that the subducted oceanic Pacific plate crust is anomalously thick (∼10 km) below southern North Island and is overlain by a ∼1.5-4.0 km thick, low Vp (4.8-5.4 km s-1) layer, interpreted as a channel of sedimentary material, that persists landwards at least as far as Kapiti Island. Distinct near vertical reflections from onshore shots identify a ∼4 km high mound of low-velocity sedimentary material that appears to underplate the overlying Australian plate crust and is likely to contribute to local rock uplift along the Axial ranges. The overriding Australian plate Moho beneath Wanganui Basin is imaged as deepening southwards and reaches a depth of at least 36.4 km. The Moho shape approximately mirrors the thickening of the basin sediments, suggestive of crustal downwarping. However, the observed crustal thickness variation is insufficient to explain the large negative Bouguer gravity anomaly (-160 mGal) centred over the basin. Partial serpentinization within the upper mantle with a concomitant density decrease is one possible way of reconciling this anomaly.

Forward Modeling of Receiver Functions to Determine Crustal Structure of the Eastern Limb in TheBushveld Complex, South Africa

NASA Astrophysics Data System (ADS)

Loza, E.; Ramirez, C.; Nyblade, A.; Durrheim, R. J.; Raveloson, A.

2016-12-01

The Bushveld Igneous Complex contains the largest layered mafic intrusion on Earth, about the size of England, and has been exploited for metals such as platinum since the 1950s. Several igneous bodies within and around the complex have been dated from 2.06 Ga, possibly representing a single massive magmatic event. The Rustenburg Layered Suite of the Bushveld Igneous Complex intruded into the Transvaal sedimentary sequence, with associated volcanic rocks of the Rooiberg Group forming the roof and part of the floor. The purpose of this study is to determine whether the Rustenburg Layered Suite is a continuous bowl-shaped formation or if it is made up of two separate dipping sheets that crop out in the western and eastern limbs. If the intrusion is connected at depth, then the Moho (crust-mantle boundary) would most likely be depressed due to the weight of the 7-8km of mafic material injected into the crust. Seismic stations were installed in the eastern and northern Bushveld in 2015 to collect teleseismic data. The use of receiver functions derived from seismic data collected since 2015 has helped determine the subsurface crustal structure of the Bushveld. Receiver functions have been used to trace the contact between the high-density mafic lower zone and the low-density Transvaal sediments. The new data gathered show the Moho boundary at about 47 km, and a 5.0 Gaussian width shows a backswing consistent with a mafic-sedimentary boundary at 8km.

New insights on shallow and deep crustal geological structures of BABEL line 7 marine reflection seismic data revealed from reprocessing

NASA Astrophysics Data System (ADS)

Shahrokhi, H.; Malehmir, A.; Sopher, D.

2012-04-01

The BABEL project (Baltic And Bothnian Echoes from the Lithosphere) was a collaboration among British, Danish, Finnish, German and Swedish geoscientists to collect deep-crustal reflection and wide-angle refraction profiles in Baltic Shield and Gulf of Bothnia. The acquisition of 2,268km of deep marine reflection seismic data was carried out in 1989. The BABEL line 7 runs in E-W direction in the Bothnian Sea, north of the Åland islands and east of the city of Gävle. Several authors presented the seismic results but with a main focus of imaging and interpreting deep crustal geological structures and the nature and the depth of Moho discontinuity along line 7. Based on this seismic data, several publications about velocity distributions within the crust, the depth and texture of Moho discontinuity and seismic reflectivity patterns in the crust were presented. Some evidence from the reflection seismic data was also presented to suggest Early Proterozoic plate tectonics in the Baltic Shield. Previous seismic images of the BABEL line 7 reflection data show a dramatic change in the reflectivity pattern from weakly reflective lower crust in the west to a more reflective lower crust in the east, which was attributed to a change from a rigid crust to a plastic crust from the west to the east. The BABEL line 7 reflection data were acquired with a total profile length of 174km, a set of 48 airguns towed at 7.5m depth, and 3000m long streamer with 60 channels spaced with 50m intervals and towed at 15m depth. Seismic data were recorded for 25s using 4ms sampling interval and 75m shot interval. Seismic data is characterized by strong source-generated noise at shallow travel times and strong but randomly distributed spurious spikes at later arrival times. In this study, we have recovered and reprocessed the seismic data along BABEL line 7. Using modern processing and imaging techniques, which were not available at the time, and with a focus on the shallow parts of the seismic data, we have managed to reveal reflections as shallow as 1s in the data. Some of these reflections appear to be a continuation of deeper reflections but now they appear to reach to the surface, allowing correlation with the near-surface geology. At least two major moderately dipping shear zones are visible in the reprocessed data in comparison with the previous results. Deeper reflections are also improved which together with the improvements in the shallow parts of the data should allow small-scale geological structures encounter along the BABEL line 7 to be refined.

Evaluating methods for controlling depth perception in stereoscopic cinematography

NASA Astrophysics Data System (ADS)

Sun, Geng; Holliman, Nick

2009-02-01

Existing stereoscopic imaging algorithms can create static stereoscopic images with perceived depth control function to ensure a compelling 3D viewing experience without visual discomfort. However, current algorithms do not normally support standard Cinematic Storytelling techniques. These techniques, such as object movement, camera motion, and zooming, can result in dynamic scene depth change within and between a series of frames (shots) in stereoscopic cinematography. In this study, we empirically evaluate the following three types of stereoscopic imaging approaches that aim to address this problem. (1) Real-Eye Configuration: set camera separation equal to the nominal human eye interpupillary distance. The perceived depth on the display is identical to the scene depth without any distortion. (2) Mapping Algorithm: map the scene depth to a predefined range on the display to avoid excessive perceived depth. A new method that dynamically adjusts the depth mapping from scene space to display space is presented in addition to an existing fixed depth mapping method. (3) Depth of Field Simulation: apply Depth of Field (DOF) blur effect to stereoscopic images. Only objects that are inside the DOF are viewed in full sharpness. Objects that are far away from the focus plane are blurred. We performed a human-based trial using the ITU-R BT.500-11 Recommendation to compare the depth quality of stereoscopic video sequences generated by the above-mentioned imaging methods. Our results indicate that viewers' practical 3D viewing volumes are different for individual stereoscopic displays and viewers can cope with much larger perceived depth range in viewing stereoscopic cinematography in comparison to static stereoscopic images. Our new dynamic depth mapping method does have an advantage over the fixed depth mapping method in controlling stereo depth perception. The DOF blur effect does not provide the expected improvement for perceived depth quality control in 3D cinematography. We anticipate the results will be of particular interest to 3D filmmaking and real time computer games.

Extent and impact of Cretaceous magmatism on the formation and evolution of Jurassic oceanic crust in the western Pacific

NASA Astrophysics Data System (ADS)

Feng, H.; Lizarralde, D.; Tominaga, M.; Hart, L.; Tivey, M.; Swift, S. A.

2015-12-01

Multi-channel seismic (MCS) images and wide-angle sonobuoy data acquired during a 2011 cruise on the R/V Thomas G. Thompson (TN272) show widespread emplacement of igneous sills and broadly thickened oceanic Layer 2 through hundreds of kilometers of oceanic crust in one of the oldest ocean basins in the western Pacific, a region known as the Jurassic Quiet Zone (JQZ). Oceanic crust from the JQZ has grown through at least two main magmatic phases: It was formed by mid-ocean ridge processes in the Jurassic (at ~170 Ma), and then it was added to by a substantial Cretaceous magmatic event (at ~75-125 Ma). The scale of Cretaceous magmatism is exemplified by massive seafloor features such as the Ontong Java Plateau, Mid-Pacific Mountains, Marshall-Gilbert Islands, Marcus-Wake Seamount Chain, and numerous guyots, seamounts, and volcaniclastic flows observed throughout the region. We use seismic data to image heavily intruded and modified oceanic crust along an 800-km-long transect through the JQZ in order to examine how processes of secondary crustal growth - including magmatic emplacement, transport, and distribution - are expressed in the structure of modified oceanic crust. We also model gravity anomalies to constrain crustal thickness and depth to the Moho. Our observations suggest that western Pacific crust was modified via the following modes of emplacement: (a) extrusive seafloor flows that may or may not have grown into seamounts, (b) seamounts formed through intrusive diking that pushed older sediments aside during their formation, and (c) igneous sills that intruded sediments at varying depths. Emplacement modes (a) and (b) tend to imply a focused, pipe-like mechanism for melt transport through the lithosphere. Such a mechanism does not explain the observed broadly distributed intrusive emplacement of mode (c) however, which may entail successive sill emplacement between igneous basement and sediments thickening oceanic Layer 2 along ~400 km of our seismic line. This mode of crustal growth seems to require broad zones of melt transport through the lithosphere and across the Moho.

Extensional Fault Evolution and its Flexural Isostatic Response During Iberia-Newfoundland Rifted Margin Formation

NASA Astrophysics Data System (ADS)

Gómez-Romeu, J.; Kusznir, N.; Manatschal, G.; Roberts, A.

2017-12-01

During the formation of magma-poor rifted margins, upper lithosphere thinning and stretching is achieved by extensional faulting, however, there is still debate and uncertainty how faults evolve during rifting leading to breakup. Seismic data provides an image of the present-day structural and stratigraphic configuration and thus initial fault geometry is unknown. To understand the geometric evolution of extensional faults at rifted margins it is extremely important to also consider the flexural response of the lithosphere produced by fault displacement resulting in footwall uplift and hangingwall subsidence. We investigate how the flexural isostatic response to extensional faulting controls the structural development of rifted margins. To achieve our aim, we use a kinematic forward model (RIFTER) which incorporates the flexural isostatic response to extensional faulting, crustal thinning, lithosphere thermal loads, sedimentation and erosion. Inputs for RIFTER are derived from seismic reflection interpretation and outputs of RIFTER are the prediction of the structural and stratigraphic consequences of recursive sequential faulting and sedimentation. Using RIFTER we model the simultaneous tectonic development of the Iberia-Newfoundland conjugate rifted margins along the ISE01-SCREECH1 and TGS/LG12-SCREECH2 seismic lines. We quantitatively test and calibrate the model against observed target data restored to breakup time. Two quantitative methods are used to obtain this target data: (i) gravity anomaly inversion which predicts Moho depth and continental lithosphere thinning and (ii) reverse post-rift subsidence modelling to give water and Moho depths at breakup time. We show that extensional faulting occurs on steep ( 60°) normal faults in both proximal and distal parts of rifted margins. Extensional faults together with their flexural isostatic response produce not only sub-horizontal exhumed footwall surfaces (i.e. the rolling hinge model) and highly rotated (60° or more) pre- and syn-rift stratigraphy, but also extensional allochthons underlain by apparent horizontal detachments. These detachment faults were never active in this sub-horizontal geometry; they were only active as steep faults which were isostatically rotated to their present sub-horizontal position.

Crustal structure in Ethiopia and Kenya from receiver function analysis: Implications for rift development in eastern Africa

NASA Astrophysics Data System (ADS)

Dugda, Mulugeta T.; Nyblade, Andrew A.; Julia, Jordi; Langston, Charles A.; Ammon, Charles J.; Simiyu, Silas

2005-01-01

Crustal structure in Kenya and Ethiopia has been investigated using receiver function analysis of broadband seismic data to determine the extent to which the Cenozoic rifting and magmatism has modified the thickness and composition of the Proterozoic crust in which the East African rift system developed. Data for this study come from broadband seismic experiments conducted in Ethiopia between 2000 and 2002 and in Kenya between 2001 and 2002. Two methods have been used to analyze the receiver functions, the H-κ method, and direct stacks of the waveforms, yielding consistent results. Crustal thickness to the east of the Kenya rift varies between 39 and 42 km, and Poisson's ratios for the crust vary between 0.24 and 0.27. To the west of the Kenya rift, Moho depths vary between 37 and 38 km, and Poisson's ratios vary between 0.24 and 0.27. These findings support previous studies showing that crust away from the Kenya rift has not been modified extensively by Cenozoic rifting and magmatism. Beneath the Ethiopian Plateau on either side of the Main Ethiopian Rift, crustal thickness ranges from 33 to 44 km, and Poisson's ratios vary from 0.23 to 0.28. Within the Main Ethiopian Rift, Moho depths vary from 27 to 38 km, and Poisson's ratios range from 0.27 to 0.35. A crustal thickness of 25 km and a Poisson's ratio of 0.36 were obtained for a single station in the Afar Depression. These results indicate that the crust beneath the Ethiopian Plateau has not been modified significantly by the Cenozoic rifting and magmatism, even though up to a few kilometers of flood basalts have been added, and that the crust beneath the rifted regions in Ethiopia has been thinned in many places and extensively modified by the addition of mafic rock. The latter finding is consistent with models for rift evolution, suggesting that magmatic segments with the Main Ethiopian Rift, characterized by dike intrusion and Quaternary volcanism, act now as the locus of extension rather than the rift border faults.

Tectonics of the Philippines and ambient regions from geophysical inversions

NASA Astrophysics Data System (ADS)

Liu, W.; Li, C.; Zhou, Z.; Fairhead, J. D.

2012-12-01

The geological study in the Philippines and ambient regions is relatively low so far for the rather scanty data and complex geological structure. Therefore it is a challenge to do the research with limited data. In this paper, an investigation of the Philippines and surrounding area has been carried out using regional magnetic and gravity anomalies. Owing to the difficulties and limitations in reduction to the pole at the low latitudes, analytical signal amplitudes of magnetic anomalies are calculated as the equivalent substitute. Application of the Parker-Oldenburg algorithm to Bouguer gravity anomalies yields a 3D Moho topography. Curie-point depths are estimated from the magnetic anomalies using a windowed wavenumber-domain algorithm. This paper aims to reveal the structure of the Manila subduction zone accurately, and moreover, to clarify the interplay between the magmatism and subduction in the Manila Trench and East Luzon Trough. On the basis of Bouguer gravity anomaly and AS(analytical signal) of magnetic anomaly, the positions of hydrated mantle wedge in the subduction zones of this area are identified in the areas charicterizd by the distribution of high-and low value of Bouguer gravity anomaly or the paralell high value of Bouguer gravity anomaly and AS. Using our inversion results together with some other published information, the boundaries of Palawan Block, Philippine Mobile Belt and Sulu-Celebes Block are defined and the collision history of PCB(Palawan continental block)-PMB (Philippine mobile belt) and PCB-Sulu Sea is also discussed. A "seismic gap" near the 14 degree north latitude on Manila Trench, mentioned in previous studies, is thought to be induced by the slab melting and plastic behavior due to the relatively high geothermal gradient. In the central Philippines, it is likely that an incipient collision-related rifting is proceeding. Furthermore, a possible new evolution model of Sulu Sea, in which the Cagayan Ridge area is thought to be the palaeo-subduction zone and volcanic arc and Palawan Trough is supposed to be a foredeep rather than an extinct trench, is presented. In addition, mantle serpentinization extent of this area is also estimated according to Curie point and Moho depths.

The 3-dimensional construction of the Rae craton, central Canada

NASA Astrophysics Data System (ADS)

Snyder, David B.; Craven, James A.; Pilkington, Mark; Hillier, Michael J.

2015-10-01

Reconstruction of the 3-dimensional tectonic assembly of early continents, first as Archean cratons and then Proterozoic shields, remains poorly understood. In this paper, all readily available geophysical and geochemical data are assembled in a 3-D model with the most accurate bedrock geology in order to understand better the geometry of major structures within the Rae craton of central Canada. Analysis of geophysical observations of gravity and seismic wave speed variations revealed several lithospheric-scale discontinuities in physical properties. Where these discontinuities project upward to correlate with mapped upper crustal geological structures, the discontinuities can be interpreted as shear zones. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can also be correlated to surface rocks. The 3.6-2.6 Ga Rae craton comprises at least three smaller continental terranes, which "cratonized" during a granitic bloom. Cratonization probably represents final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho. The peak thermotectonic event at 1.86-1.7 Ga was associated with the Hudsonian orogeny that assembled several cratons and lesser continental blocks into the Canadian Shield using a number of southeast-dipping megathrusts. This orogeny metasomatized, mineralized, and recrystallized mantle and lower crustal rocks, apparently making them more conductive by introducing or concentrating sulfides or graphite. Little evidence exists of thin slabs similar to modern oceanic lithosphere in this Precambrian construction history whereas underthrusting and wedging of continental lithosphere is inferred from multiple dipping discontinuities.

Multi-Depth-Map Raytracing for Efficient Large-Scene Reconstruction.

PubMed

Arikan, Murat; Preiner, Reinhold; Wimmer, Michael

2016-02-01

With the enormous advances of the acquisition technology over the last years, fast processing and high-quality visualization of large point clouds have gained increasing attention. Commonly, a mesh surface is reconstructed from the point cloud and a high-resolution texture is generated over the mesh from the images taken at the site to represent surface materials. However, this global reconstruction and texturing approach becomes impractical with increasing data sizes. Recently, due to its potential for scalability and extensibility, a method for texturing a set of depth maps in a preprocessing and stitching them at runtime has been proposed to represent large scenes. However, the rendering performance of this method is strongly dependent on the number of depth maps and their resolution. Moreover, for the proposed scene representation, every single depth map has to be textured by the images, which in practice heavily increases processing costs. In this paper, we present a novel method to break these dependencies by introducing an efficient raytracing of multiple depth maps. In a preprocessing phase, we first generate high-resolution textured depth maps by rendering the input points from image cameras and then perform a graph-cut based optimization to assign a small subset of these points to the images. At runtime, we use the resulting point-to-image assignments (1) to identify for each view ray which depth map contains the closest ray-surface intersection and (2) to efficiently compute this intersection point. The resulting algorithm accelerates both the texturing and the rendering of the depth maps by an order of magnitude.

Characterizing the Crustal architecture of the Parnaiba basin with passive-source seismology

NASA Astrophysics Data System (ADS)

Coelho, Diogo; Julià, Jordi; Rodríguez Tribaldos, Verónica; White, Nicky

2017-04-01

Lithospheric-scale processes, such as the origin and evolution of large cratonic basins, can create big footprints or signatures in the subsurface that can be observed by geophysical means. With a huge potential for natural resources, the equatorial margin of NE Brazil has motivated many geophysical investigations by the oil industry. Our study area is the Parnaíba Basin, one of the largest cratonic basins of the world. The main goal of our study is to provide new images of the crust and lithosphere under the basin and highlight seismic discontinuities within, in order to improve our understanding of its architecture and help constrain models for its origin and evolution. A total of 9 broadband seismographic stations were installed within the PBAP project, a collaboration among several universities and BP Energy do Brasil, along an approximately 500 km-long transect across the basin, with interstation spacing of around 50 km. The receiver function technique is probably one of the most successful methodologies in broadband seismology for imaging of the crust and lithospheric mantle in continental areas, and we estimated crustal thickness and Vp/Vs ratio of the Parnaíba Basin by developing P-wave receiver functions from the acquired dataset. We also developed one-dimensional velocity models calculated from the joint inversion of P-wave receiver function and Rayleigh dispersion curves. Results from HK-Stacking, receiver function migration and joint inversion indicate the Moho dips gently toward the depocenter of the basin, displaying up to three different behaviors: A flat Moho in the depocenter of the basin, which showed the thickest crust (>42 km) and Vp/Vs ratio values arround 1,75; A thinning crust towards the eastern flank (<38 km), bounding with the Borborema Province, with Vp/Vs ratio of 1,74; An almost flat Moho with thickness of 40 km and Vp/Vs ratio around 1,72 on the western border, bounding with the Araguaia Belt. We also noted some mid crustal reflections at 15-20 km depth indicating the presence of a mid-crustal discontinuity. The presence of this discontinuity, along with the segmentation of the Parnaíba crust, suggest that limited stretching might have occurred during the development of this cratonic basin.

Timing magma migration through the Icelandic Crust: from the Moho to the surface

NASA Astrophysics Data System (ADS)

Mutch, E. J. F.; Maclennan, J.; Edmonds, M.

2017-12-01

The rate of magma transfer throughout the crust, particularly the amount of time it takes for melt to travel from the upper mantle to the surface, is largely unknown. Only one previous study has investigated the timescales of transport of crystals that were in equilibrium with mantle melts [1]. Despite estimating timescales on the order of months to years, the depths from which these crystals were entrained is poorly constrained. Borgarhraun is an exceptionally well-characterised picrite lava flow in the Theistareykir Volcanic System of Northern Iceland. The crystal-cargo of this lava includes macrocrysts of olivine (Fo86-90), plagioclase (An84-90), clinopyroxene and spinel with much rarer wehrlitic nodules. Crystallisation has been estimated to have taken place in deep sub-Moho magma chambers ( 24 km). Melt inclusions in primitive olivine macrocrysts (Fo88-90) are the result of mixing a suite of geochemically distinct mantle melts that were CO2 undersaturated [2-3]. Zoning in the macrocrysts holds a record of concurrent crystallisation and mixing of these variable mantle melts, as well as ascent through the crust prior to eruption [4]. We have conducted a multi-phase, multi-element approach by applying finite-element diffusion models to wehrlite olivines and plagioclase macrocrysts to constrain the timescales of crystal residence and magma ascent prior to eruption. Model results suggest that at 1250 °C the timescale of final ascent was on the order of 20-50 days, whilst longer-term crystal residence times can exceed 700 years. This analysis shows that magma can ascend from the base of the crust to the surface in under a couple of months, suggesting picrites such as Borgarhraun are the result of high speed conduits to sub-Moho magma chambers. These rapid ascent timescales have important implications for the physical modelling of primitive magmas as well as for understanding the architecture of magma-plumbing systems in the temporal domain. References [1] Ruprecht, P., & Plank, T. (2013). Nature, 500(7460), 68-72. [2] Maclennan et al. (2003) Geochemistry, Geophysics, Geosystems, 4(11). [3] Hauri, E. et al. In AGU Spring Meeting Abstracts (Vol. 1, p. 03). [4] Winpenny & Maclennan (2011). Journal of Petrology, 52(9), 1791-1812.

High-resolution Teleseismic Tomography Reveals a Complex Lithospheric Structure Beneath the North Anatolian Fault

NASA Astrophysics Data System (ADS)

Papaleo, E.; Cornwell, D. G.; Rawlinson, N.

2016-12-01

We present high-resolution tomography images of a major active continental strike slip fault zone, the North Anatolian Fault (NAF) in northern Turkey. Historical seismic records show that the NAF, with a length of 1500 km and a current slip rate of 25 mm/yr, is capable of producing large magnitude earthquakes that have activated different segments of the fault in a westward progression towards the study region, where the devastating Izmit and Düzce events occurred in 1999. The NAF poses a major seismic hazard to the city of Istanbul, situated close to one of the two strands into which the fault splays east of the Sea of Marmara. In order to improve our understanding of the lower crust and upper mantle properties that influence fault dynamics throughout the seismic cycle, we constrain NAF structure across the Moho in unprecedented detail by applying teleseismic tomography to data recorded by an array of 70 temporary seismic stations deployed with 7 km spacing (Dense Array for North Anatolia, DANA). High quality recordings of teleseismic earthquakes combined with the dense nature of the array allow high-resolution (i.e. horizontal and vertical resolution of 8 and 15 km, respectively) 3D seismic imaging of the velocity structure beneath the NAF. The northern branch of the NAF coincides with an abrupt change between opposite polarity velocity anomalies and can be traced to at least Moho depths ( 36 km) with a width of ≤8 km. A similar pattern of antithetic anomalies occurs over a horizontal distance of 30-50 km below the Moho and may indicate a widening shear zone as it passes from the crust into the upper mantle. We find evidence for significant along-strike variation in NAF structure over distances of ≤20 km and interpret an east-to-west narrowing of upper mantle slow velocity anomalies (from 50 to 30 km) to represent laterally variable strain focussing within the lithosphere. Our observations are consistent with the notion that the NAF marks the boundary between compositionally distinct lithospheres with different tectonic histories and reactivates the pre-existing Intra-Pontide suture zone. We discuss our results in terms of the influence of lithosphere heterogeneity on the development and evolution of global continental strike-slip fault zones and assess the applicability of current shear zone deformation models.

Mapping snow depth distribution in forested terrain using unmanned aerial vehicles and structure-from-motion

NASA Astrophysics Data System (ADS)

Webster, C.; Bühler, Y.; Schirmer, M.; Stoffel, A.; Giulia, M.; Jonas, T.

2017-12-01

Snow depth distribution in forests exhibits strong spatial heterogeneity compared to adjacent open sites. Measurement of snow depths in forests is currently limited to a) manual point measurements, which are sparse and time-intensive, b) ground-penetrating radar surveys, which have limited spatial coverage, or c) airborne LiDAR acquisition, which are expensive and may deteriorate in denser forests. We present the application of unmanned aerial vehicles in combination with structure-from-motion (SfM) methods to photogrammetrically map snow depth distribution in forested terrain. Two separate flights were carried out 10 days apart across a heterogeneous forested area of 900 x 500 m. Corresponding snow depth maps were derived using both, LiDAR-based and SfM-based DTM data, obtained during snow-off conditions. Manual measurements collected following each flight were used to validate the snow depth maps. Snow depths were resolved at 5cm resolution and forest snow depth distribution structures such as tree wells and other areas of preferential melt were represented well. Differential snow depth maps showed maximum ablation in the exposed south sides of trees and smaller differences in the centre of gaps and on the north side of trees. This new application of SfM to map snow depth distribution in forests demonstrates a straightforward method for obtaining information that was previously only available through manual spatially limited ground-based measurements. These methods could therefore be extended to more frequent observation of snow depths in forests as well as estimating snow accumulation and depletion rates.

Geodynamics Of Mid-Tertiary Extensional Phase In Southwest Texas And Relationship With The Rio Bravo Fault

NASA Astrophysics Data System (ADS)

Le Pichon, X. T.; Husson, L.; Henry, P.

2004-12-01

Three independent sets of data lead us to conclude that gravity collapse alone cannot account for the Cenozoic evolution of the Texas margin of the GOM and that there has been since Paleocene a significant reactivation of the extension there.1) We have examined an extensive set of thermal data from wells including 2000 Reservoir temperatures offshore Texas and Louisiana. Solving for 1-D thermal and stratigraphic evolution of 166 representative wells we obtain the basal heat flow that defines the existence of a well defined anomaly centred on the Corsair Fault. The basal heat flow increases over less than 100 km from 35 mW/m2 northwest of the Corsair Fault to 55 mW/m2 on the fault. This increase is best explained by a crustal extensional episode during Upper Cenozoic as demonstrated by a simple modelization. The thermal structure results in very high temperatures at depth. The deepest wellls at 6000 m depth give a temperature larger than 200C. Below the Corsair rift, the extrapolated temperature is more than 300C at 10 000 m (6 s twt), 375C at 12.5 km (7 s twt) and close to 500C at 18.5 km (9 s twt). 2) Velocity/depth data from refraction (Ebeniro et al., 1988), from Moho inversion based on gravity and from 11s TWT seismic reflection depth sections show that the deep decollement layer is indeed very deep and very hot and that there is little if any igneous crust below the Corsair Rift. The velocity structure lead us to conclude that the major decollement that is generally identified with the Middle-Cretaceous Unconformity (MCU) and that lies at a depth of 7 s twt increasing to 9 s below the Corsair Rift plunges from 12-13 km northwest of the Corsair Fault to 18-19 km below the Corsair Rift. There the Moho is localized at 21-22km. The 3 km thick material between these two depths could potentially include the Cretaceous, Jurassic and Triassic as well as the whole igneous crust. In any case at this depth the present extrapolated temperature is about 500°C. The brittle-ductile transition at the present time is then expected to be situated within the sedimentary section and the brittle - ductile transition is probably situated between 6 and 8 s twt. The material, whatever its composition, below the main décollement in the area of the Corsair Rift must be metarmorphized and ductile.3) Field data led to the identification a major left-lateral shear zone most active during the late Eocene-Oligocene that we have called the Rio Bravo Fault zone. The fault zone had been previously described in the literature as the N120° Texas lineament assumed to have been inherited from the Jurassic opening of the Gulf of Mexico and located at the boundary between Texas and Mexico, approximately coinciding with the Rio Bravo (or Rio Grande).We demonstrate that this zone of shear was active during Oligocene from about 31°N to about 25°N. We conclude that an approximately 1000 km long left-lateral shear zone was active during mid-Tertiary with a total offset of 40-60 km. Its activity affected the Tertiary depocenters in Texas and within the Burgos Basin. It could account for the Paleocene to Oligocene extension in Southwest Texas.

A taxonomy of three species of negative velocity arrivals in the lithospheric mantle beneath the United States using Sp receiver functions

NASA Astrophysics Data System (ADS)

Foster, K.; Dueker, K.; McClenahan, J.; Hansen, S. M.; Schmandt, B.

2012-12-01

The Transportable Array, with significant supplement from past PASSCAL experiments, provides an unprecedented opportunity for a holistic view over the geologically and tectonically diverse continent. New images from 34,000 Sp Receiver Functions image lithospheric and upper mantle structure that has not previously been well constrained, significant to our understanding of upper mantle processes and continental evolution. The negative velocity gradient (NVG) found beneath the Moho has been elusive and is often loosely termed the "Lithosphere-Asthenosphere Boundary" (LAB).This label is used by some researchers to indicate a rheological boundary, a thermal gradient, an anisotropic velocity contrast, or a compositional boundary, and much confusion has arisen around what observed NVG arrivals manifest. Deconvolution across up to 400 stations simultaneously has enhanced the source wavelet estimation and allowed for more accurate receiver functions. In addition, Sdp converted phases are precursory to the direct S phase arrival, eliminating the issue of contamination from reverberated phases that add noise to Ps receiver functions in this lower-lithospheric and upper mantle depth range. We present taxonomy of the NVG arrivals beneath the Moho across the span of the Transportable Array (125° - 85° W). The NVG is classified into three different categories, primarily distinguished by the estimated temperature at the depth of the arrival. The first species of Sp NVG arrivals is found to be in the region west of the Precambrian rift hinge line, at a depth range of 70 - 90 km, corresponding to a temperature of >1150° C. This temperature and depth is predicted to be supersolidus for a 0.02% weight H2O Peridotite (Katz et al., 2004), supporting the theory that these arrivals are due to a melt-staging area (MSA), which could be correlated with the base of the thermal lithosphere. The current depth estimate of the cratonic US thermal LAB ranges from 150-220 km (Yuan and Romanowitz, 2010), and yet a pervasive arrival in our Sp and Ps images shows a NVG ranging from 80 - 110 km depth, with temperature estimates of ~800° C. Clearly internal to the lithosphere, this signal cannot be a LAB arrival. Hence, our second species of NVG is a Mid-Lithospheric Discontinuity (MLD) that we interpret as a layer of sub-solidus metasomatic minerals that have solidus in the 1000-1100°C range near three Gpa. These low solidus minerals are amphibole, phlogophite, and carbon-bearing phases. A freezing front (solidus) near three Gpa freezing front would concentrate these low velocity minerals to make a metasomatic layer over Ga time-scales to explain our NVG MLD arrivals. A third species of NVG, in the "warm" category of 950-1150° C, exists beneath the intermountain west region of Laramide shortening that extends from Montana to New Mexico. This region has experienced abundant post-Eocene alkaline magmatism. Mantle xenoliths from this region provide temperature at depth measurements which are in agreement with our surface wave velocity based temperature estimates. Thus, this NVG arrival is interpreted as a near to super-solidus metasomatic layer. Noteworthy is that a deeper arrival (150-190 km) is intermittently observed which would be more relative to the base of the thermal lithosphere.

Depth image super-resolution via semi self-taught learning framework

NASA Astrophysics Data System (ADS)

Zhao, Furong; Cao, Zhiguo; Xiao, Yang; Zhang, Xiaodi; Xian, Ke; Li, Ruibo

2017-06-01

Depth images have recently attracted much attention in computer vision and high-quality 3D content for 3DTV and 3D movies. In this paper, we present a new semi self-taught learning application framework for enhancing resolution of depth maps without making use of ancillary color images data at the target resolution, or multiple aligned depth maps. Our framework consists of cascade random forests reaching from coarse to fine results. We learn the surface information and structure transformations both from a small high-quality depth exemplars and the input depth map itself across different scales. Considering that edge plays an important role in depth map quality, we optimize an effective regularized objective that calculates on output image space and input edge space in random forests. Experiments show the effectiveness and superiority of our method against other techniques with or without applying aligned RGB information

Magnetic basement and crustal structure in the Arabia-Eurasia collision zone from a combined gravity and magnetic model

NASA Astrophysics Data System (ADS)

Mousavi, Naeim; Ebbing, Jörg

2017-04-01

In this study, we investigate the magnetic basement and crustal structure in the region of Iran by inverse and forward modeling of aeromagnetic data and gravity data. The main focus is on the definition of the magnetic top basement. The combination of multiple shallow magnetic sources and an assumed shallow Curie isotherm depth beneath the Iranian Plateau creates a complex magnetic architecture over the area. Qualitative analysis, including pseudo gravity, wavelength filtering and upward continuation allowed a first separation of probable deep and shallow features, like the Sanandaj Sirjan zone, Urumieh Dokhtar Magmatic Assemblage, Kopet Dagh structural unit and Central Iran domain. In the second step, we apply inverse modeling to generate an estimate of the top basement geometry. The initial model was established from top basement to (a) constant depth of 25 km and (b) Moho depth. The inversion result was used as starting model for more detailed modelling in 3D to evaluate the effect of susceptibility heterogeneities in the crust. Subsequently, the model was modified with respect to tectonic and geological characterization of the region. Further modification of model in regards more details of susceptibility distribution was led to separating upper crust to different magnetic domains. In addition, we refined the top basement geometry by using terrestrial gravity observation as well. The best fitting model is consistent with the Curie isotherm depth as the base of magnetization. The Curie isotherm was derived from independent geophysical-petrological model.

Seismic imaging of a mid-lithospheric discontinuity beneath Ontong Java Plateau

NASA Astrophysics Data System (ADS)

Tharimena, Saikiran; Rychert, Catherine A.; Harmon, Nicholas

2016-09-01

Ontong Java Plateau (OJP) is a huge, completely submerged volcanic edifice that is hypothesized to have formed during large plume melting events ∼90 and 120 My ago. It is currently resisting subduction into the North Solomon trench. The size and buoyancy of the plateau along with its history of plume melting and current interaction with a subduction zone are all similar to the characteristics and hypothesized mechanisms of continent formation. However, the plateau is remote, and enigmatic, and its proto-continent potential is debated. We use SS precursors to image seismic discontinuity structure beneath Ontong Java Plateau. We image a velocity increase with depth at 28 ± 4 km consistent with the Moho. In addition, we image velocity decreases at 80 ± 5 km and 282 ± 7 km depth. Discontinuities at 60-100 km depth are frequently observed both beneath the oceans and the continents. However, the discontinuity at 282 km is anomalous in comparison to surrounding oceanic regions; in the context of previous results it may suggest a thick viscous root beneath OJP. If such a root exists, then the discontinuity at 80 km bears some similarity to the mid-lithospheric discontinuities (MLDs) observed beneath continents. One possibility is that plume melting events, similar to that which formed OJP, may cause discontinuities in the MLD depth range. Plume-plate interaction could be a mechanism for MLD formation in some continents in the Archean prior to the onset of subduction.

Subduction and collision processes in the Central Andes constrained by converted seismic phases.

PubMed

Yuan, X; Sobolev, S V; Kind, R; Oncken, O; Bock, G; Asch, G; Schurr, B; Graeber, F; Rudloff, A; Hanka, W; Wylegalla, K; Tibi, R; Haberland, C; Rietbrock, A; Giese, P; Wigger, P; Röwer, P; Zandt, G; Beck, S; Wallace, T; Pardo, M; Comte, D

The Central Andes are the Earth's highest mountain belt formed by ocean-continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovicić discontinuity--generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro-eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10-20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

Improved H-κ Method by Harmonic Analysis on Ps and Crustal Multiples in Receiver Functions with respect to Dipping Moho and Crustal Anisotropy

NASA Astrophysics Data System (ADS)

Li, J.; Song, X.; Wang, P.; Zhu, L.

2017-12-01

The H-κ method (Zhu and Kanamori, 2000) has been widely used to estimate the crustal thickness and Vp/Vs ratio with receiver functions. However, in regions where the crustal structure is complicated, the method may produce uncertain or even unrealistic results, arising particularly from dipping Moho and/or crustal anisotropy. Here, we propose an improved H-κ method, which corrects for these effects first before stacking. The effect of dipping Moho and crustal anisotropy on Ps receiver function has been well studied, but not as much on crustal multiples (PpPs and PpSs+PsPs). Synthetic tests show that the effect of crustal anisotropy on the multiples are similar to Ps, while the effect of dipping Moho on the multiples is 5 times that on Ps (same cosine trend but 5 times in time shift). A Harmonic Analysis (HA) method for dipping/anisotropy was developed by Wang et al. (2017) for crustal Ps receiver functions to extract parameters of dipping Moho and crustal azimuthal anisotropy. In real data, the crustal multiples are much more complicated than the Ps. Therefore, we use the HA method (Wang et al., 2017), but apply separately to Ps and the multiples. It shows that although complicated, the trend of multiples can still be reasonably well represented by the HA. We then perform separate azimuthal corrections for Ps and the multiples and stack to obtain a combined receiver function. Lastly, the traditional H-κ procedure is applied to the stacked receiver function. We apply the improved H-κ method on 40 CNDSN (Chinese National Digital Seismic Network) stations distributed in a variety of geological setting across the Chinese continent. The results show apparent improvement compared to the traditional H-κ method, with clearer traces of multiples and stronger stacking energy in the grid search, as well as more reliable H-κ values.

Examining the interior of Llaima Volcano with receiver functions

NASA Astrophysics Data System (ADS)

Bishop, J. W.; Lees, J. M.; Biryol, C. B.; Mikesell, T. D.; Franco, L.

2018-02-01

Llaima Volcano in Chile is one of the largest and most active volcanoes in the southern Andes, with over 50 eruptions since the 1600s. After years of persistent degassing, Llaima most recently erupted in a series of violent Strombolian eruptions in 2007-2009. This period had few precursory signals, which highlights the need to obtain accurate magma storage information. While petrologic advancements have been made in understanding magma degassing and crystallization trends, a comprehensive seismic study has yet to be completed. Here, we present results of a receiver function survey utilizing a dense seismic array surrounding Llaima volcano. Application of H-κ stacking and common conversion point stacking techniques reveals a new Moho estimate and two structural anomalies beneath Llaima Volcano. We interpret a low velocity zone between 8 and 13 km depth as a newly imaged magma body.

Depth map generation using a single image sensor with phase masks.

PubMed

Jang, Jinbeum; Park, Sangwoo; Jo, Jieun; Paik, Joonki

2016-06-13

Conventional stereo matching systems generate a depth map using two or more digital imaging sensors. It is difficult to use the small camera system because of their high costs and bulky sizes. In order to solve this problem, this paper presents a stereo matching system using a single image sensor with phase masks for the phase difference auto-focusing. A novel pattern of phase mask array is proposed to simultaneously acquire two pairs of stereo images. Furthermore, a noise-invariant depth map is generated from the raw format sensor output. The proposed method consists of four steps to compute the depth map: (i) acquisition of stereo images using the proposed mask array, (ii) variational segmentation using merging criteria to simplify the input image, (iii) disparity map generation using the hierarchical block matching for disparity measurement, and (iv) image matting to fill holes to generate the dense depth map. The proposed system can be used in small digital cameras without additional lenses or sensors.

Basin-scale geothermal model calibration: experience from the Perth Basin, Australia

NASA Astrophysics Data System (ADS)

Wellmann, Florian; Reid, Lynn

2014-05-01

The calibration of large-scale geothermal models for entire sedimentary basins is challenging as direct measurements of rock properties and subsurface temperatures are commonly scarce and the basal boundary conditions poorly constrained. Instead of the often applied "trial-and-error" manual model calibration, we examine here if we can gain additional insight into parameter sensitivities and model uncertainty with a model analysis and calibration study. Our geothermal model is based on a high-resolution full 3-D geological model, covering an area of more than 100,000 square kilometers and extending to a depth of 55 kilometers. The model contains all major faults (>80 ) and geological units (13) for the entire basin. This geological model is discretised into a rectilinear mesh with a lateral resolution of 500 x 500 m, and a variable resolution at depth. The highest resolution of 25 m is applied to a depth range of 1000-3000 m where most temperature measurements are available. The entire discretised model consists of approximately 50 million cells. The top thermal boundary condition is derived from surface temperature measurements on land and ocean floor. The base of the model extents below the Moho, and we apply the heat flux over the Moho as a basal heat flux boundary condition. Rock properties (thermal conductivity, porosity, and heat production) have been compiled from several existing data sets. The conductive geothermal forward simulation is performed with SHEMAT, and we then use the stand-alone capabilities of iTOUGH2 for sensitivity analysis and model calibration. Simulated temperatures are compared to 130 quality weighted bottom hole temperature measurements. The sensitivity analysis provided a clear insight into the most sensitive parameters and parameter correlations. This proved to be of value as strong correlations, for example between basal heat flux and heat production in deep geological units, can significantly influence the model calibration procedure. The calibration resulted in a better determination of subsurface temperatures, and, in addition, provided an insight into model quality. Furthermore, a detailed analysis of the measurements used for calibration highlighted potential outliers, and limitations with the model assumptions. Extending the previously existing large-scale geothermal simulation with iTOUGH2 provided us with a valuable insight into the sensitive parameters and data in the model, which would clearly not be possible with a simple trial-and-error calibration method. Using the gained knowledge, future work will include more detailed studies on the influence of advection and convection.

New seismic Vp- and Vp/Vs- models of HUKKA 2007 wide-angle reflection and refraction profile in northern Fennoscandian Shield

NASA Astrophysics Data System (ADS)

Tiira, T.; Janik, T.; Kozlovskaya, E.; Grad, M.; Korja, A.; Komminaho, K.; Hegedüs, E.; Kovács, C. A.; Silvennoinen, H.; Brückl, E.

2012-04-01

We study the block structure within accreationary orogens. We present an example from northern part of the Fennoscandian Shield transected by deep seismic sounding profile HUKKA 2007. The 455 km long profile runs in NNW-SSE direction from Kittilä in northwestern Finnish Lapland to Kostamush in Russia near central part of the border between Finland and Russia. We present 2-D seismic velocity model (Vp and Vp/Vs ratio in the crust, depth to the Moho and depth to the intracrustal reflectors) along HUKKA 2007 wide-angle reflection and refraction profile in northern Finland. Commercial and military chemical explosions at 7 shot points were used as sources of the seismic energy. The shots were recorded by 115 recording stations deployed along the profile with an average station spacing of 3.45 km. The field recordings were cut and sorted into shot gathers. The 2-D velocity model of the HUKKA 2007 profile was developed by SEIS83 forward raytracing package using arrivals of major refracted and reflected P- and S-wave phases. In general the velocities vary in the upper crust between 5.8 and 6.1 km/s. Interesting features are three high P wave velocity (6.30-6.35 km/s) bodies in the upper crust. Two small bodies lie close to surface at first 100 km and the third one can be followed from 200 to 350 km along the profile reaching depth of 5-10 km. The central part of the profile (between 120 and 220 km) has a zone of low (lower than 6 km/s) P-wave velocity in the uppermost crust. This zone is about 4 km thick. In addition, the velocity model along the HUKKA 2007 profile shows significant difference in crustal velocity structure between the northern (up to 120 km) and southern parts of the profile. The differences in P-wave velocities and Vp/Vs ratio can be followed throughout the crust down to the Moho boundary. This suggests that the HUKKA 2007 profile transects a major terrane boundary. However, the position of this boundary with respect to major crustal units is controversial. It may be the boundary that separates the pristine parts of the Archean Karelian craton from those parts reworked in the Paleoproterozoic. Alternatively, it can be the boundary that separates the Karelian craton from the Belomorian mobile belt.

Structures within the oceanic crust of the central South China Sea basin and their implications for oceanic accretionary processes

NASA Astrophysics Data System (ADS)

Ding, Weiwei; Sun, Zhen; Dadd, Kelsie; Fang, Yinxia; Li, Jiabiao

2018-04-01

Internal structures in mature oceanic crust can elucidate understanding of the processes and mechanism of crustal accretion. In this study, we present two multi-channel seismic (MCS) transects across the northern flank of the South China Sea basin to reveal the internal structures related to Cenozoic tectono-magmatic processes during seafloor spreading. Bright reflectors within the oceanic crust, including the Moho, upper crustal reflectors, and lower crustal reflectors, are clearly imaged in these two transects. The Moho reflection displays varied character in continuity, shape and amplitude from the continental slope area to the abyssal basin, and becomes absent in the central part of the basin where abundant seamounts and seamount chains formed after the cessation of seafloor spreading. Dipping reflectors are distinct in most parts of the MCS data but generally confined to the lower crust above the Moho reflection. These lower crustal reflectors merge downward into the Moho without offsetting it, probably arising from shear zones between the crust and mantle characterized by interstitial melt, although we cannot exclude other possibilities such as brittle faulting or magmatic layering in the local area. A notable feature of these lower crustal reflector events is their opposite inclinations. We suggest the two groups of conjugate lower crustal reflector events observed between magnetic anomalies C11 and C8 were associated with two unusual accretionary processes arising from plate reorganizations with southward ridge jumps.

Seismically imaging the structural legacy of rifting and collision events in the central and eastern U.S. crust

NASA Astrophysics Data System (ADS)

Schmandt, B.; Lin, F. C.; Karlstrom, K. E.

2015-12-01

EarthScope's USArray now provides broadband seismic data across the contiguous U.S. and southeastern Canada. We used teleseismic receiver functions and surface wave tomography to map crustal structure beneath the entire array. Crust thickness was estimated with multi-mode Ps receiver function images using <0.5 Hz Ps and <0.25 Hz 2p1s and 2s1p reverberations between the free-surface and Moho. In areas of sedimentary basins or large impedance contrasts in the middle crust the reverberations alone often provide clearer images of the Moho than the Ps mode, because of interference from conversions at shallow interfaces is reduced at greater lag times. The new results enable large-scale comparison of the structural legacy of multiple rifting and collision events in eastern North America. Some Proterozoic rift segments defined by Bouguer gravity and surface geology maintain locally thin crust while others lack correlated Moho topography or are areas of locally thicker crust. Locally thin crust is found at southern end of the mid-continent rift (MCR) in northern Kansas and southern Nebraska, along the Reelfoot rift, and beneath inferred rifts in Michigan, Indiana, and Ohio. The Oklahoma aulacogen is not associated with a coherent change in crust thickness along its length, at least at a scale resolvable by USArray data and our imaging approach. The MCR extending northeast from Nebraska to Lake Superior has locally thicker crust, consistent with other recent results. We suggest that magmatic additions to the lower crust overwhelmed extension in the northern mid-continent rift, but not the rift segments further south and east. Collision events of the Grenville orogeny and Paleozoic orogens that created the Appalachian Mountains are still associated with ~45-55 km thick crust extending from the Grenville front eastward across the Appalachian Mountains to the fall line that marks the abrupt geomorphic transition to the coastal plains. Despite the ~45-55 km crust thickness long-wavelength elevations (>50 km) across this area rarely exceed 1 km. Along the fall line we find ~15-20 km of seaward thinning that is coherent from Alabama to Pennsylvania, with a transition width similar to or less than the ~70 km.

Predefined Redundant Dictionary for Effective Depth Maps Representation

NASA Astrophysics Data System (ADS)

Sebai, Dorsaf; Chaieb, Faten; Ghorbel, Faouzi

2016-01-01

The multi-view video plus depth (MVD) video format consists of two components: texture and depth map, where a combination of these components enables a receiver to generate arbitrary virtual views. However, MVD presents a very voluminous video format that requires a compression process for storage and especially for transmission. Conventional codecs are perfectly efficient for texture images compression but not for intrinsic depth maps properties. Depth images indeed are characterized by areas of smoothly varying grey levels separated by sharp discontinuities at the position of object boundaries. Preserving these characteristics is important to enable high quality view synthesis at the receiver side. In this paper, sparse representation of depth maps is discussed. It is shown that a significant gain in sparsity is achieved when particular mixed dictionaries are used for approximating these types of images with greedy selection strategies. Experiments are conducted to confirm the effectiveness at producing sparse representations, and competitiveness, with respect to candidate state-of-art dictionaries. Finally, the resulting method is shown to be effective for depth maps compression and represents an advantage over the ongoing 3D high efficiency video coding compression standard, particularly at medium and high bitrates.

Eclogitization of the Subducted Oceanic Crust and Its Implications for the Mechanism of Slow Earthquakes

NASA Astrophysics Data System (ADS)

Wang, Xinyang; Zhao, Dapeng; Suzuki, Haruhiko; Li, Jiabiao; Ruan, Aiguo

2017-12-01

The generating mechanism and process of slow earthquakes can help us to better understand the seismogenic process and the petrological evolution of the subduction system, but they are still not very clear. In this work we present robust P and S wave tomography and Poisson's ratio images of the subducting Philippine Sea Plate beneath the Kii peninsula in Southwest Japan. Our results clearly reveal the spatial extent and variation of a low-velocity and high Poisson's ratio layer which is interpreted as the remnant of the subducted oceanic crust. The low-velocity layer disappears at depths >50 km, which is attributed to crustal eclogitization and consumption of fluids. The crustal eclogitization and destruction of the impermeable seal play a key role in the generation of slow earthquakes. The Moho depth of the overlying plate is an important factor affecting the depth range of slow earthquakes in warm subduction zones due to the transition of interface permeability from low to high there. The possible mechanism of the deep slow earthquakes is the dehydrated oceanic crustal rupture and shear slip at the transition zone in response to the crustal eclogitization and the temporal stress/strain field. A potential cause of the slow event gap existing beneath easternmost Shikoku and the Kii channel is the premature rupture of the subducted oceanic crust due to the large tensional force.

Identifying Moho depths and velocity anomalies in the uppermost mantle of the Mississippi Embayment from Pn tomography and anisotropy studies

NASA Astrophysics Data System (ADS)

Basu, U.; Powell, C. A.

2017-12-01

Lateral depth variations of the Mohorovicic discontinuity, Pn velocities, and anisotropy features at uppermost mantle depths below the central U.S. are determined using Pn tomography. Excellent raypath coverage throughout the northern Mississippi Embayment (ME) is obtained using the NELE (Northern Embayment Lithosphere Experiment) and US TA (Transportable Array) datasets. High Pn velocities are present below the northern portion of the Reelfoot Rift and the New Madrid seismic zone. Prominent regions of low velocity are present to the east and north of the ME, in agreement with recent teleseismic tomography studies indicating the presence of low P- and S-wave velocities in the uppermost mantle. A prominent region of low velocity coincides with the southwestern portion of the Illinois Basin. Higher velocities are located west of the Illinois Basin and west of the Ozark Plateau. Crustal thicknesses obtained from the Pn station delays indicate thinner crust in the southern Coastal Plain and ME and thicker crust north of the ME. Strong Pn anisotropy and rotation of the fast directions are associated with the northern ME. Fast directions differ from present absolute plate motion directions and from fast directions determined from SKS splitting, suggesting the presence of multiple anisotropic layers. Parameter errors estimated using the bootstrap method are all less than 0.1 km/s for velocity and magnitude of the anisotropy.

Deep electromagnetic sounding of the lithosphere in the eastern Baltic (fennoscandian) shield with high-power controlled sources and industrial power transmission lines (FENICS experiment)

NASA Astrophysics Data System (ADS)

Zhamaletdinov, A. A.; Shevtsov, A. N.; Korotkova, T. G.; Kopytenko, Yu. A.; Ismagilov, V. S.; Petrishev, M. S.; Efimov, B. V.; Barannik, M. B.; Kolobov, V. V.; Prokopchuk, P. I.; Smirnov, M. Yu.; Vagin, S. A.; Pertel, M. I.; Tereshchenko, E. D.; Vasil'Ev, A. N.; Grigoryev, V. F.; Gokhberg, M. B.; Trofimchik, V. I.; Yampolsky, Yu. M.; Koloskov, A. V.; Fedorov, A. V.; Korja, T.

2011-01-01

The paper addresses the technique and the first results of a unique experiment on the deep tensor frequency electromagnetic sounding, the Fennoscandian Electrical conductivity from results of sounding with Natural and Controlled Sources (FENICS). In the experiment, Energy-1 and Energy-2 generators with power of up to 200 kW and two mutually orthogonal industrial 109- and 120-km-long power transmission lines were used. The sounding frequency range was 0.1-200 Hz. The signals were measured in the Kola-Karelian region, in Finland, on Svalbard, and in Ukraine at distances up to 2150 km from the source. The parameters of electric conductivity in the lithosphere are studied down to depths on the order of 50-70 km. A strong lateral homogeneity (the one-dimensionality) of a geoelectric section of the Earth's crust is revealed below depths of 10-15 km. At the same time, a region with reduced transverse crustal resistivity spread over about 80 000 square kilometers is identified within the depth interval from 20 to 40 km. On the southeast the contour of the anomaly borders the zone of deepening of the Moho boundary down to 60 km in Central Finland. The results are compared with the AMT-MT sounding data and a geodynamic interpretation of the obtained information is carried out.

Structure-aware depth super-resolution using Gaussian mixture model

NASA Astrophysics Data System (ADS)

Kim, Sunok; Oh, Changjae; Kim, Youngjung; Sohn, Kwanghoon

2015-03-01

This paper presents a probabilistic optimization approach to enhance the resolution of a depth map. Conventionally, a high-resolution color image is considered as a cue for depth super-resolution under the assumption that the pixels with similar color likely belong to similar depth. This assumption might induce a texture transferring from the color image into the depth map and an edge blurring artifact to the depth boundaries. In order to alleviate these problems, we propose an efficient depth prior exploiting a Gaussian mixture model in which an estimated depth map is considered to a feature for computing affinity between two pixels. Furthermore, a fixed-point iteration scheme is adopted to address the non-linearity of a constraint derived from the proposed prior. The experimental results show that the proposed method outperforms state-of-the-art methods both quantitatively and qualitatively.

Lithospheric Structure Beneath Taiwan From Sp Converted Waves

NASA Astrophysics Data System (ADS)

Glasgow, D.; McGlashan, N.; Brown, L.

2006-12-01

Taiwan is the product of three dimensionally complex interaction between the Eurasian Plate (EP) and the Philippine Sea plate (PSP), with the EP subducting eastward beneath the PSP in southern Taiwan while the PSP subducts northward beneath the EP in northern Taiwan. The structural emplacement of Philippine Arc lithosphere onto Chinese passive margin lithosphere is an exemplar of continental amalgamation, yet there are relatively few contraints on the geometry of lithosphere involved at depth. We have used teleseismic data recorded by the Broadband Array for Taiwan Seismology (BATS) to compute S-to-p wave receiver functions for the Taiwan region to provide new constraints on deep geometries. Moho conversions provide independent new estimates of crustal thickness, which vary from 35 to 55 km across the island in agreement with previous P to S conversion studies and local tomography. More significantly, our results suggest that the lithosphere- asthenosphere boundary (LAB) varies in depth from ca 140 km beneath northeastern Taiwan to ca 120 km beneath central Taiwan to perhaps less than 80 km beneath southern Taiwan. We attribute this along strike variation to the depression and decapitation of the Eurasian plate in the transition to northward subduction of the PSP.

The magma plumbing system in the Mariana Trough back-arc basin at 18° N

NASA Astrophysics Data System (ADS)

Lai, Zhiqing; Zhao, Guangtao; Han, Zongzhu; Huang, Bo; Li, Min; Tian, Liyan; Liu, Bo; Bu, Xuejiao

2018-04-01

Mafic magmas are common in back-arc basin, once stalled in the crust, these magmas may undergo different evolution. In this paper, compositional and textural variations of plagioclase as well as mineral-melt geothermobarometry are presented for basalts erupted from the central Mariana Trough (CMT). These data reveal crystallization conditions and we attempt a reconstruction of the magma plumbing system of the CMT. Plagioclase megacrysts, phenocrysts, microphenocrysts, microlites, olivine, spinel, and clinopyroxene have been recognized in basalt samples, using BSE images and compositional features. The last three minerals are homogeneous as microphenocrysts. Mineral-melt barometry indicates that plagioclase crystals crystallized and eventually grew into phenocrysts and megacrysts in mush zone with depth of 5-9 km, in which the normal zoning plagioclases crystallized in the interval of various batches of basic magma recharging. Plagioclase megacrysts and phenocrysts were dissolved and/or resorbed, when new basic magmas injected into the mush zone near Moho depth. It is inferred that magma extracted from the mush zone, and adiabatically ascended via different pathways. Some basaltic magmas underwent plagioclase and clinopyroxene microphenocrysts crystallization in low-pressure before eruption. Plagioclase microlites and outermost rims probably crystallized after eruption.

Integrated Analysis on Gravity and Magnetic Fields of the Hailar Basin, NE China: Implications for Basement Structure and Deep Tectonics

NASA Astrophysics Data System (ADS)

Sun, Bin; Wang, Liangshu; Dong, Ping; Wu, YongJing; Li, Changbo; Hu, Bo; Wang, Chong

2012-11-01

The Hailar Basin is one of the typical basins among the NE China Basin Groups, which is situated in the east of East Asia Orogene between the Siberia Plate and the North China Plate. Based on the detailed analysis of magnetic, gravity, petrophysical, geothermal and seismological data, we separate the Gravity and Magnetic Anomalies (GMA) into four orders using Wavelet Multi-scale Decomposition (WMD). The apparent depths of causative sources were then assessed by Power Spectrum Analysis (PSA) of each order. Low-order wavelet detail anomalies were used to study the basin's basement structure such as major faults, the basement lithology, uplifts and depressions. High-order ones were used for the inversion of Moho and Curie discontinuities using the Parker method. The results show that the Moho uplifting area of the Hailar Basin is located at the NE part of the basin, the Curie uplifting area is at the NW part, and neither of them is consistent with the basin's sedimentary center. This indicates that the Hailar Basin may differ in basin building pattern from other middle and eastern basins of the basin groups, and the Hailar Basin might be of a passive type. When the Pacific Plate was subducting to NE China, the frontier of the plate lying on the mantle transition zone didn't pass through the Great Khingan Mountains region, so there is not an obvious magma upwelling or lithospheric extension in the Hailar Basin area. Finally, based on the seismological data and results of WMD, a probable 2D crust model is derived from an across-basin profile using the 2D forward modeling of the Bouguer gravity anomaly. The results agree with those from seismic inversion, suggesting WMD is suitable for identifying major crustal density interfaces.

Thermal State, Slab Metamorphism, and Interface Seismicity in the Cascadia Subduction Zone Based On 3-D Modeling

NASA Astrophysics Data System (ADS)

Ji, Yingfeng; Yoshioka, Shoichi; Banay, Yuval A.

2017-09-01

Giant earthquakes have repeatedly ruptured the Cascadia subduction zone, and similar earthquakes will likely also occur there in the near future. We employ a 3-D time-dependent thermomechanical model that incorporates an up-to-date description of the slab geometry to study the Cascadia subduction thrust. Results show a distinct band of 3-D slab dehydration that extends from Vancouver Island to the Seattle Basin and farther southward to the Klamath Mountains in northern California, where episodic tremors cluster. This distribution appears to include a region of increased dehydration in northern Cascadia. The phenomenon of heterogeneous megathrust seismicity associated with oblique subduction suggests that the presence of fluid-rich interfaces generated by slab dehydration favors megathrust seismogenesis in the northern part of this zone. The thin, relatively weakly metamorphosed Explorer, Juan de Fuca, and Gorda Plates are associated with an anomalous lack of thrust earthquakes, and metamorphism that occurs at temperatures of 500-700°C near the Moho discontinuity may represent a key factor in explaining the presence of the associated episodic tremor and slip (ETS), which requires a young oceanic plate to subduct at a small dip angle, as is the case in Cascadia and southwestern Japan. The 3-D intraslab dehydration distribution suggests that the metamorphosed plate environment is more complex than had previously been believed, despite the existence of channeling vein networks. Slab amphibolization and eclogitization near the continental Moho depth is thus inferred to account for the resultant overpressurization at the interface, facilitating the generation of ETS and the occurrence of small to medium thrust earthquakes beneath Cascadia.

Deep seismic structure of the northeastern South China Sea: Origin of a high-velocity layer in the lower crust

NASA Astrophysics Data System (ADS)

Wan, Kuiyuan; Xia, Shaohong; Cao, Jinghe; Sun, Jinlong; Xu, Huilong

2017-04-01

We present a 2-D seismic tomographic image of the crustal structure along the OBS2012 profile, which delineates the Moho morphology and magmatic features of the northeastern South China Sea margin. The image was created by forward modeling (RayInvr) and traveltime tomographic inversion (Tomo2D). Overall, the continental crust thins seaward from 27 km to 21 km within the continental shelf across the Zhu I Depression and Dongsha Rise, with slight local thickening beneath the Dongsha Rise accompanying the increase in the Moho depth. The Dongsha Rise is also characterized by 4-7 km thick high-velocity layer (HVL) ( 7.0-7.6 km/s) in the lower crust and exhibits a relatively high velocity ( 5.5-6.4 km/s) in the upper crust with a velocity gradient lower than those of the Zhu I Depression and Tainan Basin. Across the continental slope and continent-ocean transition (COT), which contain the Tainan Basin, the crust sharply thins from 20 km to 10 km seaward and a 2-3 km thick HVL is imaged in the lower crust. We observed that volcanoes are located only within the COT, but none exist in the continental shelf; the Dongsha Rise exhibits a high magnetic anomaly zone and different geochemical characteristics from the COT. Based on those observations, we conclude that the HVL underlying the COT is probably extension related resulting from the decompression melting in the Cenozoic, whereas the HVL beneath the Dongsha Rise is probably arc related and associated with the subduction of the paleo-Pacific plate. These findings are inconsistent with those of some previous studies.

Benthic Habitat Mapping by Combining Lyzenga’s Optical Model and Relative Water Depth Model in Lintea Island, Southeast Sulawesi

NASA Astrophysics Data System (ADS)

Hafizt, M.; Manessa, M. D. M.; Adi, N. S.; Prayudha, B.

2017-12-01

Benthic habitat mapping using satellite data is one challenging task for practitioners and academician as benthic objects are covered by light-attenuating water column obscuring object discrimination. One common method to reduce this water-column effect is by using depth-invariant index (DII) image. However, the application of the correction in shallow coastal areas is challenging as a dark object such as seagrass could have a very low pixel value, preventing its reliable identification and classification. This limitation can be solved by specifically applying a classification process to areas with different water depth levels. The water depth level can be extracted from satellite imagery using Relative Water Depth Index (RWDI). This study proposed a new approach to improve the mapping accuracy, particularly for benthic dark objects by combining the DII of Lyzenga’s water column correction method and the RWDI of Stumpt’s method. This research was conducted in Lintea Island which has a high variation of benthic cover using Sentinel-2A imagery. To assess the effectiveness of the proposed new approach for benthic habitat mapping two different classification procedures are implemented. The first procedure is the commonly applied method in benthic habitat mapping where DII image is used as input data to all coastal area for image classification process regardless of depth variation. The second procedure is the proposed new approach where its initial step begins with the separation of the study area into shallow and deep waters using the RWDI image. Shallow area was then classified using the sunglint-corrected image as input data and the deep area was classified using DII image as input data. The final classification maps of those two areas were merged as a single benthic habitat map. A confusion matrix was then applied to evaluate the mapping accuracy of the final map. The result shows that the new proposed mapping approach can be used to map all benthic objects in all depth ranges and shows a better accuracy compared to that of classification map produced using only with DII.

Continental rupture and the creation of new crust in the Salton Trough rift, Southern California and northern Mexico: Results from the Salton Seismic Imaging Project

NASA Astrophysics Data System (ADS)

Han, Liang; Hole, John A.; Stock, Joann M.; Fuis, Gary S.; Kell, Annie; Driscoll, Neal W.; Kent, Graham M.; Harding, Alistair J.; Rymer, Michael J.; González-Fernández, Antonio; Lázaro-Mancilla, Octavio

2016-10-01

A refraction and wide-angle reflection seismic profile along the axis of the Salton Trough, California and Mexico, was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting. From the northern Salton Sea to the southern Imperial Valley, the crust is 17-18 km thick and approximately one-dimensional. The transition at depth from Colorado River sediment to underlying crystalline rock is gradual and is not a depositional surface. The crystalline rock from 3 to 8 km depth is interpreted as sediment metamorphosed by high heat flow. Deeper felsic crystalline rock could be stretched preexisting crust or higher-grade metamorphosed sediment. The lower crust below 12 km depth is interpreted to be gabbro emplaced by rift-related magmatic intrusion by underplating. Low upper mantle velocity indicates high temperature and partial melting. Under the Coachella Valley, sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement. Mafic rock does not exist at 12-18 km depth as it does to the south, and a weak reflection suggests Moho at 28 km depth. Structure in adjacent Mexico has slower midcrustal velocity, and rocks with mantle velocity must be much deeper than in the Imperial Valley. Slower velocity and thicker crust in the Coachella and Mexicali valleys define the rift zone between them to be >100 km wide in the direction of plate motion. North American lithosphere in the central Salton Trough has been rifted apart and is being replaced by new crust created by magmatism, sedimentation, and metamorphism.

Continental rupture and the creation of new crust in the Salton Trough rift, southern California and northern Mexico: Results from the Salton Seismic Imaging Project

USGS Publications Warehouse

Han, Liang; Hole, John A.; Stock, Joann M.; Fuis, Gary S.; Kell, Annie; Driscoll, Neal W.; Kent, Graham M.; Rymer, Michael J.; Gonzalez-Fernandez, Antonio; Aburto-Oropeza, Octavio

2016-01-01

A refraction and wide-angle reflection seismic profile along the axis of the Salton Trough, California and Mexico, was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting. From the northern Salton Sea to the southern Imperial Valley, the crust is 17-18 km thick and approximately one-dimensional. The transition at depth from Colorado River sediment to underlying crystalline rock is gradual and is not a depositional surface. The crystalline rock from ~3 to ~8 km depth is interpreted as sediment metamorphosed by high heat flow. Deeper felsic crystalline rock could be stretched pre-existing crust or higher grade metamorphosed sediment. The lower crust below ~12 km depth is interpreted to be gabbro emplaced by rift-related magmatic intrusion by underplating. Low upper-mantle velocity indicates high temperature and partial melting. Under the Coachella Valley, sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement. Mafic rock does not exist at 12-18 depth as it does to the south, and a weak reflection suggests Moho at ~28 km depth. Structure in adjacent Mexico has slower mid-crustal velocity and rocks with mantle velocity must be much deeper than in the Imperial Valley. Slower velocity and thicker crust in the Coachella and Mexicali valleys define the rift zone between them to be >100 km wide in the direction of plate motion. North American lithosphere in the central Salton Trough has been rifted apart and is being replaced by new crust created by magmatism, sedimentation, and metamorphism.

Focal Depth of the WenChuan Earthquake Aftershocks from modeling of Seismic Depth Phases

NASA Astrophysics Data System (ADS)

Luo, Y.; Zeng, X.; Chong, J.; Ni, S.; Chen, Y.

2008-12-01

After the 05/12/2008 great WenChuan earthquake in Sichuan Province of China, tens of thousands earthquakes occurred with hundreds of them stronger than M4. Those aftershocks provide valuable information about seismotectonics and rupture processes for the mainshock, particularly accurate spatial distribution of aftershocks is very informational for determining rupture fault planes. However focal depth can not be well resolved just with first arrivals recorded by relatively sparse network in Sichuan Province, therefore 3D seismicity distribution is difficult to obtain though horizontal location can be located with accuracy of 5km. Instead local/regional depth phases such as sPmP, sPn, sPL and teleseismic pP,sP are very sensitive to depth, and be readily modeled to determine depth with accuracy of 2km. With reference 1D velocity structure resolved from receiver functions and seismic refraction studies, local/regional depth phases such as sPmP, sPn and sPL are identified by comparing observed waveform with synthetic seismograms by generalized ray theory and reflectivity methods. For teleseismic depth phases well observed for M5.5 and stronger events, we developed an algorithm in inverting both depth and focal mechanism from P and SH waveforms. Also we employed the Cut and Paste (CAP) method developed by Zhao and Helmberger in modeling mechanism and depth with local waveforms, which constrains depth by fitting Pnl waveforms and the relative weight between surface wave and Pnl. After modeling all the depth phases for hundreds of events , we find that most of the M4 earthquakes occur between 2-18km depth, with aftershocks depth ranging 4-12km in the southern half of Longmenshan fault while aftershocks in the northern half featuring large depth range up to 18km. Therefore seismogenic zone in the northern segment is deeper as compared to the southern segment. All the aftershocks occur in upper crust, given that the Moho is deeper than 40km, or even 60km west of the Longmenshan fault. Absence of mid-lower crustal shocks supports the model of lower crustal flow beneath eastern Tibetan plateau, which is probably responsible for Longmenshan uplifting and hence the Wenchuan earthquake.

Imaging rifting at the lithospheric scale in the northern East African Rift using S-to-P receiver functions

NASA Astrophysics Data System (ADS)

Lavayssiere, A.; Rychert, C.; Harmon, N.; Keir, D.; Hammond, J. O. S.; Kendall, J. M.; Leroy, S. D.; Doubre, C.

2017-12-01

The lithosphere is modified during rifting by a combination of mechanical stretching, heating and potentially partial melt. We image the crust and upper mantle discontinuity structure beneath the northern East African Rift System (EARS), a unique tectonically active continental rift exposing along strike the transition from continental rifting in the Main Ethiopian rift (MER) to incipient seafloor spreading in Afar and the Red Sea. S-to-P receiver functions from 182 stations across the northern EARS were generated from 3688 high quality waveforms using a multitaper technique and then migrated to depth using a regional velocity model. Waveform modelling of data stacked in large conversion point bins confirms the depth and strength of imaged discontinuities. We image the Moho at 29.6±4.7 km depth beneath the Ethiopian plateaux with a variability in depth that is possibly due to lower crustal intrusions. The crust is 27.3±3.9 km thick in the MER and thinner in northern Afar, 17.5±0.7 km. The model requires a 3±1.2% reduction in shear velocity with increasing depth at 68.5±1.5 km beneath the Ethiopian plateaux, consistent with the lithosphere-asthenosphere boundary (LAB). We do not resolve a LAB beneath Afar and the MER. This is likely associated with partial melt near the base of the lithosphere, reducing the velocity contrast between the melt-intruded lithosphere and the partially molten asthenosphere. We identify a 4.5±0.7% increase in velocity with depth at 91±3 km beneath the MER. This change in velocity is consistent with the onset of melting found by previous receiver functions and petrology studies. Our results provide independent constraints on the depth of melt production in the asthenosphere and suggest melt percolation through the base of the lithosphere beneath the northernmost East African rift.

Gravity anomaly and crustal structure characteristics in North-South Seismic Belt of China

NASA Astrophysics Data System (ADS)

Shen, Chongyang; Xuan, Songtbai; Yang, Guangliang; Wu, Guiju

2017-04-01

The North-South Seismic Belt (NSSB) is the binary system boundary what is formed by the western Indian plate subduction pushing and the eastern west Pacific asthenosphere rising, and it is one of the three major seismic belts (Tianshan, Taiwan and NSSB) and mainly located between E102°and E107°. And it is mainly composed of topographic gradient zones, faults, cenozoic basins and strong earthquake zones, which form two distinct parts of tectonic and physical features in the west and east. The research results of geophysical and deep tectonic setting in the NSSB show that it is not only a gravity anomaly gradient zone, it is but also a belt of crustal thickness increasing sharply westward of abrupt change. Seismic tomography results show that the anomaly zone is deeper than hundreds of kilometers in the NSSB, and the composition and structure of the crust are more complex. We deployed multiple Gravity and GNSS synchronous detection profiles in the NSSB, and these profiles crossed the mainly faults structure and got thousands of points data. In the research, source analysis, density structure inversion, residual gravity related imaging and normalized full gradient methods were used, and analyzed gravity field, density and their structure features in different positions, finally obtained the crustal density structure section characteristics and depth structure differences. The research results showed that the gravity Bouguer anomaly is similar to the existing large scale result. The Bouguer anomaly is rising significantly from west to east, its trend variation coincides well with the trend change of Moho depth, which is agreeing with the material flows to the peripheral situation of the Tibetan plateau. The obvious difference changes of the residual anomaly is relative to the boundary of structure or main tectonics, it's also connected with the stop degree of the eurasian plate when the material migrates around. The density structure of the gravity profiles mainly reflects basic frame work of the regional crust structure. The earth's crust basically present three layer structure, nearly horizontally distributes, undulation of Moho is obvious, which is consistent with the results of seismic sounding and seismic array detection; in the local area, there are lower density layer zonal distribution in the earth's crust what accelerates the lateral movement in up and middle crust; when the substance of the Tibetan plateau spreads around, the integrity in up and middle crust is well, and it is basically a coupling movement together; in the lower crust, the thickness of the Tibetan plateau is outward gradually thinning, there is decoupling phenomenon in crust-mantle; The results of the gravity and the crustal density structure show that the research area can be divided into several part such as Qinghai-Tibet Plateau, Sichuan-Yunnan block, Ordos block and Alxa block, the transitional zones of the Qinghai-Tibet Plateau and Sichuan basin, and Alxa and Ordos are complex, and Moho slope is bigger, where is the part of strong tectonic activity and strong earthquakes occur easily. The research is of great significance for study the crustal deep structure, geodynamic evolution process and environment of earthquake gestation of the NSSB region.

Crustal thickness and composition beneath the High Lava Plains of Eastern Oregon from teleseismic receiver functions

NASA Astrophysics Data System (ADS)

Eagar, K. C.; Fouch, M. J.; James, D. E.; Carlson, R. W.

2009-12-01

The nature of the crust beneath the High Lava Plains of eastern Oregon is fundamental for understanding the origins of widespread Cenozoic volcanism in the region. Eruptions of flood basalts in the southern Cascadian back arc peaked ~17-15 Ma, and were followed by distributed bimodal volcanism along two perpendicular migrating tracks; the Snake River Plain and the High Lava Plains. The orientations of eruptive centers have led to several competing hypotheses about their cause, including a deep mantle plume, slab retreat and asthenospheric inflow, lithospheric delamination, and lithospheric extension. The goal of this project is to constrain the nature, geometry, and depth of the Moho across the High Lava Plains, which will shed light on questions regarding crustal influence on melt generation and differentiation and the degree of magmatic underplating. In this study, we analyze teleseismic receiver functions from 118 stations of the High Lava Plains temporary broadband array, 34 nearby EarthScope/USArray stations, and 5 other regional broadband stations to determine bulk crustal features of thickness (H) and Vp/Vs ratio (κ). Applying the H-κ stacking method, we search for the best-fitting solution of timing predictions for direct and multiple P-to-S conversions from the Moho interface. Converting Vp/Vs to Poisson ratio, which is dependent primarily upon rock composition, allows for comparison with other direct geological observations. Preliminary results show that the crust of the High Lava Plains is relatively thin (~31 km) with a very sharp gradient to thicker crust (~42 km) at the western edge of the Owyhee Plateau in southwestern Idaho. This gradient is co-located with the western margin of Precambrian North America and is in the vicinity of the Jordan Craters volcanic center. The sharp topography of the Moho might have been a factor in melt migration beneath this area. West of the High Lava Plains, the crust thickens to ~40 km into the Cascade volcanic arc. We note that these results are consistent with preliminary crustal images from the complementary active source imaging effort of the High Lava Plains project. In contrast to crustal thickening to the east and west, there appears to be no change into the Blue Mountains to the north or the northern Basin and Range to the south. Although not as sharp and well defined as the Moho relief, there is variation in bulk crustal composition between the Owyhee Plateau and the High Lava Plains as defined by Vp/Vs. In contrast to the crust of Precambrian North America and the northern Basin and Range, whose Poisson ratio (~0.26) is comparable to average continental crust (0.27), the High Lava Plains exhibits high values (~0.30) typical of more mafic bulk crustal composition. This result suggests that the crust of the High Lava Plains evolved in a fundamentally different fashion in response to widespread magmatism relative to surrounding terranes. High Poisson ratios support the suggestion of mafic underplating and high, near crustal melting temperatures, possibly explaining the occurrence of rhyolitic volcanism across the High Lava Plains.

Mantle structure beneath the western edge of the Colorado Plateau

USGS Publications Warehouse

Sine, C.R.; Wilson, D.; Gao, W.; Grand, S.P.; Aster, R.; Ni, J.; Baldridge, W.S.

2008-01-01

Teleseismic traveltime data are inverted for mantle Vp and Vs variations beneath a 1400 km long line of broadband seismometers extending from eastern New Mexico to western Utah. The model spans 600 km beneath the moho with resolution of ???50 km. Inversions show a sharp, large-magnitude velocity contrast across the Colorado Plateau-Great Basin transition extending ???200 km below the crust. Also imaged is a fast anomaly 300 to 600 km beneath the NW portion of the array. Very slow velocities beneath the Great Basin imply partial melting and/or anomalously wet mantle. We propose that the sharp contrast in mantle velocities across the western edge of the Plateau corresponds to differential lithospheric modification, during and following Farallon subduction, across a boundary defining the western extent of unmodified Proterozoic mantle lithosphere. The deep fast anomaly corresponds to thickened Farallon plate or detached continental lithosphere at transition zone depths. Copyright 2008 by the American Geophysical Union.

Mechanical behaviour of the lithosphere beneath the Adamawa uplift (Cameroon, West Africa) based on gravity data

NASA Astrophysics Data System (ADS)

Poudjom Djomani, Y. H.; Diament, M.; Albouy, Y.

1992-07-01

The Adamawa massif in Central Cameroon is one of the African domal uplifts of volcanic origin. It is an elongated feature, 200 km wide. The gravity anomalies over the Adamawa uplift were studied to determine the mechanical behaviour of the lithosphere. Two approaches were used to analyse six gravity profiles that are 600 km long and that run perpendicular to the Adamawa trend. Firstly, the coherence function between topography and gravity was interpreted; secondly, source depth estimations by spectral analysis of the gravity data was performed. To get significant information for the interpretation of the experimental coherence function, the length of the profiles was varied from 320 km to 600 km. This treatment allows one to obtain numerical estimates of the coherence function. The coherence function analysis points out that the lithosphere is deflected and thin beneath the Adamawa uplift, and the Effective Elastic Thickness is of about 20 km. To fit the coherence, a load from below needs to be taken into account. This result on the Adamawa massif is of the same order of magnitude as those obtained on other African uplifts such as Hoggar, Darfur and Kenya domes. For the depth estimation, three major density contrasts were found: the shallowest depth (4-15 km) can be correlated to shear zone structures and the associated sedimentary basins beneath the uplift; the second density contrast (18-38 km) corresponds to the Moho; and finally, the last depth (70-90 km) would be the top of the upper mantle and demotes the low density zone beneath the Adamawa uplift.

Lithospheric structure of the Eastern Iranian plateau from integrated geophysical modeling: A transect from Makran to the Turan platform

NASA Astrophysics Data System (ADS)

Entezar-Saadat, Vahid; Motavalli-Anbaran, Seyed-Hani; Zeyen, Hermann

2017-05-01

We present a 2D profile of density and temperature distribution in the lithosphere across Iran along a more than 1600 km long profile extending from the Oman Gulf in the South to the Kopeh-Dagh and the Turan platform in the North. Gravity, geoid, topography and surface heat flow data were used for modeling, assuming local isostatic equilibrium. As much as possible, crustal structure has been constrained by seismic data. Crustal thickening is found under the Taftan-Bazman volcanic-arc (up to 47 km), under the Binalud foreland (∼54 km) and beneath the Kopeh-Dagh mountains (up to 50 km) whereas thin crust has been obtained under the Oman Gulf (20 km). Moho depth under the Lut block and the Turan platform is about 40 km. The lithospheric thickness is ∼90 km under the Oman Gulf and increases slightly until the Jazmourian depression. Then the lithospheric-asthenospheric boundary (LAB) bends significantly and sinks to ∼260 km under the Taftan-Bazman volcanic-arc. The LAB depth is about 190 km beneath the Lut block. A slight increase of LAB depth under the Binalud foreland towards the North may indicate a suture zone. Under the Turan platform, the LAB depth reaches ∼210 km. We also modeled two possible positions of the deep suture zone in NE Iran (along the main Kopeh-Dagh fault or along the Atrak River) and concluded that, when the suture zone is along the Atrak River, we obtained the better fit between calculated and measured data.

A regional-scale network for geoid monitoring and satellite gravimetry validation

NASA Astrophysics Data System (ADS)

Winester, D.; Pool, D.; Kennedy, J.

2010-12-01

In the past two decades, improved measurements of acceleration due to gravity have allowed for accurate detection of temporal gravity change. Terrestrial absolute gravimeters (for example, Micro-g LaCoste FG5 or A-10) can sense changes of gravity induced by elevation or mass changes, including local effects that may bias regional studies. Satellite instrumentation (e.g. GRACE) can detect large scale mass changes on a regular basis. However, the Nyquist wave number for satellite observations is often much too small for the size of regional studies. Also, satellites are limited by their life of deployment. Both techniques are used to (in)validate change models generated from other geophysical observations including water storage(underground and glacial), geoid definition, isostatic adjustments and tectonic(magmatic and faulting)activity. The gap between terrestrial and satellite gravity observations (and between satellite missions) might be bridged by developing a terrestrial network of sites of various observation techniques that define a representative sample of a given, regional study area. This information could then be statistically extrapolated to the extent of the region. The Southern High Plains Aquifer is such a region, since it has widespread relatively uniform geology, has relatively flat topography, and is well monitored for groundwater levels and soil moisture. Each site would have extensive instrumentation for monitoring, at a minimum, gravity (periodic and continuous) using absolute and tidal gravimeters, soil moisture, precipitation, depths to water in wells, evapotranspiration, air pressure, and land surface (GPS). Where possible, the network would build upon existing, data collection infrastructure. Preferably, the region would also have seismic tomography or crustal seismic reflection observations to characterize Moho-depth mass changes and have regional Bouguer anomaly mapping. In addition to information on local hydrology and geology, data collection would allow for characterization of local seasonal corrections, earth tides, atmospheric loading and episodic slip. No test network has yet been funded, but cost and man-power can be estimated. Such a network would rely on co-operation between various federal, state, local and university groups.

Shear Wave Velocity, Depth to Bedrock, and Fundamental Resonance Applied to Bedrock Mapping using MASW and H/V Analysis

NASA Astrophysics Data System (ADS)

Gonsiewski, J.

2015-12-01

Mapping bedrock depth is useful for earthquake hazard analysis, subsurface water transport, and other applications. Recently, collaborative experimentation provided an opportunity to explore a mapping method. Near surface glacial till shear wave velocity (Vs) where data is available from an array of 3-component seismometers were studied for this experiment. Vs is related to depth to bedrock (h) and fundamental resonance (Fo); Fo = Vs/(4h). The H/V spectral peak frequency of recordings from a 3-component seismometer yields a fundamental resonance estimate. Where a suitable average Vs is established, the depth to bedrock can be calculated at every seismometer. 3-component seismometer data was provided by Spectraseis. Geophones, seismographs, and an extra 3-component seismometer were provided by Wright State University for this study. For Vs analysis, three MASW surveys were conducted near the seismometer array. SurfSeis3© was used for processing MASW data. Overtones from complicated bedrock structure and great bedrock depth are improved by combining overtones from multiple source offsets from each survey. From MASW Vs and depth to bedrock results, theoretical fundamental resonance (Fo) was calculated and compared with the H/V peak spectral frequency measured by a seismometer at selected sites and processed by Geopsy processing software. Calculated bedrock depths from all geophysical data were compared with measured bedrock depths at nearby water wells and oil and gas wells provided by ODNR. Vs and depth to bedrock results from MASW produced similar calculated fundamental resonances to the H/V approximations by respective seismometers. Bedrock mapping was performed upon verifying the correlation between the theoretical fundamental resonance and H/V peak frequencies. Contour maps were generated using ArcGIS®. Contour lines interpolated from local wells were compared with the depths calculated from H/V analysis. Bedrock depths calculated from the seismometer array correlate with the major trends indicated by the surrounding wells. A final contour map was developed from depth to bedrock measured by all wells and depths calculated from the average Vs and estimated resonance at select Spectraseis 3-component seismometers.

Ground-penetrating radar--A tool for mapping reservoirs and lakes

USGS Publications Warehouse

Truman, C.C.; Asmussen, L.E.; Allison, H.D.

1991-01-01

Ground-penetrating radar was evaluated as a tool for mapping reservoir and lake bottoms and providing stage-storage information. An impulse radar was used on a 1.4-ha (3.5-acre) reservoir with 31 transects located 6.1 m (20 feet) apart. Depth of water and lateral extent of the lake bottom were accurately measured by ground-penetrating radar. A linear (positive) relationship existed between measured water depth and ground-penetrating radar-determined water depth (R2=0.989). Ground-penetrating radar data were used to create a contour map of the lake bottom. Relationships between water (contour) elevation and water surface area and volume were established. Ground-penetrating radar proved to be a useful tool for mapping lakes, detecting lake bottom variations, locating old stream channels, and determining water depths. The technology provides accurate, continuous profile data in a relatively short time compared to traditional surveying and depth-sounding techniques.

Imaging the Crust and Upper Mantle of Taiwan with Ambient Noise and Full Waveform Tomography

NASA Astrophysics Data System (ADS)

Rodzianko, A.; Roecker, S. W.

2013-12-01

Taiwan is the result of a complex, actively deforming tectonic boundary between the Eurasian and Philippine Sea plates that provides an excellent venue for investigating processes related to arc-continent collision. The TAIGER (TAiwan Integrated GEodynamics Research) project deployed broadband and short-period seismic stations that observed passive and active sources between 2006-2008. We analyze data collected by the TAIGER deployment, supplemented by observations from the permanent BATS (Broadband Array in Taiwan for Seismology) network, to create a 3D elastic wave velocity model of the crust and upper mantle beneath Taiwan. We start by applying ambient noise tomography techniques on the dataset to create a 3D Vs model. The vertical component of continuous ambient noise is whitened and cross-correlated between stations to construct empirical Green's functions (EGFs) of Rayleigh waves, which are graded by the signal to noise (SNR) ratio prior to recovering group and phase velocities of the fundamental mode for periods between 6 and 30 seconds. We invert group and phase velocity maps on a regular grid with 5 km spacing, and combine the results to generate a 3D Vs model. This model, combined with the arrival time model of Hao et al (2012), are used as a starting model for full waveform inversion (FWI) of teleseismic body and surface waves using the 2.5D technique of Roecker et al (2010). We find that below the Central Mountain Range, the crust thickens with the Moho at ~50 km depth and with S-wave speeds ~3.0 km/s, indicating a deep crustal root. The west half of the island is generally characterized by a thinner crust and relatively lower S-wave velocities.

3D lithospheric mapping of the Iberian Peninsula and surrounding Atlantic and Mediterranean margins from 3D joint inversion of potential field and elevation data.

NASA Astrophysics Data System (ADS)

Torne, Montserrat; Zeyen, Hermann; Jimenez-Munt, Ivone; Fernandez, Manel; Vergés, Jaume

2017-04-01

We investigate the lithospheric density structure of the Iberian Peninsula and the surrounding Atlantic and Mediterranean margins from a 3D joint inversion of free-air, geoid and elevation data, based on a Bayesian approach. In addition, the crustal structure has been further constrained by incorporating about 750 Moho values from DSS investigations and RF analysis covering the entire region. Our preliminary results shows a significant lithospheric deformation along the plate boundaries, the Bay of Biscay-Pyrenees to the North and the Azores-Gibraltar to the south, where the CMB and LAB are located at depths more than 45 and 150 km, respectively. Noteworthy is the arcuate lithospheric thickening located at the westernmost end of the Gibraltar Arc system showing the presence of the NW-to-Westward retreated Gibraltar Arc slab that has given rise to the formation of the Betics-Rif Alpine belt system and the back arc Alboran basin. To the west, the stable-slightly deformed Iberian massif shows a quasi-flat CMB and LAB topography (30 to 32 km and about 110 km, respectively). The crust and mantle lithosphere thin towards the Mediterranean and Atlantic margins, with the exception of its northern margin where lithospheric thickening extends offshore to the Gulf of Biscay. In the western Mediterranean the SE-Neogene slab retreat has resulted in a significant thinning of the crust and mantle lithosphere. Thin lithosphere is also observed in the Tagus-Horseshoe abyssal plain region where the LAB shallows to less than 90 km. This work has been funded by the Spanish projects MITE (CGL2014-59516-P) and WEME-CSIC project 201330E11.

The Paleozoic - Mesozoic Mekele Sedimentary Basin in Ethiopia: An example of an exhumed IntraCONtinental Sag (ICONS) basin

NASA Astrophysics Data System (ADS)

Alemu, Tadesse; Abdelsalam, Mohamed G.; Dawit, Enkurie L.; Atnafu, Balemwal; Mickus, Kevin L.

2018-07-01

We investigated the evolution of the Mekele Sedimentary Basin (MSB) in northern Ethiopia using geologic field and gravity data. The depth to Moho and lithospheric structure beneath the basin was imaged using two-dimensional (2D) radially-averaged power spectral analysis, Lithoflex three-dimensional (3D) forward and inverse modeling, and 2D forward modeling of the Bouguer gravity anomalies. Previous studies proposed that the basin was formed as part of a multi-branched rift system related to the breakup of Gondwana. Our results show that the MSB: (1) is circular to elliptical in map view and saucer shaped in cross sectional view, (2) is filled with terrestrial and shallow marine sedimentary rocks, (3) does not significantly structurally control the sedimentation and the major faults are post-depositional, (4) is characterized by a concentric gravity minima, (5) is underlain by an unstretched crust (∼40 km thick) and thicker lithosphere (∼120 km thick). These features compare positively with a group of basins known as IntraCONtinental Sags (ICONS), especially those ICONS formed over accretionary orogenic terranes. Since the MSB is located above the Neoproterozoic accretionary orogenic terranes of the Arabian-Nubian Shield (ANS), we propose that the formation of the MSB to be related to cooling and thickening of a juvenile sub-continental lithospheric mantle beneath the ANS, which most probably provided negative buoyancy, and hence subsidence in the MSB, leading to its formation as an ICONS. The MSB could be used as an outcrop analog for information about the internal facies architecture of ICONS because it is completely exhumed due to tectonic uplift on the western flank of the Afar Depression.

Structure of the Lithosphere in Central Europe: Integrated Density Modelling

NASA Astrophysics Data System (ADS)

Bielik, M.; Grinč, M.; Zeyen, H. J.; Plašienka, D.; Pasteka, R.; Krajňák, M.; Bošanský, M.; Mikuška, J.

2014-12-01

Firstly, we present new results related to the lithospheric structure and tectonics of the Central Europe and the Western Carpathians. For geophysical study of the lithosphere in Central Europe we calculated four original 2D lithosphere-scales transects crossing this area from the West European Platform in the North to the Aegean Sea in the South and from the Adriatic Sea in the West to the East European Platform in the East. Modelling is based on the joint interpretation of gravity, geoid, topography and surface heat flow data with temperature-dependent density. Wherever possible, crustal structure is constrained by seismic data. The thickness of the lithosphere decreases from the older and colder platforms to the younger and hotter Pannonian Basin with a maximum thickness under the Eastern and Southern Carpathians. The thickness of the Carpathian arc lithosphere varies between 150 km in the North (the Western Carpathians) and about 300 km in the Vrancea zone (the Eastern and Southern Carpathian junction). In the Platform areas it is between 120 and 150 km and in the Pannonian Basin it is about 70 km. The models show that the Moesian Platform is overthrust from the North by the Southern Carpathians and from the South by the Balkanides and characterized by bending of this platform. In all transects, the thickest crust is found underneath the Carpathian Mountains or, as in the case of the Vrancea area, under their immediate foreland. The thickest crust outside the orogens is modelled for the Moesian Platform with Moho depths of up to 45 km. The thinnest crust is located under the Pannonian Basin with about 26-27 km. Secondly, our presentation deals with construction of the stripped gravity map in the Turiec Basin, which represents typical intramontane Neogene depression of the Western Carpathians. Based on this new and original gravity map corrected by regional gravity effect we were able to interpret the geological structure and tectonics of this sedimentary basin. This work was supported by the Slovak Grant Agency VEGA (grants No. 1/0095/12, 2/0067/12) and Slovak Research and Development Agency (grants No. APVV-0827-12, APVV-0194-10).

Crustal structure along the DESERT 2000 Transect inferred from 3-D gravity modelling

NASA Astrophysics Data System (ADS)

El-Kelani, R.; Goetze, H.; Rybakov, M.; Hassouneh, M.; Schmidt, S.

2003-12-01

A three-dimensional interpretation of the newly compiled Bouguer anomaly map is part of the DESERT 2000 Transect. That is multi-disciplinary and multinational project studying for first time the Dead Sea Transform (DST) fault system (DST) from the Mediterranean Sea to Saudi Arabia across the international border in the NW-SE direction. The negative Bouguer anomalies (with magnitude reached "C130 mGal), located into transform valley, are caused by the internal sedimentary basins filled by the light density young sediments (­Y10 km). A high-resolution 3-D model constrained with the seismic results reveals a possible crustal thickness and density distribution beneath the DST valley. The inferred zone of intrusion coincides with the maximum gravity anomaly over the eastern flank of the DST. The intrusion is displaced at different sectors along the NW-SE direction. The zone of the maximum crustal thinning (­30 km) is attained in the western sector at the Mediterranean. The southeastern plateau, on the other hand, shows by far the largest crustal thickness in the region (38-42 km). Linked to the left lateral movement of ~ 105 km at the boundary between the African and Arabian plate, and constrained with the DESERT 2000 seismic data, a small asymmetric topography of the Moho beneath the DST was modelled. The thickness and density of the crust suggest that a continental crust underlies the DST. The deep basins, the relatively large nature of the intrusion and the asymmetric topography of the Moho lead to the conclusion that a small-scale asthenospheric upwelling(?) might be responsible for the thinning of the crust and subsequent rifting of the Dead Sea graben during the left lateral movement.

Lithosphere mantle density of the North China Craton based on gravity data

NASA Astrophysics Data System (ADS)

Xia, B.; Artemieva, I. M.; Thybo, H.

2017-12-01

Based on gravity, seismic and thermal data we constrained the lithospheric mantle density at in-situ and STP condition. The gravity effect of topography, sedimentary cover, Moho and Lithosphere-Asthenosphere Boundary variation were removed from free-air gravity anomaly model. The sedimentary covers with density range from 1.80 g/cm3 with soft sediments to 2.40 g/cm3 with sandstone and limestone sediments. The average crustal density with values of 2.70 - 2.78 g/cm3 which corresponds the thickness and density of the sedimentary cover. Based on the new thermal model, the surface heat flow in original the North China Craton including western block is > 60 mW/m2. Moho temperature ranges from 450 - 600 OC in the eastern block and in the western block is 550 - 650 OC. The thermal lithosphere is 100 -140 km thick where have the surface heat flow of 60 - 70 mW/m2. The gravity effect of surface topography, sedimentary cover, Moho depth are 0 to +150 mGal, - 20 to -120 mGal and +50 to -200 mGal, respectively. By driving the thermal lithosphere, the gravity effect of the lithosphere-asthenosphere boundary ranges from 20 mGal to +200 mGal which shows strong correction with the thickness of the lithosphere. The relationship between the gravity effect of the lithosphere-asthenosphere boundary and the lithosphere thickness also for the seismic lithosphere, and the value of gravity effect is 0 to +220 mGal. The lithospheric mantle residual gravity which caused by lithospheric density variation range from -200 to +50 mGal by using the thermal lithosphere and from -250 to +100 mGal by driving the seismic lithosphere. For thermal lithosphere, the lithospheric mantle density with values of 3.21- 3.26 g/cm3 at in-situ condition and 3.33 - 3.38 g/cm3 at STP condition. Using seismic lithosphere, density of lithosphere ranges from 3.20 - 3.26 g/cm3 at in-situ condition and 3.31 - 3.41 g/cm3 at STP condition. The subcontinental lithosphere of the North China Craton is highly heterogeneous with Archean lithosphere at the southwestern of the Eastern Block, major the Trans-North China Orogen and western part of the Western Block. The lithospheric mantle beneath the northern part of the Eastern Block, central segment of the Trans-North China Craton and the eastern margin of the Western Block have experienced modification and replacement.

Tomographic evidence for recent extension in the Bentley Subglacial Trench and a hotspot beneath Marie Byrd Land

NASA Astrophysics Data System (ADS)

Lloyd, A. J.; Wiens, D. A.; Nyblade, A.; Anandakrishnan, S.; Aster, R. C.; Huerta, A. D.; Wilson, T. J.; Shore, P.

2013-12-01

Here we present the first regional P and S wave relative velocity models of the upper mantle beneath much of West Antarctica using P and S wave relative travel time residuals from teleseismic events recorded by seismographs from the POLENET/ANET project. 21 of the seismographs form a sparse backbone network co-located with continuously recording GPS stations at rock sites throughout West Antarctica, and 17 stations formed a seismic transect extending from the Whitmore Mountains across the West Antarctic Rift System (WARS) and into Marie Byrd Land (MBL) with a station spacing of 90-100 km. Corrections for heterogeneities above the Moho, including the ice sheet, are applied to the relative travel time residuals using the receiver function models of Chaput et al., [submitted, 2013]. Both P and S wave velocity models indicate velocities faster than the mean of the model beneath the Whitmore Mountains that may be interpreted as thicker, colder lithosphere relative to the rest of West Antarctica. Slow velocity anomalies are observed beneath the Bentley Subglacial Trench (BST) and MBL. Slow velocities extending from the Moho to the transition zone beneath MBL are centered beneath the Mt Sidley volcano and coincide with high topography that is not isostatically supported by the crust [Chaput et al., submitted, 2013]. The slowest velocities occur at 200-300 km depth and are consistent with warm, low viscosity mantle that provides topographic support for the elevated MBL volcanic dome. Poor vertical resolution, typical of body wave tomography, hampers the models ability to resolve whether the anomaly beneath MBL is strictly an upper mantle hotspot or a classic mantle plume that extends into the lower mantle. The shallow (≤ 100 km depth) slow anomaly beneath the BST coincides with a region of thin crust and likely reflects a localized region of Cenozoic extension in the WARS that may have undergone a last phase of extension in the Neogene [Garnot et al., 2013]. Anomalously high heat flow reported by Fudge et al.[2012] at the WAIS divide ice core is also consistent with recent Neogene extension and a thermal perturbation suggested by both P and S tomography models. In general, the strong heterogeneities in our models are predominantly interpreted as reflecting upper mantle temperature variations in addition to possible mantle partial melting beneath MBL.

Spatial Relationship Between Crustal Structure and Mantle Seismicity in the Vrancea Seismogenic Zone of Romania

NASA Astrophysics Data System (ADS)

Knapp, C. C.; Enciu, D. M.; Knapp, J. H.

2007-12-01

Active crustal deformation and subsidence in the Southeast Carpathian foreland has previously been attributed to active foundering of thickened continental lithosphere beneath the Carpathian bend region (Knapp et al, 2005). The present study involves integration of active and passive-source seismic data in order to place constraints on the duration, timing, and scale of crustal deformation in the Carpathian foreland, and in particular to assess the genetic relationship with the Vrancea intermediate-depth seismogenic zone (VSZ). Relocated crustal earthquakes and focal mechanisms were correlated with four deep industry seismic profiles, the reprocessed DACIA PLAN deep seismic profile, and the DRACULA (Deep Reflection Acquisition Constraining Unusual Lithospheric Activity) II and III profiles. Projection of foreland crustal hypocenters onto the deep seismic lines correlates well with previously identified crustal faults such as the Trotus and Sinaia, as well as the newly identified Ialomita Fault. Specifically, results of this study (1) image the full crustal and uppermost mantle structure of the Focsani Basin in the close proximity of the VSZ, (2) show evidence for a sub-horizontal, slightly east-dipping Moho in the vicinity of the VSZ and thinning of the crust towards the Carpathian orogen, (3) illustrate the conspicuous absence of west-dipping fabrics or structures in the crust and across the Moho, (4) present evidence that the Trotus Fault is a crustal-scale active fault with a dextral sense of motion, (5) suggest that the Paleozoic age Peceneaga-Camena and Capidava-Ovidiu Faults have not been active in post-Paleozoic time, and (6) show evidence for a new active crustal scale sinistral fault, named the Ialomita fault. Both the seismogenic Vrancea body and deformation in the Focsani Basin appear to be concentrically bound by the Trotus Fault in the north and east and the Sinaia-Ialomita Fault in the south, suggesting a coupled deformation between the VSZ and the foreland deformation, possibly accommodated on these two major fault systems. These results contradict both the "subduction-in-place" and "slab- break-off" hypotheses as feasible explanations for VSZ intermediate-depth seismicity, and lend additional support to a lithospheric delamination model to explain both the origin of the VSZ and the crustal architecture of the Southeast Carpathian foreland.

An efficient hole-filling method based on depth map in 3D view generation

NASA Astrophysics Data System (ADS)

Liang, Haitao; Su, Xiu; Liu, Yilin; Xu, Huaiyuan; Wang, Yi; Chen, Xiaodong

2018-01-01

New virtual view is synthesized through depth image based rendering(DIBR) using a single color image and its associated depth map in 3D view generation. Holes are unavoidably generated in the 2D to 3D conversion process. We propose a hole-filling method based on depth map to address the problem. Firstly, we improve the process of DIBR by proposing a one-to-four (OTF) algorithm. The "z-buffer" algorithm is used to solve overlap problem. Then, based on the classical patch-based algorithm of Criminisi et al., we propose a hole-filling algorithm using the information of depth map to handle the image after DIBR. In order to improve the accuracy of the virtual image, inpainting starts from the background side. In the calculation of the priority, in addition to the confidence term and the data term, we add the depth term. In the search for the most similar patch in the source region, we define the depth similarity to improve the accuracy of searching. Experimental results show that the proposed method can effectively improve the quality of the 3D virtual view subjectively and objectively.

Recent Seismic Experiments of OBS in the South China Sea

NASA Astrophysics Data System (ADS)

Ruan, A.; Li, J.; Wu, Z.

2012-12-01

Since 2006 some research institutions of China have carried out some important seismic experiments by using ocean bottom seismometer(OBS) in the South China Sea (SCS) and obtained many concrete progresses in modeling the crustal structure of SCS and also in understanding of its formation and evolution as well. In 2006 three wide-angle profiles were completed in the northern margin, named OBS2006-1 across the northwestern sub-basin, OBS2006-2 parallel to the sea basin boundary and OBS2006-3 across the Dongsha Rise and Chaoshan Depression respectively. In 2010 two wide- angle profiles were completed, named OBS2010-1 and OBS 2010-2 both perpendicular to the northern off-shore faulting system. During 2009-2011 four wide-angle profiles were completed in the southern margin, named OBS973-1 from southern margin to the southwestern sub-basin, OBS973-2 from Liyue Bank to the southwestern sub-basin and OBS973-3 from Xisha to the southwestern sub-basin, OBS2011-2 from Xisha to Hainan Island respectively. In 2011 two 3D seismic array of OBS were completed in the Zhongnan-Changlong sea mount chain and Huangyan-Zhenbei sea mount chain respectively. Here we present some primary but important results as follows. (1) The velocity model of OBS2006-1 indicates that the crust under the continental slope decreases from 21km to 11km, and to 7.7km in the northwestern sub-basin with Moho depth ascends from 21km to 11km. The tectonic geometry and velocity structure of the northwestern sub-basin and its margins on both sides shows symmetrical and conjugate and indicates pure shear mode of continental margin rifting mechanism. (2) The velocity model of OBS2006-3 reveals remarkable thickness with maximum 8 km of the Mesozoic sediment in Chaoshan Depression in which velocity increases downward from 4.4 km/s at top to 5.3 km/s at the bottom. The buried depth of Moho decreases from 24-25 km under Dongsha Rise to 17 km in the lower slope and an obviously velocity abnormal is detected in the upper crust of the Dongsha Rise and its velocity raises to 6.9 km/s due to the mantle underplating and magma activities. A high velocity layer (HVL) of 3-12 km thick is found in the lower crust, and its velocity is 7.1-7.4 km/s. (3) The wide angle seismic profile-OBS973-2 that extends in NW-SE direction 369km long from the northeastern Liyue Bank to the central sub-basin. It indicates that there are some small volcanoes on the top of crust in Liyue Bank and P wave velocities downward increase from 5.5~6.4km/s in the upper crust (9~10km thick) to 6.6~7.1km/s in the lower crust (11km thick). In the transition zone and sea basin P wave velocities downward increase from 5.9~6.1km/s in the upper curst (4~5km thick) to 6.6~6.9km/s in the lower crust (2~4km thick). The buried depth of Moho is 23km in Liyue Bank and 8~12km in the sea basin. The comparison of profile OBS973-2 with profile OBS2006-1 in the northern margin shows remarkable similarity between them and suggests a possibility of conjugation relationship between Liyue Bank and Zhongsha Massif. Acknowledgements This work was supported by the National Natural Science Foundation of China (91028006,40876035,41106053 and 41176046) and the National Basic Research Program of China (2007CB411701) .

Atlas of depth-duration frequency of precipitation annual maxima for Texas

USGS Publications Warehouse

Asquith, William H.; Roussel, Meghan C.

2004-01-01

Ninety-six maps depicting the spatial variation of the depth-duration frequency of precipitation annual maxima for Texas are presented. The recurrence intervals represented are 2, 5, 10, 25, 50, 100, 250, and 500 years. The storm durations represented are 15 and 30 minutes; 1, 2, 3, 6, and 12 hours; and 1, 2, 3, 5, and 7 days. The maps were derived using geographically referenced parameter maps of probability distributions used in previously published research by the U.S. Geological Survey to model the magnitude and frequency of precipitation annual maxima for Texas. The maps in this report apply that research and update depth-duration frequency of precipitation maps available in earlier studies done by the National Weather Service.

Spectrally based bathymetric mapping of a dynamic, sand‐bedded channel: Niobrara River, Nebraska, USA

USGS Publications Warehouse

Dilbone, Elizabeth; Legleiter, Carl; Alexander, Jason S.; McElroy, Brandon

2018-01-01

Methods for spectrally based mapping of river bathymetry have been developed and tested in clear‐flowing, gravel‐bed channels, with limited application to turbid, sand‐bed rivers. This study used hyperspectral images and field surveys from the dynamic, sandy Niobrara River to evaluate three depth retrieval methods. The first regression‐based approach, optimal band ratio analysis (OBRA), paired in situ depth measurements with image pixel values to estimate depth. The second approach used ground‐based field spectra to calibrate an OBRA relationship. The third technique, image‐to‐depth quantile transformation (IDQT), estimated depth by linking the cumulative distribution function (CDF) of depth to the CDF of an image‐derived variable. OBRA yielded the lowest depth retrieval mean error (0.005 m) and highest observed versus predicted R2 (0.817). Although misalignment between field and image data did not compromise the performance of OBRA in this study, poor georeferencing could limit regression‐based approaches such as OBRA in dynamic, sand‐bedded rivers. Field spectroscopy‐based depth maps exhibited a mean error with a slight shallow bias (0.068 m) but provided reliable estimates for most of the study reach. IDQT had a strong deep bias but provided informative relative depth maps. Overprediction of depth by IDQT highlights the need for an unbiased sampling strategy to define the depth CDF. Although each of the techniques we tested demonstrated potential to provide accurate depth estimates in sand‐bed rivers, each method also was subject to certain constraints and limitations.

Sub-Moho Reflectors, Mantle Faults and Lithospheric Rheology

NASA Astrophysics Data System (ADS)

Brown, L. D.

2013-12-01

One of the most unexpected and dramatic observations from the early years of deep reflection profiling of the continents using multichannel CMP techniques was the existing of prominent reflections from the upper mantle. The first of these, the Flannan thrust/fault/feature, was traced by marine profiling of the continental margin offshore Britain by the BIRPS program, which soon found them to be but one of several clear sub-crustal discontinuities in that area. Subsequently, similar mantle reflectors have been observed in many areas around the world, most commonly beneath Precambrian cratonic areas. Many, but not all, of these mantle reflections appear to arise from near the overlying Moho or within the lower crust before dipping well into the mantle. Others occur as subhorizontal events at various depths with the mantle, with one suite seeming to cluster at a depth of about 75 km. The dipping events have been variously interpreted as mantle roots of crustal normal faults or the deep extension of crustal thrust faults. The most common interpretation, however, is that these dipping events are the relicts of ancient subduction zones, the stumps of now detached Benioff zones long since reclaimed by the deeper mantle. In addition to the BIRPS reflectors, the best known examples include those beneath Fennoscandia in northern Europe, the Abitibi-Grenville of eastern Canada, and the Slave Province of northwestern Canada (e.g. on the SNORCLE profile). The most recently reported example is from beneath the Sichuan Basin of central China. The preservation of these coherent, and relatively delicate appearing, features beneath older continental crust and presumably within equally old (of not older) mantle lithosphere, has profound implications for the history and rheology of the lithosphere in these areas. If they represent, as widely believe, some form of faulting with the lithosphere, they provide corollary constraints on the nature of faulting in both the lower crust and upper mantle. The SNORCLE mantle reflectors, which can be traced deep within the early Precambrian (?) mantle by both surface (controlled source) reflection profiles and passive (receiver function) images most clearly illustrates the rheological implications of such feature. The SNORCLE events appear to root upwards into the lower crust and extend to depths approaching 200 km into the mantle. This would seem to require the preservation of undeformed mantle lithosphere for almost 2.5 billion years in this area. This preservation is clearly inconsistent with the interpretation of nearby shallower mantle interfaces as marking the modern lithosphere-asthenosphere boundary. In summary, dipping mantle reflections imply preservation of substantial thicknesses of mantle lithosphere for very long periods of time, and localization of mantle deformation during the formation of these structures along relatively narrow, discrete interfaces rather than across broad zones of diffuse deformation. .

Anomalous Subsidence at the Ocean Continent Transition of the Gulf of Aden Rifted Continental Margin

NASA Astrophysics Data System (ADS)

Cowie, Leanne; Kusznir, Nick; Leroy, Sylvie

2013-04-01

It has been proposed that some rifted continental margins have anomalous subsidence and that at break-up they were elevated at shallower bathymetries than the isostatic response predicted by classical rift models (McKenzie, 1978). The existence of anomalous syn- or early-post break-up subsidence of this form would have important implications for our understanding of the geodynamics of continental break-up and sea-floor spreading initiation. We have investigated subsidence of the young rifted continental margin of the eastern Gulf of Aden, focussing on the western Oman margin (break-up age 17.6 Ma). Lucazeau et al. (2008) have found that the observed bathymetry here is approximately 1 km shallower than the predicted bathymetry. In order to examine the proposition of an anomalous early post break-up subsidence history of the Omani Gulf of Aden rifted continental margin, we have determined the subsidence of the oldest oceanic crust adjacent to the continent-ocean boundary (COB) using residual depth anomaly (RDA) analysis corrected for sediment loading and oceanic crustal thickness variation. RDAs corrected for sediment loading using flexural backstripping and decompaction have been calculated by comparing observed and age predicted oceanic bathymetries in order to identify anomalous subsidence of the Gulf of Aden rifted continental margin. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions of Crosby and McKenzie (2009). Non-zero RDAs at the Omani Gulf of Aden rifted continental margin can be the result of non standard oceanic crustal thickness or the effect of mantle dynamic topography or a non-classical rift and break-up model. Oceanic crustal basement thicknesses from gravity inversion together with Airy isostasy have been used to predict a "synthetic" gravity RDA, in order to determine the RDA contribution from non-standard oceanic crustal thickness. Gravity inversion, used to determine crustal basement thickness, incorporates a lithosphere thermal gravity anomaly correction and uses sediment thicknesses from 2D seismic data. Reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. The difference between the sediment corrected RDA and the "synthetic" gravity derived RDA gives the component of the RDA which is not due to variations in oceanic crustal thickness. This RDA corrected for sediment loading and crustal thickness variation has a magnitude between +600m and +1000m (corresponding to anomalous uplift) and is comparable to that reported (+1km) by Lucazeau et al. (2008). We are unable to distinguish whether this anomalous uplift is due to mantle dynamic topography or anomalous subsidence with respect to classical rift model predictions.

Detailed Northern Anatolian Fault Zone crustal structure from receiver functions

NASA Astrophysics Data System (ADS)

Cornwell, D. G.; Kahraman, M.; Thompson, D. A.; Houseman, G. A.; Rost, S.; Turkelli, N.; Teoman, U.; Altuncu Poyraz, S.; Gülen, L.; Utkucu, M.

2013-12-01

We present high resolution images derived from receiver functions of the continental crust in Northern Turkey that is dissected by two fault strands of the Northern Anatolian Fault Zone (NAFZ). The NAFZ is a major continental strike-slip fault system that is comparable in length and slip rate to the San Andreas Fault Zone. Recent large earthquakes occurred towards the western end of the NAFZ in 1999 at Izmit (M7.5) and Düzce (M7.2). As part of the multi-disciplinary Faultlab project, we aim to develop a model of NAFZ crustal structure and locate deformation by constraining variations in seismic properties and anisotropy in the upper and lower crust. The crustal model will be an input to test deformation scenarios in order to match geodetic observations from different phases of the earthquake loading cycle. We calculated receiver functions from teleseismic earthquakes recorded by a rectangular seismometer array spanning the NAFZ with 66 stations at a nominal inter-station spacing of 7 km and 7 additional stations further afield. This Dense Array for North Anatolia (DANA) was deployed from May 2012 until September 2013 and we selected large events (Mw>5.5) from the high quality seismological dataset to analyze further. Receiver functions were calculated for different frequency bands then collected into regional stacks before being inverted for crustal S-wave velocity structure beneath the entire DANA array footprint. In addition, we applied common conversion point (CCP) migration using a regional velocity model to construct a migrated 3D volume of P-to-S converted and multiple energy in order to identify the major crustal features and layer boundaries. We also performed the CCP migration with transverse receiver functions in order to identify regions of anisotropy within the crustal layers. Our preliminary results show a heterogeneous crust above a flat Moho that is typically at a depth of 33 km. We do not observe a prominent step in the Moho beneath the surface locations at either of the NAFZ fault branches. We observe first-order differences in crustal structure between the crustal blocks that are separated by the faults. Each crustal block also contains regions of strong anisotropy at various depths that will be analyzed further with the full seismological dataset and compared to petrofabric analyses of exhumed fault segments. We will compare our results with other seismological imaging techniques to attempt to resolve the distribution of fault zone deformation with respect to depth. This information will be useful to other complementary Faultlab techniques that will add a detailed insight into the fault structure and dynamics of the NAFZ and contribute more broadly into ongoing research into major strike-slip fault zones.

Approach for scene reconstruction from the analysis of a triplet of still images

NASA Astrophysics Data System (ADS)

Lechat, Patrick; Le Mestre, Gwenaelle; Pele, Danielle

1997-03-01

Three-dimensional modeling of a scene from the automatic analysis of 2D image sequences is a big challenge for future interactive audiovisual services based on 3D content manipulation such as virtual vests, 3D teleconferencing and interactive television. We propose a scheme that computes 3D objects models from stereo analysis of image triplets shot by calibrated cameras. After matching the different views with a correlation based algorithm, a depth map referring to a given view is built by using a fusion criterion taking into account depth coherency, visibility constraints and correlation scores. Because luminance segmentation helps to compute accurate object borders and to detect and improve the unreliable depth values, a two steps segmentation algorithm using both depth map and graylevel image is applied to extract the objects masks. First an edge detection segments the luminance image in regions and a multimodal thresholding method selects depth classes from the depth map. Then the regions are merged and labelled with the different depth classes numbers by using a coherence test on depth values according to the rate of reliable and dominant depth values and the size of the regions. The structures of the segmented objects are obtained with a constrained Delaunay triangulation followed by a refining stage. Finally, texture mapping is performed using open inventor or VRML1.0 tools.

On-the-go mapping of soil mechanical resistance using a linear depth effect model.

USDA-ARS?s Scientific Manuscript database

An instrumented blade sensor was developed to map soil mechanical resistance as well as its change with depth. The sensor has become a part of the Integrated Soil Physical Properties Mapping System (ISPPMS), which also includes an optical and a capacitor-based sensor. The instrumented blade of the...

Seismic Imaging of the Lesser Antilles Subduction Zone Using S-to-P Receiver Functions: Insights From VoiLA

NASA Astrophysics Data System (ADS)

Chichester, B.; Rychert, C.; Harmon, N.; Rietbrock, A.; Collier, J.; Henstock, T.; Goes, S. D. B.; Kendall, J. M.; Krueger, F.

2017-12-01

In the Lesser Antilles subduction zone Atlantic oceanic lithosphere, expected to be highly hydrated, is being subducted beneath the Caribbean plate. Water and other volatiles from the down-going plate are released and cause the overlying mantle to melt, feeding volcanoes with magma and hence forming the volcanic island arc. However, the depths and pathways of volatiles and melt within the mantle wedge are not well known. Here, we use S-to-P receiver functions to image seismic velocity contrasts with depth within the subduction zone in order to constrain the release of volatiles and the presence of melt in the mantle wedge, as well as slab structure and arc-lithosphere structure. We use data from 55-80° epicentral distances recorded by 32 recovered broadband ocean-bottom seismometers that were deployed during the 2016-2017 Volatiles in the Lesser Antilles (VoiLA) project for 15 months on the back- and fore-arc. The S-to-P receiver functions are calculated using two methods: extended time multi-taper deconvolution followed by migration to depth to constrain 3-D discontinuity structure of the subduction zone; and simultaneous deconvolution to determine structure beneath single stations. In the south of the island arc, we image a velocity increase with depth associated with the Moho at depths of 32-40 ± 4 km on the fore- and back-arc, consistent with various previous studies. At depths of 65-80 ± 4 km beneath the fore-arc we image a strong velocity decrease with depth that is west-dipping. At 96-120 ± 5 km beneath the fore-arc, we image a velocity increase with depth that is also west-dipping. The dipping negative-positive phase could represent velocity contrasts related to the top of the down-going plate, a feature commonly imaged in subduction zone receiver function studies. The negative phase is strong, so there may also be contributions to the negative velocity discontinuity from slab dehydration and/or mantle wedge serpentinization in the fore-arc.

Rift Structure in Eastern Papua New Guinea From the Joint Inversion of Receiver Functions and Seismic Noise

NASA Astrophysics Data System (ADS)

Abers, G. A.; Obrebski, M. J.; Jin, G.; Eilon, Z.

2014-12-01

The recent CDPapua seismic array in the active D'Entrecasteaux-Woodlark Rift provides insights into how continental crust accommodates large extension. Here, >100 km of extension has occurred in the last 4-6 Ma, exhuming rocks from 100 km depth. To better understand the modes of deformation of the crust, we analyze shear wave velocity (Vs) distribution for a set of temporary land and ocean bottom broadband stations. We resolve the depth of the main velocity contrasts using receiver function (RF) analysis, alleviating the intrinsic trade-off between depth and velocity intrinsic by joint inversion with dispersion constraints (10 - 100 s) from earthquake surface waves and ambient noise. A transdimensional Bayesian scheme explores the model space (Vs in each layer, number of interfaces and their respective depths), minimizing the number of layers required to fit the observations given their noise level. Preliminary results suggest that the Moho is sharp in most places, with a depth of 28-38 km and 20-27 km below the Papuan Peninsula and the highly-extended D'Entracasteaux Islands, respectively. The mid-lower crust of these regions appears to be similar and consistent with felsic compositions, 3.25≤Vs≤3.5 km/s, and may represent the Owen-Stanley Metamorphic Belt or underlying continental rocks. A fast layer (3.75≤Vs≤4 km/s) is observed below the Papuan Peninsula in the 20-30 km depth range and may indicate more mafic lower crust. In contrast, faster velocities between 10 and 20km depth are modeled below the Goodenough Basin (3.75≤Vs≤4 km/s) and the Trobriand Basin (3.5≤Vs≤3.75 km/s) where rocks of the Papuan Ultramafic Belt have been suggested, although these results partly depend upon complicated signals from ocean-bottom seismometers. Well-located seismicity shows that active fault systems generally follow the boundaries between regions of different crustal velocity structure. Overall these results confirm a continental velocity structure for the onshore parts of the rift, but allow for much more mafic crust beneath intervening basins. Much of the rifting at crustal depths could have been accommodated by opening these basins.

GPU-accelerated depth map generation for X-ray simulations of complex CAD geometries

NASA Astrophysics Data System (ADS)

Grandin, Robert J.; Young, Gavin; Holland, Stephen D.; Krishnamurthy, Adarsh

2018-04-01

Interactive x-ray simulations of complex computer-aided design (CAD) models can provide valuable insights for better interpretation of the defect signatures such as porosity from x-ray CT images. Generating the depth map along a particular direction for the given CAD geometry is the most compute-intensive step in x-ray simulations. We have developed a GPU-accelerated method for real-time generation of depth maps of complex CAD geometries. We preprocess complex components designed using commercial CAD systems using a custom CAD module and convert them into a fine user-defined surface tessellation. Our CAD module can be used by different simulators as well as handle complex geometries, including those that arise from complex castings and composite structures. We then make use of a parallel algorithm that runs on a graphics processing unit (GPU) to convert the finely-tessellated CAD model to a voxelized representation. The voxelized representation can enable heterogeneous modeling of the volume enclosed by the CAD model by assigning heterogeneous material properties in specific regions. The depth maps are generated from this voxelized representation with the help of a GPU-accelerated ray-casting algorithm. The GPU-accelerated ray-casting method enables interactive (> 60 frames-per-second) generation of the depth maps of complex CAD geometries. This enables arbitrarily rotation and slicing of the CAD model, leading to better interpretation of the x-ray images by the user. In addition, the depth maps can be used to aid directly in CT reconstruction algorithms.

Depth estimation and camera calibration of a focused plenoptic camera for visual odometry

NASA Astrophysics Data System (ADS)

Zeller, Niclas; Quint, Franz; Stilla, Uwe

2016-08-01

This paper presents new and improved methods of depth estimation and camera calibration for visual odometry with a focused plenoptic camera. For depth estimation we adapt an algorithm previously used in structure-from-motion approaches to work with images of a focused plenoptic camera. In the raw image of a plenoptic camera, scene patches are recorded in several micro-images under slightly different angles. This leads to a multi-view stereo-problem. To reduce the complexity, we divide this into multiple binocular stereo problems. For each pixel with sufficient gradient we estimate a virtual (uncalibrated) depth based on local intensity error minimization. The estimated depth is characterized by the variance of the estimate and is subsequently updated with the estimates from other micro-images. Updating is performed in a Kalman-like fashion. The result of depth estimation in a single image of the plenoptic camera is a probabilistic depth map, where each depth pixel consists of an estimated virtual depth and a corresponding variance. Since the resulting image of the plenoptic camera contains two plains: the optical image and the depth map, camera calibration is divided into two separate sub-problems. The optical path is calibrated based on a traditional calibration method. For calibrating the depth map we introduce two novel model based methods, which define the relation of the virtual depth, which has been estimated based on the light-field image, and the metric object distance. These two methods are compared to a well known curve fitting approach. Both model based methods show significant advantages compared to the curve fitting method. For visual odometry we fuse the probabilistic depth map gained from one shot of the plenoptic camera with the depth data gained by finding stereo correspondences between subsequent synthesized intensity images of the plenoptic camera. These images can be synthesized totally focused and thus finding stereo correspondences is enhanced. In contrast to monocular visual odometry approaches, due to the calibration of the individual depth maps, the scale of the scene can be observed. Furthermore, due to the light-field information better tracking capabilities compared to the monocular case can be expected. As result, the depth information gained by the plenoptic camera based visual odometry algorithm proposed in this paper has superior accuracy and reliability compared to the depth estimated from a single light-field image.

Application of Airborne Hydrographic Laser Scanning for Mapping Shallow Water Riverine Environments in the Pacific Northwest, United States

NASA Astrophysics Data System (ADS)

Cooper, C.; Nayegandhi, A.; Faux, R.

2013-12-01

Small-footprint, green wavelength airborne LiDAR systems can provide seamless topography across the land-water interface at very high spatial resolution. These data have the potential to improve floodplain modeling, fisheries habitat assessments, stream restoration efforts, and other applications by continuously mapping shallow water depths that are difficult or impossible to measure using traditional ground-based or water-borne survey techniques. WSI (Corvallis, Oregon) in collaboration with Dewberry, (Tampa, Florida) and Riegl (Orlando, Florida), deployed the Riegl VQ-820-G hydrographic airborne laser scanner to map riverine and lacustrine environments from Oregon to Minnesota. Discussion will focus on the ability to accurately map depth and underwater structure, as well as riparian vegetation and terrain under different conditions. Results indicate that depth penetration varies with both water (i.e. clarity and surface conditions) and bottom conditions (i.e. substrate, depth, and landform). Depth penetration was typically limited to 1 Secchi depth or less across selected project areas. As an example, the green LiDAR system effectively mapped 83% of a shallow water river system, the Sandy River, with typical depths ranging from 0-2.5 meters. WSI will show quantitative comparisons of Green LiDAR surveys against more traditional methods such as rod or sonar surveys. WSI will also discuss advantages and limitations of Green LiDAR surveys for bathymetric modeling including survey accuracy, density, and efficiency along with data processing challenges not inherent with traditional NIR LiDAR processing.

Faulting and hydration of the Juan de Fuca plate system

NASA Astrophysics Data System (ADS)

Nedimović, Mladen R.; Bohnenstiehl, DelWayne R.; Carbotte, Suzanne M.; Pablo Canales, J.; Dziak, Robert P.

2009-06-01

Multichannel seismic observations provide the first direct images of crustal scale normal faults within the Juan de Fuca plate system and indicate that brittle deformation extends up to ~ 200 km seaward of the Cascadia trench. Within the sedimentary layering steeply dipping faults are identified by stratigraphic offsets, with maximum throws of 110 ± 10 m found near the trench. Fault throws diminish both upsection and seaward from the trench. Long-term throw rates are estimated to be 13 ± 2 mm/kyr. Faulted offsets within the sedimentary layering are typically linked to larger offset scarps in the basement topography, suggesting reactivation of the normal fault systems formed at the spreading center. Imaged reflections within the gabbroic igneous crust indicate swallowing fault dips at depth. These reflections require local alteration to produce an impedance contrast, indicating that the imaged fault structures provide pathways for fluid transport and hydration. As the depth extent of imaged faulting within this young and sediment insulated oceanic plate is primarily limited to approximately Moho depths, fault-controlled hydration appears to be largely restricted to crustal levels. If dehydration embrittlement is an important mechanism for triggering intermediate-depth earthquakes within the subducting slab, then the limited occurrence rate and magnitude of intraslab seismicity at the Cascadia margin may in part be explained by the limited amount of water imbedded into the uppermost oceanic mantle prior to subduction. The distribution of submarine earthquakes within the Juan de Fuca plate system indicates that propagator wake areas are likely to be more faulted and therefore more hydrated than other parts of this plate system. However, being largely restricted to crustal levels, this localized increase in hydration generally does not appear to have a measurable effect on the intraslab seismicity along most of the subducted propagator wakes at the Cascadia margin.

Rheological separation of the megathrust seismogenic zone and episodic tremor and slip

NASA Astrophysics Data System (ADS)

Gao, Xiang; Wang, Kelin

2017-03-01

Episodic tremor and accompanying slow slip, together called ETS, is most often observed in subduction zones of young and warm subducting slabs. ETS should help us to understand the mechanics of subduction megathrusts, but its mechanism is still unclear. It is commonly assumed that ETS represents a transition from seismic to aseismic behaviour of the megathrust with increasing depth, but this assumption is in contradiction with an observed spatial separation between the seismogenic zone and the ETS zone. Here we propose a unifying model for the necessary geological condition of ETS that explains the relationship between the two zones. By developing numerical thermal models, we examine the governing role of thermo-petrologically controlled fault zone rheology (frictional versus viscous shear). High temperatures in the warm-slab environment cause the megathrust seismogenic zone to terminate before reaching the depth of the intersection of the continental Mohorovičić discontinuity (Moho) and the subduction interface, called the mantle wedge corner. High pore-fluid pressures around the mantle wedge corner give rise to an isolated friction zone responsible for ETS. Separating the two zones is a segment of semi-frictional or viscous behaviour. The new model reconciles a wide range of seemingly disparate observations and defines a conceptual framework for the study of slip behaviour and the seismogenesis of major faults.

Geometry and Dynamics of the Mesopotamian Foreland Basin

NASA Astrophysics Data System (ADS)

Pirouz, M.; Avouac, J. P.; Gualandi, A.; Hassanzadeh, J.; Sternai, P.

2016-12-01

We have constrained the geometry of the Zagros foreland basin along the entire northern edge of the Arabian plate using subsurface data from Iran, Iraq and Syria. We use the Oligo-Miocene marine Asmari Formation and its equivalents in the region to reconstruct high resolution foreland basin geometry. This extensive carbonate platform limestone unit separates pre-collisional passive margin marine sediments from the Cenozoic foreland deposits dominated by continental sources; and therefore it can be used as a measure of post-collisional deflection. The 3D reconstructed Asmari Formation shows along-strike thickness variations of the foreland basin deposits from 1 to 6 km. The deepest part of the foreland basin coincides with the Dezful embayment in Iran, and its depth decreases on both sides. In principle the basin geometry should reflect the loading resulted from overthrusting in the Zagros fold-thrust belt, the sediment fill and dynamic stresses due to lithospheric and upper mantle deformation. To estimate these various sources of loads we analyze the basin geometry in combination with gravity, free air anomaly, and Moho depths determined from seismological observations. Our analysis suggests in particular that redistribution of surface load by surface processes is a primary controlling factor of the basin geometry. The wavelength of a foreland basin may bear little information on the elastic flexural rigidity of the lithosphere.

Heat and extension at mid- and lower crustal levels of the Rio Grande rift

NASA Technical Reports Server (NTRS)

Olsen, K. H.; Baldridge, W. S.; Callender, J. F.

1985-01-01

The process by which large amounts (50 to 200 percent) of crustal extension are produced was concisely described by W. Hamilton in 1982 and 1983. More recently, England, Sawyer, P. Morgan and others have moved toward quantifying models of lithospheric thinning by incorporating laboratory and theoretical data on rock rheology as a function of composition, temperature, and strain rate. Hamilton's description identifies three main crustal layers, each with a distinctive mechanical behavior; brittle fracturing and rotation in the upper crust, discontinuous ductile flow in the middle crust and laminar ductile flow in the lower crust. The temperature and composition dependent brittle-ductile transition essentially defines the diffuse boundary between upper and middle crust. It was concluded that the heat responsible for the highly ductile nature of the lower crust and the lensoidal and magma body structures at mid-crustal depths in the rift was infused into the crust by relatively modest ( 10 percent by mass) magmatic upwelling (feeder dikes) from Moho levels. Seismic velocity-versus-depth data, supported by gravity modeling and the fact that volumes of rift related volcanics are relatively modest ( 6000 cubic km) for the Rio Grande system, all imply velocities and densities too small to be consistent with a massive, composite, mafic intrusion in the lower crust.

Structure of the subduction system in southern Peru from seismic array data

NASA Astrophysics Data System (ADS)

Phillips, Kristin; Clayton, Robert W.; Davis, Paul; Tavera, Hernando; Guy, Richard; Skinner, Steven; Stubailo, Igor; Audin, Laurence; Aguilar, Victor

2012-11-01

The subduction zone in southern Peru is imaged using converted phases from teleseismic P, PP, and PKP waves and Pwave tomography using local and teleseismic events with a linear array of 50 broadband seismic stations spanning 300 km from the coast to near Lake Titicaca. The slab dips at 30° and can be observed to a depth of over 200 km. The Moho is seen as a continuous interface along the profile, and the crustal thickness in the back-arc region (the Altiplano) is 75 km thick, which is sufficient to isostatically support the Andes, as evidenced by the gravity. The shallow crust has zones of negative impedance at a depth of 20 km, which is likely the result of volcanism. At the midcrustal level of 40 km, there is a continuous structure with a positive impedance contrast, which we interpret as the western extent of the Brazilian Craton as it underthrusts to the west.Vp/Vs ratios estimated from receiver function stacks show average values for this region with a few areas of elevated Vp/Vs near the volcanic arc and at a few points in the Altiplano. The results support a model of crustal thickening in which the margin crust is underthrust by the Brazilian Shield.

Systems and methods that generate height map models for efficient three dimensional reconstruction from depth information

DOEpatents

Frahm, Jan-Michael; Pollefeys, Marc Andre Leon; Gallup, David Robert

2015-12-08

Methods of generating a three dimensional representation of an object in a reference plane from a depth map including distances from a reference point to pixels in an image of the object taken from a reference point. Weights are assigned to respective voxels in a three dimensional grid along rays extending from the reference point through the pixels in the image based on the distances in the depth map from the reference point to the respective pixels, and a height map including an array of height values in the reference plane is formed based on the assigned weights. An n-layer height map may be constructed by generating a probabilistic occupancy grid for the voxels and forming an n-dimensional height map comprising an array of layer height values in the reference plane based on the probabilistic occupancy grid.

Imaging Seismic Zones and Magma beneath Mount St. Helens with the iMUSH Broadband Array

NASA Astrophysics Data System (ADS)

Ulberg, C. W.; Creager, K.; Moran, S. C.; Abers, G. A.; Crosbie, K.; Crosson, R. S.; Denlinger, R. P.; Thelen, W. A.; Kiser, E.; Levander, A.; Bachmann, O.

2017-12-01

We deployed 70 broadband seismometers from 2014 to 2016 to image the seismic velocity structure beneath Mount St. Helens (MSH), Washington, as part of the collaborative imaging Magma Under St. Helens (iMUSH) project. The broadband array had a 100 km diameter centered on MSH with an average station spacing of 10 km, augmented by dozens of permanent stations. We picked P- and S-wave arrival times and also incorporated picks from the permanent network. More than 400 local events M>0.5 occurred during the deployment, providing over 12,000 P-wave and 6,000 S-wave arrival times. In addition, we incorporated 23 explosions that were part of the active-source component of iMUSH. We used the program struct3DP to invert travel times to obtain a 3-D seismic velocity model and relocated hypocenters, with travel times computed using a 3-D eikonal-equation solver. Principal features of our 3-D model include: (1) Low P- and S-wave velocities along the St. Helens seismic Zone (SHZ), striking NNW-SSE north of MSH from near the surface to where we lose resolution at 15-20 km depth. This anomaly corresponds to high conductivity as imaged by iMUSH magnetotelluric studies. The SHZ also coincides with a sharp boundary in continental Moho reflectivity that has been interpreted as the eastern boundary of a serpentinized mantle wedge (Hansen et al, 2016). We speculate that the SHZ and low velocities are related to fluids rising from the eastern boundary of the wedge; (2) A 4-5% negative P- and S-wave velocity anomaly beneath MSH at depths of 6-15 km with a quasi-cylindrical geometry and a diameter of 5 km, probably indicating a magma storage region. Based on resolution testing of similar-sized features, it is possible that the velocity anomaly we see underneath MSH is narrower and higher (i.e., more negative) amplitude; (3) A broad, high-amplitude, low P-wave velocity region below 10-km depth extending between Mount Adams and Mount Rainier along and to the east of the main Cascade arc, which is likely due to high-temperature arc crust and possible presence of melt; (4) Several anomalies associated with surface-mapped features, including high-velocity igneous units such as the Spud Mountain and Spirit Lake plutons and low velocities in the Chehalis sedimentary basin and the Indian Heaven volcanic field.

Seals map bathymetry of the Antarctic continental shelf

NASA Astrophysics Data System (ADS)

Padman, Laurie; Costa, Daniel P.; Bolmer, S. Thompson; Goebel, Michael E.; Huckstadt, Luis A.; Jenkins, Adrian; McDonald, Birgitte I.; Shoosmith, Deborah R.

2010-11-01

We demonstrate the first use of marine mammal dive-depth data to improve maps of bathymetry in poorly sampled regions of the continental shelf. A group of 57 instrumented elephant seals made on the order of 2 × 105 dives over and near the continental shelf on the western side of the Antarctic Peninsula during five seasons, 2005-2009. Maximum dive depth exceeded 2000 m. For dives made near existing ship tracks with measured water depths H<700 m, ˜30% of dive depths were to the seabed, consistent with expected benthic foraging behavior. By identifying the deepest of multiple dives within small areas as a dive to the seabed, we have developed a map of seal-derived bathymetry. Our map fills in several regions for which trackline data are sparse, significantly improving delineation of troughs crossing the continental shelf of the southern Bellingshausen Sea.

Estimating floodwater depths from flood inundation maps and topography

USGS Publications Warehouse

Cohen, Sagy; Brakenridge, G. Robert; Kettner, Albert; Bates, Bradford; Nelson, Jonathan M.; McDonald, Richard R.; Huang, Yu-Fen; Munasinghe, Dinuke; Zhang, Jiaqi

2018-01-01

Information on flood inundation extent is important for understanding societal exposure, water storage volumes, flood wave attenuation, future flood hazard, and other variables. A number of organizations now provide flood inundation maps based on satellite remote sensing. These data products can efficiently and accurately provide the areal extent of a flood event, but do not provide floodwater depth, an important attribute for first responders and damage assessment. Here we present a new methodology and a GIS-based tool, the Floodwater Depth Estimation Tool (FwDET), for estimating floodwater depth based solely on an inundation map and a digital elevation model (DEM). We compare the FwDET results against water depth maps derived from hydraulic simulation of two flood events, a large-scale event for which we use medium resolution input layer (10 m) and a small-scale event for which we use a high-resolution (LiDAR; 1 m) input. Further testing is performed for two inundation maps with a number of challenging features that include a narrow valley, a large reservoir, and an urban setting. The results show FwDET can accurately calculate floodwater depth for diverse flooding scenarios but also leads to considerable bias in locations where the inundation extent does not align well with the DEM. In these locations, manual adjustment or higher spatial resolution input is required.

Multi-frequency Optical-depth Maps And The Case For Free-free Absorption In Two Compact Symmetric Objects: 1321+410 And 0026+346

NASA Astrophysics Data System (ADS)

Perry, Thomas M.; Marr, J. M.; Read, J. W.; Taylor, G. B.

2011-01-01

We obtained VLBI observations at six frequencies of two Compact Symmetric Objects, 1321+410 and 0026+346. By comparing the lower frequency maps with spectral extrapolations of the higher frequency maps, we produced maps of the optical depth as a function of frequency. The optical-depth maps of 1321+410 are strikingly uniform, consistent with a foreground screen of absorbing gas; the optical depths as a function of frequency are consistent with free-free absorption; and no net polarization was detected. We conclude that the case for free-free absorption in 1321+410 is strong. The optical-depth maps of 0026+346 exhibit structure but the morphology does not correlate with that in the intensity maps, in conflict with that expected in the case of synchrotron self-absorption. No net polarization was detected. The frequency dependence of the optical depths does not fit well to a simple free-free absorption model, but this does not take into account possible structure in the absorbing gas on smaller scales. We conclude that free-free absorption by a thin amount of gas with structure on the scale of our maps and smaller is possible in 0026+346, although no definitive conclusion can be made. A compact feature between the lobes in 0026+346 has an inverted spectrum even at the highest frequencies, suggesting that this component is synchrotron self-absorbed. We infer this to be the location of the core. We estimate an upper limit to the magnetic field in the core of 50 Gauss at a radius of 1 pc. This research was supported by an award from the Research Corporation, a NASA NY Space Grant, and a Booth-Ferris Research Fellowship. The VLBA is operated by the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

3D Seismic Velocity Structure Around Philippine Sea Slab Subducting Beneath Kii Peninsula, Japan

NASA Astrophysics Data System (ADS)

Shibutani, T.; Imai, M.; Hirahara, K.; Nakao, S.

2013-12-01

Kii Peninsula is a part of the source area of Nankai Trough megaquakes and the region through which the strong seismic waves propagate to big cities in Kansai such as Osaka, Kyoto, Nara, Kobe, and so on. Moreover, the rupture starting point is thought to be possibly at off the peninsula. Therefore, it is important for simulations of the megaquakes and the strong motions to estimate accurately the configuration of the Philippine Sea slab and the seismic velocity structure around the slab and to investigate properties and conditions of the plate boundary surface. Deep low frequency events (DLFEs) are widely distributed from western Shikoku to central Tokai at 30 - 40 km depths on the plate boundary (Obara, 2002). Results from seismic tomography and receiver function analyses revealed that the oceanic crust of the Philippine Sea plate had a low velocity and a high Vp/Vs ratio (Hirose et al., 2007; Ueno et al., 2008). Hot springs with high 3He/4He ratios are found in an area between central Kinki and Kii Peninsula despite in the forearc region (Sano and Wakita, 1985). These phenomena suggest the process that H2O subducting with the oceanic crust dehydrates at the depths, causes the DLFEs, and moves to shallower depths. We carried out linear array seismic observations in the Kii Peninsula since 2004 in order to estimate the structure of the Philippine Sea slab and the surrounding area. We have performed receiver function analyses for four profile lines in the dipping direction of the slab and two lines in the perpendicular direction so far. We estimated three dimensional shapes of seismic velocity discontinuities such as the continental Moho, the upper surface of the oceanic crust and the oceanic Moho (Imai et al., 2013, this session). In addition, we performed seismic tomography with a velocity model embedded the discontinuities and observed travel times at stations in the linear arrays, and successfully estimated 3D seismic velocity structure around the Philippine Sea slab beneath the Kii Peninsula in higher resolutions. The results show that in the vicinity of the areas of the DLFEs low velocity anomalies (LVAs) are distributed from the oceanic crust to the mantle wedge. These LVAs are thought to be due to fluids discharged from hydrous minerals in the oceanic crust by dehydration that occurs at 30 - 40 km depths on the plate boundary. Other strong LVAs (with 5 % velocity perturbation or more) are widely distributed in the lower crust beneath northern Wakayama Prefecture where the seismicity in the upper crust is high. Since the latter LVAs continue to deeper in the mantle wedge than the former LVAs, the origin of the LVAs in the two regions might be different. No matter what the origin is, the latter LVAs beneath the northern Wakayama area are probably due to fluids too. Then the high seismicity in the area can be explained by the reduction of the effective normal stress on the fault planes due to the increase of the pore pressure in the micro cracks caused by the fluids from the LVAs.

Restoration of distorted depth maps calculated from stereo sequences

NASA Technical Reports Server (NTRS)

Damour, Kevin; Kaufman, Howard

1991-01-01

A model-based Kalman estimator is developed for spatial-temporal filtering of noise and other degradations in velocity and depth maps derived from image sequences or cinema. As an illustration of the proposed procedures, edge information from image sequences of rigid objects is used in the processing of the velocity maps by selecting from a series of models for directional adaptive filtering. Adaptive filtering then allows for noise reduction while preserving sharpness in the velocity maps. Results from several synthetic and real image sequences are given.

The suitability of lightfield camera depth maps for coordinate measurement applications

NASA Astrophysics Data System (ADS)

Rangappa, Shreedhar; Tailor, Mitul; Petzing, Jon; Kinnell, Peter; Jackson, Michael

2015-12-01

Plenoptic cameras can capture 3D information in one exposure without the need for structured illumination, allowing grey scale depth maps of the captured image to be created. The Lytro, a consumer grade plenoptic camera, provides a cost effective method of measuring depth of multiple objects under controlled lightning conditions. In this research, camera control variables, environmental sensitivity, image distortion characteristics, and the effective working range of two Lytro first generation cameras were evaluated. In addition, a calibration process has been created, for the Lytro cameras, to deliver three dimensional output depth maps represented in SI units (metre). The novel results show depth accuracy and repeatability of +10.0 mm to -20.0 mm, and 0.5 mm respectively. For the lateral X and Y coordinates, the accuracy was +1.56 μm to -2.59 μm and the repeatability was 0.25 μm.

On the resolution of shallow mantle viscosity structure using post-earthquake relaxation data: Application to the 1999 Hector Mine, California, earthquake

USGS Publications Warehouse

Pollitz, Fred F.; Thatcher, Wayne R.

2010-01-01

Most models of lower crust/mantle viscosity inferred from postearthquake relaxation assume one or two uniform-viscosity layers. A few existing models possess apparently significant radially variable viscosity structure in the shallow mantle (e.g., the upper 200 km), but the resolution of such variations is not clear. We use a geophysical inverse procedure to address the resolving power of inferred shallow mantle viscosity structure using postearthquake relaxation data. We apply this methodology to 9 years of GPS-constrained crustal motions after the 16 October 1999 M = 7.1 Hector Mine earthquake. After application of a differencing method to isolate the postearthquake signal from the “background” crustal velocity field, we find that surface velocities diminish from ∼20 mm/yr in the first few months to ≲2 mm/yr after 2 years. Viscoelastic relaxation of the mantle, with a time-dependent effective viscosity prescribed by a Burgers body, provides a good explanation for the postseismic crustal deformation, capturing both the spatial and temporal pattern. In the context of the Burgers body model (which involves a transient viscosity and steady state viscosity), a resolution analysis based on the singular value decomposition reveals that at most, two constraints on depth-dependent steady state mantle viscosity are provided by the present data set. Uppermost mantle viscosity (depth ≲ 60 km) is moderately resolved, but deeper viscosity structure is poorly resolved. The simplest model that explains the data better than that of uniform steady state mantle viscosity involves a linear gradient in logarithmic viscosity with depth, with a small increase from the Moho to 220 km depth. However, the viscosity increase is not statistically significant. This suggests that the depth-dependent steady state viscosity is not resolvably different from uniformity in the uppermost mantle.

Forearc structure in the Lesser Antilles inferred from depth to the Curie temperature and thermo-mechanical simulations

NASA Astrophysics Data System (ADS)

Gailler, Lydie; Arcay, Diane; Münch, Philippe; Martelet, Guillaume; Thinon, Isabelle; Lebrun, Jean-Frédéric

2017-06-01

Imaging deep active volcanic areas remains a challenge in our understanding of their activity and evolution, especially in subduction zones. Study of magnetic anomalies is appropriate to access such dynamics in depth. The magnetic anomaly pattern of the Lesser Antilles Arc (LAA) subduction is studied through Curie Point Depth (CPD), interpreted as the depth of the 580 °C isotherm, and developed to better assess the deep thermal structure of the arc. The depth of the estimated CPD exhibits a complex topography. Keeping in mind the overall uncertainty associated with this method, a main doming is evidenced below the Guadeloupe archipelago. Its apex is shifted towards the ancient arc, suggesting a very hot state of the fore-arc/arc domain. To better understand the LAA thermal state, we perform 2D thermo-mechanical simulations of the subduction zone. Recalling that magnetite is a serpentinization by-product, we simulate water transfer triggered by slab dehydration to test the assumption of fore-arc serpentinization suggested by the positive magnetic anomaly in the vicinity of the Guadeloupe archipelago. In this area, the subduction-induced arc lithosphere hydration and related weakening trigger a fast heating of the upper plate by basal convective removal. This process of fast arc lithosphere thinning may apply where simultaneously the volcanic arc is split in two and normal convergence is high enough. As serpentinization strongly decreases P-wave velocity, we propose a new interpretation of a published seismic profile below Guadeloupe. The seismic layer previously interpreted as the arc lower crust may rather be a layer of serpentinized mantle, as supported by spatial correlations between gravimetric and magnetic anomalies. Consequently, at the scale of Guadeloupe Island, the fore-arc Moho would be shallower than initially assumed, with a dome shape more consistent with both the extensive deformation active since the Oligocene in the inner fore-arc and the CPD doming.

Crustal velocity structure and earthquake processes of Garhwal-Kumaun Himalaya: Constraints from regional waveform inversion and array beam modeling

NASA Astrophysics Data System (ADS)

Negi, Sanjay S.; Paul, Ajay; Cesca, Simone; Kamal; Kriegerowski, Marius; Mahesh, P.; Gupta, Sandeep

2017-08-01

In order to understand present day earthquake kinematics at the Indian plate boundary, we analyse seismic broadband data recorded between 2007 and 2015 by the regional network in the Garhwal-Kumaun region, northwest Himalaya. We first estimate a local 1-D velocity model for the computation of reliable Green's functions, based on 2837 P-wave and 2680 S-wave arrivals from 251 well located earthquakes. The resulting 1-D crustal structure yields a 4-layer velocity model down to the depths of 20 km. A fifth homogeneous layer extends down to 46 km, constraining the Moho using travel-time distance curve method. We then employ a multistep moment tensor (MT) inversion algorithm to infer seismic moment tensors of 11 moderate earthquakes with Mw magnitude in the range 4.0-5.0. The method provides a fast MT inversion for future monitoring of local seismicity, since Green's functions database has been prepared. To further support the moment tensor solutions, we additionally model P phase beams at seismic arrays at teleseismic distances. The MT inversion result reveals the presence of dominant thrust fault kinematics persisting along the Himalayan belt. Shallow low and high angle thrust faulting is the dominating mechanism in the Garhwal-Kumaun Himalaya. The centroid depths for these moderate earthquakes are shallow between 1 and 12 km. The beam modeling result confirm hypocentral depth estimates between 1 and 7 km. The updated seismicity, constrained source mechanism and depth results indicate typical setting of duplexes above the mid crustal ramp where slip is confirmed along out-of-sequence thrusting. The involvement of Tons thrust sheet in out-of-sequence thrusting indicate Tons thrust to be the principal active thrust at shallow depth in the Himalayan region. Our results thus support the critical taper wedge theory, where we infer the microseismicity cluster as a result of intense activity within the Lesser Himalayan Duplex (LHD) system.

Hales discontinuity beneath India: selective appearance and a case for systematic modeling

NASA Astrophysics Data System (ADS)

Mitra, S.; Chaudhury, J.

2016-12-01

Hales discontinuity was first reported in Lake Superior at depth of 80-90 km, characterized by an increase in P-wave velocity from 8.05 to 8.45 km/s. Subsequent, worldwide studies have observed this discontinuity beneath selected continental regions and Pacific Ocean, with depth varying from 40 to 115 km. The cause for the absence of observable signal corresponding to the Hales discontinuity beneath a number of seismic stations and the large depth variation of the discontinuity are poorly understood. In the Indian subcontinent, the Hales discontinuity has been selectively imaged beneath the Southern Granulite Terrain, Eastern Dharwar, Bastar and Aravalli Cratons. These studies used low frequency P-wave receiver functions (P-RFs) to show that the Hales discontinuity corresponds to a PHs phase arriving between 7.5 and 11 s. A few studies have forwarded modeled this phase to demonstrate that this arrival is distinct from Moho reverberations and corresponds to a depth range of 75-90 km. However, these studies have ignored the effect of mid-crustal discontinuity, which had been observed in P-RF inverted crustal models beneath these stations, and its first reverberation coincide with the reported PHs. We demonstrate through forward modeling that the observed Hales discontinuity PHs can be matched by the PpPs from mid-crustal discontinuity beneath the cratons, with the exception of Hyderabad (HYB), where this discontinuity was reported to be deepest at 90 km. We perform joint inversion of the HYB P-RFs with Rayleigh wave dispersion to obtain a 32 km thick two layer crust, and Hales discontinuity at a depth of 108±2 km, with 4% increase in S-wave velocity from 4.6 to 4.8 km/s. Several mechanisms have been proposed to explain this velocity discontinuity, which include transition from spinel to garnet peridotite or changes in cation partitioning in olivine. We intend to evaluate the velocity increase based on thermoelasticity data of minerals constituting peridotite.

Spectrally-Based Bathymetric Mapping of a Dynamic, Sand-Bedded Channel: Niobrara River, Nebraska, USA

NASA Astrophysics Data System (ADS)

Dilbone, Elizabeth K.

Methods for spectrally-based bathymetric mapping of rivers mainly have been developed and tested on clear-flowing, gravel bedded channels, with limited application to turbid, sand-bedded rivers. Using hyperspectral images of the Niobrara River, Nebraska, and field-surveyed depth data, this study evaluated three methods of retrieving depth from remotely sensed data in a dynamic, sand-bedded channel. The first regression-based approach paired in situ depth measurements and image pixel values to predict depth via Optimal Band Ratio Analysis (OBRA). The second approach used ground-based reflectance measurements to calibrate an OBRA relationship. For this approach, CASI images were atmospherically corrected to units of apparent surface reflectance using an empirical line calibration. For the final technique, we used Image-to-Depth Quantile Transformation (IDQT) to predict depth by linking the cumulative distribution function (CDF) of depth to the CDF of an image derived variable. OBRA yielded the lowest overall depth retrieval error (0.0047 m) and highest observed versus predicted R2 (0.81). Although misalignment between field and image data were not problematic to OBRA's performance in this study, such issues present potential limitations to standard regression-based approaches like OBRA in dynamic, sand-bedded rivers. Field spectroscopy-based maps exhibited a slight shallow bias (0.0652 m) but provided reliable depth estimates for most of the study reach. IDQT had a strong deep bias, but still provided informative relative depth maps that portrayed general patterns of shallow and deep areas of the channel. The over-prediction of depth by IDQT highlights the need for an unbiased sampling strategy to define the CDF of depth. While each of the techniques tested in this study demonstrated the potential to provide accurate depth estimates in sand-bedded rivers, each method also was subject to certain constraints and limitations.

Automatic Depth Extraction from 2D Images Using a Cluster-Based Learning Framework.

PubMed

Herrera, Jose L; Del-Blanco, Carlos R; Garcia, Narciso

2018-07-01

There has been a significant increase in the availability of 3D players and displays in the last years. Nonetheless, the amount of 3D content has not experimented an increment of such magnitude. To alleviate this problem, many algorithms for converting images and videos from 2D to 3D have been proposed. Here, we present an automatic learning-based 2D-3D image conversion approach, based on the key hypothesis that color images with similar structure likely present a similar depth structure. The presented algorithm estimates the depth of a color query image using the prior knowledge provided by a repository of color + depth images. The algorithm clusters this database attending to their structural similarity, and then creates a representative of each color-depth image cluster that will be used as prior depth map. The selection of the appropriate prior depth map corresponding to one given color query image is accomplished by comparing the structural similarity in the color domain between the query image and the database. The comparison is based on a K-Nearest Neighbor framework that uses a learning procedure to build an adaptive combination of image feature descriptors. The best correspondences determine the cluster, and in turn the associated prior depth map. Finally, this prior estimation is enhanced through a segmentation-guided filtering that obtains the final depth map estimation. This approach has been tested using two publicly available databases, and compared with several state-of-the-art algorithms in order to prove its efficiency.

Correction techniques for depth errors with stereo three-dimensional graphic displays

NASA Technical Reports Server (NTRS)

Parrish, Russell V.; Holden, Anthony; Williams, Steven P.

1992-01-01

Three-dimensional (3-D), 'real-world' pictorial displays that incorporate 'true' depth cues via stereopsis techniques have proved effective for displaying complex information in a natural way to enhance situational awareness and to improve pilot/vehicle performance. In such displays, the display designer must map the depths in the real world to the depths available with the stereo display system. However, empirical data have shown that the human subject does not perceive the information at exactly the depth at which it is mathematically placed. Head movements can also seriously distort the depth information that is embedded in stereo 3-D displays because the transformations used in mapping the visual scene to the depth-viewing volume (DVV) depend intrinsically on the viewer location. The goal of this research was to provide two correction techniques; the first technique corrects the original visual scene to the DVV mapping based on human perception errors, and the second (which is based on head-positioning sensor input data) corrects for errors induced by head movements. Empirical data are presented to validate both correction techniques. A combination of the two correction techniques effectively eliminates the distortions of depth information embedded in stereo 3-D displays.

An efficient depth map preprocessing method based on structure-aided domain transform smoothing for 3D view generation

PubMed Central

Ma, Liyan; Qiu, Bo; Cui, Mingyue; Ding, Jianwei

2017-01-01

Depth image-based rendering (DIBR), which is used to render virtual views with a color image and the corresponding depth map, is one of the key techniques in the 2D to 3D conversion process. Due to the absence of knowledge about the 3D structure of a scene and its corresponding texture, DIBR in the 2D to 3D conversion process, inevitably leads to holes in the resulting 3D image as a result of newly-exposed areas. In this paper, we proposed a structure-aided depth map preprocessing framework in the transformed domain, which is inspired by recently proposed domain transform for its low complexity and high efficiency. Firstly, our framework integrates hybrid constraints including scene structure, edge consistency and visual saliency information in the transformed domain to improve the performance of depth map preprocess in an implicit way. Then, adaptive smooth localization is cooperated and realized in the proposed framework to further reduce over-smoothness and enhance optimization in the non-hole regions. Different from the other similar methods, the proposed method can simultaneously achieve the effects of hole filling, edge correction and local smoothing for typical depth maps in a united framework. Thanks to these advantages, it can yield visually satisfactory results with less computational complexity for high quality 2D to 3D conversion. Numerical experimental results demonstrate the excellent performances of the proposed method. PMID:28407027

Crustal structure in the southern part of Central Java based on analysis of tele-seismic receiver function using a neighbourhood algorithm

NASA Astrophysics Data System (ADS)

Ariyanto, P.; Syuhada; Rosid, S.; Anggono, T.; Januarti, Y.

2018-03-01

In this study, we applied receiver functions analysis to determine the crustal thickness, the ratio of Vp/Vs and the S wave velocity in the southern part of the Central Java. We selected tele-seismic data with magnitude more than 6 (M>6) and epicenter distance 30°-90° recorded from 3 broadband stations: UGM, YOGI, and WOJI station, as part of Indonesia-Geophone Network (IA-GE). Inversions were performed using nonlinear Neighborhood Algorithm (NA). We observed Ps phase conversion on the receiver functions corresponding to Moho depth at around 36-39 km. We also observed strong negative phase arrivals at around 10-12 s which might be associated with Indo-Australian subducting slab underneath the stations. The inversion results show the presence of low velocity zone with high Vp/Vs ratio (>1.78) in the middle crust around the study area which could be related to the Merapi-Lawu Anomaly (MLA).

On the evolution of the geothermal regime of the North China Basin

NASA Astrophysics Data System (ADS)

Wang, Ji-yang; Chen, Mo-xiang; Wang, Ji-an; Deng, Xiao

1985-12-01

Recent heat flow and regional geothermal studies indicate that the North China Basin is characterized by relatively high heat flow compared with most stable areas in other parts of the world, but lower heat flow than most active tectonic areas. Measured heat flow values range from 61 to 74 mW m -2. The temperature at a depth of 2000 m is generally in the range 75 to 85°C, but sometimes is 90°C or higher. The geothermal gradient in Cenozoic sediments is in the range 30 to 40°C/km for most of the area. The calculated temperature at the Moho is 560 and 640°C for surface heat flow values of 63 and 71 mW m -2, respectively. These thermal data are consistent with other geophysical observations for the North China Basin. Relatively high heat flow in this area is related to Late Cretaceous-Paleogene rifting as described in this paper.

Using Commercial Digital Cameras and Structure-for-Motion Software to Map Snow Cover Depth from Small Aircraft

NASA Astrophysics Data System (ADS)

Sturm, M.; Nolan, M.; Larsen, C. F.

2014-12-01

A long-standing goal in snow hydrology has been to map snow cover in detail, either mapping snow depth or snow water equivalent (SWE) with sub-meter resolution. Airborne LiDAR and air photogrammetry have been used successfully for this purpose, but both require significant investments in equipment and substantial processing effort. Here we detail a relatively inexpensive and simple airborne photogrammetric technique that can be used to measure snow depth. The main airborne hardware consists of a consumer-grade digital camera attached to a survey-quality, dual-frequency GPS. Photogrammetric processing is done using commercially available Structure from Motion (SfM) software that does not require ground control points. Digital elevation models (DEMs) are made from snow-free acquisitions in the summer and snow-covered acquisitions in winter, and the maps are then differenced to arrive at snow thickness. We tested the accuracy and precision of snow depths measured using this system through 1) a comparison with airborne scanning LiDAR, 2) a comparison of results from two independent and slightly different photogrameteric systems, and 3) comparison to extensive on-the-ground measured snow depths. Vertical accuracy and precision are on the order of +/-30 cm and +/- 8 cm, respectively. The accuracy can be made to approach that of the precision if suitable snow-free ground control points exists and are used to co-register summer to winter DEM maps. Final snow depth accuracy from our series of tests was on the order of ±15 cm. This photogrammetric method substantially lowers the economic and expertise barriers to entry for mapping snow.

Slab detachment during continental collision: Influence of crustal rheology and interaction with lithospheric delamination

NASA Astrophysics Data System (ADS)

Duretz, T.; Gerya, T. V.

2013-08-01

Collision between continents can lead to the subduction of continental material. If the crust remains coupled to the downgoing slab, a large buoyancy force is generated. This force slows down convergence and promotes slab detachment. If the crust resists to subduction, it may decouple from the downgoing slab and be subjected to buoyant extrusion. We employ two-dimensional thermo-mechanical modelling to study the importance of crustal rheology on the evolution of subduction-collision systems. We propose simple quantifications of the mechanical decoupling between lithospheric levels (σ*) and the potential for buoyant extrusion of the crust (ξ*). The modelling results indicate that a variable crustal rheological structure results in slab detachment, delamination, or the combination of both mechanisms. A strong crust provides coupling at the Moho (low σ*) and remains coherent during subduction (low ξ). It promotes deep subduction of the crust (180 km) and slab detachment. Exhumation occurs in coherent manners via eduction and thrusting. Slab detachment triggers the development of topography (> 4.5 km) close to the suture. A contrasting style of collision occurs using a weak crustal rheology. Mechanical decoupling at the Moho (high σ*) promotes the extrusion of the crust (high ξ), disabling slab detachment. Ongoing shortening leads to buckling of the crust and development of topography on the lower plate. Collisions involving rheologically layered crust allow decoupling at mid-crustal depths. This structure favours both the extrusion of upper crust and the subduction of the lower crust. Such collisions are successively affected by delamination and slab detachment. Topography develops together with the buoyant extrusion of crust onto the foreland and is further amplified by slab detachment. Our results suggest that the occurrence of both delamination (Apennines) and slab detachment (Himalayas) in orogens may indicate differences in the initial crustal structure of subducting continental plates in these regions.

A STEP fault in Central Betics, associated with lateral lithospheric tearing at the northern edge of the Gibraltar arc subduction system

NASA Astrophysics Data System (ADS)

Mancilla, Flor de Lis; Heit, Benjamin; Morales, Jose; Yuan, Xiaohui; Stich, Daniel; Molina-Aguilera, Antonio; Azañon, Jose Miguel; Martín, Rosa

2018-03-01

We study the crustal and lithospheric mantle structure under central Betics in the westernmost Mediterranean region by migrating P-receiver functions along a dense seismic profile (∼2 km interstation distance). The profile, North-South oriented, probes the crustal structure of different geological units, from the Alboran domain in the south with metamorphic rocks, through the External Zones with sedimentary rocks to the Variscan terrains of the Iberian Massif in the north. From north to south, the Moho depth increases from ∼30 km to ∼46 km underneath the Guadix basin, due to the underthrusting of the Iberian crust below the Alboran crust, and suddenly shallows to ∼30 km underneath the Internal Zones with a step of 17 km. This sharp Moho step correlates well with a lithospheric step of ∼40 km, where the thickness of the lithosphere changes abruptly from ∼100 km in the north to ∼50 km in the south. We interpret this sharp and prominent lithospheric step as the termination of the Iberian lithosphere caused by a near-vertical STEP (Subduction-Transform-Edge-Propagator) fault that continues towards the surface as a positive flower tectonic structure of crustal scale. This STEP fault is located at the northern edge of the narrow Westernmost Mediterranean subduction system facilitating the slab rollback motion towards the west. The sharp termination of the Iberian lithosphere occurs under the contact between the Alpujarride and the Nevado-Filabride complexes of the Alboran domain in an ENE-WSW right-lateral transpressive shear zone. The thickest crust and lithosphere do not correlate with the highest topography along the profile suggesting that this high topography is a combined effect of the positive flower structure, and the push up of the asthenosphere produced by the removal of the Iberian lithosphere.

The Effect of Ignoring Earth Curvature on Near-Regional Traveltime Tomography and Earthquake Hypocentral Determination

NASA Astrophysics Data System (ADS)

Bai, Chao-ying; Li, Xing-wang; Wang, Di; Greenhalgh, Stewart

2017-12-01

Earthquake hypocenter determination and traveltime tomography with local earthquake data are normally conducted using a Cartesian coordinate system and assuming a flat Earth model, but for regional and teleseismic data Earth curvature is incorporated and a spherical coordinate system employed. However, when the study region is from the local to near-regional scale (1°-4°), it is unclear what coordinate system to use and what kind of incorrect anomalies or location errors might arise when using the Cartesian coordinate frame. In this paper we investigate in a quantitative sense through two near-regional crustal models and five different inversion methods, the hypocenter errors, reflector perturbation and incorrect velocity anomalies that can arise due to the selection of the wrong coordinate system and inversion method. The simulated inversion results show that the computed traveltime errors are larger than 0.1 s when the epicentral distance exceeds 150 km, and increases linearly with increasing epicentral distance. Such predicted traveltime errors will result in different patterns of incorrect velocity anomalous structures, a perturbed Moho interface for traveltime tomography and source position which deviate for earthquake locations. The maximum magnitude of a velocity image artifact is larger than 1.0% for an epicentral distance of less than 500 km and is up to 0.9% for epicentral distances of less than 300 km. The earthquake source location error is more than 2.0 km for epicentral distances less than 500 km and is up to 1.5 km for epicentral distances less than 300 km. The Moho depth can be in error by up 1.0 km for epicentral distances of less than 500 km but is less than 0.5 km at distances below 300 km. We suggest that spherical coordinate geometry (or time correction) be used whenever there are ray paths at epicentral distances in excess of 150 km.

Seismic imaging of the upper mantle beneath the northern Central Andean Plateau: Implications for surface topography

NASA Astrophysics Data System (ADS)

Ward, K. M.; Zandt, G.; Beck, S. L.; Wagner, L. S.

2015-12-01

Extending over 1,800 km along the active South American Cordilleran margin, the Central Andean Plateau (CAP) as defined by the 3 km elevation contour is second only to the Tibetan Plateau in geographic extent. The uplift history of the 4 km high Plateau remains uncertain with paleoelevation studies along the CAP suggesting a complex, non-uniform uplift history. As part of the Central Andean Uplift and the Geodynamics of High Topography (CAUGHT) project, we use surface waves measured from ambient noise and two-plane wave tomography to image the S-wave velocity structure of the crust and upper mantle to investigate the upper mantle component of plateau uplift. We observe three main features in our S-wave velocity model including (1), a high velocity slab (2), a low velocity anomaly above the slab where the slab changes dip from near horizontal to a normal dip, and (3), a high-velocity feature in the mantle above the slab that extends along the length of the Altiplano from the base of the Moho to a depth of ~120 km with the highest velocities observed under Lake Titicaca. A strong spatial correlation exists between the lateral extent of this high-velocity feature beneath the Altiplano and the lower elevations of the Altiplano basin suggesting a potential relationship. Non-uniqueness in our seismic models preclude uniquely constraining this feature as an uppermost mantle feature bellow the Moho or as a connected eastward dipping feature extending up to 300 km in the mantle as seen in deeper mantle tomography studies. Determining if the high velocity feature represents a small lithospheric root or a delaminating lithospheric root extending ~300 km into the mantle requires more integration of observations, but either interpretation shows a strong geodynamic connection with the uppermost mantle and the current topography of the northern CAP.

Detailed interpretation of aeromagnetic data from the Patagonia Mountains area, southeastern Arizona

USGS Publications Warehouse

Bultman, Mark W.

2015-01-01

Euler deconvolution depth estimates derived from aeromagnetic data with a structural index of 0 show that mapped faults on the northern margin of the Patagonia Mountains generally agree with the depth estimates in the new geologic model. The deconvolution depth estimates also show that the concealed Patagonia Fault southwest of the Patagonia Mountains is more complex than recent geologic mapping represents. Additionally, Euler deconvolution depth estimates with a structural index of 2 locate many potential intrusive bodies that might be associated with known and unknown mineralization.

Staged storage and magma convection at Ambrym volcano, Vanuatu

NASA Astrophysics Data System (ADS)

Sheehan, Fionnuala; Barclay, Jenni

2016-08-01

New mineral-melt thermobarometry and mineral chemistry data are presented for basaltic scoriae erupted from the Mbwelesu crater of Ambrym volcano, Vanuatu, during persistent lava lake activity in 2005 and 2007. These data reveal crystallisation conditions and enable the first detailed attempt at reconstruction of the central magma plumbing system of Ambrym volcano. Pressures and temperatures of magma crystallisation at Ambrym are poorly constrained. This study focuses on characterising the magma conditions underlying the quasi-permanent lava lakes at the basaltic central vents, and examines petrological evidence for magma circulation. Mineral-melt equilibria for clinopyroxene, olivine and plagioclase allow estimation of pressures and temperatures of crystallisation, and reveal two major regions of crystallisation, at 24-29 km and 11-18 km depth, in agreement with indications from earthquake data of crustal storage levels at c. 25-29 km and 12-21 km depth. Temperature estimates are 1150-1170 °C for the deeper region, and 1110-1140 °C in the mid-crustal region, with lower temperatures of 1090-1100 °C for late-stage crystallisation. More primitive plagioclase antecrysts are thought to sample a slightly more mafic melt at sub-Moho depths. Resorption textures combined with effectively constant mafic mineral compositions suggest phenocryst convection in a storage region of consistent magma composition. In addition, basalt erupted at Ambrym has predominantly maintained a constant composition throughout the volcanic succession. This, coupled with recurrent periods of elevated central vent activity on the scale of months, suggest frequent magmatic recharge via steady-state melt generation at Ambrym.

Crustal structure beneath the Kenya Rift from axial profile data

USGS Publications Warehouse

Mechie, J.; Keller, Gordon R.; Prodehl, C.; Gaciri, S.; Braile, L.W.; Mooney, W.D.; Gajewski, D.; Sandmeier, K.-J.

1994-01-01

Modelling of the KRISP 90 axial line data shows that major crustal thinning occurs along the axis of the Kenya Rift from Moho depths of 35 km in the south beneath the Kenya Dome in the vicinity of Lake Naivasha to 20 km in the north beneath Lake Turkana. Low Pn velocities of 7.5-7.7 km/s are found beneath the whole of the axial line. The results indicate that crustal extension increases to the north and that the low Pn velocities are probably caused by magma (partial melt) rising from below and being trapped in the uppermost kilometres of the mantle. Along the axial line, the rift infill consisting of volcanics and a minor amount of sediments varies in thickness from zero where Precambrian crystalline basement highs occur to 5-6 km beneath the lakes Turkana and Naivasha. Analysis of the Pg phase shows that the upper crystalline crust has velocities of 6.1-6.3 km/s. Bearing in mind the Cainozoic volcanism associated with the rift, these velocities most probably represent Precambrian basement intruded by small amounts of igneous material. The boundary between the upper and lower crusts occurs at about 10 km depth beneath the northern part of the rift and 15 km depth beneath the southern part of the rift. The upper part of the lower crust has velocities of 6.4-6.5 km/s. The basal crustal layer which varies in thickness from a maximum of 2 km in the north to around 9 km in the south has a velocity of about 6.8 km/s. ?? 1994.

Gravity signals from the lithosphere in the Central European Basin System

NASA Astrophysics Data System (ADS)

Yegorova, T.; Bayer, U.; Thybo, H.; Maystrenko, Y.; Scheck-Wenderoth, M.; Lyngsie, S. B.

2007-01-01

We study the gravity signals from different depth levels in the lithosphere of the Central European Basin System (CEBS). The major elements of the CEBS are the Northern and Southern Permian Basins which include the Norwegian-Danish Basin (NDB), the North-German Basin (NGB) and the Polish Trough (PT). An up to 10 km thick sedimentary cover of Mesozoic-Cenozoic sediments, hides the gravity signal from below the basin and masks the heterogeneous structure of the consolidated crust, which is assumed to be composed of domains that were accreted during the Paleozoic amalgamation of Europe. We performed a three-dimensional (3D) gravity backstripping to investigate the structure of the lithosphere below the CEBS. Residual anomalies are derived by removing the effect of sediments down to the base of Permian from the observed field. In order to correct for the influence of large salt structures, lateral density variations are incorporated. These sediment-free anomalies are interpreted to reflect Moho relief and density heterogeneities in the crystalline crust and uppermost mantle. The gravity effect of the Moho relief compensates to a large extent the effect of the sediments in the CEBS and in the North Sea. Removal of the effects of large-scale crustal inhomogeneities shows a clear expression of the Variscan arc system at the southern part of the study area and the old crust of Baltica further north-east. The remaining residual anomalies (after stripping off the effects of sediments, Moho topography and large-scale crustal heterogeneities) reveal long wavelength anomalies, which are caused mainly by density variations in the upper mantle, though gravity influence from the lower crust cannot be ruled out. They indicate that the three main subbasins of the CEBS originated on different lithospheric domains. The PT originated on a thick, strong and dense lithosphere of the Baltica type. The NDB was formed on a weakened Baltica low-density lithosphere formed during the Sveco-Norwegian orogeny. The major part of the NGB is characterized by high-density lithosphere, which includes a high-velocity lower crust (relict of Baltica passive margin) overthrusted by the Avalonian terrane. The short wavelength pattern of the final residuals shows several north-west trending gravity highs between the Tornquist Zone and the Elbe Fault System. The NDB is separated by a gravity low at the Ringkøbing-Fyn high from a chain of positive anomalies in the NGB and the PT. In the NGB these anomalies correspond to the Prignitz (Rheinsberg anomaly), the Glueckstadt and Horn Graben, and they continue further west into the Central Graben, to join with the gravity high of the Central North Sea.

The Architecture of A Variscan Collisional Crust, As Revealed By The Iberseis Seismic Reflection Profile In Southwest Iberia

NASA Astrophysics Data System (ADS)

Simancas, F.; Carbonell, R.; Gonzalez-Lodeiro, F.; Perez-Estaun, A.; Ayarza, P.; Juhlin, C.; Azor, A.; Saez, R.; Martinez-Poyatos, D.; Pascual, E.

The recently acquired IBERSEIS Seismic Reflection Profile runs across major do- mains of the Variscan Orogen in SW Iberia. Geological studies indicate that the seis- mically surveyed region has been built up from three terranes, namely the South Por- tuguese Zone (SPZ), the Ossa-Morena Zone (OMZ) and the Central Iberian Zone (CIZ). These terranes became sutured after a complex, mainly transpressive (left- lateral), collisional history in Devonian-Carboniferous time. The deep seismic reflec- tion profile IBERSEIS has successfully imaged the sutures between these terranes as well as the structure of their crust. The following main features emerge from the pre- liminary integration of seismic and geological data: 1) The suture between the SPZ and OMZ terranes, marked by oceanic amphibolites, appears at present as a north- dipping left-lateral thrust merging in a mid-crustal detachment; the continuity of this suture-contact in the lower crust is not well defined in the seismic image. 2) The OMZ/CIZ suture, a shear zone with eclogites, is clearly imaged in the upper crust as a band of reflectivity dipping to the NE which, after a flat geometry in the middle crust, may continue downwards to the Moho as NE-dipping lower crustal reflections. 3) The SPZ upper crust has an imbricate structure merging into a mid-crustal detachment at constant depth in the surveyed profile. 4) The structure of the OMZ upper crust is dominated by large-scale recumbent folds affected by late upright folds, as fore- seen by geology and fully confirmed by the seismic image. 5) A general mid-crustal detachment exists in the whole surveyed area, whose geometry varies from a sharp detachment-level in the SPZ to a pinching and swelling horizontal band of reflectivity -a melting layer?- in the OMZ; in any case, a strong decoupling between upper and lower crust characterizes this transect of the Variscan orogen. 6) The lower crust of the SPZ has an intense seismic fabric, in accordance with the consideration of this ter- rane as an external orogenic domain with discrete shear bands preserved in the whole crust. 7) The lower crust of the OMZ is much less reflective than the lower crust of the SPZ. 8) The Moho is flat all along the surveyed area, which means that crustal 1 roots formed during the collisional processes were eliminated later on, probably in Late Carboniferous-Permian times. Despite the disturbance due to the generation of a post-orogenic flat Moho, the IBERSEIS seismic image seems to be a good snapshot of the Variscan collision, with very minor reworking by alpine processes. 2

Global significance of a sub-Moho boundary layer (SMBL) deduced from high-resolution seismic observations

USGS Publications Warehouse

Fuchs, K.; Tittgemeyer, M.; Ryberg, T.; Wenzel, F.; Mooney, W.

2002-01-01

We infer the fine structure of a sub-Moho boundary layer (SMBL) at the top of the lithospheric mantle from high-resolution seismic observations of Peaceful Nuclear Explosions (PNE) on superlong-range profiles in Russia. Densely recorded seismograms permit recognition of previously unknown features of teleseismic propagation of the well known Pn and Sn phases, such as a band of incoherent, scattered, high-frequency seismic energy, developing consistently from station to station, apparent velocities of sub-Moho material, and high-frequency energy to distances of more than 3000 km with a coda band, incoherent at 10 km spacing and yet consistently observed to the end of the profiles. Estimates of the other key elements of the SMBL were obtained by finite difference calculations of wave propagation in elastic 2D models from a systematic grid search through parameter space. The SMBL consists of randomly distributed, mild velocity fluctuations of 2% or schlieren of high aspect ratios (???40) with long horizontal extent (???20 km) and therefore as thin as 0.5 km only; SMBL thickness is 60-100 km. It is suggested that the SMBL is of global significance as the physical base of the platewide observed high-frequency phases Pn and Sn. It is shown that wave propagation in the SMBL waveguide is insensitive to the background velocity distribution on which its schlieren are superimposed. This explains why the Pn and Sn phases traverse geological provinces of various age, heat flow, crustal thickness, and tectonic regimes. Their propagation appears to be independent of age. temperature, pressure, and stress. Dynamic stretching of mantle material during subduction or flow, possibly combined with chemical differentiation have to be considered as scale-forming processes in the upper mantle. However, it is difficult to distinguish with the present sets of Pn/Sn array data whether (and also where) the boundary layer is a frozen-in feature of paleo-processes or whether it is a response to an on-going processes; nevertheless, the derived quantitative estimates of the SMBL properties provide important constraints for any hypothesis on scale-forming processes. Models to be tested by future numerical and field experiments are, for example, repeated subduction-convection stretching of oceanic lithosphere (marble-cake model) and schlieren formation at mid-ocean ridges. It is also proposed that the modeling of the observed blocking of Sn and Pn propagation at active plate margins offers a new tool to study the depth range of tectonics below the crust-mantle boundary. Finally, the deduced schlieren structure of the SMBL closes an important scale gap of three to four orders of magnitude between structural dimensions studied in petrological analysis of mantle samples (xenoliths or outcrop of oceanic lithosphere) and those imaged in classical seismological studies of the lithosphere.

Crustal structure and deformation under the Longmenshan and its surroundings revealed by receiver function data

NASA Astrophysics Data System (ADS)

Sun, Ya; Liu, Jianxin; Zhou, Keping; Chen, Bo; Guo, Rongwen

2015-07-01

The convergence of India and Eurasia and the obstruction from the rigid Sichuan Basin cause the Longmenshan (LMS) to have the steepest topographic gradient at the eastern margin of the Tibetan Plateau. However, the mechanisms of surface uplift are still controversial. In this paper, we estimate the crustal structure and deformation under the LMS and its surroundings by analyzing a large amount of receiver function data recorded by regional seismic networks of the China Earthquake Administration. We apply a comprehensive splitting measurement technique on Ps conversion phase at the Moho (Moho Ps splitting) to calculate crustal anisotropy from azimuthal variations of receiver functions. Our results show that most of the seismic stations beneath the LMS area exhibit significant seismic anisotropy with the splitting time of 0.22-0.94 s and a fast polarization direction of NW-SE, while less or even no crustal anisotropy has been observed under the Sichuan Basin. Comparing the fast polarization directions of Moho Ps splitting with the indicators of lithospheric deformation (such as shear wave splitting, absolute plate motion, and global positioning system) imply a consistent tendency of deformation between the lower crust and upper mantle, but decoupling deformation in the crust beneath the LMS area. We further compare Moho Ps splitting time to that estimated from previous SKS splitting, indicating that crustal anisotropy is an important source of the SKS splitting time in this study area. In addition, a thick crust (>50 km) with high Vp/Vs values (1.74-1.86) is also observed using the H-κ stacking method. These seismic observations are consistent with the scenario that the LMS area has been built by the lower crustal flow. Combined with the seismic reflection/refraction profile and geology studies, we further suggest that the lower crustal flow may extrude upward into the upper crust along the steeply dipping strike faults under the LMS area, resulting in the surface uplift of the LMS.

Optimizing visual comfort for stereoscopic 3D display based on color-plus-depth signals.

PubMed

Shao, Feng; Jiang, Qiuping; Fu, Randi; Yu, Mei; Jiang, Gangyi

2016-05-30

Visual comfort is a long-facing problem in stereoscopic 3D (S3D) display. In this paper, targeting to produce S3D content based on color-plus-depth signals, a general framework for depth mapping to optimize visual comfort for S3D display is proposed. The main motivation of this work is to remap the depth range of color-plus-depth signals to a new depth range that is suitable to comfortable S3D display. Towards this end, we first remap the depth range globally based on the adjusted zero disparity plane, and then present a two-stage global and local depth optimization solution to solve the visual comfort problem. The remapped depth map is used to generate the S3D output. We demonstrate the power of our approach on perceptually uncomfortable and comfortable stereoscopic images.

Rapid 2D-to-3D conversion

NASA Astrophysics Data System (ADS)

Harman, Philip V.; Flack, Julien; Fox, Simon; Dowley, Mark

2002-05-01

The conversion of existing 2D images to 3D is proving commercially viable and fulfills the growing need for high quality stereoscopic images. This approach is particularly effective when creating content for the new generation of autostereoscopic displays that require multiple stereo images. The dominant technique for such content conversion is to develop a depth map for each frame of 2D material. The use of a depth map as part of the 2D to 3D conversion process has a number of desirable characteristics: 1. The resolution of the depth may be lower than that of the associated 2D image. 2. It can be highly compressed. 3. 2D compatibility is maintained. 4. Real time generation of stereo, or multiple stereo pairs, is possible. The main disadvantage has been the laborious nature of the manual conversion techniques used to create depth maps from existing 2D images, which results in a slow and costly process. An alternative, highly productive technique has been developed based upon the use of Machine Leaning Algorithm (MLAs). This paper describes the application of MLAs to the generation of depth maps and presents the results of the commercial application of this approach.

VizieR Online Data Catalog: Herschel nearby isolated low-mass clouds maps (Sadavoy+, 2018)

NASA Astrophysics Data System (ADS)

Sadavoy, S. I.; Keto, E.; Bourke, T. L.; Dunham, M. M.; Myers, P. C.; Stephens, I. W.; di, Francesco J.; Webb, K.; Stutz, A. M.; Launhardt, R.; Tobin, J. J.

2018-05-01

For all the sources listed in table1, maps of dust temperature and optical depth at 353GHz for all globules as fits files. For all the sources listed in table1, maps of dust temperature, optical depth at 353GHz, and corrected Herschel intensities are available as fits files. The intensity maps contain labels to indicate the reliability of their intensity corrections with Group A as the most reliable, Group B as somewhat reliable, and Group C as least reliable. See paper for details. (3 data files).

Correlation mapping microscopy

NASA Astrophysics Data System (ADS)

McGrath, James; Alexandrov, Sergey; Owens, Peter; Subhash, Hrebesh M.; Leahy, Martin J.

2015-03-01

Changes in the microcirculation are associated with conditions such as Raynauds disease. Current modalities used to assess the microcirculation such as nailfold capillaroscopy are limited due to their depth ambiguity. A correlation mapping technique was recently developed to extend the capabilities of Optical Coherence Tomography to generate depth resolved images of the microcirculation. Here we present the extension of this technique to microscopy modalities, including confocal microscopy. It is shown that this correlation mapping microscopy technique can extend the capabilities of conventional microscopy to enable mapping of vascular networks in vivo with high spatial resolution.

Evaluating the potential for remote bathymetric mapping of a turbid, sand-bed river: 2. Application to hyperspectral image data from the Platte River

USGS Publications Warehouse

Legleiter, C.J.; Kinzel, P.J.; Overstreet, B.T.

2011-01-01

This study examined the possibility of mapping depth from optical image data in turbid, sediment-laden channels. Analysis of hyperspectral images from the Platte River indicated that depth retrieval in these environments is feasible, but might not be highly accurate. Four methods of calibrating image-derived depth estimates were evaluated. The first involved extracting image spectra at survey point locations throughout the reach. These paired observations of depth and reflectance were subjected to optimal band ratio analysis (OBRA) to relate (R2 = 0.596) a spectrally based quantity to flow depth. Two other methods were based on OBRA of data from individual cross sections. A fourth strategy used ground-based reflectance measurements to derive an OBRA relation (R2 = 0.944) that was then applied to the image. Depth retrieval accuracy was assessed by visually inspecting cross sections and calculating various error metrics. Calibration via field spectroscopy resulted in a shallow bias but provided relative accuracies similar to image-based methods. Reach-aggregated OBRA was marginally superior to calibrations based on individual cross sections, and depth retrieval accuracy varied considerably along each reach. Errors were lower and observed versus predicted regression R2 values higher for a relatively simple, deeper site than a shallower, braided reach; errors were 1/3 and 1/2 the mean depth for the two reaches. Bathymetric maps were coherent and hydraulically reasonable, however, and might be more reliable than implied by numerical metrics. As an example application, linear discriminant analysis was used to produce a series of depth threshold maps for characterizing shallow-water habitat for roosting cranes. ?? 2011 by the American Geophysical Union.

Evaluating the potential for remote bathymetric mapping of a turbid, sand-bed river: 2. application to hyperspectral image data from the Platte River

USGS Publications Warehouse

Legleiter, Carl J.; Kinzel, Paul J.; Overstreet, Brandon T.

2011-01-01

This study examined the possibility of mapping depth from optical image data in turbid, sediment-laden channels. Analysis of hyperspectral images from the Platte River indicated that depth retrieval in these environments is feasible, but might not be highly accurate. Four methods of calibrating image-derived depth estimates were evaluated. The first involved extracting image spectra at survey point locations throughout the reach. These paired observations of depth and reflectance were subjected to optimal band ratio analysis (OBRA) to relate (R2 = 0.596) a spectrally based quantity to flow depth. Two other methods were based on OBRA of data from individual cross sections. A fourth strategy used ground-based reflectance measurements to derive an OBRA relation (R2 = 0.944) that was then applied to the image. Depth retrieval accuracy was assessed by visually inspecting cross sections and calculating various error metrics. Calibration via field spectroscopy resulted in a shallow bias but provided relative accuracies similar to image-based methods. Reach-aggregated OBRA was marginally superior to calibrations based on individual cross sections, and depth retrieval accuracy varied considerably along each reach. Errors were lower and observed versus predicted regression R2 values higher for a relatively simple, deeper site than a shallower, braided reach; errors were 1/3 and 1/2 the mean depth for the two reaches. Bathymetric maps were coherent and hydraulically reasonable, however, and might be more reliable than implied by numerical metrics. As an example application, linear discriminant analysis was used to produce a series of depth threshold maps for characterizing shallow-water habitat for roosting cranes.

A calibration method immune to the projector errors in fringe projection profilometry

NASA Astrophysics Data System (ADS)

Zhang, Ruihua; Guo, Hongwei

2017-08-01

In fringe projection technique, system calibration is a tedious task to establish the mapping relationship between the object depths and the fringe phases. Especially, it is not easy to accurately determine the parameters of the projector in this system, which may induce errors in the measurement results. To solve this problem, this paper proposes a new calibration by using the cross-ratio invariance in the system geometry for determining the phase-to-depth relations. In it, we analyze the epipolar eometry of the fringe projection system. On each epipolar plane, the depth variation along an incident ray induces the pixel movement along the epipolar line on the image plane of the camera. These depth variations and pixel movements can be connected by use of the projective transformations, under which condition the cross-ratio for each of them keeps invariant. Based on this fact, we suggest measuring the depth map by use of this cross-ratio invariance. Firstly, we shift the reference board in its perpendicular direction to three positions with known depths, and measure their phase maps as the reference phase maps; and secondly, when measuring an object, we calculate the object depth at each pixel by equating the cross-ratio of the depths to that of the corresponding pixels having the same phase on the image plane of the camera. This method is immune to the errors sourced from the projector, including the distortions both in the geometric shapes and in the intensity profiles of the projected fringe patterns.The experimental results demonstrate the proposed method to be feasible and valid.

Evidence of focused fluid flow associated to the gas hydrate wedge on the angolan margin

NASA Astrophysics Data System (ADS)

Casenave, Viviane; Imbert, Patrice; Gay, Aurélien

2013-04-01

The Lower Congo basin, offshore south west Africa, is a prolific petroleum province, which has been extensively investigated and exploited for more than 30 years. The study area is located above a producing oil and gas field, the hydrocarbons being trapped in turbidite channels on a tectonic horst. The work is based on the analysis of 3-D seismic and site survey data (2D AUV, grab samples and ROV photos) above a deeper oil and gas field called Moho. The analysis of this seismic data set reveals numerous evidence of focused fluid flow through the Mio-Pliocene interval, including present-day seafloor seep features and shallow buried paleo-seeps, indicating past activity of the system. The main fluid migration-related structures are the followings: 1. Stacked amplitude anomalies, interpreted as the result of vertical migrations of gas are pervasive. Most of these seep features seem to correspond to fossil events as they are interpreted as successive precipitation at the seafloor of patches of seep carbonates (MDAC, Methane Derived Authigenic Carbonates) stacked during the activity of a seep. 2. Another phenomenon of gas migration through the sediment pile is visible on the seismic data of the Moho area: it is the BSR (Bottom Simulating Reflector) located above a horst. The BSR is formed by 2 patches, which cover a small area about 1.5 km² for the largest and 0.5 km2 for the smallest. These two BSRs are located under a depth of water included between 600 and 700 m, into the BSR wedging area. 3. A 'spider morphology' is visible on the seafloor. It corresponds to depressions forming variable-sized furrows oriented slightly oblique to the slope dip direction, directly above the upslope limit of the BSR patches. ROV photos and movies from these furrows showed the presence of seep carbonates and of bacterial carpets, linked with methane leak at the seafloor. A similar 'spider morphology' was also identified in subsurface, at 20 ms under the seafloor, further down the slope, in present-day water depth ranging from 750 to 850 m. These buried depressions cover a stripe in that depth range all over the area covered by the 3D seismic data. These two observations, made both on the seafloor and on it subsurface, seem to correspond to the same phenomenon of fluid expulsion, for the views of the seismic morphology similarities, but in different periods. It is interpreted as a result of a downward migration of the BSR, because of the last sea-level rise, which would have meant an upslope migration of the intersection of the BSR with the seafloor. Based on the evidence of gas hydrate dissociation phenomenon in the Lower Congo Basin, the pinch-out of the BSR may be considered as a natural laboratory for investigating a possible massive greenhouse gas release due to global warming.

Pixel-based parametric source depth map for Cerenkov luminescence imaging

NASA Astrophysics Data System (ADS)

Altabella, L.; Boschi, F.; Spinelli, A. E.

2016-01-01

Optical tomography represents a challenging problem in optical imaging because of the intrinsically ill-posed inverse problem due to photon diffusion. Cerenkov luminescence tomography (CLT) for optical photons produced in tissues by several radionuclides (i.e.: 32P, 18F, 90Y), has been investigated using both 3D multispectral approach and multiviews methods. Difficult in convergence of 3D algorithms can discourage to use this technique to have information of depth and intensity of source. For these reasons, we developed a faster 2D corrected approach based on multispectral acquisitions, to obtain source depth and its intensity using a pixel-based fitting of source intensity. Monte Carlo simulations and experimental data were used to develop and validate the method to obtain the parametric map of source depth. With this approach we obtain parametric source depth maps with a precision between 3% and 7% for MC simulation and 5-6% for experimental data. Using this method we are able to obtain reliable information about the source depth of Cerenkov luminescence with a simple and flexible procedure.

Two-component Thermal Dust Emission Model: Application to the Planck HFI Maps

NASA Astrophysics Data System (ADS)

Meisner, Aaron M.; Finkbeiner, Douglas P.

2014-06-01

We present full-sky, 6.1 arcminute resolution maps of dust optical depth and temperature derived by fitting the Finkbeiner et al. (1999) two-component dust emission model to the Planck HFI and IRAS 100 micron maps. This parametrization of the far infrared thermal dust SED as the sum of two modified blackbodies serves as an important alternative to the commonly adopted single modified blackbody dust emission model. We expect our Planck-based maps of dust temperature and optical depth to form the basis for a next-generation, high-resolution extinction map which will additionally incorporate small-scale detail from WISE imaging.

Bayesian depth estimation from monocular natural images.

PubMed

Su, Che-Chun; Cormack, Lawrence K; Bovik, Alan C

2017-05-01

Estimating an accurate and naturalistic dense depth map from a single monocular photographic image is a difficult problem. Nevertheless, human observers have little difficulty understanding the depth structure implied by photographs. Two-dimensional (2D) images of the real-world environment contain significant statistical information regarding the three-dimensional (3D) structure of the world that the vision system likely exploits to compute perceived depth, monocularly as well as binocularly. Toward understanding how this might be accomplished, we propose a Bayesian model of monocular depth computation that recovers detailed 3D scene structures by extracting reliable, robust, depth-sensitive statistical features from single natural images. These features are derived using well-accepted univariate natural scene statistics (NSS) models and recent bivariate/correlation NSS models that describe the relationships between 2D photographic images and their associated depth maps. This is accomplished by building a dictionary of canonical local depth patterns from which NSS features are extracted as prior information. The dictionary is used to create a multivariate Gaussian mixture (MGM) likelihood model that associates local image features with depth patterns. A simple Bayesian predictor is then used to form spatial depth estimates. The depth results produced by the model, despite its simplicity, correlate well with ground-truth depths measured by a current-generation terrestrial light detection and ranging (LIDAR) scanner. Such a strong form of statistical depth information could be used by the visual system when creating overall estimated depth maps incorporating stereopsis, accommodation, and other conditions. Indeed, even in isolation, the Bayesian predictor delivers depth estimates that are competitive with state-of-the-art "computer vision" methods that utilize highly engineered image features and sophisticated machine learning algorithms.

The Moho as a magnetic boundary

NASA Technical Reports Server (NTRS)

Wasilewski, P. J.; Thomas, H. H.; Mayhew, M. A.

1979-01-01

Magnetic data are presented for mantle derived rocks: peridtites from St. Pauls rocks, dunite xenoliths from the kaupulehu flow in Hawaii, as well as peridolite, dunite and eclogite xenoliths from Roberts Victor, Dutoitspan, Kilbourne Hole, and San Carlos diatremes. The rocks are paramagnetic or very weakly ferromagnetic at room temperature. Saturation magnetization values range from 0.013 emu/gm to less than 0.001 emu/gm. A review of pertinent literature dealing with analysis of the minerals in mantle xenoliths provides evidence that metals and primary Fe3O4 are absent, and that complex CR, Mg, Al, and Fe spinels dominate the oxide mineralogy. All of the available evidence supports the magnetic results, indicating that the seismic MOHO is a magnetic boundary.

Structure of Kilauea's southwest rift zone and western south flank defined by relocated earthquakes

NASA Astrophysics Data System (ADS)

Rinard, Bethany D.

This study is the first detailed seismic investigation of the southwest rift and western south flank of Kilauea Volcano. Earthquakes outline the tectonic and magmatic systems of the volcano. In this study, more than 4800 earthquakes from the years 1981--2001 were relocated with a double-difference method, and almost 500 were relocated with cross-correlation. The result is a much-improved image of Kilauea's south flank structure. The shallowest of the earthquakes on Kilauea (<5km) are usually related to magma movement, and occur almost exclusively in the actively intruded rift. The few tectonic earthquakes that occur at this depth are along the Koae and Hilina Fault systems. Focal mechanisms indicate that the shallow events on the Hilina system have [normal, right-lateral] oblique-slip motion. Beneath the entire south flank are earthquakes that occur on a decollement, located at a depth of 7--10km. The inland-dipping decollement structure is clearly imaged with this new data set. Earthquakes on the volcano's south flank normal faults appear to extend downward to the decollement. Earthquakes at intermediate depths image the decollement, a plane that dips inland. This is the boundary between the volcano and the old oceanic crust beneath it. Movement on faults at decollement depths of 7--10km have [right-lateral thrust] oblique-slip motion. When intrusions occur in the rift zones, the flank is forced seaward along the decollement. Since the decollement dips inland, the south flank must move up an incline as it slides seaward. Hawaii also experiences deep (>25km) earthquakes, which are the most intriguing events in this study. These earthquakes are significant because the Moho is located at a depth of 13--15km, so they are clearly occurring in the mantle. The deep events examined in this study are tectonic earthquakes, not attributable to melt migration. A high strain rate in the mantle, largely due to the geologically rapid formation of the island that has quickly increased the load on the underlying mantle, may account for the occurrence of these deep earthquakes. Focal mechanisms indicate [normal, right-lateral] oblique-slip motion on faults below 25km depth.

Enhanced in vivo visualization of the microcirculation by topical application of fructose solution confirmed with correlation mapping optical coherence tomography

NASA Astrophysics Data System (ADS)

Enfield, Joey; McGrath, James; Daly, Susan M.; Leahy, Martin

2016-08-01

Changes within the microcirculation can provide an early indication of the onset of a plethora of ailments. Various techniques have thus been developed that enable the study of microcirculatory irregularities. Correlation mapping optical coherence tomography (cmOCT) is a recently proposed technique, which enables mapping of vasculature networks at the capillary level in a noninvasive and noncontact manner. This technique is an extension of conventional optical coherence tomography (OCT) and is therefore likewise limited in the penetration depth of ballistic photons in biological media. Optical clearing has previously been demonstrated to enhance the penetration depth and the imaging capabilities of OCT. In order to enhance the achievable maximum imaging depth, we propose the use of optical clearing in conjunction with the cmOCT technique. We demonstrate in vivo a 13% increase in OCT penetration depth by topical application of a high-concentration fructose solution, thereby enabling the visualization of vessel features at deeper depths within the tissue.

Updates to Enhanced Geothermal System Resource Potential Estimate

DOE Office of Scientific and Technical Information (OSTI.GOV)

Augustine, Chad

The deep EGS electricity generation resource potential estimate maintained by the National Renewable Energy Laboratory was updated using the most recent temperature-at-depth maps available from the Southern Methodist University Geothermal Laboratory. The previous study dates back to 2011 and was developed using the original temperature-at-depth maps showcased in the 2006 MIT Future of Geothermal Energy report. The methodology used to update the deep EGS resource potential is the same as in the previous study and is summarized in the paper. The updated deep EGS resource potential estimate was calculated for depths between 3 and 7 km and is binned inmore » 25 degrees C increments. The updated deep EGS electricity generation resource potential estimate is 4,349 GWe. A comparison of the estimates from the previous and updated studies shows a net increase of 117 GWe in the 3-7 km depth range, due mainly to increases in the underlying temperature-at-depth estimates from the updated maps.« less

Update to Enhanced Geothermal System Resource Potential Estimate: Preprint

DOE Office of Scientific and Technical Information (OSTI.GOV)

Augustine, Chad

2016-10-01

The deep EGS electricity generation resource potential estimate maintained by the National Renewable Energy Laboratory was updated using the most recent temperature-at-depth maps available from the Southern Methodist University Geothermal Laboratory. The previous study dates back to 2011 and was developed using the original temperature-at-depth maps showcased in the 2006 MIT Future of Geothermal Energy report. The methodology used to update the deep EGS resource potential is the same as in the previous study and is summarized in the paper. The updated deep EGS resource potential estimate was calculated for depths between 3 and 7 km and is binned inmore » 25 degrees C increments. The updated deep EGS electricity generation resource potential estimate is 4,349 GWe. A comparison of the estimates from the previous and updated studies shows a net increase of 117 GWe in the 3-7 km depth range, due mainly to increases in the underlying temperature-at-depth estimates from the updated maps.« less

Flooding Hazard Maps of Different Land Uses in Subsidence Area

NASA Astrophysics Data System (ADS)

Lin, Yongjun; Chang, Hsiangkuan; Tan, Yihchi

2017-04-01

This study aims on flooding hazard maps of different land uses in the subsidence area of southern Taiwan. Those areas are low-lying due to subsidence resulting from over pumping ground water for aquaculture. As a result, the flooding due to storm surges and extreme rainfall are frequent in this area and are expected more frequently in the future. The main land uses there include: residence, fruit trees, and aquaculture. The hazard maps of the three land uses are investigated. The factors affecting hazards of different land uses are listed below. As for residence, flooding depth, duration of flooding, and rising rate of water surface level are factors affecting its degree of hazard. High flooding depth, long duration of flooding, and fast rising rate of water surface make residents harder to evacuate. As for fruit trees, flooding depth and duration of flooding affects its hazard most due to the root hypoxia. As for aquaculture, flooding depth affects its hazard most because the high flooding depth may cause the fish flush out the fishing ponds. An overland flow model is used for simulations of hydraulic parameters for factors such as flooding depth, rising rate of water surface level and duration of flooding. As above-mentioned factors, the hazard maps of different land uses can be made and high hazardous are can also be delineated in the subsidence areas.

A promising tool for subsurface permafrost mapping-An application of airborne geophysics from the Yukon River Basin, Alaska

USGS Publications Warehouse

Abraham, Jared E.

2011-01-01

In the area of Fort Yukon, the AEM survey shows elevated resistivities extending to depth, likely indicative of thick permafrost. This depth corresponds well to observations from a borehole drilled in the area in the late 1990s, which detected permafrost to a depth of about 100 meters (Clark and others, 2009). In contrast to the area of Fort Yukon, the Yukon River and its floodplain are not associated with deep resistive sediments, suggesting a lack of deep permafrost, at least within the depth range of the AEM mapping (fig. 3).

Method for the Preparation of Hazard Map in Urban Area Using Soil Depth and Groundwater Level

NASA Astrophysics Data System (ADS)

Kim, Sung-Wook; Choi, Eun-Kyeong; Cho, Jin Woo; Lee, Ju-Hyoung

2017-04-01

The hazard maps for predicting collapse on natural slopes consists of a combination of topographic, hydrological, and geological factors. Topographic factors are extracted from DEM, including aspect, slope, curvature, and topographic index. Hydrological factors, such as distance to drainage, drainage density, stream-power index, and wetness index are most important factors for slope instability. However, most of the urban areas are located on the plains and it is difficult to apply the hazard map using the topography and hydrological factors. In order to evaluate the risk of collapse of flat and low slope areas, soil depth and groundwater level data were collected and used as a factor for interpretation. In addition, the reliability of the hazard map was compared with the disaster history of the study area (Gangnam-gu and Yeouido district). In the disaster map of the disaster prevention agency, the urban area was mostly classified as the stable area and did not reflect the collapse history. Soil depth, drainage conditions and groundwater level obtained from boreholes were added as input data of hazard map, and disaster vulnerability increased at the location where the actual collapse points. In the study area where damage occurred, the moderate and low grades of the vulnerability of previous hazard map were 12% and 88%, respectively. While, the improved map showed 2% high grade, moderate grade 29%, low grade 66% and very low grade 2%. These results were similar to actual damage. Keywords: hazard map, urban area, soil depth, ground water level Acknowledgement This research was supported by a Grant from a Strategic Research Project (Horizontal Drilling and Stabilization Technologies for Urban Search and Rescue (US&R) Operation) funded by the Korea Institute of Civil Engineering and Building Technology.

The spatial sensitivity of Sp converted waves—scattered-wave kernels and their applications to receiver-function migration and inversion

NASA Astrophysics Data System (ADS)

Mancinelli, N. J.; Fischer, K. M.

2018-03-01

We characterize the spatial sensitivity of Sp converted waves to improve constraints on lateral variations in uppermost-mantle velocity gradients, such as the lithosphere-asthenosphere boundary (LAB) and the mid-lithospheric discontinuities. We use SPECFEM2D to generate 2-D scattering kernels that relate perturbations from an elastic half-space to Sp waveforms. We then show that these kernels can be well approximated using ray theory, and develop an approach to calculating kernels for layered background models. As proof of concept, we show that lateral variations in uppermost-mantle discontinuity structure are retrieved by implementing these scattering kernels in the first iteration of a conjugate-directions inversion algorithm. We evaluate the performance of this technique on synthetic seismograms computed for 2-D models with undulations on the LAB of varying amplitude, wavelength and depth. The technique reliably images the position of discontinuities with dips <35° and horizontal wavelengths >100-200 km. In cases of mild topography on a shallow LAB, the relative brightness of the LAB and Moho converters approximately agrees with the ratio of velocity contrasts across the discontinuities. Amplitude retrieval degrades at deeper depths. For dominant periods of 4 s, the minimum station spacing required to produce unaliased results is 5 km, but the application of a Gaussian filter can improve discontinuity imaging where station spacing is greater.

The crustal structure of the Cocos ridge off Costa Rica

NASA Astrophysics Data System (ADS)

Walther, Christian H. E.

2003-03-01

The submarine Cocos ridge in the northwestern Panamá basin, a bathymetric feature more than 1000-km long and 250-500 km broad, is about 2 km shallower than the adjacent basin. It is generally interpreted as the trace of the Galápagos hot spot. Two 127- and 260-km long seismic wide-angle sections were recorded along and across this ridge, offshore the Osa peninsula, Costa Rica. Crustal thickening is seen everywhere along the sections. On the northwestern outer ridge flank, increased thickness is exclusively attributed to the upper crust and expressed by 2-km thick flow basalts. The Quepos plateau caps the upper crust in this area. Toward the center of the Cocos ridge, the Moho deepens from 11-12 to 21 km depth and crustal thickening is almost entirely attributed to the lower crust which makes up 80% of the crust and is three times the thickness of normal oceanic lower crust. It is homogeneously structured and the velocities which range from 6.5 km/s at the top to 7.35 km/s at the base are comparable to normal lower crust under these depth conditions and suggest no differences to a gabbroic rock composition. Similarities to the crustal velocity structure of Iceland, central Kerguelen plateau, and Broken ridge are consistent with a formation of this 13-15 Ma old Cocos ridge segment by excessive magmatism in a near-plate boundary setting.

Laboratory derived constraints on electrical conductivity beneath Slave craton

NASA Astrophysics Data System (ADS)

Bagdassarov, Nikolai S.; Kopylova, Maya G.; Eichert, Sandrine

2007-04-01

The depth profile of the electrical conductivity, σ(d), beneath the Central Slave craton (Canada) has been reconstructed with the help of laboratory measurements carried out on peridotite xenoliths. σ(T) of xenoliths was determined in the piston-cylinder apparatus at 1 and 2 GPa and from 600 to 1150 °C. σ(T) of xenoliths follows the Arrhenius dependence with the activation energy, E, varying from 2.10 to 1.44 eV depending on temperature range and the Mg-number. The calculated xenolith geotherm and the suggested lithology beneath the Central Slave have been used to constrain σ(d) as follows: σ(d) in the crust varies between 0.5×10-5 and 10-3 S/m; the lithospheric σ(d) sharply decreases below the Moho at 39.4 km to 0.5×10-8 S/m, which corresponds to 460 °C, and then gradually increases with the depth d to 0.5×10-2 S/m. The modeled MT-response of the constrained σ(d) profile has been compared with MT-observations [Jones, A.G., Lezaeta, P., Ferguson, I.J., Chave, A.D., Evans, R.L., Garcia, X., Spratt J., 2003. The electrical structure of the Slave craton. Lithos, 71, 505-527]. The general trend of the calculated MT-response based on the σ(d) model mimics the MT-inversion of the field data from the Central Slave.

Shaded seafloor relief, backscatter strength, and surficial geology; German Bank, Scotian Shelf, offshore Nova Scotia

USGS Publications Warehouse

Todd, B.J.; Valentine, Page C.

2010-01-01

This map is part of a three-map series of German Bank, located on the Scotian Shelf off southern Nova Scotia.  This map is the product of a number of surveys (1997-2003) that used a multibeam sonar system to map 5321 km2 of the seafloor.  Other surveys collected geological data for scientific interpretation.  This map sheet shows the seafloor topography of German Bank in shaded-relief view and seafloor depth (coded by colour) at a scale of 1:1000,000.  Topographic contours generated from the multibeam data are shown (in white) on the colour-coded multibeam topography at a depth interval of 20 m.  Bathymetic contours (in blue) outside the multibeam survey area, presented at a depth interval of 10 m, are from the Natural Resource Map series (Canadian Hydrographic Service, 1967, 1971a, 1971b, 1972). Sheet 2 shows coloured backscatter strength in shaded-relief view.  Sheet 3 shows seafloor topography in shaded-relief view with colour-coded surficial geological units.

Sampling strategies to improve passive optical remote sensing of river bathymetry

USGS Publications Warehouse

Legleiter, Carl; Overstreet, Brandon; Kinzel, Paul J.

2018-01-01

Passive optical remote sensing of river bathymetry involves establishing a relation between depth and reflectance that can be applied throughout an image to produce a depth map. Building upon the Optimal Band Ratio Analysis (OBRA) framework, we introduce sampling strategies for constructing calibration data sets that lead to strong relationships between an image-derived quantity and depth across a range of depths. Progressively excluding observations that exceed a series of cutoff depths from the calibration process improved the accuracy of depth estimates and allowed the maximum detectable depth ($d_{max}$) to be inferred directly from an image. Depth retrieval in two distinct rivers also was enhanced by a stratified version of OBRA that partitions field measurements into a series of depth bins to avoid biases associated with under-representation of shallow areas in typical field data sets. In the shallower, clearer of the two rivers, including the deepest field observations in the calibration data set did not compromise depth retrieval accuracy, suggesting that $d_{max}$ was not exceeded and the reach could be mapped without gaps. Conversely, in the deeper and more turbid stream, progressive truncation of input depths yielded a plausible estimate of $d_{max}$ consistent with theoretical calculations based on field measurements of light attenuation by the water column. This result implied that the entire channel, including pools, could not be mapped remotely. However, truncation improved the accuracy of depth estimates in areas shallower than $d_{max}$, which comprise the majority of the channel and are of primary interest for many habitat-oriented applications.

An image-space parallel convolution filtering algorithm based on shadow map

NASA Astrophysics Data System (ADS)

Li, Hua; Yang, Huamin; Zhao, Jianping

2017-07-01

Shadow mapping is commonly used in real-time rendering. In this paper, we presented an accurate and efficient method of soft shadows generation from planar area lights. First this method generated a depth map from light's view, and analyzed the depth-discontinuities areas as well as shadow boundaries. Then these areas were described as binary values in the texture map called binary light-visibility map, and a parallel convolution filtering algorithm based on GPU was enforced to smooth out the boundaries with a box filter. Experiments show that our algorithm is an effective shadow map based method that produces perceptually accurate soft shadows in real time with more details of shadow boundaries compared with the previous works.

Improving depth maps of plants by using a set of five cameras

NASA Astrophysics Data System (ADS)

Kaczmarek, Adam L.

2015-03-01

Obtaining high-quality depth maps and disparity maps with the use of a stereo camera is a challenging task for some kinds of objects. The quality of these maps can be improved by taking advantage of a larger number of cameras. The research on the usage of a set of five cameras to obtain disparity maps is presented. The set consists of a central camera and four side cameras. An algorithm for making disparity maps called multiple similar areas (MSA) is introduced. The algorithm was specially designed for the set of five cameras. Experiments were performed with the MSA algorithm and the stereo matching algorithm based on the sum of sum of squared differences (sum of SSD, SSSD) measure. Moreover, the following measures were included in the experiments: sum of absolute differences (SAD), zero-mean SAD (ZSAD), zero-mean SSD (ZSSD), locally scaled SAD (LSAD), locally scaled SSD (LSSD), normalized cross correlation (NCC), and zero-mean NCC (ZNCC). Algorithms presented were applied to images of plants. Making depth maps of plants is difficult because parts of leaves are similar to each other. The potential usability of the described algorithms is especially high in agricultural applications such as robotic fruit harvesting.

Depth-color fusion strategy for 3-D scene modeling with Kinect.

PubMed

Camplani, Massimo; Mantecon, Tomas; Salgado, Luis

2013-12-01

Low-cost depth cameras, such as Microsoft Kinect, have completely changed the world of human-computer interaction through controller-free gaming applications. Depth data provided by the Kinect sensor presents several noise-related problems that have to be tackled to improve the accuracy of the depth data, thus obtaining more reliable game control platforms and broadening its applicability. In this paper, we present a depth-color fusion strategy for 3-D modeling of indoor scenes with Kinect. Accurate depth and color models of the background elements are iteratively built, and used to detect moving objects in the scene. Kinect depth data is processed with an innovative adaptive joint-bilateral filter that efficiently combines depth and color by analyzing an edge-uncertainty map and the detected foreground regions. Results show that the proposed approach efficiently tackles main Kinect data problems: distance-dependent depth maps, spatial noise, and temporal random fluctuations are dramatically reduced; objects depth boundaries are refined, and nonmeasured depth pixels are interpolated. Moreover, a robust depth and color background model and accurate moving objects silhouette are generated.

Using Gravity Inversion to Estimate Antarctic Geothermal Heat Flux

NASA Astrophysics Data System (ADS)

Vaughan, Alan P. M.; Kusznir, Nick J.; Ferraccioli, Fausto; Leat, Phil T.; Jordan, Tom A. R. M.; Purucker, Michael E.; (Sasha) Golynsky, A. V.; Rogozhina, Irina

2014-05-01

New modelling studies for Greenland have recently underlined the importance of GHF for long-term ice sheet behaviour (Petrunin et al. 2013). Revised determinations of top basement heat-flow for Antarctica and adjacent rifted continental margins using gravity inversion mapping of crustal thickness and continental lithosphere thinning (Chappell & Kusznir 2008), using BedMap2 data have provided improved estimates of geothermal heat flux (GHF) in Antarctica where it is very poorly known. Continental lithosphere thinning and post-breakup residual thicknesses of continental crust determined from gravity inversion have been used to predict the preservation of continental crustal radiogenic heat productivity and the transient lithosphere heat-flow contribution within thermally equilibrating rifted continental and oceanic lithosphere. The sensitivity of present-day Antarctic top basement heat-flow to initial continental radiogenic heat productivity, continental rift and margin breakup age has been examined. Recognition of the East Antarctic Rift System (EARS), a major Permian to Cretaceous age rift system that appears to extend from the continental margin at the Lambert Rift to the South Pole region, a distance of 2500 km (Ferraccioli et al. 2011) and is comparable in scale to the well-studied East African rift system, highlights that crustal variability in interior Antarctica is much greater than previously assumed. GHF is also important to understand proposed ice accretion at the base of the EAIS in the GSM and its links to sub-ice hydrology (Bell et al. 2011). References Bell, R.E., Ferraccioli, F., Creyts, T.T., Braaten, D., Corr, H., Das, I., Damaske, D., Frearson, N., Jordan, T., Rose, K., Studinger, M. & Wolovick, M. 2011. Widespread persistent thickening of the East Antarctic Ice Sheet by freezing from the base. Science, 331 (6024), 1592-1595. Chappell, A.R. & Kusznir, N.J. 2008. Three-dimensional gravity inversion for Moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction. Geophysical Journal International, 174 (1), 1-13. Ferraccioli, F., Finn, C.A., Jordan, T.A., Bell, R.E., Anderson, L.M. & Damaske, D. 2011. East Antarctic rifting triggers uplift of the Gamburtsev Mountains. Nature, 479, 388-392. Petrunin, A., Rogozhina, I., Vaughan, A. P. M., Kukkonen, I. T., Kaban, M., Koulakov, I., Thomas, M. (2013): Heat flux variations beneath central Greenland's ice due to anomalously thin lithosphere. - Nature Geoscience, 6, 746-750.

Mapping the seafloor geology offshore of Massachusetts

USGS Publications Warehouse

Barnhardt, Walter A.; Andrews, Brian D.

2006-01-01

Geologic and bathymetric maps help us understand the evolutionary history of the Massachusetts coast and the processes that have shaped it. The maps show the distribution of bottom types (for example, bedrock, gravel, sand, mud) and water depths over large areas of the seafloor. In turn, these two fundamental parameters largely determine the species of flora and fauna that inhabit a particular area. Knowledge of bottom types and water depths provides a framework for mapping benthic habitats and managing marine resources. The need for coastal–zone mapping to inform policy and management is widely recognized as critical for mitigating hazards, creating resource inventories, and tracking environmental changes (National Research Council, 2004; U.S. Commission on Ocean Policy, 2004).

Planarity constrained multi-view depth map reconstruction for urban scenes

NASA Astrophysics Data System (ADS)

Hou, Yaolin; Peng, Jianwei; Hu, Zhihua; Tao, Pengjie; Shan, Jie

2018-05-01

Multi-view depth map reconstruction is regarded as a suitable approach for 3D generation of large-scale scenes due to its flexibility and scalability. However, there are challenges when this technique is applied to urban scenes where apparent man-made regular shapes may present. To address this need, this paper proposes a planarity constrained multi-view depth (PMVD) map reconstruction method. Starting with image segmentation and feature matching for each input image, the main procedure is iterative optimization under the constraints of planar geometry and smoothness. A set of candidate local planes are first generated by an extended PatchMatch method. The image matching costs are then computed and aggregated by an adaptive-manifold filter (AMF), whereby the smoothness constraint is applied to adjacent pixels through belief propagation. Finally, multiple criteria are used to eliminate image matching outliers. (Vertical) aerial images, oblique (aerial) images and ground images are used for qualitative and quantitative evaluations. The experiments demonstrated that the PMVD outperforms the popular multi-view depth map reconstruction with an accuracy two times better for the aerial datasets and achieves an outcome comparable to the state-of-the-art for ground images. As expected, PMVD is able to preserve the planarity for piecewise flat structures in urban scenes and restore the edges in depth discontinuous areas.

Automatic detection of artifacts in converted S3D video

NASA Astrophysics Data System (ADS)

Bokov, Alexander; Vatolin, Dmitriy; Zachesov, Anton; Belous, Alexander; Erofeev, Mikhail

2014-03-01

In this paper we present algorithms for automatically detecting issues specific to converted S3D content. When a depth-image-based rendering approach produces a stereoscopic image, the quality of the result depends on both the depth maps and the warping algorithms. The most common problem with converted S3D video is edge-sharpness mismatch. This artifact may appear owing to depth-map blurriness at semitransparent edges: after warping, the object boundary becomes sharper in one view and blurrier in the other, yielding binocular rivalry. To detect this problem we estimate the disparity map, extract boundaries with noticeable differences, and analyze edge-sharpness correspondence between views. We pay additional attention to cases involving a complex background and large occlusions. Another problem is detection of scenes that lack depth volume: we present algorithms for detecting at scenes and scenes with at foreground objects. To identify these problems we analyze the features of the RGB image as well as uniform areas in the depth map. Testing of our algorithms involved examining 10 Blu-ray 3D releases with converted S3D content, including Clash of the Titans, The Avengers, and The Chronicles of Narnia: The Voyage of the Dawn Treader. The algorithms we present enable improved automatic quality assessment during the production stage.

GlobalSoilMap France: High-resolution spatial modelling the soils of France up to two meter depth.

PubMed

Mulder, V L; Lacoste, M; Richer-de-Forges, A C; Arrouays, D

2016-12-15

This work presents the first GlobalSoilMap (GSM) products for France. We developed an automatic procedure for mapping the primary soil properties (clay, silt, sand, coarse elements, pH, soil organic carbon (SOC), cation exchange capacity (CEC) and soil depth). The procedure employed a data-mining technique and a straightforward method for estimating the 90% confidence intervals (CIs). The most accurate models were obtained for pH, sand and silt. Next, CEC, clay and SOC were found reasonably accurate predicted. Coarse elements and soil depth were the least accurate of all models. Overall, all models were considered robust; important indicators for this were 1) the small difference in model diagnostics between the calibration and cross-validation set, 2) the unbiased mean predictions, 3) the smaller spatial structure of the prediction residuals in comparison to the observations and 4) the similar performance compared to other developed GlobalSoilMap products. Nevertheless, the confidence intervals (CIs) were rather wide for all soil properties. The median predictions became less reliable with increasing depth, as indicated by the increase of CIs with depth. In addition, model accuracy and the corresponding CIs varied depending on the soil variable of interest, soil depth and geographic location. These findings indicated that the CIs are as informative as the model diagnostics. In conclusion, the presented method resulted in reasonably accurate predictions for the majority of the soil properties. End users can employ the products for different purposes, as was demonstrated with some practical examples. The mapping routine is flexible for cloud-computing and provides ample opportunity to be further developed when desired by its users. This allows regional and international GSM partners with fewer resources to develop their own products or, otherwise, to improve the current routine and work together towards a robust high-resolution digital soil map of the world. Copyright © 2016 Elsevier B.V. All rights reserved.

A Review of Depth and Normal Fusion Algorithms

PubMed Central

Štolc, Svorad; Pock, Thomas

2018-01-01

Geometric surface information such as depth maps and surface normals can be acquired by various methods such as stereo light fields, shape from shading and photometric stereo techniques. We compare several algorithms which deal with the combination of depth with surface normal information in order to reconstruct a refined depth map. The reasons for performance differences are examined from the perspective of alternative formulations of surface normals for depth reconstruction. We review and analyze methods in a systematic way. Based on our findings, we introduce a new generalized fusion method, which is formulated as a least squares problem and outperforms previous methods in the depth error domain by introducing a novel normal weighting that performs closer to the geodesic distance measure. Furthermore, a novel method is introduced based on Total Generalized Variation (TGV) which further outperforms previous approaches in terms of the geodesic normal distance error and maintains comparable quality in the depth error domain. PMID:29389903

Lithologic boundaries from gravity and magnetic anomalies over Proterozoic Dalma volcanics

NASA Astrophysics Data System (ADS)

Yadav, Pramod Kumar; Adhikari, P. K.; Srivastava, Shalivahan; Maurya, Ved P.; Tripathi, Anurag; Singh, Shailendra; Singh, Roshan K.; Bage, Ashish K.

2018-03-01

Dalma volcanics (DVs) has intruded the older Singhbhum Group of Metapelites. Despite DVs being rich in mineralisation, its boundaries are not clearly demarcated. Gravity and magnetic surveys have been attempted for mapping the boundaries in DVs. These surveys were made in the northern fringe of the DVs over an area of ˜ 0.70 km2 along 13 parallel lines at 50 m spacing. The data was acquired at ˜ 25 m spacing. The surveys were taken for determination of lithological boundaries, depths and nature of causative source using Euler depth solutions and radially averaged power spectrum (RAPS). Residual anomaly maps of gravity and magnetic intensity show the same trend as that of Bouguer gravity anomaly and total magnetic intensity anomaly map indicating towards shallow sources. The magnetic map in general follows the same pattern as that of gravity anomaly maps. The map shows coincident high gravity and magnetic anomalies. These anomalies together with resistivity signatures confirm that the northern fringe of DVs hosts volcanogenic massive sulphide settings. The Euler depth solution delineated the lateral boundaries and nature of the source. It seems that the source is of spherical nature lying within a depth range of 25-40 m. The obtained lithological (vertical) units from RAPS are between Lower DVs, Upper DVs and Singhbhum Group Metapelites at depths of ˜ 15, ˜ 25 and ˜ 40 m, respectively. The metallogeny is associated with the Upper DVs and the corresponding delineated lithological (vertical) unit is indicative of the top of the ore body. Good agreement is observed with the geological succession from the drilling data and resistivity data. The findings suggest that the northern fringe of DVs could be a preferred target for drilling.

Regional P wave velocity structure of the Northern Cascadia Subduction Zone

USGS Publications Warehouse

Ramachandran, K.; Hyndman, R.D.; Brocher, T.M.

2006-01-01

This paper presents the first regional three-dimensional, P wave velocity model for the Northern Cascadia Subduction. Zone (SW British Columbia and NW Washington State) constructed through tomographic inversion of first-arrival traveltime data from active source experiments together with earthquake traveltime data recorded at permanent stations. The velocity model images the structure of the subducting Juan de Fuca plate, megathrust, and the fore-arc crust and upper mantle. Beneath southern Vancouver Island the megathrust above the Juan de Fuca plate is characterized by a broad zone (25-35 km depth) having relatively low velocities of 6.4-6.6 km/s. This relative low velocity zone coincides with the location of most of the episodic tremors recently mapped beneath Vancouver Island, and its low velocity may also partially reflect the presence of trapped fluids and sheared lower crustal rocks. The rocks of the Olympic Subduction Complex are inferred to deform aseismically as evidenced by the lack of earthquakes withi the low-velocity rocks. The fore-arc upper mantle beneath the Strait of Georgia and Puget Sound is characterized by velocities of 7.2-7.6 km/s. Such low velocities represent regional serpentinization of the upper fore-arc mantle and provide evidence for slab dewatering and densification. Tertiary sedimentary basins in the Strait of Georgia and Puget Lowland imaged by the velocity model lie above the inferred region of slab dewatering and densification and may therefore partly result from a higher rate of slab sinking. In contrast, sedimentary basins in the Strait of Juan de Fuca lie in a synclinal depression in the Crescent Terrane. The correlation of in-slab earthquake hypocenters M>4 with P wave velocities greater than 7.8 km/s at the hypocenters suggests that they originate near the oceanic Moho of the subducting Juan de Fuca plate. Copyright 2006 by the American Geophysical Union.

Constraints on upper mantle Vp/Vs ratio variations beneath eastern North China from receiver function tomography

NASA Astrophysics Data System (ADS)

Si, Shaokun; Tian, Xiaobo; Gao, Rui

2017-05-01

To detect the thinning, modification, and replacement of the basement of the lithosphere is a key step in understanding the destruction mechanism of the North China lithosphere. The difference of the basement of the lithosphere is mainly displayed by the variation of the peridotite composition and its physical state. Vp/Vs ratio (hereafter referred to as velocity ratio) is more sensitive to this change than Vp or Vs alone. By means of the strong dependence of the travel-time of the wave converted at the 410-km discontinuity (P410s) observed in the receiver function (RF) on the velocity ratio in the upper mantle, we developed a new mapping method to constrain the velocity ratio between the Moho and 410-km discontinuity. Using the RFs extracted from 246 broadband stations beneath the North China Craton (NCC), we obtained a high-resolution velocity ratio image of the upper mantle. The abnormal velocity ratio indicates a strong lateral variation of the mineral composition in the upper mantle beneath North China. Two low-velocity-ratio patches are imaged at the top of the upper mantle and the 410 km depth, respectively. The former may be related to the orthopyroxene enrichment in the lithospheric mantle, and the latter may reflect the stagnant Pacific slab in the mantle transition zone (MTZ). A prominent high-velocity-ratio anomaly is also imaged in the upper mantle beneath the Shaanxi-Shanxi rift system in the central NCC, with the highest anomaly reaching 10%. We speculate that the high velocity ratio of upper mantle is related to convective flow due to slab dehydration in the MTZ. The dehydration of the retained slab in the MTZ results in partial melting and upwelling of upper mantle materials. Such convective flow and their melting are closely related to the Cenozoic basalt eruption in the northern section of the Shaanxi-Shanxi rift system.

Lithosphere and Asthenosphere Structure of the Western Mediterranean and Northwest Africa from Rayleigh Wave tomography and Ps Receiver Functions

NASA Astrophysics Data System (ADS)

Palomeras, Imma; Thurner, Sally; Levander, Alan; Bezada, Maximiliano; Villasenor, Antonio; Humphreys, Eugene; Carbonell, Ramon; Gallart, Josep

2013-04-01

Since Cenozoic times the Western Mediterranean has been affected by complex subduction and slab rollback, during African-European convergence. The deformed region occupies a wide area from the Atlas mountains in northwest Africa to the southern Iberian Massif in Spain. Evolutionary models of the Western Mediterranean invoke extensive slab rollback and compression, as well as likely upper mantle delamination/convective drip scenarios during formation of the Alboran domain, the Betics, Rif, and Atlas Mountains. We report on a multidisciplinary, international investigation of the Alboran System and surrounding areas. In this study we have analyzed teleseismic data from the roughly 240 temporary and permanent broadband seismographs operated in this region by more than a dozen different cooperating research groups. Here we present combined results from Rayleigh wave tomography and Ps receiver functions. Receiver functions were made in 3 frequency bands (2 Hz, 1 Hz, 0.5 Hz) using iterative time-domain and water-level frequency-domain methods. We measured Rayleigh phase velocities using the two-plane-wave method and finite-frequency kernels to remove complications due to multi-pathing and to improve lateral resolution, respectively. The resulting 3D shear velocity model was used to create 3D image volumes of the Ps receiver functions. The RF and tomography images are consistent with one another and withteleseismic body wave tomography (Bezada et al., submitted) Our results show high velocities from ~70 km to 230 km depth in an elliptical area just west of the Gibraltar straits which is interpreted as a near vertical slab beneath the Alboran Domain and the adjacent Spanish continental margin. The surface wave results map out the top of a 600+ km deep nearly vertical slab seen in the P body wave tomography. The RF images suggest that the top of this slab is still attached to the Alboran domain Moho beneath Gibraltar, a complex region where lower crustal velocities (

Expading fluvial remote sensing to the riverscape: Mapping depth and grain size on the Merced River, California

NASA Astrophysics Data System (ADS)

Richardson, Ryan T.

This study builds upon recent research in the field of fluvial remote sensing by applying techniques for mapping physical attributes of rivers. Depth, velocity, and grain size are primary controls on the types of habitat present in fluvial ecosystems. This thesis focuses on expanding fluvial remote sensing to larger spatial extents and sub-meter resolutions, which will increase our ability to capture the spatial heterogeneity of habitat at a resolution relevant to individual salmonids and an extent relevant to species. This thesis consists of two chapters, one focusing on expanding the spatial extent over which depth can be mapped using Optimal Band Ratio Analysis (OBRA) and the other developing general relations for mapping grain size from three-dimensional topographic point clouds. The two chapters are independent but connected by the overarching goal of providing scientists and managers more useful tools for quantifying the amount and quality of salmonid habitat via remote sensing. The OBRA chapter highlights the true power of remote sensing to map depths from hyperspectral images as a central component of watershed scale analysis, while also acknowledging the great challenges involved with increasing spatial extent. The grain size mapping chapter establishes the first general relations for mapping grain size from roughness using point clouds. These relations will significantly reduce the time needed in the field by eliminating the need for independent measurements of grain size for calibrating the roughness-grain size relationship and thus making grain size mapping with SFM more cost effective for river restoration and monitoring. More data from future studies are needed to refine these relations and establish their validity and generality. In conclusion, this study adds to the rapidly growing field of fluvial remote sensing and could facilitate river research and restoration.

SU-E-J-197: Investigation of Microsoft Kinect 2.0 Depth Resolution for Patient Motion Tracking

DOE Office of Scientific and Technical Information (OSTI.GOV)

Silverstein, E; Snyder, M

2015-06-15

Purpose: Investigate the use of the Kinect 2.0 for patient motion tracking during radiotherapy by studying spatial and depth resolution capabilities. Methods: Using code written in C#, depth map data was abstracted from the Kinect to create an initial depth map template indicative of the initial position of an object to be compared to the depth map of the object over time. To test this process, simple setup was created in which two objects were imaged: a 40 cm × 40 cm board covered in non reflective material and a 15 cm × 26 cm textbook with a slightly reflective,more » glossy cover. Each object, imaged and measured separately, was placed on a movable platform with object to camera distance measured. The object was then moved a specified amount to ascertain whether the Kinect’s depth camera would visualize the difference in position of the object. Results: Initial investigations have shown the Kinect depth resolution is dependent on the object to camera distance. Measurements indicate that movements as small as 1 mm can be visualized for objects as close as 50 cm away. This depth resolution decreases linearly with object to camera distance. At 4 m, the depth resolution had decreased to observe a minimum movement of 1 cm. Conclusion: The improved resolution and advanced hardware of the Kinect 2.0 allows for increase of depth resolution over the Kinect 1.0. Although obvious that the depth resolution should decrease with increasing distance from an object given the decrease in number of pixels representing said object, the depth resolution at large distances indicates its usefulness in a clinical setting.« less

Snow Depth Mapping at a Basin-Wide Scale in the Western Arctic Using UAS Technology

NASA Astrophysics Data System (ADS)

de Jong, T.; Marsh, P.; Mann, P.; Walker, B.

2015-12-01

Assessing snow depths across the Arctic has proven to be extremely difficult due to the variability of snow depths at scales from metres to 100's of metres. New Unmanned Aerial Systems (UAS) technology provides the possibility to obtain centimeter level resolution imagery (~3cm), and to create Digital Surface Models (DSM) based on the Structure from Motion method. However, there is an ongoing need to quantify the accuracy of this method over different terrain and vegetation types across the Arctic. In this study, we used a small UAS equipped with a high resolution RGB camera to create DSMs over a 1 km2 watershed in the western Canadian Arctic during snow (end of winter) and snow-free periods. To improve the image georeferencing, 15 Ground Control Points were marked across the watershed and incorporated into the DSM processing. The summer DSM was subtracted from the snowcovered DSM to deliver snow depth measurements across the entire watershed. These snow depth measurements were validated by over 2000 snow depth measurements. This technique has the potential to improve larger scale snow depth mapping across watersheds by providing snow depth measurements at a ~3 cm . The ability of mapping both shallow snow (less than 75cm) covering much of the basin and snow patches (up to 5 m in depth) that cover less than 10% of the basin, but contain a significant portion of total basin snowcover, is important for both water resource applications, as well as for testing snow models.

A 3-D crustal and uppermost mantle model of the western US from receiver functions and surface wave dispersion derived from ambient noise and teleseismic earthquakes

NASA Astrophysics Data System (ADS)

Shen, W.; Schulte-Pelkum, V.; Ritzwoller, M. H.

2011-12-01

The joint inversion of surface wave dispersion and receiver functions was proven feasible on a station by station basis more than a decade ago. Joint application to a large number of stations across a broad region such as western US is more challenging, however, because of the different resolutions of the two methods. Improvements in resolution in surface wave studies derived from ambient noise and array-based methods applied to earthquake data now allow surface wave dispersion and receiver functions to be inverted simultaneously across much of the Earthscope/USArray Transportable Array (TA), and we have developed a Monte-Carlo procedure for this purpose. As a proof of concept we applied this procedure to a region containing 186 TA stations in the intermountain west, including a variety of tectonic settings such as the Colorado Plateau, the Basin and Range, the Rocky Mountains, and the Great Plains. This work has now been expanded to encompass all TA stations in the western US. Our approach includes three main components. (1) We enlarge the Earthscope Automated Receiver Survey (EARS) receiver function database by adding more events within a quality control procedure. A back-azimuth-independent receiver function and its associated uncertainties are constructed using a harmonic stripping algorithm. (2) Rayleigh wave dispersion curves are generated from the eikonal tomography applied to ambient noise cross-correlation data and Helmoholtz tomography applied to teleseismic surface wave data to yield dispersion maps from 8 sec to 80 sec period. (3) We apply a Metropolis Monte Carlo algorithm to invert for the average velocity structure beneath each station. Simple kriging is applied to interpolate to the discrete results into a continuous 3-D model. This method has now been applied to over 1,000 TA stations in the western US. We show that the receiver functions and surface wave dispersion data can be reconciled beneath more than 80% of the stations using a smooth parameterization of both crustal and uppermost mantle structure. After the inversion, a 3-D model for the crust and uppermost mantle to a depth of 150 km is constructed for this region. Compared with using surface wave data alone, uncertainty in crustal thickness is much lower and as a result, the lower crustal velocity is better constrained given a smaller depth-velocity trade-off. The new 3-D model including Moho depth with attendant uncertainties provides the basis for further analysis on radial anisotropy and geodynamics in the western US, and also forms a starting point for other seismological studies such as body wave tomography and receiver function CCP analysis.

A Case For Free-free Absorption In The GPS Sources 1321+410 And 0026+346

NASA Astrophysics Data System (ADS)

Marr, Jonathan M.; Perry, T. M.; Read, J. W.; Taylor, G. B.

2010-05-01

We report on the results of VLBI observations of two gigahertz-peaked spectrum sources, 1321+410 and 0026+346, at five frequencies bracketing the spectral peaks. By comparing the three lower-frequency flux-density maps with extrapolations of the high frequency spectra we obtained maps of the optical depths as a function of frequency. The morphologies of the optical depth maps of 1321+410, at all frequencies, are strikingly uniform, consistent with there being a foreground screen of absorbing gas. We also find that the flux densities across the map fit free-free absorption spectra within the uncertainties. The required free-free optical depths are satisfied with reasonable gas parameters (ne 4000 cm-3, T 104 K, and L 1 pc). We conclude that the case for free-free absorption in 1321+410 is strong. In 0026+346, there is a compact feature with an inverted spectrum at the highest frequencies which we take to be the core. The optical depth maps, even excluding the possible core component, exhibit a noticeable amount of structure, but the morphology does not correlate with that in the flux-density maps, as would be expected if the absorption was due to synchrotron self-absorption. Additionally, the spectra (except at the core component) are consistent with free-free absorption, to within the uncertainties, and require column depths about one half of that in 1321+410. We conclude that free-free absorption by a relatively thin amount of gas with structure apparent on the scale of our maps in 0026+346 is likely, although the case is weaker than in 1321+410. This research was supported by an award from the Research Corporation, a NASA NY Space Grant, and by a Booth-Ferris Research Fellowship. The VLBA is operated by the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Lithospheric architecture of the Levant Basin (Eastern Mediterranean region): A 2D modeling approach

NASA Astrophysics Data System (ADS)

Inati, Lama; Zeyen, Hermann; Nader, Fadi Henri; Adelinet, Mathilde; Sursock, Alexandre; Rahhal, Muhsin Elie; Roure, François

2016-12-01

This paper discusses the deep structure of the lithosphere underlying the easternmost Mediterranean region, in particular the Levant Basin and its margins, where the nature of the crust, continental versus oceanic, remains debated. Crustal thickness and the depth of the lithosphere-asthenosphere boundary (LAB) as well as the crustal density distribution were calculated by integrating surface heat flow data, free-air gravity anomaly, geoid and topography. Accordingly, two-dimensional, lithospheric models of the study area are discussed, demonstrating the presence of a progressively attenuated crystalline crust from E to W (average thickness from 35 to 8 km). The crystalline crust is best interpreted as a strongly thinned continental crust under the Levant Basin, represented by two distinct components, an upper and a lower crust. Further to the west, the Herodotus Basin is believed to be underlain by an oceanic crust, with a thickness between 6 and 10 km. The Moho under the Arabian Plate is 35-40 km deep and becomes shallower towards the Mediterranean coast. It appears to be situated at depths ranging between 20 and 23 km below the Levant Basin and 26 km beneath the Herodotus Basin, based on our proposed models. At the Levantine margin, the thinning of the crust in the transitional domain between the onshore and the offshore is gradual, indicating successive extensional regimes that did not reach the beak up stage. In addition, the depth to LAB is around 120 km under the Arabian and the Eurasian Plates, 150 km under the Levant Basin, and it plunges to 180 km under the Herodotus Basin. This study shows that detailed 2D lithosphere modeling using integrated geophysical data can help understand the mechanisms responsible for the modelled lithospheric architecture when constrained with geological findings.

Geometries of geoelectrical structures in central Tibetan Plateau from INDEPTH magnetotelluric data

NASA Astrophysics Data System (ADS)

Vozar, J.; Jones, A. G.; Le Pape, F.

2012-12-01

Magnetotelluric (MT) data collected on N-S profiles crossing the Banggong-Nujiang Suture (BNS), which separates the Qiangtang and Lhasa Terranes in central Tibet, as a part of InterNational DEep Profiling of Tibet and the Himalaya project (INDEPTH) are modeled by 2D, 3D inversion codes and 1D petro-physical package LitMod. The modeling exhibits regional resistive and conductive structures correlated with ShuangHu Suture, Tanggula Mountains and strike-slip faults like BengCo-Jiali fault in the south. The BNS is not manifested in the geoelectrical models as a strong crustal regional structure. The strike direction azimuth of mid and lower crustal structures estimated from horizontal slices from 3D modeling (N110°E) is slightly different from one estimated by 2D strike analysis (N100°E). Orientation of crustal structures is perpendicular to convergence direction in this area. The deepest lower crustal conductors are correlated to areas with maximum Moho depth obtained from satellite gravity data. The anisotropic 2D modeling reveals that lower crustal conductor in Lhasa Terrane is anisotropic. This anisotropy can be interpreted as a proof for crustal channel flow below Lhasa Terrane. But same Lhasa lower crust conductor from isotropic 3D modeling can be interpreted more likely as 3D lower Indian crust structure, located to the east from line 500, than geoelectrical anisotropic crustal flow. From deep electromagnetic sounding, supported by independent integrated petro-physical investigation, we can estimate the next upper-mantle conductive layer at depths from 200 km to 250 km below the Lhasa Terrane and less resistive Tibetan lithosphere below the Qiangtang Terrane with conductive upper-mantle in depths about 120 km.

Crustal structure along the geosciences transect from Altay to Altun Tagh

USGS Publications Warehouse

Wang, Y.-X.; Han, G.-H.; Jiang, M.; Yuan, X.-C.; Mooney, W.D.; Coleman, R.G.

2004-01-01

Based upon the P- and S-wave data acquired along the geoscience transect from Altay to Altun Tagh in Northwest China, the crustal structures of velocities and Poisson's ratio are determined. The crustal velocity structure features an obvious three-layer structure with velocities of 6. 0 ??? 6. 3km/s, 6. 3 ??? 6. 6km/s and 6.9 ??? 7. Okm/s from surface to depth, respectively. The crustal thickness along the. entire profile is mostly 50km with the thickest crust (56km) beneath the Altay and the thinnest (46km) beneath the Junggar basin. The velocities underlying Moho are 7.7 to 7.8km/s between the Tianshan and the Junggar basin, and 7.9 to 8.0km/s below the Altay Mountains and eastern margin of the Tarim basin. The southern half of the profile, including the eastern Tianshan Mountains and eastern margin of the Tarim basin, shows low P-wave velocities and ?? = 0. 25 to a depth, of 30km, which suggests a quartz-rich, granitic upper crustal composition. The northern half of the profile below the Altay Mountains and Junggar Accretional Belt has a higher Poisson's ratio of ?? = 0.26 ??? 0.27 to a depth of 30km, indicative of an intermediate crustal composition, The entire profile is underlain by a 15 to 30km thick high-velocity (6.9 ??? 7.0km/s; ?? = 0. 26 - 0.28) lower crustal layer that we interpret to have a bulk composition of mafic granulite. At the southern end of the profile a 5km-thick midcrustal low-velocity layer ( Vp, = 5.9km/s, ?? = 0.25) underlies the Tianshan and the region to the south, and may be indicative of granitic intrusive in Late Paleozoic.

Relocalizing a historical earthquake using recent methods: The 10 November 1935 Earthquake near Montserrat, Lesser Antilles

NASA Astrophysics Data System (ADS)

Niemz, P.; Amorèse, D.

2016-03-01

This study investigates the hypothesis of Feuillet et al. (2011) that the hypocenter of the seismic event on November 10, 1935 near Montserrat, Lesser Antilles (MS 6 1/4) (Gutenberg and Richter, 1954) was mislocated by other authors and is actually located in the Montserrat-Havers fault zone. While this proposal was based both on a Ground Motion Prediction Equation and on the assumption that earthquakes in this region are bound to prominent fault systems, our study relies on earthquake localization methods using arrival times of the International Seismological Summary (ISS). Results of our methodology suggest that the hypocenter was really located at 16.90° N, 62.53° W. This solution is about 25 km north-west of the location proposed by Feuillet et al. (2011) within the Redonda fault system, northward of the Montserrat-Havers fault zone. As depth phases that contribute valuable insights to the focal depth are not included in the ISS data set and the reassociation of these phases is difficult, the error in depth is high. Taking into account tectonic constraints and the vertical extend of NonLinLoc's uncertainty area of the preferred solution we assume that the focus is most probably in the lower crust between 20 km and the Moho. Our approach shows that the information of the ISS can lead to a reliable solution even without an exhaustive search for seismograms and station bulletins. This is encouraging for a better assessment of seismic and tsunami hazard in the Caribbean, Mexico, South and Central America, where many moderate to large earthquakes occurred in the first half of the 20th century. The limitations during this early phase of seismology which complicate such relocations are described in detail in this study.

Anisotropic tomography of the Atlantic ocean

NASA Astrophysics Data System (ADS)

Silveira, G.; Stutzmann, E.

2003-04-01

We present a regional tri-dimensional model of the Atlantic Ocean with anisotropy. The model, derived from Rayleigh and Love phase velocity measurements, is defined from the Moho down to 300 km depth with a lateral resolution of about 500 km and is presented in terms of average isotropic S-wave velocity, azimuthal anisotropy and transverse isotropy. The cratons beneath North America, Brazil and Africa are clearly associated with fast S-wave velocity anomalies. The Mid Atlantic Ridge is a shallow structure in the North Atlantic corresponding to a negative velocity anomaly down to about 150 km depth. In contrast, the ridge negative signature is visible in the South Atlantic down to the deepest depth inverted, that is 300~km depth. This difference is probably related to the presence of hot-spots along or close to the ridge axis in the South Atlantic and may indicate a different mechanism for the ridge between the North and South Atlantic. Negative velocity anomalies are clearly associated with hot-spots from the surface down to at least 300km depth, they are much broader that the supposed size of the hot-spots and seem to be connected along a North-South direction. Down to 100 km depth, a fast S-wave velocity anomaly is extenting from Africa into the Atlantic Ocean within the zone defined as the Africa superswell area. This result indicates that the hot material rising from below does not reach the surface in this area but may be pushing the lithosphere upward. In most parts of the Atlantic, the azimuthal anisotropy directions remain stable with increasing depth. Close to the ridge, the fast S-wave velocity direction is roughly parallel to the sea floor spreading direction. The hot-spot anisotropy signature is striking beneath Bermuda, Cape Verde and Fernando Noronha islands where the fast S-wave velocity direction seems to diverge radially from the hot-spots. The Atlantic average radial anisotropy is similar to that of the PREM model, that is positive down to about 220 km, but with slightly smaller amplitude and null deeper. Cratons have a lower than average radial anisotropy. As for the velocities, there is a difference between North and South Atlantic. Most hot-spots and the South Atlantic ridge are associated with positive radial anisotropy perturbation whereas the North atlantic ridge corresponds to negative radial anisotropy perturbation.

Integrating Depth and Image Sequences for Planetary Rover Mapping Using Rgb-D Sensor

NASA Astrophysics Data System (ADS)

Peng, M.; Wan, W.; Xing, Y.; Wang, Y.; Liu, Z.; Di, K.; Zhao, Q.; Teng, B.; Mao, X.

2018-04-01

RGB-D camera allows the capture of depth and color information at high data rates, and this makes it possible and beneficial integrate depth and image sequences for planetary rover mapping. The proposed mapping method consists of three steps. First, the strict projection relationship among 3D space, depth data and visual texture data is established based on the imaging principle of RGB-D camera, then, an extended bundle adjustment (BA) based SLAM method with integrated 2D and 3D measurements is applied to the image network for high-precision pose estimation. Next, as the interior and exterior elements of RGB images sequence are available, dense matching is completed with the CMPMVS tool. Finally, according to the registration parameters after ICP, the 3D scene from RGB images can be registered to the 3D scene from depth images well, and the fused point cloud can be obtained. Experiment was performed in an outdoor field to simulate the lunar surface. The experimental results demonstrated the feasibility of the proposed method.

Mapping Curie temperature depth in the western United States with a fractal model for crustal magnetization

USGS Publications Warehouse

Bouligand, C.; Glen, J.M.G.; Blakely, R.J.

2009-01-01

We have revisited the problem of mapping depth to the Curie temperature isotherm from magnetic anomalies in an attempt to provide a measure of crustal temperatures in the western United States. Such methods are based on the estimation of the depth to the bottom of magnetic sources, which is assumed to correspond to the temperature at which rocks lose their spontaneous magnetization. In this study, we test and apply a method based on the spectral analysis of magnetic anomalies. Early spectral analysis methods assumed that crustal magnetization is a completely uncorrelated function of position. Our method incorporates a more realistic representation where magnetization has a fractal distribution defined by three independent parameters: the depths to the top and bottom of magnetic sources and a fractal parameter related to the geology. The predictions of this model are compatible with radial power spectra obtained from aeromagnetic data in the western United States. Model parameters are mapped by estimating their value within a sliding window swept over the study area. The method works well on synthetic data sets when one of the three parameters is specified in advance. The application of this method to western United States magnetic compilations, assuming a constant fractal parameter, allowed us to detect robust long-wavelength variations in the depth to the bottom of magnetic sources. Depending on the geologic and geophysical context, these features may result from variations in depth to the Curie temperature isotherm, depth to the mantle, depth to the base of volcanic rocks, or geologic settings that affect the value of the fractal parameter. Depth to the bottom of magnetic sources shows several features correlated with prominent heat flow anomalies. It also shows some features absent in the map of heat flow. Independent geophysical and geologic data sets are examined to determine their origin, thereby providing new insights on the thermal and geologic crustal structure of the western United States.

Heat flow, seismic cut-off depth and thermal modeling of the Fennoscandian Shield

NASA Astrophysics Data System (ADS)

Veikkolainen, Toni; Kukkonen, Ilmo T.; Tiira, Timo

2017-12-01

Being far from plate boundaries but covered with seismograph networks, the Fennoscandian Shield features an ideal test laboratory for studies of intraplate seismicity. For this purpose, this study applies 4190 earthquake events from years 2000-2015 with magnitudes ranging from 0.10 to 5.22 in Finnish and Swedish national catalogues. In addition, 223 heat flow determinations from both countries and their immediate vicinity were used to analyse the potential correlation of earthquake focal depths and the spatially interpolated heat flow field. Separate subset analyses were performed for five areas of notable seismic activity: the southern Gulf of Bothnia coast of Sweden (area 1), the northern Gulf of Bothnia coast of Sweden (area 2), the Swedish Norrbotten and western Finnish Lapland (area 3), the Kuusamo region of Finland (area 4) and the southernmost Sweden (area 5). In total, our subsets incorporated 3619 earthquake events. No obvious relation of heat flow and focal depth exists, implying that variations of heat flow are primarily caused by shallow lying heat producing units instead of deeper sources. This allows for construction of generic geotherms for the range of representative palaeoclimatically corrected (steady-state) surface heat flow values (40-60 mW m-2). The 1-D geotherms constructed for a three-layer crust and lithospheric upper mantle are based on mantle heat flow constrained with the aid of mantle xenolith thermobarometry (9-15 mW m-2), upper crustal heat production values (3.3-1.1 μWm-3) and the brittle-ductile transition temperature (350 °C) assigned to the cut-off depth of seismicity (28 ± 4 km). For the middle and lower crust heat production values of 0.6 and 0.2 μWm-3 were assigned, respectively. The models suggest a Moho temperature range of 460-500 °C.

Weak depth and along-strike variations in stretching from a multi-episodic finite stretching model: Evidence for uniform pure-shear extension in the opening of the South China Sea

NASA Astrophysics Data System (ADS)

Chen, Lin; Zhang, Zhongjie; Song, Haibin

2013-12-01

The South China Sea is widely believed to have been opened by seafloor spreading during the Cenozoic. The details of its lithospheric extension are still being debated, and it is unknown whether pure, simple, or conjunct shears are responsible for the opening of the South China Sea. The depth-dependent and along-strike extension derived from the single-stage finite stretching model or instantaneous stretching model is inconsistent with the observation that the South China Sea proto-margins have experienced multi-episodic extension since the Late Cretaceous. Based on the multi-episodic finite stretching model, we present the amount of lithosphere stretching at the northern continental margin of the South China Sea for different depth scales (upper crust, whole crust and lithosphere) and along several transects. The stretching factors are estimated by integrating seven deep-penetration seismic profiles, the Moho distribution derived from gravity modeling, and the tectonic subsidence data for 41 wells. The results demonstrate that the amount of stretching increases rapidly from 1.1 at the continent shelf to over 3.5 at the lower slope, but the stretching factors at the crust and lithosphere scales are consistent within error (from the uncertainty in paleobathymetry and sea-level change). Furthermore, the along-strike variation in stretching factor is within the range of 1.11-1.9 in west-east direction, accompanied by significant west-east differences in the thickness of high-velocity layers (HVLs) within the lowermost crust. This weak along-strike variation of the stretching factor is most likely produced by the preexisting contrasts in the composition and thermal structure of the lithosphere. The above observations suggest that the continental extension in the opening of the South China Sea mainly takes the form of a uniform pure shear rather than depth-dependent stretching.

Evaluating the potential for remote bathymetric mapping of a turbid, sand-bed river: 1. field spectroscopy and radiative transfer modeling

USGS Publications Warehouse

Legleiter, Carl J.; Kinzel, Paul J.; Overstreet, Brandon T.

2011-01-01

Remote sensing offers an efficient means of mapping bathymetry in river systems, but this approach has been applied primarily to clear-flowing, gravel bed streams. This study used field spectroscopy and radiative transfer modeling to assess the feasibility of spectrally based depth retrieval in a sand-bed river with a higher suspended sediment concentration (SSC) and greater water turbidity. Attenuation of light within the water column was characterized by measuring the amount of downwelling radiant energy at different depths and calculating a diffuse attenuation coefficient, Kd. Attenuation was strongest in blue and near-infrared bands due to scattering by suspended sediment and absorption by water, respectively. Even for red wavelengths with the lowest values of Kd, only a small fraction of the incident light propagated to the bed, restricting the range of depths amenable to remote sensing. Spectra recorded above the water surface were used to establish a strong, linear relationship (R2 = 0.949) between flow depth and a simple band ratio; even under moderately turbid conditions, depth remained the primary control on reflectance. Constraints on depth retrieval were examined via numerical modeling of radiative transfer within the atmosphere and water column. SSC and sensor radiometric resolution limited both the maximum detectable depth and the precision of image-derived depth estimates. Thus, although field spectra indicated that the bathymetry of turbid channels could be remotely mapped, model results implied that depth retrieval in sediment-laden rivers would be limited to shallow depths (on the order of 0.5 m) and subject to a significant degree of uncertainty.