Science.gov

Sample records for grenville orogenic cycle

  1. The Grenville Orogenic Cycle (ca. 1350-1000 Ma): an Adirondack perspective

    NASA Astrophysics Data System (ADS)

    McLelland, James; Daly, J. Stephen; McLelland, Jonathan M.

    1996-11-01

    The Adirondack Mountains are characterized by three major events that took place during the interval ca. 1350-1000 Ma. The earliest of these is the arc-related Elzevirian Orogeny (ca. 1350-1185 Ma) during which substantial volumes of juvenile calc-alkaline crust were added to the Adirondacks as well as to the northwest segment of the Central Metasedimentary Belt. Data from the southwestern United States as well as from Ireland and Baltica indicate that Elzevirian magmatism and orogeny were of global dimensions. Within the southwestern sector of the Grenville Province, the Elzevirian Orogeny culminated at ca. 1185 Ma when accretion of all outboard terranes was completed. Compressional orogeny related to this convergence resulted in overthickened crust and lithosphere which subsequently delaminated giving rise to orogen collapse and AMCG magmatism that swept southeastward from the Frontenac Terrane into the Adirondack Highlands during the interval ca. 1180-1130 Ma. Localized compressional events within neighboring parts of the Grenville Province emphasize the continued existence of contraction during this interval, although crustal extension caused local in sedimentary basins in which were deposited the Flinton and the St. Boniface Groups. The Adirondacks have not yet provided any record of events within the interval ca. 1125-1100 Ma, although there is evidence of contraction elsewhere in the southwestern Grenville Province at that time. At 1100-1090 Ma the northern Adirondack Highlands were invaded by mildly A-type hornblende granites (Hawkeye suite) that are interpreted to be the result of local crustal thinning contemporaneous with rifting and mafic magmatism taking place in the Midcontinent rift. Immediately following, at ca. 1090 Ma, the global-scale continental collision of the Ottawan Orogeny was initiated. Strong convergence, deformation, and metamorphism continued to at least ca. 1070 Ma, and rocks older than this are profoundly affected by this event

  2. The Grenville orogenic cycle of southern Laurentia: Unraveling sutures, rifts, and shear zones as potential piercing points for Amazonia

    NASA Astrophysics Data System (ADS)

    Bartholomew, Mervin J.; Hatcher, Robert D., Jr.

    2010-01-01

    The magnetic anomaly map of North America serves as a useful base from which to attempt palinspastic reconstruction of terranes accreted during the Elzevirian orogeny (1250-1200 Ma); the Shawinigan (1200-1150 Ma), Ottawan (1080-1020 Ma), and Rigolet (1020-1000 Ma) phases of the Grenvillian orogeny; and post-Grenvillian magmatism (760-600 Ma) and deformation prior to Iapetan rifting at 565 Ma. Accreted terranes had unique histories prior to amalgamation and share common tectonic events afterwards. Comparisons with magnetic signatures of the Paleozoic craton-craton suture, sutures of accreted terranes, and the Jurassic rifted-margin for the southern-central Appalachians provide a basis for discriminating among alternative Grenvillian sutures beneath the Appalachian orogen. The Elzevirian suture is partially preserved beneath the Appalachians where it separates the Reading Prong terrane from Laurentia (i.e., Adirondacks and composite-arc terrane and Canadian Grenville Province). The Shawinigan suture is partially preserved in the Llano area (Texas), but separated the now-fragmented and allochthonous Amazonian (as indicated from Pb-isotope data) blocks of the outboard Blue Ridge terrane from the Reading Prong terrane in the Appalachians. Isolated blocks of the Sauratown Mountains terrane are interpreted as outboard of the Blue Ridge terrane, but were also accreted during the Shawinigan phase. Within present-day Laurentia, the only fragment of a terrane believed to have been accreted during the main Ottawan phase is the Mars Hill terrane (North Carolina-Tennessee). This suggests that the outboard Ottawan suture may have served as the locus of Iapetan rifting along much of Laurentia. The Rigolet phase (1020-1000 Ma) is characterized by widespread "Basin and Range" type extension (NW-SE) associated with sinistral or dextral movement on the NY-AL lineament, mobilization of core-complexes (Adirondack Highlands), and AMCG magmatism along the outboard flank of the extensional

  3. Possible Subcrustal Anisotropic Fabric Beneath the Grenville Orogen

    NASA Astrophysics Data System (ADS)

    Frederiksen, A. W.; Miong, S.; Eaton, D. W.

    2004-05-01

    The Proterozoic Grenville Orogen, which forms the southeastern edge of the Canadian Shield, is the result of extensive crustal shortening and deformation during the interval 1.3-0.98 Ga. The degree to which this crustal deformation is reflected in the underlying mantle is uncertain, though LITHOPROBE detection of a preserved subduction zone (Calvert et al., 1995) in adjacent Archean terranes indicates that relict Precambrian features are preserved below the crust. A number of permanent CNSN stations (e.g., SADO, GAC, KGNO, etc.) are located on or near the Grenville, and have large archives of teleseismic data available; the more recent deployment of the dense Ontario POLARIS network provides additional constraints. We examine teleseismic receiver functions from POLARIS and CNSN stations for transverse-component energy beneath the Moho; early results indicate a complex structure involving multiple anisotropic domains that do not correlate with SKS splitting results.

  4. Exhumed Himalayan-type syntaxis in the Grenville orogen, northeastern Laurentia

    NASA Astrophysics Data System (ADS)

    Gates, Alexander E.; Valentino, David W.; Chiarenzelli, Jeffrey R.; Solar, Gary S.; Hamilton, Michael A.

    2004-04-01

    A deep-seated analog of the syntaxis developed in the Tibetan Plateau occurs in the Grenville Orogen of eastern Laurentia. During the final assembly of Rodinia, Amazonia collided with Laurentia and produced a series of large, conjugate, transcurrent, shear systems and pervasive strike-slip deformation that overprinted compressional structures related to the Ottawan Orogeny (the last orogenic phase of what is considered Grenvillian). A northeast-striking dextral system at least 35-km wide developed in the Reading Prong of New York (locally known as the Hudson Highlands), New Jersey, and Pennsylvania. U-Pb SHRIMP zircon geochronology and Ar/Ar thermochronology on the lowest grade cataclasites constrain the age of movement between 1008 and 876 Ma. A 60-km-wide, east-west striking, sinistral shear system developed across the central Adirondack Highlands. This system overprints rocks with granulite-facies metamorphic assemblages containing ca. 1050 Ma metamorphic zircons and is cut by a swarm of 950 Ma leucogranites. The timing, geometric relationships, and shear sense of the Adirondacks and Reading Prong shear systems suggest a conjugate system within a syntaxis with bulk compression directed ENE-WSW. This tectonic scenario invokes a component of strike-parallel deformation during the Ottawan Orogeny and provides a kinematic mechanism for an otherwise enigmatic, synchronous, late (ca. 930 Ma) extensional event including the Carthage-Colton mylonite zone in the northwest Adirondacks and Canada.

  5. How does the deep orogenic crust deform? The example of the Central Gneiss Belt (CGB), Grenville Province, Ontario.

    NASA Astrophysics Data System (ADS)

    Culshaw, Nicholas; Gerbi, Christopher; Marsh, Jeffrey; Slagstad, Trond

    2013-04-01

    The CGB may be understood as the product of deep crustal nappe flow requiring high strains during thickening- (P1) and flow- (P2) related phases. We attempt to describe, in terms of structural geometry and rheology, how these deformations are accommodated. The CGB consists of domains (thrust sheets) primarily composed of Proterozoic allochthonous and parautochthonous granitoid continental arc protoliths. These were either deformed at high grade (polycyclic) or never deformed (monocyclic) prior to granulite and amphibolite facies deformation in the Grenville orogeny. Grenville-age structure: polycyclic rocks do not melt and in P1-2 form narrow gneissic shear zones transposing pre-Grenville fabric or uniform domain-wide transposition gneissosity. Monocyclic rocks interlayed with polycylic form thick uniform migmatite sheets. Monocyclic domains not associated with polycyclic units form nappe complexes of highly strained granulite gneiss (P1) or migmatite sheets (P2). At later stages of progressive deformation accompanying nappe flow (P2), gneissosity of monocyclic rocks may be deformed coplanar with P1 gneissosity or form spectacular shear zone systems (amphibolite facies-on-granulite facies, or amphibolite-on-amphibolite). Overall, tracts of uniform deformation (gneiss domains/thrust sheets) are dominant over discrete shear zones. Grenville-age rheology: the largest scale rheological gradient, that between the Archean foreland, showing minor Grenville deformation, and highly deformed Proterozoic arc rocks, depends on the latters' protolith age and history as upper plate in a convergent margin. On the oceanward orogen margin, contrasting arc and back-arc properties resulted in P1 granulite- to amphibolite grade juxtaposition of upper- and deep crustal lithologies. At smaller scale, melting of fertile monocyclic granitoids is a major process, controlling formation of the large uniformly deformed thrust sheets (few internal strain gradients) and low competency layers

  6. A reconsideration of Pan African orogenic cycle in the Anti-Atlas Mountains, Morocco

    NASA Astrophysics Data System (ADS)

    Hefferan, Kevin; Soulaimani, Abderrahmane; Samson, Scott D.; Admou, Hassan; Inglis, Jeremy; Saquaque, Ali; Latifa, Chaib; Heywood, Neil

    2014-10-01

    For over 50 years the Pan African orogeny has been recognized as a Neoproterozoic tectonothermal episode affecting West Africa 800-550 Ma. As such, the Pan African events are similar to the Appalachian orogenic cycle extending from ∼1100 to 250 Ma and the Cordilleran orogenic cycle of 350 Ma to the present. A significant difference is that the Appalachian orogenic cycle has long been recognized as consisting of separate Grenville, Taconic, Acadian and Alleghenian orogenies. Similarly, the Cordilleran orogenic cycle consists of distinct Antler, Sonoma, Nevadan, Sevier, Laramide and ongoing Cascadian-Andean orogenies. Failure to distinguish individual tectonic events in the Anti-Atlas Mountains has been attributable to the dearth of radiometric dates in this region. Since 2000, precision geochronologic dating in the Anti-Atlas Mountains, Morocco, has provided a means by which it is now appropriate to designate a Pan African orogenic cycle consisting of separate distinct orogenic events. We herein propose the following distinct orogenic events in the Anti-Atlas Mountains of Morocco: Iriri-Tichibanine orogeny (760-700 Ma), Bou Azzer orogeny (680-640 Ma) and the WACadomian orogeny (620-555 Ma).

  7. The Carthage-Colton mylonite zone (Adirondack Mountains, New York): The site of a cryptic suture in the Grenville Orogen

    SciTech Connect

    Mezger, K.; Pluijm, B.A. van der; Essene, E.J.; Halliday, A.N. )

    1992-09-01

    U-Pb ages were determined on metamorphic sphenes and monazites from the Late Proterozoic Adirondack Highlands and Lowlands in the vicinity of the Carthage-Colton mylonite zone. Monazites were extracted from metapelites, and sphenes were separated from marbles, calc-silicate gneisses, and granite gneisses in order to determine the timing and the duration of metamorphism as well as the cooling histories for rocks on either side of the mylonite zone. Monazite ages from the Lowlands range from 1,171-1,137 Ma; sphene ages in the Lowlands range from 1,156-1,103 Ma, those from the Highlands immediately to the east of the mylonite zone range from 1,050-982 ma. The ages indicate that the last high-grade metamorphism in the Highlands is ca. 100 m.y. younger than in the Lowlands and that both terranes had separate cooling histories at least until ca. 1,000 Ma. Sphenes from within the Carthage-Colton mylonite zone yield ages of about 1,098 Ma, which are distinct from sphene ages on either side of the shear zone. The mineral ages, structures, and metamorphic histories suggests that the Carthage-Colton mylonite zone is a fundamental tectonic boundary within the Proterozoic Grenville Orogen of North America.

  8. A Reconsideration of Pan African Orogenic Cycle in the Anti-Atlas Mountains, Morocco

    NASA Astrophysics Data System (ADS)

    Hefferan, K. P.; Soulaimani, A.; Samson, S. D.; Admou, H.; Inglis, J.; Saquaque, A.; Heywood, N. C.

    2013-12-01

    The term 'Pan African' orogeny was first proposed in 1964 for a tectonothermal event in Africa ~ 500+/- 50 Ma. Over the past 50 years, the Pan African orogeny has been extended to as much as ~1050-450 Ma and recognized in other Gondwanan continents where regional names such as Brasiliano (South America), Adelaidean (Australian) and Bearmore (Antarctica) have been applied. The Pan African time span of ~500 million years is much longer than any Phanerozoic orogeny. However, it does correlate with time ranges of well defined Phanerozoic orogenic cycles such as the Appalachian cycle, extending from ~1,100 to 250 Ma, and the Cordilleran cycle of ~350 Ma to the present. A significant difference of course is that the Appalachian orogenic cycle has long been recognized as consisting of separate Grenville, Taconic, Acadian and Alleghenian orogenies. Similarly, the Mesozoic-Cenozoic Cordilleran orogenic cycle consists of distinct Antler, Sonoma, Nevadan, Sevier, Laramide and ongoing Cascadian-Andean orogenies. Until recently, the absence of precise geochronology in West Africa has prevented a more refined analysis of individual orogenic events within the Pan-African orogenic cycle. Since 2000, precision geochronologic dating by various researchers in the Anti-Atlas Mountains, Morocco, has provided critical data by which it is now appropriate to designate a Pan African orogenic cycle consisting of three separate orogenic events. We herein propose the following distinct orogenic events in the Anti-Atlas Mountains of Morocco: Iriri-Tichibanine orogeny (750-700 Ma), Bou Azzer orogeny (660-640 Ma) and the WACadomian orogeny (620-580 Ma). Generalized tectonic map of the Anti-Atlas Mountain inliers (Adapted from Ennih and Liégeois, 2008). Geodynamic model of the Pan African orogenic cycle in the Anti-Atlas Mountains. Modified from Walsh et al. (2012) and incorporating ideas from Thomas et al. (2002) and El Hadi et al. (2012).

  9. The fate of olivine in the lower crust: Pseudomorphs after olivine in coronitic metagabbro from the Grenville Orogen, Ontario

    NASA Astrophysics Data System (ADS)

    Kendrick, J. L.; Jamieson, R. A.

    2016-09-01

    Orthopyroxene-oxide symplectites after olivine are among the most enigmatic features of corona assemblages in metagabbros. Two coronitic metagabbro bodies from the Algonquin suite in the Grenville Orogen, Ontario, contain exceptionally well preserved orthopyroxene + Fe-Ti oxide symplectite formed during prograde Ottawan (ca. 1060 Ma) granulite-facies metamorphism. Based on textural evidence, we propose a new hypothesis for the formation of these symplectites. Under oxidising conditions associated with fluid infiltration, magmatic olivine and ilmenite underwent a coupled reaction whereby magnetite produced by oxidation of olivine replaced adjacent igneous ilmenite. Ilmenite was re-precipitated as a fine-grained intergrowth with orthopyroxene and some magnetite in the former olivine sites. This hypothesis is supported by textural evidence showing partial replacement of magmatic ilmenite by magnetite and a close spatial association between magmatic oxides and orthopyroxene + Fe-Ti oxide symplectite, which locally radiates from ilmenite into olivine. Measured orthopyroxene/oxide ratios in the symplectite (20-35% oxides) agree with the ratio predicted from the proposed reaction (ca. 30%). Coronas and pseudomorphs formed during high-grade metamorphism, with increasing fO2 interpreted to result from fluid infiltration at near-peak conditions of ca. 13 kbar, 800 °C. The same samples contain red-brown fine-grained aggregates interpreted as iddingsite pseudomorphs after olivine. Raman spectroscopy suggests that the iddingsite consists largely of amorphous silica and Fe-hydroxide; textural evidence indicates that it formed by late-stage oxidation and hydration of olivine that survived earlier metamorphism. The unusual co-occurrence of granulite-facies pseudomorphs after olivine with an alteration product formed at near-surface conditions indicates that some olivine may survive protracted high-grade metamorphism in environments where fluid access is limited.

  10. Cooling and inferred uplift/erosion history of the Grenville Orogen, Ontario: Constraints from sup 40 Ar/ sup 39 Ar thermochronology

    SciTech Connect

    Cosca, M.A.

    1989-01-01

    Thermochronological ({sup 40}Ar/{sup 39}Ar) data are presented from 76 mineral separates of hornblende, muscovite, biotite, phlogopite, and K-feldspar. Samples were selected from regionally metamorphosed gneiss, amphibolite, metasediment, marble, metagabbro and pegmatite across the two major metamorphic belts of the Grenville Province, the Central Metasedimentary Belt (CMB) and the Central Gneiss Belt (CGB). When combined with published temperature estimates for closure to argon diffusion in the phases analyzed, cooling rates from {approximately}500 C to {approximately}120 C of 1-4 C/MA are calculated across the entire Grenville Province of Ontario. Regional uplift/erosion rates for the Grenville Orogen of Ontario have been estimated from the {sup 40}Ar/{sup 39}Ar data, a retrograde P-T path for rocks of the CGB, and an upper time constraint provided by flat, overlying Cambro-Ordovician sediments. Twenty-two of the hornblendes used for thermochronology have been quantitatively analyzed for major elements by microprobe, Fe{sup 2+}/Fe{sup 3+} by wet chemistry, and for H{sub 2}O by manometric measurement. Water activities calculated from hornblende equilibria are typically low (<0.01) because of the exponential dilutions in hornblende (tremolite) activity required by present activity-composition models. An oxyamphibole component of 25% further reduces any amphibole component and the H{sub 2}O activity by as much as 50% below that calculated with simplifying assumption. These findings indicate that different amphibole normalization schemes have a marked effect on the activity calculated for a specific amphibole or H{sub 2}O, and should be carefully evaluated.

  11. Reconstructing deep crustal dynamics in a large, hot orogen: Application of integrated zircon petrochronology and petrological modeling to the Canadian Grenville Province

    NASA Astrophysics Data System (ADS)

    Marsh, Jeffrey; Culshaw, Nicholas

    2014-05-01

    The timing and conditions of high pressure (HP) metamorphism, crustal anatexis, and shear zone formation are of primary importance in understanding lithosphere-scale dynamics of collisional orogens. Within the western Grenville Province, Ontario, Canada, a number of structural and metamorphic relationships are preserved that represent specific orogenic stages or processes. Lower allochthonous domains contain variably retrogressed mafic complexes that typically retain vestages of earlier HP metamorphic assemblages (e.g. garnet porphyroblasts, kyanite, and rutile) within a matrix dominated by incomplete decompression reactions (e.g. pseudomorphous diopside + sodic plagioclase intergrowths after omphacite and concentric coronas of aluminous minerals surrounding kyanite). Zircon from these samples yield U-Pb ages between 1085-1097 Ma, and exhibit REE characteristics consistent with crystallization in an eclogite-facies (garnet-rich, plagioclase-poor) mineral assemblage. REE partitioning between zircon and garnet suggests zircon growth coincided with the latter stages of garnet growth. Titanium concentrations in zircon constrain crystallization temperatures between ~678-736 °C, whereas Zr concentration in rutile yield crystallization temperatures of ~705-740 °C (for rutile inclusions in garnet) and 742-764 °C (for rutile in the matrix). Intersection of zircon and rutile crystallization temperature with the calculated stability field for the HP assemblage (Grt+Cpx+Ky+Rt+Zrn±Hbl) yields minimum pressures of ~15 kbar. Thus, HP metamorphism apparently occurred at ca. 1090 Ma across the orogen, at minimum depths of ~53 km and T ~700±50 °C, yielding a geothermal gradient of < 15 °C/km. Widespread high-T, medium-P metamorphism and migmatization in the surrounding gneisses apparently began around 1090-1080 Ma, suggesting a rapidly evolving thermal field synchronous with the exhumation of HP rocks to mid-crustal depths. Zircon U-Pb ages and REE patterns from a large

  12. Visualizing the sedimentary response through the orogenic cycle using multi-dimensional scaling

    NASA Astrophysics Data System (ADS)

    Spencer, C. J.; Kirkland, C.

    2015-12-01

    Changing patterns in detrital provenance through time have the ability to resolve salient features of an orogenic cycle. Such changes in the age spectrum of detrital minerals can be attributed to fluctuations in the geodynamic regime (e.g. opening of seaways, initiation of subduction and arc magmatism, and transition from subduction to collisional tectonics with arrival of exotic crustal material). These processes manifest themselves through a variety of sedimentary responses due to basin formation, transition from rift to drift sedimentation, or inversion and basement unroofing. This generally is charted by the presence of older detrital zircon populations during basement unroofing events and is followed by a successive younging in the detrital zircon age signature either through arrival of young island arc terranes or the progression of subduction magmatism along a continental margin. The sedimentary response to the aforementioned geodynamic environment can be visualized using a multi-dimensional scaling approach to detrital zircon age spectra. This statistical tool characterizes the "dissimilarity" of age spectra of the various sedimentary successions, but importantly also charts this measure through time. We present three case studies in which multi-dimensional scaling reveals additional useful information on the style of basin evolution within the orogenic cycle. The Albany-Fraser Orogeny in Western Australia and Grenville Orogeny (sensu stricto) in Laurentia demonstrate clear patterns in which detrital zircon age spectra become more dissimilar with time. In stark contrast, sedimentary successions from the Meso- to Neoproterozoic North Atlantic Region reveal no consistent pattern. Rather, the North Atlantic Region reflects a signature consistent with significant zircon age communication due to a distal position from an orogenic front, oblique translation of terranes, and complexity of the continental margin. This statistical approach provides a mechanism to

  13. Sensitive high resolution ion microprobe (SHRIMP) detrital zircon geochronology provides new evidence for a hidden neoproterozoic foreland basin to the Grenville Orogen in the eastern Midwest, U.S.A

    USGS Publications Warehouse

    Santos, J.O.S.; Hartmann, L.A.; McNaughton, N.J.; Easton, R. M.; Rea, R.G.; Potter, P.E.

    2002-01-01

    A sensitive high resolution ion microprobe (SHRIMP) was used in combination with backscattered electron (BSE) and cathodoluminescence (CL) images to determine the age of detrital zircons from sandstones in the Neoproterozoic Middle Run Formation of the eastern Midwest, United States. Eleven samples from seven drill cores of the upper part of the Middle Run Formation contain detrital zircons ranging in age from 1030 to 1982 Ma (84 analyses), with six distinctive modes at 1.96, 1.63, 1.47, 1.34, 1.15, and 1.08 Ga. This indicates that most, but not all, of the zircon at the top of the Middle Run Formation was derived from the Grenville Orogen. The youngest concordant detrital zircon yields a maximum age of 1048 ?? 22 Ma for the Middle Run Formation, indicating that the formation is younger than ca. 1026 Ma minus the added extra time needed for later uplift, denudation, thrusting, erosion, and transport to southwestern Ohio. Thus, as judged by proximity, composition, thickness, and geochronology, it is a North American equivalent to other Neoproterozoic Grenvillian-derived basins, such as the Torridon Group of Scotland and the Palmeiral Formation of South America. An alternate possibility, although much less likely in our opinion, is that it could be much younger, any time between 1048 ?? 22 Ma and the deposition of the Middle Cambrian Mount Simon Sandstone at about 510 Ma, and still virtually almost all derived from rocks of the Grenville Orogen.

  14. Juvenile Middle Proterozoic crust in the Adirondack Highlands, Grenville province, northeastern North America

    SciTech Connect

    Daly, J.S. ); McLelland, J.M. )

    1991-02-01

    Nd isotope data indicate that minimal amounts of significantly older crust have contributed to the genesis of the oldest (ca. 1.3-13.5 Ga) plutons in the Adirondack Highlands. These are magmatic arc tonalites with positive initial {epsilon}{sub Nd} values and Sm-Nd depleted mantle model ages (t{sub DM}) that are within 70 m.y. of the time of their crystallization. Granitoids of the anorthosite-mangerite-charnockite-granite suite, dated at 1,156-1,134 Ma, as well as the 1,100-1,050 Ma plutons, associated with the Ottawan phase of the Grenvillian orogenic cycle, also have positive initial {epsilon}{sub Nd} values and t{sub DM} ages similar to the tonalites. Derivation of both groups of granitoids by crustal melting of the magmatic arc is consistent with the available isotopic and geochemical data. Juvenile late Middle Proterozoic crust that formed during or just prior to the Grenville cycle appears to dominate the southwestern Grenville province as well as the Grenville inliers to the south. In contrast, most of the contiguous Grenville province in Canada comprises largely reworked older crust.

  15. Multiple thermal events in the Grenvillian orogenic cycle: geochronologic evidence from the northern Reading Prong, New York-New Jersey

    SciTech Connect

    Grauch, R.I.; Aleinikoff, J.N.

    1985-01-01

    The Grenville terrane of the northern Reading Prong, New York-New Jersey, has a far more complex thermal history than previously recognized. U-Pb zircon ages record at least five discrete thermal events between about 1300 and 900 Ma: (1) 1230 +/- 20 Ma--metamorphism of paragneiss; (2) 1150 +/- 30 Ma--crystallization of Storm King-type granite, protoliths of a leucogneiss, and a member of the Fordham Gneiss; (3) 1090 +/- 10 Ma--crystallization of a hornblende gneiss and the Canopus pluton; (4) 1015 +/- Ma--regional metamorphism, crystallization of the Canada Hill granite and pegmatitic alaskites; (5) 965 +/- 10 Ma--crystallization of pegmatites, formation of a monazite-xenotime rock, and closure of the U-Pb whole-rock system. Another problem that arises from these results is the past correlation throughout the Reading Prong of lithologic sequences whose zircon ages apparently do not agree. This complex sequence of Middle Proterozoic thermal events in the central Appalachians correlates fairly well with events recorded in the Grenville terrane of eastern Ontario, where at least two major orogenic events between 1300 and 900 Ma have been identified. The fit with identified events in the Adirondacks of northern New York and with Precambrian events of the southern Appalachians is not as good, but the early approx. = 1230 Ma and the late approx. = 960 Ma events seem to be present in both regions.

  16. Sedimentary basinal responses to a Late Precambrian Wilson Cycle: the Damara Orogen and Nama Foreland, Namibia

    NASA Astrophysics Data System (ADS)

    Stanistreet, Ian G.; Kukla, Peter A.; Henry, George

    In the Damara Orogen sedimentary basinal responses are important in recording the evolution of the fold belt. Here we integrate sedimentological patterns and tectonics to characterise the basin development of both the pre- to syn-orogenic Damara Sequence and the syn- to post-orogenic Nama Group. The evolution of an entire Late Proterozoic Wilson Cycle involved initial rifting, with the opening of two oceanic arms through convergence to collision and foreland basin development. Rift initiation (stage 1) took place along old tectonic weaknesses and extensional rift basins (stage 2) were filled by continental sediments and alkaline/bimodal volcanics. Two oceanic openings occurred: (i) the Adamastor Ocean (stage 3) produced a break-up unconformity and eastward transgression over the Kalahari and Congo Cratons; and (ii) the Khomas Sea gulf subsequently developed betwen the two cratons (stage 4) and is associated with break-up unconformities, and ultimately the development of mature shleves (stage 5). In the latter opening, we envisage an anticlockwise rotation of the Kalahari Craton with respect to the Congo Craton. During convergence the closing Khomas Sea produced an accretionary prism/arc/retro-arc system (stage 6) and the first deformation phase in the Southern Zone. The Khomas Orogeny records the collision between the Kalahari and Congo Cratons (stage 7) including the obduction of oceanic elements onto the Kalahari Craton foreland, and caused the second and third deformation phases in the Southern Zone and first and second deformation phases in the Central Zone. A peripheral foreland basin and peripheral bulge on the Kalahari Craton resulted, which respectively contained and affected the marine and fluvial Nama Group sedimentation. A complementary hinterland basin accepted Mulden Group sediments on the Congo Craton. Ultimately the collision of the South American continent with the newly reconstituted African foreland (stage 8) caused the Adamastor Orogeny and

  17. Paleomagnetism and Thermochronometry of the Grenville Province, Canada

    NASA Astrophysics Data System (ADS)

    Dunlop, D. J.

    2009-05-01

    The Grenville Province, a marginal orogen of the Canadian Precambrian Shield, extends through southern Ontario, Québec and Labrador, with equivalents in Fennoscandia. Before the Iapetus Ocean opened, the Canadian and Fennoscandian orogens were contiguous. Age-equivalent rocks in small exposures in the Appalachians, Texas and Mexico suggest that the subsurface extent of Grenvillia is comparable to that of the Superior Province, the Archean core of the Canadian Shield. It was therefore a substantial continent in its own right at the time it collided with the Superior, Southern and Nain Provinces. However, most of our paleomagnetic knowledge comes from a narrow zone on the (present-day) northern margin of Grenvillia, comprising the Grenville Front Tectonic Zone of reactivated rocks as old as Archean, the Central Gneiss Belt (CGB) with deeply buried and exhumed Proterozoic rocks, and the Central Medisedimentary Belt (CMB), containing a core of less deeply buried rocks where some pre-collisional paleomagnetic information has survived. 40Ar/39Ar thermochronometry has been vital in determining the age relationships of paleomagnetic overprints (often two, occasionally even three, in individual formations) and establishing the Grenville Track of the Laurentian APWP. Most 40Ar/39Ar mineral ages are in the 850 to 1000 Ma range but there is enough separation between key tiepoint poles to permit clockwise and counterclockwise alternatives for the Grenville "Loop". Of great interest are the short track segments defined by mean poles for different domains of the CGB and terranes of the CMB because these may record the sequence of assemblage and/or burial and uplift. Unfortunately the profound contrast between uplift ages of different CMB terranes, as indicated by their hornblende, mica and feldspar 40Ar/39Ar data, is not borne out by the relatively minor differences in mean poles for the same terranes. The "Grenville problem" is therefore still not laid to rest, and furthermore

  18. Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids.

    PubMed

    Camacho, Alfredo; Lee, James K W; Hensen, Bastiaan J; Braun, Jean

    2005-06-30

    Collision tectonics and the associated transformation of continental crust to high-pressure rocks (eclogites) are generally well-understood processes, but important contradictions remain between tectonothermal models and petrological-isotopic data obtained from such rocks. Here we use 40Ar-39Ar data coupled with a thermal model to constrain the time-integrated duration of an orogenic cycle (the burial and exhumation of a particular segment of the crust) to be less than 13 Myr. We also determine the total duration of associated metamorphic events to be approximately 20 kyr, and of individual heat pulses experienced by the rocks to be as short as 10 years. Such short timescales are indicative of rapid tectonic processes associated with catastrophic deformation events (earthquakes). Such events triggered transient heat advection by hot fluid along deformation (shear) zones, which cut relatively cool and dry subducted crust. In contrast to current thermal models that assume thermal equilibrium and invoke high ambient temperatures in the thickened crust, our non-steady-state cold-crust model satisfactorily explains several otherwise contradictory geological observations. PMID:15988516

  19. The Influence of Glacial-Interglacial Cycles on the Erosion of Orogens

    NASA Astrophysics Data System (ADS)

    Yanites, Brian; Ehlers, Todd

    2010-05-01

    The evolution of mountain topography and sediment flux to adjacent basins is dictated by variations in the rates of rock-uplift, climate, lithology, and vegetation. Currently, many mountainous settings are in a state of a ‘glacial hangover' whereby Quaternary glaciation has dramatically altered catchment morphology and produced non-equilibrium conditions with respect to the environmental conditions preceding this major climatic transition. In this study, we investigate transients in mountain erosion and morphology due to glacial-interglacial cycles imposed on landscapes previously dominated by fluvial and hillslope processes. In our approach, we use a surface process model to produce an equilibrium fluvial landscape for rock uplift rates between 0.25-1.0 mm/yr. The landscapes are then subjected to repeated glacial cycles of different periodicity and intensity. Variations in predicted glacial basal sliding velocity, erosion, topography and sediment flux are tracked. Results indicate that glacial processes increase rates of valley bottom erosion by one to two orders of magnitude higher than fluvial processes, a result consistent with low-temperature thermochronological data from a number of glaciated catchments worldwide. Increased rates of hillslope and ridgetop erosion occur in response to increased glacial erosion and lag behind the onset of glaciation, thereby producing a complicated history of local relief. The timescale of this lag can vary by orders of magnitude and depends on model parametrization. We also find that two broad effects compete to control the evolution of sediment leaving such an orogen: 1) the topographic disequilibrium with glacial processes acts to initially increase sediment production, but as the topography readjusts, the disequilibrium wanes; 2) the initial geometry of the drainage basin is inefficient at providing ice to the sliding portions of the glaciers, thus impeding erosion early on, but as the topography becomes more

  20. Deep Crustal Metamorphic Carbon Cycling in Collisional Orogens: What do we Really Know?

    NASA Astrophysics Data System (ADS)

    Ague, J.

    2012-12-01

    The classic study of Bowen (1940) showed that CO2 is lost during metamorphic heating of common carbonate-bearing rocks. Despite tremendous progress in our understanding of devolatilization since Bowen's time, the fact remains that the metamorphic CO2 flux from active mountain belts is one of the most poorly-understood components of the global C cycle. A basic question is: do large fluid fluxes accompany collisional orogenesis? It has been argued that metamorphic rocks are overpressured and, thus, permeability is low and fluid fluxes are limited. In contrast, field-based studies commonly estimate large time-integrated fluxes of the order of 103 m3 m-2 or more. These opposing viewpoints can be reconciled by recognizing that large fluxes can pass through rocks on geologic timescales even if permeability is low, and that transient events such as fracturing can greatly increase permeability and facilitate flow. Large regional fluxes make possible significant CO2 transfer from the deep roots of mountain belts to the shallow hydrosphere and atmosphere. Important prograde CO2 release processes include "internally-buffered" reactions, fluid infiltration, carbonate mineral dissolution, and oxidation of graphite (or diamond). The relative roles of these processes, particularly the latter two, are topics of active research. Regardless of process, however, field-based results commonly show prograde loss of 10 to 15 kg CO2 per 100 kg rock for a range of carbonate mineral-bearing lithologies from sub-greenschist to upper amphibolite facies metamorphism. Could this CO2 make a real difference to C cycling? A new assessment of the mean loss of CO2 from metamorphic sequences in New England, USA, for example, yields a minimum release rate of about 1018 mol Ma-1 during the Acadian orogeny (Devonian). This is comparable to Kerrick and Caldeira's (1993) threshold needed to affect global atmospheric temperatures. Flow around large, deep-seated intrusions could easily produce even larger

  1. Paleomagnetism and Monazite Dating of Grenville Rocks, Adirondack Mountains, NY

    NASA Astrophysics Data System (ADS)

    Brown, L.; McEnroe, S.; Jercinovic, M.

    2003-12-01

    Paleomagnetic studies on three rock units from the Adirondack Highlands, New York State yield stable magnetic directions. Electron microprobe monazite geochronology suggests a strong ca. 1050 Ma signature, corresponding to Ottawan granulite-facies metamorphism. Remnants of older (ca. 1130-1190 Ma) monazite, consistent with early-Grenville tectonomagmatic events are also documented. There is no evidence of younger (<1050 Ma) events with the exception of partial alteration (with Ca-enrichment) of some monazite. Sillimanite-microcline gneisses (gms) of the far-western Highlands, associated with negative aeromagnetic anomalies, exhibit strong stable magnetization dominated by titanohematite with abundant exsolutions of ilmenite, pyrophanite, rutile and spinel. Mean magnetic directions for 14 sites are I-62.8, D=289.2 and a-95=7.6. Sampled in the central Highlands is the post-orogenic fayalite ferro-hedenbergite Wanakena Granite. Samples contain magnetite with ilmenite oxy-exsolution, occurring as discrete grains and inclusions in silicates. Directions from the Wanakena are steeply negative with westerly declinations (I=-76.4, D=296.7, a-95=4.4, N=7). The Marcy meta-anorthosite was sampled in the central and eastern Highlands, although many of these sites proved unstable. Stable results were combined with unpublished data from Rob Hargraves for 13 sites (I=-64.4, D=286.2, a-95=9.1). Over half of the anorthosites and one gms site have normal directions; all Wanakena sites are reversed. Combined anorthosites and gms units give a pole position of 20S/151E; the Wanakena pole is at -29S/132E. Both poles fall in the southern extent of the Grenville loop. The thermodynamically constrained equilibrium phase diagram for ilm-hem predicts that very fine exsolution, most likely responsible for the stable magnetization of the gms rocks, starts to form around 390C, well below the conditions of granulite grade metamorphism. The abundant lamellae provide a stable NRM through the

  2. The Midcontinent Rift and Grenville connection

    SciTech Connect

    Cambray, F.W.; Fujita, K. . Dept. of Geological Sciences)

    1994-04-01

    The Mid-Proterozoic, Midcontinent Rift System (MRS) is delineated by an inverted U shaped gravity and magnetic anomaly. It terminates in southeast Michigan but a less continuous series of anomalies and sediments, the Eastcontinent Rift occur on a north-south line through Ohio and Kentucky. The geometry allows for a north-south opening, the Lake Superior section being orthogonal to opening, the western arm transtensional and the north-south trending eastern arm a transform boundary offset by pull-apart basins. The opening and closing of the rift overlaps in time with the Grenville Orogeny. Grenville age rocks can also be found in the Llano uplift of Texas. The authors propose a model to explain the temporal and geographic association of the opening and closing of the MRS with the Grenville Orogeny that involves irregular suturing between two continental masses. Initiation of Grenville suturing, associated with south dipping subduction, in the northeast and in the Llano area of Texas would leave portion of unclosed ocean in between. Tensional stresses in the continental crust adjacent to the oceanic remnant could lead to its fragmentation and the formation of the MRS. The remaining oceanic lithosphere would eventually subduct, limiting the opening of the MRS. Continued convergence of the plates would induce compressional stresses thus accounting for the deformation of the MRS. An analogy is made with more recent opening of the Red Sea, Gulf of Aden Rift System in association with irregular collision along the Zagros-Bitlis Sutures.

  3. Linking collisional and accretionary orogens during Rodinia assembly and breakup: Implications for models of supercontinent cycles

    NASA Astrophysics Data System (ADS)

    Cawood, Peter A.; Strachan, Robin A.; Pisarevsky, Sergei A.; Gladkochub, Dmitry P.; Murphy, J. Brendan

    2016-09-01

    Periodic assembly and dispersal of continental fragments has been a characteristic of the solid Earth for much of its history. Geodynamic drivers of this cyclic activity are inferred to be either top-down processes related to near surface lithospheric stresses at plate boundaries or bottom-up processes related to mantle convection and, in particular, mantle plumes, or some combination of the two. Analysis of the geological history of Rodinian crustal blocks suggests that internal rifting and breakup of the supercontinent were linked to the initiation of subduction and development of accretionary orogens around its periphery. Thus, breakup was a top-down instigated process. The locus of convergence was initially around north-eastern and northern Laurentia in the early Neoproterozoic before extending to outboard of Amazonia and Africa, including Avalonia-Cadomia, and arcs outboard of Siberia and eastern to northern Baltica in the mid-Neoproterozoic (∼760 Ma). The duration of subduction around the periphery of Rodinia coincides with the interval of lithospheric extension within the supercontinent, including the opening of the proto-Pacific at ca. 760 Ma and the commencement of rifting in east Laurentia. Final development of passive margin successions around Laurentia, Baltica and Siberia was not completed until the late Neoproterozoic to early Paleozoic (ca. 570-530 Ma), which corresponds with the termination of convergent plate interactions that gave rise to Gondwana and the consequent relocation of subduction zones to the periphery of this supercontinent. The temporal link between external subduction and internal extension suggests that breakup was initiated by a top-down process driven by accretionary tectonics along the periphery of the supercontinent. Plume-related magmatism may be present at specific times and in specific places during breakup but is not the prime driving force. Comparison of the Rodinia record of continental assembly and dispersal with that

  4. Chronologic constraints on the tectonic evolution of the Wilson Lake terrane of the Grenville Province, Canada

    NASA Astrophysics Data System (ADS)

    Reno, B. L.; Korhonen, F. J.; Stout, J. H.; Waight, T.

    2010-12-01

    The Wilson Lake terrane in central Labrador, Canada is one of a number of terranes that make up the Grenville Province, representing the northern extent of the Grenville Orogen in North America. Many of these terranes record evidence of two orogenies: the Labradorian Orogeny at ca. 1710-1600 Ma, and the Grenville Orogeny at ca. 1080-980 Ma. The rocks in the Wilson Lake terrane are interpreted to have been subjected to peak pressures of ~0.95 GPa and ~930°C during the Labradorian Orogeny (Korhonen et al., in prep., Stability of sapphirine + quartz in the Wilson Lake terrane: calculated equilibria in NCKFMASHTO). The final amalgamation of the Wilson Lake terrane over the underlying Parautochthonous Belt is interpreted to have occurred during the Grenville Orogeny, when the terrane was subjected to a lower-T (500-350°C) overprinting. However, petrologic and chronologic evidence for the Grenville orogeny is limited in the Wilson Lake terrane. Here we present results from a monazite chemical (U-Th)-Pb chronologic study in order to provide constraints on the metamorphic history of the Wilson Lake terrane. Monazite was analyzed in samples of orthopyroxene + sillimanite + quartz bearing and sapphirine + quartz bearing gneisses from throughout the Wilson Lake terrane. These samples contain two distinct populations of monazite: 1) a population of large (up to ~500 μm) monazite exhibits distinct core and rim zoning in yttrium X-ray compositional maps, and occurs predominately in the melanosome of the rocks, and 2) a population of smaller (up to ~50 μm) unzoned monazite rarely occurs in quartz-rich layers of the rocks. In a majority of the melanosome-hosted monazite, (U-Th)-Pb chemical ages yield cores and rims with statistically similar Labradorian ages of ca. 1705-1675 Ma. However, one sample from the middle of the terrane yields monazite grains with Labradorian age cores (ca. 1710 Ma) and post-Labradorian rims (ca. 1590 Ma). Monazite from the second, quartz

  5. Structural investigation of the Grenville Province by radar and other imaging and nonimaging sensors

    NASA Technical Reports Server (NTRS)

    Lowman, P. D., Jr.; Blodget, H. W.; Webster, W. J., Jr.; Paia, S.; Singhroy, V. H.; Slaney, V. R.

    1984-01-01

    The structural investigation of the Canadian Shield by orbital radar and LANDSAT, is outlined. The area includes parts of the central metasedimentary belt and the Ontario gneiss belt, and major structures as well-expressed topographically. The primary objective is to apply SIR-B data to the mapping of this key part of the Grenville orogen, specifically ductile fold structures and associated features, and igneous, metamorphic, and sedimentary rock (including glacial and recent sediments). Secondary objectives are to support the Canadian RADARSAT project by evaluating the baseline parameters of a Canadian imaging radar satellite planned for late in the decade. The baseline parameters include optimum incidence and azimuth angles. The experiment is to develop techniques for the use of multiple data sets.

  6. Acadian orogen Which Acadian orogen

    SciTech Connect

    Ludman, A. )

    1993-03-01

    The Acadian orogeny is widely viewed as the climactic event in much of the Northern Appalachians, resulting from the closing of one or more ocean basins and the accretion of the Avalonian continent to ancestral North America. Unconformities have traditionally dated the Acadian orogeny as Middle Devonian in the east and Late Middle to Late Devonian in the western part of the orogen. The recent recognition that several post-Acadian'' plutons are actually Late Silurian suggests that the tectonic models are too simplistic; that accretion previously attributed to a Devonian (Acadian) event may have been a two-stage process: Late Silurian (Salinic ) and Acadian sensu stricto; that orogenic activity may have been continuous from Taconian through Acadian'' times. Differences between the records of the Taconic (Ordovician) and Acadian orogenies suggest different plate interactions: near-orthogonal subduction for the former vs transcurrent faulting for the latter as the dominant mechanisms. Distribution of Salinic'' unconformities across the Northern Appalachian orogen in Maine suggests that accretion of Avalon may have been completed by that Silurian event, but faunal provincialities displayed by fossils as young as Early Devonian in terranes bordering the Fredericton Trough argue for later (Devonian) accretion. A further complication is the imprecise knowledge of the extent to which post-Devonian tectonism is responsible for the current terrane configuration in the Northern Appalachians. Alleghenian folding and thrusting is well-established in some areas where post-Acadian (Carboniferous) strata are present, and is now being documented in older rocks in eastern Maine as well: transcurrent faulting as young as post-Mesozoic( ) along the Norumbega Fault Zone; east-directed thrusting involving the Early Devonian Center Pond pluton.

  7. Grenville foreland thrust belt hidden beneath the eastern US midcontinent

    SciTech Connect

    Hauser, E.C. )

    1993-01-01

    Grenville foreland thrust structures are observed beneath the eastern US midcontinent on COCORP (Consortium for Continental Reflection Profiling) line OH-1 and a short seismic line in southwest Ohio. These structures represent the first evidence for a significant Grenville foreland thrust belt preserved in eastern North America. On the COCORP lines, the structures include a thrust ramp anticline and an associated asymmetric syncline. The Grenville front tectonic zone appears to truncate these foreland structures, indicating a later, second phase expressed as a deeply penetrating, out-of-sequence thrust zone associated with the main uplift of the Grenville province on the east. A short, shallow seismic line in southwestern Ohio reveals an east-dipping sequence of prominently layered rocks that may lie above a footwall ramp to a deeper Grenville thrust fault. A drill hole into the less reflective top of this dipping sequence encountered unmetamorphosed sedimentary rocks like those increasingly reported from other drill holes in southwestern Ohio and adjacent states. Although possibly part of a late Precambrian (Keweenawan ) rift, these clastic sedimentary rocks may instead preserve evidence of a heretofore unrecognized Grenville foreland basin in eastern North America. Alternatively these Precambrian sedimentary rocks together with an underlying, but yet undrilled, strongly layered sequence may correlate with similarly layered rocks observed on COCORP and industrial seismic lines within the Middle Proterozoic granite-rhyolite province to the west in Indiana and Illinois and indicate that unmetamorphosed sedimentary material is an important constituent of the granite-rhyolite province. 25 refs., 6 figs.

  8. The Grenville Front Tectonic Zone: Results from the 1986 Great Lakes Onshore Seismic Wide-Angle Reflection and Refraction Experiment

    NASA Astrophysics Data System (ADS)

    Epili, Duryodhan; Mereu, Robert F.

    1991-09-01

    The Grenville Front, which marks the orogenic boundary between the Archean Superior Structural Province and the much younger Grenville Province to the southeast, is one of the major tectonic features of the Canadian Shield. Within Canada, it is approximately 1900 km in length extending from the north shore of Lake Huron across Ontario and Quebec to Labrador. In 1986, a major coincident onship near-vertical reflection and onshore wide-angle reflection/refraction experiment (GLIMPCE-Great Lakes International Multidisciplinary Program on Crustal Evolution) was conducted along a series of lines across the Great lakes. One of the lines, line J, ran across Georgian Bay and Lake Huron for a distance of 350 km and crossed the Grenville Front Tectonic Zone (GFTZ). The seismic signals from the air gun array source were well recorded by the onshore stations up to distances of 250 km with a seismic trace spacing of 50-62.5 m. The GFTZ had a profound effect on the nature of the reflector patterns observed on the onshore seismic sections. Data recorded by the stations on the east end of the line indicate that the crustal P phases are very complex and form a "shinglelike" pattern of reflected waves. Data recorded by stations at the center and at the western end of the line show that the Pg phases are normal and lack the shinglelike appearance. This character of arrivals was also observed on the corresponding S wave sections. A combined P and S wave forward modeling analysis shows that the GFTZ is composed of bands of reflectors dipping at angles of 20°-35° extending to the lower crust. These reflectors were also well imaged on the coincident near-vertical reflection data. Reflectors under the Britt domain to the east of the GFTZ have a shallower dip than those along the zone. The structure of the crust under the Manitoulin terrane to the west of the GFTZ is laterally homogeneous with a major intracrustal reflector at a depth of 17-20 km below the surface. Poisson's ratio is

  9. Ophiolites in the Xing'an-Inner Mongolia accretionary belt of the CAOB: Implications for two cycles of seafloor spreading and accretionary orogenic events

    NASA Astrophysics Data System (ADS)

    Song, Shuguang; Wang, Ming-Ming; Xu, Xin; Wang, Chao; Niu, Yaoling; Allen, Mark B.; Su, Li

    2015-10-01

    The Xing'an-Inner Mongolia accretionary belt in the southeastern segment of the Central Asian Orogenic Belt (CAOB) was produced by the long-lived subduction and eventual closure of the Paleo-Asian Ocean and by the convergence between the North China Craton and the Mongolian microcontinent. Two ophiolite belts have been recognized: the northern Erenhot-Hegenshan-Xi-Ujimqin ophiolite belt and the southern Solonker-Linxi ophiolite belt. Most basalts in the northern ophiolite belt exhibit characteristics of normal-type to enriched-type mid-ocean ridge basalt affinities with depleted Nd isotopic composition (ɛNd(t) > +5), comparable to modern Eastern Pacific mid-ocean ridge basalts. Most basaltic rocks in the southern belt show clear geochemical features of suprasubduction zone-type oceanic crust, probably formed in an arc/back-arc environment. The inferred back-arc extension along the Solonker-Linxi belt started at circa 280 Ma. Statistics of all the available age data for the ophiolites indicates two cycles of seafloor spreading/subduction, which gave rise to two main epochs of magmatic activity at 500-410 Ma and 360-220 Ma, respectively, with a gap of ~50 million years (Myr). The spatial and temporal distribution of the ophiolites and concurrent igneous rocks favor bilateral subduction toward the two continental margins in the convergence history, with final collision at ~230-220 Ma. In the whole belt, signals of continental collision and Himalayan-style mountain building are lacking. We thus conclude that the Xing'an-Inner Mongolia segment of the CAOB experienced two cycles of seafloor subduction, back-arc extension, and final "Appalachian-type" soft collision.

  10. Nd isotopes and the origin of Grenville-age rocks in Texas: Implications for Proterozoic evolution of the United States mid-continent region

    SciTech Connect

    Patchett, P.J.; Ruiz, J. )

    1989-11-01

    Nd isotopic data were obtained for Precambrian Grenville-belt rocks in Texas. The samples represent most components of the crust of the Llano, Van Horn, and Franklin Mountains exposed terranes. Almost all Precambrian igneous, metamorphic, and sedimentary rocks from the three regions document addition to North America of mantle-derived crustal materials in the 1.6-1.0 Ga interval. The exception is a quartzite from the westernmost (Franklin Mountains) exposure, which was derived from {approximately}1.8 Ga crust of the southwestern United States. The initial {epsilon}{sub Nd} values of all rocks except the quartzite lie in the +1 to +6 range for igneous/metamorphic ages of 1.37 to 1.06 Ga. These results can be interpreted in two ways: (1) as documenting 0-20% additions of older crustal material to mantle-derived products 1.4-1.0 Ga ago; or (2) as documenting derivation of the Grenville exposures by recycling of older crustal protoliths separated from the mantle 1.6-1.3 Ga ago. The Nd data in isolation do not resolve these two interpretations. The model mantle separation ages (T{sub DM}) of the rocks are very similar to published values of granulites in Mexico and Virginia: all these regions of the United States and Mexico show a strong peak of T{sub DM} around 1.4 Ga ago. If the ages represent older crustal protoliths, then they would have formed coevally with the 1.5-1.3 Ga Granite-Rhyolite Terranes of the continental interior USA. This would imply that the Granite-Rhyolite Terranes were formed during orogenic/accretionary processes in the adjoining Grenville Belt, and are not anorogenic in association, as conventionally assumed.

  11. Structural and U/Pb chronology of superimposed folds, Adirondack Mountains: implications for the tectonic evolution of the Grenville Province

    NASA Astrophysics Data System (ADS)

    Kusky, Timothy M.; Loring, David P.

    2001-10-01

    The Grenville Province of eastern Laurentia formed during Mesoproterozoic events that led to the formation of the supercontinent of Rodinia. Structural analysis and U-Pb geochronology of the Piseco antiform in the southern Adirondack Highlands part of the Grenville Province has shed new light on the geology and structural history of this orogen. An outcrop positioned on the northern limb of the Piseco antiform near Wells, NY, contains a major F 1 fold refolded by regional F 2 and F 3 axes. This is the first recognized large-scale F 1 fold in the Adirondack Highlands. Foliated leucocratic gneiss and deformed metagabbro dikes in the core of the antiform have yielded single-grain U-Pb zircon ages of 1172.4±2.2 and 1165.4±1.3 Ma, respectively. Discordant pegmatitic veins that crosscut the foliation in the metagabbro have yielded an estimated 207Pb/ 206Pb age of 1052.4 Ma. The geochronological data have several important implications. The 1172 Ma age of the Wells leucocratic gneiss is virtually identical with that of the syntectonic Hyde School gneiss, Rockport granite, and Wellesley granite from the Adirondack Lowlands and Frontenac terrane of the Central Metasedimentary Belt, thus extending the known distribution of magmas of this age to the southern Adirondack Highlands. We relate these syntectonic granitoids and F 1 folds to the suturing of the ca. 1350-1250 Ma Adirondack Highlands-Green Mountain arc to the composite arc terrane and Laurentia at 1220-1170 Ma. The 1165 Ma deformed metagabbro contains the S 2 but not the S 1 foliation, and we correlate the metagabbro with the ca. 1170-1125 Ma AMCG suite, interpreted to have formed in response to orogenic collapse and delamination following the collision. The Wells leucocratic gneiss and gabbro were folded by reclined isoclinal F 2 folds and upright F 3 folds, and cut by apparently undeformed ca. 1052.4 Ma pegmatitic dikes. Since the discordant pegmatites cut the S 1, S 2, and S 3 foliations which postdate the ca

  12. The Central Metasedimentary Belt (Grenville Province) as a failed back-arc rift zone: Nd isotope evidence

    NASA Astrophysics Data System (ADS)

    Dickin, A. P.; McNutt, R. H.

    2007-07-01

    Nd isotope data are presented for granitoid orthogneisses from the Central Metasedimentary Belt (CMB) of the Grenville Province in order to map the extent of juvenile Grenvillian-age crust within this orogenic belt that is composed mostly of older crustal terranes. The data reveal a 150 km-wide belt of juvenile crust in Ontario, but this belt contains a block of pre-Grenvillian crust (containing the Elzevir pluton) which yields an estimated crustal formation age of 1.5 Ga. The recognition of an older block within the CMB has profound implications for its structure and tectonic evolution, because it implies that juvenile Grenvillian crust, apparently forming a wide NE-SW belt, is in fact distributed in two narrower segments with approximately N-S strike. We suggest that the CMB comprises an en echelon series of ensimatic rift segments, created by back-arc spreading behind a continental margin arc. These rift segments extend southwards (in the subsurface) into the northeastern Unites States. The rift segments contain abundant marble outcrops, consistent with marine incursion into the rift zone, and these deposits also continue northwards into a 'Marble domain' of the CMB in Quebec. However, crustal formation ages in the latter domain are largely pre-Grenvillian, implying that the Quebec rift segment was ensialic. Hence, we interpret the CMB in Ontario and Quebec as the northern termination of a failed back-arc rift zone.

  13. Differential unroofing within the central metasedimentary Belt of the Grenville Orogen: constraints from 40Ar/39Ar thermochronology

    USGS Publications Warehouse

    Cosca, M.A.; Essene, E.J.; Kunk, M.J.; Sutter, J.F.

    1992-01-01

    An 40Ar/39Ar thermochronological investigation of upper greenschist to granulite facies gneiss, amphibolite and marble was conducted in the Central Metasedimentary Belt (CMB), Ontario, to constrain its cooling history. Incremental 40Ar/39Ar release spectra indicate that substantial differential unroofing occurred in the CMB between ??? 1000 and ??? 600 Ma. A consistent pattern of significantly older hornblende and phlogopite 40Ar/3Ar cooling ages on the southeast sides of major northeast striking shear zones is interpreted to reflect late displacement due to extensional deformation. Variations in hornblende 40Ar/39Ar age plateaus exceeding 200 Ma occur over distances less than 50 km with major age discontinuities occurring across the Robertson Lake shear zone and the Sharbot Lake mylonite zone which separate the Sharbot Lake terrane from the Elzevir and Frontenac terranes. Extensional displacements of up to 14 km are inferred between the Frontenac and Elzevir terranes of the CMB. No evidence for significant post argon-closure vertical displacement is indicated in the vicinity of the Perth Road mylonite within the Frontenac terrane. Variations of nearly 100 Ma in phlogopite 40Ar/39Ar plateau ages occur in undeformed marble on either side of the Bancroft Shear Zone. Phlogopites from sheared and mylonitized marble within the shear zone yield 40Ar/39Ar diffusional loss profiles, but have older geologically meaningless ages thought to reflect incorporation of excess argon. By ??? 900 Ma, southeast directed extension was occurring throughout the CMB, possibly initiated along previous zones of compressional shearing. An easterly migration of active zones of extension is inferred, possibly related to an earlier, overall easterly migration of active zones of regional thrusting and easterly migration of an ancient subduction zone. The duration of extensional shearing is not well constrained, but must have ceased before ??? 600 Ma as required by the deposition of overlying undeformed Cambrian and/or Ordovician sedimentary rocks. ?? 1992 Springer-Verlag.

  14. The Precambrian of Transangaria, Yenisei Ridge (Siberia): Neoproterozoic microcontinent, Grenville-age orogen, or reworked margin of the Siberian craton?

    NASA Astrophysics Data System (ADS)

    Kuzmichev, Alexander B.; Sklyarov, Eugene V.

    2016-01-01

    The Yenisei Ridge was traditionally perceived as an uplifted segment of the western Siberian craton affected by Neoproterozoic collision events. However, the suggestions for Archaean or Palaeoproterozoic ('Siberian') basement in Transangaria have not been confirmed by reliable geochronological data. A new view regards most of the Ridge, namely, its Transangarian segment, to be an exotic Neoproterozoic terrane that collided with Siberia in the late Neoproterozoic. This paper presents new U-Pb SHRIMP zircon ages demonstrating that Archaean rocks (2611 ± 12 Ma) actually exist in this territory. We also provide a review of published U-Pb zircon ages for igneous and metamorphic rocks of Transangaria together with our new age data. This geochronological dataset clarifies the geology of the Yenisei Ridge and leads to new conclusions, as follows. (1) It is likely that Transangaria was originally underlain by an Archaean-Palaeoproterozoic basement, similar to that of the Siberian craton. (2) Geochronological data do not confirm the idea of widespread "Greenvillian age" granitoides in Transangaria. (3) The Neoproterozoic evolution of the Yenisei Ridge segment of the Siberian craton margin includes the following events. (i) Collision of an unidentified terrane with the western margin (in recent coordinates) of the Siberian craton during 900-855 Ma. The colliding terrane is no longer present in the current structure. (ii) Dextral shearing during 830-800 Ma may have been caused by counter-clockwise rotation of the Siberian craton. (iii) Extensional conditions prevailed during 800-700 Ma. The Isakovka oceanic basin formed at this time interval. (iv) Thrusting of the Isakovka island arc and accretionary prism onto the Siberian margin occurred during the late Neoproterozoic (650-630 Ma) and caused high-pressure metamorphism.

  15. Timing and conditions of high-pressure metamorphism in the western Grenville Province: Constraints from accessory mineral composition and phase equilibrium modeling

    NASA Astrophysics Data System (ADS)

    Marsh, Jeffrey H.; Culshaw, Nicholas G.

    2014-07-01

    Previous geochronological analyses of high pressure (HP) metamorphic rocks in the western Grenville Province, Ontario, Canada have yielded precise U-Pb zircon ages; however, uncertainty has remained as to whether these ages represent the timing of HP metamorphism or the granulite/amphibolite facies overprint accompanying exhumation to a hot middle orogenic crust. Detailed study of these HP rocks, involving garnet, rutile, and zircon trace element analysis, phase equilibrium modeling, and zircon U-Pb geochronology, has yielded much improved constraints on the timing and conditions of HP metamorphism. Zircon from five of the six HP samples yield anchored discordia upper intercept and 207Pb/206Pb weighted average ages between 1097 and 1085 Ma, and typically have trace element compositions consistent with growth in a garnet-rich, plagioclase-poor eclogite-type assemblage (i.e. no negative Eu anomaly and flat HREE trends). Titanium-in-zircon and Zr-in-rutile thermometry indicates that the range of zircon crystallization temperatures for most samples (643-767 °C) is close to that of rutile inclusions in garnet (668-753 °C) and matrix rutile (690-772 °C). Phase relations in a pseudosection calculated for the sample that best preserves the HP assemblage indicate that: (1) the stability field for the inclusions observed in garnet and kyanite is between 11.5 < P < 14 kbar and 600 < T < 700 °C, and (2) zircon and rutile crystallization temperatures intersect the inferred HP assemblage field (Grt + Cpx + Ky + Rt + Hbl + Qtz) and garnet and kyanite modal isopleths at P > ~ 15 kbar, indicating that the ca. 1090 Ma zircon ages date metamorphism at eclogite facies conditions. Thus, the deep burial of mafic lower crust that resulted in HP metamorphism in the western CGB occurred just prior to the main "Ottawan" phase of continental collision in the western Grenville Province (ca. 1080-1040 Ma).

  16. Crustal velocity structure of the Superior and Grenville Provinces of the southeastern Canadian Shield

    NASA Astrophysics Data System (ADS)

    Winardhi, Sonny

    In the summer of 1992, the Lithoprobe Abitibi-Grenville Seismic Refraction Experiment was conducted across the Grenville and Superior Provinces of the Southeastern Canadian Shield. More than 17,000 seismograms were collected during the survey from 44 shot points at an average shot spacing of 30 km and station spacing of 1-1.5 km. Each shot was recorded by 417 instruments, 47 of which were three-component systems which were placed strategically across the Grenville Front. The dense ray coverage in the area allowed us to make a detailed study of the variation of crustal velocities using seismic refraction tomography. The shallow velocity structure under the profiles was mapped using a delay-time analysis of the Psb{g} phases. A study of wide-angle reflected waves from the Moho (Psb{m}P) was carried out to constrain the thickness of the crust and the nature of the crust-mantle boundary. Three-component analysis was also employed to explore the anisotropic nature of the upper crust in the vicinity of the Grenville Front Tectonic Zone. The 2-D tomographic inversion was performed using an iterative conjugate gradient technique, whereas its corresponding uncertainty in the velocity determinations were estimated using a combination of the very fast simulated annealing and calculation of the ray-path density. From the three-component data set, polarization and propagation vectors were extracted. Angular deviations between these two directions (5-15 degrees) were then inverted to give parameters describing the anisotropic nature of the uppermost crust in the vicinity of the receivers. The latter was performed using a non-linear inversion technique utilizing the very fast simulated annealing. The delay-time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another, with the largest velocity values occurring underneath the Central Gneiss and the Central Metasedimentary Belts

  17. Reworked pre-Grenville crust and timing of Grenville orogenesis in the southeastern Llano Uplift, Texas: Results from U-Pb geochronometry

    SciTech Connect

    Reese, J.F.; Roback, R.C.; Walker, N.W. . Geological Sciences)

    1992-01-01

    Zircon geochronometry of metaigneous units in the southeastern Llano Uplift (1) show that 1355-1215 Ma rocks were tectonically reworked during the Grenville orogeny, (2) further constrain the timing of Grenville orogenesis along the southern margin of North America, and (3) necessitate revision of previously proposed stratigraphic and structural relations within the Llano Supergroup. Multigrain, abraded zircon fractions from a sample of quartzofeldspathic Valley Spring Gneiss (VSG) define a discordia trajectory with an upper intercept of 1355 [plus minus] 3 Ma, interpreted as the igneous crystallization age of the protolith. This age is the oldest thus far documented for Llano Supergroup protoliths and is substantially older than a recently reported age (1232 [plus minus]4 Ma) from a different part of the VSG. The lithologic and chronometric similarities of the old VSG to rocks to the Mid-continent Granite-Rhyolite terrane raise the possibility of genetic relations between these units. Thus components of the VSG are tentatively interpreted to represent a deformed and metamorphosed part of the southernmost Mid-Continent Granite-Rhyolite terrane. Alternatively, this rock could be part of an ensialic arc built upon the margin of southern North America and later deformed during the Grenville orogeny. Analyses from four abraded zircon fractions from a quartzofeldspathic gneiss in the structurally overlying Packsaddle Schist yield an upper intercept of 1215 [plus minus] 3 Ma, interpreted as the igneous protolith crystallization age. This age is the youngest yet determined for a polydeformed metamorphic rock in the Llano Uplift and more tightly brackets the timing of Grenville deformation in the southeastern Llano Uplift to post-1215 Ma and pre-1098 Ma.

  18. Seismic images of a tectonic subdivision of the Greenville Orogen beneath lakes Ontario and Erie

    USGS Publications Warehouse

    Forsyth, D. A.; Milkereit, B.; Davidson, A.; Hanmer, S.; Hutchinson, Deborah R.; Hinze, W. J.; Mereu, R. F.

    1994-01-01

    New seismic data from marine air-gun and Vibroseis profiles in Lake Ontario and Lake Erie provide images of subhorizontal Phanerozoic sediments underlain by a remarkable series of easterly dipping reflections that extends from the crystalline basement to the lower crust. These reflections are interpreted as structural features of crustal-scale subdivisions within the Grenville Orogen. Broadly deformed, imbricated, and overlapping thrust sheets within the western Central Metasedimentary Belt are succeeded to the west by a complex zone of easterly dipping, apparent thrust faults that are interpreted as a southwest subsurface extension of the boundary zone between the Central Metasedimentary Belt and the Central Gneiss Belt. The interpreted Central Metasedimentary Belt boundary zone has a characteristic magnetic anomaly that provides a link from the adjacent ends of lakes Ontario and Erie to structures exposed 150 km to the north. Less reflective, west-dipping events are interpreted as structures within the eastern Central Gneiss Belt. The seismic interpretation augments current tectonic models that suggest the exposed ductile structures formed at depth as a result of crustal shortening along northwest-verging thrust faults. Relatively shallow reflections across the boundary region suggest local, Late Proterozoic extensional troughs containing post-Grenville sediments, preserved possibly as a result of pre-Paleozoic reactivation of basement structures.

  19. Deep crustal structure and seismic expression of the central Appalachian orogenic belt

    SciTech Connect

    Herman, G.C. Rutgers Univ., New Brunswick, NJ )

    1992-03-01

    Deep-seismic reflection profiles across parts of the Central Appalachian orogenic belt indicate that the crust here includes an exposed Paleozoic fold-and-thrust belt that is mainly soled in Proterozoic Grenville basement. Translation strain in the foreland and Highlands resulting from Paleozoic orogenesis is as much as 25 km, exclusive of layer-parallel-shortening penetrative strains. This estimate is limited by the slight structural relief beneath the Pocono plateau and by the lack of extensive Cambrian-Ordovician cover beneath the sole thrust. Earlier estimates of translation strain are much higher. To the southwest, this parautochthonous region is separated by an unrecognized structural transition from the more allochthonous parts of the central and southern Appalachian overthrust belt, marked by significantly larger translation strains. Crustal architecture in the hinterland part of the region is poorly understood due to data gaps.

  20. Oral Histories in Meteoritics and Planetary Science--XVI: Grenville Turner

    NASA Astrophysics Data System (ADS)

    Sears, Derek W. G.

    2012-03-01

    In this interview, Grenville Turner (Fig. 1) recounts how he became interested in meteorites during postdoctoral research with John Reynolds at the University of California, Berkeley, after completing a DPhil with Ken Mayne at the University of Oxford. At Berkeley, he worked on xenon isotopes with fellow students Bob Pepin and Craig Merrihue, but Reynolds' insistence that they analyze all the inert gases in their samples meant that they also made important contributions to Ne isotope studies and potassium-argon dating leading to the Ar-Ar technique. In 1964, Grenville obtained a teaching position at the University of Sheffield where he developed his own laboratory for inert gas isotope measurements. After the return of samples from the Moon by the Apollo program, he became involved in determining the chronology of volcanism and major impacts on the Moon. In 1988, Grenville and his team moved to the University of Manchester as part of a national reorganization of earth science departments. During the post Apollo years, Grenville's interest turned to the development of new instrumentation (resonance ionization mass spectrometry and the ion microprobe), and to problems in terrestrial isotope geochemistry, particularly the source of inert gases in fluid inclusions. He received the Leonard Medal of the Meteoritical Society in 1999, and he has also received awards from the Royal Society, the European Association of Geochemistry, and the Royal Astronomical Society.

  1. Crimean orogene: A nappe interpretation

    SciTech Connect

    Popadyuk, I.V.; Smirnov, S.E. )

    1993-09-01

    On the International Tectonic Map of Europe, the Crimean orogene presents a structure that has no analog in the Alpine orogenic belt. The Crimean mountain system lacks nappe structures of Alpine age. Its geosynclinal folding deformation is restricted to the Tavria flysch formation, previously dated as Triassic-Liassic. Therefore, the Crimean orogene was considered as the Kimmerian tectonotype. In our model, the Crimean orogene is characterized by nappe structures. The Yayla nappe is composed to Upper Jurassic and Neocomian sediments and is located below the Tavrian nappe, which consists of the Tavrian flysch formation. We dated the age of the Tavrian formation as Hauterivian-Aptian on the basis of published ammonite finds in the stratotype section. Nappe displacements are dated roughly as Austrian (albian) by the occurrence of upper-middle Albian sediments below the nappes and the Upper Cretaceous age of their neoautochthonous sedimentary cover. These north-vergent nappes have a horizontal displacement of about 20 km. In the eastern part of the orogene, these nappe structures were overprinted by Laramide-Savic (Paleocene and end Oligocene-early Miocene) deformations, as evident in the Kerch peninsula. The nature of these late deformation is not clear. The hydrocarbon potential of the prenapping autochthonous series has not yet been studied. Exploration for oil and gas in the Crimean sector of the Black Sea must take the suggested model and its implications into account.

  2. Accretionary orogens through Earth history

    USGS Publications Warehouse

    Cawood, Peter A.; Kroner, A.; Collins, W.J.; Kusky, T.M.; Mooney, W.D.; Windley, B.F.

    2009-01-01

    Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero. ?? The Geological Society of London 2009.

  3. River Valley pluton, Ontario - A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

    NASA Technical Reports Server (NTRS)

    Ashwal, Lewis D.; Wooden, Joseph L.

    1989-01-01

    This paper presents Nd, Sr, and Pb isotopic data indicating a late-Archean/early-Proterozoic age for the River Valley anorthositic pluton of the southwestern Grenville Province of Sudbury, Ontario. Pb-Pb isotopic data on 10 whole-rock samples ranging in composition from anorthosite to gabbro yield an age of 2560 + or - 155 Ma. The River Valley pluton is thus the oldest anorthositic intrusive yet recognized within the Grenville Province. The Sm-Nd isotopic system records an age of 2377 + or - 68 Ma. High Pb-208/Pb-204 of deformed samples relative to igneous-textured rocks implies Th introduction and/or U loss during metamorphism in the River Valley area. Rb-Sr data from igneous-textured and deformed samples and from mineral separates give an age of 2185 + or - 105 Ma, indicating substantial disturbance of the Rb-Sr isotopic system.

  4. U-Pb zircon dates of morin anorthosite suite rocks, Grenville Province, Quebec

    SciTech Connect

    Doig, R. )

    1991-09-01

    U-Pb zircon ages of samples of anorthosite, pyroxene monzodiorite (jotunite), and pyroxene quartz monzonite (quartz mangerite) of the Morin anorthosite complex, Grenville Province, Quebec, are 1155 {plus minus} 3, 1146 {plus minus} 4 and 1135 {plus minus} 3 Ma, respectively. These dates are very similar to available dates for equivalent units of the Lac St-Jean and Adirondack anorthosite suite occurrences and slightly predate estimates of 1075-1100 Ma for peak metamorphic conditions during the Grenville orogeny in this region. {sup 87}Sr/{sup 86}Sr initial ratios of 0.7048-0.7051 for the three Morin units sampled permit a comagmatic origin if the different emplacement or cooling times can be reconciled. The mangerite sampled cannot have been formed by fusion of the upper crust, but may have been derived from a relatively juvenile crust at depth. All three units have interacted with continental crust, given the likely depleted nature of the mantle in this region.

  5. Detrital mineral chronology of the Uinta Mountain Group: Implications for the Grenville flood in southwestern Laurentia

    USGS Publications Warehouse

    Mueller, P.A.; Foster, D.A.; Mogk, D.W.; Wooden, J.L.; Kamenov, George D.; Vogl, J.J.

    2007-01-01

    Numerous studies have shown that large quantities of Grenville-age detritus dominate Neo-proterozoic to Cambrian arenites in southwest Laurentia (southwestern United States). U-Pb ages and Hf isotopic compositions of zircons and 40Ar/39Ar ages of white mica from clastic sedimentary rocks of the Neoproterozoic Uinta Mountain Group also indicate significant Mesoproterozoic detritus mixed with a variably abundant Archean component. Zircons with ages representative of the Paleoproterozoic basement in the eastern Uinta Mountains or the younger Paleoproterozoic rocks of the adjacent Yavapai-Mazatzal terranes were not observed. A limited range of initial ??Hf (???90% between -3 and +3) for Mesoproterozoic zircons suggests derivation from a source region (or regions) characterized by mixing between juvenile and reworked older crust during Grenville orogenesis. The enriched Grenville-age basement proposed to underlie much of southeastern North America may be this source based on similarities of Hf isotopic data from Mesoproterozoic zircons in Mississippi River sand and available paleocurrent data. If so, then disruption of this supply in the Cambrian may be related to Iapetan rifting and, perhaps, the separation of the Precordillera terrane from Laurentia. ?? 2007 The Geological Society of America.

  6. Time-space focused intrusion of genetically unrelated arc magmas in the early Paleozoic Ross-Delamerian Orogen (Morozumi Range, Antarctica)

    NASA Astrophysics Data System (ADS)

    Rocchi, S.; Di Vincenzo, G.; Dini, A.; Petrelli, M.; Vezzoni, S.

    2015-09-01

    The growth of continental crust in accretionary orogenic belts takes place through repeated cycles of subduction-accretion of rock units from continental and oceanic magmatic arcs, supra-subduction zone backarcs and forearcs loaded with continent-derived materials. An ancient example relevant to magmatic arc accretion models is represented by the remnants of the Cambrian-Ordovician Ross Orogen in the Morozumi Range, Victoria Land (Antarctica). There, late Neoproterozoic phyllites host an intrusive complex which preserves a remarkably uncommon record of genetically unrelated magma pulses emplaced under a variable stress regime in a short time span: (1) a dominant K-feldspar-phyric granite, (2) fine-grained dioritic stocks and dykes, (3) a peraluminous granite; and (4) a tonalitic-granodioritic dyke swarm. Laserprobe U-Pb zircon dates cluster at late Cambrian times for all these units, yet they carry differential cargoes of relict cores. Unique geochemical-isotopic signatures for both the less evolved magmas (diorite and dyke tonalite) and the most acidic ones (granite and peraluminous granite) indicate that each one of them originated from distinct sources at depth. Additionally, field relationships and chemical evolutionary trends testify for a variety of shallow level open-system processes, such as magma mingling/mixing between diorite and main granite magmas, as well as progressive incorporation of the host schists by the dyke tonalite magma. In summary, crustal growth in the Morozumi intrusive complex was contributed by fresh mantle magma issuing from the metasomatised mantle wedge, while the production of other melts did recycle different crustal portions/layers: the main granite derived from Grenville-age granulitic lower crust; the peraluminous granite from late Proterozoic upper crust, and the tonalite magmas derived from subduction erosion-enriched subarc mantle and evolved by ingestion of local metasedimentary rocks. Overall, the Morozumi intrusive complex

  7. Petrologic, geochemical, and geochronologic constraints on the tectonic evolution of the southern Appalachian orogen, Blue Ridge Province of western North Carolina

    NASA Astrophysics Data System (ADS)

    Anderson, Eric Douglas

    2011-07-01

    The Blue Ridge Province of western North Carolina contains a wide variety of metamorphosed igneous and sedimentary rocks that record the tectonic effects of Precambrian and Paleozoic orogenic cycles. Tectonic interpretations of the events that led to the present configuration are varied and often conflicting. This investigation examines metamorphosed mafic rocks that are widely interpreted to have formed during the closure of ocean basins. Metabasites, and specifically eclogites, have a tendency to mark tectonic sutures and frequently preserve pressure (P), temperature (T), and age data (t) that can be gleaned from mineral equilibria and U-Pb isotopic compositions. As such, the examination of the metabasites is considered the key to understanding the orogenic history of the southern Blue Ridge where these metabasites occur. Chapter 2 is an investigation of the retrograde reactions related to the decompression of sodic pyroxenes that react to form diopside-plagioclase-hornblende-quartz symplectites as stability fields are overstepped during isothermal decompression. In Chapter 3 metabasites from the central and eastern Blue Ridge are re-examined and P-T pathways of these lithologies are determined. The argument is made that the Taconic orogeny of the Blue Ridge is the result of a continent-continent collision event that culminated in a mega-melange that coincides with the Cullowhee terrane and the eastern Blue Ridge melange of western North Carolina. Chapter 4 contains the results of a geochronological investigation of the Precambrian basement complex of the eastern Great Smoky Mountains. Chapter 5 is a whole rock geochemical study of the same basement complex. In Chapter 6, a potential lithologic correlation between the southern Blue Ridge basement and the Arequipa-Antofalla block of Peru is discussed. The geologic history of western South America from the Mesoproterozoic through Cambrian is summarized, a potential isotope-based lithologic correlation is proposed

  8. Paleozoic Orogens of Mexico and the Laurentia-Gondwana Connections: an Update

    NASA Astrophysics Data System (ADS)

    Ortega-Gutierrez, F.

    2009-05-01

    The present position of Mexico in North America and the fixist tectonic models that prevailed prior to the seventies of the past century, have considered the main Paleozoic tectonic systems of Mexico as natural extensions of the orogens that fringed the eastern and southern sides of the Laurentian craton. Well known examples of pre-Mesozoic orogens in Mexico are the Oaxacan, Acatlan, and Chiapas polymetamorphic terranes, which have been correlated respectively with the Grenville and Appalachian-Ouachitan orogens of eastern North America. Nonetheless, several studies conducted during the last decade in these Mexican orogenic belts, have questioned their Laurentian connections, regarding northwestern Gondwana instead as the most plausible place for their birth and further tectonic evolution. This work pretends to approach the problem by briefly integrating the massive amount of new geological information, commonly generated through powerful dating methods such as LA-ICPM-MS on detrital zircon of sedimentary and metasedimentary units in the Paleozoic crustal blocks, which are widely exposed in southern and southeastern Mexico. The Acatlan Complex bears the closest relationships to the Appalachian orogenic system because it shows thermotectonic evidence for opening and closure of the two main oceans involved in building the Appalachian mountains in eastern Laurentia, whereas two other Paleozoic terranes in NW and SE Mexico, until recently rather geologically unknown, may constitute fundamental links between the Americas for the last-stage suturing and consolidation of western Pangea. The buried basement of the Yucatan platform (400,000 squared km) on the other hand, remains as one of the most relevant problems of tectonostratigraphic correlations across the Americas, because basement clasts from the Chicxulub impact ejecta reveal absolute and Nd-model ages that suggest close Gondwanan affinities. Major changes in the comprehension of the Paleozoic orogens in Mexico

  9. Nappe-Bounding Shear Zones Initiated On Syn-Tectonic, Pegmatite-Filled Extensional Shear Fractures During Deep-Crustal Nappe Flow In A Large Hot Orogen

    NASA Astrophysics Data System (ADS)

    Culshaw, Nicholas; Gerbi, Christopher; Marsh, Jeffrey; Regan, Peter

    2014-05-01

    The Central Gneiss Belt (CGB) of the Proterozoic western Grenville Province is an extensive exposure of the mid-crustal levels (upper amphibolite facies, lesser granulites) of a large hot orogen. Numerical models give a credible prediction of structure and metamorphism accompanying CGB deep-crustal nappe flow and define a temporal framework based on four developmental phases: thickening, heating, nappe-flow and post convergence extensional spreading. These phases are diachronous in direction of orogen propagation and imply a spatial framework: externides (close to orogen-craton boundary) containing moderately inclined thickening and/or extensional structures, and internides containing thickening structures overprinted by sub-horizontal nappe flow structures, which may be locally overprinted by those due to extensional spreading. Although on average of granitoid composition, CGB nappes differ in rheology, varying from fertile and weak (unmetamorphosed before Grenville, meltable) to infertile and strong (metamorphosed at high grade before Grenville, unmeltable) or mixed fertile-infertile protoliths. Deformation style varies from diffuse in fertile nappes, weakened by pervasive melting, to localised in shear zones on boundaries or interiors of infertile nappes. Specifically, in terms of deformation phase and location within the orogen, shear zones occur as: thickening structures of externides, early thickening- and later overprinting nappe-flow structures of infertile internide nappes, and extension-related shear zones in externides and internides. Many of the nappe-flow shear zones of the internides are associated with pegmatites. One example has been recognized of a preserved progression from small-scale fracture arrays to regional shear zone. The sequence is present on a km-scale and initiates in the interior of a nappe of layered granulite with arrays of pegmatite filled extensional-shear fractures (mm to cm width) displaying amphibolized margins. The fracture

  10. The initiation of orogenic margin reverse faulting

    NASA Astrophysics Data System (ADS)

    Bailey, R. C.

    2002-04-01

    Laboratory values of rock friction coefficients suggest that reverse faulting should be very difficult to initiate by simple horizontal compression of the crust. Values of stresses required by Andersonian faulting may be an order of magnitude higher than those actually present in orogenic margins. A simple stress balance calculation shows that the effect of the excess lithostatic pressure under an elevated orogen, if transmitted laterally through a crustal ductile layer to the orogenic margin, is to provide sufficient hydraulic lift under the orogen flanks to initiate reverse faulting by direct lift, even with rock friction coefficients of order 0.8. The required orogenic elevation above surrounding ``normal'' lithosphere is about one fifth of the thickness of the brittle crust of the orogen. This elevation may be as small as 2 km in tectonically active regions. The mechanism works even in the absence of regional lithospheric compressive stresses.

  11. Imaging the mantle lithosphere of the Precambrian Grenville Province: large-scale electrical resistivity structures

    NASA Astrophysics Data System (ADS)

    Adetunji, Ademola Q.; Ferguson, Ian J.; Jones, Alan G.

    2015-05-01

    The resistivity structure of the lithospheric mantle beneath the Proterozoic Grenville Province in southern Ontario, Canada is investigated using 84 magnetotelluric (MT) sites divided into four profiles. Depth-based regional geoelectric dimensionality analyses of the MT responses indicate that the mantle lithosphere north of Lake Ontario can be subdivided into upper (45-150 km) and deeper (>200 km) lithospheric mantle layers with regional strike azimuths of N85°E (±5°) and N65°E (±5°), respectively. MT responses from the Grenville Front and the northwest part of the Central Gneiss Belt are compatible with the presence of 2-D resistivity structures but farther to the southeast, in the southeast part of the Central Gneiss Belt and Central Metasedimentary Belt, they suggest the presence of localized 3-D structures. 2-D inversion of distortion-free MT responses images a large scale very resistive (>20 000 Ω m) region that extends 300 km southeast of the Grenville Front and for at least 800 km along-strike in the lithospheric mantle beneath the Grenville Province. This feature is interpreted to be Superior Province lithosphere and the corresponding N85°E geoelectric strike to be associated with the fabric of the Superior Province. The base of the resistor reaches depths of 280 km on two of the three MT profiles north of Lake Ontario and this depth is interpreted to be the base of the lithosphere. A large region of enhanced conductivity in the lower lithosphere, spatially correlated with decreased seismic velocity, is bounded to the northwest by a subvertical resistivity anomaly located near the Kirkland Lake and Cobalt kimberlite fields. The enhanced conductivity in the lower lithosphere is attributed to refertilization by fluids associated with Cretaceous kimberlite magmatism and can be explained by water content in olivine of 50 wt ppm in background areas with higher values in a localized anomaly beneath the kimberlite fields. Farther to the southeast the

  12. Granitoids and Crustal Growth in the East- Kunlun Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Mo, X.; Luo, Z.; Deng, J.; Yu, X.; Liu, C.; Yuan, W.; Bi, X.

    2011-12-01

    1MO Xuanxue, 1LUO Zhaohua, 1DENG Jinfu,1YU Xuehui, 2LIU Chengdong, 1Yuan Wanming, 1Bi Xianmei 1China University of Geosciences, Beijing, 100083; 2Donghua University of Technology, Nanchang, 330013 Abstract: The East- Kunlun orogenic belt (abbrev. EKOB) is one of major tectono-magmatic belts in the Tibetan plateau. Four stages of granitoid plutonism took place in Precambrian (the Proterozoic), early Paleozoic (∈-D3), late Paleozoic- early Mesozoic (D3-T3), and late Mesozoic- Cenozoic (after early Jurassic), respectively. Among them, the late Paleozoic- early Mesozoic, especially Triassic granitoids are predominant. The basement of EKOB formed in late Paleo- Proterozoic. The early Paleozoic tectono-magmatic event sequence is comparable to that in the North- Qilian orogenic belt, and became a part of the Qilian- East- Kunlun Caledonian orogenic system. However, EKOB had been involved in the Paleo- Tethyan tectonic regime during late Paleozoic- early Mesozoic period, and yield an integrated Variscian- Indo-Chinese orogenic cycle, similar to the Sanjiang Paleo-Tethyan orogeny. The south- Kunlun suture zone then defined the main boundary between the north and the south China. Subsequently, EKOB has been involved in the Tibetan collisional orogenic system in the Cenozoic since Indo- Eurasia collision. While the continental crust of EKOB mainly formed in late Paleo- Proterozoic, juvenile crust was also generated in the Phanerozoic time, similar to the Xing'anling- Mongolia, the Gangdese and the Andes orogenic belts. There are abundant conclusive evidences of underplating and magma mixing in the East- Kunlun granitoid belt (Fig.1). Initiate values of 87Sr/86Sr of the East- Kunlun granitoids are mostly less than 0.710 and ɛNd(t)values of them range from -9.2 to +3.6. Those imply that inputting of mantle materials and mixing between mantle- and crust- derived materials played an important role in crustal generation and evolution in EKOB during the Phanerozoic time

  13. Ages and petrogenetic significance of igneous mangerite-charnockite suites associated with massif anorthosites, Grenville Province

    SciTech Connect

    Emslie, R.F.; Hunt, P.A. )

    1990-03-01

    U-Pb ages of zircon fractions of major anorthosite-mangerite-charnockite-granite (AMCG) igneous suites imply that this magmatism inaugurated what is widely regarded as the Grenvillian event between about 1.16 and 1.12 Ga ago over about two-thirds of the Grenville Province east, northeast, and southeast of the Central Metasedimentary Belt. Pre-Grenvillian AMCG suites about 1.36 and 1.64 Ga old have much more restricted distribution. An apparent time lag of about 0.05 to 0.10 Ga is indicated between culmination of AMCG magmatism and the widely recognized Grenvillian metamorphic peak (about 1.10 to 1.03 Ga), perhaps the most distinctive hallmark of the Grenville event. The time lag is consistent with conductive heating of thick subcontinental lithosphere that began with initiation of AMCG magmatism and continued until geotherms rose sufficiently to produce granulites in much of the lower to middle crust. Tectonic crustal thickening did not likely occur until later in the sequence of events, perhaps after some cooling from the metamorphic peak. Compressive forces were externally applied, possibly at a distant plate margin, while the continental lithosphere was still thermally weakened from preceding magmatic-metamorphic culminations.

  14. Composition of the crust in the Grenville and Appalachian Provinces of North America inferred from VP/VS ratios

    USGS Publications Warehouse

    Musacchio, G.; Mooney, W.D.; Luetgert, J.H.; Christensen, N.I.

    1997-01-01

    We use the ratios between P and S wave velocities (VP/VS), derived from seismic refraction data, to infer the composition of the crust in the Grenville and the Appalachian Provinces of North America. The crust exhibits VP/VS increasing with depth from 1.64 to 1.84; there is a clear distinction between the Grenville Province (average VP/VS=1.81) and the Appalachian Province (average VP/VS=1.73) which persists at all depths. The boundary between these provinces is east dipping extending for 100 km east of the Champlain thrust. In the Appalachian Province the increase in VP/VS ratios with depth from 1.67 to 1.74 ?? 0.02 may reflect a normal decrease of silica content in the continental crust. In the Grenville Province beneath the Central Granulite Terrane, an anomalous VP/VS ratio of 1.82 ?? 0.02 is observed extending to a depth of 10 km; this correlates with the abundance of Ca-plagioclase in the Marcy Anorthosite. At greater depth (15-20 km), where seismic lamination and high electrical conductivity is observed, VP/VS is 1-84 ?? 0.02 and correlates with the Tahawus Complex, a layered mafic intrusion. Within the 25-km-thick lower crust of the Grenville Province the VP/VS is 1-84 ?? 0.02 and P-velocity is 7.0 ?? 0.1 km/s, which are typical for plagioclase-bearing rocks (gabbro-norite). The high VP/VS ratio in the Grenville Province has not been reported in crust of any other age. Since the Grenville Province contains 75% of the world's known anorthosites, high VP/VS ratio is related to high plagioclase. We suggest that the composition of the Grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid-Proterozoic. Copyright 1997 by the American Geophysical Union.

  15. And the Variscan Orogen Buckled

    NASA Astrophysics Data System (ADS)

    Pastor-Galán, D.; Groenewegen, T.; Gutiérrez-Alonso, G.; Langereis, C. G.

    2013-12-01

    Oroclines are the largest scale folds in nature, and as folds can be produced by bending or by buckling. The most commonly invoked bending mechanisms are indentation (the Himalayan syntaxes) and slab roll-back (The Calabria Arc) whereas buckling usually are commonly related with collision of the apex of ribbon continents along strike (Alaskan oroclines). In Western Europe the tectonostratigraphic zonation of Variscan orogen shows a complex 'S' shape pattern recently interpreted as a double orocline consisting of a northern and southern arc. The northern arc, known as Cantabria-Asturias Arc or Cantabrian Orocline, was developed after closure of the Rheic Ocean and the building and collapse of the Variscan orogenic edifice and, therefore, is considered post-Variscan in age. On the other hand, neither the geometry nor the kinematics of the so-called Central Iberian orocline, situated at the south of the Iberian peninsula are properly known. However, it seems reasonable to think that both oroclines developed at the same time as other coupled oroclines, such as the New England oroclines or the Carpathian oroclines. The particular paleogeography of the Variscan belt in Pangea and the kinematics of the oroclinal formation make impossible the mechanisms of indentation or buckling of a ribbon continent. The occurrence of an intense syn- and slightly post-Cantabrian orocline magmatic event (310-290 Ma) has been linked to the development of the orocline(s) due to the particular spatial-temporal distribution of these post-tectonic granitoids and its isotopic signature which would imply that the oroclines (if synchronous) are thick-skinned. This magmatic pulse has also been interpreted as due to slab roll-back. We present widespread evidences of buckling around the whole orocline at different lithospheric levels and new insights of the particular geometry of the southern orocline which is difficult to reconcile with a roll-back related origin. Therefore, a major change in the

  16. Continental deformation and the mid-lithospheric discontinuity along the Grenville Front

    NASA Astrophysics Data System (ADS)

    Abrahams, L.; Long, M. D.; Ford, H. A.; Wirth, E. A.

    2015-12-01

    The existence of a mid-lithospheric discontinuity (MLD) within the stable continental mantle lithosphere of North America has been well established, but its interpretation remains difficult. Recent work with Ps receiver functions has found evidence for anisotropic structure at MLD depths within the western portion of the Granite-Rhyolite Province, suggesting that the MLD is the result of deformation associated with the formation of the continent. The last significant deformation to occur within the province was approximately 1.3-0.9 Ga and impacted the lithosphere east of the Grenville Front. In this study we analyzed six stations east of the front using Ps receiver functions in order to characterize anisotropy associated with the MLD in the region. Transverse and radial component Ps receiver functions were calculated for six stations (ACSO, BINY, ERPA, MCWV, SSPA, TZTN) using a multi-taper correlation technique and binned as a function of back azimuth and of epicentral distance. All six stations analyzed displayed significant positive phase energy on the radial component at ~6 seconds, which was interpreted as the Moho. At four of the six stations (ACSO, MCWV, SSPA, TZTN) the Moho showed moderate to significant complexity. At stations MCWV, SSPA, TZTN, all located along the Appalachian margin, there was significant transverse component energy at crustal depths with both two- and four-lobed anisotropy patterns observed. While ACSO, BINY, and ERPA displayed evidence of isotropic and/ or anisotropic crustal structure, a coherent pattern in back azimuth could not be established. The radial component receiver functions also exhibited negative phase energy, interpreted as the MLD, between 7.5 and 12.5 seconds (or ~80 to 120 km), at five of the six stations, with the exception of BINY, where no negative phase was observed. The transverse component receiver functions at stations ACSO, ERPA, SSPA and TZTN, also displayed a two-lobed pattern in back azimuth at MLD depths

  17. River Valley pluton, Ontario: A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

    USGS Publications Warehouse

    Ashwal, L.D.; Wooden, J.L.

    1989-01-01

    The River Valley pluton is a ca. 100 km2 body of anorthositic and gabbroic rocks located about 50 km northeast of Sudbury, Ontario. The pluton is situated entirely within the Grenville Province, but its western margin is a series of imbricate thrust faults associated with the Grenville Front Tectonic Zone. It is dominated by coarse leuconorite and leucogabbro, with lesser anorthosite, gabbro, and rare ultramafics. Igneous textured rocks are abundant and consist of plagioclase (An60-70) charged with Fe-Ti oxide inclusions, low Ca pyroxene (orthopyroxene and/or inverted pigeonite) and augite. The most unfractionated rocks are minor olivine gabbros with Fo70-80. A variety of deformed and recrystallized equivalents of the igneous-textured rocks is also present, and these are composed largely of calcic plagioclase and hornblende. Ten samples, including both igneous and deformed lithologies give a Pb-Pb whole-rock isochron of 2560??155Ma, which is our best estimate of the time of primary crystallization. The River Valley pluton is thus the oldest anorthositic intrusive yet reported from the Grenville Province, but is more calcic and augitic than typical massifs, and lacks their characteristic Fe-Ti oxide ore deposits. The River Valley body may be more akin to similar gabbro-anorthosite bodies situated at the boundary between the Archean Superior Province and Huronian supracrustal belt of the Southern Province west of the Grenville Front. An Sm-Nd isochron from 3 igneous-textured leucogabbros and an augite mineral separate gives 2377 ?? 68 Ma, implying slight disturbance of the Sm-Nd whole-rock-mineral system during later metamorphism. The Rb-Sr system has been substantially disturbed, giving an age of 2185 ?? 105 Ma, which is similar to internal Pb-Pb isochron ages of 2165 ?? 130 Ma and 2100 ?? 35 Ma for two igneous-textured rocks. It is uncertain whether these ages correspond to a discrete event at this time or represent a partial resetting of the Rb-Sr and Pb

  18. River Valley pluton, Ontario: A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

    SciTech Connect

    Ashwal, L.D. ); Wooden, J.L. )

    1989-03-01

    The River Valley pluton is a ca. 100 km{sup 2} body of anorthositic and gabbroic rocks located about 50 km northeast of Sudbury, Ontario. The pluton is situated entirely within the Grenville Province, but its western margin is a series of imbricate thrust faults associated with the Grenville Front Tectonic Zone. It is dominated by coarse leuconorite and leucogabbro, with lesser anorthosite, gabbro, and rare ultramafics. Igneous textured rocks are abundant and consist of plagioclase (An{sub 60-70}) charged with Fe-Ti oxide inclusions, low Ca pyroxene (orthopyroxene and/or inverted pigeonite) and augite. The most unfractionated rocks are minor olivine gabbros with Fo{sub 70-80}. A variety of deformed and recrystallized equivalents of the igneous-textured rocks is also present, and these are composed largely of calcic plagioclase and hornblende. An Sm-Nd isochron from 3 igneous-textured leucogabbros and an augite mineral separate gives 2,377 {plus minus} 68 Ma, implying slight disturbance of the Sm-Nd whole-rock-mineral system during later metamorphism. The Rb-Sr system has been substantially disturbed, giving an age of 2,185 {plus minus} 105 Ma, which is similar to internal Pb-Pb isochron ages of 2,165 {plus minus} 130 Ma and 2,100 {plus minus} 35 Ma for two igneous-textured rocks. Initial isotopic ratios for the River Valley pluton correspond to single-stage model parameters of {mu} = 8.06, {epsilon}{sub Nd} = O to {minus}3, and I{sub Sr} = 0.7015 to 0.7021. Collectively, these suggest either an enriched mantle source or crustal contamination of a mantle-derived magma. The crustal component involved must have been older and more radiogenic than the majority of rocks exposed at the surface in the nearby Superior Province.

  19. Convergence rate controls seismicity styles in collision orogens

    NASA Astrophysics Data System (ADS)

    Dal Zilio, Luca; van Dinther, Ylona; Gerya, Taras

    2016-04-01

    The 25 April 2015 Mw 7.8 Gorkha earthquake in Nepal resulted from the unzipping of the previously locked Main Himalayan Thrust (MHT) fault, along which the Himalayan wedge is thrust over India. Strong ground shaking caused the collapse of more than half a million homes, killing more than 8500 people. Can such a large magnitude event also occur within the populated European Alps? Or is there a distinctly different seismicity pattern in different orogens? We show that their long-term seismicity patterns are indeed different and that their differences can be explained by a single parameter: their convergence rate. To do so we present the first self-consistent seismic cycle model for continental collisional margins. We use the viscoelastoplastic continuum Seismo-Thermo-Mechanical model (STM) validated for seismic cycle applications against a laboratory model (van Dinther et al., 2013a) and natural observations (van Dinther et al., 2013b), which includes Drucker-Prager plasticity and spontaneous rupture events governed by strongly rate-dependent friction. The 2-D model setup consists of two continental plates separated by an oceanic plate, in which the incipient subduction phase is followed by collisional orogeny. Results show the physically consistent spontaneous emergence of complex rupture paths, both on and off the main frontal thrust. These off-main frontal thrust events within the upper and lower plate complement the main frontal thrust seismicity leading to a Gutenberg-Richter frequency-magnitude distribution. This is a key observational feature of seismicity, which is typically not reproduced in seismic cycle models. The range of simulated b-values agrees with natural ranges, as we observe values from 0.97 up to 1.25 for convergence rates decreasing from 5 to 1 cm/yr. Decreasing convergence rates thus lead to relatively larger amounts of smaller earthquakes (increasing b-value) and lower maximum magnitudes. This change in b-value also observed to corresponds to

  20. The Western Sierras Pampeanas: Protracted Grenville-age history (1330-1030 Ma) of intra-oceanic arcs, subduction-accretion at continental-edge and AMCG intraplate magmatism

    NASA Astrophysics Data System (ADS)

    Rapela, C. W.; Pankhurst, R. J.; Casquet, C.; Baldo, E.; Galindo, C.; Fanning, C. M.; Dahlquist, J. M.

    2010-01-01

    basic amphibolites with geochemical fingerprints of emplacement in a more mature crust, and (ii) a 1027 ± 17 Ma TTG juvenile suite, which is the youngest Grenville-age magmatic event registered in the Western Sierras Pampeanas. The geodynamic history in both study areas reveals a complex orogenic evolution, dominated by convergent tectonics and accretion of juvenile oceanic arcs to the continent.

  1. Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution

    NASA Astrophysics Data System (ADS)

    Fritz, H.; Abdelsalam, M.; Ali, K. A.; Bingen, B.; Collins, A. S.; Fowler, A. R.; Ghebreab, W.; Hauzenberger, C. A.; Johnson, P. R.; Kusky, T. M.; Macey, P.; Muhongo, S.; Stern, R. J.; Viola, G.

    2013-10-01

    The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world´s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ∼600-500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600-550 Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ∼550-480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution of

  2. Orogenic Response to Augmented Erosion Associated with Northern Hemisphere Glaciation: The St. Elias Orogen of Alaska

    NASA Astrophysics Data System (ADS)

    Berger, A. L.; Gulick, S. P.; Spotila, J. A.; Worthington, L. L.; Upton, P.; Jaeger, J. M.; Pavlis, T. L.

    2009-12-01

    Active orogens are thought to behave as internally deforming critical-taper wedges that are in rough long-term equilibrium with regional boundary conditions. Deformation, kinematics, and the thermal evolution of orogenic systems are therefore believed to be significantly influenced by spatial and temporal variations in climate. The exact role of cryosphere-geosphere interactions in natural systems, however, continues to be elusive. Here we quantify the spatial patterns of denudation and deformation, and their temporal variations, in the heavily glaciated St Elias orogen in southern Alaska. The tractable size and high latitude of the St. Elias orogen provides an ideal setting to address the real world orogenic response to focused denudation and climate change. Independent of any known change in regional tectonic convergence rate, the St. Elias orogen was subjected to the most severe transition in climate during Cenozoic time, the Late Neogene and Pleistocene onset and advance of glaciers. Low-temperature bedrock thermochronometry, thermokinematic modelling, and offshore seismic reflection and borehole data demonstrate an association between augmented glacial denudation and orogenic evolution. Coeval with the onset of enhanced glacier coverage in mid-Pleistocene time, onshore denudation and offshore sedimentation accelerated ~ten-fold, with the highest rates of exhumation (4 km/Myr (±25%)) located around a narrow zone where the mean Quaternary glacial equilibrium line altitude (ELA) intersects mean topography on the windward flank of the orogen. This climatically driven mass redistribution coincided with the transfer of strain away the seaward deformation front as well as accelerated motion along both a backthrust running the length of the orogen and a series of forethrusts that lie beneath the zone of highest glacial flux. In a cause and effect response, the expansion of glaciers thus appears to have driven an orogen scale structural reorganization leading to the

  3. Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution☆

    PubMed Central

    Fritz, H.; Abdelsalam, M.; Ali, K.A.; Bingen, B.; Collins, A.S.; Fowler, A.R.; Ghebreab, W.; Hauzenberger, C.A.; Johnson, P.R.; Kusky, T.M.; Macey, P.; Muhongo, S.; Stern, R.J.; Viola, G.

    2013-01-01

    The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world́s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara–Congo–Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian–Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite–Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650–620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo–Tanzania–Bangweulu Cratons and the Zimbabwe–Kalahari Craton. They closed during the ∼600–500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600–550 Ma extension is recorded in the Arabian–Nubian Shield and the Eastern Granulite–Cabo Delgado Nappe Complex. Later ∼550–480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings

  4. Paleomagnetism of the Grenville diabase dyke swarm and implications for the mid Vendian paleolatitude of Laurentia

    NASA Astrophysics Data System (ADS)

    Buchan, K. L.; Ernst, R. E.; Kumarapeli, P. S.

    2004-05-01

    The Vendian-early Cambrian drift of Laurentia is important for theories of `Snowball Earth' and the continental breakup that formed the Iapetus Ocean. However, estimates of Laurentia's paleolatitude in this period differ widely. Some authors have proposed that Laurentia remained in low latitude throughout this period, whereas others have supported rapid drift of the continent from low to high and back to low latitude. To assist in evaluating these models, a paleomagnetic study was conducted on the mid Vendian Grenville dyke swarm of southeastern Laurentia. This 700 km long swarm was emplaced along the Ottawa graben, an aulacogen associated with rifting that preceded the opening of the Iapetus Ocean. The swarm was the subject of an early paleomagnetic study by Murthy (1971). More recently, U-Pb baddeleyite and zircon ages of ca. 590 Ma have been described for three Grenville dykes (Kamo et al. 1995). At one of these sites, on the `Mattawa' dyke, a positive paleomagnetic baked contact test was also reported (Hyodo and Dunlop 1993). In that detailed test thermoremanent overprinting in the zone of hybrid magnetization was shown to match that expected from heat conduction for a cooling dyke. Nevertheless, Hyodo and Dunlop suggested that the steep down remanence in the dyke, although primary, was likely acquired during a geomagnetic excursion because it did not appear to fit the then-available polar wander path. In our study, paleomagnetic sampling was carried out at 36 sites, including all three dated locations. A detailed analysis has been completed for the dated sites and preliminary analysis for the remaining sites. A stable steep down remanence was obtained for all samples in the Mattawa dyke, and in most samples from a second dated site. The third dated site is less stably magnetized and has not yielded a usable remanence direction. Ten additional sites yield stable steep down or occasionally steep up remanences. The presence of a steep remanence in two dated dykes

  5. Mineral-chemical and isotopic studies of Namaqualand granulites, South Africa: A grenville analogue

    NASA Astrophysics Data System (ADS)

    Clifford, Tom N.; Stumpfl, Eugen F.; Burger, Alwyn J.; McCarthy, Terence S.; Rex, David C.

    1981-10-01

    The northwestern part of South Africa and southern South-West Africa/Namibia is amongst the most extensive granulite terranes in Africa. This work reports the results of electron microprobe studies of minerals from two-pyroxene, cordieriteorthopyroxene (-gedrite) (-sapphirine) and garnet and/or cordierite parageneses from Namaqualand, in the N.W. Cape Province of South Africa. Determined PT conditions of prograde metamorphism based on thermodynamic calculations are 800°-900° C and ca. 6-7 Kb; and it is argued that rocks of unusual composition, notably cordierite-orthopyroxene rocks, are restites after the extraction of granitic liquid from former argillites. This interpretation is consistent with previously published data on similar rocks, and with McCarthy's (1976) suggestion of extensive partial melting in the quartzofeldspathic rocks in the area. U-Pb isotopic studies of some 50 zircon fractions have been carried out and confirm an age of 1,200 m.y. for the high-grade regional metamorphism; but certain zircon populations record inherited ages greater than 1,700 m.y. Garnet-sillimanite rocks that contain retrograde kyanite reflect PT conditions of 550°-650° C and ca. 7-8 Kb; and constituent biotite has yielded a K-Ar age of ca. 950 m.y. These data, the regional stratigraphy and structure, and the mineralisation are compared with data from the Grenville Province of Canada. Notable similarities are the possible basement-cover relationships, and the calendar of tectonothermal events, while differences include the important stratiform base-metal mineralisation in the supracrustal sequence in Namaqualand, and the Cu-mineralisation in hypersthenebearing intrusives, emplaced some 1,100 m.y. ago, that are areally, and believed to be genetically, related to the granulite facies metamorphic regime.

  6. 13C 12C exchange between calcite and graphite: A possible thermometer in Grenville marbles

    USGS Publications Warehouse

    Valley, J.W.; O'Neil, J.R.

    1981-01-01

    The fractionation of 13C between calcite and graphite, ??(Cc-Gr). is consistently small (2.6-4.8 permil) in 34 assemblages from upper amphibolite- and granulite-facies marbles of the Grenville Province. In 25 samples from the Adirondack Mountains, New York, it decreases regularly with increasing metamorphic temperature. The fractionations are independent of absolute ??13C values of calcite (-2.9 to +5.0). For T = 600-800??C, the Adirondack data are described by ??(Cc-Gr) = -0.00748T (??C) + 8.68. This good correlation between ?? and T suggests that carbon isotope equilibrium was attained in these high-grade marbles and that the theoretical calculations of this fractionation by Bottinga are approximately 2 permil too large in this temperature range. Because of the relatively high temperature sensitivity suggested by these results and by Bottinga's calculations, and the pressure independence of isotope fractionation, ??(Cc-Gr) may provide a very good thermometer for high-grade marbles. Comparison of this field calibration for ??(Cc-Gr) vs temperature with results from other terranes supports the utility of ??(Cc-Gr) for geothermometry and suggests that graphite is much more sluggish to exchange than calcite, that exchange between calcite and graphite occurs at temperatures as low as 300??C, and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500-600??C. Because 13C exchange is an unavoidable metamorphic process at temperatures above 300??C, high values of ??13C(Gr) in moderate- to high-grade carbonate-bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite. ?? 1981.

  7. Refining Rodinia: New Paleomagnetic Results From Amazonia and Paleogeographic Implications For The Grenville Orogeny.

    NASA Astrophysics Data System (ADS)

    Tohver, E.; van der Pluijm, B. A.; van der Voo, R.

    The Grenville province of eastern Laurentia is commonly considered to be the product of continental collision between ancestral North America and an as yet unidentified continent. New paleogeographic information for the Amazon craton in early Grenvil- lian times was determined from a new paleomagnetic pole based on the hypabyssal gabbros and flat-lying basalts of the Nova Floresta Fm. found in the western Brazil- ian state of Rondonia. Measurement of the anisotropy of magnetic susceptibility of the gabbros reveals a flat-lying fabric, suggesting an undeformed, igneous body. A paleomagnetic pole (n = 16 sites, Plat = 24.6N, Plon. = 164.6E, A95 = 5.5, Q = 5) is calculated from a steep, characteristic remanence (ChRM) that is inferred to be primary. This ChRM is isolated at applied field >30 mT and is probably carried by magnetite present in large, oxyexsolved titanomagnetites or igneous reaction rims. Emplacement of the body and acquisition of magnetization is dated by 40Ar/39Ar analysis of igneous biotite and plagioclase, both phases yielding ages of ca.1.2 Ga. Comparison of the position of Amazonia with that of ancestral North America deter- mined from the Laurentian APWP from 1.3 - 1.15 Ga suggests that Amazonia may have collided with the southernmost portion of Laurentia at ca.1.2 Ga. The timing of this collision is in agreement with geochronological constraints on the timing of de- formation in the Llano segment of Laurentia as well as observed deformation of the western Amazon craton.

  8. Exploration of the Banda orogen, Indonesia

    SciTech Connect

    Harris, R.A. ); Audley-Charles, M.G. ); Tobing, S.L. )

    1990-06-01

    An integrated geologic evaluation of the late Miocene to present Banda orogen and its petroleum potential is in progress. The focus of the investigation involves sedimentary sections of the NW Australian continental margin incorporated into the Banda orogen. The islands of Timor and Seram, and several other islands throughout the Banda orogenic arc, are the closest exposed lithostratigraphic equivalents of the hydrocarbon producing NW Australian subsurface stratigraphy. Hydrocarbon occurrences are well documented on many of the islands, and over 13 million barrels of oil have been produced from the Bula field in Seram. Recent studies of seeps in Timor and associated stratigraphic, structural, and geochemical relations indicate that (1) oil and gas seeps occur throughout the Permian to Pliocene stratigraphic section; (2) bituminous limestones of the Triassic Aituti Formation are the most likely source for many of the seeps; (3) preliminary geochemical analyses of the oils indicate a mixed terrestrial and marine organic source, and suggest a genetic link between the Aitutu oils and oils seeping from Tertiary reservoirs; (4) triterpane and sterane parameters indicate some of these oils are highly mature; (5) palynomorph coloration, conodont alteration, and clay mineralogy data yield paleotemperature estimates in the range of hydrocarbon generation from much of the pre-rift sequence (Permian-Jurassic); (6) structural modeling suggests that the initial decollement zone of the orogenic wedge formed near the Jurassic breakup unconformity of the underthrust NW Australian margin, imbricating the post-rift sequence; (7) in Timor and Seram the decollement has stepped to deeper levels involving prerift sequences that form structural culminations in the core of the orogen. The culminations are expressed as topographic highs as in New Guinea and Oman.

  9. Shear Zone Development and Rheology in the Deep Orogenic Crust

    NASA Astrophysics Data System (ADS)

    Marsh, J. H.; Johnson, S. E.; Gerbi, C. C.; Culshaw, N. G.

    2008-12-01

    Within the Central Gneiss Belt (CGB) of the southwestern Grenville Province, Ontario, Canada, a number of allocthonous lithotectonic domains are juxtaposed along crustal-scale shear zones. Extensive exposure of variably reworked granulites of the interior Parry Sound domain (iPSD) has enabled investigation of the structural and petrologic character of domain-bounding shear zones within the deep orogenic crust. Recent detailed mapping and structural data collected along the southwestern margin of the iPSD is consistent with the suggestion of Culshaw et al. (in prep) that spaced outcrop-scale shear zones have coalesced and progressively reworked layered granulites into a transposed amphibolite-facies tectonite. The tectonites comprise the Twelve Mile Bay Shear Zone (TMBSZ), which separates the iPSD from para-autocthonous rocks to the south. This study investigates the grain- and outcrop-scale mechanisms involved in shear zone development and attempts to quantify the associated changes in rock rheology. Northwest of TMBSZ, samples collected across individual outcrop-scale shear zones (i.e., across large strain gradients) have distinct differences in mineralogy and microstructure. In mafic layers the original granulite texture and cpx + opx + pl + hbl +/- grt assemblage is commonly retained away from the shear zones within unsheared "panels". With proximity to the shear zones pyroxenes and garnet are progressively consumed in hydration reactions producing hornblende and biotite, which define a new planar foliation within the highly attenuated and deflected layering. Felsic layers generally have only minor mineralogical changes across the zones, but develop an increasingly intense and recrystallized structural fabric into the sheared margin. The shear zones are commonly cored by variably deformed pegmatite dikes that were emplaced prior to, or during the early stages of shearing. Evidence for incipient shear zone formation along mineralized fracture sets that cut

  10. High-pressure metamorphism in the southern New England Orogen: Implications for long-lived accretionary orogenesis in eastern Australia

    NASA Astrophysics Data System (ADS)

    Phillips, G.; Offler, R.; Rubatto, D.; Phillips, D.

    2015-09-01

    New geochemical, metamorphic, and isotopic data are presented from high-pressure metamorphic rocks in the southern New England Orogen (eastern Australia). Conventional and optimal thermobarometry are augmented by U-Pb zircon and 40Ar/39Ar phengite dating to define pressure-temperature-time (P-T-t) histories for the rocks. The P-T-t histories are compared with competing geodynamic models for the Tasmanides, which can be summarized as (i) a retreating orogen model, the Tasmanides formed above a continuous, west dipping, and eastward retreating subduction zone, and (ii) a punctuated orogen model, the Tasmanides formed by several arc accretion, subduction flip, and/or transference events. Whereas both scenarios are potentially supported by the new data, an overlap between the timing of metamorphic recrystallization and key stages of Tasmanides evolution favors a relationship between a single, long-lived subduction zone and the formation, exhumation, and exposure of the high-pressure rocks. By comparison with the retreating orogen model, the following links with the P-T-t histories emerge: (i) exhumation and underplating of oceanic eclogite during the Delamerian Orogeny, (ii) recrystallization of underplated and exhuming high-pressure rocks at amphibolite facies conditions coeval with a period of rollback, and (iii) selective recrystallization of high-pressure rocks at blueschist facies conditions, reflecting metamorphism in a cooled subduction zone. The retreating orogen model can also account for the anomalous location of the Cambrian-Ordovician high-pressure rocks in the Devonian-Carboniferous New England Orogen, where sequential rollback cycles detached and translated parts of the leading edge of the overriding plate to the next, younger orogenic cycle.

  11. Alkali-calcic and alkaline post-orogenic (PO) granite magmatism: petrologic constraints and geodynamic settings

    NASA Astrophysics Data System (ADS)

    Bonin, Bernard; Azzouni-Sekkal, Abla; Bussy, François; Ferrag, Sandrine

    1998-12-01

    The end of an orogenic Wilson cycle corresponds to amalgamation of terranes into a Pangaea and is marked by widespread magmatism dominated by granitoids. The post-collision event starts with magmatic processes still influenced by subducted crustal materials. The dominantly calc-alkaline suites show a shift from normal to high-K to very high-K associations. Source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle, affected by dehydration melting and generating more and more K- and LILE-rich magmas. In the vicinity of intra-crustal magma chambers, anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves. The post-collision event ends with emplacement of bimodal post-orogenic (PO) suites along transcurrent fault zones. Two suites are defined, (i) the alkali-calcic monzonite-monzogranite-syenogranite-alkali feldspar granite association characterised by [biotite+plagioclase] fractionation and moderate [LILE+HFSE] enrichments and (ii) the alkaline monzonite-syenite-alkali feldspar granite association characterised by [amphibole+alkali feldspar] fractionation and displaying two evolutionary trends, one peralkaline with sodic mafic mineralogy and higher enrichments in HFSE than in LILE, and the other aluminous biotite-bearing marked by HFSE depletion relative to LILE due to accessory mineral precipitation. Alkali-calcic and alkaline suites differ essentially in the amounts of water present within intra-crustal magma chambers, promoting crystallisation of various mineral assemblages. The ultimate enriched and not depleted mantle source is identical for the two PO suites. The more primitive LILE and HFSE-rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere. Present rock compositions are a mixture of major mantle contribution and various crustal components

  12. GEOPHYSICS. Layered deformation in the Taiwan orogen.

    PubMed

    Huang, T-Y; Gung, Y; Kuo, B-Y; Chiao, L-Y; Chen, Y-N

    2015-08-14

    The underthrusting of continental crust during mountain building is an issue of debate for orogens at convergent continental margins. We report three-dimensional seismic anisotropic tomography of Taiwan that shows a nearly 90° rotation of anisotropic fabrics across a 10- to 20-kilometer depth, consistent with the presence of two layers of deformation. The upper crust is dominated by collision-related compressional deformation, whereas the lower crust of Taiwan, mostly the crust of the subducted Eurasian plate, is dominated by convergence-parallel shear deformation. We interpret this lower crustal shearing as driven by the continuous sinking of the Eurasian mantle lithosphere when the surface of the subducted plate is coupled with the orogen. The two-layer deformation clearly defines the role of subduction in the formation of the Taiwan mountain belt. PMID:26273051

  13. SVM-based base-metal prospectivity modeling of the Aravalli Orogen, Northwestern India

    NASA Astrophysics Data System (ADS)

    Porwal, Alok; Yu, Le; Gessner, Klaus

    2010-05-01

    The Proterozoic Aravalli orogen in the state of Rajasthan, northwestern India, constitutes the most important metallogenic province for base-metal deposits in India and hosts the entire economically viable lead-zinc resource-base of the country. The orogen evolved through near-orderly Wilson cycles of repeated extensional and compressional tectonics resulting in sequential opening and closing of intracratonic rifts and amalgamation of crustal domains during a circa 1.0-Ga geological history from 2.2 Ga to 1.0 Ga. This study develops a conceptual tectonostratigraphic model of the orogen based on a synthesis of the available geological, geophysical and geochronological data followed by deep-seismic-reflectivity-constrained 2-D forward gravity modeling, and links it to the Proterozoic base-metal metallogeny in the orogen in order to identify key geological controls on the base-metal mineralization. These controls are translated into exploration criteria for base-metal deposits, validated using empirical spatial analysis, and used to derive input spatial variables for model-based base-metal prospectivity mapping of the orogen. A support vector machine (SVM) algorithm augmented by incorporating a feature selection procedure is used in a GIS environment to implement the prospectivity mapping. A comparison of the SVM-derived prospectivity map with the ones derived using other established models such as neural-networks, logistic regression, and Bayesian weights-of-evidence indicates that the SVM outperforms other models, which is attributed to the capability of the SVM to return robust classification based on small training datasets.

  14. Where the Caledonides crosses the Grenville: The Grenvillian Glenelg Inlier as an allochthonous pip within a fold-nappe complex in the Scottish Caledonides

    NASA Astrophysics Data System (ADS)

    Krabbendam, M.; Ramsay, J. G.; Leslie, A. G.; Tanner, P. W. G.; Dietrich, D.; Goodenough, K. M.

    2012-04-01

    The Grenvillian (1100 - 990 Ma) and Caledonian (470-420 Ma) orogenies represent pivotal tectonic events in the evolution of Laurentia and Baltica. Significantly, these two orogenic belts lie at a high angle to one another, with an inferred intersection in NW Scotland. This relationship is most readily examined at the Glenelg Inlier of NW Scotland, a basement gneiss inlier within the Scottish Caledonides nappe pile. This inlier contains a Western Glenelg Inlier, composed of orthogneiss with no record of Grenville metamorphism, and a separate Eastern Glenelg Inlier, comprising both ortho-and paragneisses that experienced Grenvillian eclogite-facies metamorphism. The two components of the Glenelg Inlier are interleaved and/or infolded with locally unconformable, basal Moine Supergroup metasediments, deposited (just) after Grenvillian orogenesis. The inlier and the metasediments are now located in the hanging wall of the well-studied Caledonian Moine Thrust. Despite decades of research and classical structural studies, the overall geometry and structural evolution of the Glenelg Inlier and the surrounding Moine metasediments remain elusive. The synthesis presented here is based upon both new, and hitherto unpublished, mapping. The Glenelg Inlier and enclosing Moine were deformed by three generations of major ductile fold structures (F1-F3). In areas of medium strain, away from the basement inliers, F2 and F3 large-scale structures face and verge towards the west, and record coaxial interference patterns. In areas of higher strain, F2 fold axes were rotated into parallelism with the (westerly) transport direction. Subsequent refolding of these F2 folds by west-vergent (N-S trending) F3 folds led in some areas to high-angle, non-coaxial fold interference patterns, including dome-and-basin structures. On structural grounds, both F2 and F3 are thought to be of Caledonian age. An approximate restoration of the F2 and F3 folds reveals the pre-F2 basement-cover architecture

  15. The Honey Brook Upland: Multiple Accessory Phase Parageneses in a Grenville Terrane and Associated Cover Sequence

    NASA Astrophysics Data System (ADS)

    Pyle, J. M.

    2004-05-01

    The Honey Brook Upland (HBU) of southeastern Pennsylvania is the only Grenville-age AMCG suite exposed between the Adirondacks and the anorthosite-bearing terranes of central and eastern Virginia. Several distinct accessory phase parageneses in the HBU and its Paleozoic metasedimentary cover sequence help: 1) to constrain the timing of known events affecting the HBU and cover; 2) to identify previously unknown events, and; 3) to elucidate the T-x conditions of the distinct metamorphic events. Granulite-facies gneisses (charnockites, mangerites) associated with the Honey Brook anorthosite contain primary Zrn, Aln, and Ap, but also texturally late Mnz in Bt+Hbl coronas around Opx, and (with Xno) as oriented acicular inclusions in primary Ap. The latter texture is interpreted as evidence of metasomatic infiltration (Harlov et al., Am Min, 2002), and Mnz-Xno pairs in Ap yield temperatures of 450° C-500° C. Amphibolite-facies felsic gneisses (metavolcanics) contain primary Zrn, Mnz, and Ap; Mnz rims are commonly replaced by Aln+Ep. Mnz in the metavolcanics has three distinct compositional domains; moderate-Y, low-Th cores, low-Y, high-Th outboards, and rare low-Y, low-Th rims. Garnets in the metavolcanics are only slightly zoned in Y (1000-800 ppm core to rim); Grt rims are typically replaced by Ep+Ms. Pairing of Grt compositions and Mnz core domains yields temperatures of 500° C-520° C, indicating preservation of prograde Mnz. The association of Qtz+Plg+Ms symplectites with high-Th Mnz domains suggests that this domain is representative of peak metamorphic conditions (650° C-750° C). Coexisting Mnz and Xno are found in metaquartzite (Chickies Formation) at the base of the HBU cover sequence; epitaxial overgrowths of Xno on oscillatory-zoned Zrn imply anin situ origin for Xno. Mnz-Xno pairs in the metaquartzite yield equilibration temperatures of 300° C-400° C. A provisional sequence of events in the HBU and cover, as determined from chemical Mnz ages, is as

  16. SW U. S. diabase province: A 1. 1-Ga intrusion event of middle Grenville and middle Keweenawan age

    SciTech Connect

    Conway, C.M.; Elston, D.P. ); Wrucke, C.T. )

    1993-02-01

    Diabase in the southwestern US intrudes Middle Proterozoic stratified rocks as sills and Early and Middle Proterozoic crystalline rocks as subhorizontal sheets and subvertical dikes. It is discontinuous in a broad belt extending from western Texas to southeastern California. The best known intrusions are sills in Middle Proterozoic strata in Death Valley, Grand Canyon, and central Arizona. Sparse to rare dikes in some of these strata trend mostly north but range from north-northeast to west-northwest. Diabase dikes widespread in crystalline rocks in western Arizona and adjacent parts of southeastern California strike from north to west-northwest, but are predominantly northwesterly. Dikes and sheets are also present in crystalline rocks in the southern Pinaleno Mountains, southeastern Arizona, where dikes strike west-northwest. The northwest trend of the diabase province and prevalent northwesterly trend of dikes in crystalline rocks suggest that intrusion was controlled by an approximately horizontal least compressive stress field roughly parallel to the Grenville Front. Radiometric ages of Arizona and California diabase indicate emplacement at [approximately]1,100 Ma. Paleomagnetic poles from diabase sills and enclosing stratified rocks in Arizona correlate with poles reported from middle and early-late Keweenawan rocks of Lake Superior. Emplacement of the diabase coincides with: (1) the middle Keweenawan eruptive and intrusive episode of the Midcontinent Rift System; (2) a major episode of (middle) Grenville thrusting and deformation documented in the Van Horn area; and (3) a time of abrupt reversal in North American apparent polar wander. These interrelated manifestations presumably arose in response to a major episode of plate interaction and collision between North American and a plate that encroached from the southeast.

  17. Lead and barium sources in Cambrian siliciclastics and sediment provenance of a sector of the Taconic Orogen, Quebec: a mixing scenario based on Pb-isotopic evidence

    USGS Publications Warehouse

    Schrijver, K.; Zartman, R.E.; Williams-Jones, A. E.

    1994-01-01

    To test the hypothesis that siliciclastic rocks constituted the major source of Pb and Ba in barite-galena deposits of the Taconic Orogen, we determined Pb-isotope ratios in galena, barren rocks and contained minerals, as well as concentrations of Pb, U, Th and Ba in the latter (detrital feldspars, sandstones, mudstones, rock clasts and carbonate cements and clasts). Ranges in 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb of 28 galena samples are 17.96-18.05, 15.56-15.59 and 37.75-37.93, respectively; ranges for 41 barren rocks and minerals are 16.17-23.31, 15.26-15.86 and 35.98-42.51, respectively. The lowest ratios are in feldspar, and the highest in carbonate and mudstone. Values of the mudstones samples overlap those of galena when corrected for in situ decay of U and Th since galena precipitation (???450 Ma). We thus propose that mudstones constituted a source of lead. Corrected ratios for anomalously Pb-rich mudstones are virtually identical to galena-Pb ratios and may be due to contamination by lead-bearing brines. Assuming that burial diagenesis did not disturb the Pb-isotope values of sandstones, these rocks contributed only a minor fraction of lead to the galena, estimated at ???20% for one deposit. The source of barite-Ba was probably perthite. Low Ba and Pb concentrations of sandstone adjacent to this deposit, compared to high concentrations remote from it, support leaching of barium (and minor lead) from feldspar penecontemporaneous with feldspar dissolution. Geological data indicate that the provenance of the siliciclastic rocks was mainly from Grenville terrane. A comparison of our Pb-isotopic data for Taconic perthite with those of Grenville K-feldspar, as well as ratios of trace elements, support this provenance for both sandstones and mudstones. The presence of carbonate platforms peripheral to the orogen, and the Middle Ordovician-Middle Devonian depositional range of the studied and Mississippi Valley type deposits north (Newfoundland) and south (U

  18. Neoproterozoic, Paleozoic, and Mesozoic granitoid magmatism in the Qinling Orogen, China: Constraints on orogenic process

    NASA Astrophysics Data System (ADS)

    Wang, Xiaoxia; Wang, Tao; Zhang, Chengli

    2013-08-01

    The Qinling Orogen is one of the main orogenic belts in Asia and is characterized by multi-stage orogenic processes and the development of voluminous magmatic intrusions. The results of zircon U-Pb dating indicate that granitoid magmatism in the Qinling Orogen mainly occurred in four distinct periods: the Neoproterozoic (979-711 Ma), Paleozoic (507-400 Ma), and Early (252-185 Ma) and Late (158-100 Ma) Mesozoic. The Neoproterozoic granitic magmatism in the Qinling Orogen is represented by strongly deformed S-type granites emplaced at 979-911 Ma, weakly deformed I-type granites at 894-815 Ma, and A-type granites at 759-711 Ma. They can be interpreted as the products of respectively syn-collisional, post-collisional and extensional setting, in response to the assembly and breakup of the Rodinia supercontinent. The Paleozoic magmatism can be temporally classified into three stages of 507-470 Ma, 460-422 Ma and ˜415-400 Ma. They were genetically related to the subduction of the Shangdan Ocean and subsequent collision of the southern North China Block and the South Qinling Belt. The 507-470 Ma magmatism is spatially and temporally related to ultrahigh-pressure metamorphism in the studied area. The 460-422 Ma magmatism with an extensive development in the North Qinling Belt is characterized by I-type granitoids and originated from the lower crust with the involvement of mantle-derived magma in a collisional setting. The magmatism with the formation age of ˜415-400 Ma only occurred in the middle part of the North Qinling Belt and is dominated by I-type granitoid intrusions, and probably formed in the late-stage of a collisional setting. Early Mesozoic magmatism in the study area occurred between 252 and 185 Ma, with the cluster in 225-200 Ma. It took place predominantly in the western part of the South Qinling Belt. The 250-240 Ma I-type granitoids are of small volume and show high Sr/Y ratios, and may have been formed in a continental arc setting related to subduction

  19. Mantle refertilization and magmatism in old orogenic regions: The role of late-orogenic pyroxenites

    NASA Astrophysics Data System (ADS)

    France, Lydéric; Chazot, Gilles; Kornprobst, Jacques; Dallai, Luigi; Vannucci, Riccardo; Grégoire, Michel; Bertrand, Hervé; Boivin, Pierre

    2015-09-01

    Pyroxenites and garnet pyroxenites are mantle heterogeneities characterized by a lower solidus temperature than the enclosing peridotites; it follows that they are preferentially involved during magma genesis. Constraining their origin, composition, and the interactions they underwent during their subsequent evolution is therefore essential to discuss the sources of magmatism in a given area. Pyroxenites could represent either recycling of crustal rocks in mantle domains or mantle originated rocks (formed either by olivine consuming melt-rock reactions or by crystal fractionation). Petrological and geochemical (major and trace elements, Sr-Nd and O isotopes) features of xenoliths from various occurrences (French Massif-Central, Jordan, Morocco and Cameroon) show that these samples represent cumulates crystallized during melt percolation at mantle conditions. They formed in mantle domains at pressures of 1-2 GPa during post-collisional magmatism (possibly Hercynian for the French Massif-Central, and Panafrican for Morocco, Jordan and Cameroon). The thermal re-equilibration of lithospheric domains, typical of the late orogenic exhumation stages, is also recorded by the samples. Most of the samples display a metasomatic overprint that may be either inherited or likely linked to the recent volcanic activity that occurred in the investigated regions. The crystallization of pyroxenites during late orogenic events has implications for the subsequent evolution of the mantle domains. The presence of large amounts of mantle pyroxenites in old orogenic regions indeed imparts peculiar physical and chemical characteristics to these domains. Among others, the global solidus temperature of the whole lithospheric domain will be lowered; in turn, this implies that old orogenic regions are refertilized zones where magmatic activity would be enhanced.

  20. Ambient tremors in a collisional orogenic belt

    USGS Publications Warehouse

    Chuang, Lindsay Yuling; Chen, Kate Huihsuan; Wech, Aaron G.; Byrne, Timothy; Peng, Wei

    2014-01-01

    Deep-seated tectonic tremors have been regarded as an observation tied to interconnected fluids at depth, which have been well documented in worldwide subduction zones and transform faults but not in a collisional mountain belt. In this study we explore the general features of collisional tremors in Taiwan and discuss the possible generation mechanism. In the 4 year data, we find 231 ambient tremor episodes with durations ranging from 5 to 30 min. In addition to a coseismic slip-induced stress change from nearby major earthquake, increased tremor rate is also highly correlated with the active, normal faulting earthquake swarms at the shallower depth. Both the tremor and earthquake swarm activities are confined in a small, area where the high attenuation, high thermal anomaly, the boundary between high and low resistivity, and localized veins on the surfaces distributed, suggesting the involvement of fluids from metamorphic dehydration within the orogen.

  1. Tectonothermal history in the Mattawa area, Ontario, Canada, deduced from paleomagnetism and Ar-40/Ar-39 dating of a Grenville dike

    NASA Astrophysics Data System (ADS)

    Hyodo, Hironobu; York, Derek; Dunlop, David J.

    1993-10-01

    The paleomagnetic ambient has been determined from samples at various distances from a Grenville diabase dike cutting late Protozoic tonalitic gneiss near Mattawa in the Grenville Province, Canada. Using the Ar-40/Ar-39 technique, the intrusion age of the dike is estimated to be 570 +/- 3 Ma from a 0.5-mm-diameter chilled margin chip. The paleo-ambient temperature of the country rock at the time of intrusion, 184 +/- 40 C was calculated from Jaeger's (1964) one-dimensional heat conduction model. The burial depth of the presently exposed rocks is estimated to be 6.5 +/- 1.7 km, assuming a surface temperature of 15 C and a geothermal gradient of 26 C/km. This depth at 570 Ma is discordant with the presence of Ordovician shallow-water (less than 100 m) limestones about 200 km distant, suggesting that differential uplift may have occurred between the two areas.

  2. Palaeomagnetism of The Stoer Group (neoprotrozoic), NW Scotland: Deposition and Magnetisation Prior To Culmination of The Grenville Orogeny

    NASA Astrophysics Data System (ADS)

    Darabi, M. H.; Piper, J. D. A.

    The Stoer Group redbeds comprise the lower of two groups making up the Torridonian Supergroup. It was deposited on Lewisian metamorphic rocks of the Laurentian Shield and deformed before deposition of the Torridon Group. A recent Rb/Sr age of 1199 Ma is associated with a large error of 70 Ma and we have restudied the palaeomag- netism with a view to refining the age and tectonic setting of deposition. AMS results show a primary sedimentary fabric with imbrication indicating a westerly source con- sistent with sedimentary evidence. Study of penecontemporaneous slumps shows that remanence postdates early stages of consolidation but study of blocks which have slipped into the base of the Torridon Group shows that it was fixed by the time of later sedimentation. A fold test also suggests that it predates the folding episode. Mag- netisations resident in magnetite and hematite show an increase from shallower to steeper inclinations upwards through the succession. We interpret this as an increase in inclination of the palaeofield during the time of lithification recorded by primary magnetite, which was soon after partially oxidised to hematite. Palaeopoles show a time sequence, which correlates with the Gardar APW Track at ca. 1160 Ma and we attribute folding of the Stoer Group to the culminating stages of tectonism at 1100 Ma in the Grenville Belt formerly sited to the south.

  3. The U-Pb age and Hf isotope record of detrital zircons from Paleozoic sedimentary rocks of the proto-Andean accretionary orogen

    NASA Astrophysics Data System (ADS)

    Bahlburg, Heinrich

    2010-05-01

    The global record of detrital zircon ages is frequently interpreted as reflecting cycles of supercontinent assembly and dispersal. When peaks in zircon age distributions are combined with data on the relative production of juvenile crust through time, a coincidence between the data sets is discernible for events mainly in the Archean and Paleoproterozoic. Hawkesworth et al. (2009) recently concluded that peaks in U-Pb detrital ages (n=7000; Campbell and Allen, 2008) and juvenile crust production do not always coincide on a global scale, and that a link to supercontinent cycles may in fact not exist. The reason for this is seen in the poor preservation potential of juvenile magmatic arc crust of accretionary orogens relative to continental collisional systems. This is significant as juvenile crust tends to be produced at active margins where it is also prone to destruction by subduction erosion. Granitic magmatic systems in collisional mountain belts, which later may become prolific sources of detrital zircons when eroded, are often formed by partial melting of preexisting crust. They may be volumetrically smaller than arc systems but have a better preservation potential due to their position in the interior of the evolving orogenic belts. The difference in formation and preservation potential is seen as a potential bias of the detrital zircon record towards collisional mountain belts. In contrast, non-collisional accretionary orogens should have detrital zircon age records free of this bias. The SW Amazonia Orogenic System evolved by the episodic accretion of marginal orogenic belts to Amazonia from c. 2.3 to 0.9 Ga. From 0.9 to 0.25 Ga it was superseded by the Andean portion of the Terra Australis accretionary orogen. Recent reconsiderations of paleomagnetic data (e.g. Evans et al., 2010) together with the tectonic evolution of these orogens indicate that they reflect a c. 2 Ga long orogenic history free of major continental collisions. A compilation of U

  4. Paleozoic orogens in New England, USA

    USGS Publications Warehouse

    Robinson, P.; Tucker, R.D.; Bradley, D.; Berry, H.N., IV; Osberg, P.H.

    1998-01-01

    Stratigraphy and isotope geochronology in the crystalline core of the Appalachians suggest revised interpretations of the extent, nature and timing of Paleozoic orogens in New England. Five major episodes of magmatism, deformation, and high-grade regional metamorphism are recognized: Taconian (455-442 Ma), Acadian (423-385 Ma), Neo-Acadian (366-350 Ma), Late Pennsylvanian (300-290 Ma) and Alleghanian (280-260 Ma). In the Taconian, the passive margin of Laurentia was subducted below a complex magmatic arc lasting from 480 to 442 Ma, founded in part on continental crust of a Medial New England terrane with possible affinities with Amazonia. Questions about Medial New England involve its coherence as a single plate, and the nature of its underlying crust. The Acadian began in Late Silurian as a collision between the amalgamated Laurentia-Medial New England and outer belts of Composite Avalon along a cryptic suture in coastal Maine, and progressed northwestward to the Connecticut Valley basin by mid-Devonian. Tonalitic-granitic magmatism and up to granulite-facies metamorphism culminated in Early Devonian, possibly tied to lithospheric detachment below the subducting northwestern plate and consequent asthenosphere upwelling. Newly discovered Neo-Acadian Late Devonian to Early Mississippian tonalitic-granitic magmatism, up to granulite-facies metamorphism, and severe deformation in central Massachusetts took place in a plate context poorly understood. Late Pennsylvanian effects include magmatism, metamorphism, and deformation near south New England gneiss domes and the Sebago batholith, and development of the right-lateral Norumbega fault system. Permian Alleghanian effects include penetrative deformation, granitic intrusions and up to sillimanite-grade metamorphism of Pennsylvanian beds in southeastern New England. These last two episodes relate to the arrival of Africa.

  5. Mesoproterozoic syntectonic garnet within Belt Supergroup metamorphic tectonites: Evidence of Grenville-age metamorphism and deformation along northwest Laurentia

    USGS Publications Warehouse

    Nesheim, T.O.; Vervoort, J.D.; McClelland, W.C.; Gilotti, J.A.; Lang, H.M.

    2012-01-01

    Northern Idaho contains Belt-Purcell Supergroup equivalent metamorphic tectonites that underwent two regional deformational and metamorphic events during the Mesoproterozoic. Garnet-bearing pelitic schists from the Snow Peak area of northern Idaho yield Lu-Hf garnet-whole rock ages of 1085??2. Ma, 1198??79. Ma, 1207??8. Ma, 1255??28. Ma, and 1314??2. Ma. Garnet from one sample, collected from the Clarkia area, was micro-drilled to obtain separate core and rim material that produced ages of 1347??10. Ma and 1102??47. Ma. The core versus rim ages from the micro-drilled sample along with the textural and spatial evidence of the other Lu-Hf garnet ages indicate two metamorphic garnet growth events at ~. 1330. Ma (M1) and ~. 1080. Ma (M2) with the intermediate ages representing mixed ages. Some garnet likely nucleated and grew M1 garnet cores that were later overgrown by younger M2 garnet rims. Most garnet throughout the Clarkia and Snow Peak areas are syntectonic with a regional penetrative deformational fabric, preserved as a strong preferred orientation of metamorphic matrix minerals (e.g., muscovite and biotite). The syntectonic garnets are interpreted to represent one regional, coeval metamorphic and deformation event at ~. 1080. Ma, which overlaps in time with the Grenville Orogeny. The older ~. 1330. Ma ages may represent an extension of the East Kootenay Orogeny described in western Canada. These deformational and metamorphic events indicate that western Laurentia (North America) was tectonically active in the Mesoproterozoic and during the assembly of the supercontinent Rodinia. ?? 2011 Elsevier B.V.

  6. Mesoproterozoic syntectonic garnet within Belt Supergroup metamorphic tectonites: Evidence of Grenville-age metamorphism and deformation along northwest Laurentia

    NASA Astrophysics Data System (ADS)

    Nesheim, Timothy O.; Vervoort, Jeffrey D.; McClelland, William C.; Gilotti, Jane A.; Lang, Helen M.

    2012-03-01

    Northern Idaho contains Belt-Purcell Supergroup equivalent metamorphic tectonites that underwent two regional deformational and metamorphic events during the Mesoproterozoic. Garnet-bearing pelitic schists from the Snow Peak area of northern Idaho yield Lu-Hf garnet-whole rock ages of 1085 ± 2 Ma, 1198 ± 79 Ma, 1207 ± 8 Ma, 1255 ± 28 Ma, and 1314 ± 2 Ma. Garnet from one sample, collected from the Clarkia area, was micro-drilled to obtain separate core and rim material that produced ages of 1347 ± 10 Ma and 1102 ± 47 Ma. The core versus rim ages from the micro-drilled sample along with the textural and spatial evidence of the other Lu-Hf garnet ages indicate two metamorphic garnet growth events at ~ 1330 Ma (M1) and ~ 1080 Ma (M2) with the intermediate ages representing mixed ages. Some garnet likely nucleated and grew M1 garnet cores that were later overgrown by younger M2 garnet rims. Most garnet throughout the Clarkia and Snow Peak areas are syntectonic with a regional penetrative deformational fabric, preserved as a strong preferred orientation of metamorphic matrix minerals (e.g., muscovite and biotite). The syntectonic garnets are interpreted to represent one regional, coeval metamorphic and deformation event at ~ 1080 Ma, which overlaps in time with the Grenville Orogeny. The older ~ 1330 Ma ages may represent an extension of the East Kootenay Orogeny described in western Canada. These deformational and metamorphic events indicate that western Laurentia (North America) was tectonically active in the Mesoproterozoic and during the assembly of the supercontinent Rodinia.

  7. Linking magmatism with collision in an accretionary orogen

    PubMed Central

    Li, Shan; Chung, Sun-Lin; Wilde, Simon A.; Wang, Tao; Xiao, Wen-Jiao; Guo, Qian-Qian

    2016-01-01

    A compilation of U-Pb age, geochemical and isotopic data for granitoid plutons in the southern Central Asian Orogenic Belt (CAOB), enables evaluation of the interaction between magmatism and orogenesis in the context of Paleo-Asian oceanic closure and continental amalgamation. These constraints, in conjunction with other geological evidence, indicate that following consumption of the ocean, collision-related calc-alkaline granitoid and mafic magmatism occurred from 255 ± 2 Ma to 251 ± 2 Ma along the Solonker-Xar Moron suture zone. The linear or belt distribution of end-Permian magmatism is interpreted to have taken place in a setting of final orogenic contraction and weak crustal thickening, probably as a result of slab break-off. Crustal anatexis slightly post-dated the early phase of collision, producing adakite-like granitoids with some S-type granites during the Early-Middle Triassic (ca. 251–245 Ma). Between 235 and 220 Ma, the local tectonic regime switched from compression to extension, most likely caused by regional lithospheric extension and orogenic collapse. Collision-related magmatism from the southern CAOB is thus a prime example of the minor, yet tell-tale linking of magmatism with orogenic contraction and collision in an archipelago-type accretionary orogen. PMID:27167207

  8. Linking magmatism with collision in an accretionary orogen

    NASA Astrophysics Data System (ADS)

    Li, Shan; Chung, Sun-Lin; Wilde, Simon A.; Wang, Tao; Xiao, Wen-Jiao; Guo, Qian-Qian

    2016-05-01

    A compilation of U-Pb age, geochemical and isotopic data for granitoid plutons in the southern Central Asian Orogenic Belt (CAOB), enables evaluation of the interaction between magmatism and orogenesis in the context of Paleo-Asian oceanic closure and continental amalgamation. These constraints, in conjunction with other geological evidence, indicate that following consumption of the ocean, collision-related calc-alkaline granitoid and mafic magmatism occurred from 255 ± 2 Ma to 251 ± 2 Ma along the Solonker-Xar Moron suture zone. The linear or belt distribution of end-Permian magmatism is interpreted to have taken place in a setting of final orogenic contraction and weak crustal thickening, probably as a result of slab break-off. Crustal anatexis slightly post-dated the early phase of collision, producing adakite-like granitoids with some S-type granites during the Early-Middle Triassic (ca. 251–245 Ma). Between 235 and 220 Ma, the local tectonic regime switched from compression to extension, most likely caused by regional lithospheric extension and orogenic collapse. Collision-related magmatism from the southern CAOB is thus a prime example of the minor, yet tell-tale linking of magmatism with orogenic contraction and collision in an archipelago-type accretionary orogen.

  9. Rotund versus skinny orogens: Well-nourished or malnourished gold?

    USGS Publications Warehouse

    Goldfarb, R.J.; Groves, D.I.; Gardoll, S.

    2001-01-01

    Orogenic gold vein deposits require a particular conjunction of processes to form and be preserved, and their global distribution can be related to broad-scale, evolving tectonic processes throughout Earth history. A heterogeneous distribution of formation ages for these mineral deposits is marked by two major Precambrian peaks (2800-2555 Ma and 2100-1800 Ma), a singular lack of deposits for 1200 m.y. (1800-600 Ma), and relatively continuous formation since then (after 600 Ma). The older parts of the distribution relate to major episodes of continental growth, perhaps controlled by plume-influenced mantle overturn events, in the hotter early Earth (ca. 1800 Ma or earlier). This worldwide process allowed preservation of gold deposits in cratons, roughly equidimensional, large masses of buoyant continental crust. Evolution to a less episodic, more continuous, modern-style plate tectonic regime led to the accretion of volcano-sedimentary complexes as progressively younger linear orogenic belts sorrounding the margins of the more buoyant cratons. The susceptibility of these linear belts to uplift and erosion can explain the overall lack of orogenic gold deposits at 1800-600 Ma, their exposure in 600-50 Ma orogens, the increasing importance of placer deposits back through the Phanerozoic since ca. 100 Ma, and the absence of gold deposits in orogenic belts younger than ca. 50 Ma.

  10. Linking magmatism with collision in an accretionary orogen.

    PubMed

    Li, Shan; Chung, Sun-Lin; Wilde, Simon A; Wang, Tao; Xiao, Wen-Jiao; Guo, Qian-Qian

    2016-01-01

    A compilation of U-Pb age, geochemical and isotopic data for granitoid plutons in the southern Central Asian Orogenic Belt (CAOB), enables evaluation of the interaction between magmatism and orogenesis in the context of Paleo-Asian oceanic closure and continental amalgamation. These constraints, in conjunction with other geological evidence, indicate that following consumption of the ocean, collision-related calc-alkaline granitoid and mafic magmatism occurred from 255 ± 2 Ma to 251 ± 2 Ma along the Solonker-Xar Moron suture zone. The linear or belt distribution of end-Permian magmatism is interpreted to have taken place in a setting of final orogenic contraction and weak crustal thickening, probably as a result of slab break-off. Crustal anatexis slightly post-dated the early phase of collision, producing adakite-like granitoids with some S-type granites during the Early-Middle Triassic (ca. 251-245 Ma). Between 235 and 220 Ma, the local tectonic regime switched from compression to extension, most likely caused by regional lithospheric extension and orogenic collapse. Collision-related magmatism from the southern CAOB is thus a prime example of the minor, yet tell-tale linking of magmatism with orogenic contraction and collision in an archipelago-type accretionary orogen. PMID:27167207

  11. Strong imprint of past orogenic events on the thermochronological record

    NASA Astrophysics Data System (ADS)

    Braun, Jean

    2016-06-01

    Using a simple solution to the heat conduction equation, I show how, at the end of an orogenic event, the relaxation of isotherms from a syn-orogenic advection-dominated geometry to a post-orogenic conduction-dominated geometry leads to the creation of a thick iso-age crustal layer. Subsequent erosion of this layer yields peculiar age-elevation profiles and detrital age distributions that cannot be easily interpreted using traditional techniques. I illustrate these points by using a simple analytical solution of the heat equation as well as a transient, three-dimensional numerical model. I also demonstrate that the age of the end of an orogenic event is so strongly imprinted in the thermochronological record that it erases most of the information pertaining to the orogenic phase itself and the subsequent isostatically-driven exhumation. The concept is used to explain two thermochronological datasets from the Himalayas and demonstrate that their most likely interpretation involves the sudden interruption of extremely fast exhumation accommodated by movement along the South Tibetan Detachment in the Higher Himalayas around 15 Ma.

  12. Cumberland batholith, Trans-Hudson Orogen, Canada: Petrogenesis and implications for Paleoproterozoic crustal and orogenic processes

    NASA Astrophysics Data System (ADS)

    Whalen, Joseph B.; Wodicka, Natasha; Taylor, Bruce E.; Jackson, Garth D.

    2010-06-01

    Large volume, plutonic belts, such as the ˜ 221,000 km 2, ca. 1.865-1.845 Ga Cumberland batholith (CB) of the Trans-Hudson Orogen in Canada, are major components of Paleoproterozoic orogenic belts. In many cases, they have been interpreted as continental arc batholiths. The petrogenesis and tectonic context of the CB and implications for crustal growth and recycling are interpreted herein based on a 900 km geochemical-isotopic (Nd-O) transect across it and into granitoid plutons within bounding Archean cratons in central and southern Baffin Island. The mainly granulite grade CB, emplaced over an age span of between 14 and 24 Ma, consists mainly of high-K to shoshonitic monzogranite and granodiorite, but also includes low- and medium-K granitoid rocks. Metaluminous to slightly peraluminous compositions and δ 18O (VSMOW) values (+ 6 to + 10‰) indicate derivation from infracrustal (I-type) sources. ɛ Nd 1.85 Ga signatures (- 12 to - 2) of both mafic and felsic units suggest a dominance of evolved sources. Isotopic signatures in the interior of the CB (- 2 to - 7) are more radiogenic than those within Archean domains in central (- 8 to - 15) and southern (- 5 to - 19) Baffin Island. The isotopic transect is interpreted as 'imaging' an accreted microcontinental block (Meta Incognita) and bounding Archean cratons. The CB includes granites of arc, within-plate (A-type) and post-collisional affinity and volumetrically minor mafic rocks with both arc and non-arc features. (La/Yb) CN and Sr/Y values range from < 1 to 225 and < 1 to 611, respectively. In these respects, some CB granitoid rocks resemble Paleozoic adakitic granites, interpreted as partial melts of greatly thickened crust within post-collisional settings, such as Tibet. Thus, the CB likely encompasses various non-consanguineous magmatic suites generated at deep- to mid-crustal depths. Although CB granitoid rocks undoubtedly had important crustal sources, it is hard to assess the relative contribution of

  13. Seismogenic nodes in the Mediterranean orogenic belt

    NASA Astrophysics Data System (ADS)

    Gorshkov, A.; Panza, G.; Soloviev, A.; Aoudia, A.

    2003-04-01

    The central segment of the Mediterranean orogenic belt (the Apennines, Alps, Carpathians, Balkanides, and Dinarides) has been studied to identify seismogenic nodes, specific structures formed at the intersections of fault zones. The nodes have been delineated with the morphostructural zoning (MSZ) based on the concept that the lithosphere is built-up by different-scale blocks, separated by mobile boundaries. With MSZ we compiled the morphostructural map (scale 1:1,000,000) for the study region using the GIS technology. The map shows the hierarchical block-structure of the region, the boundary zones bounding blocks, and the loci of the nodes. Three-level hierarchy has been established for the blocks and their boundaries. All the recorded M >= 6.0 earthquakes nucleate at the nodes delineated by MSZ, i.e. ignoring the seismic record. The nodes capable of M >= 6.5 earthquakes are identified with the criteria of high seismicity nodes, previously derived from pattern recognition in the Pamirs -Tien Shan region. In the study region, with the employed criteria 29 out of the 33 nodes, hosting the observed M >= 6.5 events, have been classified as prone to M >= 6.5 earthquakes. With the exception of the Carpatho-Balkanides system, where the recognition procedure is inapplicable due to the insufficient number of sample nodes for the learning stage, we recognized the seismogenic nodes (D), prone to M >= 6.0 earthquakes, with the pattern recognition algorithm CORA-3. The recognition is performed using geomorphic, morphostructural, and gravity parameters relevant to seismicity. The majority of D nodes is associated with the first and second rank boundaries, i.e. larger earthquakes originate at the boundaries of larger blocks. Characteristic traits of D nodes selected by CORA-3 suggest an increased fragmentation of the crust and high intensity of tectonic movements in the D nodes vicinities. We identify a number of D nodes, where strong earthquakes are not recorded till present

  14. Pn anisotropic tomography under the entire Tienshan orogenic belt

    NASA Astrophysics Data System (ADS)

    Zhou, Zhigang; Lei, Jianshe

    2015-11-01

    We present a new anisotropic tomography of the uppermost mantle under the Tienshan orogenic belt and surrounding regions using a number of Pn arrival-time data hand-picked from portable seismic stations and chosen from the Xinjiang provincial observation bulletins and the EHB datasets. Our results exhibit prominent lateral heterogeneities in the study region. Distinct low-velocity anomalies are visible under the tectonically active regions, such as the Tienshan orogenic belt and western Kunlun Mountains, whereas pronounced high-velocity anomalies are imaged beneath the stable blocks, such as the Kazakh shield, the Junggar, Tarim, Qaidam, and Turpan-Hami basins, and the Tajik depression. Most strong earthquakes (Ms > 7.0) are mainly distributed along the transition zone of high to low velocity anomalies, suggesting a possible correlation between the strong earthquakes and the upper mantle structure. The fast directions of Pn anisotropy beneath the Tienshan orogenic belt are generally parallel to its striking orientation, whereas those beneath Pamir show a northward arc-shaped distribution. The Pn fast-velocity directions on the boundaries of the Kazakh shield and the Tarim and Junngar basins are approximately perpendicular to the strike of the Tienshan orogenic belt. By integrating with previous findings, our results suggest that the Tarim and Kazakh lithospheric materials could have underthrusted beneath the Tienshan orogenic belt that leads to the hot mantle material upwelling under the Tienshan orogenic belt, which is attributable to the Indo-Asian collision. These dynamic processes could play important roles in the Tienshan mountain building.

  15. Grenville age of basement rocks in Cape May NJ well: New evidence for Laurentian crust in U.S. Atlantic Coastal Plain basement Chesapeake terrane

    USGS Publications Warehouse

    Sheridan, R.E.; Maguire, T.J.; Feigenson, M.D.; Patino, L.C.; Volkert, R.A.

    1999-01-01

    The Chesapeake terrane of the U.S. mid-Atlantic Coastal Plain basement is bounded on the northwest by the Salisbury positive gravity and magnetic anomaly and extends to the southeast as far as the Atlantic coast. It underlies the Coastal Plain of Virginia, Maryland, Delaware and southern New Jersey. Rubidium/Strontium dating of the Chesapeake terrane basement yields an age of 1.025 ?? 0.036 Ga. This age is typical of Grenville province rocks of the Middle to Late Proterozoic Laurentian continent. The basement lithologies are similar to some exposed Grenville-age rocks of the Appalachians. The TiO2 and Zr/P2O5 composition of the metagabbro from the Chesapeake terrane basement is overlapped by those of the Proterozoic mafic dikes in the New Jersey Highlands. These new findings support the interpretation that Laurentian basement extends southeast as far as the continental shelf in the U.S. mid-Atlantic region. The subcrop of Laurentian crust under the mid-Atlantic Coastal Plain implies unroofing by erosion of the younger Carolina (Avalon) supracrustal terrane. Dextral-transpression fault duplexes may have caused excessive uplift in the Salisbury Embayment area during the Alleghanian orogeny. This extra uplift in the Salisbury area may have caused the subsequent greater subsidence of the Coastal Plain basement in the embayment.

  16. The Capricorn Orogen Passive source Array (COPA) in Western Australia

    NASA Astrophysics Data System (ADS)

    Gessner, K.; Yuan, H.; Murdie, R.; Dentith, M. C.; Johnson, S.; Brett, J.

    2015-12-01

    COPA is the passive source component of a multi-method geophysical program aimed at assessing the mineral deposits potential of the Proterozoic Capricorn Orogen. Previous results from the active source surveys, receiver functions and magnetotelluric studies show reworked orogenic crust in the orogen that contrasts with more simple crust in the neighbouring Archean cratons, suggesting progressive and punctuated collisional processes during the final amalgamation of the Western Australian craton. Previous seismic studies are all based on line deployment or single station analyses; therefore it is essential to develop 3D seismic images to test whether these observations are representative for the whole orogen. With a careful design that takes advantage of previous passive source surveys, the current long-term and short-term deployments span an area of approximately 500 x 500 km. The 36-month total deployment can guarantee enough data recording for 3D structure imaging using body wave tomography, ambient noise surface wave tomography and P- and S-wave receiver function Common Conversion Point (CCP) stacking techniques. A successive instrument loan from the ANSIR national instrument pool, provided 34 broadband seismometers that have been deployed in the western half of the orogen since March 2014. We expect approximately 40-km lateral resolution near the surface for the techniques we propose, which due to low frequency nature of earthquake waves will degrade to about 100 km near the base of the cratonic lithosphere, which is expected at depths between 200 to 250 km. Preliminary results from the first half of the COPA deployment will be presented in the light of the hypotheses that 1) distinct crustal blocks can be detected continuously throughout the orogen (using ambient noise/body wave tomography); 2) distinct lithologies are present in the crust and upper mantle across the orogen (using receiver function CCP images); and 3) crustal and lithosphere deformation along

  17. Curved orogen and syntaxes formation during subduction and collision

    NASA Astrophysics Data System (ADS)

    Bajolet, F.; Replumaz, A.; Faccenna, C.; Lainé, R.

    2012-04-01

    The sustained convergence between India and Asia with successive stages of oceanic subduction, continental subduction and continental collision has lead to the formation of the Tibetan plateau while the Himalayan orogenic front acquired an arcuate shape convex toward the South. The Indian plate is bounded by north-south strike-slip faults, which accommodate a large indentation of Asia, between two oceanic subductions, beneath Makran to the west, beneath Indonesia to the east. Two syntaxes formed at both east and west termination of the Himalayan orogenic front at the transition between Indian and Asian plates. In order to better understand this particular configuration, we performed analog experiments at the Laboratory of Experimental Tectonics of Roma TRE to simulate, at the scale of the mantle - lithosphere system, the mechanics of the indentation process. The configuration is set to drive the India indenter towards the Asian continent with a motor-controlled-piston, to simulate far field stresses necessary for indentation. In particular, we test (1) which geometry and rheological parameters favor arcuate orogen and syntaxes formation, (2) what are the consequences on the topography of both the orogenic front and the plateau, and (3) how they relate with the subduction/collision dynamics. The setup is composed of a subducting and an overriding plate made of visco-elastic silicone putty, floating on low-viscosity syrup simulating the asthenosphere. The subducting plate simulates an oceanic lithosphere followed by a continental indenter (analog for the Indian craton) flanked or not by oceans (analog for Makran and Indonesian domains), while the upper continental plate simulates the Tibetan plateau. Results show that the curvature of the orogen and syntaxes' formation are primarily controlled by the strength and gravitational potential energy of the upper plate, and the shape of the subducting plate. A relatively strong upper plate flanked by oceans leads to a

  18. Proterozoic orogens in southern Peninsular India: Contiguities and complexities

    NASA Astrophysics Data System (ADS)

    Chetty, T. R. K.; Santosh, M.

    2013-12-01

    The Precambrian terranes of southern Peninsular India have been central to discussions on the history of formation and breakup of supercontinents. Of particular interest are the Proterozoic high grade metamorphic orogens at the southern and eastern margins of the Indian shield, skirting the 3.4 Ga Dharwar craton which not only preserve important records of lower crustal processes and lithospheric geodynamics, but also carry imprints of the tectonic framework related to the assembly of the major Neoproterozoic supercontinents - Rodinia and Gondwana. These Proterozoic orogens are described as Southern Granulite Terrane (SGT) in the southern tip and the Eastern Ghats Mobile Belt (EGMB) in the eastern domains of the peninsula. The contiguity of these orogens is broken for a distance of ˜400 km and disappears in the Bay of Bengal. These orogens expose windows of middle to lower crust with well-preserved rock records displaying multiple tectonothermal events and multiphase exhumation paths.Recent studies in these orogens have led to the recognition of discrete crustal blocks or terranes separated by major shear zone systems, some of which represent collisional sutures. The SGT and EGMB carry several important features such as fold-thrust tectonics, regional granulite facies metamorphism of up to ultrahigh-temperature conditions in some cases, multiple P-T paths, development of lithospheric shear zones, emplacement of ophiolites, presence of alkaline and anorthositic complexes, development of crustal-scale "flower structures", transpressional strains, and reactivation tectonics. A heterogeneous distribution of different metamorphic and magmatic assemblages with distinct spatial and temporal strain variations in shaping the fabric elements in different blocks is identified. Both EGMB and SGT share a common transpressional deformation history during the latest Neoproterozoic characterized by the steepening of the initial low angle crustal scale structures leading to a

  19. Neoproterozoic to Paleozoic Geological Evolution of Mongolia: Constraints on Modes of "Crustal Growth" in the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Macdonald, F. A.; Bold, U.; Smith, E.; Olin, P. H.; Crowley, J. L.; Schmitz, M. D.

    2012-12-01

    The Central Asian Orogenic Belt (CAOB) is widely considered the largest area of Phanerozoic juvenile crustal growth on Earth. However, the timing and nature of the orogenic events in the core of the CAOB in Mongolia has remained poorly constrained due to a dearth of detailed geological and geochronological studies. To bridge this gap and test models of crustal growth, here we refine the sequencing of geological events by focusing on the formation and destruction of Neoproterozoic and Paleozoic tectonic basins. Mongolia's basins record a complete Neoproterozoic to Cambrian Wilson cycle with rifting of the Mongolian continent at ca. 700 Ma, the development of a Cryogenian to Ediacaran thermally subsiding passive margin, an arc-continent collision at ca. 520 Ma, and a continent-arc-continent collision at ca. 500 Ma. During this collisional orogeny, that is the Cambrian Altaids, crustal growth occurred largely through the obduction of ophiolites. Rifting of the southern margin occurred during the Ordovician Period, with the development of a Silurian passive margin. Oblique northwest-dipping subduction was initiated during the Devonian and resulted in a transpressional accretionary orogen. The CAOB culminated with a continent-arc-continent collision and the accretion of the North China and Tarim Blocks in the latest Permian. The Devonian to early Permian accretionary orogen is associated not only with voluminous plutonism, but also, major translational structures oblique to the margin resulting in the appearance of many accreted terranes. These data are consistent with existing coarse Hf and Nd isotopic data, but also provide a framework for future detailed studies. Although our geological constraints suggest distinct periods of apparent crustal growth through either collisional or accretionary orogenies, net crustal growth after accounting for recycling is equivocal.

  20. Structural inversion of the Tamworth Belt: Insights into the development of orogenic curvature in the southern New England Orogen, Australia

    NASA Astrophysics Data System (ADS)

    Phillips, G.; Robinson, J.; Glen, R.; Roberts, J.

    2016-05-01

    The middle to late Permian Hunter Bowen Event is credited with the development of orogenic curvature in the southern New England Orogen, yet contention surrounds the structural dynamics responsible for the development of this curvature. Debate is largely centred on the roles of orogen parallel strike-slip and orogen normal extension and contraction to explain the development of curvature. To evaluate the dynamic history of the Hunter Bowen Event, we present new kinematic reconstructions of the Tamworth Belt. The Tamworth Belt formed as a Carboniferous forearc basin and was subsequently inverted during the Hunter Bowen Event. Kinematic reconstructions of the Tamworth Belt are based on new maps and cross-sections built from a synthesis of best-available mapping, chronostratigraphic data and new interpretations of depth-converted seismic data. The following conclusions are made from our study: (i) the Hunter Bowen Event was dominantly driven by margin normal contraction (east-west shortening; present-day coordinates), and; (ii) variations in structural style along the strike of the Tamworth Belt can be explained by orthogonal vs. oblique inversion, which reflects the angular relationship between the principal shortening vector and continental-arc margin. Given these conclusions, we suggest that curvature around the controversial Manning Bend was influenced by the presence of primary curvature in the continental margin, and that the Hastings Block was translated along a sinistral strike-slip fault system that formed along this oblique (with respect to the regional east-west extension and convergence direction) part of the margin. Given the available temporal data, the translation of the Hastings Block took place in the Early Permian (Asselian) and therefore preceded the Hunter Bowen Event. Accordingly, we suggest that the Hunter Bowen Event was dominantly associated with enhancing curvature that was either primary in origin, or associated with fault block translation

  1. Field Vectors to Metamorphosed Ores: A Prelude to Finding Currently Concealed Volcano-Plutonic Arc Settings and Their Mineral Deposits in The Grenville Province

    NASA Astrophysics Data System (ADS)

    Corriveau, L.; Bonnet, A.; van Breemen, O.

    2004-05-01

    Recent mineral deposits synthesis highlights the largely barren nature of the high-grade metamorphic terrains of the Canadian Shield in terms of large mining camps. No where is the gap most startling than in the Grenville Province even though a lot of its Paleo- to Mesoproterozoic crust consists of magmatic arcs renown worldwide to host IOCG, VHMS and Porphyry Cu deposits. All these deposit types have significant alteration halos that can serve as vectors to ore. The use of such vectors forced a complete reinterpretation of the nature of the La Romaine domain in the eastern Grenville Province. Mapped in the 70's as being a metasedimentary basin with >500 km2 of meta-arkose and minor pelite, quartzite, conglomerate and marble, the domain is herein reassessed as a major 1.5 Ga Pinwarian continental magmatic arc fertile in Cu-sulphides and Fe-oxides mineralizing systems. The original markers used to prognosticate a sedimentary origin can now be demonstrated to be a series of rhyolitic to dacitic lapillistone, sericitized tuff with Al nodules and veins, Al gneiss locally with lapilli textures, garnetite, ironstones and calc-silicate rocks. The distribution, paragenesis and mode of the Al-, Fe- and Ca-rich units significantly depart from those of normal metasediments but are very diagnostic of metamorphosed hydrothermal alteration zones and meta-exhalites. Mapping alteration vectors provided clues to search for and find the volcanic rocks concealed among the composite granitic gneiss, the zones of hydrothermal leaching (e.g., sericitic, argillic and advanced argillic alterations) and discharge, the cap rocks, and the Cu mineralization. Spatial and stratigraphic relationships provided a means to compare their settings with ore deposit models. Roof pendants of Ba-rich meta-exhalite in surrounding 1.5 Ga granitic plutons and intrusion of 1495 Ma Qtz-Kfs porphyry across hydrothermally altered 1500 Ma tuffs attest to coeval hydrothermal activity and sub-volcanic plutons. The

  2. Sr, Nd, and Pb isotopes in Proterozoic intrusives astride the Grenville Front in Labrador - Implications for crustal contamination and basement mapping

    NASA Technical Reports Server (NTRS)

    Ashwal, L. D.; Wooden, J. L.; Emslie, R. F.

    1986-01-01

    Trace element and Pb, Sr, and Nd isotopic compositions of anorthosites and related rocks, and of younger mafic dikes from Harp Lake and Mealy Mountains in Labrador, Canada are estimated and compared. The effects of crustal component contaminants on the isotopic compositions of the dikes are investigated. The correlation between the isotopic data and the crustal contamination model is studied. It is observed that for Harp Lake the initial Sr ratios are higher and the Nd values are lower than Mealy samples, and the data do not correspond to the crustal contamination model; however, the Pb isotope data favor a crustal contamination model. It is noted that the Labrador segment of the Grenville Front appears to coincide with the southern margin of the Archean North Atlantic craton, and may represent a pre mid-Proterozoic suture.

  3. Taconic plate kinematics as revealed by foredeep stratigraphy, Appalachian Orogen

    USGS Publications Warehouse

    Bradley, D.C.

    1989-01-01

    Destruction of the Ordovician passive margin of eastern North America is recorded by an upward deepening succession of carbonates, shales, and flysch. Shelf drowning occurred first at the northern end of the orogen in Newfoundland, then at the southern end of the orogen in Georgia, and finally in Quebec. Diachronism is attributed to oblique collision between an irregular passive margin, that had a deep embayment in Quebec, and at least one east dipping subduction complex. The rate of plate convergence during collision is estimated at 1 to 2 cm/yr, and the minimum width of the ocean that closed is estimated at 500 to 900 km. The drowning isochron map provides a new basis for estimating tectonic transport distances of four of these allochthons (about 165 to 450 km), results not readily obtained by conventional structural analysis. -Author

  4. Flow of deep crust in orogens, associated surface dynamics, and the stabilization of continents

    NASA Astrophysics Data System (ADS)

    Teyssier, C. P.; Whitney, D. L.; Mulch, A.; Rey, P. F.

    2013-12-01

    Mountain building throws continental crust into an unstable state; subsequent stabilization of continental crust takes various forms, but flow of low-viscosity crust is the most common. Some of this low-viscosity crust remains at depth- it is the crust we see in the deep portions of Archean and Proterozoic cratons, typically granulite-migmatite terrains that have recorded ~10 kbar pressure, 600-800°C temperature, and intense deformation dominated by subhorizontal fabrics. In some places though, this deep crust reached the surface during the orogenic cycle. This is the case in the North American Cordillera where the deep crust leaked toward the surface and formed a series of metamorphic complexes that are cored by migmatite domes. Within the domes, complex structural overprints and decompression metamorphic paths indicate large-magnitude horizontal and vertical flow of partially molten crust relative to mantling rocks. No matter how the crust reached partial melting (thermal relaxation and/or heating) during continental under-thrusting, crustal thickening, lithosphere foundering, slab break-off, or slab window, the end result is one of an orogenic crust that contains a low viscosity layer at depth. This layer is mobile and opportunistic: it flows laterally and therefore helps keep a flat Moho; it may flow from a thick plateau and thicken the foreland region (mechanism of plateau growth); it fills gaps that open in the upper crust and therefore enhances orogenic collapse by transferring material from deep to shallow levels; ultimately, flow of this layer stabilizes the crust and may bring the end of orogeny. Thermal and mechanical numerical modeling can help evaluate quantitatively the relative importance of crust thickness, geothermal gradients, and tectonic boundary conditions in the evolution of orogenic systems. In the simple case of steady extension of a layered crust, results show that upper-crust extension is dynamically linked to lower crustal flow until

  5. Paleozoic tectonics of the Ouachita Orogen through Nd isotopes

    SciTech Connect

    Gleason, J.D.; Patchett, P.J.; Dickinson, W.R.; Ruiz, J. . Dept. of Geosciences)

    1992-01-01

    A combined isotopic and trace-element study of the Late Paleozoic Ouachita Orogenic belt has the following goals: (1) define changing provenance of Ouachita sedimentary systems throughout the Paleozoic; (2) constrain sources feeding into the Ouachita flysch trough during the Late Paleozoic; (3) isolate the geochemical signature of proposed colliding terranes to the south; (4) build a data base to compare with possible Ouachita System equivalents in Mexico. The ultimate aim is to constrain the tectonic setting of the southern margin of North America during the Paleozoic, with particular emphasis on collisional events leading to the final suturing of Pangea. Nd isotopic data identify 3 distinct groups: (1) Ordovician passive margin sequence; (2) Carboniferous proto-flysch (Stanley Fm.), main flysch (Jackfork and Atoka Fms.) and molasse (foreland Atoka Fm.); (3) Mississippian ash-flow tuffs. The authors interpret the Ordovician signature to be essentially all craton-derived, whereas the Carboniferous signature reflects mixed sources from the craton plus orogenic sources to the east and possibly the south, including the evolving Appalachian Orogen. The proposed southern source is revealed by the tuffs to be too old and evolved to be a juvenile island arc terrane. They interpret the tuffs to have been erupted in a continental margin arc-type setting. Surprisingly, the foreland molasse sequence is indistinguishable from the main trough flysch sequence, suggesting the Ouachita trough and the craton were both inundated with sediment of a single homogenized isotopic signature during the Late Carboniferous. The possibility that Carboniferous-type sedimentary dispersal patterns began as early as the Silurian has important implications for the tectonics and paleogeography of the evolving Appalachian-Ouachita Orogenic System.

  6. Jurassic sedimentary basins in the Central Asian orogenic belt

    SciTech Connect

    Bebeshev, I.I.

    1995-05-01

    The principal stages of development of Jurassic sedimentary basins (from their origin to the end of their existence) in the Central Asian orogenic belt are considered. The interrelations of the basins with the surrounding paleorises are investigated. Paleogeographic maps are compiled representing the evolution of paleolandscapes and revealing their interrelations in space and time for each stage. Areas with the highest prospects for coal are found.

  7. The crust-mantle interaction in continental subduction channels: Zircon evidence from orogenic peridotite in the Sulu orogen

    NASA Astrophysics Data System (ADS)

    Li, Hai-Yong; Chen, Ren-Xu; Zheng, Yong-Fei; Hu, Zhaochu

    2016-02-01

    A combined secondary ion mass spectrometer and laser ablation-(multicollector)-inductively coupled plasma mass spectrometer study of zircon U-Pb ages, trace elements, and O and Hf isotopes was carried out for orogenic peridotite and its host gneiss in the Sulu orogen. Newly grown zircon domains exhibit weak zoning or no zoning, relatively low Th/U ratios (<0.1), low heavy rare earth element (HREE) contents, steep middle rare earth element-HREE patterns, negative Eu anomalies, and negative to low δ18O values of -11.3 to 0.9‰ and U-Pb ages of 220 ± 2 to 231 ± 4 Ma. Thus, these zircons would have grown from metasomatic fluids during the early exhumation of deeply subducted continental crust. The infiltration of metasomatic fluids into the peridotite is also indicated by the occurrence of hydrous minerals such as amphibole, serpentine, and chlorite. In contrast, relict zircon domains exhibit magmatic zircon characteristics. Their U-Pb ages and trace element and Hf-O isotope compositions are similar to those for protolith zircons from ultrahigh-pressure metamorphic rocks in the Dabie-Sulu orogenic belt. Thus, these relict magmatic zircons would be physically transported into the peridotite by metasomatic fluids originated from the deeply subducted continental crust. Therefore, the peridotite underwent metasomatism by aqueous solutions derived from dehydration of the deeply subducted continental crust during the early exhumation. It is these crustally derived fluids that would have brought not only such chemical components as Zr and Si but also tiny zircon grains from the deeply subducted crustal rocks into the peridotite at the slab-mantle interface in continental subduction channels. As such, the orogenic peridotite records the crust-mantle interaction at the deep continental subduction zone.

  8. Orogenic structural inheritance and rifted passive margin formation

    NASA Astrophysics Data System (ADS)

    Salazar Mora, Claudio A.; Huismans, Ritske S.

    2016-04-01

    Structural inheritance is related to mechanical weaknesses in the lithosphere due to previous tectonic events, e.g. rifting, subduction and collision. The North and South Atlantic rifted passive margins that formed during the breakup of Western Gondwana, are parallel to the older Caledonide and the Brasiliano-Pan-African orogenic belts. In the South Atlantic, 'old' mantle lithospheric fabric resulting from crystallographic preferred orientation of olivine is suggested to play a role during rifted margin formation (Tommasi and Vauchez, 2001). Magnetometric and gravimetric mapping of onshore structures in the Camamu and Almada basins suggest that extensional faults are controlled by two different directions of inherited older Brasiliano structures in the upper lithosphere (Ferreira et al., 2009). In the South Atlantic Campos Basin, 3D seismic data indicate that inherited basement structures provide a first order control on basin structure (Fetter, 2009). Here we investigate the role of structural inheritance on the formation of rifted passive margins with high-resolution 2D thermo-mechanical numerical experiments. The numerical domain is 1200 km long and 600 km deep and represents the lithosphere and the sublithospheric mantle. Model experiments were carried out by creating self-consistent orogenic inheritance where a first phase of orogen formation is followed by extension. We focus in particular on the role of varying amount of orogenic shortening, crustal rheology, contrasting styles of orogen formation on rifted margin style, and the time delay between orogeny and subsequent rifted passive formation. Model results are compared to contrasting structural styles of rifted passive margin formation as observed in the South Atlantic. Ferreira, T.S., Caixeta, J.M., Lima, F.D., 2009. Basement control in Camamu and Almada rift basins. Boletim de Geociências da Petrobrás 17, 69-88. Fetter, M., 2009. The role of basement tectonic reactivation on the structural evolution

  9. Late-stage orogenic processes: How to link surface motion with distinct lithospheric processes

    NASA Astrophysics Data System (ADS)

    Neubauer, F.; Heberer, B.

    2009-04-01

    There is still a lack of knowledge of surface expression caused by deep-seated lithospheric processes, and how such processes could be distinguished from other, e.g. climate-induced, surface processes like denudation. Surface expressions of deep-seated lithospheric processes in convergent settings are expected to have been long-lived and to show large wave-length structures creating a dynamic topography (Wortel and Spakman, 2000; Cloetingh and Ziegler, 2007). Resulting continent-continent collisional orogens are bivergent, and the principal vergency of collisional orogens is controlled by the previous subduction of oceanic lithosphere (Beaumont et al., 1996). A number of tectonic processes are shown to be active during late orogenic phases and these processes particularly result in specific patterns of surface uplift and denudation of the evolving orogens as well as subsidence in the associated foreland basin. A number of these processes are not fully understood. Late-stage orogenic processes include, among others, slab break-off, slab delamination respectively of lithospheric roots, back-thrusting, tectonic indentation and consequent orogen-parallel lateral extrusion and formation of Subduction-Transform Edge Propagator (STEP) faults acting on the subducting lithosphere (Molnar and Tapponnier, 1975; Wortel and Spakman, 2000; Ratschbacher et al., 1991; Govers and Wortel, 2005). Here, we discuss these processes mainly in terms of their near-surface geological expressions within the orogen and the associated foreland basins, and how these processes could be distinguished by such geological features. We also show distinct theoretical models applied to the arcuate Alpine-Balkan-Carpathian-Dinaric system, which is driven by the oblique convergence of Africa-Europe. Slab-break-off results in lateral orogen-parallel migration of sharp subsidence in a linear belt in front of the slab window, coupled subsidence and subsequent uplift/basin inversion of peripheral foreland

  10. Sand fairway mapping as a tool for tectonic restoration in orogenic belts

    NASA Astrophysics Data System (ADS)

    Butler, Rob

    2016-04-01

    geomorphology. The stratigraphic records are preserved in many parts of the Alpine-Mediterranean region. Examples are drawn from the Eo-Oligocene of the western Alps and the early Miocene of the Maghreb-Apennine system to illustrate how turbidite sedimentology, linked to studies of basin structure, can inform understanding of tectonic processes on regional and local scales. In both examples, sediment was delivered across deforming basin arrays containing contractional structures, sourced from beyond the immediate orogenic segments. The depositional systems show that multiple structures were active in parallel, rather than develop in any particular sequence. Both systems show that significant deformation occurs, emerging to the syn-orogenic surface ahead of the main orogenic wedge. The cycling of uplift and subsidence of "massifs" can be significantly more complex that the histories resolved from thermochronological data alone.

  11. Exhumation at orogenic indentor corners under long-term glacial conditions: Example of the St. Elias orogen, Southern Alaska

    NASA Astrophysics Data System (ADS)

    Spotila, James A.; Berger, Aaron L.

    2010-07-01

    Syntaxial bends in convergent plate boundaries, or indentor corners, display some of the most intriguing deformation patterns on Earth and are type localities for "aneurysms" of coupled erosion, thermal weakening, and strain. The St. Elias orogen in Alaska is a small, young convergent system that has been dominated by a glacial climate for much of its history and exhibits two prominent indentor corners that are not well understood. We have added 40 new apatite (U-Th)/He ages to the already extensive dataset for the low-temperature cooling history of this orogen to constrain the pattern of exhumation in these indentor corners. Ages from the western syntaxis show minor variation across the structural hinge, suggesting that the bend has little effect on the pattern of exhumation and that structures, including the Bagley fault, connect smoothly from the orogen core to the subduction zone to the southwest. Rock uplift on the north flank of the range appears to increase steadily towards the eastern syntaxis, which represents the apex in the right-angle bend between a transform fault in the south and the collision zone in the west. Based on age-elevation relationships, zones of relative rock uplift can be defined in which the Mt. Logan massif, or the area just north of the eastern syntaxis, experienced ˜ 4.8 km greater rock uplift than background levels northwest of the western syntaxis. A bulge in relative rock uplift is symmetric about the hinge in the eastern indentor corner. However, rates of denudation in this bulge are not as rapid as the core of the fold and thrust belt and are lower than those implied by detrital cooling ages from beneath the Seward Glacier. This implies that a large bull's eye of ultra-rapid (˜ 5 mm/yr) exhumation does not occur and that the subpopulation of young detrital ages may be sourced from a narrow transpressional zone along the Fairweather fault. Unlike the Himalayan syntaxes, it thus appears that an aneurysm of coupled erosion

  12. Rapid exhumation of deep crust in an obliquely convergent orogen: The Kaoko Belt of the Damara Orogen

    NASA Astrophysics Data System (ADS)

    Foster, David A.; Goscombe, Ben D.; Gray, David R.

    2009-08-01

    The exhumation of deep crustal rocks and juxtaposition of structural-metamorphic domains from different depths in a transpressional orogen may occur during the prograde evolution of the orogen by vertical extrusion or during the retrograde evolution of the orogen via extension. Metamorphic petrology, kinematics, and thermochronology of strike-slip shear zones in the Kaoko Belt of the Damara Orogen are used to evaluate extrusion and extension processes in this transpressional orogen. Mineral assemblages and deformation mechanisms record shearing at pressures of 4-6 kbar and temperatures of ˜550°C for the Three Palms mylonite zone, 600-650°C for the Purros mylonite zone, and ˜630-700°C for the Village and Khumib mylonite zones. The Three Palms mylonite zone, which separates the accreted Coastal Terrane from the former passive margin of the Congo Craton, exhibits progressive deformation during decreasing temperatures through lower greenschist facies and into the brittle field, with consistent oblique normal shear indicators. Lower-temperature fabrics and brittle features also overprint the Village mylonite zone. The 207Pb-206Pb (titanite) and 40Ar/39Ar (hornblende, muscovite, and biotite) data indicate rapid cooling, at rates of 30-100°C/Ma, of all high-grade structural domains in the core of the Kaoko Belt between circa 535 and 525 Ma, which is about 20-30 Ma younger then peak metamorphism. The 40Ar/39Ar data from synkinematic muscovite fish in the retrograde shear zone assemblages indicate that the Khumib, Village, and Three Palms mylonite zones were actively deforming at temperatures below 350°C at circa 530-524 Ma. These data indicate that the high-grade metamorphic rocks of the Kaoko were rapidly exhumed and juxtaposed after the main transpressive deformation by oblique extension. Reactivation of the shear zones and tectonic exhumation of high-grade structural domains from beneath the accreted Coastal Terrane was caused by collision in the Damara Belt and

  13. Thermo-kinematic evolution of the Annapurna-Dhaulagiri Himalaya, central Nepal: The Composite Orogenic System

    NASA Astrophysics Data System (ADS)

    Parsons, A. J.; Law, R. D.; Lloyd, G. E.; Phillips, R. J.; Searle, M. P.

    2016-04-01

    The Himalayan orogen represents a "Composite Orogenic System" in which channel flow, wedge extrusion, and thrust stacking operate in separate "Orogenic Domains" with distinct rheologies and crustal positions. We analyze 104 samples from the metamorphic core (Greater Himalayan Sequence, GHS) and bounding units of the Annapurna-Dhaulagiri Himalaya, central Nepal. Optical microscopy and electron backscatter diffraction (EBSD) analyses provide a record of deformation microstructures and an indication of active crystal slip systems, strain geometries, and deformation temperatures. These data, combined with existing thermobarometry and geochronology data are used to construct detailed deformation temperature profiles for the GHS. The profiles define a three-stage thermokinematic evolution from midcrustal channel flow (Stage 1, >700°C to 550-650°C), to rigid wedge extrusion (Stage 2, 400-600°C) and duplexing (Stage 3, <280-400°C). These tectonic processes are not mutually exclusive, but are confined to separate rheologically distinct Orogenic Domains that form the modular components of a Composite Orogenic System. These Orogenic Domains may be active at the same time at different depths/positions within the orogen. The thermokinematic evolution of the Annapurna-Dhaulagiri Himalaya describes the migration of the GHS through these Orogenic Domains and reflects the spatial and temporal variability in rheological boundary conditions that govern orogenic systems.

  14. Late-orogenic, post-orogenic, and anorogenic granites: Distinction by major-element and trace-element chemistry and possible origins

    SciTech Connect

    Rogers, J.J.W.; Greenberg, J.K. )

    1990-05-01

    Granites classified into four categories based solely on tectonics of occurrence and associated rock types also have compositional characteristics that are consistent within groups and different among groups. Orogenically related granites include late-orogenic varieties (LO) associated with calc-alkaline batholiths, and post-orogenic varieties (PO), which occur in broad zones of isolated diapiric plutons in recently deformed orogenic belts. Inclined REE patterns, moderate Sr contents, and K{sub 2}O-SiO{sub 2} relationships show that late-orogenic granites formed by fractionation of plagioclase, clinopyroxene, and amphibole from calcalkaline magmas. Flatter REE patterns and K{sub 2}O contents near 5%, plus the absence of associated magmatic rocks, indicate that the post-orogenic granites developed by partial melting of subduction-produced mafic/intermediate magmatic rocks. Both the late- and post-orogenic granites can be part of material newly added to continental crust as a result of orogeny. Anorogenic granites in anorthosite/rapakivi complexes (AR) or alkaline ring complexes (RC) have LIL contents too high to have been equilibrated with a mafic mineral assemblage. These anorogenic rocks probably formed by partial melting of preexisting sialic crust and do not represent new crustal increment.

  15. Tectonic and Topographic Effects of Collision Between Two Orogens

    NASA Astrophysics Data System (ADS)

    Forte, A. M.; Cowgill, E.

    2013-12-01

    Although collisions between small orogens may be common during the early stages of orogeny as small, intervening basins and back-arcs are consumed during progressive collision, the structural, topographic and stratigraphic expressions of such events are poorly understood. Investigation of the Greater Caucasus (GC) and Lesser Caucasus (LC) in the Arabia-Eurasia collision zone, suggests that incipient collision can drive an orogen out of steady state, causing significant structural and topographic departures from the predictions of a simple steady-state bivergent orogenic wedge model. The W-NW striking GC form the northern margin of the Arabia-Eurasia collision between the Black and Caspian Seas and represent the main locus of NE-SW shortening within the central portion (40° to 50°E) of the collision. Strong tectonic and climatic gradients characterize the GC along their 1000 km length, including an eastward increase in GPS-derived convergence rates from 2 to 10 mm/yr, and an eastward decrease in mean annual precipitation from 100 to 20 cm/yr. The existence of the GC as a significant topographic and structural feature is recent, with rapid exhumation beginning at 5 Ma. Importantly, the first order architecture of the GC remains enigmatic. Previous workers suggest the range is well explained as a bivergent orogenic wedge with a south-facing pro-wedge. However both the location of active structures and the topographic expression of the range significantly depart from predictions of a standard, steady-state, bivergent orogenic wedge model. Topographically, the GC are atypically symmetric across-strike, with nearly equal retro- and pro-wedge widths. Also unexpected is the consistent location of the topographic crest of the range north,of the main drainage divide and towards the retro-wedge by up to 40 km in the center of the range. Stratigraphic records along the SE margin of the GC indicate Plio-Pleistocene-aged alluvial fans were an order of magnitude larger than

  16. Asymmetric gravitational spreading - Analogue experiments on the Svecofennian orogen

    NASA Astrophysics Data System (ADS)

    Nikkilä, Kaisa; Korja, Annakaisa; Koyi, Hemin; Eklund, Olav

    2015-04-01

    Over-thickened orogenic crust may suffer from rheological, gravitational and topographical unbalancing resulting in discharging via gravitational spreading. If the thickened orogen is also hot, then increased temperature may reduce the viscosity of the crust that may induce large-scale horizontal flow. The effect of flow on the crustal architecture has previously been modeled with symmetric two-way spreading or asymmetric one- or two-way spreading (like channel flow) experiments. Most models do not take into account of the contrasting mechanical properties of the juxtaposed terranes. We have made analogue experiments to study gravitational one-way spreading and the interplay between two crustal blocks with contrasting rheological properties. The models are 3 cm thick replicas of 60 km thick crust. They have three horizontal layers representing strong lower, weak middle and brittle upper crust. The models have cuts to study the effect of inherited crustal-scale weakness zones. The experiments have been conducted within a large centrifuge in the Hans Ramberg Tectonic Laboratory at Uppsala University. The analogue models propose that asymmetric, unilateral flow has different effect on the contrasting crustal units, in both horizontal and vertical directions. The laterally heterogeneous crust flows towards the direction of extension, and it rotates and extends the pre-existing weakness zones. The weakness zones facilitate exhumation and they increase strain rate. The weakness zones split the crust into subblocks, which stretch individually and which may show signatures of compression or rotation. The changes in thickness of the model reflect changes in the layers, which may thin or thicken depending on the mechanical properties of crustal layers. A consequence of this the total amount of flattening is less than the model extension. The results are compared to geophysical and geological data from Precambrian Svecofennian orogen in Fennoscandia. The comparison suggest

  17. Predictive mapping of prospectivity for orogenic gold in Uganda

    NASA Astrophysics Data System (ADS)

    Herbert, Sarah; Woldai, Tsehaie; Carranza, Emmanuel John M.; van Ruitenbeek, Frank J. A.

    2014-11-01

    Integration of enhanced regional geo-datasets has facilitated new geological interpretation and modelling of prospectivity for orogenic gold in southwestern Uganda. The geo-datasets include historical geological maps, geological field data, digital terrain models, Landsat TM data and airborne geophysical data. The study area, bordered by the western branch of the East African Rift, covers a range of different aged terranes including the Archaean basement gneisses, Palaeoproterozoic volcano-sedimentary Buganda Toro Belt, Mesoproterozoic clastic sedimentary Karagwe Ankolean Belt and several outliers of undeformed Neoproterozoic sediments. The mineral systems approach to practical exploration targeting requires a framework to link conceptual models of mineralisation with available data. A conceptual model requires good understanding of key processes and their timing within the geodynamic history of an area. The challenge is that processes cannot be mapped, only their results or effects. In this study, a district-scale (1:100,000) investigation is considered appropriate given the scarcity of geological information and the absence of world-renowned gold deposits in southwestern Uganda. At this scale of orogenic gold mineral systems understanding, evidence for the source of gold, active pathways and the physical traps are considered critical. Following the mineral system approach, these processes critical to orogenic gold systems are translated into district-scale mappable proxies using available regional-scale datasets. Tectono-stratigraphic domains, mantle indicators and gold occurrences represent the “source of gold” as a critical process. Zones of hydrothermal alteration were extracted from radiometric data, structures involved in the orogenies and terrane contacts were extracted to represent the active pathway as a critical process and finally the physical throttle is represented by rheological contrasts and geological complexity. Then, the knowledge

  18. A Paleozoic Japan-type subduction-accretion system in the Beishan orogenic collage, southern Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Song, Dongfang; Xiao, Wenjiao; Windley, Brian F.; Han, Chunming; Tian, Zhonghua

    2015-05-01

    Magmatic arcs ascribed to oceanic lithosphere subduction played a dominant role in the construction of the accretionary Central Asian Orogenic Belt (CAOB). The Beishan orogenic collage, situated between the Tianshan Orogen to the west and the Inner Mongolia Orogen to the east, is a key area to understanding the subduction and accretionary processes of the southern CAOB. However, the nature of magmatic arcs in the Beishan and the correlation among different tectonic units along the southern CAOB are highly ambiguous. In order to investigate the subduction-accretion history of the Beishan and put a better spatial and temporal relationship among the tectonic belts along the southern CAOB, we carried out detailed field-based structural geology and LA-ICP-MS zircon U-Pb geochronological as well as geochemical studies along four cross-sections across crucial litho-tectonic units in the central segment of the Beishan, mainly focusing on the metamorphic assemblages and associated plutons and volcanic rocks. The results show that both the plutonic and volcanic rocks have geochemical characteristics similar to those of subduction-related rocks, which favors a volcanic arc setting. Zircons from all the plutonic rocks yield Phanerozoic ages and the plutons have crystallization ages ranging from 464 ± 2 Ma to 398 ± 3 Ma. Two volcanic-sedimentary rocks yield zircons with a wide age range from Phanerozoic to Precambrian with the youngest age peaks at 441 Ma and 446 Ma, estimated to be the time of formation of the volcanic rocks. These new results, combined with published data on ophiolitic mélanges from the central segment of the Beishan, favor a Japan-type subduction-accretion system in the Cambrian to Carboniferous in this part of the Paleo-Asian Ocean. The Xichangjing-Niujuanzi ophiolite probably represents a major suture zone separating different tectonic units across the Beishan orogenic collage, while the Xiaohuangshan-Jijitaizi ophiolitic mélange may represent a

  19. Plume-orogenic lithosphere interaction recorded in the Haladala layered intrusion in the Southwest Tianshan Orogen, NW China

    NASA Astrophysics Data System (ADS)

    He, Peng-Li; Huang, Xiao-Long; Xu, Yi-Gang; Li, Hong-Yan; Wang, Xue; Li, Wu-Xian

    2016-03-01

    The plume-orogenic lithosphere interaction may be common and important for the generation of large igneous provinces. The information regarding such a process is recorded by the Haladala gabbroic intrusion (~300 Ma), the largest layered ultramafic-mafic intrusion hosting V-Ti magnetite deposits in the Southwest Tianshan Orogen, NW China. The Haladala gabbros exhibit unfractionated chondrite-normalized rare earth element patterns with negative Nb and Ta anomalies and positive Pb anomaly on the primitive mantle-normalized multielement variation diagram. They are characterized by low initial Sr isotopes, slightly decoupled but high positive bulk rock ɛNd(t) and ɛHf(t), and high 207Pb/204Pb and 208Pb/204Pb relative to 206Pb/204Pb, delineating a DUPAL signature in the sources. The Haladala gabbros cannot be arc or postcollisional magmatism, given the lack of hydrous minerals and low K contents, respectively. This is further supported by the relatively low oxygen fugacity required for the gradual enrichment of V-Ti magnetite during the magma fractionation and by an overall anhydrous mantle source suggested by troctolite mineral assemblage (olivine + plagioclase). The emplacement age of the Haladala gabbros is identical to that of the Wajilitag kimberlites in the Tarim's interior, which have been interpreted as the first magmatic expression of the Tarim mantle plume. We thus propose that the Haladala gabbroic intrusion was generated in a hybrid geodynamic setting in which the Southwest Tianshan Orogen was impacted by an upwelling mantle plume. In this sense, the Haladala layered gabbroic intrusion records the early phase of magmatism of the Tarim plume, which was preferentially emplaced in a lithospheric weak zone.

  20. Post-orogenic thermal evolution of newborn Archean continents

    NASA Astrophysics Data System (ADS)

    Jaupart, C.; Mareschal, J.-C.

    2015-12-01

    The post-orogenic thermal evolution of newborn cratons in the Archean is marked by high-temperature metamorphism and plutonic activity that lag accretion by several tens of million years. The source of the heat that is required remains controversial. Here, we show that such late activity is consistent with the thermal evolution of new continental crust that adjusts to heat released by radioactive decay. Quantitative results depend on the total amount of radioactive elements in the newborn crust. Using heat flow and heat production data from the Archean Superior Province of the Canadian Shield, we show that temperatures ≈800-900 °C were reached in the lower crust a few tens of million years after the final accretion event. The timing of post-orogenic metamorphism is sensitive to the thermal structure acquired at the end of accretion. For the Superior Province, the relatively short time-lag between the end of accretion and metamorphism suggests that the lithosphere was thin or had been heated up by sustained magma percolation.

  1. Continuation of the New England Orogen, Australia, beneath the Queensland Plateau and Lord Howe rise

    USGS Publications Warehouse

    Mortimer, N.; Hauff, F.; Calvert, A.T.

    2008-01-01

    Greywacke, argillite, greyschist and hypabyssal igneous rocks have been obtained from an Ocean Drilling Program core on the Queensland Plateau and from xenoliths in a volcanic breccia dredged from the crest of the Lord Howe Rise. Low to intermediate detrital quartz contents, 260-240 Ma K-Ar ages, and only moderately radiogenic Sr and Nd isotope compositions, suggest a correlation with the New England Orogen of eastern Australia, rather than with Australia's Lachlan Orogen or other adjacent geological provinces. Our results indicate that the New England Orogen terranes continue towards New Zealand at least as far as the southern Lord Howe Rise. The projected offshore boundaries of the major east Australian orogens are now known with more confidence, and do not appear to require any major cross-orogen offsets.

  2. Field evidence of hydraulic connections between bedrock aquifers and overlying granular aquifers: examples from the Grenville Province of the Canadian Shield

    NASA Astrophysics Data System (ADS)

    Richard, Sandra K.; Chesnaux, Romain; Rouleau, Alain; Morin, Roger; Walter, Julien; Rafini, Silvain

    2014-12-01

    Field evidence of hydraulic connections between a bedrock aquifer and an overlying granular aquifer in the Canadian Shield (Grenville Province) is presented. This issue is rarely considered and investigated despite its important hydraulic and chemical consequences and its widespread occurrence worldwide. The methodology employed is based on complementary field tests conducted at specific experimental sites instrumented both in the rock and in the overlying deposits. One of the bedrock sites revealed a natural hydraulic connection with the overlying granular aquifer caused by the weathered surface of the uppermost bedrock. Another site revealed an artificial hydraulic connection between the bedrock and the granular aquifer created by an improperly sealed casing. A regional study showed that hydraulic connections yield an erroneous interpretation of the true hydraulic properties of the tested aquifer. The detection of hydraulic connections is therefore essential to properly define well-capture areas and contamination conditions. It is recommended to practitioners that pumping tests be performed as well as hydrochemical comparisons of each existing aquifer unit. Falling-head permeability tests are also helpful in verifying the quality of the seal at the bedrock-casing contact. More effective procedural controls and better well-construction practices are necessary to reduce the risks of cross-contamination induced by defective seals.

  3. Regional and local controls on mineralization and pluton emplacement in the Bondy gneiss complex, Grenville Province, Canada interpreted from aeromagnetic and gravity data

    NASA Astrophysics Data System (ADS)

    Dufréchou, G.; Harris, L. B.; Corriveau, L.; Antonoff, V.

    2015-05-01

    The Bondy gneiss complex in the Grenville Province of Southwest Quebec hosts a mineralized iron oxide- and copper-rich hydrothermal system. The northern part of the complex overlies the lithospheric-scale Mont-Laurier lineament and is cut by the regional Mont-Laurier South shear zone interpreted from Bouguer gravity. A sinistral 6 km wide strike-slip corridor defined by several second-order shears (the Mont-Laurier South shear zone) in the complex was identified from geophysical data, including a new high-resolution airborne magnetic survey, and field observations. The spatial association of a metamorphosed alteration system, several pre- to post-metamorphic mineralized zones and mafic intrusions within the Mont-Laurier South shear zone suggests that (i) underlying basement structures controlled hydrothermal fluid migration during the formation of epithermal-IOCG mineralization and associated alteration system before ca. 1.2 Ga high-grade metamorphism and penetrative ductile deformation in the complex; (ii) post-metamorphic reactivation allowed magma ascent and pluton emplacement in the complex and adjacent supracrustal rocks within dilatational sites; and (iii) brittle-ductile shears that postdate high-grade metamorphism provided channel ways for fluid migration associated with magnetite-related mineralization. Although the complex does not host an economic mineral deposit, the role between structures at different levels and the combination of gravity and aeromagnetics at different scales provides an example of an approach for mineral exploration in similar high grade gneiss terrains.

  4. The Andes as a peripheral orogen of the breaking-up Pangea

    NASA Astrophysics Data System (ADS)

    Lomize, M. G.

    2008-05-01

    Formation conditions of the peripheral orogen are expressed most fully in the Central Andes, a mountain system almost not yielding in height to the Himalayan-Tibetan system but formed at the margin of ocean without any relations to intercontinental collision. The marine transgression and rejuvenation of subduction in the Early Jurassic during the origination of foldbelt at the margin of Pangea marked the transition to a new supercontinental cycle, and the overall further evolution began and continues now in the frame of the first half of this cycle. The marginal position of this belt above the subduction zone, the rate and orientation of convergence of the lithospheric plates, the age of “absolute” movement of the continental plate, variation in slab velocity, and subduction of heterogeneities of the oceanic crust were the crucial factors that controlled the evolution of the marginal foldbelt. At the stage of initial subsidence (Jurassic-Mid-Cretaceous), during extension of the crust having a moderate thickness (30-35 km), the Andean continental margin comprises the full structural elements of an ensialic island arc that resembled the present-day Sunda system. These conditions changed with the separation and onset of the western drift of the South American continent. Being anchored in the mantle and relatively young, the slab of the Andean subduction zone served as a stop that brought about compression that controlled the subsequent evolution. Due to the contribution of deep magma sources along with marine sediments and products of tectonic erosion removed to a depth, the growth of crust above the subduction zone was favorable for heating of the crust. By the middle Eocene, when compression enhanced owing to the acceleration of subduction, the thermal evolution of the crust had already prepared the transition to the orogenic stage of evolution, i.e., to the progressive viscoplastic shortening and swelling of the mechanically weakened lower crust and the

  5. Crustal Development in the Northeast Asian Orogenic Belt and its comparison with the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Jahn, Bor-ming

    2016-04-01

    The Northeast Asian Orogenic Belt is a Mesozoic-Cenozoic accretionary orogenic collage, and it constitutes the northern and principal part of the "Nipponides" (Sengor and Natal'in, 1996). The tectonic framework was formed in Mesozoic and Cenozoic, and it continues to evolve along the modern Pacific arc-trench systems. Generally, a oceanward younging of tectonic units may be discerned, but such a simple pattern is disrupted in many places by extensive strike-slip faulting, most of which is left-lateral. In this talk, the issue of crustal development in the sector of Sikhote-Alin and Japanese Islands will be discussed based on the geochemical and isotopic analyses of granitoids that intruded in various tectonostratigraphic terrains. The majority of granitoids in the NE Asian Orogenic Belt formed from Jurassic to late Cenozoic, with Cretaceous as the dominant period of granitic magmatism and tectonothermal events. A few Early Paleozoic granitic rocks (500 to 450 Ma) have been identified in SW Japan (Kurosegawa Belt) as well as in NE Japan (Kitakami Belt), among them the ca. 500 Ma diorites and tonalites of southern Kitakami are the oldest rocks in Japan and interpreted as the first TTG crust of proto-Japan (Isozaki et al., 2015). Cretaceous granitoids are widespread in Sikhote-Alin and in NE and SW Japan. However, granitoids were emplaced only in the Cenozoic in Sakhalin (ca. 44 - 42 Ma) and Hokkaido (45, 37 and 18 Ma). Most granitoids from Sikhote-Alin are of I-type and have ISr = 0.7040 to 0.7083, and ɛNd(T) = +3.0 to -6.0 (mostly 0 to -5). The Sr-Nd isotopic data fall within the range of granitoids from SW Japan (0.704 to 0.712; +5.0 to -13.0), and the data of Cretaceous granitoids from Sikhote-Alin and SW Japan overlap almost completely. The Cenozoic granitoids of Hokkaido are characterized by ISr = 0.7044 - 0.7061, ɛNd(T) = +1.0 to +4.7, and Sm-Nd model-1 ages = 400-1000 Ma. This is remarkably similar to the Sakhalin granitoids with ISr = 0.7047 - 0.7050,

  6. Magnetic constraints on the thermal evolution of a collapsing orogen

    NASA Astrophysics Data System (ADS)

    Platzman, Ellen

    2002-06-01

    The thermal evolution of an orogen undergoing late-stage extension was investigated using rock magnetic properties of a suite of mafic dyke rocks affected by greenschist facies metamorphism in the internal zones of the Betic Cordillera, southern Spain. The natural remanent magnetization (NRM) in the dykes intruded into the lowest geological unit is made up of up to three components. The lowest temperature component (LT) is in the direction of the present day magnetic field and is believed to be a chemical remanent magnetization (CRM) or viscous remanent magnetization (VRM) acquired in the recent magnetic field. The intermediate temperature (IT) component unblocked between 200°C and 450°C is thought to be largely a thermoviscous overprint acquired during metamorphism. This component is carried by either primary or authogenic sulfides and low-unblocking temperature magnetite. The component of magnetization with the highest blocking temperature (HT) is isolated above 450°C and is interpreted as the primary component of remanent magnetization. It is most likely that this component is carried by magnetite that resides in the plagioclase and has been shielded from the metamorphism, which transformed most of the original magnetite to metamorphic amphibole, chlorite and biotite. Thermal demagnetization of these dykes separates the IT overprint from the HT primary remanence at a sharp junction occurring at 450°C. For single domain grains this translates to a peak palaeotemperature in the natural sample of approximately 370°C, which is close to the estimated temperature experienced by the greenschist facies country rocks (400°C). In contrast, results obtained from a dyke that is intruded into an overlying weakly metamorphosed geological unit, indicates that temperatures only reached about 175°C in this unit. These results are consistent with temperatures deduced from geological constraints and they imply that between 4 and 6 km of section has been removed between the

  7. Stress Dynamics of Magma Activity during Orogenic Evolution: An Example from Kinmen Island, SE China

    NASA Astrophysics Data System (ADS)

    Chen, Ping-Chuan; Yeh, En-Chao; Lin, Jian-Wei; Lee, Chi-Yu; Chen, Rou-Fei; Lin, Wayne; Hsieh, Pei-Shan; Lin, Cheng-Kuo; Iizuka, Yoshiyuki

    2016-04-01

    During orogeny, a mountain belt experienced different orogenic stages with various conditions of temperature, pressure, stress and fluid pressure. The speculation, that the orogenic stresses evolve from reverse faulting via strike-slip faulting to normal faulting stress regimes corresponding to syn-orogenic, post-orogenic and an-orogenic stage respectively, has been proposed but has not been proved yet. Here we report the study of dikes from Kinmen Island can shed light on understanding the stress evolution of orogeny. The Kinmen Island, located in the southeastern continental margin of Mainland China, cropped out the middle to lower crust of NE structural grain, which was experienced deformation and metamorphism during Late Yenshanian Orogeny(LYO). Based on previous studies of geochemistry, geochronology, and P-T conditions, various types of dike have been identified. They are syn-orognic dikes of amphibolite (130-110Ma), post-orogenic dikes of pegmatite and aplite (110-100Ma), and an-orogenic dike of gabbro (94-76Ma). During syn-orogenic stage of LYO, dike intrusion appeared as low-angle dip, which reflected that reverse faulting regime and horizontal maximum stress direction in E-W orientation. In post-orogenic stage, stress would be divided into two sub-stages. The early one was strike-slip faulting regime and horizontal maximum stress was in NW-SE orientation. The late one was normal faulting regime and horizontal maximum stress direction returned to E-W orientation. Finally, an-orogenic dike intrusion striked NE-SW with vertical dip, which displayed that normal faulting regime and NE-SW horizontal maximum stress direction. Our observation is consistent with the expected stress evolution during orogeny. Deviatoric stresses from new findings were decreased at early post-orogenic stage but increased after late post-orogenic stage, indicating the change of fluid pressure ratio with time. The spatiotemporal variation of stress field might be suggested the change

  8. Proterozoic collisional tectonism in the Trans-Hudson orogen, Saskatchewan

    SciTech Connect

    Bickford, M.E.; Chiarenzelli, J.R.; Van Schmus, W.R. ); Collerson, K.D. ); Lewry, J.F. )

    1990-01-01

    Isotopic and structural data from the juvenile Reindeer zone of the Trans-Hudson orogen, northern Saskatchewan, indicate a pre-1.85 Ga thermotectonic event, possibly reflecting arc-continent collision, followed by a more extensive, nappe-forming, ca. 1.83-1.80 Ga thermotectonism during terminal continent-continent collision. Preliminary data from the adjacent, ensialic Cree Lake zone suggest high-grade reworking of Archean crust by the pre-1.85 Ga event. In the Rae province to the west, high-grade metamorphism and reworking of Archean crust occurred about 2.0 Ga and may be related to the formation of the coeval Taltson magmatic zone.

  9. Simultaneous Miocene Extension and Shortening in the Himalayan Orogen

    NASA Astrophysics Data System (ADS)

    Hodges, K. V.; Parrish, R. R.; Housh, T. B.; Lux, D. R.; Burchfiel, B. C.; Royden, L. H.; Chen, Z.

    1992-11-01

    The South Tibetan detachment system separates the high-grade metamorphic core of the Himalayan orogen from its weakly metamorphosed suprastructure. It is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front. Geochronologic data from the Rongbuk Valley, north of Qomolangma (Mount Everest) in southern Tibet, demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago, an interval characterized by large-scale crustal thickening at lower structural levels. These data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of Himalayan tectonics in Miocene time.

  10. Seismic anisotropy and mantle creep in young orogens

    USGS Publications Warehouse

    Meissner, R.; Mooney, W.D.; Artemieva, I.

    2002-01-01

    Seismic anisotropy provides evidence for the physical state and tectonic evolution of the lithosphere. We discuss the origin of anisotropy at various depths, and relate it to tectonic stress, geotherms and rheology. The anisotropy of the uppermost mantle is controlled by the orthorhombic mineral olivine, and may result from ductile deformation, dynamic recrystallization or annealing. Anisotropy beneath young orogens has been measured for the seismic phase Pn that propagates in the uppermost mantle. This anisotropy is interpreted as being caused by deformation during the most recent thermotectonic event, and thus provides information on the process of mountain building. Whereas tectonic stress and many structural features in the upper crust are usually orientated perpendicular to the structural axis of mountain belts, Pn anisotropy is aligned parallel to the structural axis. We interpret this to indicate mountain-parallel ductile (i.e. creeping) deformation in the uppermost mantle that is a consequence of mountain-perpendicular compressive stresses. The preferred orientation of the fast axes of some anisotropic minerals, such as olivine, is known to be in the creep direction, a consequence of the anisotropy of strength and viscosity of orientated minerals. In order to explain the anisotropy of the mantle beneath young orogens we extend the concept of crustal 'escape' (or 'extrusion') tectonics to the uppermost mantle. We present rheological model calculations to support this hypothesis. Mountain-perpendicular horizontal stress (determined in the upper crust) and mountain-parallel seismic anisotropy (in the uppermost mantle) require a zone of ductile decoupling in the middle or lower crust of young mountain belts. Examples for stress and mountain-parallel Pn anisotropy are given for Tibet, the Alpine chains, and young mountain ranges in the Americas. Finally, we suggest a simple model for initiating mountain parallel creep.

  11. Young orogenic gold mineralisation in active collisional mountains, Taiwan

    NASA Astrophysics Data System (ADS)

    Craw, Dave; Upton, Phaedra; Yu, Bing-Sheng; Horton, Travis; Chen, Yue-Gau

    2010-10-01

    Gold-bearing vein systems in the high mountains of Taiwan are part of the youngest tectonic-hydrothermal system on Earth. Tectonic collision initiated in the Pliocene has stacked Eocene-Miocene marine sedimentary rocks to form steep mountains nearly 4 km high. Thinner portions of the sedimentary pile (˜5 km) are currently producing hydrocarbons in a fold and thrust belt, and orogenic gold occurs in quartz veins in thicker parts of the pile (˜10 km) in the Slate Belt that underlies the mountains. Metamorphic fluids (2-5 wt.% NaCl equivalent) are rising from the active greenschist facies metamorphic zone and transporting gold released during rock recrystallisation. Metamorphic fluid flow at the Pingfengshan historic gold mine was focussed in well-defined (4 km3) fracture zones with networks of quartz veins, whereas large surrounding volumes of rock are largely unveined. Gold and arsenopyrite occur in several superimposed vein generations, with ankeritic alteration of host rocks superimposed on chlorite-calcite alteration zones as fluids cooled and became out of equilibrium with the host rocks. Mineralising fluids had δ18O near +10‰, δ13C was between -1‰ and -6‰ and these fluids were in isotopic equilibrium with host rocks at ˜350°C. Ankeritic veins were emplaced in extensional sites in kink fold axial surfaces, formed as the rock mass was transported laterally from compressional to extensional regimes in the orogen. Rapid exhumation (>2 mm/year) of the Slate Belt is causing a widespread shallow conductive thermal anomaly without igneous intrusions. Meteoric water is penetrating into the conductive thermal anomaly to contribute to crustal fluid flow and generate shallow boiling fluids (˜250°C) with fluid temperature greater than rock temperature. The meteoric-hydrothermal system impinges on, but causes only minor dilution of, the gold mineralisation system at depth.

  12. On the orogenic status of the Central Alps

    NASA Astrophysics Data System (ADS)

    Rahn, M. K.

    2003-04-01

    In a simplified way, the history of every orogen can be divided into an initial stage of formation, an intermediate stage of maturity and a final stage of decay. For the Central European Alps, the initial stage started with the continent-continent collision in the Eocene. The intermediate stage may be divided into several substages of quiescence and further growth. The youngest stage of growth, the Grindelwald stage, has led to shortening and thrusting within the Central crystalline massifs (Aar, Gotthard) and to major exhumation of these units and possibly a northward thrusting of the overlying sediments (Helvetic nappes) onto more external basin sediments (Molasse Basin). During the last 10 myr, however, the Central Alps are characterized by the formation of dome structures aligned along a major thrust zone (Penninic front thrust) that seems to have acted in a normal sense since that time. A contoured compilation of fission track apatite ages reveals the absence of structures with compressional movements during that time interval. Compression was restricted to the northern Alpine border and to the Jura Mountains, an external chain of thin-skin tectonic folds and thrust faults, which is separated from the Alpine body by some tens of kilometres of tectonically undisturbed sediments of the Molasse Basin. Folding and thrusting in the Jura Mountains, starting some time between 9 and 5 Ma and being completed at about 3.5 Ma, was contemporaneous to exhumation and erosion in the Molasse Basin. The Alpine border thrust, where Helvetic sediments thrust on top of Molasse Basin sediments towards N to NW has been active at around 10 Ma. Younger, N-directed thrusting can also be showm from boreholes within nearby Molasse sediments. During the last few myr, the Central Alps, however, seem to have been dominated by normal faulting and doming, indicating that the general status of the Central Alps is that of a quiet or even decaying orogen.

  13. Terrestrial cosmogenic nuclide surface exposure dating of the oldest glacial successions in the Himalayan orogen: Ladakh Range, northern India

    USGS Publications Warehouse

    Owen, L.A.; Caffee, M.W.; Bovard, K.R.; Finkel, R.C.; Sharma, M.C.

    2006-01-01

    Terrestrial cosmogenic nuclide surface exposure dating of moraine boulders and alluvial fan sediments define the timing of five glacial advances over at least the last five glacial cycles in the Ladakh Range of the Transhimalaya. The glacial stages that have been identified are: the Indus Valley glacial stage, dated at older than 430 ka; the Leh glacial stage occurring in the penultimate glacial cycle or older; the Karglacial stage, occurring during the early part of the last glacial cycle; the Bazgo glacial stage, at its maximum during the middle of the last glacial cycle; and the early Holocene Khalling glacial stage. The exposure ages of the Indus Valley moraines are the oldest observed to date throughout the Himalayan orogen. We observe a pattern of progressively more restricted glaciation during the last five glacial cycles, likely indicating a progressive reduction in the moisture supply necessary to sustain glaciation. A possible explanation is that uplift of Himalayan ranges to the south and/or of the Karakoram Mountains to the west of the region may have effectively blocked moisture supply by the south Asian summer monsoon and mid-latitude westerlies, respectively. Alternatively, this pattern of glaciation may reflect a trend of progressively less extensive glaciation in mountain regions that has been observed globally throughout the Pleistocene. ?? 2006 Geological Society of America.

  14. Tracking basement cross-strike discontinuities in the Indian crust beneath the Himalayan orogen using gravity data - relationship to upper crustal faults

    NASA Astrophysics Data System (ADS)

    Godin, Laurent; Harris, Lyal B.

    2014-07-01

    The Himalaya is the result of the on-going convergence and collision of India and Asia. The internal configuration and processes that govern the rise of the Himalayan Mountains and Tibetan Plateau are crucial to understand continental collision zones. However, knowledge of the prior configuration of the colliding plates is equally important, since inherited (pre-orogenic/basement) structures can undeniably influence the development of the orogenic architecture throughout the orogen's cycle of collision and eventual collapse. Three northeast-trending palaeotopographic ridges of faulted Precambrian Indian basement underlie the Ganga basin south of the Himalaya. Our paper illustrates a crustal-scale fault origin for these ridges and succeeds in determining how far north beneath the Himalayan system they extend and how they ultimately govern the location of upper crustal faults in southern Tibet. Spectrally filtered EGM2008 Bouguer gravity data and edges in its horizontal gradient at different source depths (`gravity worms') over northern Peninsular India, the Himalaya and southern Tibet reveal several continuous Himalayan cross-strike discontinuities interpreted to represent crustal faults. Gravity lineaments in Peninsular India coincide with edges of the Precambrian basement ridges and megakinks up to 100 km wide develop in foreland cover sequences between the interpreted basement faults. The interpreted basement faults project northward beneath the Himalayan system and southern Tibet. Our results suggest that several active Himalayan cross-strike faults, such as the ones related to many graben in southern Tibet, are rooted in the underplated Indian lower crust or step en échelon along interpreted basement faults. Our interpretation thus suggests that south Tibet graben are spatially related to deep-seated crustal-scale faults rooted in the underplated Indian crust. These major discontinuities partition the Himalayan range into distinct zones, and could ultimately

  15. Late-stage orogenic processes: How to link surface motion with distinct lithospheric processes

    NASA Astrophysics Data System (ADS)

    Neubauer, F.; Heberer, B.

    2009-04-01

    There is still a lack of knowledge of surface expression caused by deep-seated lithospheric processes, and how such processes could be distinguished from other, e.g. climate-induced, surface processes like denudation. Surface expressions of deep-seated lithospheric processes in convergent settings are expected to have been long-lived and to show large wave-length structures creating a dynamic topography (Wortel and Spakman, 2000; Cloetingh and Ziegler, 2007). Resulting continent-continent collisional orogens are bivergent, and the principal vergency of collisional orogens is controlled by the previous subduction of oceanic lithosphere (Beaumont et al., 1996). A number of tectonic processes are shown to be active during late orogenic phases and these processes particularly result in specific patterns of surface uplift and denudation of the evolving orogens as well as subsidence in the associated foreland basin. A number of these processes are not fully understood. Late-stage orogenic processes include, among others, slab break-off, slab delamination respectively of lithospheric roots, back-thrusting, tectonic indentation and consequent orogen-parallel lateral extrusion and formation of Subduction-Transform Edge Propagator (STEP) faults acting on the subducting lithosphere (Molnar and Tapponnier, 1975; Wortel and Spakman, 2000; Ratschbacher et al., 1991; Govers and Wortel, 2005). Here, we discuss these processes mainly in terms of their near-surface geological expressions within the orogen and the associated foreland basins, and how these processes could be distinguished by such geological features. We also show distinct theoretical models applied to the arcuate Alpine-Balkan-Carpathian-Dinaric system, which is driven by the oblique convergence of Africa-Europe. Slab-break-off results in lateral orogen-parallel migration of sharp subsidence in a linear belt in front of the slab window, coupled subsidence and subsequent uplift/basin inversion of peripheral foreland

  16. Structure and physical properties of the Earth's crust in the Central Metasedimentary Belt, Grenville Province, Ontario from near vertical seismic reflection data

    NASA Astrophysics Data System (ADS)

    Roy, Baishali

    The Central Metasedimentary Belt (CMB), located in the Southwestern Grenville Province, has undergone large scale tectonic movements during the Grenville orogeny. These large scale deformations have resulted in crustal scale faults and mylonite zones in these terranes. This work focuses on studying the structure and physical properties of the earth's crust from near vertical seismic reflection data from Lithoprobe reflection Lines 32 and 33 located in the southwestern CMB. These lines trend in a NW-SE manner roughly perpendicular to the major lineaments. The data was initially processed by Seismic Data Processors Ltd. (Calgary). Their processed sections from Line 32 and 33 do not give a clear subsurface picture, even though reflections were present in the raw data. Examination of the data revealed that the signal to noise ratio was not improved by CDP (common depth point) stacking, as they should, due to misalignment of signals even after static, NMO (normal move-out) and residual static corrections. This improper alignment is almost certainly due to lateral heterogeneity. The first part of the study is aimed at developing a Pattern Recognition method to improve the CDP stacking in spite of the misalignment. The signal to noise ratio was greatly improved by reduction of incoherent noise. The signals were then energy stacked in a moving time window, adding positive and negative peaks separately and thereby eliminating destructive addition. Results using this method show substantial improvements in the subsurface image. Reprocessing has helped in the interpretation of Lines 32 and 33. South east dipping reflectors are observed throughout Line 32. This supports the earlier geological and seismological studies which suggest northward directed thrusting. Reprocessed results suggest the gradual soling of the south east dipping reflectors into the lower crust, contrary to a mid-crustal decollement presented in earlier interpretations. In contrast the lower crust is

  17. The Grenville orogeny in the Llano Uplift, Texas: A record of collision and contraction along the southern margin of North America

    SciTech Connect

    Reese, J.F. . Dept. of Geological Sciences)

    1993-02-01

    Precambrian metamorphic rocks in the SE Llano Uplift record NE-directed ductile thrusting and regional-scale polyphase folding. This deformation is in response to Grenville-age shortening and crustal thickening associated with the collision of a south-lying tectonic block with the southern margin of North America. In the SE Llano Uplift, the most intense and pervasive deformational event, D2, is characterized in the Packsaddle Schist (PS) and Valley Spring Gneiss (VSG) by SSE-plunging, NE-verging isoclinal folds (F2) with an associated SW-dipping axial planar metamorphic layering (S2), and SW-dipping mylonite zones with kinematic indicators showing top-to-the-NE motion. In the Red Mountain and augen-bearing Big Branch gneisses, D2 structures are SW-dipping mylonite zones parallel to S2, and a SW-plunging stretching lineation. Taken together, this suite of structures indicates tectonic transport was to the NE, perpendicular to the NW trending regional structural and metamorphic grain. D2 structures were reoriented by at least two later phases of folding. Timing of all ductile deformation in the SE Llano Uplift is constrained from post-1,215 Ma (deformed PS) to pre-1,098 Ma (undeformed melarhyolite dike). From south to north, metatonalitic, arc-derived Big Branch Gneiss ([approximately] 1,303 Ma) and older mafic schist country rock, previously interpreted as possible ophiolitic melange, structurally overlie much younger, lithologically heterogeneous PS units (1,248-1,215 Ma), previously considered as arc flank deposits. In turn, the PS has been tectonically emplaced above the predominantly felsic VSG (1,270-1,232 Ma). The presence of older zircons in the VSG, of similar age ([approximately]1360 Ma) to Western Granite-Rhyolite Terrane rocks to the north, suggests that the VSG formed in a settling proximal to North America.

  18. Anisotropy of magnetic susceptibility versus lattice- and shape-preferred orientation in the Lac Tio hemo-ilmenite ore body (Grenville province, Quebec)

    NASA Astrophysics Data System (ADS)

    Bolle, Olivier; Charlier, Bernard; Bascou, Jérôme; Diot, Hervé; McEnroe, Suzanne A.

    2014-08-01

    The Lac Tio hemo-ilmenite ore body crops out in the outer portion of the 1.06 Ga Lac Allard anorthosite, a member of the Havre-Saint-Pierre anorthosite suite from the Grenville province of North America. It is made up of ilmenitite (commonly with more than 95% hemo-ilmenite) associated with noritic lithologies and anorthosite. The present study compares the magnetic fabric of the ore body, as deduced from anisotropy of magnetic susceptibility (AMS) measurements, with the crystallographic and shape fabrics, obtained from lattice-preferred orientation (LPO) and shape-preferred orientation (SPO) measurements made using electron backscattered diffraction (EBSD) and 3D image analysis, respectively. Room-temperature hysteresis measurements, thermomagnetic curves and values of the bulk magnetic susceptibility reveal a magnetic mineralogy dominated by a mixed contribution of hemo-ilmenite and magnetite. The hemo-ilmenite grains display a LPO characterized by a strong preferred orientation of the basal (0001) plane of ilmenite along which hematite was exsolved. This LPO and the magnetic fabric fit well (angle between the crystallographic c-axis and the axis of minimum susceptibility ≤ ca. 15° for most samples), and the latter is thus strongly influenced by the hemo-ilmenite magneto-crystalline anisotropy. A magnetite SPO, concordant with the hemo-ilmenite LPO, may also influence and even dominate the magnetic fabric. The rock shape fabric is coaxial with the magnetic fabric that can thus be used to perform detailed structural mapping. Interpretation of the magnetic fabric and field structural data suggests that the Lac Tio ore body would be a sag point at the margin of the Lac Allard anorthosite, deformed by ballooning during the final stage of diapiric emplacement of the anorthosite body.

  19. Magmatism in the Carolina terrane: Isotopic evidence for a Grenville-age source for Late Proterozoic volcanics and a mantle source for Silurian Concord syenite

    SciTech Connect

    Kozuch, M.; Heatherington, A.L.; Mueller, P.A. . Dept. of Geology); Offield, T.W.; Koeppen, R.P.; Klein, T.L. )

    1992-01-01

    Rhyolitic to andesitic volcanic rocks from the central portion of the Carolina slate belt in North Carolina were analyzed for Sr and Nd isotopic composition and dated by U-Pb zircon geochronology. Samples were from the greenschist-facies Late Proterozoic Albemarle Group, Uwharrie Formation, and the informal Virginia sequence. A rhyolite from the Cid Formation of the Albemarle Group dated by U-Pb zircon geochronology yielded a Pb-207/Pb-206 age of 575 [+-] 7.6 Ma, consistent with its position below strata containing the Late Proterozoic trace fossil Pteridinium and above rocks previously dated at 586 [+-] 10 Ma. Rb-Sr isotopic analyses of late Proterozoic rocks showed average initial Sr-87/Sr-86 ratios of approximately 0.704, indicating a moderately depleted source for these samples. E[sub ND] values at 600 Ma are moderately positive (+0.7 [minus] +2.3) and T(DM) values range from 1.19--1.04 Ga. These isotopic data, along with major and trace element data, suggest that andesites and rhyolites of the Carolina slate belt may have formed by partial melting of attenuated, Grenville-aged continental lithosphere during a 600 Ma episode of arc volcanism. In contrast, Sr and Nd data for the younger ([approximately]400 Ma) Concord pluton indicate it was derived from a depleted mantle source (Sr-87/Sr-86 = 0.7021 and E[sub ND] = +0.4 at 400 Ma) without significant involvement of older lithosphere (T(DM) = 370 Ma).

  20. Evidence for the importance of ductile shear in regional fabric development in Grenville-age gneisses of the Beaver Creek region, Northwest Lowlands, New York State

    SciTech Connect

    Tewksbury, B.; Culbertson, H.; Marcoline, J.; Walvoord, M. . Dept. of Geology)

    1993-03-01

    In the Beaver Creek region of the Northwest Lowlands, Brown (1989) has described Grenville-age metasedimentary and metaigneous rocks as showing a prominent regional foliation, early southeastward emplacement of a nappe complex (the North Gouverneur Nappe), 2 subsequent generations of folds, and late regional faulting along the Beaver Creek, Pleasant Lake, and Hickory-Mud Lakes faults. The authors examined a variety of units across the Beaver Creek region, including a granitic augen gneiss immediately west of the Beaver Creek Fault Zone, an alaskitic gneiss immediately below Brown's (1989) North Gouverneur Nappe Sole Fault, a biotitic granitic gneiss within the body of Brown's North Gouverneur Nappe, and hornblende augen gneisses and metasediments adjacent to the granitic gneisses. Each of the granitic units has moderately well-developed to extremely well-developed quartz ribbon lineations, and all show at least 2 ductile shear fabrics. Shear fabrics are present as well in the hornblende augen gneisses but are essentially absent in most of the metasedimentary lithologies, even those immediately adjacent to well-lineated, sheared granitic gneiss. The earliest shear fabrics exhibit spectacular quartz ribbon lineations, sigma grains, and, in the hornblende augen gneiss, shear bands. Granitic gneisses in the Beaver Creek Region show shear fabrics in addition to the main fabric in the rock. A second, variably-recovered shear fabric with quartz ribbons and well-developed sigma grains with core and mantle structure overprints the main shear fabric and shows largely the same sense of shear. The authors suggest further that a regional kinematic model for the Beaver Creek region must take into account significant, protracted regional shear, perhaps including formation of sheath folds, as in the Hyde School Gneiss at Payne Lake and Dobbs Creek.

  1. Controls of erosional denudation in the orogen on foreland basin evolution: The Oligocene central Swiss Molasse Basin as an example

    NASA Astrophysics Data System (ADS)

    Schlunegger, Fritz; Jordan, Teresa E.; Klaper, Eva Maria

    1997-10-01

    A high-resolution three-dimensional reconstruction of the 25-m.y.-old central Swiss Molasse Basin reveals two sedimentary domains separated by a ˜5-km-wide flood-plain. The proximal domain of the basin attained a width of 20 km, and its basement is steeply flexed (6°-7° dip). Petrographic data indicate that it was filled by sediment from the Rigi dispersal system derived from the central Alps of eastern Switzerland and by locally sourced bajadas. In contrast, the distal sedimentary domain, located farther north, was gently dipping (<2°) and was filled by the meandering Lac Léman and Honegg dispersal systems. Chronological data reveal that sedimentation in the northern proximal part of the basin started at ˜27 Ma, when sediment supply to the basin started to increase. Deflection of the foreland plate at ˜25 Ma is successfully simulated by flexural modeling of the thrust load and the sediment load. The model reveals that the Lac Léman and Honegg dispersal systems are located on a buried flexural bulge. Furthermore, it shows that burial and suppression of the flexural bulge at ˜27 Ma as well as an increase of the basin wavelength were controlled by the contemporaneous increase in the sediment supply rate of the Rigi system. The model presented suggests that the tectonic subsidence of the Molasse Basin was mainly controlled by tectonic events in the northern part of the orogen, within ˜70 km distance from the tip of the orogenic wedge. Crustal thickening in this part of the orogen is reflected in the proximal Molasse by sedimentary cycles characterized by an increase in the sediment accumulation rates up section and by the presence of locally sourced bajada fans at the top of each cycle. Although south vergent back thrusting along the Insubric Line ˜150 km south of the foreland basin contributed little to flexure, it resulted in an increase of the sediment supply to the foreland basin. This is reflected in the Molasse by coarsening and thickening upward

  2. Crustal structure and evolution of the Trans-Hudson orogen: Results from seismic reflection profiling

    NASA Astrophysics Data System (ADS)

    Baird, D. J.; Nelson, K. D.; Knapp, J. H.; Walters, J. J.; Brown, L. D.

    1996-04-01

    A 400-km-long deep seismic reflection transect across northeastern Montana and northern North Dakota reveals the crustal-scale structural fabric of the Early Proterozoic Trans-Hudson orogen beneath the Williston basin. Comparison with deep seismic reflection data across the Canadian portion of the same orogen ˜700 km to the north reveals first-order similarities in crustal architecture but documents significant along-strike variation in orogenic evolution. Both transects display a broad crustal-scale antiform axial to the orogen. In the north, geologic data suggest that this antiform is cored by an Archean microcontinent. In the south, west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the Archean Wyoming craton. Within the Wyoming craton, the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the Early Proterozoic Trans-Hudson orogen as interpreted from potential field and drill core data. On the basis of subsurface geochronologic data from the crystalline basement and by analogy with the Glennie domain within the exposed Trans-Hudson orogen in Canada, we suggest that the southern antiform is cored by an Archean crustal fragment that was caught up in the terminal collision of the Wyoming and Superior cratons during Hudsonian orogeny. The eastern side of the Trans-Hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections. We interpret the easterly dip of these reflections as evidence that the Superior province was thrust westward over the interludes of the orogen during terminal collision. Although juvenile Early Proterozoic terranes characterize the exposed segment of the Trans-Hudson orogen in Canada, limited drill core information within the Dakota segment of the orogen shows a predominance of granulitic

  3. Tectonic stress evolution in the Pan-African Lufilian Arc and its foreland (Katanga, DRC): orogenic bending, late orogenic extensional collapse and transition to rifting

    NASA Astrophysics Data System (ADS)

    Kipata, M. L.; Delvaux, D.; Sebagenzi, M. N.; Cailteux, J.; Sintubin, M.

    2012-04-01

    Between the paroxysm of the Lufilian orogeny at ~ 550 Ma and the late Neogene to Quaternary development of the south-western branch of the East African rift system, the tectonic evolution of the Lufilian Arc and Kundelungu foreland in the Katanga region of the Democratic Republic of Congo remains poorly unknown although it caused important Cu-dominated mineral remobilizations leading to world-class ore deposits. This long period is essentially characterized by brittle tectonic deformations that have been investigated by field studies in open mines spread over the entire arc and foreland. Paleostress tensors were computed for a database of 1450 fault-slip data by interactive stress tensor inversion and data subset separation, and the relative succession of 8 brittle deformation events established. The oldest brittle structures observed are related to the Lufilian brittle compressional climax (stage 1). They have been re-oriented during the orogenic bending that led to the arcuate shape of the belt. Unfolding the stress directions from the first stage allows to reconstruct a consistent NE-SW direction of compression for this stage. Constrictional deformation occurred in the central part of the arc, probably during orogenic bending (Stage 2). After the orogenic bending, a sequence of 3 deformation stages marks the progressive onset of late-orogenic extension: strike-slip deformations (stages 3-4) and late-orogenic arc-parallel extension (stage 5). It is proposed that these 3 stages correspond to orogenic collapse. In early Mesozoic, NW-SE compression was induced by a transpressional inversion, interpreted as induced by far-field stresses generated at the southern active margin of Gondwana (stage 6). Since then, this region was affected by rift-related extension, successively in a NE-SW direction (stage 7, Tanganyika trend) and NW-SE direction (stage 8, Moero trend).

  4. The geometries and development of late orogenic strike-slip faults

    NASA Astrophysics Data System (ADS)

    Anderson, M. W.; Peacock, D. C. P.

    2003-04-01

    Strike-slip faults are commonly the final phase of contraction in orogenic belts, occurring when the folds have locked-up and the thrusts have become too steep for further displacement to occur. Where the maximum stress axis (sigma1) is perpendicular to the orogenic belt (i.e. pure shear), the strike-slip faults are conjugate and represent strike-perpendicular shortening and necessary strike-parallel extension. An example of such behaviour occurs in the steeply dipping Silurian sandstones and shales in Kirkcudbrightshire, in the Southern Uplands of Scotland. If sigma1 is at ~ 70°; to the strike of the orogenic belt, the conjugate strike-slip faults become asymmetric. One set of strike-slip faults occurs at a high angle to the strike of bedding, and the conjugate set has a ramp-flat trajectory across steeply dipping beds. An example of this behaviour occurs in the steeply dipping Silurian turbidites at Ardglass, in the Longford-Down terrane of Northern Ireland. If sigma1 is at a lower angle to the orogenic belt (i.e. transpression), one set of strike-slip faults tends to dominate. In some cases, the dominant strike-slip fault set is parallel to the strike of the orogenic belt, the Caledonian-age Great Glen Fault of Scotland being such an example. In other cases, block rotation occurs on the set of strike-slip faults that are at a high angle to the orogenic belt, as in the Variscan Orogenic Belt of SW England. An annulus model is presented to illustrate the variations in geometries of late-orogenic strike-slip faults from pure shear to transpression.

  5. Zagros fold belt: orogenic accretion from obduction to collision

    NASA Astrophysics Data System (ADS)

    Vergés, Jaume; Saura, Eduard; Casciello, Emilio; Fernandez, Manel; Garcia-Castellanos, Daniel; Jiménez-Munt, Ivone; Torne, Montserrat; Villaseñor, Antonio

    2014-05-01

    The Zagros orogenic system comprises an exceptionally wide deformation zone between Arabia and Eurasia, embracing the entire Iran, and resulting from the closure of the Neotethys Ocean through its protracted NE-dipping subduction beneath Eurasia. The ~2000-km long, NW-SE trending Zagros fold belt is at the front of this orogenic system formed by the Sanandaj-Sirjan Zone and the Urumieh-Dokhtar Magmatic Arc, which are parallel to the main tectonic grain and have different geodynamic significance. The Zagros fold belt deforms 10-12-km thick Arabian sedimentary cover, which records compressive deformation since Late Cretaceous times. These tectonic events and their sequence have been studied in greater detail in the last ten years, mainly due to the profusion of dating of the syntectonic marine and non-marine sediments in the foreland basin. Despite these new data, and taking in account that there is a general consensus that the Zagros orogeny occurred during the complete consumption of the Neotethys Ocean, tectonic interpretations differ and ages of major geodynamic events remain controversial. Our studies confirm that the early Amiran foreland basin depocenter migrated from Campanian to Eocene (c. 83-52.7 Ma) after the onset of young Tethyan intra-oceanic obduction on top of the Arabian plate margin at the Cenomanian-Turonian boundary (~93 Ma). This migration is coeval with a mild but far-reaching deformation as indicated by punctuated growth strata patterns. A younger deformation event shaped the present geometry of the magnificent Zagros fold belt, overprinting the previous phase. Deformation along the High Zagros Fault was active from 20 Ma to at least 7.5 Ma. Folding in the Lurestan was active from at least ~13.5 Ma in the NE, migrating to the SW where it possibly terminated at about 2.5-1.5 Ma. In the Fars, deformation onset is dated at 14.5 Ma migrating SW-wards to the Persian Gulf coastline where the folds are still active. We propose a simple 2D kinematic

  6. Extreme mass flux from the glaciated, collisional St. Elias Orogen: Preliminary results from IODP Expedition 341 (Invited)

    NASA Astrophysics Data System (ADS)

    Gulick, S. P.; Jaeger, J. M.

    2013-12-01

    Integrated Ocean Drilling Program Expedition 341 drilled a cross-margin transect to investigate the linkages between global climate change, modification of the dynamics of surficial processes, and subsequent tectonic responses. The Gulf of Alaska (GoA) borders the St. Elias orogen, the highest coastal mountain range on Earth. Exp. 341 drilled five sites within a regional seismic reflection grid that spans from the distal Surveyor Fan to the continental shelf. More than 3000 m of high-quality core coupled with seismic reflection profiles collected with nested vertical resolution allows us to address the major objectives of drilling in the GoA. These objectives were to: 1) document the tectonic response of an active orogenic system to late Miocene to recent climate change; 2) establish the timing of advance/retreat phases of the northern Cordilleran ice sheet to test its relation to dynamics of other global ice sheets; 3) implement an expanded source-to-sink study of the interactions between glacial, tectonic, and oceanographic processes responsible for creation of one of the thickest Neogene high-latitude continental margin sequences; 4) understand the dynamics of productivity, nutrients, freshwater input to the ocean, and ocean circulation in the northeast Pacific and their role in the global carbon cycle, and 5) document the spatial and temporal behavior of the geomagnetic field at extremely high temporal resolution in an under-sampled region of the globe. The Exp. 341 cross-margin transect discovered transitions in sediment accumulation rates from >100 m/Ma at the distal site to > 1000 m/Ma in the proximal fan, slope and on the continental shelf that provide a telescoping view of strata formation from the Miocene to the Holocene. Complete recovery and development of spliced sedimentary records of the Pleistocene through Holocene were achieved at the distal, proximal, and slope Sites U1417, U1418, and U1419, respectively, because of exceptional piston core

  7. Patterns of glaciation and topographic hypsometry across semi-arid western Himalayan-Tibetan orogen (Invited)

    NASA Astrophysics Data System (ADS)

    Dortch, J.; Hughes, P.; Owen, L. A.; Murari, M. K.; Caffee, M. W.

    2013-12-01

    It has been hypothesized that the reduction of mountain glacier extent throughout late Quaternary glacial cycles may reflect adjustment of topographic hypsometry and lowering of accumulation area via glacial erosion. The Himalayan-Tibetan orogen represents a good test case due to high relief, fast rates of erosion, and recently developed regional glacial chronologies. In particular, analysis of all terrestrial cosmogenic nuclide ages of moraines in the drylands of the western Himalayan-Tibetan orogen show 16 regional glacial stages extending back >300 ka; stages older than the global Last Glacial Maximum (gLGM) broadly correlate with strong monsoons, while younger stages correlate with northern hemisphere climatic events. These patterns suggest that frequent glacial advances are driven by mid-latitude westerlies within each glacial cycle, but that each cycle is punctuated by more extensive advances driven by the monsoon. This raises the question of which style of glaciation has the greatest effect on the shaping and incision of topography: frequent, less-extensive variations in glaciation, or infrequent more-extensive advances (the latter being akin to punctuated equilibrium)? To investigate this question, changes in glacier position based on moraines and climatic proxies are used to model changes in equilibrium line altitude (ELA), which is then compared to topographic hypsometry in catchments were the glacial buzzsaw hypothesis has been previously invoked. The loss in resolution due to the decrease in moraine preservation through time must be accounted for. Thus, the MIS curve and monsoon index are combined to fit to the regional chronology with the view that ages represents the minimum age of deglaciation. A best fit curve is used as a proxy for glacial extent to estimate higher-frequency fluctuations for the pre-gLGM portion of the regional chronology. ELAs are then calculated for selected glaciers at various extents to enable the determination of the average

  8. How Orogen-scale Exhumed Strike-slip Faults Initiate

    NASA Astrophysics Data System (ADS)

    Cao, S.; Neubauer, F.

    2015-12-01

    Orogen-scale strike-slip faults present one the most important geodynamic processes affecting the lithosphere-asthenosphere system. In specific subtypes, faulting is virtually initiated along hot-to-cool boundaries, e.g. at such of hot granite intrusions or metamorphic core complexes to cool country rocks. Such fault zones are often subparallel to mountain ranges and expose a wide variety of mylonitic, cataclastic and non-cohesive fault rocks, which were formed at different structural levels of the crust and are stacked within each other ("telescoping"). Exhumation of rocks is, therefore, a common feature of such strike-slip faults implying major transtensive and/or transpressive processes accompanying pure strike-slip motion. The hot-to-cool thermal structure across the fault zone significantly influences the physical fault rock properties. One major question is how and where a major strike-slip initiates and further development. Here, we propose a model in which major continental exhumed strike-slip faults potentially evolve along rheologically weak zones such as plutons or margins of metamorphic complexes. As an example, we propose a model for the Ailao Shan-Red River (ASRR) fault, SE Asia, which initiated along the edge of a plutonic belt and evolved in response to India-Asia collision with four tectonic phases.

  9. Tectonic evolution of the western Kunlun orogenic belt, western China

    NASA Astrophysics Data System (ADS)

    Zhihong, Wang

    2004-11-01

    The western Kunlun orogenic belt in western China evolved through the development of a large subduction-accretionary complex, including flysch sediments and granitic plutons, and by collision of three terranes, namely the North and South Kunlun and Karakorum-Qiangtang blocks from the early Paleozoic to the early Mesozoic. North-dipping subduction of the Paleo-Tethys ocean beneath the Kunlun terranes, which may have commenced in the Cambrian, produced an early Paleozoic Andean-type magmatic arc on the South Kunlun, and a marginal back arc basin, represented by the early Paleozoic Oytag-Kudi ophiolite belt, between the North and South Kunlun. A northward subduction zone consumed the basin and the young, hot upper plate lithosphere was obducted southward onto the South Kunlun following closure of the basin. This resulted in collision of the North and South Kunlun in the early Devonian. Continuous northward subduction of Paleo-Tethys resulted in the development of a Carboniferous-Triassic magmatic arc, and a back arc rifting sequence composed of the Carboniferous to Permian carbonates and clastic sediments on the North and South Kunlun terranes. The Paleo-Tethys ocean finally closed in the late Triassic-early Jurassic, when the Kunlun and Karakorum-Qiangtang blocks were accreted, with the Kara-Kunlun accretionary prism marking their suture zone.

  10. Late Paleozoic orogenic episodes, Trans-Pecos Texas

    SciTech Connect

    Muehlberger, W.R. . Dept. of Geological Sciences)

    1993-04-01

    The onset of the Ouachita orogeny is marked by the absence of rock of Early Mississippian age. This is interpreted to be the result of the narrowing of the trans-equatorial seaway between North America and the oncoming South American plate. Foredeep filling (Pulse I) began in Early Meramecian time with northward-prograding Tesnus Fm siliciclastic turbidites. Pulse 2 began in Atokan time with northward-prograding siliciclastic turbidites of the Haymond Fm. The thrust belt to the south had now intersected the edge of the NAm carbonate platform as shown by clasts of Middle Cambrian carbonates, as well as Ouachita facies clasts, and rounded clasts of Precambrian quartzite, metarhyolite, and gneiss of unknown source. During Late Pennsylvanian time deltaic sediments prograded across the Marathon region into the southern Permian Basin (Gaptank Fm). In Early Wolfcampian time (Pulse 3), all these units were translated on the Dugout Creek thrust, then (Pulse 4 ) translated finally on a frontal imbricate before deposition of Upper Wolfcampian units across the eroded toes of the thrust sheets. Permian Basin orogenic phases parallel those outlined above. Transgressive clastics from the south and the subdivision of the earlier broad, shallow Tobosa Basin into the uplifts and basins that characterize the Permian Basin began in the Mississippian and became prominent structural units by Late Mississippian time.

  11. Tectono-metamorphic evolution of the internal zone of the Pan-African Lufilian orogenic belt (Zambia): Implications for crustal reworking and syn-orogenic uranium mineralizations

    NASA Astrophysics Data System (ADS)

    Eglinger, Aurélien; Vanderhaeghe, Olivier; André-Mayer, Anne-Sylvie; Goncalves, Philippe; Zeh, Armin; Durand, Cyril; Deloule, Etienne

    2016-01-01

    The internal zone of the Pan-African Lufilian orogenic belt (Zambia) hosts a dozen uranium occurrences mostly located within kyanite micaschists in a shear zone marking the contact between metasedimentary rocks attributed to the Katanga Neoproterozoic sedimentary sequence and migmatites coring domes developed dominantly at the expense of the pre-Neoproterozoic basement. The P-T-t-d paths reconstructed for these rocks combining field observations, microstructural analysis, metamorphic petrology and thermobarometry and geochronology indicate that they have recorded burial and exhumation during the Pan-African orogeny. Both units of the Katanga metasedimentary sequence and pre-Katanga migmatitic basement have underwent minimum peak P-T conditions of ~ 9-11 kbar and ~ 640-660 °C, dated at ca. 530 Ma by garnet-whole rock Lu-Hf isochrons. This suggests that this entire continental segment has been buried up to a depth of 40-50 km with geothermal gradients of 15-20 °C.km- 1 during the Pan-African orogeny and the formation of the West Gondwana supercontinent. Syn-orogenic exhumation of the partially molten root of the Lufilian belt is attested by isothermal decompression under P-T conditions of ~ 6-8 kbar at ca. 530-500 Ma, witnessing an increase of the geothermal gradients to 25-30 °C·km- 1. Uranium mineralizations that consist of uraninite and brannerite took place at temperatures ranging from ~ 600 to 700 °C, and have been dated at ca. 540-530 Ma by U-Pb ages on uraninite. The main uranium deposition thus occurred at the transition from the syn-orogenic burial to the syn-orogenic exhumation stages and has been then partially transposed and locally remobilized during the post-orogenic exhumation accommodated by activation of low-angle extensional detachment.

  12. Orogenic plateau growth in the Zagros of Iran

    NASA Astrophysics Data System (ADS)

    Allen, Mark; Blanc, Eric; Saville, Christopher

    2010-05-01

    This paper concerns how Turkish-Iranian plateau grows by incorporating the Zagros fold and thrust belt. The plateau's tectonic boundary can be defined as the limit of significant seismogenic thrusting, which occurs close to the regional 1 km elevation contour. The geomorphic boundary is less distinct, but occurs northeast of the limit of active thrusting, because of a time lag during which mountainous relief converts to the subdued plateau geomorphology. Most of the High Zagros and ~25,000 km2 of the Zagros Simple Folded Zone behave as part of the plateau. The Dezful Embayment is a low strain zone in the western Zagros Simple Folded Zone, implying locally strong basement. Deformation is correspondingly more intense northeast of the Embayment, where the highest elevations and steepest slopes in the Zagros occur. As a consequence of the Embayment, lateral plateau growth is more limited in the western Zagros than the east (Fars). A more uniform structure across the Fars region has produced a lower orogenic taper, and a wider region of the Zagros behaves as part of the Turkish-Iranian plateau. Climatic variation along the Zagros is likely to act as a positive feedback on this tectonic variation, although the rates are not well-constrained. Relatively high orographic precipitation northeast of the Dezful Embayment promotes exhumation. The more arid climate in the Fars region should subdue exhumation, implying quicker crustal thickening and elevation for any given shortening. This enhances lateral plateau growth. Regional plateau elevations >1 km may relate to underlying warm and partially molten mantle.

  13. Distribution of strain rates in the Taiwan orogenic wedge

    NASA Astrophysics Data System (ADS)

    Mouthereau, F.; Fillon, C.; Ma, K.-F.

    2009-07-01

    To constrain the way Eurasian crust is accreted to the Taiwan orogenic wedge we investigate the present-day 3D seismogenic deformation field using the summation of 1129 seismic moment tensors of events ( Mw > 4) covering a period of 11 years (1995 to 2005). Based on the analysis of the principal strain-rate field, including dilatation and maximum shear rates, we distinguish four domains. Domain I comprises the Coastal Plain and the Western Foothills. It is mainly contractional in both the horizontal plane and in cross-section. Domain II comprises the eastern Western Foothills, the Hsuehshan Range and the Backbone Range. It is characterized by the highest contraction rates of 10 - 6 yr - 1 in association with area expansion in cross-section and area contraction in the horizontal plane. Domain III corresponds to the Central Range. It is characterized by area contraction in cross-section and area expansion in the horizontal plane. The maximum contractional axis is typically low and plunges ~ 30°E. Extension is larger, horizontal and strikes parallel to the axis of the mountain range. Domain IV corresponding to the Coastal Range and offshore Luzon Arc shows deformation patterns similar to domain II. This seismogenic strain-rate field, which is found in good agreement with the main features of the geodetic field, supports shortening within a thick wedge whose basal décollement is relatively flat and located in the middle-to-lower crust > 20 km. The east plunges of maximum strain-rate axes below the Central Range argue for the development of top-to-the-east transport of rocks resulting from the extrusion of the whole crust along west-dipping crustal-scale shear zones. The study of seismogenic strain rates argues that the initiation of subduction reversal has already started in the Taiwan collision domain.

  14. Propagation tectonics and multiple accretionary processes of the Qinling Orogen

    NASA Astrophysics Data System (ADS)

    Dong, Yunpeng; Zhang, Xiaoning; Liu, Xiaoming; Li, Wei; Chen, Qing; Zhang, Guowei; Zhang, Hongfu; Yang, Zhao; Sun, Shengsi; Zhang, Feifei

    2015-05-01

    The Qinling Orogen was built through collision between the North China and South China Blocks. Previous detailed geological, geochemical and geochronological investigations revealed that the mountain range can be divided into four tectonic units with distinct tectono-lithostratigraphy, which are, from north to south, the southern sector of the North China Block, North Qinling Belt, South Qinling Belt and northern sector of the South China Block, separated by the Kuanping, Shangdan and Mianlue sutures. According to the petrology, geochemistry and geochronology of ophiolitic mélanges and related magmatic rocks, as well as the features of sedimentary units, we think that the North China Block, the North Qinling Belt and the South China Block were originally independent continental units while the South Qinling Belt had been the northern part of the South China Block. These units experienced three episodes of accretionary tectonic processes and amalgamation from south to north. The Neoproterozoic accretion took place along the Luonan-Luanchuan Fault and Kuanping ophiolitic mélange belt as a result of southward subduction and subsequent collision between the North Qinling and North China Blocks during ca. 1.0-0.8 Ga related to the formation of the supercontinent of Rodinia. The Paleozoic accretion occurred along the Shangdan suture resulted from northward subduction of oceanic lithosphere in the Early Paleozoic and subsequent continental subduction in the Late Paleozoic. Late Triassic accretion took place along the Mianlue suture between the South Qinling and South China Blocks due to northward subduction of the Mianlue oceanic lithosphere during the Permian-Early Triassic and subsequent collision in the Late Triassic. After the Late Triassic collision along the Mianlue suture the whole Qinling Mountain range entered the phase of intense intracontinental deformation.

  15. Early Cretaceous extensional reworking of the Triassic HP-UHP metamorphic orogen in Eastern China

    NASA Astrophysics Data System (ADS)

    Lin, Wei; Ji, Wenbin; Faure, Michel; Wu, Lin; Li, Qiuli; Shi, Yonghong; Scharer, Urs; Wang, Fei; Wang, Qingchen

    2015-11-01

    Corresponding to the Early Mesozoic continental subduction between the North China Block (NCB) and the South China Block (SCB), the Tongbaishan-Hong'an-Dabieshan-Sulu massifs are famous for their HP-UHP metamorphism. More than 50% of the HP-UHP Orogenic Belt was significantly reworked by Early Cretaceous extensional tectonics. This Early Cretaceous event with a fast cooling period, at 130-120 Ma, superimposed on the Early Mesozoic HP-UHP orogenic belt and intensively changed the architecture of this orogen. Each individual segment documents different Early Cretaceous extensional structures, namely the central Tongbaishan domain is a metamorphic core complex (MCC) represented by an A-type non-cylindrical antiform; the central Dabieshan domain is a typical Cordilleran-type migmatite-cored MCC; the Southern Sulu UHP domain is a "wedge-shaped" structure exhumed by a simple detachment fault. These late stage extensional structures expose the previous HP-UHP orogenic belt as fragments along the NCB-SCB boundary. The geodynamic setting of this Early Cretaceous extensional tectonics along the HP-UHP orogen is a part of a 1000 km-scale crustal extension belt that is widespread in eastern Eurasia continent from Trans-Baikal to the central part of SCB. Convective erosion or delamination of the mantle lithosphere might be considered as a possible mechanism for mantle removal.

  16. Early Cretaceous extensional reworking of the Triassic HP-UHP metamorphic orogen in Eastern China

    NASA Astrophysics Data System (ADS)

    Lin, W.; Ji, W.; Faure, M.; Wu, L.; Li, Q. L.; Shi, Y.; Scharer, U.; Wang, F.; Wang, Q.

    2015-12-01

    Corresponding to the Early Mesozoic continental subduction between the North China Block (NCB) and the South China Block (SCB), the Tongbaishan-Hong'an-Dabieshan-Sulu massifs are famous for their HP-UHP metamorphism. More than 50% of the HP-UHP Orogenic Belt was significantly reworked by Early Cretaceous extensional tectonics. This Early Cretaceous event with a fast cooling period, at 130-120 Ma, superimposed on the Early Mesozoic HP-UHP orogenic belt and intensively changed the architecture of this orogen. Each individual segment documents different Early Cretaceous extensional structures, namely the central Tongbaishan domain is a metamorphic core complex (MCC) represented by an A-type non-cylindrical antiform; the central Dabieshan domain is a typical Cordilleran-type migmatite-cored MCC; the Southern Sulu UHP domain is a "wedge-shaped" structure exhumed by a simple detachment fault. These late stage extensional structures expose the previous HP-UHP orogenic belt as fragments along the NCB-SCB boundary. The geodynamic setting of this Early Cretaceous extensional tectonics along the HP-UHP orogen is a part of a 1000 km-scale crustal extension belt that is widespread in eastern Eurasia continent from Trans-Baikal to the central part of the South China Block. Convective erosion or delamination of the mantle lithosphere might be considered as a possible mechanism for mantle removal.

  17. Crustal Architecture along BABEL and FIRE profiles - Insight in the Growth of the Svecofennian Orogen

    NASA Astrophysics Data System (ADS)

    Korja, Annakaisa

    2016-04-01

    The Precambrian Svecofennian orogen is characterized by LP- HT metamorphism and voluminous granitoid magmatism that usually develop in transitional to plateau stages of a collisional orogeny. Deep seismic reflection profiles BABEL and FIRE have been interpreted using PURC concepts: prowedge, retrowedg, uplifted plug, subduction conduit and elevated plateau. BABEL profiles image a transitional orogen with several nuclei displaying prowedge-uplifted plug-retrowedge architecture above paleo-subduction conduits. Prowedge and -continent are on the south-southwestern side and retrowedge and -continent on the north-northwestern side. This implies a long-lived southwesterly retreating convergent margin, where transitional accretionary orogens have developed. FIRE1-3 profiles images a hot orogen with a pronounced super-infra structure, typical of an elevated plateau stage, below the Central Finland Granitoid Complex. Large volumes of granitoid intrusions suggest large scale melting of the middle and/or lower crust. Reflection structures, analogue and numerical modeling suggest midcrustal flow. The plateau is flanked by prowedges that are characterized by HT-LP migmatite belts. The Svecofennian orogeny has progressed to an elevated plateau stage in the thickest core of the orogen, west of the arc-continent collision zone.

  18. Mechanical model for subduction-collision tectonics of Alpine-type compressional orogens

    NASA Astrophysics Data System (ADS)

    Beaumont, Christopher; Ellis, Susan; Hamilton, Juliet; Fullsack, Philippe

    1996-08-01

    Alpine-type orogens are characterized by three distinct convergent tectonic phases: subduction with deformation that has primarily single vergence, a transition from subduction to collision, and continental collision with double vergence. Although the Cenozoic history of the European Alps has additional complexities, a mechanical explanation for these three phases would provide the necessary crustal-scale framework in which to develop an understanding of the smaller-scale processes. We present results from a simple numerical model, which explain the mechanics of these three phases as a consequence of the changing buoyancy of the lithosphere subducted beneath the orogen. The development and exhumation of a subduction complex, suture zone, and basement nappe stack (Piemont suture, Penninic Nappes); the presence of a crustal-scale back fold and thrust (Insubric Line); and uplift of basement on the pro- (European) side of the orogen (external basement massifs) may be explained as a simple consequence of changing dynamics during the transition from subduction to collision.

  19. Neogene coupling between Kuqa Basin and Southern Tien Shan Orogen, Northwestern China.

    PubMed

    He, Guang-Yu; Chen, Han-Lin

    2004-08-01

    Based on the sedimentary and subsiding features of Kuqa foreland basin, this paper presents the following characteristics of Neogene coupling relationship between Kuqa Basin and Southern Tien Shan Orogen, Northwestern China: (1) The Southern Tien Shan Orogen underwent Neogene uplifting of 4 km in height and the Kuqa Basin underwent Neogene subsidence of 4-6 km in depth accordingly beginning in 25 Ma; (2) The Southern Tien Shan Orogen moved continuously toward the Kuqa Basin, with largest structural shortening rate of greater than 53.7%, and the north boundary of the Kuqa Basin retreated continuously southward accordingly since the Miocene; (3) There are two subsidence centers with high subsiding rates and large subsiding extent, located in the eastern and western Kuqa Basin respectively, with the subsiding maximizing in the deposition period of Kuqa Formation. PMID:15236483

  20. Architecture of orogenic belts and convergent zones in Western Ishtar Terra, Venus

    NASA Technical Reports Server (NTRS)

    Head, James W.; Vorderbruegge, R. W.; Crumpler, L. S.

    1989-01-01

    Linear mountain belts in Ishtar Terra were recognized from Pioneer-Venus topography, and later Arecibo images showed banded terrain interpreted to represent folds. Subsequent analyses showed that the mountains represented orogenic belts, and that each had somewhat different features and characteristics. Orogenic belts are regions of focused shortening and compressional deformation and thus provide evidence for the nature of such deformation, processes of crustal thickening (brittle, ductile), and processes of crustal loss. Such information is important in understanding the nature of convergent zones on Venus (underthrusting, imbrication, subduction), the implications for rates of crustal recycling, and the nature of environments of melting and petrogenesis. The basic elements of four convergent zones and orogenic belts in western Ishtar Terra are identified and examined, and then assess the architecture of these zones (the manner in which the elements are arrayed), and their relationships. The basic nomenclature of the convergent zones is shown.

  1. Remagnetization and orogenic fluids: testing the hypothesis in the central Appalachians

    NASA Astrophysics Data System (ADS)

    Elmore, R. Douglas; Kelley, Jeff; Evans, Mark; Lewchuk, Michael T.

    2001-03-01

    Palaeomagnetic and geochemical studies on Devonian sedimentary rocks in the Valley and Ridge province in West Virginia indicate that the hypothesized relationship between orogenic fluids and remagnetization is not straightforward. The Helderberg Group, an aquitard, and the Oriskany Formation, a palaeoaquifer, both contain similar, synfolding late Palaeozoic chemical remanent magnetizations (CRMs) residing in magnetite. The results from the Oriskany Formation are consistent with remagnetization by orogenic fluids because the unit contains geochemical evidence for the migration of such fluids. In contrast, there is no geochemical evidence for the widespread migration of orogenic fluids into the Helderberg Group and thus the CRM cannot be related to such fluids. Another remagnetization mechanism, which remains elusive, must be responsible for the CRM in the Helderberg Group.

  2. Orogenic plateau magmatism of the Arabia-Eurasia collision zone

    NASA Astrophysics Data System (ADS)

    Allen, M. B.; Neill, I.; Kheirkhah, M.; van Hunen, J.; Davidson, J. P.; Meliksetian, Kh.; Emami, M. H.

    2012-04-01

    generally applicable as melt triggers. Enigmatic lavas are erupted over the thick lithosphere of Kurdistan Province, Iran. These alkali basalts and basanites have the chemical characteristics of small degree (<1%) melts in the garnet stability field. Most possess supra-subduction zone chemistry (La/Nb = 1-3), but this signature is highly variable. Similar La/Nb variability occurs in the basic lavas of Damavand volcano in the Alborz Mountains of northern Iran. Modelling suggests the depletion of residual amphibole during the progression of partial melting can explain the observed La/Nb range. This melting may occur as the result of lithospheric thickening. At depths of ~90 km, amphibole-bearing peridotite crosses an experimentally-determined "backbend" in its solidus. Melting can continue while the source remains hydrated. Such "compression" melting may apply to parts of other orogenic plateaux, including Tibet.

  3. Crustal Shear Wave Anisotropy in the Taiwan Orogen

    NASA Astrophysics Data System (ADS)

    Rau, R.; Yang, C.

    2002-12-01

    Crustal shear wave anisotropy is analyzed in seismograms from local earthquakes (1.5 < M < 4.5) recorded at 75 permanent network stations in Taiwan during the period between 1991 and 2000. We investigate the origin of Taiwan crustal anisotropy by analyzing splitting in recorded shear waves, which can be characterized by a fast polarization direction and a time delay between fast and slow shear waves. Particle motion analysis and cross-correlation method are applied to estimate the splitting parameters of local shear waves. Clear evidence of shear wave splitting with split times of 0.02 to 0.2 s is found in about 75% of the stations studied. Stations in Coastal Plain, the foreland basin, show consistent alignment of the fast polarization directions that are parallel to the directions of local maximum horizontal compressive stress. Around the Chukou fault in the foothills region, fast shear-wave polarization directions of over 20 earthquakes recorded at station TWL are normal to the direction of local maximum horizontal compressive stress and parallel to the NE-SW trend of this east-dipping thrust fault. Except TWL, all the stations in foothills show large scatter in measured shear-wave polarizations. In the southern Central Range, two stations with a stable polarization direction of NNE-SSW, which is consistent with the local preferential mineral orientation, are found near the ChaoChou fault system. Two persistent polarization directions of fast shear wave are observed in a station (STY) near the boundary between the foothills and the southern Central Range: NW-SE direction for earthquakes located beneath the foothills and NE-SW direction for earthquakes coming under the Central Range. Our study indicates that the crustal anisotropy beneath the Taiwan orogen cannot be simply explained by the hypothesis of extensive dilatancy anisotropy (EDA), where parallel alignment of fluid-filled fractures produces the anisotropy. Other factors, such as intrinsic rock anisotropy

  4. The Effects of Gravitational Instability on the Tectonic Evolution of Continental Orogens

    NASA Astrophysics Data System (ADS)

    Gemmer, L.; Houseman, G. A.

    2005-05-01

    Convergent continental orogens have been the research focus of numerous geological and geophysical surveys and dynamical modeling studies, but the mechanisms controlling the evolution of these systems are still under debate. The temporal and spatial distribution of tectonic processes in orogenic regions is complex, and the interplay between buoyancy forces and regional tectonics in some of these systems is still not well understood. In several cases the extensional collapse of mountain belts is associated with dramatic thinning of the mantle part of the lithosphere, more so than the crust. A key region for investigating these processes is the Carpathian-Pannonian system of eastern Europe, where rapid extension took place in the Pannonian Basin simultaneously with contractional deformation in the surrounding orogens. Previous studies show that gravitational instabilities may play a fundamental role in the tectonics of mountain ranges. In general, the lithosphere is colder and thereby denser than the underlying asthenosphere. Under some circumstances this may cause the lithosphere to sink into the underlying asthenosphere. We use dynamic numerical models to investigate how such gravitational instabilities may affect the evolution of continental orogens. We show how a crust initially thickened by localized convergence may promote lithospheric gravitational instabilities that cause the collapse of high topography and focused, depth-variable lithospheric thinning developing simultaneously with contractional deformation and lithospheric downwelling in the adjacent areas. We investigate the relative importance of buoyancy and regional tectonics in convergent continental orogens and show how density and viscosity (Newtonian or non-Newtonian) affect the evolution of a model system. We investigate the distribution and amplitude of lithospheric downwelling and examine how the instability develops adjacent to the corner of an initially rectangular region of thickened crust in

  5. Reconciling the detrital zircon record and crustal growth within juvenile accretionary orogens

    NASA Astrophysics Data System (ADS)

    Spencer, C. J.; Cawood, P. A.; Roberts, N. M. W.

    2014-12-01

    Ancient cratons are generally characterised by Archaean cores surrounded by Proterozoic accretionary belts with large volumes of juvenile crust. Their crustal growth histories provide important insights into the genesis of continents and orogenic evolution. Whole-rock and detrital zircon isotopic studies are often used to deduce those histories, but the extent to which representative lithologies within the orogens are reliably sampled for such studies is not well established. This is especially true in cases where juvenile, zircon-poor mafic crust comprises a significant proportion of an orogen such as the East African (0.8-0.5 Ga), Namaqua-Natal (1.2-1.0 Ga), Trans-Hudson (1.9-1.8 Ga), and Kola (2.5 Ga). In particular, the Mesoproterozoic Namaqua-Natal orogenic belt (NNO) fringing the Kalahari Craton is a case in point in which Nd isotopic studies of whole-rock outcrop samples and U-Pb-Hf isotopic studies of detrital zircons from sediments of the Orange River (which drains the NNO) show different crust-formation ages and proportions of new and reworked crustal material. We hypothesise that this discrepancy is due to biasing of the detrital zircon record towards felsic rocks. Understanding the representative nature of the crustal archive preserved in detrital zircons remains critical for many studies of crustal evolution. We present data that: (a) addresses the scale of potential bias within an accretionary orogen containing large proportions of juvenile material, (b) demonstrates how the whole-rock and detrital zircon records can be reconciled for the Namaqua-Natal orogen to start, and (c) can be used to evaluate the effect of zircon bias on previous crustal growth models.

  6. Plate tectonics and orogenic research after 25 years: Synopsis of a Tethyan perspective

    NASA Astrophysics Data System (ADS)

    Şengör, A. M. Celâl

    1991-02-01

    Orogeny, the process by which the earth's prominent mountain ranges are constructed, is herein defined as a collective term for convergent margin processes. The recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt has grown gradually during the last two centuries. Investigation of orogenic belts along cross-sections reveals that there are a large number of types of orogenic belts. These are divided into four main orders ( transpressional, subduction-controlled, obduction-controlled, and collision-controlled) consisting of two superfamilies, eight families, and twenty genera. Cross-sectional studies of orogenic belts show that the cross-sectional area during orogeny is not conserved. Similarly, map-view studies of orogenic belts reveal that an absolute minimum of 60% (by length) of them display significant strike-slip motion along their trend which leads also to a non-conservation of the cross-sectional area during orogeny. Thus, rigorous line and area balancing across orogenic belts now is not possible. Large orogenic belts are commonly made up of tectonic collages of microcontinents, island arcs, and accretionary complexes, generally disrupted to form smaller, fault-bounded tectonic entities of diverse sorts. The recently developed "terrane analysis" was developed to aid the study of these but it resembles the early concepts of Alpine nappes and is found to be a retrogressive step in tectonic research mainly because of its disclaim of most genetic connotations. The temporal aspects of orogeny have been debated for over 200 years in terms of continuous vs. world-wide, synchronously episodic orogeny. Plate tectonics has provided a rigorous rationale and something approaching a consensus for continuous orogeny. I conclude that there are as yet no shortcuts to establishing the kinematics of continental deformation except by the traditional methods of field geology aided by relevant

  7. Crustal Structure of the Dabie Orogenic Belt (eastern China) Inferred from Gravity and Magnetic Data

    NASA Astrophysics Data System (ADS)

    Yang, Y.; Li, Y.

    2012-12-01

    This study investigates the utility of the potential fields (gravity and magnetics) in orogenic belt settings as observed in the Dabie orogenic belt. The methods applied are Euler deconvolution on the magnetic data, continuous wavelet transform (CWT), and the gravity forward modeling. We devise a 500km long profile I, which trends N21°E and begins in the north within the Hefei basin of the North China craton, passes through the Dabie orogen, and ends in the Yangtze craton. The Euler deconvolution on the magnetic signal provides depth estimation for magnetized sources along the profile. The CWT method is then adopted to characterize the underground density interfaces of gravity field, which provides a useful constraint for the forward modeling of the profile I. Finally we present a crustal cross section through the Dabie orogenic belt, derived from the gravity forward modeling results with integration of source depth estimations from the gravity and magnetic data. As indicated by the forward gravity modeling results, there are low density bodies in the central orogen, which are probably due to the over-thickened crustal materials formed during the continuous convergence between the North China craton (NCC) and the Yangtze craton after break-off of the subducted slab. After the delamination or mountain root removal in the Dabie orogen, the asthenosphere upwelled to replace the volume previously occupied by the delaminated lithosphere. Under the extensional environment in the early Cretaceous, the upwelling mantle was metasomatised and partially melted to produce the parental magma of the post-collisional mafic-ultramafic intrusive rocks. Two anomalous denser materials with low magnetization in the lower crust and upper mantle of the Dabie orogen can be recognized in the CWT-based spectral analysis result on the magnetic data, which may be inferred as sequence of mass transport from the metasomatised mantle. Figure 1. A profile view from the Dabie orogen along the

  8. Basin-mountain structures and hydrocarbon exploration potential of west Junggar orogen in China

    NASA Astrophysics Data System (ADS)

    Wu, X.; Qi, X.; Zheng, M.

    2015-12-01

    Situated in northern Xinjiang, China, in NE-SW trend, West Junggar Orogen is adjacent to Altai fold belt on the north with the Ertix Fault as the boundary, North Tianshan fold belt on the south with the Ebinur Lake Strike-slip Fault as the boundary, and the Junggar Basin on the southeast with Zaire-Genghis Khan-Hala'alat fold belt as the boundary. Covering an area of about 10×104 km2 in China, there are medium and small intermontane basins, Burqin-Fuhai, Tacheng, Hefeng and Hoxtolgay, distributing inside the orogen. Tectonically West Junggar Orogen lies in the middle section of the Palaeo-Asian tectonic domain where the Siberia, Kazakhstan and Tarim Plates converge, and is the only orogen trending NE-SW in the Palaeo-Asian tectonic domain. Since the Paleozoic, the orogen experienced pre-Permian plate tectonic evolution and post-Permian intra-plate basin evolution. Complex tectonic evolution and multi-stage structural superimposition not only give rise to long term controversial over the basin basement property but also complex basin-mountain coupling relations, structures and basin superimposition modes. According to analysis of several kinds of geological and geophysical data, the orogen was dominated by compressive folding and thrust napping from the Siberia plate in the north since the Late Paleozoic. Compressive stress weakened from north to south, corresponding to subdued vertical movement and enhanced horizontal movement of crustal surface from north to south, and finally faded in the overthrust-nappe belt at the northwest margin of the Junggar Basin. The variation in compressive stress is consistent with the surface relief of the orogen, which is high in the north and low in the south. There are two kinds of basin-mountain coupling relationships, i.e. high angle thrusting and overthrusting and napping, and two kinds of basin superimposition modes, i.e. inherited and progressive, and migrating and convulsionary modes. West Junggar orogen has rich oil and gas

  9. Reconciling competing models for the tectono-stratigraphic zonation of the Variscan orogen in Western Europe

    NASA Astrophysics Data System (ADS)

    Murphy, J. Brendan; Quesada, Cecilio; Gutiérrez-Alonso, Gabriel; Johnston, Stephen T.; Weil, Arlo

    2016-06-01

    The Late Paleozoic Variscan orogen in Europe is the result of convergence and collision between Laurussia and Gondwana during closure of the Rheic Ocean. The orogen is divided into tectonostratigraphic zones that have a distinct curvature (Ibero-Armorican Arc, IAA) and record the Late Cambrian-Early Ordovician opening of the Rheic Ocean, the migration of terranes from the Gondwanan margin towards Laurussia, as well as the closure of that ocean and development of the IAA. Three models have emerged to explain the distribution of tectonostratigraphic zones:

  10. Trans-Hudson Orogen of North America and Himalaya-Karakoram-Tibetan Orogen of Asia: Structural and thermal characteristics of the lower and upper plates

    NASA Astrophysics Data System (ADS)

    St-Onge, Marc R.; Searle, Michael P.; Wodicka, Natasha

    2006-08-01

    The Trans-Hudson Orogen (THO) of North America and the Himalaya-Karakoram-Tibetan Orogen (HKTO) of Asia preserve a Paleoproterozoic and Cenozoic record, respectively, of continent-continent collision that is notably similar in scale, duration and character. In THO, the tectonothermal evolution of the lower plate involves (1) early thin-skinned thrusting and Barrovian metamorphism, (2) out-of-sequence thrusting and high-T metamorphism, and (3) fluid-localized reequilibration, anatexis, and leucogranite formation. The crustal evolution of the Indian lower plate in HKTO involves (1) early subduction of continental crust to ultrahigh pressure (UHP) eclogite depths, (2) regional Barrovian metamorphism, and (3) widespread high-T metamorphism, anatexis, and leucogranite formation. The shallow depths of the high-T metamorphism in HKTO are consistent with early to mid-Miocene ductile flow of an Indian lower plate midcrustal channel, from beneath the southern Tibetan Plateau to the Greater Himalaya. Melt weakening of the lower plate in THO is not observed at a similar scale probably due to the paucity of pelitic lithologies. Tectonothermal events in the upper plate of both orogens include precollisional accretion of crustal blocks, emplacement of Andean-type plutonic suites, and high-T metamorphism. Syncollisional to postcollisional events include emplacement of garnet-biotite-muscovite leucogranites, anatectic granites, and sporadic metamorphism (up to 90 Myr following the onset of collision in THO). Comparing the type and duration of tectonothermal events for THO and HKTO supports the notion of tectonic uniformitarianism for at least the later half of dated Earth history and highlights the complementary nature of the rock record in an older "exhumed" orogen compared to one undergoing present-day orogenesis.

  11. How does the mid-crust accommodate deformation in large, hot collisional orogens? A review of recent research in the Himalayan orogen

    NASA Astrophysics Data System (ADS)

    Cottle, John M.; Larson, Kyle P.; Kellett, Dawn A.

    2015-09-01

    The presence of hot, weak crust is a central component of recent hypotheses that seek to explain the evolution of continent-continent collisions, and in particular may play an important role in accommodating the >3000 km of convergence within the Himalaya-Tibetan collision over the last ˜55 Myr. Models that implicate flow of semi-viscous midcrustal rocks south toward the front of the Himalayan orogen, 'channel flow', are able to account for many geologic observations in the Himalaya, while alternative models of collision, particularly 'thrust-wedge taper', demonstrate that much of the observed geology could have formed in the absence of a low-viscosity mid-crustal layer. Several recent studies, synthesized here, have prompted a shift from initial assumptions that channel flow and thrust-wedge taper processes are by definition mutually exclusive. These new studies reveal the presence of several tectonometamorphic discontinuities in the midcrust that appear to reflect a continuum of deformation in which both channel- and wedge-type processes operate in spatially and temporally distinct domains within the orogen, and further, that the system may migrate back and forth between these types of behavior. This continuum of deformation styles within the collisional system is of crucial importance for explaining the evolution of the Himalayan orogen and, hence, for understanding the evolution of Earth's many continent-continent collision zones.

  12. The eye of the field geologist and the mind of the tectonician: one view of dynamic crustal rheology in convergent orogens

    NASA Astrophysics Data System (ADS)

    Brown, M.

    2004-05-01

    -depleted rocks, implying that they were quasi-continuously drained. Studies of migmatitic granulites demonstrate that melt migrates from grain boundaries to mesoscale networks of structures (mm to m) to steeply-inclined conduits recorded by rod or tabular granite intrusions (m to dm). Melt loss from lower crust yields residual rocks composed of strong minerals (feldspar, pyroxene and garnet) with only minor melt on grain boundaries. Thus, weakening of lower crust due to melting is followed by its strengthening. Around the brittle-to-viscous transition zone granite accumulates in subhorizontal tabular plutons, which implies transient presence of significantly weaker layers in shallow orogenic crust; these are potential detachment horizons. Field studies of exhumed orogens suggest deformation commonly is laterally, transversely and vertically diachronous, reflecting the spatial and temporal variation in the weakening-to-strengthening cycle. There may be important sub-horizontal movement horizons, which allow (partial) decoupling of crustal layers. At upper-to-middle crustal levels rocks are metamorphosed in greenschist-amphibolite facies, with local enhancement by pluton-advected heat to amphibolite-granulite facies and thrust-style brittle-ductile deformation (e.g., Acadian, NH). Rocks from middle crustal levels are in amphibolite facies and have penetrative steep fabrics (e.g., Acadian, western ME) or exhibit a complex network of shallowly- and steeply-dipping fabrics (e.g., St. Malo, France). Rocks from lower crustal levels are in amphibolite-granulite facies and have shallow fabrics due to crustal flow, although these may be steepened by later deformation including core-complex formation (e.g., S. Brittany, France).

  13. An Early Cretaceous garnet pressure-temperature path recording synconvergent burial and exhumation from the hinterland of the Sevier orogenic belt, Albion Mountains, Idaho

    NASA Astrophysics Data System (ADS)

    Kelly, Eric D.; Hoisch, Thomas D.; Wells, Michael L.; Vervoort, Jeffrey D.; Beyene, Mengesha A.

    2015-08-01

    Rocks may undergo complex pressure-temperature ( P- T) histories during orogenesis in response to alternating episodes of synconvergent burial and exhumation. In this study, chemical zoning in garnets combined with textural and chemical evidence from the schist of Willow Creek in the Albion Mountains of south-central Idaho (USA), reveals a complex P- T path during the early stages of Sevier orogenesis. The distribution of quartz inclusions combined with internal resorption features establishes a hiatus in garnet growth. Chemical zoning was simulated using a G-minimization approach to yield a P- T path consisting of three distinct pressure changes during increasing temperature, defining an "N" shape. Lu-Hf isochron ages from multiple garnet fractions and whole-rock analyses in two samples are 132.1 ± 2.4 and 138.7 ± 3.5 Ma. The samples were collected from the hanging wall of the Basin-Elba thrust fault and yielded results similar to those previously obtained from the footwall. This leads to several conclusions: (1) Both the hanging wall and footwall experienced the same metamorphic event, (2) the paths document a previously unrecognized crustal thickening and synorogenic extension cycle that fills an important time gap in the shortening history of the Sevier retroarc, suggesting progressive eastward growth of the orogen rather than a two-stage history, and (3) episodes of extensional exhumation during protracted convergent orogenesis are increasingly well recognized and highlight the dynamic behavior of orogenic belts.

  14. Post-collisional magmatism in the central East African Orogen: The Maevarano Suite of north Madagascar

    USGS Publications Warehouse

    Goodenough, K.M.; Thomas, Ronald J.; De Waele, B.; Key, R.M.; Schofield, D.I.; Bauer, W.; Tucker, R.D.; Rafahatelo, J.-M.; Rabarimanana, M.; Ralison, A.V.; Randriamananjara, T.

    2010-01-01

    Late tectonic, post-collisional granite suites are a feature of many parts of the Late Neoproterozoic to Cambrian East African Orogen (EAO), where they are generally attributed to late extensional collapse of the orogen, accompanied by high heat flow and asthenospheric uprise. The Maevarano Suite comprises voluminous plutons which were emplaced in some of the tectonostratigraphic terranes of northern Madagascar, in the central part of the EAO, following collision and assembly during a major orogeny at ca. 550 Ma. The suite comprises three main magmatic phases: a minor early phase of foliated gabbros, quartz diorites, and granodiorites; a main phase of large batholiths of porphyritic granitoids and charnockites; and a late phase of small-scale plutons and sheets of monzonite, syenite, leucogranite and microgranite. The main phase intrusions tend to be massive, but with variably foliated margins. New U-Pb SHRIMP zircon data show that the whole suite was emplaced between ca. 537 and 522 Ma. Geochemically, all the rocks of the suite are enriched in the LILE, especially K, and the LREE, but are relatively depleted in Nb, Ta and the HREE. These characteristics are typical of post-collisional granitoids in the EAO and many other orogenic belts. It is proposed that the Maevarano Suite magmas were derived by melting of sub-continental lithospheric mantle that had been enriched in the LILE during earlier subduction events. The melting occurred during lithospheric delamination, which was associated with extensional collapse of the East African Orogen. ?? 2009 Natural Environment Research Council.

  15. Relating orogen width to shortening, erosion, and exhumation during Alpine collision

    NASA Astrophysics Data System (ADS)

    Rosenberg, C. L.; Berger, A.; Bellahsen, N.; Bousquet, R.

    2015-06-01

    We investigate along-strike width changes of the thickened, accreted lower plate (TALP) in the Central and in the Eastern Alps. We set the width of the TALP in relation to the inferred amount of collisional shortening and exhumation along six orogen-scale cross sections. Taking the present-day, along-strike gradients in the amount of collisional shortening to represent the temporal evolution of the collisional wedge, it may be concluded that the cross-sectional area of the TALP diminishes during ongoing shortening, indicating that the erosional flux outpaced the accretionary flux. Higher amounts of collisional shortening systematically coincide with smaller widths of the TALP and dramatic increases of the reconstructed eroded rock column. Higher amounts of shortening also coincide with larger amplitudes of orogen-scale, upright folds, with higher exhumation and with higher exhumation rates. Hence, erosion did play a major role in reducing by >30 km the vertical crustal thickness in order to accommodate and allow shortening by folding. Long-term climate differences cannot explain alternating changes of width by a factor of almost 2 along straight segments of the orogen on length scales less than 200 km, as observed from the western Central Alps to the easternmost Eastern Alps. Sedimentary or paleontological evidences supporting such paleo-climatic differences are lacking, suggesting that erosional processes did not directly control the width of the orogen.

  16. Crustal structure and orogenic material budget in the west central Pyrenees

    NASA Astrophysics Data System (ADS)

    Teixell, Antonio

    1998-06-01

    Surface and subsurface data are combined to construct a crustal-scale cross section of the western central Pyrenees (France and Spain) at the boundary between the European and Iberian plates. The position of Moho reflections in the ECORS-Arzacq reflection profile suggests a tectonic wedge of European crust and upper mantle had indented the Iberian plate at lower crustal levels. The European wedge is overlain by an upper, Iberian wedge thus constituting a double (stacked) wedge geometry. The upper wedge was delaminated and deformed giving rise to the Pyrenean orogenic prism, manifested as a bivergent fan in upper crustal levels. The underthrust lower Iberian plate has been imaged to depths of 55-60 km, but crustal budget considerations based on a palinspastic reconstruction require that this continental root subducted to depths up to 90 km. Total orogenic contraction calculated from surface structures is about 75-80 km, which was accomplished between the latest Cretaceous and the early Miocene at an averaged rate of 1.2 mm/yr. Consistent with these moderate values, exhumation of the orogen is much less than in the more shortened eastern parts of the range. Paleozoic basement and preorogenic Mesozoic rocks are little eroded, and much of the exhumation involved cannibalization of early foreland basins, which once covered the entire, poorly emergent orogen at this transect. This resulted in a continuous process of sediment recycling and, coupled with a considerable lateral arrival of material, a bulk negative erosion-sedimentation budget.

  17. Paleomagnetic data from the New England Orogen (eastern Australia) and implications for oroclinal bending

    NASA Astrophysics Data System (ADS)

    Shaanan, Uri; Rosenbaum, Gideon; Pisarevsky, Sergei; Speranza, Fabio

    2015-11-01

    Orogenic curvatures (oroclines) are common in modern and ancient orogens, but the geodynamic driving forces of many oroclines remain controversial. Here we focus on the New England oroclines of eastern Australia, the formation of which had been previously broadly constrained to the Early-Middle Permian. This time interval encompasses periods of both back-arc extension (at ~ 300-280 Ma) and subsequent contractional deformation (Hunter-Bowen Orogeny) that commenced at ~ 270 Ma along the paleo-Pacific and Gondwanan subduction plate boundary. We present new paleomagnetic data from volcanic rocks that were extruded during the transition from extension to contraction (at ~ 272 Ma), and we show that the oroclinal structure must have formed prior to the emplacement of the volcanic rocks. Our results thus indicate that oroclinal bending in the southernmost New England Orogen has been completed prior to the onset of Middle Permian contractional deformation. It is therefore concluded that the oroclines have likely formed during back-arc extension, and that a major contribution to the orogenic curvature was driven by trench retreat.

  18. Lithosphere structure of the west Qinling orogenic belt revealed by deep seismic reflection profile

    NASA Astrophysics Data System (ADS)

    Wang, H.

    2009-12-01

    The west Qinling orogen located in the northeastern margin of the Qinghai-Tibet plateau, is transformation zone between the N-S-trending and E-W-trending tectonics in the Chinese continent. Further study of the fine crust structure of the west Qinling orogen and its relationships with surrounding basins have very important significance for understanding tectonic response of the northeastern margin of the plateau about collision convergence of the Indian block and Asian block and learning formation and evolution of the plateau. In 2009, we reprocessed the data of the Tangke-Hezuo deep seismic reflection profiles collected in 2004 across the west Qinling orogen and the northern Songpan block. The new results show the lithosphere fine structure of the west Qinling orogen. Reflection features indicate that an interface at 6.0-7.0s (TWT) divided the crust into the upper and lower crust, whose structural style and deformation are totally different. Integrating geological data, we deduce that the interface at 6.0-7.0s (depth with 18-21 km) was the basement detachment, which made deformation decoupled of the upper and lower crust. The multi-layered reflections in the upper crust reveal the sedimentary covers of the west Qinling orogen, disclose the thickness of the various structure layer and deformation degree, and provide a basis for the prospective evaluation of a multi-metallic mineral and energy exploration. The north dipping strong reflection characteristics of the lower crust in the west Qinling orogen constituted imbricate structure, such imbricate structural features provide seismology evidence for researching the west Qinling thrusting toward the northern Songpan block, and have great significance for studying formation and evolution of the Songpan-Garze structure. Moho reflections are observed around 17.0-17.2s, characterized by nearly horizontal reflections, which implies the west Qinling orogen underwent an intense extension post orogeny caused the lithosphere

  19. Transient and Steady-State Kinematic Response to Erosional Forcing in an Orogenic Wedge: Sandbox Perspective

    NASA Astrophysics Data System (ADS)

    Cruz, L.; Teyssier, C.; Annia, F.; Take, A.

    2005-12-01

    The evolution of orogens is highly affected by surface processes that control mass distribution. Transportation and redistribution of mass at the Earth's surface modifies the gravitational load and alters the stress field and kinematics within orogens. We explore the role of asymmetric erosion, indenter dip angle, and flux steady/non-steady state in determining the patterns of deformation and exhumation in doubly-sided orogenic wedges. In our analogue model, shortening of the orogen is driven by rigid indenters, represented by Plexiglas wedged blocks (35 and 70 degrees) that deform a non-cohesive dry Coulomb material (walnut shells) representing crustal material. Three end-member erosional scenarios are considered. In the first case, erosion is not applied, and thus the doubly-sided orogenic wedge evolves without restraints (non-steady state). In the second case, erosion is concentrated solely on the indenters side of the orogen (retrowedge), and in the third case, erosion is focused on the flank opposite to the indenter side (prowedge). In the last two cases, steady-state conditions were present in the middle stages of shortening. Strain and exhumation were calculated using displacement fields from 2D particle image velocimetry (PIV analysis). In the three cases, the model deforms as a combination of lateral compaction and localization of strain in shear bands. In the early stages of deformation, a "pop-up" structure develops, bounded by a fore-shear on the front and a back-shear toward the indenter. As deformation continues, a new fore-shear develops, and the previous one remains inactive and is passively pushed up the wedge. In the case of no erosion, the old fore-shears rotate slightly toward the indenter, and the shear bands evolve to steeply dipping structures. In the case of retrowedge erosion, the old fore-shears back rotate toward the indenter, and the shear bands evolve to shallowly dipping structures. In the case of prowedge erosion, old fore

  20. Tectonic controls of Mississippi Valley-type lead-zinc mineralization in orogenic forelands

    USGS Publications Warehouse

    Bradley, D.C.; Leach, D.L.

    2003-01-01

    Most of the world's Mississippi Valley-type (MVT) zinc-lead deposits occur in orogenic forelands. We examine tectonic aspects of foreland evolution as part of a broader study of why some forelands are rich in MVT deposits, whereas others are barren. The type of orogenic foreland (collisional versus Andean-type versus inversion-type) is not a first-order control, because each has MVT deposits (e.g., Northern Arkansas, Pine Point, and Cevennes, respectively). In some MVT districts (e.g., Tri-State and Central Tennessee), mineralization took place atop an orogenic forebulge, a low-amplitude (a few hundred meters), long-wavelength (100-200 km) swell formed by vertical loading of the foreland plate. In the foreland of the active Banda Arc collision zone, a discontinuous forebulge reveals some of the physiographic and geologic complexities of the forebulge environment, and the importance of sea level in determining whether or not a forebulge will emerge and thus be subject to erosion. In addition to those on extant forebulges, some MVT deposits occur immediately below unconformities that originated at a forebulge, only to be subsequently carried toward the orogen by the plate-tectonic conveyor (e.g., Daniel's Harbour and East Tennessee). Likewise, some deposits are located along syn-collisional, flexure-induced normal and strike-slip faults in collisional forelands (e.g., Northern Arkansas, Daniel's Harbour, and Tri-State districts). These findings reveal the importance of lithospheric flexure, and suggest a conceptual tectonic model that accounts for an important subset of MVT deposits-those in the forelands of collisional orogens. The MVT deposits occur both in flat-lying and in thrust-faulted strata; in the latter group, mineralization postdated thrusting in some instances (e.g., Picos de Europa) but may have predated thrusting in other cases (e.g., East Tennessee).

  1. Possible polyphase metamorphic evolution of high grade metabasic rocks from the Songshugou ophiolite, Qinling orogen, China

    NASA Astrophysics Data System (ADS)

    Belic, Maximilian; Hauzenberger, Christoph; Dong, Yunpeng; Chen, Danling

    2014-05-01

    cycle or developed during a separate event. The financial support by Eurasia-Pacific Uninet is gratefully acknowledged. Dong, Y.P., Zhou, M.F., Zhang, G.W., Zhou, D.W., Liu, L., Zhang, Q., 2008. The Grenvillian Songshugou ophiolite in the Qinling Mountains, Central China: implications for the tectonic evolution of the Qinling orogenic belt. Journal of Asian Earth Science 32 (5-6), 325-335. Zhang, Z.J., 1999. Metamorphic evolution of garnet-clinopyroxene-amphibole rocks from the Proterozoic Songshugou mafic-ultramafic complex, Qinling Mountains, central China. The Island Arc, 8, 259-280.

  2. Thermochemical cycles

    NASA Technical Reports Server (NTRS)

    Funk, J. E.; Soliman, M. A.; Carty, R. H.; Conger, W. L.; Cox, K. E.; Lawson, D.

    1975-01-01

    The thermochemical production of hydrogen is described along with the HYDRGN computer program which attempts to rate the various thermochemical cycles. Specific thermochemical cycles discussed include: iron sulfur cycle; iron chloride cycle; and hybrid sulfuric acid cycle.

  3. Recycling of crustal materials through study of ultrahigh-pressure minerals in collisional orogens, ophiolites, and mantle xenoliths: A review

    NASA Astrophysics Data System (ADS)

    Liou, Juhn G.; Tsujimori, Tatsuki; Yang, Jingsui; Zhang, R. Y.; Ernst, W. G.

    2014-12-01

    Newly recognized occurrences of ultrahigh-pressure (UHP) minerals including diamonds in ultrahigh-temperature (UHT) felsic granulites of orogenic belts, in chromitites associated with ophiolitic complexes, and in mantle xenoliths suggest the recycling of crustal materials through deep subduction, mantle upwelling, and return to the Earth's surface. This circulation process is supported by crust-derived mineral inclusions in deep-seated zircons, chromites, and diamonds from collision-type orogens, from eclogitic xenoliths in kimberlites, and from chromitities of several Alpine-Himalayan and Polar Ural ophiolites; some of these minerals contain low-atomic number elements typified by crustal isotopic signatures. Ophiolite-type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone. In addition to ringwoodite and inferred stishovite, a number of nanometric minerals have been identified as inclusions employing state-of-the-art analytical tools. Reconstitution of now-exsolved precursor UHP phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier UHP conditions. For example, Tibetan chromites containing exsolution lamellae of coesite + diopside suggest that the original chromitites formed at P > 9-10 GPa at depths of >250-300 km. The precursor phase most likely had a Ca-ferrite or a Ca-titanite structure; both are polymorphs of chromite and (at 2000 °C) would have formed at minimum pressures of P > 12.5 or 20 GPa respectively. Some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin, including zircon, feldspars, garnet, kyanite, andalusite, quartz, and rutile; the zircons possess much older U-Pb ages than the time of ophiolite formation. These UHP mineral-bearing chromitite hosts evidently had a deep-seated evolution prior to extensional mantle upwelling and partial

  4. The deep structure of Alpine-type orogens: how important is rift-inheritance?

    NASA Astrophysics Data System (ADS)

    Tugend, Julie; Manatschal, Gianreto; Mohn, Geoffroy

    2016-04-01

    Collisional belts are commonly thought to result from the closure of oceanic basins and subsequent inversion of former rifted margins. The formation and evolution of collisional belts should therefore be closely interlinked with the initial architecture of former rifted margins. Reflection and refraction seismic data from present-day magma-poor rifted margins show the omnipresence of hyperextended domains (severely thinned continental crust (<10 km) and/or exhumed serpentinized mantle with minor magmatic additions) between unequivocal continental and oceanic domains. Integrating these new observations and exploring their impact on mountain building processes may result in alternative interpretations of the lithospheric structure of collisional orogens. We focus on the Pyrenees and Western to Central Alps, respectively resulting from the inversion of a Late Jurassic to Mid Cretaceous and an Early to Mid Jurassic rift system eventually floored by hyperextended crust, exhumed mantle or proto-oceanic crust. The rift-related pre-collisional architecture of the Pyrenees shows many similarities with that proposed for the Alps; although the width of the hyperextended and in particular of the proto-oceanic domains is little constrained. Contrasting with the Pyrenees, remnants of these domains are largely affected by orogeny-related deformation and show a HP-LT to HT-MP metamorphic overprint in the Alps. Nevertheless, in spite of the occurrence of these highly deformed and metamorphosed rocks constituting the internal parts of the Alps, the overall crustal and lithospheric structure looks surprisingly comparable. High resolution tomographic images across both orogens unravel the occurrence of a velocity anomaly dipping underneath the internal domains and progressively attenuated at depth that we interpret as former hyperextended domains subducted/underthrusted during collision. This interpretation contrasts with the classical assumption that the subducted material is made of

  5. Testing models of orogen exhumation using zircon (U-Th)/He thermochronology: Insight from the Ligurian Alps, Northern Italy

    NASA Astrophysics Data System (ADS)

    Maino, Matteo; Dallagiovanna, Giorgio; Dobson, Katherine J.; Gaggero, Laura; Persano, Cristina; Seno, Silvio; Stuart, Finlay M.

    2012-08-01

    Testing models of orogen exhumation requires precise constraint of the time-temperature paths of the exhumed rocks. The zircon (U-Th)/He (ZHe) thermochronometer has a closure temperature of ~ 140-210 °C, and can provide temporal constraints on the exhumation history of rocks through a thermal range which crucially corresponds the onset of brittle behaviour of an exhuming orogen. We performed ZHe analyses to test the existing contradictory models for the exhumation of the Ligurian Alps. The ZHe ages indicate a very rapid (1.3-6.8 mm/yr) and southward migrating exhumation of the orogen between ~ 32 and 25 Ma. These high exhumation rates are unique within the Alpine belt and cannot be reconciled with existing geodynamic models of the evolution of the Ligurian Alps. We propose a model of tectonic denudation via detachment accomplished in the shallowest crust (< 5 km) as a result of the last orogenic phase of extension.

  6. Linkages of Erosion, Tectonics, and Climate in a Glacial Setting: Lessons Learned in Alaska's St. Elias Orogen

    NASA Astrophysics Data System (ADS)

    Spotila, J. A.; Berger, A. L.

    2009-12-01

    A microplate collision in the southern Alaskan syntaxis provides useful analogies and lessons for the erosive-tectonic coupling that occur at Earth’s other great orogens. The St. Elias orogen is a relatively small collision produced by impingement of the Yakutat terrane in the transform-subduction crook of the northernmost Pacific-North America plate boundary. Due to convergence rates comparable to those in the Himalaya and a severe maritime glacial climate, deformation and rock uplift in this orogen rival Earth’s most extreme tectonic environments. Ongoing multidisciplinary investigations have documented several key phenomena that are relevant to orogenic processes worldwide. Previously obtained bedrock thermochronometry has revealed that glaciers exert an important control on tectonic crustal efflux. Long-term denudation is focused where mean Quaternary glacial ELA intersects the windward flank of the orogen. Quaternary enhancement of glaciation also appears to have dramatically accelerated denudation and forced a reorganization of orogenic wedge architecture. These results imply that glaciers are more significant to orogenic belts than implied by the topographic “buzz saw” hypothesis, in that they actually dictate long-term patterns of strain and mass transfer at the orogen-scale. New data refine the spatial and temporal patterns of exhumation in the orogen and enable more robust assessment of flux steady-state, or how exhumation has kept pace with tectonic crustal influx. Results suggest that, despite the presence of such an efficient erosive system, a component of tectonic influx is likely accommodated by subduction or lateral advection via intraplate transform faults. We suggest that a continuous dextral fault connects the mapped Totschunda-Denali and Fairweather faults, enabling a significant component of plate convergence to bypass the orogen. Finally, the refined pattern of exhumation sheds light onto the possible existence of tectonic

  7. Axial Belt Provenance: modern river sands from the core of collision orogens

    NASA Astrophysics Data System (ADS)

    Resentini, A.; Vezzoli, G.; Paparella, P.; Padoan, M.; Andò, S.; Malusà, M.; Garzanti, E.

    2009-04-01

    Collision orogens have a complex structure, including diverse rock units assembled in various ways by geodynamic processes. Consequently, orogenic detritus embraces a varied range of signatures, and unravelling provenance of clastic wedges accumulated in adjacent foreland basins, foredeeps, or remnant-ocean basins is an arduous task. Dickinson and Suczek (1979) and Dickinson (1985) recognized the intrinsically composite nature of orogenic detritus, but did not attempt to establish clear conceptual and operational distinctions within their broad "Recycled Orogenic Provenance". In the Alpine and Himalayan belts, the bulk of the detritus is produced by focused erosion of the central backbone of the orogen, characterized by high topography and exhumation rates (Garzanti et al., 2004; Najman, 2006). Detritus derived from such axial nappe pile, including slivers of thinned continental-margin lithosphere metamorphosed at depth during early collisional stages, has diagnostic general features, which allows us to define an "Axial Belt Provenance" (Garzanti et al., 2007). In detail, "Axial Belt" detrital signatures are influenced by metamorphic grade of source rocks and relative abundance of continental versus oceanic protoliths, typifying distinct subprovenances. Metasedimentary cover nappes shed lithic to quartzolithic detritus, including metapelite, metapsammite, and metacarbonate grains of various ranks; only amphibolite-facies metasediments supply abundant heavy minerals (e.g., almandine garnet, staurolite, kyanite, sillimanite, diopsidic clinopyroxene). Continental-basement nappes shed hornblende-rich quartzofeldspathic detritus. Largely retrogressed blueschist to eclogite-facies metaophiolites supply albite, metabasite and foliated antigorite-serpentinite grains, along with abundant heavy minerals (epidote, zoisite, clinozoisite, lawsonite, actinolitic to barroisitic amphiboles, glaucophane, omphacitic clinopyroxene). Increasing metamorphic grade and deeper

  8. Association of orogenic activity with the Ordovician radiation of marine life

    NASA Technical Reports Server (NTRS)

    Miller, A. I.; Mao, S.

    1995-01-01

    The Ordovician radiation of marine life was among the most substantial pulses of diversification in Earth history and coincided in time with a major increase in the global level of orogenic activity. To investigate a possible causal link between these two patterns, the geographic distributions of 6576 individual appearances of Ordovician vician genera around the world were evaluated with respect to their proximity to probable centers of orogeny (foreland basins). Results indicate that these genera, which belonged to an array of higher taxa that diversified in the Middle and Late Ordovician (trilobites, brachiopods, bivalves, gastropods, monoplacophorans), were far more diverse in, and adjacent to, foreland basins than they were in areas farther removed from orogenic activity (carbonate platforms). This suggests an association of orogeny with diversification at that time.

  9. Basin-mountain structures and hydrocarbon exploration potential of west Junggar orogen in China

    NASA Astrophysics Data System (ADS)

    Wu, Xiaozhi; He, Dengfa; Qi, Xuefeng

    2016-04-01

    Situated in northern Xinjiang, China, in NE-SW trend, West Junggar Orogen is adjacent to Altai fold belt on the north with the Ertix Fault as the boundary, North Tianshan fold belt on the south with the Ebinur Lake Strike-slip Fault as the boundary, and the Junggar Basin on the southeast with Zaire-Genghis Khan-Hala'alat fold belt as the boundary. Covering an area of about 10×104 km2 in China, there are medium and small intermontane basins, Burqin-Fuhai, Tacheng, Hefeng and Hoxtolgay, distributing inside the orogen. Tectonically West Junggar Orogen lies in the middle section of the Palaeo-Asian tectonic domain where the Siberia, Kazakhstan and Tarim Plates converge, and is the only orogen trending NE-SW in the Palaeo-Asian tectonic domain. Since the Paleozoic, the orogen experienced pre-Permian plate tectonic evolution and post-Permian intra-plate basin evolution. Complex tectonic evolution and multi-stage structural superimposition not only give rise to long term controversial over the basin basement property but also complex basin-mountain coupling relations, structures and basin superimposition modes. According to analysis of several kinds of geological and geophysical data, the orogen was dominated by compressive folding and thrust napping from the Siberia plate in the north since the Late Paleozoic. Compressive stress weakened from north to south, corresponding to subdued vertical movement and enhanced horizontal movement of crustal surface from north to south, and finally faded in the overthrust-nappe belt at the northwest margin of the Junggar Basin. The variation in compressive stress is consistent with the surface relief of the orogen, which is high in the north and low in the south. There are two kinds of basin-mountain coupling relationships, i.e. high angle thrusting and overthrusting and napping, and two kinds of basin superimposition modes, i.e. inherited and progressive, and migrating and convulsionary modes. West Junggar orogen has rich oil and gas

  10. Early proterozoic evolution of the saskatchewan craton and its allochthonous coyer, trans-hudson Orogen

    USGS Publications Warehouse

    Chiarenzelli, J.; Aspler, L.; Villeneuve, M.; Lewry, J.

    1998-01-01

    The composition, chronology, and structural relations of the Saskatchewan Craton and enveloping mylonitic rocks exposed in basement windows of the Glennie Domain, Trans-Hudson Orogen, have been determined by geochemical, geochronologic, and structural studies accompanying detailed field mapping. Basement windows lie along the hinge zone of a regional crustal culmination and consist mostly of 2.4-2.5 Ga felsic plutonic rocks enveloped by the Nistowiak Thrust. The Nistowiak Thrust is a folded, 1-2 km thick, upper amphibolite facie??s mylonite zone formed during emplacement of the Flin Flon-Glennie Complex across the Saskatchewan Craton. It is likely correlative to the Pelican Thrust, which envelops basement windows in the Hanson Lake Block -100 km to the east. An internal high strain zone within the overlying nappe pile, the Guncoat Thrust, is composed primarily of mylonitized porphyroclastic pelitic and psammitic migmatites. U-Pb geochronological results suggest calc-alkaline plutonism from 1889-1837 Ma, thrust stacking, peak metamorphism and associated anatexis between 1837 and 1809 Ma, isotopic closure of titanite at 1790-1772 Ma, and intrusion of late granitic rocks at 1770-1762 Ma. This is in agreement with ages from the Hanson Lake Block, and La Ronge, Kisseynew, and Flin-Flon domains in Saskatchewan and Manitoba, and from the Ungava-Baffin portion of Trans-Hudson Orogen, suggesting broadly synchronous thermotectonic processes along a strike length of 2000 km. We speculate that the Saskatchewan Craton, rather than representing an exotic continental fragment, rifted from the Superior and/or Hearne Provinces at ca. 2.1 Ga and that the Trans-Hudson Orogen is an internal orogen. In this scenario the Maniwekan Ocean, developed between the Rae-Hearne and Superior cratons, opened and closed about similar pole(s) of plate motion. ?? 1998 by The University of Chicago. All rights reserved.

  11. The structure, stratigraphy, tectonostratigraphy, and evolution of the southernmost part of the Appalachian Orogen

    SciTech Connect

    Higgins, M.W.; Atkins, R.L.; Crawford, T.J.; Crawford, R.F. III; Brooks, R.; Cook, R.B.

    1988-01-01

    The southernmost part of the Appalachian Orogen is composed of three stacks of folded thrust sheets, which preserve rocks formed in a variety of environments that virtually spanned the Iapetus Ocean. All of the metamorphism, deformation, and plutonism in the southernmost Appalachians can be related to the movement of the thrust sheets and stacks. Thrusting took place continuously from Early Ordovician through Carboniferous time. An account is given of the origins, assembly, transport, and arrival of the thrust sheets and stacks.

  12. Central Appalachian Exotic Terranes and Exposures of Former Orogenic Middle Crust

    NASA Astrophysics Data System (ADS)

    Martin, A. J.

    2015-12-01

    In the northern and southern Appalachians, rocks that formed Paleozoic orogenic middle crust mostly are exposed within or directly inboard of terranes that originated near Gondwana. Most outcrops of the Paleozoic orogenic middle crust of the eastern edge of Laurentia occur adjacent to these exotic terranes. However, a narrow belt of Paleozoic orogenic middle crust is exposed in the Piedmont of the central Appalachians despite the absence of recognized exotic terranes. The presence of these deformed, amphibolite facies rocks raises the questions: "Did central Appalachian orogeny occur in the absence of accreted exotic terranes?" and, more generally, "Is exotic terrane collision required for exhumation of Appalachian former middle crust?" Previous U/Pb isotopic dating of spots in detrital zircon revealed the presence of Gondwanan terranes in three locations in the central Appalachians: central Virginia, central Maryland, and southeastern Pennsylvania. Two new samples collected near the discovery locations in Virginia and Maryland yielded prominent peaks in zircon U/Pb age distributions at ca. 630-610 Ma, confirming the Gondwanan affinity of these rocks. Hf isotopic compositions of spots in these upper Neoproterozoic zircon grains range to both more and less depleted than spots in zircon from the few possible Laurentian granitic sources, consistent with derivation of the zircon from Gondwana. Abundant 1700-1000 Ma detrital zircon rules out the West Africa Craton as a potential source; Amazonia is the most likely ultimate source of the zircon. The extent of the exotic terrane(s) in the central Appalachian Piedmont remains enigmatic due to uncertain connections between isolated exposures of the terrane(s). Nevertheless, the discovery of one or more exotic terranes in the central Appalachian Piedmont underscores the relationship between exotic terranes and exposed former middle crust in the Appalachians. This relationship may be a feature of several other major

  13. The role of lateral lithospheric strength heterogeneities in orogenic plateau growth: Insights from 3-D thermo-mechanical modeling

    NASA Astrophysics Data System (ADS)

    Chen, Lin; Gerya, Taras V.

    2016-04-01

    Preexisting lateral variations in crustal thickness and lithospheric thermal state are documented for the formation of some orogenic plateaux. Here we use high-resolution 3-D thermo-mechanical simulations to investigate the influence of preexisting lateral lithospheric strength heterogeneity on the growth of orogenic plateau. The modeling results illustrate an episodic scenario for plateau growth: (1) an early rapid growth stage, characterized by rapid surface uplift and intensive crustal buckling and thickening; (2) an outward spreading stage, characterized by significant lateral expansion of the plateau edges; and (3) a mature stage, characterized by the development of the intracrustal partial melting and subduction of the surrounding lithosphere under the plateau. Sensitivity analyses indicate that lateral variation in crustal thickness favors outward spreading of orogenic plateau, while lateral variation in geothermal gradient favors crustal buckling. The model in absence of lateral strength heterogeneity leads to progressive migration of orogenic belt. Our models show that the plateau's lower crust is largely coupled with underlying lithospheric mantle and does not flow into the surrounding lithospheres, casting doubt on the lower crust flow model. We suggest that the Himalayan-Tibetan orogenic system can be best understood within the framework that the proto-southern Asian margin was fairly weak prior to the India-Asia collision to steer the formation of a large hot orogenic plateau there.

  14. Formation of Secondary Lherzolite and Refertilization of the Subcontinental Lithospheric Mantle: The Record of Orogenic Peridotites

    NASA Astrophysics Data System (ADS)

    Garrido, Carlos J.; Varas-Reus, María Isabel; Bodinier, Jean-Louis; Marchesi, Claudio; Bosch, Delphine; Hidas, Károly

    2016-04-01

    Correlations observed between major and minor transition elements in tectonically-emplaced orogenic peridotites have classically been ascribed to variable degrees of melt extraction. There is a growing body of evidence indicating that these chemical variations mostly reflect melt redistribution and near solidus reactions superimposed onto previous melting depletion events. Here we will assess this hypothesis using a large database of peridotites from orogenic peridotites in the westernmost Mediterranean (Ronda and Beni Bousera peridotites). We show that lherzolite samples show some trends in major elements and modal variations that are inconsistent with their interpretation as depleted MORB mantle (DMM). These trends are more consistent with the secondary formation of lherzolites by refertilization processes involving a least two different near-solidus, melt-processes: refertilization by pyroxenite-derived melts and by hydrous melts leading, respectively, to secondary lherzolites with Ol/Opx and Cpx/Opx ratios greater than those expected from residues from a primitive upper mantle source. Together with their N-MORB, LREE-depleted pattern, their fertile lherzolitic composition may have been acquired as a result of melt-rock interaction processes associated with the thermomechanical erosion of lithospheric mantle by asthenosphere. Major refertilization of depleted subcontinental mantle is an alternative to the small degrees of melt extraction to account for LREE depletion in otherwise fertile orogenic lherzolites.

  15. Window into the Caledonian orogen: Structure of the crust beneath the East Shetland platform, United Kingdom

    USGS Publications Warehouse

    McBride, J.H.; England, R.W.

    1999-01-01

    Reprocessing and interpretation of commercial and deep seismic reflection data across the East Shetland platform and its North Sea margin provide a new view of crustal subbasement structure beneath a poorly known region of the British Caledonian orogen. The East Shetland platform, east of the Great Glen strike-slip fault system, is one of the few areas of the offshore British Caledonides that remained relatively insulated from the Mesozoic and later rifting that involved much of the area around the British Isles, thus providing an "acoustic window" into the deep structure of the orogen. Interpretation of the reflection data suggests that the crust beneath the platform retains a significant amount of its original Caledonian and older architecture. The upper to middle crust is typically poorly reflective except for individual prominent dipping reflectors with complex orientations that decrease in dip with depth and merge with a lower crustal layer of high reflectivity. The three-dimensional structural orientation of the reflectors beneath the East Shetland platform is at variance with Caledonian reflector trends observed elsewhere in the Caledonian orogen (e.g., north of the Scottish mainland), emphasizing the unique tectonic character of this part of the orogen. Upper to middle crustal reflectors are interpreted as Caledonian or older thrust surfaces that were possibly reactivated by Devonian extension associated with post-Caledonian orogenic collapse. The appearance of two levels of uneven and diffractive (i.e., corrugated) reflectivity in the lower crust, best developed on east-west-oriented profiles, is characteristic of the East Shetland platform. However, a north-south-oriented profile reveals an interpreted south-vergent folded and imbricated thrust structure in the lower crust that appears to be tied to the two levels of corrugated reflectivity on the east-west profiles. A thrust-belt origin for lower crustal reflectivity would explain its corrugated

  16. The tectonic frame of the Variscan Alleghanian orogen in Southern Europe and Northern Africa

    NASA Astrophysics Data System (ADS)

    Simancas, J. Fernando; Tahiri, Abdelfatah; Azor, Antonio; Lodeiro, Francisco González; Martínez Poyatos, David J.; El Hadi, Hassan

    2005-04-01

    By confronting different geological and geophysical data, we attempt to reconstruct the Variscan-Alleghanian orogenic belt, with especial emphasis on the links between Iberia, northwest Africa, and northeast America. The northern Iberia transect corresponds to the rifted margin of Gondwana, inverted during the Variscan orogeny and overthrust in its westernmost sector by a pile of allochthonous units, some of them with oceanic affinity and witnessing an orogenic suture. The southern Iberia section reveals two sutures at both boundaries of a terrane closely tied to Gondwana, namely the Ossa-Morena Zone. The southern boundary of this zone (i.e., the contact with the South Portuguese Zone) is enhanced by amphibolites with oceanic affinity and probably represents the suture of the Rheic Ocean. The Moroccan Variscides can be divided into: (i) a western external zone, namely the Coastal Block and the Central Massif; (ii) an internal zone, namely the Eastern Meseta; (iii) an eastern external zone represented in the Anti Atlas region; and (iv) the African cratonic foreland. Since neither ophiolites nor eclogites crop out along this transect of the orogen, we consider it to be an incomplete transect, lacking the main suture zone (Rheic Ocean). Stratigraphic and faunal affinities between the Moroccan Meseta, on the one hand, and the Central Iberian, Western Asturian-Leonese, and Cantabrian zones of Iberia, on the other hand, suggest that they may well have been part of a common pre-orogenic domain at the margin of Gondwana. On the contrary, there are no counterparts in Morocco for the Ossa-Morena Zone. Thus, the northern Iberia palaeogeographic zones would prolong in the Moroccan Meseta through an eastward arcuate structure, while the suture of the Rheic Ocean would do so offshore Morocco. In our reconstruction, an Avalonian spur (Grand Banks indentor), which included the Caledonian Sehoul block in northern Morocco and the crust of the South Portuguese Zone in SW Iberia

  17. Unravelling the impact of inheritance within the Wilson Cycle: a combined mapping and numerical modelling approach

    NASA Astrophysics Data System (ADS)

    Chenin, Pauline; Manatschal, Gianreto; Lavier, Luc

    2015-04-01

    Our study aims to unravel how structural, lithological and thermal heterogeneities may influence both orogenic and rift systems within the Wilson Cycle. To do this, we map first-order rift structural domains, timing of the main rift events as well as major heterogeneities and structures inherited from previous orogenies. Besides, we design numerical modelling experiments to investigate the relationships highlighted from the comparison of these maps. We apply this approach to the North Atlantic region, which underwent two major orogenic phases during the Palaeozoic: (1) the Caledonian orogeny - now extending from United-Kingdom to northern Norway and Eastern Greenland - resulted from the Late Ordovician closure of the large Iapetus ocean (> 2 000 km) and smaller Tornquist Seaway. It was followed by purely mechanical extensional orogenic collapse; (2) the Variscides of Southwestern Europe were essentially built from the Devono-Carboniferous suturing of several small oceanic basins (< 200 km) in addition to the large Rheic Ocean. The subsequent orogenic collapse was accompanied by significant magmatic activity, which resulted in mafic underplating and associated mantle depletion over the whole orogenic area. Our study is twofolds: On the one hand, we investigate how the size and maturity of the intervening oceanic basins affect subduction and orogeny, considering two end-members: (a) immature oceanic basins defined as hyperextended rift systems that never achieved steady state seafloor spreading; and (b) mature oceans characterized by a self-sustained magmatic system forming homogeneous oceanic crust. On the other hand, we study how post-orogenic collapse-related underplating and associated mantle depletion may impact subsequent rifting depending on the thermal state (e.g. the duration of relaxation time between the magmatic episode and the onset of rifting). Our results highlight a very different behaviour of the North Atlantic rift with respect to the Caledonian and

  18. Late Paleozoic closure of the Ob-Zaisan Ocean along the Irtysh/Chara shear zone and implications for arc amalgamation and oroclinal bending in the western Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Li, Pengfei; Sun, Min; Rosenbaum, Gideon

    2016-04-01

    The Irtysh/Chara Shear Zone is one of the largest strike-slip systems in the Central Asian Orogenic Belt (CAOB). It records collisional processes of the peri-Siberian orogenic system with the West Junggar-Kazakhstan-Tianshan orogenic system following the closure of the Ob-Zaisan Ocean, but the exact timing of these events remains enigmatic. We conducted detailed structural analysis along the Irtysh Shear Zone (NW China), which together with new geochronological data allows us to reconstruct the tectonic evolution during the final closure of the Ob-Zaisan Ocean. Our results showed that subduction-accretion processes lasted at least until the Late Carboniferous in the Chinese Altai and the East/West Junggar. The subsequent arc amalgamation is characterized by a cycle of crustal thickening, orogenic collapse and transpressional thickening. On a larger scale, the West Junggar- Kazakhstan -Tianshan orogenic system defines a U-shape oroclinal structure (e.g. Xiao et al., 2010). A major phase of oroclinal bending that involved ~110° rotation may have occurred during the Late Devonian to Early Carboniferous (Levashova et al., 2012). Previous authors have linked oroclinal bending with the late Paleozoic amalgamation of the western CAOB, and proposed that a quasi-linear West Junggar- Kazakhstan -Tianshan orogenic system was buckled during the convergence of the Siberian and Tarim cratons following the closure of the Ob-Zaisan Ocean (in the north) and the South Tianshan Ocean (in the south) (e.g. Abrajevitch et al., 2008). This model, however, is not supported by our new data that constrain the closure of the Ob-Zaisan Ocean to the Late Carboniferous. Alternatively, we propose that oroclinal bending may have involved two phases of bending, with the ~110° rotation in the Late Devonian to Early Carboniferous possibly associated with trench retreat. Further tightening may have occurred in response to the convergence of the Siberian and Tarim cratons during the Late

  19. Rift inheritance in orogenes: a case study from the Western Pyrenees

    NASA Astrophysics Data System (ADS)

    Masini, E.; Manatschal, G.; Tugend, J.; Kusznir, N. J.; Flament, J.

    2012-12-01

    In plate tectonics, there is a general assumption that rifted margins represent most of the former material accreted into collisional orogenic prisms. In this regard, the former architecture, structures and composition of rifted margins, i.e. the pre-orogenic inheritances, play undoubtedly a major role during tectonic inversion. Studies have shown that rifted margins are more complex than a succession of tilted blocks. Indeed, the discovery of hyper-extended domains, where low-angle detachments replace high-angle normal faults and mantle material is exhumed to the seafloor implies a revision of the margin's template used in orogenic models. Because of overprint, the role of rift inheritance in orogenes remains often underestimated. The Pyrenees, located along the Iberian-European plate boundary, can be considered as one of the best places to study the reactivation of hyper-extended rifts. In this orogen, the Late Cretaceous and Tertiary convergence overprints a Latest Jurassic to Lower Cretaceous intracontinental rift linked with the opening of the North Atlantic. There, Albian hyper-extended rift basins developed where deep crustal and mantle rocks were exhumed along low-angle detachments to the seafloor. In this work we discuss the example of the Mauléon-Arzacq domain, which escaped from the most pervasive deformation because of its specific location between the western termination of the chain and the Bay of Biscay oceanic domain. Combining field study with subsurface geophysical and drillhole data, we show that the overall rift domain is asymmetric. The northern European upper plate is on the hangingwall of low-angle detachment systems affecting the southern Iberian Lower plate. The upper plate records depth-dependent crustal thinning and the development of a syn-rift sag basin. In contrast, the lower plate resulted from the hyper-extension of Iberian continental crust accommodated at the surface by two diachronous top-basement detachment systems. The first

  20. Structural controls on Tertiary orogenic gold mineralization during initiation of a mountain belt, New Zealand

    NASA Astrophysics Data System (ADS)

    Craw, D.; Begbie, M.; MacKenzie, D.

    2006-10-01

    Two types of structurally controlled hydrothermal mineralization have occurred during folding of fissile schist in southern New Zealand: fold-related mineralization and normal fault-related mineralization. Both types have the same mineralogy and textures, and are dominated by quartz-ankerite veins and silicified breccias with ankeritic alteration. Most mineralized zones are thin (centimetre scale), although host schist is commonly impregnated with ankerite up to 20 m away. Thick (up to 5 m wide) mineralized zones are generally gold-bearing and contain pyrite and arsenopyrite with stibnite pods locally. Some of these auriferous zones have been extensively mined historically despite rugged topography and difficult access. Mineralization occurred during regional tectonic compression in the initial stages of development of the Southern Alps mountain belt at the Pacific-Australian plate boundary in the Miocene. Most of the gold-bearing deposits occur in east to south-east, striking normal faults that cut across mesoscopic folds in a belt that coincides with the southern termination of a regional-scale north trending antiform. Mineralized zones have similar structural control and relative timing to a nearby swarm of Miocene lamprophyre dykes and carbonatites. Limited stable isotopic data (C and O) and trace element geochemistry suggest that there was probably no genetic link between the igneous activity and gold mineralization. However, these two types of fluid flow have been controlled by the same tectonically created crustal plumbing system. This Miocene hydrothermal activity and gold deposition demonstrates that orogenic (mesothermal) mineralization can occur during the inception of an orogenic belt, not just in the latter stages as is commonly believed. These Miocene structures have been preserved in the orogen because the locus of uplift has moved northwards, so the early-formed gold deposits have not yet been structurally overprinted or eroded.

  1. Chemical remagnetization and paleomagnetic dating of fluid migration events: Testing the orogenic fluid hypothesis

    SciTech Connect

    Douglas, E.R. )

    1993-11-01

    Remagnetization, the acquisition of a secondary magnetization, is now recognized as a widespread phenomenon in sedimentary rocks. The recognition that many sedimentary rocks were remagnetized long after deposition has not only led to a reevaluation of the paleomagnetic database, but also has led to a new application of the paleomagnetic method to other areas of earth science. Many secondary magnetizations are tangible evidence of a chemical event caused by rock-fluid interactions. Recent studies demonstrate that isolation of a chemical remanent magnetization (CRM) and comparison of the corresponding pole position to the apparent polar wander path can provide essential constraints on the timing of a diagenetic event. Many CRMs can be spatially correlated with orogenic belts and temporally related to orogeny. In this talk several case studies will be presented where paleomagnetic and geochemical results are used to date diagenetic events and test the role of orogenic fluids as agents of remagnetization. The Ordovician Viola Limestone contains a pervasive Pennsylvanian CRM and a localized Permian CRM that occurs in a halo around veins mineralized by saline radiogenic fluids. The Permian CRM can be related to alteration by the basinal fluids. The pervasive CRM, which is similar to many other CRMs that have been related to orogenic fluids, occurs in relatively unaltered limestone. The acquisition of this CRM was caused by an as yet unidentified chemical mechanism that was not triggered by externally-derived fluids. Preliminary results, however, suggest a connection between the CRM and organic matter in the limestone. Radiogenic Cambrian-Ordovician dolomites near the Ouachita Mountains in southern Oklahoma that have been altered by basinal fluids contain a late Paleozoic CRM. Understanding the origin of these CRMs will require tests of mechanisms that do not rely on externally-derived fluids.

  2. Porphyry molybdenum deposits in the Tianshan-Xingmeng orogenic belt, northern China

    NASA Astrophysics Data System (ADS)

    Zeng, Qingdong; Qin, Kezhang; Liu, Jianming; Li, Guangming; Zhai, Mingguo; Chu, Shaoxiong; Guo, Yunpeng

    2015-06-01

    Molybdenum (Mo) exploration activity in China has increased tremendously over the past decade, and China is now known to have the largest Mo reserves in the world. The Tianshan-Xingmeng orogenic belt, the second largest Mo metallogenic belt, possesses over 8.2 Mt of Mo reserves. Porphyry Mo deposits contain 99 % of the Mo reserves in the Tianshan-Xingmeng orogenic belt; other Mo deposits contain 1 % of the Mo reserves. Five subtypes of the porphyry Mo deposits can be distinguished by deposit associations, such as Mo, Mo-Cu, Mo-W, Mo-Pb-Zn-Ag, and Cu-Mo deposits. These porphyry Mo deposits are formed at different stages: during the Ordovician, Devonian, Carboniferous, Late Permian, Triassic, Jurassic, and Cretaceous Periods. The polystage porphyry Mo mineralizations indicate that polystage tectonic-magmatic activity occurred in the orogenic belt. The Ordovician-Carboniferous porphyry Cu-Mo deposits are formed in an island-arc setting; the Late Permian porphyry Mo deposits are formed in a syn-collisional tectonic setting; and the Triassic porphyry Mo deposits are formed in a syn-collisional to post-collisional tectonic setting. The Ordovician-Triassic porphyry deposits are related to the Paleo-Asian Ocean tectonic system. The Jurassic porphyry Mo deposits are formed at the eastern margin of the Asian continent and are associated with a Paleo-Pacific plate-subduction tectonic setting. Cretaceous porphyry Mo deposits are formed in a lithospheric thinning setting and are related to the rollback of the Paleo-Pacific subduction plate.

  3. Noble gases fingerprint a metasedimentary fluid source in the Macraes orogenic gold deposit, New Zealand

    NASA Astrophysics Data System (ADS)

    Goodwin, Nicholas R. J.; Burgess, Ray; Craw, Dave; Teagle, Damon A. H.; Ballentine, Chris J.

    2016-04-01

    The world-class Macraes orogenic gold deposit (˜10 Moz resource) formed during the late metamorphic uplift of a metasedimentary schist belt in southern New Zealand. Mineralising fluids, metals and metalloids were derived from within the metasedimentary host. Helium and argon extracted from fluid inclusions in sulphide mineral grains (three crush extractions from one sample) have crustal signatures, with no evidence for mantle input (R/Ra = 0.03). Xenon extracted from mineralised quartz samples provides evidence for extensive interaction between fluid and maturing organic material within the metasedimentary host rocks, with 132Xe/36Ar ratios up to 200 times greater than air. Similarly, I/Cl ratios for fluids extracted from mineralised quartz are similar to those of brines from marine sediments that have interacted with organic matter and are ten times higher than typical magmatic/mantle fluids. The Macraes mineralising fluids were compositionally variable, reflecting either mixing of two different crustal fluids in the metasedimentary pile or a single fluid type that has had varying degrees of interaction with the host metasediments. Evidence for additional input of meteoric water is equivocal, but minor meteoric incursion cannot be discounted. The Macraes deposit formed in a metasedimentary belt without associated coeval magmatism, and therefore represents a purely crustal metamorphogenic end member in a spectrum of orogenic hydrothermal processes that can include magmatic and/or mantle fluid input elsewhere in the world. There is no evidence for involvement of minor intercalated metabasic rocks in the Macraes mineralising system. Hydrothermal fluids that formed other, smaller, orogenic deposits in the same metamorphic belt have less pronounced noble gas and halogen evidence for crustal fluid-rock interaction than at Macraes, but these deposits also formed from broadly similar metamorphogenic processes.

  4. Erosion and Sediment Transport Across Pronounced Topographic and Climatic Gradients in the Himalayan Orogen

    NASA Astrophysics Data System (ADS)

    Strecker, M. R.; Bookhagen, B.

    2014-12-01

    The interaction between the NW-directed trajectories of moisture transport associated with the Indian Summer Monsoon circulation and the high topography of the Himalayan orogen results in one of the most efficient orographic barriers on Earth. The steep topographic gradients, the impact of focused rainfall along the southern flank of the range, and northward shifts of rainfall during frequent intensified Indian Summer Monsoons are responsible for an efficient erosional regime, with some of the highest known erosion rates. The spatiotemporal correlation between various topographic, tectonic, climatic, and exhumational phenomena in this region has resulted in the formulation of models of possible long-term erosional and tectonic feedback processes that drive the lateral expansion and vertical growth of the mountain belt. However, despite an increase in thermochronologic, cosmogenic radionuclide, and sedimentological datasets that help explain the underlying mechanisms, the true nature of these relationships is still unclear and controversies particularly exist concerning the importance of the different forcing factors that drive exhumation and, ultimately, deformation. Here, we synthesize and assess these controversies with observations from studies conducted perpendicular to and along strike of the orogen, and combine them with our new basin-wide erosion-rate data from the Sutlej Valley in the NW Himalaya. In our regional comparison we highlight the importance of large river systems and climate-controlled aspects of weathering concerning fluvial mass distribution as there appear to be positive feedbacks between tectonics and surface processes. In contrast, observations from smaller catchments along the orogenic front suggest a negative correlation. Similar to other environments with steep topographic and climatic gradients, our observations from the Sutlej catchments emphasize that erosional processes in the Himalayan realm are most efficient in geomorphic

  5. Linked basin sedimentation and orogenic uplift: The Neogene Barinas basin sediments derived from the Venezuelan Andes

    NASA Astrophysics Data System (ADS)

    Erikson, Johan P.; Kelley, Shari A.; Osmolovsky, Peter; Verosub, Kenneth L.

    2012-11-01

    The Venezuelan Andes are an asymmetric, doubly vergent orogen that is flanked on its southeastern side by the Barinas basin. Analyses of sedimentary facies, sandstone petrography, apatite fission-tracks, and magnetostratigraphy were completed on a 1750-m section of the syn-orogenic Neogene Parángula and Río Yuca formations in the Barinas side foothills of the Venezuelan Andes. Our sedimentary facies analyses record a progression of sedimentary environments from floodplain and floodplain channel deposits through the 560-m thick Parángula Formation transitioning to distal alluvial fan deposits in the lower Río Yuca Formation and finally to an alternation of distal alluvial fan and two, ˜100-m thick organic-rich lacustrine deposits in the upper third of the section. Major- and minor-mineral petrographic analysis reveals unroofing of the Venezuelan Andes, with quartz arenite composition low in the section succeeded by metamorphic and igneous clasts and potassium feldspar appearing near the base of the Río Yuca Formation. Apatite fission-track (AFT) analysis of sandstones and pebbles generated ages of 11.2 ± 1.3 - 13.8 ± 2.0 Ma over ˜1100 m of stratigraphic section. Thermal modeling of the detrital AFT and vitrinite data from the lower Río Yuca Formation indicates exhumation of the source area was occurring by 12-13 Ma, surface exposure at 10-9 Ma, maximum burial by 4-2 Ma and exhumation of the sedimentary package starting 3-2 Ma. Accumulation of the Río Yuca Formation is contemporaneous with a basinward migration of the deformation front. Regional considerations indicate that the Venezuelan Andes evolved from a primarily singly vergent orogen to its current double vergence over the interval of Neogene-Quaternary sedimentation.

  6. Tandem suturing and disarticulation of the Taiwan orogen revealed by its neotectonic elements

    NASA Astrophysics Data System (ADS)

    Shyu, J. Bruce H.; Sieh, Kerry; Chen, Yue-Gau

    2005-04-01

    Taiwan's numerous active faults and folds demarcate distinct eastern and western neotectonic belts. The western belt results from the attachment and subsequent detachment of a sliver of continental lithosphere to the Eurasian continental margin. The eastern belt is the product of the same continental sliver docking with and then separating from the Luzon volcanic arc. Thus, the active Taiwan orogen is a tandem suturing and tandem disengagement of a volcanic arc and a continental sliver to and from the Eurasian continental margin. This progressive suturing and separation is a superb, living demonstration of the fundamental weakness of lithospheric sutures. Furthermore, this neotectonic architecture provides the basis for understanding the Taiwan's seismic sources.

  7. Flake tectonics in the Sulu orogen in eastern China as revealed by seismic tomography

    NASA Astrophysics Data System (ADS)

    Xu, Peifen; Liu, Futian; Ye, Kai; Wang, Qingchen; Cong, Bolin; Chen, Hui

    2002-05-01

    Seismic tomographic image reveals a crocodile-like P-waves velocity structure beneath the Sulu orogenic belt, which marks the subduction/collision zone between the Sino-Korean block (SK) and Yangtze block (YZ) in eastern China. It may imply that the upper crust of the YZ was detached from its lower crust and thrust over the SK for a maximum of ~400 km in the Sulu region, whereas the remnant of the subducted Yangtze lithosphere was lay beneath the SK. This crustal detached structure (flake tectonics) might have occurred after the Triassic subduction/collision.

  8. A new lithostratigraphic framework for the Anti-Atlas Orogen, Morocco

    NASA Astrophysics Data System (ADS)

    Thomas, R. J.; Fekkak, A.; Ennih, N.; Errami, E.; Loughlin, S. C.; Gresse, P. G.; Chevallier, L. P.; Liégeois, J.-P.

    2004-06-01

    A new lithostratigraphic nomenclatural framework is proposed for the Anti-Atlas Orogen of Morocco, to replace the previous chronostratigraphic scheme. All the lithostratigraphic units of the Anti-Atlas Orogen are well represented in the Sirwa, Kerdous and Sarhro inliers and examples from these areas are given prominence, though correlates in other areas are given, where known. The oldest, Palaeoproterozoic, rocks of the cratonic basement form the northern part of the West African Craton and can be subdivided into a series of Complexes (made up of constituent Groups, Suites etc.), depending on geographical outcrop (e.g. the Zenaga and Kerdous Complexes in the Sirwa and Kerdous inliers respectively). The Neoproterozoic rocks deposited on this basement are termed the Anti-Atlas Supergroup, within which five groups of volcano-sedimentary units ( Taghdout, Jbel Lkst, Sarhro, Iriri, and Bou Azzer Groups) and various intrusive igneous rocks are recognised (e.g. Ifzwane and Toudma Suites). These units are related to the earliest passive margin, oceanic and island-arc development phases of the orogen between ˜800 and 660 Ma. The earliest Pan-African deformation (at ˜660 Ma) resulted from closure of the ocean basin, SW-directed thrusting and accretion of the island-arc remnants. Continued prolonged convergence gave rise to a sinistral transpressional regime in which late syn- to post-tectonic continental volcanic and clastic sequences of the Ouarzazate Supergroup were deposited. The first rocks of this new tectonic regime were deposited in narrow, tectonically active, strike-slip pull-apart rift basins ( Bou Salda, Mgouna, Tafrawt and Anzi Groups) associated with intrusion of high-K calc-alkaline granitoid batholiths (e.g. Assarag, Bardouz Suites) and the juxtaposition of ophiolitic remnants in major strike-slip shear belts. These were followed by the deposition of the much more areally extensive continental volcano-sedimentary molasse, the volcanic components of which

  9. A crustal model of the ultrahigh-pressure Dabie Shan orogenic belt, China, derived from deep seismic refraction profiling

    USGS Publications Warehouse

    Wang, Chun-Yong; Zeng, Rong-Sheng; Mooney, W.D.; Hacker, B.R.

    2000-01-01

    We present a new crustal cross section through the east-west trending ultrahigh-pressure (UHP) Dabie Shan orogenic belt, east central China, based on a 400-km-long seismic refraction profile. Data from our profile reveal that the cratonal blocks north and south of the orogen are composed of 35-km-thick crust consisting of three layers (upper, middle, and lower crust) with average seismic velocities of 6.0±0.2 km/s, 6.5±0.1 km/s, and 6.8±0.1 km/s. The crust reaches a maximum thickness of 41.5 km beneath the northern margin of the orogen, and thus the present-day root beneath the orogen is only 6.5 km thick. The upper mantle velocity is 8.0±0.1 km/s. Modeling of shear wave data indicate that Poisson's ratio increases from 0.24±0.02 in the upper crust to 0.27±0.03 in the lower crust. This result is consistent with a dominantly felsic upper crustal composition and a mafic lower crustal composition within the amphibolite or granulite metamorphic facies. Our seismic model indicates that eclogite, which is abundant in surface exposures within the orogen, is not a volumetrically significant component in the middle or lower crust. Much of the Triassic structure associated with the formation of the UHP rocks of the Dabie Shan has been obscured by post-Triassic igneous activity, extension and large-offset strike-slip faulting. Nevertheless, we can identify a high-velocity (6.3 km/s) zone in the upper (<5 km depth) crustal core of the orogen which we interpret as a zone of ultrahigh-pressure rocks, a north dipping suture, and an apparent Moho offset that marks a likely active strike-slip fault.

  10. A comparative analysis of pre-Silurian crustal building blocks of the northern and the southern Appalachian orogen

    USGS Publications Warehouse

    Hibbard, J.P.; van Staal, C.R.; Rankin, D.W.

    2007-01-01

    The New York promontory serves as the divide between the northern and southern segments of the Appalachian orogen. Antiquated subdivisions, distinct for each segment, implied that they had lithotectonic histories that were independent of each other. Using new lithotectonic subdivisions we compare first order features of the pre-Silurian orogenic 'building blocks' in order to test the validity of the implication of independent lithotectonic histories for the two segments. Three lithotectonic divisions, termed here the Laurentian, Iapetan, and the peri-Gondwanan realms, characterize the entire orogen. The Laurentian realm, composed of native North American rocks, is remarkably uniform for the length of the orogen. It records the multistage Neoproterozoic-early Paleozoic rift-drift history of the Appalachian passive margin, formation of a Taconic Seaway, and the ultimate demise of both in the Middle Ordovician. The Iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the Iapetus Ocean, between Laurentia and Gondwana. In the northern segment, the realm is divisible on the basis of stratigraphy and faunal provinciality into peri-Laurentian and peri-Gondwanan tracts that were amalgamated in the Late Ordovician. South of New York, stratigraphic and faunal controls decrease markedly; rock associations are not inconsistent with those of the northern Appalachians, although second-order differences exist. Exposed exotic crustal blocks of the peri-Gondwanan realm include Ganderia, Avalonia, and Meguma in the north, and Carolinia in the south. Carolinia most closely resembles Ganderia, both in early evolution and Late Ordovician-Silurian docking to Laurentia. Our comparison indicates that, to a first order, the pre-Silurian Appalachian orogen developed uniformly, starting with complex rifting and a subsequent drift phase to form the Appalachian margin, followed by the consolidation of Iapetan components and ending with accretion of the peri

  11. Zirconolite, zircon and monazite-(Ce) U-Th-Pb age constraints on the emplacement, deformation and alteration history of the Cummins Range Carbonatite Complex, Halls Creek Orogen, Kimberley region, Western Australia

    NASA Astrophysics Data System (ADS)

    Downes, Peter J.; Dunkley, Daniel J.; Fletcher, Ian R.; McNaughton, Neal J.; Rasmussen, Birger; Jaques, A. Lynton; Verrall, Michael; Sweetapple, Marcus T.

    2016-04-01

    In situ SHRIMP U-Pb dating of zirconolite in clinopyroxenite from the Cummins Range Carbonatite Complex, situated in the southern Halls Creek Orogen, Kimberley region, Western Australia, has provided a reliable 207Pb/206Pb age of emplacement of 1009 ± 16 Ma. Variably metamict and recrystallised zircons from co-magmatic carbonatites, including a megacryst ~1.5 cm long, gave a range of ages from ~1043-998 Ma, reflecting partial isotopic resetting during post-emplacement deformation and alteration. Monazite-(Ce) in a strongly foliated dolomite carbonatite produced U-Th-Pb dates ranging from ~900-590 Ma. Although the monazite-(Ce) data cannot give any definitive ages, they clearly reflect a long history of hydrothermal alteration/recrystallisation, over at least 300 million years. This is consistent with the apparent resetting of the Rb-Sr and K-Ar isotopic systems by a post-emplacement thermal event at ~900 Ma during the intracratonic Yampi Orogeny. The emplacement of the Cummins Range Carbonatite Complex probably resulted from the reactivation of a deep crustal structure within the Halls Creek Orogen during the amalgamation of Proterozoic Australia with Rodinia over the period ~1000-950 Ma. This may have allowed an alkaline carbonated silicate magma that was parental to the Cummins Range carbonatites, and generated by redox and/or decompression partial melting of the asthenospheric mantle, to ascend from the base of the continental lithosphere along the lithospheric discontinuity constituted by the southern edge of the Halls Creek Orogen. There is no evidence of a link between the emplacement of the Cummins Range Carbonatite Complex and mafic large igneous province magmatism indicative of mantle plume activity. Rather, patterns of Proterozoic alkaline magmatism in the Kimberley Craton may have been controlled by changing plate motions during the Nuna-Rodinia supercontinent cycles (~1200-800 Ma).

  12. Timing and climatic drivers for glaciation across monsoon-influenced regions of the Himalayan-Tibetan orogen

    NASA Astrophysics Data System (ADS)

    Murari, Madhav K.; Owen, Lewis A.; Dortch, Jason M.; Caffee, Marc W.; Dietsch, Craig; Fuchs, Markus; Haneberg, William C.; Sharma, Milap C.; Townsend-Small, Amy

    2014-03-01

    Mapping and thirty-eight 10Be terrestrial cosmogenic nuclide (TCN) ages help define the timing of glaciation in the monsoon-influenced Greater Himalaya in central Garwhal, India. Glacial landforms in central Garwhal are present only within a few kilometers of the present glaciers and all date to less than ˜12.5 ka. This suggests that the Lateglacial and/or Holocene glacial advances were more extensive than their predecessors and hence destroyed or buried evidence for earlier glaciation or that other processes, such as intensive fluvial erosion and/or hillslope mass movements, have destroyed evidence of earlier glaciation. Prominent laterofrontal moraines date to the Lateglacial, the Early Holocene, and the Neoglaciation. Moraines next to the active ice and boulders on contemporary glaciers date to 101-102 years before present. This suggests only a minor glacial advance during the Little Ice Age occurred in central Garhwal. These young ages indicate that inheritance of TCNs in areas that were recently glaciated is very small and likely has little effect when considering TCN ages on moraines older than the global Last Glacial Maximum. The new 10Be ages are combined with 1081 recalculated 10Be ages from previous studies to develop a regional framework of glaciation across the monsoon-influenced and adjacent regions of the Himalayan-Tibetan orogen. We separate what appears to be continuous glaciation into 27 regional glacial stages (plus 2 tentative glacial stages) that are termed monsoonal Himalayan-Tibetan stages (MOHITS). The regional glacial stages cover a wide chronologic range that includes: five regional glacial stages older than the Last Glacial cycle (MOHITS 13 at 483 ± 38 ka to MOHITS 5E at 122 ± 15 ka); thirteen regional glacial stages within the Last Glacial cycle (MOHITS 5B at 91 ± 15 ka to MOHITS 2A at 12.9 ± 0.9 ka); and eleven regional glacial stages during the Holocene (MOHITS 1k at 11.4 ± 0.7 ka to MOHITS 1A at 0.4 ± 0.1 ka). There are strong

  13. Geodynamic implications for the formation of the Betic-Rif orogen from magnetotelluric studies

    NASA Astrophysics Data System (ADS)

    Martí, A.; Queralt, P.; Roca, E.; Ledo, J.; Galindo-ZaldíVar, J.

    2009-01-01

    Magnetotelluric data from the central Betics mountains (Spain) have been used to determine the electrical resistivity of the crust after a three-dimensional (3D) interpretive approach. At shallow levels (<2 km), the resulting model shows good correlation between the geoelectric structures and the geologic units. At greater depths (>3 km), the most striking and well-resolved feature of the model is an upper-middle crust conductive body, located at the core of the Internal Betics antiform. This approximately 14-km-thick body is interpreted as basic or ultrabasic rocks containing a conducting mineral phase. Its structural location above the sole thrust of the Betic orogen and beneath the Nevado-Filábride complex confirms the presence of a major suture zone between this complex and the autochthonous Iberian plate. This suture may correspond to an ancient oceanic or transitional domain developed between Iberia and the Alboran Domain during the opening of the Tethys Ocean, partially subducted and closed during the development of the Betic orogen. The possible geodynamic scenarios for the Betics have been reconsidered, taking into account this new constraint.

  14. Ediacaran 2,500-km-long synchronous deep continental subduction in the West Gondwana Orogen.

    PubMed

    Ganade de Araujo, Carlos E; Rubatto, Daniela; Hermann, Joerg; Cordani, Umberto G; Caby, Renaud; Basei, Miguel A S

    2014-01-01

    The deeply eroded West Gondwana Orogen is a major continental collision zone that exposes numerous occurrences of deeply subducted rocks, such as eclogites. The position of these eclogites marks the suture zone between colliding cratons, and the age of metamorphism constrains the transition from subduction-dominated tectonics to continental collision and mountain building. Here we investigate the metamorphic conditions and age of high-pressure and ultrahigh-pressure eclogites from Mali, Togo and NE-Brazil and demonstrate that continental subduction occurred within 20 million years over at least a 2,500-km-long section of the orogen during the Ediacaran. We consider this to be the earliest evidence of large-scale deep-continental subduction and consequent appearance of Himalayan-scale mountains in the geological record. The rise and subsequent erosion of such mountains in the Late Ediacaran is perfectly timed to deliver sediments and nutrients that are thought to have been necessary for the subsequent evolution of sustainable life on Earth. PMID:25319269

  15. Devonian sedimentation in the Xiqingshan Mountains: Implications for paleogeographic reconstructions of the SW Qinling Orogen

    NASA Astrophysics Data System (ADS)

    Yan, Zhen; Aitchison, Jonathan C.; Fu, Changlei; Guo, Xianqing; Xia, Wenjing; Niu, Manlan

    2016-08-01

    The Qinling Orogen between the North China and Yangtze plates comprises various accreted and collisional terranes, recording the evolution of the Proto-Tethyan Ocean and the formation of east Asia. Knowledge of the provenance and tectonic setting of a thick succession of E-W striking Devonian sedimentary rocks sandwiched tectonically between the Shangdan and Mianlue ophiolitic suture zones in this belt is essential to understanding the evolution of the Qinling Orogen and the assembly of the North China and Yangtze plates during the Late Paleozoic. Systematic studies of the Devonian sedimentary facies in the Xiqingshan Mountains indicate the northern margin of the South Qinling belt was uplifted prior to the Early Devonian. Lower Devonian sandstones are dominated by feldspathic litharenite (Q12-31F18-37L40-65), indicative of a continental arc derivation. Detrital zircon U-Pb data demonstrate that the Lower and Middle Devonian sandstones were derived respectively from ca. 770 Ma and ca. 400 Ma felsic arc-related magmatic sources. Devonian sedimentation in the Xiqingshan Mountains is interpreted to have occurred in a foreland basin between the Yangtze plate and the North Qinling continental arc.

  16. Modelling the thermal evolution of orogens: where’s the heat?

    NASA Astrophysics Data System (ADS)

    Healy, D.; Clark, C.

    2009-12-01

    Our understanding of heat transfer during orogenesis has largely been shaped by the seminal work of England & Thompson (1984). These authors explored P-T-t paths in thickened crust as a function of radiogenic heat production, mantle heat flow, thermal conductivity and rates of erosion. In this contribution we incorporate recent experimental data on the temperature dependence of thermal conductivity and field observations of heat producing elements in exposed orogenic roots into 1D thermal models of regional metamorphism. We identify distinct tectonic processes responsible for heat flow perturbations in thickened crust and compare the temporal evolution of temperature with depth (pressure) for each case. Several recent papers have presented ad hoc combinations of these processes and we seek to clarify the contribution of each process to the total heat orogenic heat budget. We present sensitivity analyses to key model parameters for each case and discuss the scope for separating the relative contribution of each process from field data such as metamorphic field gradients and pressure-temperature-time paths.

  17. Shallow, old, and hydrologically insignificant fault zones in the Appalachian orogen

    NASA Astrophysics Data System (ADS)

    Malgrange, Juliette; Gleeson, Tom

    2014-01-01

    The permeability of fault zones impacts diverse geological processes such as hydrocarbon migration, hydrothermal fluid circulation, and regional groundwater flow, yet how fault zones affect groundwater flow at a regional scale (1-10 km) is highly uncertain. The objective of this work is to determine whether faults affect regional patterns of groundwater flow, by using radioactive radon and chloride to quantify groundwater discharge to lakes underlain by faults and not underlain by faults. We sampled lakes overlying the Paleozoic Appalachian fold and thrust belt in the Eastern Townships in Québec, and compared our results to a previous study in a crystalline watershed in the Canadian Shield. The field data was analyzed with an analytical geochemical mixing model. The uncertainties of model parameters were assessed in a sensitivity analysis using Monte Carlo simulation, and the difference between lakes tested with statistical analysis. While the model results indicate non-negligible groundwater discharge for most of the lakes in the Paleozoic orogen, the difference between the groundwater discharge rate into the lakes located on faults and the other lakes is not statistically significant. However, the groundwater discharge rate to lakes in the Paleozoic orogeny is significantly higher than lakes that overlay crystalline bedrock, which is consistent with independent estimates of permeability. The rate of groundwater discharge is not significantly enhanced or diminished around the thrust fault zones, suggesting that in a regional scale, permeability of fault zones is not significantly different from the bedrock permeability at shallow depth in this old, tectonically- inactive orogen.

  18. Sediment-hosted/orogenic gold mineral systems exploration using PALSAR remote sensing data in Peninsular Malaysia

    NASA Astrophysics Data System (ADS)

    Beiranvand Pour, Amin; Hashim, Mazlan

    2016-06-01

    The Bentong-Raub Suture Zone (BRSZ) is genetically related to the sediment-hosted/orogenic gold deposits associated with the major lineaments and form-lines in the Central Gold Belt of Peninsular Malaysia. In this investigation, the Phased Array type L-band Synthetic Aperture Radar (PALSAR) satellite remote sensing data were used to analyse major geological structures in Peninsular Malaysia and provide detailed characterization of lineaments and form-lines in the BRSZ, as well as its implications for sediment-hosted/orogenic gold exploration in tropical environments. The pervasive array of N-S faults in the study area and surrounding terrain is mainly linked to the N-S trending of the BRSZ Suture Zone. N-S striking lineaments are often cut by younger NE-SW and NW-SE-trending lineaments. Three generations of folding event have been discerned from remote sensing structural analysis. Gold mineralized trends lineaments are associated with the intersection of N-S, NE-SW, NNW-SSE and ESE-WNW faults and curvilinear features in shearing and alteration zones. Compressional tectonics structures such as NW-SE trending thrust, ENE-WSW oriented faults in mylonite and phyllite, recumbent folds and asymmetric anticlines in argillite are high potential zones for gold prospecting.

  19. Crustal structure of the alaska range orogen and denali fault along the richardson highway

    USGS Publications Warehouse

    Fisher, M.A.; Pellerin, L.; Nokleberg, W.J.; Ratchkovski, N.A.; Glen, J.M.G.

    2007-01-01

    A suite of geophysical data obtained along the Richardson Highway crosses the eastern Alaska Range and Denali fault and reveals the crustal structure of the orogen. Strong seismic reflections from within the orogen north of the Denali fault dip as steeply as 25?? north and extend downward to depths between 20 and 25 km. These reflections reveal what is probably a shear zone that transects most of the crust and is part of a crustal-scale duplex structure that probably formed during the Late Cretaceous. These structures, however, appear to be relict because over the past 20 years, they have produced little or no seismicity despite the nearby Mw = 7.9 Denali fault earthquake that struck in 2002. The Denali fault is nonreflective, but we interpret modeled magnetotelluric (MT), gravity, and magnetic data to propose that the fault dips steeply to vertically. Modeling of MT data shows that aftershocks of the 2002 Denali fault earthquake occurred above a rock body that has low electrical resistivity (>10 ohm-m), which might signify the presence of fluids in the middle and lower crust. Copyright ?? 2007 The Geological Society of America.

  20. The early Cretaceous orogen-scale Dabieshan metamorphic core complex: implications for extensional collapse of the Triassic HP-UHP orogenic belt in east-central China

    NASA Astrophysics Data System (ADS)

    Ji, Wenbin; Lin, Wei; Faure, Michel; Shi, Yonghong; Wang, Qingchen

    2016-03-01

    The Dabieshan massif is famous as a portion of the world's largest HP-UHP metamorphic belt in east-central China that was built by the Triassic North-South China collision. The central domain of the Dabieshan massif is occupied by a huge migmatite-cored dome [i.e., the central Dabieshan dome (CDD)]. Origin of this domal structure remains controversial. Synthesizing previous and our new structural and geochronological data, we define the Cretaceous Dabieshan as an orogen-scale metamorphic core complex (MCC) with a multistage history. Onset of lithospheric extension in the Dabieshan area occurred as early as the commencement of crustal anatexis at the earliest Cretaceous (ca. 145 Ma), which was followed by primary (early-stage) detachment during 142-130 Ma. The central Dabieshan complex in the footwall and surrounding detachment faults recorded a consistently top-to-the-NW shearing. It is thus inferred that the primary detachment was initiated from a flat-lying detachment zone at the middle crust level. Removal of the orogenic root by delamination at ca. 130 Ma came into the extensional climax, and subsequently isostatic rebound resulted in rapid doming. Along with exhumation of the footwall, the mid-crustal detachment zone had been warped as shear zones around the CDD. After 120 Ma, the detachment system probably experienced a migration accommodated to the crustal adjustment, which led to secondary (late-stage) detachment with localized ductile shearing at ca. 110 Ma. The migmatite-gneiss with HP/UHP relicts in the CDD (i.e., the central Dabieshan complex) was product of the Cretaceous crustal anatexis that consumed the deep-seated part of the HP-UHP slices and the underlying para-autochthonous basement. Compared with the contemporaneous MCCs widely developed along the eastern margin of the Eurasian continent, we proposed that occurrence of the Dabieshan MCC shares the same tectonic setting as the "destruction of the North China craton". However, geodynamic trigger

  1. The Tyrrhenian stage geodinamic evolution of Apenninic-Maghrebian orogen (Southern Apennines and Sicily)

    NASA Astrophysics Data System (ADS)

    Lentini, F.; Carbone, S.; Barreca, G.

    2009-04-01

    In the Central Mediterranean region the foreland domains are represented by two continental blocks, the Apulian Block to the north and the Pelagian Block to the south, respectively belonging to the Adria and to the Africa plates. They are separated since Permo-Triassic times by the oceanic crust of the Ionian Sea. The Apenninic-Maghrebian orogen is located between two oceanic crusts: the old Ionian crust, at present time subducting beneath the Calabrian Arc, and the new crust of the opening Tyrrhenian Sea. The orogenic belt is represented by a multilayer allochthonous edifice, composed of the Calabride Chain (CC) tectonically overlying the Apenninic-Maghrebian Chain (AMC), which in turn overthrust onto the Upper Miocene and Pliocene top-levels of a deep seated thrust system, originating by the deformation of the innermost carbonates of the Pelagian/Apulian blocks (External Thrust System: ETS). The AMC tectonic units derive from the orogenic transport during Oligo-Miocene times of sedimentary sequences deposited in palaeogeographical domains located between the Europe and the Afro-Adriatic plates. These units are composed of Meso-Cenozoic shallow-water carbonate successions detached from a continental type crust sector, the Panormide/Apenninic Block, recognizable by means of seismic lines shot in the Tyrrhenian offshore of Southern Apennines and Northern Sicily. The Meso-Cenozoic basinal units, that compose the AMC, can be distinguished into two main groups of sequences, originally located on oceanic crusts separated by the Panormide/Apenninic Block: the external ones (Ionides) related to an original basin belonging to branches of the Ionian Palaeobasin involved in the orogenesis, and the internal ones ascribed to the Alpine Tethys (Sicilide Units). The terrigenous deposits of the basinal sequences belonging to the Ionides are represented by Tertiary foreland/foredeep deposits, whose relationships with the substratum are occasionally preserved, although large

  2. Late Mesozoic deformations of the Verkhoyansk-Kolyma orogenic belt, Northeast Russia

    NASA Astrophysics Data System (ADS)

    Fridovsky, Valery

    2016-04-01

    The Verkhoyansk-Kolyma orogenic belt marks the boundary between the Kolyma-Omolon superterrane (microcontinent) and the submerged eastern margin of the North Asian craton. The orogenic system is remark able for its large number of economically viable gold deposits (Natalka, Pavlik, Rodionovskoe, Drazhnoe, Bazovskoe, Badran, Malo-Tarynskoe, etc.). The Verkhoyansk - Kolyma orogenic belt is subdivided into Kular-Nera and the Polousny-Debin terranes. The Kular-Nera terrane is mainly composed of the Upper Permian, Triassic, and Lower Jurassic black shales that are metamorphosed at lower greenschist facies conditions. The Charky-Indigirka and the Chai-Yureya faults separate the Kular-Nera from the Polousny-Debin terrane that is predominantly composed of the Jurassic flyschoi dturbidites. The deformation structure of the region evolved in association with several late Mesozoic tectonic events that took place in the north-eastern part ofthe Paleo-Pacific. In Late Jurassic-Early Cretaceous several generations of fold and thrust systems were formed due to frontal accretion of the Kolyma-Omolon superterrane to the eastern margin of the North Asian craton.Thrusting and folding was accompanied by granitic magmatism, metamorphic reworking of the Late Paleozoic and the Early Mesozoic sedimentary rocks, and formation of Au-Sn-W mineralization. Three stages of deformation related to frontal accretion can be distinguished. First stage D1 has developed in the north-eastern part of the Verkhoyansk - Kolyma orogenic belt. Early tight and isoclinal folds F1 and assosiated thrusts are characteristic of D1. Major thrusts, linear concentric folds F2 and cleavage were formed during D2. The main ore-controlling structures are thrust faults forming imbricate fan systems. Frontal and oblique ramps and systems of bedding and cross thrusts forming duplexes are common. It is notable that mineralized tectonized zones commonly develop along thrusts at the contacts of rocks of contrasting competence

  3. Fragile cycles

    NASA Astrophysics Data System (ADS)

    Bonatti, Ch.; Díaz, L. J.

    We study diffeomorphisms f with heterodimensional cycles, that is, heteroclinic cycles associated to saddles p and q with different indices. Such a cycle is called fragile if there is no diffeomorphism close to f with a robust cycle associated to hyperbolic sets containing the continuations of p and q. We construct a codimension one submanifold of Diff(S×S) that consists of diffeomorphisms with fragile heterodimensional cycles. Our construction holds for any manifold of dimension ⩾4.

  4. Kinematics of post-orogenic extension and exhumation of the Taku Schist, NE Peninsular Malaysia

    NASA Astrophysics Data System (ADS)

    Md Ali, M. A.; Willingshofer, E.; Matenco, L.; Francois, T.; Daanen, T. P.; Ng, T. F.; Taib, N. I.; Shuib, M. K.

    2016-09-01

    Recent studies imply that the formation and evolution of many SE Asian basins was driven by extensional detachments or systems of low-angle normal faults that created significant crustal exhumation in their footwalls. In this context, the architecture of the Triassic Indosinian orogen presently exposed in Peninsular Malaysia is compatible with significant extension post-dating the orogenic event. In this study we performed a kinematic analysis based on fieldwork and microstructural observations in the Taku Schist, Kemahang granite and the surrounding Gua Musang sediments of northern Peninsular Malaysia in order to shed light on processes related to the build-up and subsequent demise of the Indosinian orogen. The first three phases of deformation were related to an overall period of E-W oriented contraction and burial metamorphism. These phases of deformation are characterized by isoclinal folding with flat lying axial plane cleavages (D1), asymmetrical folding, top-to-the-W-SW shearing (D2) and upright folding (D3). All are in general agreement with observations of the previously inferred Permo-Triassic Indosinian orogeny. During these times, the Taku Schist, a sequence of Paleozoic clastic sediments with mafic intercalations was metamorphosed to amphibolite facies. These rocks are most likely equivalent to the ones exposed in the Bentong-Raub suture zone. Structural relations suggest that the Triassic Kemahang pluton is syn-kinematic, which provides important constraints for the timing of these contractional events. We demonstrate that the overall shortening was followed by a hitherto undescribed extension in NW-SE direction resulting in the formation of a large-scale detachment, the Taku detachment, in northern Peninsular Malaysia. Extension probably reactivated the former subduction plane as a detachment and exhumed previously buried and metamorphosed rocks of similar lithological composition to the neighboring Bentong-Raub suture zone. Such a mechanism is

  5. The Palaeoproterozoic crustal evolution: evidences from granulite-gneiss belts, collisional and accretionary orogens

    NASA Astrophysics Data System (ADS)

    Mints, M. V.; Konilov, A. N.

    2003-04-01

    The Palaeoproterozoic juvenile assemblages were emplaced within two types of mobile belts: (1) high-grade or "granulite-gneiss" belts; (2) low- and medium-grade volcano-sedimentary and volcano-plutonic belts. Type (1) belts resulted from plume-induced heating, magmatism, emergence of riftogenic basins and volcano-tectonic depressions, their filling with rift-type sediments and juvenile but strongly contaminated lavas and ash-flow deposits, high-grade recrystallization of the lower- and mid-crustal assemblages including the filling of the basins and depressions that followed in intraplate and back-arc settings, and final thrusting and exhumation caused by collision-related tectonism. Type (2) belts represent sutures containing MORB- and arc-related assemblages, together with initial rift-related assemblages formed during evolution of the short-lived, mainly Red Sea-type oceans (intracontinental collisional orogens) and systems of oceanic, island-arc and back-arc terranes amalgamated along continent margins (peripheral accretionary orogens). Palaeoproterozoic history can be subdivided into five periods: (1) 2.51-2.44 Ga superplume activity and displacement of Fennoscandia; (2) 2.44-2.0 (2.11) Ga quiescent within-plate development complicated by local plume- and plate tectonics-related processes; (3) a 2.0-1.95 Ga superplume event; (4) 1.95-1.75 (1.71) Ga combined plume- and plate tectonics-related evolution, resulting in the partial disruption of the continental crust, and formation of accretionary orogens along some margins of the supercontinent and rebirth of the supercontinent entity, and (5) < 1,75 Ga post- and anorogenic magmatism and metamorphism. Magmatic and thermal activity during the early Palaeoproterozoic was almost exclusively concentrated within Laurentia (comprising North American and Fennoscandian cratons). In contrast, late Palaeoproterozoic assemblages are distributed within all continents. The simultaneous appearance of within-plate plume

  6. Tectonic processes during oblique collision: Insights from the St. Elias orogen, northern North American Cordillera

    USGS Publications Warehouse

    Pavlis, T.L.; Picornell, C.; Serpa, L.; Bruhn, R.L.; Plafker, G.

    2004-01-01

    Oblique convergence in the St. Elias orogen of southern Alaska and northwestern Canada has constructed the world's highest coastal mountain range and is the principal driver constructing all of the high topography in northern North America. The orogen originated when the Yakutat terrane was excised from the Cordilleran margin and was transported along margin-parallel strike-slip faults into the subduction-transform transition at the eastern end of the Aleutian trench. We examine the last 3 m.y. of this collision through an analysis of Euler poles for motion of the Yakutat microplate with respect to North America and the Pacific. This analysis indicates a Yakutat-Pacific pole near the present southern triple junction of the microplate and' predicts convergence to dextral-oblique convergence across the offshore Transition fault, onland structures adjacent to the Yakutat foreland, or both, with plate speeds increasing from 10 to 30 mm/yr from southeast to northwest. Reconstructions based on these poles show that NNW transport of the collided block into the NE trending subduction zone forced contraction of EW line elements as the collided block was driven into the subduction-transform transition. This suggests the collided block was constricted as it was driven into the transition. Constriction provides an explanation for observed vertical axis refolding of both earlier formed fold-thrust systems and the collisional suture at the top of the fold-thrust stack. We also suggest that this motion was partially accommodated by lateral extrusion of the western portion of the orogen toward the Aleutian trench. Important questions remain regarding which structures accommodated parts of this motion. The Transition fault may have accommodated much of the Yakutat-Pacific convergence on the basis of our analysis and previous interpretations of GPS-based geodetic data. Nonetheless, it is locally overlapped by up to 800 m of undeformed sediment, yet elsewhere shows evidence of young

  7. Ain't No Mountain High Enough: Reconstructing Paleoelevation of Eroded Orogens

    NASA Astrophysics Data System (ADS)

    Mulch, A.; Teyssier, C.; Chamberlain, C. P.; Vennemann, T.; Cosca, M.; Wells, M.

    2004-12-01

    Quantitative paleoelevation reconstructions are a key element in understanding the relationships among uplift, erosion, and the demise of orogens. We present a new approach of determining paleoelevation based on the stable isotope composition of meteoric water that interacted with detachment mylonite during exhumation and uplift. Provided that this interaction is precisely dated and temporally linked to the stable isotope record in syntectonic basins this approach relates the tectonic, sedimentary and climate history of the orogen. Hydrogen, oxygen and Ar-Ar data from extensional detachments of the Shuswap, Kettle and Raft River core complexes and their adjacent basins, allow us to reconstruct discrete episodes in the Cenozoic elevation history of the North American Cordillera. Our data are consistent with a southward migration of a topographic high from Eocene to Miocene times, as reflected in the sequential onset of detachment faulting and extensional collapse in the Cordilleran hinterland. The integrated oxygen and hydrogen isotope data show that meteoric waters infiltrating the early to middle Eocene (49.0 - 47.0 Ma) eastern detachments of the Shuswap (British Columbia) and Kettle (Washington) core complexes had very negative hydrogen isotope values (-135 and -120 permil, respectively). These results require mean elevations in excess of 4000 m immediately preceding the timing of extensional deformation. Preliminary hydrogen (meteoric fluid compositions of -100 to -110 permil) and oxygen isotope data from Oligocene and Miocene mylonites of the Raft River complex (Utah) indicate that at lower latitudes high elevations persisted until Oligocene/Miocene times, a conclusion consistent with stable isotope data derived from syntectonic sedimentary deposits in the adjacent Elko Basin (Nevada). The coupled Eocene to Miocene data sets derived from rocks that at the time were at several km depth indicate that crustal thickening prior to detachment faulting resulted in

  8. Lawsonite Blueschists in Recycled Mélange Involved in K-Rich Orogenic Magmatism

    NASA Astrophysics Data System (ADS)

    Wang, Y.; Prelevic, D.; Foley, S. F.; Buhre, S.; Galer, S. J. G.

    2014-12-01

    The origin of K-rich orogenic magmatism in the Alpine-Himalayan belt and its relationship to the large-scale elevations in several massifs of the orogen is controversial, particularly the significance of the widespread presence of a geochemical signal typical for recycled continental crust. Two competing scenarios invoke direct melting of continental crust during deep intercontinental subduction and removal of heavily metasomatised mantle lithosphere by delamination into the convecting mantle. Here we investigate the coupling of high Th/La ratio with crustal isotopic signatures in K-rich orogenic lavas that does not occur in volcanic rocks from other collisional environments to distinguish between these two models. High-pressure experimental results on a phyllite representing upper crustal composition and a detailed mineral and geochemical study of blueschists from Tavşanlı mélange, Turkey, indicate that this geochemical fingerprint originates by melting of subducted mélange. Melting of crust at the top of the subducted continental lithosphere cannot produce observed fingerprint, whereas lawsonites, especially those with terrigenous sediment origin from blueschists with high Th/La can. Lawsonites that grow in various components of a subduction mélange inherit the geochemical characteristics of either oceanic or continental protoliths. It is currently uncertain whether those carrying the high Th/La signature originate by direct melting of continental blocks in the mélange or by the introduction of supercritical fluids from lawsonite blueschist of continental origin that infiltrate oceanic sediment blocks. Either way, the high Th/La is later released into subsequently formed melts. This confirms the supposition that lawsonite is the main progenitor of the high Th/La and Sm/La ratio. However, lawsonite must break down completely to impart this unique feature to subsequent magmas. The source regions of the potassic volcanic rocks consist of blueschist facies m

  9. New constraints on fluid sources in orogenic gold deposits, Victoria, Australia

    NASA Astrophysics Data System (ADS)

    Fu, Bin; Kendrick, Mark A.; Fairmaid, Alison M.; Phillips, David; Wilson, Christopher J. L.; Mernagh, Terrence P.

    2012-03-01

    Fluid inclusion microthermometry, Raman spectroscopy and noble gas plus halogen geochemistry, complemented by published stable isotope data, have been used to assess the origin of gold-rich fluids in the Lachlan Fold Belt of central Victoria, south-eastern Australia. Victorian gold deposits vary from large turbidite-hosted `orogenic' lode and disseminated-stockwork gold-only deposits, formed close to the metamorphic peak, to smaller polymetallic gold deposits, temporally associated with later post-orogenic granite intrusions. Despite the differences in relative timing, metal association and the size of these deposits, fluid inclusion microthermometry indicates that all deposits are genetically associated with similar low-salinity aqueous, CO2-bearing fluids. The majority of these fluid inclusions also have similar 40Ar/36Ar values of less than 1500 and 36Ar concentrations of 2.6-58 ppb (by mass) that are equal to or much greater than air-saturation levels (1.3-2.7 ppb). Limited amounts of nitrogen-rich fluids are present at a local scale and have the highest measured 40Ar/36Ar values of up to 5,700, suggesting an external or distinct source compared to the aqueous fluids. The predominance of low-salinity aqueous-carbonic fluids with low 40Ar/36Ar values, in both `orogenic' and `intrusion-related' gold deposits, is attributed to fluid production from common basement volcano-sedimentary sequences and fluid interaction with sedimentary cover rocks (turbidites). Aqueous fluid inclusions in the Stawell-Magdala deposit of western Victoria (including those associated with N2) preserve mantle-like Br/Cl and I/Cl values. In contrast, fluid inclusions in deposits in the eastern structural zones, which contain more abundant shales, have elevated molar I/Cl ratios with maximum values of 5,170 × 10-6 in the Melbourne Zone. Br/I ratios in this zone range from 0.5 to 3.0 that are characteristic of fluid interaction with organic-rich sediments. The maximum I/Cl and characteristic

  10. South Tien Shan orogenic belt: structure, magmatism and gold mineralization (Uzbekistan)

    NASA Astrophysics Data System (ADS)

    Koneev, Rustam; Seltmann, Reimer

    2014-05-01

    The Southern Tien Shan represents one of the key units of the Central Asian orogenic belt in Uzbekistan. Together with the Beltau-Kurama volcano-plutonic arc it formed as a result of subduction of the crust under the Turkistan paleoocean and the Kazakhstan continent, followed by collision and post-collisional strike-slip processes. The Southern Tien Shan is of particular interest due to its gold mineralisation. It hosts the giant Muruntau gold deposit and the large gold deposits of Amantaitau, Daugyztau, Myutenbay (Kyzylkum ore district) and Charmitan, Guzhumsay, Urtalik (Nurata ore district). The Middle Tienshan hosts within the Beltau-Kurama volcano-plutonic arc the Kurama ore district with the giant Kalmakyr Cu-Au porphyry and large epithermal Au-Ag deposits of Kochbulak and Kyzylalma. Yakubchuk et al. (2005) and others stress that the largest ore clusters are confined by the intersections of volcano-plutonic belts and transform faults in result of sinistral strike slip dislocations during the Permo-Carboniferous. Others believe that the ore giants are in addition controlled by hot spots - a mantle plume, superimposed on the crust architecture shaped by the subduction processes. Zircon U-Pb geochronology of main intrusive massifs of Uzbekistan (CERCAMS data) showed that granitoid magmatism is predominantly of postcollisional age, manifested in the accretionary units at 270-290 Ma, whereas subduction magmatism prevails as characteristic in the volcano-plutonic arc at 300-320 Ma. Determination of sulphide mineralization ages using Os-Re method (CERCAMS data), are respectively 283-289 Ma and 298-314 Ma. The studies were performed in the framework of IGCP- 592. References 1. Yakubchuk A.S., Shatov V.V., Kirwin D. et al., (2005) Gold and base metal metallogeny of the Central Asian Orogenic supercollage: Society of Economic Geologists, Inc. Economic Geology, 100th, Anniversary Volume, 1035-1068. 2. Groves, D.I., Goldfarb, R.J., Gebre-Mariam, M., Hagemann, S.G. and

  11. Tectonic evolution of the Montagne Noire and a possible orogenic model for syncollisional exhumation of deep rocks, Variscan belt, France

    NASA Astrophysics Data System (ADS)

    Aerden, Domingo G. A. M.

    1998-02-01

    Structural analysis in the Montagne Noire, including a study of inclusion trail patterns in andalusite, garnet, and staurolite porphyroblasts, reveals a succession of five foliations in alternately steep and flat-lying orientations. This is inferred to reflect multiple gravitational collapse phases that intervened during continuous plate convergence. Initial crustal thickening by foliation development and folding (D1) was responsible for eclogite-facies metamorphism in the gneissic basement. Subsequent thrusting and recumbent folding (D2) generated a subhorizontal crenulation cleavage during Namuro-Westphalian times, while previous steep structures became rotated. Petrological and microstructural evidence indicate that thrust nappes were emplaced by a gravitational spreading mechanism, while undergoing internal thinning. An important amount of basement exhumation took place synchronously. These observations form the basis for a new orogenic model in which thermal reequilibration of a previously thickened orogen induces gravitational instability in the upper crust. Collapse of the upper crust causes it to spreadout horizontally over continuously converging relatively cool plates thereby creating low-angle thrust nappes. The upper crustal thinning facilitates simultaneous extrusion of lower crust in vertical root zones at the orogen center, which on reaching the higher part of the orogen can be incorporated in the nappes and displaced horizontally. Following such a mechanism of nappe tectonics, renewed horizontal shortening (D3) produced the Montagne Noire gneissic dome by upright folding of nappe-related foliations with orogen-parallel stretching. Previous crustal thinning had led to an increased heat flux between basement and cover so that during D3, cover sequences were still rising in temerature, while basement rocks had already commenced their retrograde path during D2. Peak-metamorphic conditions in the cover triggered a second gravitational collapse during

  12. Menstrual Cycle

    MedlinePlus

    ... Pregnancy This information in Spanish ( en español ) The menstrual cycle Day 1 starts with the first day of ... drop around Day 25 . This signals the next menstrual cycle to begin. The egg will break apart and ...

  13. Hillslope response to knickpoint migration in the Southern Appalachians: Implications for the evolution of post-orogenic landscapes

    USGS Publications Warehouse

    Wegmann, S.F.G.; Franke, K.L.; Hughes, S.; Lewis, R.Q.; Lyons, N.; Paris, P.; Ross, K.; Bauer, J.B.; Witt, A.C.

    2011-01-01

    The southern Appalachians represent a landscape characterized by locally high topographic relief, steep slopes, and frequent mass movement in the absence of significant tectonic forcing for at least the last 200 Ma. The fundamental processes responsible for landscape evolution in a post-orogenic landscape remain enigmatic. The non-glaciated Cullasaja River basin of south-western North Carolina, with uniform lithology, frequent debris flows, and the availability of high-resolution airborne lidar DEMs, is an ideal natural setting to study landscape evolution in a post-orogenic landscape through the lens of hillslope-channel coupling. This investigation is limited to channels with upslope contributing areas >2.7 km2, a conservative estimate of the transition from fluvial to debris-flow dominated channel processes. Values of normalized hypsometry, hypsometric integral, and mean slope vs elevation are used for 14 tributary basins and the Cullasaja basin as a whole to characterize landscape evolution following upstream knickpoint migration. Results highlight the existence of a transient spatial relationship between knickpoints present along the fluvial network of the Cullasaja basin and adjacent hillslopes. Metrics of topography (relief, slope gradient) and hillslope activity (landslide frequency) exhibit significant downstream increases below the current position of major knickpoints. The transient effect of knickpoint-driven channel incision on basin hillslopes is captured by measuring the relief, mean slope steepness, and mass movement frequency of tributary basins and comparing these results with the distance from major knickpoints along the Cullasaja River. A conceptual model of area-elevation and slope distributions is presented that may be representative of post-orogenic landscape evolution in analogous geologic settings. Importantly, the model explains how knickpoint migration and channel- hillslope coupling is an important factor in tectonically-inactive (i

  14. Paleozoic accretionary orogenesis in the eastern Beishan orogen: constraints from zircon U-Pb and 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Ao, Songjian; Xiao, Wenjiao; Windley, Brian; Mao, Qigui

    2016-04-01

    The continental growth mechanism of the Altaids in Central Asia is still in controversy between models of continuous subduction-accretion versus punctuated accretion by closure of multiple oceanic basins. The Beishan orogenic belt, located in the southern Altaids, is a natural laboratory to address this controversy. Key questions that are heavily debated are: the closure time and subduction polarity of former oceans, the emplacement time of ophiolites, and the styles of accretion and collision. This paper reports new structural data, zircon ages and Ar-Ar dates from the eastern Beishan Orogen that provide information on the accretion process and tectonic affiliation of various terranes. Our geochronological and structural results show that the younging direction of accretion was northwards and the subduction zone dipped southwards under the northern margin of the Shuangyingshan micro-continent. This long-lived and continuous accretion process formed the Hanshan accretionary prism. Our field investigations show that the emplacement of the Xiaohuangshan ophiolite was controlled by oceanic crust subduction beneath the forearc accretionary prism of the Shuangyingshan-Mazongshan composite arc to the south. Moreover, we address the age and terrane affiliation of lithologies in the eastern Beishan orogen through detrital zircon geochronology of meta-sedimentary rocks. We provide new information on the ages, subduction polarities, and affiliation of constituent structural units, as well as a new model of tectonic evolution of the eastern Beishan orogen. The accretionary processes and crustal growth of Central Asia were the result of multiple sequences of accretion and collision of manifold terranes. Reference: Ao, S.J., Xiao, W., Windley, B.F., Mao, Q., Han, C., Zhang, J.e., Yang, L., Geng, J., Paleozoic accretionary orogenesis in the eastern Beishan orogen: Constraints from zircon U-Pb and 40Ar/39Ar geochronology. Gondwana Research, doi: http://dx.doi.org/10.1016/j

  15. Deformation and metamorphism in an eclogite-bearing shear zone within the Sveconorwegian Orogen, Sweden

    NASA Astrophysics Data System (ADS)

    Tual, Lorraine; Möller, Charlotte; Pinan-Llamas, Aranzazu

    2013-04-01

    The Eastern Segment is the counterpart to the Parautochthonous Belt in the Grenville Province, and represents a part of the Fennoscandian Shield that was thoroughly reworked during late stages of the Sveconorwegian orogeny (~1.1-0.9 Ga). Relics of eclogite occur as lenses in high-grade gneisses in a part of the southern Eastern Segment. These rocks constitute evidence of a high-pressure event at c. 0.98 Ga, followed by regional deformation and metamorphism in the granulite and upper amphibolite facies. Structural and petrological data combined with airborne magnetic anomalies have been used to define the structure and the relationship between deformation and metamorphism related to the Sveconorwegian orogeny. Along the southern and intensely deformed boundary of the eclogite domain, the Ullared Zone, three main deformation phases (D1-D3) have been identified. D1 is identified as a lithotectonic and gneissic layering with locally preserved isoclinal folds. These structures were overprinted by a major deformation phase (D2), which affected heterogeneously the entire southern Eastern Segment. D2 is characterized by asymmetric tight to isoclinal folds, commonly with a well-developed axial planar mineral fabric. The folds are associated with shear sub-parallel to their axial planes in areas where the deformation was intense. They are interpreted as the consequence of tectonic extrusion of eclogitic-bearing crust into mid-crustal levels, resulting in decompression and retrogression of eclogite under high-pressure granulite and upper amphibolite conditions. This deformation resulted in strong E-W to WNW-ESE stretching, associated with systematic top-the-east sense of shear. Late open upright folding (D3) with predominant NNE-SSW axes is superimposed on D2. Ongoing studies aim at constraining the P-T-t path of the eclogite-bearing unit and surrounding units by multiequilibrium thermobarometry and pseudosections. The metamorphic evolution will be linked to the structural

  16. Drainage Pattern, Along-Strike Topography and Three-Dimensional Construction of the Himalayan orogen

    NASA Astrophysics Data System (ADS)

    Yin, A.

    2005-12-01

    Past studies on the Himalayan orogen have mostly emphasized its 2-D evolution in cross-section view. As a result, how the orogen has grown in 3-D remains poorly understood. For example, it is not clear if the >1500-km long Main Central Thrust (MCT) and Main Boundary Thrust (MBT) were initiated synchronously along the whole orogen or started at one segment and then propagated laterally during subsequent fault motion. To clarify this issue, I examined the Himalayan drainage pattern, along-strike topography, and geometry and kinematics of growing contractional structures across both the Himalayan front and the Shillong Plateau. The main observations may be summarized as follows. (1) The five rivers that cut across the Himalaya are arranged approximately symmetric with respect to the Himalayan-arc axis (~85° E): the Arun River (87° E) is in the middle with the Sutlej (77° E) and Indus (72° E) Rivers in the west and the Subansiri (93° E) and Yalu-Brahmaputra (96° E) Rivers in the east. (2) Between the eastern and western syntaxes, south-flowing drainages east of 85.5° E are consistently deflected to the east by east-growing anticlines and thrusts, whereas drainages west of 85.5°E are deflected systematically to the west by west-growing anticlines and thrusts along the Himalayan front. The only region where no drainage deflection is observed is the Bhutan Himalaya. There all rivers flow straight across the Himalayan front. (3) The deflected drainage pattern indicates that the Shillong Plateau south of the eastern Himalaya has been growing westward. (4) The along-crest Himalayan topographic profile concaves downward, starting from ~5200 m just inside the two syntaxes and reaching 8848 m at Mt. Everest (~87° E). (5) There are a total of 17 major growing contractional structures in the Main Frontal Thrust Zone (MFTZ), with 10 in the west and 7 in the east. Each structure has a length between ~20 km and >150 km, but the west-growing structures in the west are

  17. Fault dating in the Canadian Rocky Mountains: Evidence for late Cretaceous and early Eocene orogenic pulses

    USGS Publications Warehouse

    van der Pluijm, B.A.; Vrolijk, P.J.; Pevear, D.R.; Hall, C.M.; Solum, J.

    2006-01-01

    Fault rocks from the classic Rocky Mountain foreland fold-and-thrust belt in south-western Canada were dated by Ar analysis of clay grain-size fractions. Using X-ray diffraction quantification of the detrital and authigenic component of each fraction, these determinations give ages for individual faults in the area (illite age analysis). The resulting ages cluster around 72 and 52 Ma (here called the Rundle and McConnell pulses, respectively), challenging the traditional view of gradual forward progression of faulting and thrust-belt history of the area. The recognition of spatially and temporally restricted deformation episodes offers field support for theoretical models of critically stressed wedges, which result in geologically reasonable strain rates for the area. In addition to regional considerations, this study highlights the potential of direct dating of shallow fault rocks for our understanding of upper-crustal kinematics and regional tectonic analysis of ancient orogens. ?? 2006 Geological Society of America.

  18. The Western Chugach-St. Elias Orogen, Alaska: Strain Partitioning and the Effect of Glacial Erosion

    NASA Astrophysics Data System (ADS)

    Berger, A. L.; Spotila, J. A.

    2006-12-01

    The ongoing collision between the Yakutat terrane and the North American plate in southeastern Alaska's St. Elias orogen is a modern analog for the tectonic processes which produced, and shaped, much of the Cordillera. With convergence rates comparable to that of the Himalaya (>4 cm/yr), a young and dynamic zone of thin-skinned interplate deformation has constructed the highest coastal relief on Earth, and given rise to the second and third highest peaks in North America (5,959 and 5,489 m). The orogen receives upwards of 4 m precipitation annually, has been heavily glaciated for the last 5 Ma, and contains some of the fastest short-term erosion rates known. Over the last few years, evidence has steadily mounted that within such tectonic settings, climate and tectonics exist as a coupled system (i.e. Taiwan and Nanga Parbat). Our ongoing research, aimed at quantifying spatial patterns in exhumation rate as well as the location of active structures within the western half of the St. Elias orogen, bolsters this new paradigm. Bedrock ([U-Th]/He) cooling ages in apatite show that exhumation is currently focused on the windward side of the orogen. Time- averaged, long-term, exhumation rates near the coast are generally ~2-3 mm/yr, versus <0.5 mm/yr on the leeward side of the range. However, the rapid exhumation rates along the windward flank are not spatially uniform with the highest rates measured thus far >~5.5 mm/yr (0.4 Ma cooling age) situated near the Bering and Steller Glaciers. This locus of exhumation could reflect a redistribution of strain by focused erosion beneath these large outlet glaciers. Yet, the structural mechanism of this focused strain is still speculative. Pairs of helium ages spanning the foot-wall and hanging-wall of the Chugach-St. Elias thrust, the suture between the North American plate and colliding Yakutat terrane, imply that the thrust became inactive at some time between 2 and 5 Ma. Because of the coincidence in timing between this

  19. Flood magnitude frequency and lithologic control on bedrock river incision in post-orogenic terrain

    NASA Astrophysics Data System (ADS)

    Jansen, John D.

    2006-12-01

    Mixed bedrock-alluvial rivers-bedrock channels lined with a discontinuous alluvial cover-are key agents in the shaping of mountain belt topography by bedrock fluvial incision. Whereas much research focuses upon the erosional dynamics of such rivers in the context of rapidly uplifting orogenic landscapes, the present study investigates river incision processes in a post-orogenic (cratonic) landscape undergoing extremely low rates of incision (< 5 m/Ma). River incision processes are examined as a function of substrate lithology and the magnitude and frequency of formative flows along Sandy Creek gorge, a mixed bedrock-alluvial stream in arid SE-central Australia. Incision is focused along a bedrock channel with a partial alluvial cover arranged into riffle-pool macrobedforms that reflect interactions between rock structure and large-flood hydraulics. Variations in channel width and gradient determine longitudinal trends in mean shear stress ( τb) and therefore also patterns of sediment transport and deposition. A steep and narrow, non-propagating knickzone (with 5% alluvial cover) coincides with a resistant quartzite unit that subdivides the gorge into three reaches according to different rock erodibility and channel morphology. The three reaches also separate distinct erosional styles: bedrock plucking (i.e. detachment-limited erosion) prevails along the knickzone, whereas along the upper and lower gorge rock incision is dependent upon large formative floods exceeding critical erosion thresholds ( τc) for coarse boulder deposits that line 70% of the channel thalweg (i.e. transport-limited erosion). The mobility of coarse bed materials (up to 2 m diameter) during late Holocene palaeofloods of known magnitude and age is evaluated using step-backwater flow modelling in conjunction with two selective entrainment equations. A new approach for quantifying the formative flood magnitude in mixed bedrock-alluvial rivers is described here based on the mobility of a key

  20. Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway.

    PubMed

    Spengler, Dirk; van Roermund, Herman L M; Drury, Martyn R; Ottolini, Luisa; Mason, Paul R D; Davies, Gareth R

    2006-04-13

    The buoyancy and strength of sub-continental lithospheric mantle is thought to protect the oldest continental crust (cratons) from destruction by plate tectonic processes. The exact origin of the lithosphere below cratons is controversial, but seems clearly to be a residue remaining after the extraction of large amounts of melt. Models to explain highly melt-depleted but garnet-bearing rock compositions require multi-stage processes with garnet and clinopyroxene possibly of secondary origin. Here we report on orogenic peridotites (fragments of cratonic mantle incorporated into the crust during continent-continent plate collision) from Otrøy, western Norway. We show that the peridotites underwent extensive melting during upwelling from depths of 350 kilometres or more, forming a garnet-bearing cratonic root in a single melting event. These peridotites appear to be the residue after Archaean aluminium depleted komatiite magmatism. PMID:16612379

  1. Lithotectonic framework and continental collisional reworking in the eastern part of the Sveconorwegian orogen

    NASA Astrophysics Data System (ADS)

    Andersson, Jenny; Möller, Charlotte; Stephens, Michael B.

    2013-04-01

    The eastern part of the Sveconorwegian orogen in Scandinavia exposes a transition from the foreland in the Fennoscandian Shield to the east, across a metamorphosed and uplifted parautochthonous belt (Eastern Segment) to a major crustal-scale shear belt - the Mylonite Zone - and into the overlying allochthonous belt (Idefjorden terrane). The section is characterized by medium to high P/T metamorphism of continental crust, distinctive for continental collisional orogeny. However, the timing of metamorphism and the structural build-up differ significantly between the Eastern Segment and the Idefjorden terrane. The lower tectonic levels of the Eastern Segment hosts a c. 4000 km2 semi-continuous metamorphic complex composed of amphibolite, upper amphibolite and high-pressure granulite facies orthogneisses and metabasites which record pressures of 0.8-1.2 GPa and temperatures of 680-800°C. Relict eclogite facies rocks (pressures >>1.5 GPa) occur within a large scale (>1200km^2) fold structure. Protoliths of the orthogneisses are dominantly 1.7 Ga old granites-syenitoids that can be directly linked to rocks of equivalent age and composition in the foreland, immediately east of the Sveconorwegian orogen. High- and medium-pressure metamorphism in the Eastern Segment is dated at 0.99-0.97 Ga. Regional-scale partial melting and ductile deformation with folding and stretching along E-W-trending axes took place at 0.98-0.96 Ga. Undeformed, felsic dykes that cut ductile deformational fabrics formed at 0.96-0.94 Ga. The Eastern Segment is also affected by large-scale, open and upright folding along N-S-trending axes. Dolerites intruded at 0.98-0.95 Ga; these dykes are also affected by ductile strain. A conspicuous, up to 5 km wide and ≥ 450 km long, gently west-dipping to sub-vertical, ductile shear belt - the Mylonite Zone - separates the Eastern Segment from western allochthonous belts. This shear belt accommodated transpressive deformation with reverse, top

  2. The Velay dome (French Massif Central): melt generation and granite emplacement during orogenic evolution

    NASA Astrophysics Data System (ADS)

    Ledru, P.; Courrioux, G.; Dallain, C.; Lardeaux, J. M.; Montel, J. M.; Vanderhaeghe, O.; Vitel, G.

    2001-12-01

    This paper is a synthesis of available data on the Velay dome that include both small- and large-scale lithologic and structural mapping, strain analysis, isotope geochemistry, geochronology and pressure-temperature estimates. The Velay dome, one of the largest granite-migmatite domes of the Variscan Belt, formed during orogenic collapse at around 300 Ma. Its study allows an assessment of the thermal and geodynamic context leading to voluminous crustal anatexis of the Variscan orogenic crust. A first melting stage developed in connection with south-verging thrust zones during the Early Carboniferous, leading to a crustal thickening estimated at 20 km minimum. The involvement of fertile lithologies and the intrusion of plutons of deep origin contributed to the development of water-saturated melts. The volume of biotite granite extracted from melt during this period was limited. The second phase of melting, corresponded to generalized melting of gneiss achieved by biotite-dehydration melting reactions and accompanied by the generation of cordierite-bearing granites. At this stage, crustal-scale detachment faults were active and partially obliterated the earlier structures. The new structures were progressively tilted to the vertical at the margin of the Velay dome due to the southward and lateral ballooning of the granitic dome. The reconstructed P, T path indicate that the large volume of melt produced was a consequence of a significant increase in temperature at the onset of biotite dehydration melting. At the base of the crust, this melting event is coeval with granulite facies metamorphism associated to underplating of mantle-derived magmas as suggested by the geochemical signature of Late Paleozoic lower crustal xenoliths sampled by Cenozoic volcanoes and with the isotopic signature of the late granitic intrusions. Accordingly, it is proposed that asthenospheric upwelling was responsible for the temperature increase favoring melting of hydrous minerals.

  3. Active Tectonics in crossroads of an evolving orogen and morphological consequences: Anatolia

    NASA Astrophysics Data System (ADS)

    Koral, Hayrettin

    2016-04-01

    Anatolia lies in a curved setting of the active Alpine Mountain Range and is located in crossroads of the European and Asian terrains. It is one of the fastest deforming land in the world, manifested by seismicity, characteristic landforms and GPS measurements. Active tectonics in Anatolia provides not only a comparable geological model for the past orogens, but also a laboratory case for morphological consequences of an orogenic processes. Anatolia comprise different tectonic subsettings with its own characteristics. Northern part is influenced by tectonic characteristics of the Black Sea Basin, the Pontides and the Caucasian Range; northwestern part by the Balkanides; eastern-southeastern part by the Bitlis-Zagros suture; and south-southwestern part by the eastern Mediterranean subduction setting. Much of its present tectonic complexity was inherited from the convergence dominant plate tectonic setting of the platelets prior to the Middle-Neogene. Beginning about 11 Ma ago, the deformed and uplifted landmass unable to accommodate further deformation in Anatolia and ongoing tectonic activity gave rise to rearrangement of tectonic forces and westerly translational movements. Formation of major strike-slip faults in Anatolia including the North and East Anatolian Faults and a new platelet called the Anatolian Plate are the consequences of this episode. Such change in the tectonic regime has led to modification of previously-formed landscape, modification and sometimes termination of previously-formed basins. Evidence is present in the Plio-Quaternary stratigraphy, tectonic characteristics and morphology of the well-studied areas. This presentation will discuss active tectonic features of the northwestern, southwestern and eastern Anatolian subsettings and their influence on morphology that is closely related to sites of pre-historical human settlement.

  4. Asymmetric vs. symmetric deep lithospheric architecture of intra-plate continental orogens

    NASA Astrophysics Data System (ADS)

    Calignano, Elisa; Sokoutis, Dimitrios; Willingshofer, Ernst; Gueydan, Frédéric; Cloetingh, Sierd

    2015-08-01

    The initiation and subsequent evolution of intra-plate orogens, resulting from continental plate interior deformation due to transmission of stresses over large distances from the active plate boundaries, is controlled by lateral and vertical strength contrasts in the lithosphere. We present lithospheric-scale analogue models combining 1) lateral strength variations in the continental lithosphere, and 2) different vertical rheological stratifications. The experimental continental lithosphere has a four-layer brittle-ductile rheological stratification. Lateral heterogeneity is implemented in all models by increased crustal strength in a central narrow block. The main investigated parameters are strain rate and strength of the lithospheric mantle, both playing an important role in crust-mantle coupling. The experiments show that the presence of a strong crustal domain is effective in localizing deformation along its boundaries. After deformation is localized, the evolution of the orogenic system is governed by the mechanical properties of the lithosphere such that the final geometry of the intra-plate mountain depends on the interplay between crust-mantle coupling and folding versus fracturing of the lithospheric mantle. Underthrusting is the main deformation mode in case of high convergence velocity and/or thick brittle mantle with a final asymmetric architecture of the deep lithosphere. In contrast, lithospheric folding is dominant in case of low convergence velocity and low strength brittle mantle, leading to the development of a symmetric lithospheric root. The presented analogue modelling results provide novel insights for 1) strain localization and 2) the development of the asymmetric architecture of the Pyrenees.

  5. Crustal channel flows: 2. Numerical models with implications for metamorphism in the Himalayan-Tibetan orogen

    NASA Astrophysics Data System (ADS)

    Jamieson, Rebecca A.; Beaumont, Christopher; Medvedev, Sergei; Nguyen, Mai H.

    2004-06-01

    Results from a thermal-mechanical model (HT1) that includes midcrustal channel flow are compatible with many features of the Himalayan-Tibetan system. Radioactive self-heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot, low-viscosity midcrustal channel and a broad plateau. Channel material, corresponding to the Greater Himalayan Sequence (GHS), flows outward from beneath the plateau in response to topographically induced differential pressure. At the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler, newly accreted material corresponding to the Lesser Himalayan Sequence (LHS). The model channel is bounded by coeval thrust and normal sense ductile shear zones, interpreted to represent the Main Central Thrust (MCT) zone and South Tibetan Detachment system, respectively. Inverted metamorphism associated with the model MCT zone results from distributed ductile shear along the MCT and extrusion of the hot channel. A variety of model P-T-t path styles, resembling those observed in the GHS and LHS, are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model. Predicted times of peak metamorphism, cooling, and erosion of metamorphic facies are generally compatible with observations, although model GHS cooling ages are too young. The times of M1 and M2 metamorphic "events" observed in the GHS correspond to model times of maximum burial and maximum heating, respectively. The results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens.

  6. Cross-sectional anatomy and geodynamic evolution of the Central Pontide orogenic belt (northern Turkey)

    NASA Astrophysics Data System (ADS)

    Hippolyte, J.-C.; Espurt, N.; Kaymakci, N.; Sangu, E.; Müller, C.

    2016-01-01

    Geophysical data allowed the construction of a ~250-km-long lithospheric-scale balanced cross section of the southern Black Sea margin (Espurt et al. in Lithosphere 6:26-34, 2014). In this paper, we combine structural field data, stratigraphic data, and fault kinematics analyses with the 70-km-long onshore part of the section to reconstruct the geodynamic evolution of the Central Pontide orogen. These data reveal new aspects of the structural evolution of the Pontides since the Early Cretaceous. The Central Pontides is a doubly vergent orogenic wedge that results from the inversion of normal faults. Extensional subsidence occurred with an ENE-trend from Aptian to Paleocene. We infer that the Black Sea back-arc basin also opened during this period, which was also the period of subduction of the Tethys Ocean below the Pontides. As in the Western Pontides, the Cretaceous-Paleocene subsidence was interrupted from Latest Albian to Coniacian time by uplift and erosion that was probably related to a block collision and accretion in the subduction zone. The restoration of the section to its pre-shortening state (Paleocene) shows that fault-related subsidence locally reached 3600 m within the forearc basin. Structural inversion occurred from Early Eocene to Mid-Miocene as a result of collision and indentation of the Pontides by the Kırşehir continental block to the south, with 27.5 km (~28 %) shortening along the section studied. The inversion was characterized by NNE-trending shortening that predated the Late Neogene dextral escape of Anatolia along the North Anatolian Fault and the modern stress field characterized by NW-trending compression within the Eocene Boyabat basin.

  7. 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.

  8. The Oligocene Alps: Domal unroofing and drainage development during early orogenic growth

    NASA Astrophysics Data System (ADS)

    Garzanti, Eduardo; Malusà, Marco G.

    2008-04-01

    Tertiary evolution of the Alps-Apennines orogenic couple, but also for general relationships among tectonic activity, relief formation, drainage development, erosion distribution, and long-distance sediment transfer during continental collision and orogenic growth.

  9. Groundwater flow within a sub-aerial orogenic wedge subject to depth-dependent permeability structure

    NASA Astrophysics Data System (ADS)

    Pollyea, R.; Van Dusen, E.; Fischer, M. P.

    2014-12-01

    In recent years, investigators have revisited the problem of basin-scale fluid flow with an emphasis on depth-dependent permeability, which is a frequently observed geological phenomenon that is seldom accounted for in basin-scale flow models. These recent investigations have shown that depth-dependent permeability at the basin scale strongly influences the relationship between sub-basin and regional-scale flow paths. Here, we revisit topography driven fluid flow within a foreland basin using a numerical modeling experiment designed to assess first-order fluid system behavior when permeability decreases systematically with depth. Critical taper theory is invoked to define two-dimensional basin geometry, and three sub-aerially exposed orogenic wedge models are presented with critical taper angles of 2°, 4°, and 10°. To assess the combined influence of topographic slope and depth-dependent permeability, a constant rate infiltration is applied at the wedge surface and a transient simulation is performed within each model for 500,000 years. Our results suggest that fluid system structure within the narrowly tapering orogenic wedge (2°) is explained by recent investigations applying depth-decaying permeability to the classic Tóth basin; however, increasing topographic slope beyond 3° results in a fundamentally different fluid system architecture. As topographic slope increases, fluid system structure is characterized by (1) dominant regional flow paths and little, if any, sub-basin scale fluid circulation, (2) shallow meteoric water penetration, (3) a stratified distribution of groundwater residence time without pronounced stagnation points. Moreover, for a given detachment slope, these effects become more pronounced as topographic gradient increases.

  10. Zircon dating of Neoproterozoic and Cambrian ophiolites in West Mongolia and implications for the timing of orogenic processes in the central part of the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Jian, Ping; Kröner, Alfred; Jahn, Bor-ming; Windley, Brian F.; Shi, Yuruo; Zhang, Wei; Zhang, Fuqin; Miao, Laicheng; Tomurhuu, Dondov; Liu, Dunyi

    2014-06-01

    We present new isotopic and trace element data to review the geochronological/geochemical/geological evolution of the central part of the Central Asian Orogenic Belt (CAOB), and find a fundamental geological problem in West Mongolia, which has traditionally been subdivided into northwestern early Paleozoic (formerly Caledonian) and southerly late Paleozoic (formerly Hercynian) belts by the Main Mongolian Lineament (MML). We resolve this problem with SHRIMP zircon dating of ophiolites and re-evaluation of much published literature. In Northwest Mongolia the Dariv-Khantaishir ophiolite marks the boundary between the Lake arc in the west and the Dzabkhan-Baydrag microcontinent in the east. Zircons from a microgabbro and four plagiogranites yielded weighted mean 206Pb/238U ages of 568 ± 5 Ma, 567 ± 4 Ma, 560 ± 8 Ma (Dariv), 573 ± 8 Ma and 566 ± 7 Ma (Khantaishir) that we interpret as reflecting the time of ophiolite formation (ca. 573-560 Ma). Metamorphic zircons from an amphibolite on a thrust boundary between the Khantaishir ophiolite and the Dzabkhan-Baydrag microcontinent formed at 514 ± 8 Ma, which we interpret as the time of overthrusting. In South Mongolia the Gobi Altai ophiolite and the Trans-Altai Gurvan Sayhan-Zoolen forearc with an ophiolite basement were investigated. Zircons of a layered gabbro (lower ophiolite crust) and a leucogabbro (mid-upper crust) of the Gobi Altai ophiolite yielded crystallization ages of 523 ± 5 Ma and 518 ± 6 Ma. The age data constrain the formation time of ophiolite within ca. 523-518 Ma. Zircons from four samples of the Gurvan Sayhan-Zoolen forearc, with similar hybrid adakite-boninite affinities, yielded 519 ± 4 Ma for an anorthosite, ≥ 512 ± 4 Ma for a hornblendite and 520 ± 5 and 511 ± 5 Ma for two diorites. The ophiolite basement has an upper age limit of 494 ± 6 Ma, determined by dating a tonalite dike cutting the Zoolen ophiolite. Integrating available zircon ages as well as geochemical and geological data

  11. Orogenic gold: Common or evolving fluid and metal sources through time

    NASA Astrophysics Data System (ADS)

    Goldfarb, Richard J.; Groves, David I.

    2015-09-01

    Orogenic gold deposits of all ages, from Paleoarchean to Tertiary, show consistency in chemical composition. They are the products of aqueous-carbonic fluids, with typically 5-20 mol% CO2, although unmixing during extreme pressure fluctuation can lead to entrapment of much more CO2-rich fluid inclusions in some cases. Ore fluids are typically characterized by significant concentrations of CH4 and/or N2, common estimates of 0.01-0.36 mol% H2S, a near-neutral pH of 5.5, and salinities of 3-7 wt.% NaCl equiv., with Na > K > > Ca,Mg. This fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation. Nevertheless, there are broad ranges in more robust fluid-inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths. Much of the reported isotopic and noble gas data is inconsistent between deposits, leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces. Fluid stable isotope values are commonly characterized by the following ranges: (1) δ18O for Precambrian ores of + 6 to + 11‰ and for Phanerozoic ores of + 7 to + 13‰; (2) δD and δ34S values that are extremely variable; (3) δ13C values that range from - 11 to - 2‰; and (4) δ15N of + 10 to + 24‰ for the Neoarchean, + 6.5 to + 12‰ for the Paleoproterozoic, and + 1.5 to + 10‰ for the Phanerozoic. Secular variations in large-scale Earth processes appear to best explain some of the broad ranges in the O, S, and N data. Fluid:rock interaction, particularly in ore trap areas, may cause important local shifts in the O, S, and C ratios. The extreme variations in δD mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion

  12. The Tyrrhenian stage geodinamic evolution of Apenninic-Maghrebian orogen (Southern Apennines and Sicily)

    NASA Astrophysics Data System (ADS)

    Lentini, F.; Carbone, S.; Barreca, G.

    2009-04-01

    In the Central Mediterranean region the foreland domains are represented by two continental blocks, the Apulian Block to the north and the Pelagian Block to the south, respectively belonging to the Adria and to the Africa plates. They are separated since Permo-Triassic times by the oceanic crust of the Ionian Sea. The Apenninic-Maghrebian orogen is located between two oceanic crusts: the old Ionian crust, at present time subducting beneath the Calabrian Arc, and the new crust of the opening Tyrrhenian Sea. The orogenic belt is represented by a multilayer allochthonous edifice, composed of the Calabride Chain (CC) tectonically overlying the Apenninic-Maghrebian Chain (AMC), which in turn overthrust onto the Upper Miocene and Pliocene top-levels of a deep seated thrust system, originating by the deformation of the innermost carbonates of the Pelagian/Apulian blocks (External Thrust System: ETS). The AMC tectonic units derive from the orogenic transport during Oligo-Miocene times of sedimentary sequences deposited in palaeogeographical domains located between the Europe and the Afro-Adriatic plates. These units are composed of Meso-Cenozoic shallow-water carbonate successions detached from a continental type crust sector, the Panormide/Apenninic Block, recognizable by means of seismic lines shot in the Tyrrhenian offshore of Southern Apennines and Northern Sicily. The Meso-Cenozoic basinal units, that compose the AMC, can be distinguished into two main groups of sequences, originally located on oceanic crusts separated by the Panormide/Apenninic Block: the external ones (Ionides) related to an original basin belonging to branches of the Ionian Palaeobasin involved in the orogenesis, and the internal ones ascribed to the Alpine Tethys (Sicilide Units). The terrigenous deposits of the basinal sequences belonging to the Ionides are represented by Tertiary foreland/foredeep deposits, whose relationships with the substratum are occasionally preserved, although large

  13. The boundary between the eastern and western domains of the Pyrenean Orogen: a Cenozoic triple junction zone in Iberia?

    NASA Astrophysics Data System (ADS)

    Tavani, S.

    2012-04-01

    The Cantabrian Transitional Area (CTA) is located in the eastern portion of the Cantabrian Mountain Range of the northern Spain. It represents the most important internal boundary within the Upper Cretaceous to Cenozoic E-W elongated Pyrenean Orogen. In the south-verging portion of this orogen, the CTA divides the western thick-skinned Cantabrian Domain, which accommodated for a limited portion of the total N-S oriented orogenic shortening, from the Pyrenean realm to the east, where the south-verging frontal structures are characterised by a marked thiN-Skin style of deformation, and significantly contributed to accommodate the total shortening. In the Cantabrian Transitional Area, Cenozoic syn-orogenic left-lateral, right-lateral and reverse dip-slip movements have occurred along different directions, postdating early-orogenic extensional structures. The latter indicate that the southern portion of the study area formed the eastern termination of the northward concave roughly E-W oriented proto Duero Foreland Basin. This basin was flanked to the north by the thick-skinned proto Cantabrian Belt, which included in its easternmost part the northern portion of the Cantabrian Transitional Area. Onset of right-lateral strike-slip tectonics along the WNW-ESE striking Ubiernal-Venatniella Fault System, which locates to the SW of the CTA and crosses the entire Cantabrian Belt and its formerly southern foreland basin, caused the dislocation of the belt-foredeep system. Contextually, thiN-Skinned structures belonging to the eastern domain of the Pyrenean Orogen laterally propagated and incorporated the eastern part of the proto Duero Foreland Basin. Coexistence of right-lateral and reverse movements to the west and to the east, respectively, determined the onset of an intrabelt compression at the boundary between the Cantabrian and Pyrenean domains, which was the ultimate act of the fusion of the two domains into a single orogen. Paradoxically, this fusion has basically

  14. Rivers, re-entrants, and 3D variations in orogenic wedge development: a case study of the NW Indian Himalaya

    NASA Astrophysics Data System (ADS)

    Webb, A. G.; Yu, H.; Hendershott, Z.

    2010-12-01

    Orogenic wedges are standard elements of collisional plate tectonics, from accretionary prisms to retro-arc basins. Recent study of orogenic wedge development has focused on links between mechanisms of internal deformation and surface processes. Models of orogenic wedges are commonly presented in the cross-section plane, which is generally effective as wedges largely develop via plane strain. The 3rd dimension can be utilized to explore effects of differences in controlling parameters on wedge evolution. We are investigating a stretch of the western Himalayan orogenic wedge that has two prominent changes in along-strike morphology: (1) a tectonic window (the Kullu Window) that appears to be strongly influenced by erosion along the 3rd largest river in the Himalayan system, the Sutlej River and (2) the Kangra Re-entrant, the largest re-entrant along the Himalayan arc. In addition to the along-strike heterogeneity, a key advantage of the proposed study area is its rich stratigraphy, with the most known diversity in the Himalayan arc. The stratigraphic wealth, combined with the along-strike heterogeneity in exposure level, offers a high resolution view of regional structural geometry. Our preliminary reconstructions suggest that the Sutlej River erosion increases the exposure depth and shortening budget across a narrow segment of the orogen, strongly warping the Kullu Window. Previous models have suggested that the out-of-sequence Munsiari thrust is the main structure associated with Kullu window formation, while our work suggests that most of this uplift and warping is accomplished by antiformal stacking of basement thrust horses. Late Miocene ages (U-Pb ages of zircons and Th-Pb ages of monazites) from a leucogranite in the core of the Kullu Window along the Sutlej River further suggests that this segment of the orogen represents a middle ground between plane strain orogenic wedge development and a tectonic aneurysm model. We have constructed a palinspastic

  15. On protolith-, metamorphic overprint, microstructure and rheology of mineral assemblages in orogenic peridotites of the central Scandinavian Caledonides

    NASA Astrophysics Data System (ADS)

    Gilio, Mattia; Clos, Frediano; Van Roermund, Herman L. M.

    2013-04-01

    The Scandinavian Caledonides (SC) are a deeply eroded Alpine-type orogenic belt formed by closure of the Iapetus ocean and collision between Baltica and Laurentia (500-380 Ma). The SC consists of a stack of Nappe Complexes (from bottom to top called Lower, Middle, Upper and Uppermost Allochthons) thrusted to the east over the Baltic Shield (Brueckner and Van Roermund, 2004; Gee et al., 2008). Fossil lithospheric mantle fragments, called orogenic peridotites, have been found within the (upper part of) middle, upper and uppermost Allochthons, as well as in the reworked basement gneisses (a.o Western Gneiss Complex (WGC)) along the Norwegian west coast. They occur as isolated lenses that contain diverse mineral parageneses and/or bulk rock compositions. Crustal incorporation of orogenic peridotite is classically interpreted to be the result of plate collisional processes related to orogeny (Brueckner and Medaris, 2000). The WGC and parts of the upper part of the Middle Allochthon (a.o. Seve Nappe Complex (SNC) in N Jämtland/S Västerbotten, central Sweden), are well known for the occurrence of high (HP) and ultrahigh pressure (UHP) metamorphic terranes (of Caledonian age). The (U)HPM evidence clearly demonstrates the deep metamorphic origin of these rocks interpreted to be caused by continental subduction and/or collision. Other metamorphic rocks (of Caledonian age) exposed in allochthonous nappes are solely characterised by greenschist-, amphibolite- and/or MP granulite "facies" mineral assemblages that can be interpreted, in the absence of retrogression, to have formed in less deeply subducted (and/or metamorphic) environments. This duality in metamorphic "facies" allows for a discrimination (at least theoretically) between "deep" versus "shallow" rooted nappes (in central parts of the Scandinavian Caledonides). Conform this reasoning, this duality should also be present within the Caledonian mineral assemblages (= metamorphic overprint) of orogenic peridotites (in

  16. U Pb, Hf and O isotope evidence for two episodes of fluid-assisted zircon growth in marble-hosted eclogites from the Dabie orogen

    NASA Astrophysics Data System (ADS)

    Wu, Yuan-Bao; Zheng, Yong-Fei; Zhao, Zi-Fu; Gong, Bing; Liu, Xiaoming; Wu, Fu-Yuan

    2006-07-01

    A combined study of internal structure, U-Pb age, and Hf and O isotopes was carried out for metamorphic zircons from ultrahigh-pressure eclogite boudins enclosed in marbles from the Dabie orogen in China. CL imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain, respectively. The metamorphic zircons have low Th and U contents with low Th/U ratios, yielding two groups of 206Pb/ 238U age at 245 ± 3 to 240 ± 2 Ma and 226 ± 4 to 223 ± 2 Ma, respectively. Anomalously high δ 18O values were obtained for refractory minerals, with 9.9 to 21.4‰ for garnet and 16.9‰ for zircon. This indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction-zone metamorphism. Most of the zircons are characterized by very low 176Lu/ 177Hf ratios of 0.000001-0.000028, indicating their growth in association with garnet recrystallization. A few of them falling within the older age group have comparatively high 176Lu/ 177Hf ratios of 0.000192-0.000383, suggesting their growth prior to the formation of garnet in the late stage of subduction. The variations in the Lu/Hf ratios for zircons can thus be used to correlate with garnet growth during eclogite-facies metamorphism. In either case, the zircons have variable ɛHf ( t) values for individual samples, suggesting that their protolith is heterogeneous in Hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle. Nevertheless, a positive correlation exists between 206Pb/ 238U ages and Lu-Hf isotope ratios for the metamorphically recrystallized zircons, suggesting that eclogite-facies metamorphism in the presence of fluid has the identical effect on zircon Lu-Hf and U-Th-Pb isotopic systems. We conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh-pressure metamorphism, whereas the younger zircons

  17. Sediment budgets by detrital geochronology and new perspectives in understanding orogenic erosion (solicited)

    NASA Astrophysics Data System (ADS)

    Malusà, Marco Giovanni

    2014-05-01

    Low-temperature thermochronological data provide an estimate of the average exhumation rate from the partial retention zone to the surface. The timescale the rate is referred to largely depends on the distribution of thermochronological ages in the study area, which may be inadequate to constrain fast orogenic processes. New perspectives to investigate orogenic erosion and its linkage with climate and tectonics are provided by quantitative sediment budgets based on detrital geochronological analyses, which allows a much greater time resolution chiefly depending on available biostratigraphic constraints. Detrital samples derived from the mixing of geochronologically distinguishable detrital sources are expected to yield polymodal grain-age distributions that include different grain-age populations. The relative size between populations depends on (i) the relative size of the catchment areas, (ii) the mineral fertility of the parent bedrock, and (iii) the erosion rate in each catchment. If we know two of these factors, then we can calculate the third (Resentini & Malusà, 2012). A reliable approach to mineral fertility quantification thus represents a fundamental prerequisite for any sediment budget based on single-mineral data. In a temperate/cold climate where chemical weathering is minor, mineral fertility in the source rocks can be effectively determined by measuring the mineral concentration in the sediment they produce, provided that a range of potential sources of bias are properly accounted for and minimized (Malusà et al., 2013). Specifically, this approach requires that no significant modification has been induced by hydrodynamic processes in the natural environment (as tested by basic principles of hydraulic sorting), and that no bias is later introduced during mineral separation. Orogen-scale fertility maps are thus easily produced for different minerals thanks to the analysis of a reasonably low number of modern sand samples. Potentials and pitfalls of

  18. Coupled delamination and indentor-escape tectonics in the southern part of the c. 650-500 Ma East African/Antarctic Orogen

    NASA Astrophysics Data System (ADS)

    Jacobs, J.; Thomas, R. J.; Ueda, K.; Kleinhanns, I.; Emmel, B.; Kumar, R.; Engvik, A.; Bingen, B.

    2009-12-01

    The East African/Antarctic Orogen (EAAO) is one of the largest orogenic belts on the planet, resulting from the collision of various parts of East and West- Protogondwana between 620 and 550 Ma. The central and southern parts of the orogen are typified by high-grade rocks, representing the overprinted margins of the various colliding continental blocks. The southern third of this Himalayan-type orogen can be interpreted in terms of a lateral tectonic escape model, similar to the situation presently developing in SE-Asia. One of the escape-related shear zones of the EAAO is exposed as the approximately 20 km wide Heimefront transpression zone in western Dronning Maud Land (Antarctica). During Gondwana break-up, the southern part of the EAAO broke up into a number of microplates (Falkland, Ellsworth-Haag and Filchner blocks). These microplates probably represent shear zone-bound blocks, which were segmented by tectonic translation during lateral tectonic extrusion. The southern part of the EAAO is also typified by large volumes of late-tectonic A2-type granitoids that intruded at c. 530-490 Ma, and can constitute up to 50% of the exposed basement. They are likely the consequence of delamination of the orogenic root and the subsequent influx of hot asthenospheric mantle during tectonic escape. The intrusion of these voluminous melts into the lower crust was accompanied by orogenic collapse. The A2-type magmatism appears to terminate along the Lurio Belt in northern Mozambique. Therefore, the Lurio Belt could represent an accommodation zone, separating an area to the south in which the orogen underwent delamination of the orogenic root, and an area to the north, where the orogenic keel is still present. Erosional unroofing of the northern EAAO is documented by the remnants of originally extensive areas covered by Cambro-Ordovician molasse-type clastic sedimentary rocks throughout North Africa and Arabia, testifying to the size of this mega-orogen. Whilst the EAAO

  19. Kinematics of back-arc extension driven by the interference of subduction and/or collisional zones: application to a number of Mediterranean orogens

    NASA Astrophysics Data System (ADS)

    Matenco, L.; Cloetingh, S.

    2013-12-01

    The large number and distribution of roll-back systems in Mediterranean orogens infers the possibility of interacting extensional back-arc deformation driven by different slabs. Classical models of orogenic evolution assume that such back arc basins form in the hinterland of orogens, collapsing the upper plate above oceanic subduction zones. This is a common characteristic thought to apply to all low-topography orogens of Mediterranean type driven by the fast roll-back of genetically related slabs, or to other analogues such as the Miocene to recent evolution of the SE Asia subduction zones. This extension may take place far at the interior of the upper plate, as is the case in various segments of the Carpathians or in the core of the SE Asian domain, but in most cases of the Dinarides, Apennines or Hellenides it take place superposed or far into the foreland when compared with the position of oceanic suture zones. Mediterranean orogens often diverge from the typical scenario by widespread extensional deformation taking place during moments of continent-continent collision and by the interference of such deformation driven by different subduction zones. For instance, the formation of the Pannonian back-arc basin is generally related to the rapid Miocene roll-back of a slab attached to the European continent. The present-day extensional geometry of the Pannonian back-arc formed essentially during the Carpathians collision and was also driven by an additional Middle Miocene roll-back of a Dinaridic slab. In other orogenic systems, the back-arc extension takes place during continent-continent collision along major detachments that are located in the core of the orogen (Rif, Betics), in the accreted crustal material of the lower plate (Apennines, Dinarides) or even in a presumed former fore-arc (Aegean, Sunda-Banda arc). In all these subduction zones, collision has largely duplicated crustal blocks from the lower plate and has gradually shifted subduction zone far

  20. Sources of ore-forming fluids and formation environments of orogenic Au deposits in the Main Uralian Fault zone (Southern Urals)

    NASA Astrophysics Data System (ADS)

    Znamenskii, S. E.; Puchkov, V. N.; Michurin, S. V.

    2015-09-01

    The analysis of stable S, C, and O isotopes in minerals combined with the results of structural studies of orogenic gold deposits in carbonaceous shales of the Main Uralian Fault in the South Urals reveals that orogenic gold mineralization was formed during two stages of Late Paleozoic collisional deformations: early (thrust formation) and late (wrench faulting). The leading role in hydrothermal ore-forming systems of the first stage belonged to fluids of metamorphic origin, while at the second sage they were magmatogenic.

  1. Clinoform deposition across a boundary between orogenic front and foredeep - an example from the Lower Cretaceous in Arctic Alaska

    USGS Publications Warehouse

    Houseknecht, David W.; Wartes, Marwan A.

    2013-01-01

    The Lower Cretaceous Fortress Mountain Formation occupies a spatial and temporal niche between syntectonic deposits at the Brooks Range orogenic front and post-tectonic strata in the Colville foreland basin. The formation includes basin-floor fan, marine-slope and fan-delta facies that define a clinoform depositional profile. Texture and composition of clasts in the formation suggest progressive burial of a tectonic wedge-front that included older turbidites and mélange. These new interpretations, based entirely on outcrop study, suggest that the Fortress Mountain Formation spans the boundary between orogenic wedge and foredeep, with proximal strata onlapping the tectonic wedge-front and distal strata downlapping the floor of the foreland basin. Our reconstruction suggests that clinoform amplitude reflects the structural relief generated by tectonic wedge development and load-induced flexural subsidence of the foreland basin.

  2. Late Cenozoic orogenic history of Western Qinling inferred from sedimentation of Tianshui basin, northeastern margin of Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Wang, Xiuxi; Li, Jijun; Song, Chunhui; Zattin, Massimiliano; Zhao, Zhijun; Zhang, Jun; Zhang, Yong; He, Kuang

    2012-07-01

    The Western Qinling orogenic belt marks the northeastern margin of the Tibetan Plateau. Its late Cenozoic orogenic history is recorded in an excellent sedimentary sequence exposed in the Tianshui sub-basin of the Longzhong basin. According to the magnetostratigraphic analysis from the Yaodian and Lamashan sections, we speculate that the late Cenozoic Tianshui basin accumulated lacustrine/floodplain deposits from ~14.8 to ~2.6 Ma. In addition, detrital apatite fission-track thermochronologic and paleocurrent data reveal that the detritus of the Tianshui basin mostly derived from the Western Qinling and that the youngest population age represents a ~14 Ma volcanic intrusion, which can be related to the lithospheric deformation and uplift of the Tibetan Plateau. Furthermore, two stages of variations in depositional facies and average accumulation rates were attributed to the pulse uplift and deformation of the Western Qinling at 9.2-7.4 and ~3.6 Ma.

  3. Tale of three cratons: Tectonostratigraphic anatomy of the Damara orogen in northwestern Namibia and the assembly of Gondwana

    NASA Astrophysics Data System (ADS)

    Prave, Anthony R.

    1996-12-01

    The Neoproterozoic Damara orogen in Namibia records the Gondwanan assembly of the Congo Kalahari Rio de la Plata cratons. Sedimentological and stratigraphic analyses of the Otavi (mostly carbonate) and Mulden (siliciclastic molasse) Groups exposed along the southwestern margin of the Congo craton indicate that foreland-basin orogenesis began during middle Otavi (ca. 600 750 Ma); this time frame is earlier than that for the Nama foreland (ca. 550 Ma) on the Kalahari craton. Evidence for this interpretation includes (1) an intra-Otavi angular unconformity; (2) concomitant development of increasing accommodation space, basin segmentation, and backstepping depositional systems composed mostly of deep-basin and slope hemipelagic and sediment gravity-flow deposits; and (3) a near reversal in paleocurrents (initially northward off the Congo craton, then switching to southeastward from a west-northwestern orogen). Thus, Congo Rio de la Plata suturing predated Congo-Kalahari suturing during the assembly of Gondwana.

  4. Upper Messinian conglomerates in Calabria, southern Italy: Response to orogenic wedge adjustment following Mediterranean sea-level changes

    NASA Astrophysics Data System (ADS)

    Decelles, P. G.; Cavazza, W.

    1995-09-01

    Widespread uppermost Miocene conglomerate and sandstone along the Apenninic-Maghrebian orogenic belt in the central Mediterranean region cannot be explained as a result of the Messinian base-level falls. Along the Ionian coast of Calabria, southern Italy, these rocks were deposited in marine fan deltas and rest in angular unconformity or disconformity upon the internal part of the Calabrian accretionary wedge. We propose that the upper Messinian deposits were produced by internal shortening of the Calabrian accretionary wedge as it compensated for the decrease in upper surface slope caused by flexural rebound as the ˜3.4-km-thick Ionian water mass evaporated. Latest Miocene-Pliocene marine inundation reloaded the basin, restored the wedge to a critical state, and caused the rear part of the wedge again to become tectonically stable. This isostatically driven mechanism could explain widespread latest Messinian thrust faults and coarse siliciclastic deposits along much of the Apenninic-Maghrebian orogen.

  5. Modelling temporal gravity changes through the south of the Taiwan Orogen

    NASA Astrophysics Data System (ADS)

    Mouyen, Maxime; Masson, Frédéric; Mouthereau, Frédéric; Hwang, Cheinway; Cheng, Ching-Chung

    2010-05-01

    The Taiwan mountain belt results from the collision between Philippine Sea and Eurasian plates. Taiwan island experiences high tectonic deformation due to fast convergence between the two plates. It has been and is still widely studied and is often considered as a natural laboratory for orogeny studies. Since 2006, the French-Taiwanese scientific project AGTO (Absolute Gravity in the Taiwanese Orogen) measures the gravity change along a transect through the south of the island. It includes 10 absolute and 45 relative gravity measurements sites. The aim of this project is to validate the use of temporal gravity data for tectonic purposes. In particular, this method should be interesting to monitor deep mass transfers involved in the Taiwanese orogeny. Deep tectonic processes occuring in Taiwan are indeed still discussed, as shown by the existence of several tectonic hypotheses, and gravity can bring useful contribution to this discussion. The value of g in a particular place physically depends on the density distribution around this place. Change of this density distribution will result in a change of g, to which we try to give a tectonic meaning. However it is worth noting that other factors, like hydrology, might also be responsible for temporal g variations. Gravity modelling should therefore provide significant help in interpreting measurements. First, it can be used to estimate non-tectonic factors like hydrology, erosion or landslides, which both are supposed to modify g value through time. Albeit interesting, these effects must be properly removed from our measures before attempting any tectonic interpretation. Second, modelling is a valuable step in this study as it can help to propose deep mass transfers hypothesis constrained by gravity data and in accordance with Taiwan tectonic context. In this work, we present results of both types, computed for the south of the Taiwan orogen. Water effects on gravity have been estimated using rainfall data and global

  6. Moho depth model from GOCE gravity gradient data for the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Guy, Alexandra; Holzrichter, Nils; Ebbing, Jörg

    2016-04-01

    GOCE gravity gradient data are used together with published seismic data to determine the Moho geometry and the isostatic state of the Central Asian Orogenic Belt (CAOB). The CAOB is an accretionary orogen formed during the Palaeozoic at the periphery of the Siberian cratonic nucleus by the successive amalgamation of different types of crust (cratonic, oceanic, passive margin, magmatic arc, back-arc, ophiolites, accretionary wedge) followed by an oroclinal bending during Permian-Triassic times. This large area was and is still of great interest for geoscientific studies mainly because of its potential in mineral and fossil resources and also for its outstanding, but still misunderstood, geodynamic evolution. However, the geophysical investigations remain scarce due to the remoteness of the area. A systematic analysis of the crustal thickness has been omitted yet, although the geometry of the crust-mantle boundary (Moho boundary) provides crucial information on the evolution of the lithosphere and on the coupling between upper mantle and the crust - particularly interesting for oroclinal bending processes. In this study, the gravity gradient data of GOCE are used to investigate the topography of the Moho for Mongolia and its surroundings. In addition, we used inversion of gravity data and calculation of the isostatic Moho from topographic data to the World Gravity Map (WGM) 2012 satellite-terrestrial model of the Earth's gravity anomalies and these results are compared together with those obtained for the GOCE gravity data. The results of the gravity inversion are constrained by the few xenolith studies and the seismic data available: the receiver function seismic method for north and central Mongolia, deep seismic sounding and seismic reflection profiles in northern China; and tomography in southern Siberia. Then, the effects of isostatic compensation are evaluated by the comparison between the results of the gravity inversion and the isostatic Moho. Finally, a 3D

  7. Polyphase deformation of a Paleozoic metamorphosed subduction-accretionary complex in Beishan Orogen, southern Altaids

    NASA Astrophysics Data System (ADS)

    Song, Dongfang; Xiao, Wenjiao

    2014-05-01

    The Lebaquan Complex in central Beishan plays a significant role in understanding the subduction-accretion-collision processes of the Beishan orogenic collage, southern Altaids. This complex is a polyphase deformed upper greenschist to lower amphibolite facies metamorphic rock assemblages composed of metasedimentary sequence, gneissic plutons, metacherts, amphibolites and marbles, with multiple generations of syn-tectonic leucogranite-pegmatite and post-tectonic mafic dykes. Lithologic and geochemical characteristics show the Lebaquan Complex is a forearc-arc complex. Four stages of deformation can be recognized for the Lebaquan Complex. The D1 deformation is defined by pervasively developed foliations (S1) defined by alternative metamorphic layering and intrafolia isoclinal folds of quartz veins (F1). The D2 deformation is indicated by crenulation cleavage (S2) mainly developed in the schists, extensively-developed tight to open folds and asymmetric folds (F2) and pinch and swell structures. The D3 deformation is characterized by high-strain ductile shearing, which modified earlier structures. Small-scale asymmetric folds, σ-type porphyroclasts and S-C fabrics indicate dextral sense of shearing in east-west direction. The D4 deformation is represented by ductile to brittle structures including open/gentle folds, kink folds and small scale thrust faults which overprint earlier deformations. The overall deformation of this complex indicates a geodynamic setting change from initial north-south directed strong compression to later east-west directed transpression and finally extension. LA-ICP-MS zircon U-Pb dating was performed on key lithologies. Youngest age peak of detrital zircons from a garnet-mica-quartz schist is ~424 Ma, constraining the depositional age for the protolith of the metasedimentary sequence. A syn-deformation leucogranitic dyke that underwent D2 but did not experience D1 and a post-deformation mafic dyke that intrudes all the lithologies were

  8. Crustal recycling through intraplate magmatism: Evidence from the Trans-North China Orogen

    NASA Astrophysics Data System (ADS)

    He, Xiao-Fang; Santosh, M.

    2014-12-01

    The North China Craton (NCC) preserves the history of crustal growth and craton formation during the early Precambrian followed by extensive lithospheric thinning and craton destruction in the Mesozoic. Here we present evidence for magma mixing and mingling associated with the Mesozoic tectonic processes from the Central NCC, along the Trans-North China Orogen, a paleo suture along which the Eastern and Western Blocks were amalgamated at end of Paleoproterozoic. Our investigations focus on two granitoids - the Chiwawu and the Mapeng plutons. Typical signatures for the interaction of mafic and felsic magmas are observed in these plutons such as: (1) the presence of diorite enclaves; (2) flow structures; (3) schlierens; (4) varying degrees of hybridization; and (5) macro-, and micro-textures. Porphyritic feldspar crystals show numerous mineral inclusions as well as rapakivi and anti-rapakivi textures. We present bulk chemistry, zircon U-Pb geochronology and REE data, and Lu-Hf isotopes on the granitoids, diorite enclaves, and surrounding basement rocks to constrain the timing of intraplate magmatism and processes of interaction between felsic and mafic magmas. Our LA-ICP-MS zircon U-Pb data show that the pophyritic granodiorite was emplaced at 129.7 ± 1.0 Ma. The diorite enclaves within this granodiorite show identical ages (128.2 ± 1.5 Ma). The basement TTG (tonalite-trondhjemite-granodiorite) gneisses formed at ca. 2.5 Ga coinciding with the major period of crustal accretion in the NCC. The 1.85 Ga age from zircons in the gabbro with positive Hf isotope signature may be related to mantle magmatism during post-collisional extension following the assembly of the Western and Eastern Blocks of the NCC along the Trans-North China Orogen. Our Hf isotope data indicate that the Neoarchean-Paleoproterozoic basement rocks were derived from complex sources of both juvenile magmas and reworked ancient crust, whereas the magma source for the Mesozoic units are dominantly

  9. Magnetic signatures of the orogenic crust of the Patagonian Andes with implication for planetary exploration

    NASA Astrophysics Data System (ADS)

    Díaz Michelena, Marina; Kilian, Rolf

    2015-11-01

    The Patagonian Andes represent a good scenario of study because they have outcrops of diverse plutonic rocks representative of an orogenic crust on Earth and other planets. Furthermore, metamorphic surface rocks provide a window into deeper crustal lithologies. In such remote areas, satellite and aerial magnetic surveys could provide important geological information concerning exposed and not exposed rocks, but they integrate the magnetic anomalies in areas of kilometres. For the southernmost Andes long wavelength satellite data show clear positive magnetic anomalies (>+100 nT) for the Patagonian Batholith (PB), similar as parts of the older martian crust. This integrated signal covers regions with different ages and cooling histories during magnetic reversals apart from the variability of the rocks. To investigate the complex interplay of distinct magnetic signatures at short scale, we have analysed local magnetic anomalies across this orogen at representative sites by decimeter-scale magnetic ground surveys. As expected, the investigated sites have positive and negative local anomalies. They are related to surface and subsurface rocks, and their different formation and alternation processes including geomagnetic inversions, distinct Curie depths of the magnetic carriers, intracrustal deformation among other factors. Whole rock chemistry (ranging from 45 to >80 wt.% SiO2 and from 1 to 18 wt.% FeOtot.), magnetic characteristics (susceptibilities, magnetic remanence and Königsberger ratios) as well as the composition and texture of the magnetic carriers have been investigated for representative rocks. Rocks of an ultramafic to granodioritic intrusive suite of the western and central PB contain titanomagnetite as major magnetic carrier. Individual magnetic signatures of these plutonic rocks reflect their single versus multidomain status, complex exolution processes with ilmenite lamella formations and the stoichiometric proportions of Cr, Fe and Ti in the oxides. At

  10. A Late Cretaceous Orogen Triggering the Tertiary Rifting of the West Sunda Plate; Andaman Sea Region

    NASA Astrophysics Data System (ADS)

    Sautter, B.; Pubellier, M. F.; Menier, D.

    2015-12-01

    Rifted Basins often develop in internal zones of orogenic belts, although the latter may not be easy to unravel. We chose the example of the super-stretched Andaman sea region affected by several stages of rifting in the internal zone of a composite collage of allochthonous terranes. We made use of a set of geophysical, geochronological and structural data to analyze the rifting evolution and reconstruct the previous compressional structures. - Starting in the late Oligocene the East Andaman Basin opened as a back arc in a right-lateral pull- apart. The rifting propagated Westward to the central Andaman basin in the Middle Miocene, and to the oceanic spreading stage in the Pliocene. - An early extension occurred in the Paleogene, marked by widespread opening of isolated continental basins onshore Malay Peninsula and offshore Andaman Shelf and Malacca Straits. The rifting was accommodated by LANF's along preexisting weakness zones such as hinges of folds and granitic batholiths. Continuous extension connected the isolated basins offshore, whereas onshore, the grabens remained confined. There, AFT data show an uplift phase around 30Ma. In the Late Cretaceous, a major deformation occurred oblique to the pre-existing Indosinian basement fabrics. The convergence was partitioned into thrusting and uplift of the Cretaceous volcanic arc in Thailand and Myanmar, inversion of Mesozoic basins, and coeval wrenching responsible for large phacoid-shaped crustal slivers bounded by wide strike slip fault zones. The slivers share similar characteristics: a thick continental core of lower Paleozoic sedimentary basins units surrounded by Late Cretaceous granitoids. Radiometric data and fission tracks indicate a widespread thermal anomaly in all West Sunda Plate synchronous to a strong uplift. In the Latest Mesozoic, the Western Margin of Sunda plate was subjected to a major E-W compression, accommodated by oblique conjugate strike slip faults, leading to the formation of a large

  11. Continental Construction and Crustal Growth of Central Asian Orogenic Belt in Mongolia

    NASA Astrophysics Data System (ADS)

    Schulmann, K.; Guy, A.; Lehmann, J.; Seltmann, R.; Lexa, O.

    2013-12-01

    The geophysical and geochemical data document crustal growth and continent construction processes responsible for episodic formation of Central Asian Orogenic Belt (CAOB) crust in Mongolia. Here, well-defined Cambrian suture rims the western borders of Proterozoic continental fragments in Mongolia and Russia (1200 to 700 Ma). This arcuate suture zone is defined by numerous ophiolites (800-560 Ma) originated by Early Cambrian obduction of young (Late Proterozoic) oceanic basins and magmatic arcs. The juvenile crust further south is built by gneisses a metasediments dated at 530-470 Ma interpreted as a Cambro-Ordovician accretionary prism intruded by juvenile Devono-Carboniferous Japan type magmatic arcs. The southerly mantle fragments covered by Silurian cherts, Devonian basaltic volcanics and late Devonian volcanoclastic sediments are interpreted as an oceanic crust of back arc and oceanic arc affinity. Following scenario is proposed: 1) Proterozoic basement blocks formed N-S trending zone documented by the shape of Early Cambrian suture, 2) Devono-Carboniferous magmatic arcs were emplaced during E-W shortening event along western margin of this zone leading to moderate crustal shortening, 3) the intra oceanic ophiolites were thrust over Devonian volcanoclastics during Late Carboniferous in the E-W direction. All that suggests that the early Devonian (Phillipiny sea type) back arc spreading was replaced by 50 to 80 My lasting E-W compressive regime during prolonged activity of north trending Mongol-Okhotsk Pacific type subduction zone. The change in tectonic movements from E-W to N-S is responsible for oroclinal bending of trail of Mongolian microcontinents and scissor-like closure of Mongol Okhotsk (Pacific ocean embayment) ocean. We conclude that both crustal growth and crustal construction are episodic in the CAOB. The former process occurred in two main episodes related to Pacific subduction, while the continental construction is a complex sequence of two peri

  12. Modelling late-orogenic collapse: A combined thermo-mechanical and chemical approach

    NASA Astrophysics Data System (ADS)

    Kaislaniemi, Lars; van Hunen, Jeroen; Bouilhol, Pierre; Allen, Mark B.

    2015-04-01

    We have explored the geodynamics of late-orogenic collapse via numerical models to study the effects of lithospheric thinning on mantle and lower crustal melting. Our goal is to better understand the formation of migmatitic terrains and the role of mantle melt in generating the geochemical spectrum of granitoids observed during the lithospheric re-equilibration. These numerical models combine finite elements thermo-mechanical modelling coupled with a Gibbs free energy minimization strategy that allow tracking melt composition. Our models assume a "jelly sandwich" strength profile of the lithosphere and the existence of sub-lithospheric small-scale convection caused by the increased post-subductional mantle water contents. The models take into account the viscosity lowering effects of water and partial melts, and dynamically adjust the composition of the mantle and crustal residue after extraction of partial melts. Effects of mantle magmas underplating the crust have been studied. Using Gibbs energy minimization allows for precise tracking of the crustal melt composition (major oxides) as a function of time and location. Our results show that minor lithosphere thinning causing low degree lower crustal melting can grow into large scale lithospheric mantle delamination via a positive feedback mechanism between the thinning of the lithosphere and the strength weakening by the partial melts in the lower crust. The melt percolation threshold (percentage of melt at which melts are extracted) affects this feedback mechanism: Extracting melts disable the weakening effect by the partial melts, but also removes most of the water in the source, leaving behind a depleted high viscosity residue layer. For this reason we also found that underplating of mantle melts below the crust initially enhances the positive feedback in lithosphere thinning, but overall has no major - or has even negative - impact on the lithosphere thinning and crustal melting. If asthenospheric melts are

  13. Quantifying transient erosion of orogens with detrital thermochronology from syntectonic basin deposits

    NASA Astrophysics Data System (ADS)

    Rahl, Jeffrey M.; Ehlers, Todd A.; van der Pluijm, Ben A.

    2007-04-01

    The evolution of an orogen is marked by phases of topographic growth, equilibrium, and decay. During these phases erosion rates vary in response to temporal and spatial changes in climate, topographic relief and slope, and deformation. Detrital thermochronometer cooling-age data collected from syntectonic basin deposits are a promising tool for quantifying erosion histories during orogenic evolution. Previous studies typically assume steady-state erosion for interpreting detrital data, although in many situations this assumption is not justified. Here we present a new numerical modeling approach that predicts thermochronometer cooling ages in a stratigraphic section where sediment is sourced from a region with a temporally variable erosion history. Multiple thermochronometer cooling ages are predicted at different stratigraphic horizons as a function of variable erosion histories, rock cooling rates in the hinterland, and thermophysical material properties and boundary conditions. The modeling approach provides the context for the interpretation of natural data, including geologically realistic situations with a temporally varying erosion rate. The results of three end-member hinterland erosion histories are explored: (1) steady-state erosion; (2) increasing erosion rate with time; and (3) decreasing erosion rate with time. Results indicate that for steady erosion rates between 0.2 and 1.0 mm/yr, up to 30 m.y. will pass following a change in erosion rate before the detrital ages have adjusted to reflect a new erosion regime. In simulations with transient erosion, the estimation of erosion rates from a detrital record using assumption of thermal steady-state will generally be in error, often by as much as - 25 to 100%. These results highlight that assumptions of steady erosion in mountain belts should be used with caution. Application of the model to thermochronometer cooling ages preserved in syntectonic sediments sourced from the Nanga Parbat region, Himalaya

  14. Syn-Uralian orogenic heavy mineral provenance analysis from southeastern Taimyr, Arctic Russia

    NASA Astrophysics Data System (ADS)

    Zhang, X.; Pease, V.; Scott, R. A.

    2012-12-01

    The Taimyr peninsula is on the northern margin of the Siberian craton and divides into the southern, central and northern NE - SW trending domains. The southern Taimyr domain represents the passive margin of Siberia and is dominated by a Paleozoic, extending into the early Mesozoic, succession. The central Taimyr domain accreted to the southern domain during Late Precambrian time, followed by collision with the northern Taimyr domain of Baltican affinity during the Late Paleozoic as part of Uralian orogenesis. The Carboniferous - Permian sedimentary succession, which was deposited during the later stage of Uralian Orogeny, can provide crucial information about the extent of contemporaneous Uralian orogensis and its influence on the tectonic evolution of southern Taimyr. Three Carboniferous - Permian samples from southeastern Taimyr were analyzed for petrography and heavy mineral analysis to define their sedimentary and provenance characteristics. The Upper Carboniferous to Lower Permian sample (VP10-25, Turuzovskya Formation, C2-P1tr), the Lower Permian sample (VP10-14, Sokolinskaya Formation, P1sk) and the Upper Permian sample (VP10-12, Baykurskaya Formation, P2bk)) classify as subarkose, lithic arkose and feldspathic litharenite, respectively- they record decreasing sediment maturity through time. While all the samples represent a 'recycled orogen' source based on QtFL plots, the C2-P1tr sample represents a recycled quartzite, while the P1sk sample plots within the mixed field, and the P2bk sample is transitional on QmFLt plots. According to the heavy mineral analysis results, the C2-P1tr sample and P1sk sample show great similarity in heavy mineral assemblage, dominated by zircon, apatite and rutile. The P2bk sample shows distinct differences, containing apatite, tourmaline, garnet and zircon. The prominent increase of garnet suggests a metamorphic source. These similarities and variations among the three samples are also shown in other discrimination diagrams

  15. Early Mesozoic granitoid and rhyolite magmatism of the Bureya Terrane of the Central Asian Orogenic Belt: Age and geodynamic setting

    NASA Astrophysics Data System (ADS)

    Sorokin, A. A.; Kotov, A. B.; Kudryashov, N. M.; Kovach, V. P.

    2016-09-01

    Early Mesozoic granitoids and volcanic rocks are widespread throughout the structures of all of the continental massifs in the eastern part of the Central Asian Orogenic Belt, although its tectonic setting is not yet clear. Generally, they are associated with subduction and plume processes or rifting. Such uncertainty is mostly explained by the unequal investigation of Early Mesozoic magmatism. This paper presents the results of geochemical, Sm-Nd isotope, and U-Pb geochronologic (ID-TIMS) studies of "key-type" Early Mesozoic magmatic rock complexes of the Bureya Terrane. This is one of the largest continental massifs in the eastern Central Asian Orogenic Belt and knowledge of its geological structure is of fundamental importance in understanding the history of its formation. It has been established that the leucogranites of the Altakhtinsky Complex and the trachyrhyolites of the Talovsky Complex are practically coeval (~ 209-208 Ma). The subalkaline leucogranites of the Kharinsky Complex have a slightly younger age of ~ 199 Ma. These data correspond to the general stage of Early Mesozoic magmatic and metamorphic events (236-180 Ma) in most continental massifs in the eastern Central Asian Orogenic Belt. We believe that large-scale Early Mesozoic events were related to the amalgamation of the continental massifs of the eastern Central Asian Orogenic Belt into a single continental structure (the Amur superterrane or microcontinent Amuria) and collision with the North Asian Craton. It should be noted that the collision processes were followed by crustal thickening, thus creating the conditions for metamorphism and formation of magmatic rock complexes of various geochemical types.

  16. Role of Neogene Exhumation and Sedimentation on Critical-Wedge Kinematics in the Zagros Orogenic Belt, Northeastern Iraq, Kurdistan

    NASA Astrophysics Data System (ADS)

    Koshnaw, R. I.; Horton, B. K.; Stockli, D. F.; Barber, D. E.; Tamar-Agha, M. Y.; Kendall, J. J.

    2014-12-01

    The Zagros orogenic belt and foreland basin formed during the Cenozoic Arabia-Eurasia collision, but the precise histories of shortening and sediment accumulation remain ambiguous, especially at the NW extent of the fold-thrust belt in Iraqi Kurdistan. This region is characterized by well-preserved successions of Cenozoic clastic foreland-basin fill and deformed Paleozoic-Mesozoic hinterland bedrock. The study area provides an excellent opportunity to investigate the linkage between orogenic wedge behavior and surface processes of erosion and deposition. The aim of this research is to test whether the Zagros orogenic wedge advanced steadily under critical to supercritical wedge conditions involving in-sequence thrusting with minimal erosion or propagated intermittently under subcritical condition involving out-of-sequence deformation with intense erosion. These endmember modes of mountain building can be assessed by integrating geo/thermochronologic and basin analyses techniques, including apatite (U-Th)/He thermochronology, detrital zircon U-Pb geochronology, stratigraphic synthesis, and seismic interpretations. Preliminary apatite (U-Th)/He data indicate activation of the Main Zagros Fault (MZF) at ~10 Ma with frontal thrusts initiating at ~8 Ma. However, thermochronometric results from the intervening Mountain Front Flexure (MFF), located between the MZF and the frontal thrusts, suggest rapid exhumation at ~6 Ma. These results suggest that the MFF, represented by the thrust-cored Qaradagh anticline, represents a major episode of out-of-sequence deformation. Detrital zircon U-Pb analyses from the Neogene foreland-basin deposits show continuous sediment derivation from sources to the NNE in Iraq and western Iran, suggesting that out-of-sequence thrusting did not significantly alter sedimentary provenance. Rather, intense hinterland erosion and recycling of older foreland-basin fill dominated sediment delivery to the basin. The irregular distribution of

  17. A Major Out of Sequence Fault in Central Range and Its Implication to Mountain Building Process of Taiwan Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Lee, Y. H.

    2015-12-01

    A Major Out of Sequence Fault in Central Range and Its Implication to Mountain Building Process of Taiwan Orogenic Belt Yuan-Hsi Lee1, Wei Lo2, Wei-Hau Wang1, Tim-Byrne 3, Ruey-Juen Rau 41. Department of Earth and Environmental Sciences, National Chung Cheng University, Taiwan, R.O.C. 2. Department of Materials and Mineral Resources Engineering, Taipei, National Taipei University of Technology, Taiwan, R.O.C. 3. Center for Integrative Geosciences, University of Connecticut, Storrs, CT, USA 4. Department of Earth Science, National Chen-Kung University, Taiwan, R.O.C. Taiwan mountain belt results from collision between Eurasia continental crust and Philippine Sea plate that result in exposing the metamorphic complex with high exhumation rate in eastern Central Range of Taiwan orogenic belt. In this study we combine with field survey, zircon fission track (ZFT), metamorphic grade, and tomography data to identify there exists a major out of sequence fault (MOSF) in eastern Central Range of Taiwan orogenic belt. This MOSF extends from north to south of eastern central Range with several segments and the total length is more than 250 km. The ZFT shows total annealing age of ca.1-3 Ma on the hanging wall and partial annealing ages on the foot wall. The seismicity data indicates the MOSF is still active from central to southern central Range. We consider that the MOSF is related with crustal channel flow in depth. To the western side of crustal flow it shows thrusting mechanism associated with MOSF and the normal faults (or normal shearing zone) develop in eastern side of the crustal channel flow. This crustal channel flow is also related with exposing the metamorphic complex in Central Range that is important mechanism for the mountain building process of Taiwan orogenic belt.

  18. The contorted New England Orogen (eastern Australia): New evidence from U-Pb geochronology of early Permian granitoids

    NASA Astrophysics Data System (ADS)

    Rosenbaum, Gideon; Li, Pengfei; Rubatto, Daniela

    2012-02-01

    A series of sharp bends (oroclines) are recognized in the Paleozoic to early Mesozoic New England Orogen of eastern Australia. The exact geometry and origin of these bends is obscured by voluminous magmatism and is still debated. Here we present zircon U-Pb ages that confirm the lateral continuation of early Permian (296-288 Ma) granitoids and shed new light on the oroclinal structure. Orogenic curvature is defined by the alignment of early Permian granitoids parallel to the structural grain of the orogen, as well as the curved geometry of sub-vertical deformation fabrics, forearc basin terranes, and serpentinite outcrops. Alternative geometrical interpretations may involve two bends (Texas and Coffs Harbour Oroclines), three bends (+Manning Orocline), or even four bends (+Nambucca Orocline). We argue that the model involving four bends is most consistent with available data, although further kinematic constraints are required to confirm the existence of the Manning and Nambucca Oroclines. A subsequent phase of younger magmatism (<260 Ma) cuts across the curved structural grain, providing a minimum age constraint for orocline development. Assuming a structure of four oroclines, we suggest a tentative tectonic model that involves an early stage of subduction curvature during slab rollback at 300-285 Ma, followed by bending associated with dextral transpression. A final tightening of the curved structures was possibly obtained by E-W shortening during the late Permian to Triassic (265-230 Ma) Hunter-Bowen orogeny.

  19. Cycle Analysis

    Energy Science and Technology Software Center (ESTSC)

    2012-03-20

    1. The Cycle Analysis code is an Microsoft Excel code that performs many different types of thermodynamic cycle analysis for power producing systems. The code will calculate the temperature and pressure and all other thermodynamic properties at the inlet and outlet of each component. The code also calculates the power that is produced, the efficiency, and the heat transported in the heater, gas chiller and recuperators. The code provides a schematic of the loop andmore » provides the temperature and pressure at each location in the loop. The code also provides a T-S (temperature-entropy) diagram of the loop and often it provides an pressure enthalpy plot as well. 2. This version of the code concentrates on supercritical CO2 power cycles, but by simply changing the name of the working fluid many other types of fluids can be analyzed. The Cycle Analysis code provided here contains 18 different types of power cycles. Each cycle is contained in one worksheet or tab that the user can select. The user can change the yellow highlighted regions to perform different thermodynamic cycle analysis.« less

  20. Tectonic setting of the low-grade metamorphic rocks of the Dabie Orogen, central eastern China

    NASA Astrophysics Data System (ADS)

    Xu, Shutong; Wu, Weiping; Lu, Yiqun; Wang, Dehua

    2012-04-01

    The tectonic setting on both the northern and southern sides of the Dabie Mountains reveals that low-grade metamorphic rocks are important constituents produced by the subduction of the oceanic crust prior to collision between the Sino-Korean and Yangtze cratons. The Zhangbaling Group/Mulanshan schist is a pre-Ordovician oceanic crust. The Sujiahe and Xinyang/Foziling Groups are trench sediments of the Ordovician-Devonian age, and constitute an accretionary prism associated with subduction. The Yangshan coal measures/Meishan Group was a forearc basin sediment of Carboniferous age, and was overthrust by the accretionary prism during collision. The Susong Group is composed of passive continental margin sediments of the Yangtze craton. Backarc basin sediments are postulated to be concealed by Mesozoic-Cenozoic sediments to the north of the Dabie Mountains. High-ultrahigh pressure terrains are exotic tectonic slices exhumed from depths, located between low-grade metamorphic rocks, and disturb the integrity of the earlier subduction orogen. Subduction occurred during the Ordovician to Devonian periods, and collision initiated at the beginning of the Permian.

  1. A tear fault boundary between the Taiwan orogen and the Ryukyu subduction zone

    NASA Astrophysics Data System (ADS)

    Lallemand, Serge E.; Liu, Char-Shine; Font, Yvonne

    1997-06-01

    More than twenty seismic lines were analysed across the intersection of the Taiwan orogen and the Ryukyu arc-forearc system. The main results of this study concern the recognition of two superposed sedimentary basins that were previously unknown. We have named the recent one the Hoping Basin and the older, tectonized one the Suao Basin. The Suao Basin, containing sediments more than 3 km thick, records subsidence of about 3 km on its southwestern portion after being shortened in the direction of plate convergence. On the basis of seismicity, gravity, present-day plate kinematics as well as seismic reflection data, we relate the dramatic subsidence of the Suao Basin to the initiation of a WNW-trending tear fault within the Philippine Sea plate. This tear fault decouples the subducting Philippine Sea plate to the northeast and the apparent overthrusting Philippine Sea plate in the Coastal Range to the southwest. Back-arc rifting may thus occur in the westernmost section of the Okinawa trough above the sinking Philippine Sea slab while lithospheric shortening prevails south of Hualien. As a consequence, the triangular area located southwest of the tear fault, between the Taiwan coastline and the toe of the Yaeyama ridge, no longer belongs to the Ryukyu forearc. It is now passively transported with the non-subducting portion of the Philippine Sea plate. We propose an evolutionary model for the relative chronology of tectono-sedimentary events in this region over the last 5 m.y.

  2. Polyphase Neoproterozoic orogenesis within the east Africa- Antarctica orogenic belt in central and northern Madagascar

    USGS Publications Warehouse

    Key, R.M.; Pitfield, P.E.J.; Thomas, Ronald J.; Goodenough, K.M.; Waele, D.; Schofield, D.I.; Bauer, W.; Horstwood, M.S.A.; Styles, M.T.; Conrad, J.; Encarnacion, J.; Lidke, D.J.; O'connor, E. A.; Potter, C.; Smith, R.A.; Walsh, G.J.; Ralison, A.V.; Randriamananjara, T.; Rafahatelo, J.-M.; Rabarimanana, M.

    2011-01-01

    Our recent geological survey of the basement of central and northern Madagascar allowed us to re-evaluate the evolution of this part of the East Africa-Antarctica Orogen (EAAO). Five crustal domains are recognized, characterized by distinctive lithologies and histories of sedimentation, magmatism, deformation and metamorphism, and separated by tectonic and/or unconformable contacts. Four consist largely of Archaean metamorphic rocks (Antongil, Masora and Antananarivo Cratons, Tsaratanana Complex). The fifth (Bemarivo Belt) comprises Proterozoic meta-igneous rocks. The older rocks were intruded by plutonic suites at c. 1000 Ma, 820-760 Ma, 630-595 Ma and 560-520 Ma. The evolution of the four Archaean domains and their boundaries remains contentious, with two end-member interpretations evaluated: (1) all five crustal domains are separate tectonic elements, juxtaposed along Neoproterozoic sutures and (2) the four Archaean domains are segments of an older Archaean craton, which was sutured against the Bemarivo Belt in the Neoproterozoic. Rodinia fragmented during the early Neoproterozoic with intracratonic rifts that sometimes developed into oceanic basins. Subsequent Mid- Neoproterozoic collision of smaller cratonic blocks was followed by renewed extension and magmatism. The global 'Terminal Pan-African' event (560-490 Ma) finally stitched together the Mid-Neoproterozoic cratons to form Gondwana. ?? The Geological Society of London 2011.

  3. Partial melting of deeply subducted eclogite from the Sulu orogen in China.

    PubMed

    Wang, Lu; Kusky, Timothy M; Polat, Ali; Wang, Songjie; Jiang, Xingfu; Zong, Keqing; Wang, Junpeng; Deng, Hao; Fu, Jianmin

    2014-01-01

    We report partial melting of an ultrahigh pressure eclogite in the Mesozoic Sulu orogen, China. Eclogitic migmatite shows successive stages of initial intragranular and grain boundary melt droplets, which grow into a three-dimensional interconnected intergranular network, then segregate and accumulate in pressure shadow areas and then merge to form melt channels and dikes that transport magma to higher in the lithosphere. Here we show, using zircon U-Pb dating and petrological analyses, that partial melting occurred at 228-219 Myr ago, shortly after peak metamorphism at 230 Myr ago. The melts and residues are complimentarily enriched and depleted in light rare earth element (LREE) compared with the original rock. Partial melting of deeply subducted eclogite is an important process in determining the rheological structure and mechanical behaviour of subducted lithosphere and its rapid exhumation, controlling the flow of deep lithospheric material, and for generation of melts from the upper mantle, potentially contributing to arc magmatism and growth of continental crust. PMID:25517619

  4. A Numerical SImulation of the Goshen Tornado Observed During Vortex 2: An Orogenic Convective System

    NASA Astrophysics Data System (ADS)

    Tripoli, G. J.; Trevorrow, S. T.; Büker, M.

    2014-12-01

    A numerical simulation of the Goshen 2009 tornado observed during Vortex 2 is presented. The simulation is initialized from the North American Regional Reanalysis data set on the scale of the North American continent. The observed Goshen County tornadic supercell evolves in the simulation from the regional slope flow embedded an a favorable synoptic environment. A supercell and tornado were produced by the simulation in the approximate location and precise time that observations were made of the event by the Vortex 2 team. This was withoiut additional assimilation of high resolution observations beyond the standard data contributing to the 1500 UTC 35 km NARR analysis 7 hours prior to the observed tornado. The simulation can be and is compared directly to Vortex 2 Doppler observations. The process of orogenic storm organization driven by the interaction of topography and the large scale environment is discussed. It is hypothesized that the unusually high predictability of the timing and location as well as the system structure and attendant weather including tornado activity was the result of a highly predictable diurnal slope flow environment that constrained and so reduced the uncertainty of the simulation outcome.

  5. Timing of Mississippi Valley-type mineralization: Relation to Appalachian orogenic events

    SciTech Connect

    Kesler, S.E.; van der Pluijm, B.A. )

    1990-11-01

    Although Mississippi Valley-type deposits in Lower Ordovician carbonate rocks of the Appalachian orogen are commonly interpreted to have been precipitated by basinal brines, the timing of brine migration remains poorly known. Late Paleozoic K-Ar isotopic ages on authigenic K-feldspar, which is widespread in Appalachian carbonate rocks, as well as evidence of paleomagnetic overprints of similar age, have focused attention on the possibility that these Mississippi Valley-type deposits formed as a result of late Paleozoic deformation. Geologic and geochemical similarities among most of these deposits, from Georgia to Newfoundland, including unusually high sphalerite/galena ratios, isotopically heavy sulfur, and relatively nonradiogenic lead, suggest that they are coeval. Sphalerite sand that parallels host-rock layering in many of the deposits indicates that mineralization occurred before regional deformation. Although the late Paleozoic age of deformation in the southern Appalachians provides little constraint on the age of Mississippi Valley-type mineralization, deformation of these deposits in the Newfoundland Appalachians is early to middle Paleozoic in age. Thus, if Ordovician-hosted, Appalachian Mississippi Valley-type deposits are coeval, they must have formed by middle Paleozoic time and cannot be the product of a late Paleozoic fluid-expulsion event. This hypothesis has important implications for basin evolution, fluid events, and remagnetization in the Appalachians.

  6. Partial melting of deeply subducted eclogite from the Sulu orogen in China

    PubMed Central

    Wang, Lu; Kusky, Timothy M.; Polat, Ali; Wang, Songjie; Jiang, Xingfu; Zong, Keqing; Wang, Junpeng; Deng, Hao; Fu, Jianmin

    2014-01-01

    We report partial melting of an ultrahigh pressure eclogite in the Mesozoic Sulu orogen, China. Eclogitic migmatite shows successive stages of initial intragranular and grain boundary melt droplets, which grow into a three-dimensional interconnected intergranular network, then segregate and accumulate in pressure shadow areas and then merge to form melt channels and dikes that transport magma to higher in the lithosphere. Here we show, using zircon U–Pb dating and petrological analyses, that partial melting occurred at 228–219 Myr ago, shortly after peak metamorphism at 230 Myr ago. The melts and residues are complimentarily enriched and depleted in light rare earth element (LREE) compared with the original rock. Partial melting of deeply subducted eclogite is an important process in determining the rheological structure and mechanical behaviour of subducted lithosphere and its rapid exhumation, controlling the flow of deep lithospheric material, and for generation of melts from the upper mantle, potentially contributing to arc magmatism and growth of continental crust. PMID:25517619

  7. Structural characteristics of the central Ogcheon Belt, South Korea: orogen-parallel tectonic transport model

    NASA Astrophysics Data System (ADS)

    Kihm, You Hong; Kim, Jeong Hwan

    The rocks in the central Ogcheon Belt, South Korea, underwent three deformational phases (D 1, D 2 and D 3) during the Mesozoic Era. In the study area, NW- and NE-trending geological structures such as thrusts and folds are juxtaposed. The NW- and NE-trending folds deform isoclinal folds (F 1) with axial planar slaty cleavage (S 1), and have axial planar crenulation cleavage. All of these folds are overprinted by brittle faults (D 3). It is inferred from field data that the NW- and NE-trending structures are products of a single deformational phase (D 2). A very large-scale inclined NE-vergent isoclinal fold (F 1) produced an irregular boundary of Precambrian basement and a basement promontory, which acted as a structural obstacle against subsequent deformational phases. During the E-vergent phase (D 2), deformation partitioning occurred due to the irregular block boundary, and orogen-parallel and -orthogonal structures were produced. The D 3 phase is recognized as large-scale E-W trending structures including folds and faults. Thus, the structural evolution of the central Ogcheon Belt is related to the clockwise rotation of the maximum compressive stress axis from NE-SW to N-S during the Mesozoic Era. This study shows that the shape of colliding boundary is a very important factor in controlling the structural pattern and evolution in the study area.

  8. New seismic images of the crust in the central Trans-Hudson Orogen of Saskatchewan

    NASA Astrophysics Data System (ADS)

    Pandit, B. I.; Hajnal, Z.; Stauffer, M. R.; Lewry, J.; Ashton, K. E.

    1998-05-01

    A reprocessing program to enhance the correlation between the surface geology and the seismic data has been completed for seismic line 9 (eastern 100 km) and line 10 in the central region of the Trans-Hudson Orogen of Saskatchewan, Canada. The new seismic images through lateral continuity of reflectivity provide sufficient detail to resolve the discrepancy between the low-dipping, layer-parallel and dextral-reverse nature of the Sturgeon-Weir shear zone (line 9) observed in the field and its steeply dipping (apparent) normal displacement character interpreted on the basis of the initial processing. Furthermore, the new interpretation provides a strong confirmation of the role of Pelican Thrust as a major detachment zone — the main `sole thrust' — along which juvenile allochthons have been carried across the Archaean microcontinental block. The images are also refined enough to suggest: (a) a boundary within the Pelican Thrust between its internal and external suites; (b) a possible boundary separating a lower (older?) Archaean basement from its upper (younger?) counterpart; and (c) sub-Moho events (M2) which reveal possible involvement of the upper mantle in the collisional tectonic process in addition to the well defined Moho (M1) which probably represents the youngest of the post-collisional detachments.

  9. Contrasting Metamorphic Record of Heat Production Anomalies in the Penokean Orogen of Northern Michigan.

    PubMed

    Attoh

    2000-05-01

    It is proposed that the contrasting metamorphic mineral assemblages of the isolated amphibolite facies metamorphic highs in the Penokean orogen of northern Michigan may be caused by different heat production rates in the Archean basement. This hypothesis is based on concentrations of K, U, and Th in the Archean basement gneisses and Paleoproterozoic metasediments that indicate significant contribution of radiogenic heating during Penokean metamorphism. Heat production was anomalously high ( approximately 10.6 µWm-3) where andalusite-bearing mineral assemblages indicate that high temperatures were attained at shallow crustal levels ( approximately 550 degrees -600 degrees C at approximately 3 kbar). In contrast, where exposed metamorphic rocks indicate peak temperatures of 600 degrees -650 degrees C at 6-7 kbar, heat production in the Archean basement was lower ( approximately 3.7 µWm-3). The effect of heat production rates on the metamorphic pressure-temperature paths was tested with numerical thermal models. The calculations show (1) that if the heat production rate, where andalusite-bearing assemblages formed, was significantly <6.0 µWm-3, the estimated pressure at peak temperatures (PTmax) would be much higher and lie in the sillimanite or kyanite stability fields; and (2) differences between PTmax estimates for the metamorphic highs based on thermobarometry can be reproduced if thermal history involved significant crustal thickening as well as moderate unroofing rates. PMID:10769161

  10. Strong Lg-wave attenuation in the Middle East continental collision orogenic belt

    NASA Astrophysics Data System (ADS)

    Zhao, Lian-Feng; Xie, Xiao-Bi

    2016-04-01

    Using Lg-wave Q tomography, we construct a broadband crustal attenuation model for the Middle East. The QLg images reveal a relationship between attenuation and geological structures. Strong attenuation is found in the continental collision orogenic belt that extends from the Turkish and Iranian plateau to the Pamir plateau. We investigate the frequency dependence of QLg in different geologic formations. The results illustrate that QLg values generally increase with increasing frequency but exhibit complex relationships both with frequency and between regions. An average QLg value between 0.2 and 2.0 Hz, QLg (0.2-2.0 Hz), may be a critical index for crustal attenuation and is used to infer the regional geology. Low-QLg anomalies are present in the eastern Turkish plateau and correlate well with low Pn-velocities and Cenozoic volcanic activity, thus indicating possible partial melting within the crust in this region. Very strong attenuation is also observed in central Iran, the Afghanistan block, and the southern Caspian Sea. This in line with the previously observed high crustal temperature, high-conductivity layers, and thick marine sediments in these areas, suggests the high Lg attenuation is caused by abnormally high tectonic and thermal activities.

  11. Morphometric properties of the trans-Himalayan river catchments: Clues towards a relative chronology of orogen-wide drainage integration

    NASA Astrophysics Data System (ADS)

    Ghosh, Parthasarathi; Sinha, Sayan; Misra, Arindam

    2015-03-01

    The geomorphological evolution of the Himalayan mountain belt both in terms of crustal deformation and concomitant erosion by surface processes has been suggested to have a profound influence on a number of earth system processes and has been extensively researched through a number of different techniques. The huge catchments of the trans-Himalayan rivers are the product of long-term fluvial erosion of the landscape. This work attempts to understand their evolution through a study of drainage network, morphology, and internal organization of the smaller watersheds nested within each catchment. Using morphometric techniques applied to an orogen-wide digital elevation data grid, we characterized the drainage network structure and catchment of all the 18 trans-Himalayan rivers situated between the exits of the Indus and Brahmaputra rivers and constructed rectangular approximations of the catchment geometries. With the help of catchment dimensions measured transverse and parallel to the strike of the orogen, and by analyzing the dimension and spatial dispositions of the rectangular approximations, we demonstrate that the trans-Himalayan catchment shapes cannot be explained only as a product of the headward enlargement of drainage networks on a topographic slope, or orogenic taper. Within individual catchments we identified the existence of drainage components (watersheds) that are organized in a systematic manner with respect to the first-order physiographic features of the Himalayas, formed at different periods of geological time. Each of them shows distinct morphometric characteristics that are indicative of differences in processes and / or time scale involved in their formation. The hypsometric properties of the watersheds occupying the upper part of the catchments suggest that they are the remnants of pre-orogenic drainage that became confined to the leeward side of the Himalayas before the advent of monsoon circulation. The shape and organization of the

  12. Cambro-Ordovician Granites in the Araçuaí Belt, in Brazil: snapshots from a late orogenic collapse

    NASA Astrophysics Data System (ADS)

    De Campos, Cristina P.; Mendes, Júlio Cesar; de Medeiros, Silvia Regina; Ludka, Isabel P.

    2014-05-01

    Along the Brazilian Coast, surrounding the São Francisco Craton and adjacent mobile belts, deep segments of a Neoproterozoic orogen (Araçuaí-West Congo) generated over 120 Ma of successive magmatic episodes of granitic magmatism. The c.630-585 Ma calc-alkaline magmatic arc consists of metatonalite to metagranodiorite, with metadioritic to noritic facies and enclaves. During the syn-collisional and crustal thickening stage (c. 585 to 560 Ma) S-type metagranites have been built by dehydration melting of a diverse package of sediments. Around 545-525 Ma late orogenic crustal remelting formed mostly non-foliated garnet-cordierite leucogranites. In the post-orogenic stage (c. 510-480 Ma) inversely zoned calc-alkaline to alkaline plutons intruded previous units. This work will focus on the youngest post-orogenic magmatism. It will present the state of the art by reviewing structural measurements, detailed mapping of flow patterns and additional geochemical and isotopic data. The architecture of around 10 plutons, ranging from c. 20 to 200 km2 in surface area, unravels deep mushroom- to funnel-like magma chambers and/or conduits. Available data point towards different compositional domains, which are interfingered in complex concentric layers, so that, each pluton depicts a unique internal flow pattern. In the silica-richer structures concentric fragmented or folded layers of granite, in a hybrid K-gabbroic/dioritic matrix, contrast with predominantly homogeneous K-basaltic to gabbroic regions. These may be separated by magmatic shear zones where mixing is enhanced, also resulting in hybrid compositions. Sharp and pillow-like contacts between granitic and K-basaltic rocks locally depict a frozen-in situation of different intrusive episodes. In the silica-poorer plutonic bodies gradational contacts are more frequent and may be the result of convection enhanced diffusion. For all plutons, however, mostly sub-vertical internal contacts between most- and least

  13. Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study

    NASA Astrophysics Data System (ADS)

    Beltrando, Marco; Manatschal, Gianreto; Mohn, Geoffroy; Dal Piaz, Giorgio Vittorio; Vitale Brovarone, Alberto; Masini, Emmanuel

    2014-04-01

    Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-extended crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a pre-orogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor during Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments from the hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction mélange dynamics or deposition of sedimentary mélanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the

  14. Low-temperature thermochronology of the northern Thomson Orogen: Implications for exhumation of basement rocks in NE Australia

    NASA Astrophysics Data System (ADS)

    Verdel, Charles; Stockli, Daniel; Purdy, David

    2016-01-01

    The Tasmanides of eastern Australia record much of the Phanerozoic tectonic development of the retreating Pacific-Australia plate boundary and are an oft-cited example of an orogen that has undergone "tectonic mode switching." To begin to constrain the timing of exhumation of basement rocks that are now exposed in portions of the NE Tasmanides, we measured apatite and zircon (U-Th)/He ages from the Thomson Orogen and overlying Paleozoic strata in the back-arc of the New England Orogen in NE Australia. Zircon (U-Th)/He ages from basement samples (including those recovered from boreholes at depths of up to 1.1 km) are characterized by large inter- and intra-sample variability and range from approximately 180 Ma (Early Jurassic) to 375 Ma (Late Devonian). (U-Th)/He zircon ages from several individual samples are negatively correlated with effective uranium (eU), a pattern that is also true of the dataset as a whole, suggesting that variations in U and Th zoning and radiation damage are partially responsible for the age variability. The oldest zircon (U-Th)/He cooling ages coincide with the formation of regionally extensive Late Devonian-early Carboniferous back-arc basins, suggesting that Late Devonian extension played a significant role in exhumation of parts of the northern Thomson Orogen. Apatite (U-Th)/He ages from a basement sample and a late Permian sandstone in the overlying Bowen Basin, which are also marked by intra-sample variability and age-eU correlations, span from the Early Cretaceous through Oligocene, in general agreement with previous apatite fission track data. In conjunction with observations of key geologic relationships and prior K-Ar and 40Ar/39Ar data, our results suggest four overall phases in the thermal history of the northern Thomson Orogen: (1) Cambrian-early Silurian metamorphism during the Delamerian and Benambran Orogenies; (2) protracted cooling during the Late Devonian through mid-Permian that likely resulted from extensional

  15. The evolution of a Gondwanan collisional orogen: A structural and geochronological appraisal from the Southern Granulite Terrane, South India

    NASA Astrophysics Data System (ADS)

    Plavsa, Diana; Collins, Alan S.; Foden, John D.; Clark, Chris

    2015-05-01

    Gondwana amalgamated along a suite of Himalayan-scale collisional orogens, the roots of which lace the continents of Africa, South America, and Antarctica. The Southern Granulite Terrane of India is a generally well-exposed, exhumed, Gondwana-forming orogen that preserves a record of the tectonic evolution of the eastern margin of the East African Orogen during the Ediacaran-Cambrian (circa 600-500 Ma) as central Gondwana formed. The deformation associated with the closure of the Mozambique Ocean and collision of the Indian and East African/Madagascan cratonic domains is believed to have taken place along the southern margin of the Salem Block (the Palghat-Cauvery Shear System, PCSS) in the Southern Granulite Terrane. Investigation of the structural fabrics and the geochronology of the high-grade shear zones within the PCSS system shows that the Moyar-Salem-Attur shear zone to the north of the PCSS system is early Paleoproterozoic in age and associated with dextral strike-slip motion, while the Cauvery shear zone (CSZ) to the south of the PCSS system can be loosely constrained to circa 740-550 Ma and is associated with dip-slip dextral transpression and north side-up motion. To the south of the proposed suture zone (the Cauvery shear zone), the structural fabrics of the Northern Madurai Block suggest four deformational events (D1-D4), some of which are likely to be contemporaneous. The timing of high pressure-ultrahigh temperature metamorphism and deformation (D1-D3) in the Madurai Block (here interpreted as the southern extension of Azania) is constrained to circa 550-500 Ma and interpreted as representing collisional orogeny and subsequent orogenic collapse of the eastern margin of the East African Orogen. The disparity in the nature of the structural fabrics and the timing of the deformation in the Salem and the Madurai Blocks suggest that the two experienced distinct tectonothermal events prior to their amalgamation along the Cauvery shear zone during the

  16. Microstructural analysis of the Greater Himalayan Sequence, Annapurna-Dhaulagiri Himalaya, central Nepal: Channel Flow and Orogen-parallel deformation.

    NASA Astrophysics Data System (ADS)

    Parsons, A. J.; Phillips, R. J.; Lloyd, G. E.; Searle, M. P.; Law, R. D.

    2014-12-01

    Knowledge of deformation processes that occur in the lithosphere during orogenesis can be gained from microstructural analysis of exhumed terranes and shear zones. Here, we use Crystallographic Preferred Orientation (CPO) and Anisotropy of Magnetic Susceptibility (AMS) data to reveal the kinematic evolution of the metamorphic core of the Himalayan orogen, the Greater Himalayan Sequence (GHS). The Himalayan orogen is commonly explained with models of channel flow, which describe the GHS as a partially molten, rheologically weak mid crustal channel. Extrusion of the channel was facilitated by coeval reverse- and normal-sense shear zones, at the lower and upper channel margins respectively. Whilst many thermobarometric studies support the occurrence of channel flow, the spatial and temporal distribution of strain within the GHS is one aspect of the model that is yet to be fully resolved. We present a quantified strain proxy profile for the GHS in the Annapurna-Dhaulagiri region of central Nepal and compare our results with the kinematic predictions of the channel flow model. Samples were collected along a NS transect through the Kali Gandaki valley of central Nepal for CPO and AMS analysis. Variations in CPO strength are used as a proxy for relative strain magnitude, whilst AMS data provide a proxy for strain ellipsoid shape. Combining this information with field and microstructural observations and thermobarometric constraints reveals the kinematic evolution of the GHS in this region. Low volumes of leucogranite and sillimanite bearing rocks and evidence of reverse-sense overprinting normal-sense shearing at the top of the GHS suggest that channel flow was not as intense as model predictions. Additionally, observed EW mineral lineations and oblate strain ellipsoid proxies in the Upper GHS, indicative of three dimensional flattening and orogen parallel stretching, cannot be explained by current channel flow models. Whilst the results do not refute the occurrence of

  17. Cycling injuries.

    PubMed Central

    Cohen, G. C.

    1993-01-01

    Bicycle-related injuries have increased as cycling has become more popular. Most injuries to recreational riders are associated with overuse or improper fit of the bicycle. Injuries to racers often result from high speeds, which predispose riders to muscle strains, collisions, and falls. Cyclists contact bicycles at the pedals, seat, and handlebars. Each is associated with particular cycling injuries. Images Figure 1 Figure 3 Figure 4 Figure 5 PMID:8471908

  18. Nutrient cycling.

    PubMed

    Bormann, F H; Likens, G E

    1967-01-27

    The small-watershed approach to problems of nutrient cycling has these advantages. (i) The small watershed is a natural unit of suitable size for intensive study of nutrient cycling at the ecosystem level. (ii) It provides a means of reducing to a minimum, or virtually eliminating, the effect of the difficult-to-measure variables of geologic input and nutrient losses in deep seepage. Control of these variables makes possible accurate measurement of nutrient input and output (erosion) and therefore establishes the relationship of the smaller ecosystem to the larger biospheric cycles. (iii) The small-watershed approach provides a method whereby such important parameters as nutrient release from minerals (weathering) and annual nutrient budgets may be calculated. (iv) It provides a means of studying the interrelationships between the biota and the hydrologic cycle, various nutrient cycles, and energy flow in a single system. (v) Finally, with the small-watershed system we can test the effect of various land-management practices or environmental pollutants on nutrient cycling in natural systems. PMID:17737551

  19. On protolith-, metamorphic overprint, microstructure and rheology of mineral assemblages in orogenic peridotites of the central Scandinavian Caledonides

    NASA Astrophysics Data System (ADS)

    Gilio, Mattia; Clos, Frediano; Van Roermund, Herman L. M.

    2013-04-01

    The Scandinavian Caledonides (SC) are a deeply eroded Alpine-type orogenic belt formed by closure of the Iapetus ocean and collision between Baltica and Laurentia (500-380 Ma). The SC consists of a stack of Nappe Complexes (from bottom to top called Lower, Middle, Upper and Uppermost Allochthons) thrusted to the east over the Baltic Shield (Brueckner and Van Roermund, 2004; Gee et al., 2008). Fossil lithospheric mantle fragments, called orogenic peridotites, have been found within the (upper part of) middle, upper and uppermost Allochthons, as well as in the reworked basement gneisses (a.o Western Gneiss Complex (WGC)) along the Norwegian west coast. They occur as isolated lenses that contain diverse mineral parageneses and/or bulk rock compositions. Crustal incorporation of orogenic peridotite is classically interpreted to be the result of plate collisional processes related to orogeny (Brueckner and Medaris, 2000). The WGC and parts of the upper part of the Middle Allochthon (a.o. Seve Nappe Complex (SNC) in N Jämtland/S Västerbotten, central Sweden), are well known for the occurrence of high (HP) and ultrahigh pressure (UHP) metamorphic terranes (of Caledonian age). The (U)HPM evidence clearly demonstrates the deep metamorphic origin of these rocks interpreted to be caused by continental subduction and/or collision. Other metamorphic rocks (of Caledonian age) exposed in allochthonous nappes are solely characterised by greenschist-, amphibolite- and/or MP granulite "facies" mineral assemblages that can be interpreted, in the absence of retrogression, to have formed in less deeply subducted (and/or metamorphic) environments. This duality in metamorphic "facies" allows for a discrimination (at least theoretically) between "deep" versus "shallow" rooted nappes (in central parts of the Scandinavian Caledonides). Conform this reasoning, this duality should also be present within the Caledonian mineral assemblages (= metamorphic overprint) of orogenic peridotites (in

  20. Silurian granulite-facies metamorphism, and coeval magmatism and crustal growth in the Tongbai orogen, central China

    NASA Astrophysics Data System (ADS)

    Wang, Hao; Wu, Yuan-Bao; Gao, Shan; Zhang, Hong-Fei; Liu, Xiao-Chi; Gong, Hu-Jun; Peng, Min; Wang, Jing; Yuan, Hong-Lin

    2011-07-01

    It is intriguing whether the formation of granulite in an orogenic belt is related to magmatism and crustal growth. The Tongbai orogen is one of the particular areas in the Qinling-Tongbai-Dabie-Sulu orogen that contains both granulites and associated arc magmatic rocks, and thus is an optimal place to address this issue. However, the formation age and genesis of the granulites and their relations to the magmatic rocks have not yet been well constrained. In situ LA (MC)-ICPMS U-Pb dating, and trace element and Lu-Hf isotope analyses were undertaken on zircon grains from gneiss, granulite, gabbro and granodiorite samples. Most zircons in three granulite samples and a gneiss sample have core-rim structures. The cores have oval or rounded shapes, oscillatory zones, high Th/U ratios and variable U-Pb ages, suggesting a detrital genesis. The youngest group of the detrital zircons yields a weighted mean 206Pb/238U age of 450 ± 5 Ma, which is interpreted as the maximum depositional age of their protoliths. A major age cluster is ca. 450-490 Ma, corresponding to prolonged arc magmatism in the northern Tongbai and Qinling orogens. Another group of ages range from 660 to 950 Ma, implying that the Qinling Group of the Tongbai orogen belongs to the South China Block (SCB). Metamorphic zircons in the granulites have weak or no zoning, low Th/U ratios and REE contents, negative Eu anomalies, and relatively flat HREE patterns, which indicate that they formed contemporaneously with garnet and plagioclase, and thus under granulite-facies metamorphic conditions. Their weighted mean age of 424 ± 4 Ma is taken as the best estimated age of the granulite-facies metamorphism. Zircons in a leucosome sample have significant negative Eu anomalies and relatively flat HREE patterns with a formation age of 428 ± 4 Ma, indicating partial melting coeval with the granulite-facies metamorphism. Metamorphic zircons in the gneiss show apparently negative Eu anomalies, and variable Th/U ratios and

  1. Review of global 2.1-1.8 Ga orogens: implications for a pre-Rodinia supercontinent

    NASA Astrophysics Data System (ADS)

    Zhao, Guochun; Cawood, Peter A.; Wilde, Simon A.; Sun, Min

    2002-11-01

    Available lithostratigraphic, tectonothermal, geochronological and paleomagnetic data from 2.1-1.8 Ga collisional orogens and related cratonic blocks around the world have established connections between South America and West Africa; Western Australia and South Africa; Laurentia and Baltica; Siberia and Laurentia; Laurentia and Central Australia; East Antarctica and Laurentia, and North China and India. These links are interpreted to indicate the presence of a supercontinent existing before Rodinia, referred to herein as Columbia, a name recently proposed by Rogers and Santosh [Gondwana Res. 5 (2002) 5] for a Paleo-Mesoproterozoic supercontinent. In this supercontinent, the Archean to Paleoproterozoic cratonic blocks were welded by the global 2.1-1.8 Ga collisional belts. The cratonic blocks in South America and West Africa were welded by the 2.1-2.0 Ga Transamazonian and Eburnean Orogens; the Kaapvaal and Zimbabwe Cratons in southern Africa were collided along the ˜2.0 Ga Limpopo Belt; the cratonic blocks of Laurentia were sutured along the 1.9-1.8 Ga Trans-Hudson, Penokean, Taltson-Thelon, Wopmay, Ungava, Torngat and Nagssugtoqidian Orogens; the Kola, Karelia, Volgo-Uralia and Sarmatia (Ukrainian) Cratons in Baltica (Eastern Europe) were joined by the 1.9-1.8 Ga Kola-Karelia, Svecofennian, Volhyn-Central Russian and Pachelma Orogens; the Anabar and Aldan Cratons in Siberia were connected by the 1.9-1.8 Ga Akitkan and Central Aldan Orogens; the East Antarctica and an unknown continental block were joined by the Transantarctic Mountains Orogen; the South and North Indian Blocks were amalgamated along the Central Indian Tectonic Zone; and the Eastern and Western Blocks of the North China Craton were welded together by the ˜1.85 Ga Trans-North China Orogen. The existence of Columbia is consistent with late Paleoproterozoic to Mesoproterozoic sedimentary and magmatic records. The ˜2.0 Ga fluvio-deltaic deposits have been found in all cratonic blocks in South

  2. Vapor Compression Cycle Design Program (CYCLE_D)

    National Institute of Standards and Technology Data Gateway

    SRD 49 NIST Vapor Compression Cycle Design Program (CYCLE_D) (PC database for purchase)   The CYCLE_D database package simulates the vapor compression refrigeration cycles. It is fully compatible with REFPROP 9.0 and covers the 62 single-compound refrigerants . Fluids can be used in mixtures comprising up to five components.

  3. Progressive orogenic deformation recorded in a mylonite sample from the Dent Blanche Basal Thrust (Pennine Alps)

    NASA Astrophysics Data System (ADS)

    Kirst, Frederik; Froitzheim, Nikolaus; Nagel, Thorsten

    2013-04-01

    The Dent Blanche Basal Thrust (DBBT) represents an Alpine reverse fault along which the continental Dent Blanche Nappe was thrust onto the ocean-derived Combin Zone. Ductile NW-directed shearing associated with Early Tertiary nappe emplacement was localized along this fault resulting in the formation of intensely sheared rocks. These basal mylonites have locally been overprinted by later deformation phases associated with post-nappe compression. In the following, microstructures of a mylonite sample from a locality near the village Ollomont in the Valpelline of Italy are described and correlated with orogen-scale deformational events. The penetrative mylonitic foliation (S1) dips to the E (88/23) and is defined by a distinct metamorphic layering consisting of alternating, sub-mm thin layers of quartz and fine-grained silicates, mainly white mica ± epidote ± feldspar. The mylonitic stretching lineation (L1) is an aggregate lineation that dips to the ESE (119/19). In some parts, a grain shape preferred orientation of quartz grains defines an oblique foliation which probably developed as a result of combined subgrain rotation and grain boundary migration during simple shear deformation (Passchier & Trouw, 2005). The orientation of elongated quartz grains indicates top-NW shearing and is interpreted to reflect the bulk shear sense during nappe emplacement (D1). This type of dynamically recrystallised quartz also indicates deformation at temperatures around 500°C (Stipp et al., 2002). In most parts of the Ollomont mylonite, a secondary foliation (S2) and lineation (L2) are produced by a SE-dipping crenulation cleavage with NE-dipping (47/17) axes. In some spots, this non-penetrative feature is only slightly developed and gradational whereas in others it is a discrete compressional crenulation cleavage with large fold amplitudes in microlithons. This secondary foliation is interpreted to have formed as a result of late-stage orogen-perpendicular (NW-SE) shortening (D2

  4. From orogenic collapse to rifting ; structures of the South China Sea

    NASA Astrophysics Data System (ADS)

    Pubellier, M.; Chan, L. S.; Chamot Rooke, N.; Shen, W.; Ringenbach, J. C.

    2009-04-01

    The opening of the South China Sea has been a matter of debate for many years because of its internal structure, the differences between the conjugate margins and the variations of rifting and spreading directions. Although it is considered as being a back-arc basin, it is not sitting directly above a subduction zone, and the rifting process lasted for an unusually long duration. Among the specific characteristics is the early phase of rifting which took place early in place of the former Yanshanian andean-type mountain range. This stage is marked by narrow basins filled with deformed conglomerate, and initiated around 70My ago within a framework where the oblique subduction marked by igneous activity and ductile wrench faults, was replaced by orogenic collapse. The rifting stage is marked by Eocene syntectonic normal faults and occasional volcanics centres and has proceeded from NW-SE to NS extension. The NW stretching created at least two aborted basins which remained at rift stage. Extension was followed by spreading from 33 to ~20 Ma in the South China Sea. The ocean floor spreading also changed direction to NW-SE with a propagator inside the Sunda shelf from 20 to 17My ago. However the propagator opening implies that deformation is also taken by rifting around a southern wedge which in turn created strain inside the thinned crust. Another extension parallel to the margin is also observed althought the spreading was in process. The southward motion of the southern conjugate margin was later accommodated by its subduction beneath the NW Borneo wedge until completion of the Proto South China Sea subduction. Variations of rifting spreading through time and variations of structural styles are discussed in terms of boundary forces acting to the SE.

  5. Post-orogenic exhumation history of a Variscan mid-crustal basement in Galicia (NW Spain)

    NASA Astrophysics Data System (ADS)

    Grobe, Rene; Alvarez-Marrón, Joaquina; Glasmacher, Ulrich A.; Stuart, Finlay; Castañeda-Zarauz, A.

    2010-05-01

    The present study aims to quantify the complex post-orogenic history of cooling, denudation, and long-term landscape evolution of a mid-crustal section of Variscan basement in Galicia (NW Spain). We use apatite fission-track and apatite (U-Th)/He thermochronological techniques combined with time-temperature (t-T) path modelling using the software code HeFTy©. The topography is characterized by an extensive, low relief area at ~500 m elevation in central Galicia, and a WNW-ESE ridge that reaches up to 1000 m to the North. The area experienced two major tectonic events since the end of the Variscan orogeny in the Late Palaeozoic: 1) continental break-up and Mesozoic rifting leading to the opening of the Atlantic Ocean and the Bay of Biscay, and 2) limited convergence between Iberia and Eurasia since Middle Eocene times. Apatite fission-track ages range from 68.1 ± 5.0 Ma to 174.5 ± 7.7 Ma and apatite (U-Th)/He ages range from 73.6 ± 5.4 to 147.1 ± 16.6 Ma. Age-elevation plots and t-T path modelling suggest a tectonothermal evolution with faster exhumation associated to faulting during Mesozoic rifting. In particular, two major fault systems trending WNW-ESE and NNE-SSW, the As Pontes and the Lugo faults respectively separate areas with the fastest exhumation around 115 Ma from areas with overall slow exhumation since 200-150 Ma. A landscape of subdued topography in central Galicia was acquired prior to Eocene convergence. The higher elevation areas along the northern ridge formed since Middle Eocene times due to fault reactivation and minor exhumation occurred along the fault escarpment.

  6. Vestiges of The Peri-rodinian Ocean: Sliced, Diced, Recycled But Preserved In Younger Orogens

    NASA Astrophysics Data System (ADS)

    Murphy, J. B.; Nance, R. D.; Keppie, J. D.; Dostal, J.

    Just as the amalgamation and dispersal of Pangea exerted a first-order influence on tectonothermal events in the Phanerozoic, Middle to late Proterozoic global scale tec- tonics were profoundly influenced by the amalgamation and subsequent breakup of the supercontinent Rodinia. Most tectonic studies of the evolution of Rodinia concentrate on the near-field effects of these events such as the collisional orogenies that result in its amalgamation and the sedimentary rift-drift record of its breakup. However, ves- tiges of the far field effects of these events are also preserved and their tectonothermal evolution can provide additional constraints on supercontinent configuration and the timing of breakup. During the time interval of Rodinian amalgamation, for exam- ple, ensimatic subduction and plume activity in the peri-Rodinian ocean resulted in the formation of oceanic crust with ca. 1.0 Ga Sm-Nd depleted mantle model (TDM) ages. Vestiges of this crust are now preserved in the terranes that subsequently accreted along the margins of the dispersing continents, including the peri-Gondwanan terranes (e.g. Avalonia, Carolina, Iberia) of eastern North America and Europe, the Tocantins orogenic belt of Brazil, and the terranes of the Arabian-Nubian Shield. The Neopro- terozoic tectonothermal evolution of these terranes was a far-field response to Rodinia breakup, in the same way that the Mesozoic and Cenozoic evolution of western North America was a far-field response to the breakup of Pangea. Hence, the Neoproterozoic histories of these terranes are geodynamically linked to that of Rodinia and so provide constraints on its configuration and the timing of its major tectonothermal events.

  7. Understanding erosion rates in the Himalayan orogen: A case study from the Arun Valley

    NASA Astrophysics Data System (ADS)

    Olen, Stephanie M.; Bookhagen, Bodo; Hoffmann, Bernd; Sachse, Dirk; Adhikari, D. P.; Strecker, Manfred R.

    2015-10-01

    Understanding the rates and pattern of erosion is a key aspect of deciphering the impacts of climate and tectonics on landscape evolution. Denudation rates derived from terrestrial cosmogenic nuclides (TCNs) are commonly used to quantify erosion and bridge tectonic (Myr) and climatic (up to several kiloyears) time scales. However, how the processes of erosion in active orogens are ultimately reflected in 10Be TCN samples remains a topic of discussion. We investigate this problem in the Arun Valley of eastern Nepal with 34 new 10Be-derived catchment-mean denudation rates. The Arun Valley is characterized by steep north-south gradients in topography and climate. Locally, denudation rates increase northward, from <0.2 mm yr-1 to ~1.5 mm yr-1 in tributary samples, while main stem samples appear to increase downstream from ~0.2 mm yr-1 at the border with Tibet to 0.91 mm yr-1 in the foreland. Denudation rates most strongly correlate with normalized channel steepness (R2 = 0.67), which has been commonly interpreted to indicate tectonic activity. Significant downstream decrease of 10Be concentration in the main stem Arun suggests that upstream sediment grains are fining to the point that they are operationally excluded from the processed sample. This results in 10Be concentrations and denudation rates that do not uniformly represent the upstream catchment area. We observe strong impacts on 10Be concentrations from local, nonfluvial geomorphic processes, such as glaciation and landsliding coinciding with areas of peak rainfall rates, pointing toward climatic modulation of predominantly tectonically driven denudation rates.

  8. Origin of Relief in a Collisional Orogen: the Case of the Canadian Rockies

    NASA Astrophysics Data System (ADS)

    Osborn, G.; Stockmal, G.; Haspel, R.

    2005-12-01

    Contrary to popular assumption, the mountainous local relief of the Canadian Rockies probably did not originate with the thrusting that characterized the Laramide Orogeny; rather, that relief is a post-orogenic phenomenon. The magnitude of present local relief in the Rockies thrust-and-fold belt (TFB) depends the relative erosional resistance of rocks exposed at the surface. The mountains consist of high ridges of well-indurated Paleozoic and Proterozoic rocks carried by thrust faults, alternating with valleys developed in softer Mesozoic clastic rocks, whereas in the foothills, where the less-resistant Mesozoic rocks are at the surface, relief is subdued. Mesozoic rocks originally blanketed the entire width of what is now the TFB, and local physiography of the belt at the end of Laramide (EOL) time must have depended on whether the Mesozoic rocks had been erosionally removed to expose the underlying, resistant Paleozoic rocks. A reconstruction of the TFB using critical-taper theory generally agrees with reconstructions from earlier stratigraphic and paleothermometry studies: what are now the front ranges in the eastern Rockies were still covered with Mesozoic rocks at the EOL. Hence this part of the belt may have comprised a high-elevation upland of relatively low relief, perhaps broadly similar to the Tibetan Plateau. Generation of modern relief, including the escarpment at the mountain front, had to await stripping of Mesozoic rocks and incision of rivers into harder substrates in post-EOL time. In the 'main ranges' in the western part of the TFB, thrusting and consequent erosion began earlier. Mesozoic cover was partly stripped off by EOL time, and probably locally in early Cretaceous time, so that part of the belt probably displayed 'mountainous' relief then. But the evolution there probably was the same, from original high-elevation, low-relief plateau to later rugged mountains.

  9. Uplift mechanism of orogens inferred from GRACE temporal gravity changes - example of Qinghai-Tibet

    NASA Astrophysics Data System (ADS)

    Braitenberg, C. F.; Shum, C. K.

    2015-12-01

    Orogenic areas are subject to uplift and horizontal deformation as observed by present-day global positioning system and repeated leveling measurements. Crustal mass is conservative and less dense than the mantle, thus the horizontal shortening must be accompanied by crustal thickening and horizontal extrusion. According to the level of isostatic compensation, the thickening is partitioned into topographic uplift and Moho deepening. We investigate the mass change induced gravity signal and discuss whether this signal could be detected using terrestrial or satellite gravity observations. An example is the Qinghai-Tibet plateau, for which we model crustal thickening and calculate the expected gravity signal. The predictions are compared with present-day gravity changes observed by GRACE and with published in situ absolute gravity rates. It is found that the crustal thickening signal cannot be neglected and that it contributes significantly to the observed signal. Those studies with focus on the glacier and hydrologic mass fluxes should be aware that, if neglected, the crustal signal could introduce a significant bias. The GRACE observations give a positive gravity rate over central Tibetan Plateau, unexplained by the hydrologic or cryospheric signals, and a negative rate over the Himalayas and at its foothill, which is attributable to a prolonged hydrologic drought and/or depletion of groundwater. Our model shows that the positive gravity rate could be explained by elevation uplift, and a stable or upwelling Moho. The negative gravity change signal is due primarily to the strong elevation-gradient at the foothill of the Himalayas, and to an uplift accompanied by crustal thickening and Moho lowering. Finally, we place constraints and requirements on future gravity missions, for the feasibility to more accurately observe this signal, and to separate it from the background hydrologic and cryospheric processes.

  10. New geomorphic data on the active Taiwan orogen: A multisource approach

    NASA Technical Reports Server (NTRS)

    Deffontaines, B.; Lee, J.-C.; Angelier, J.; Carvalho, J.; Rudant, J.-P.

    1994-01-01

    A multisource and multiscale approach of Taiwan morphotectonics combines different complementary geomorphic analyses based on a new elevation model (DEM), side-looking airborne radar (SLAR), and satellite (SPOT) imagery, aerial photographs, and control from independent field data. This analysis enables us not only to present an integrated geomorphic description of the Taiwan orogen but also to highlight some new geodynamic aspects. Well-known, major geological structures such as the Longitudinal Valley, Lishan, Pingtung, and the Foothills fault zones are of course clearly recognized, but numerous, previously unrecognized structures appear distributed within different regions of Taiwan. For instance, transfer fault zones within the Western Foothills and the Central Range are identified based on analyses of lineaments and general morphology. In many cases, the existence of geomorphic features identified in general images is supported by the results of geological field analyses carried out independently. In turn, the field analyses of structures and mechanisms at some sites provide a key for interpreting similar geomorphic featues in other areas. Examples are the conjugate pattern of strike-slip faults within the Central Range and the oblique fold-and-thrust pattern of the Coastal Range. Furthermore, neotectonic and morphological analyses (drainage and erosional surfaces) has been combined in order to obtain a more comprehensive description and interpretation of neotectonic features in Taiwan, such as for the Longitudinal Valley Fault. Next, at a more general scale, numerical processing of digital elevation models, resulting in average topography, summit level or base level maps, allows identification of major features related to the dynamics of uplift and erosion and estimates of erosion balance. Finally, a preliminary morphotectonic sketch map of Taiwan, combining information from all the sources listed above, is presented.

  11. Geological and thermochronological studies of the Dashui gold deposit, West Qinling Orogen, Central China

    NASA Astrophysics Data System (ADS)

    Zeng, Qingtao; Evans, Noreen J.; McInnes, Brent I. A.; Batt, Geoffrey E.; McCuaig, Campbell T.; Bagas, Leon; Tohver, Eric

    2013-03-01

    The Dashui gold deposit is a structurally controlled, Carlin-type gold deposit hosted by recrystallised limestone in the West Qinling Orogen of Central China. The major, structurally late east-trending Dashui Fault forms the hanging wall to the gold mineralisation at the Dashui mine and defines the contact between Middle Triassic limestone and a steeply dipping overlying succession of Middle Triassic argillaceous limestone, dolomite, and sandstone. Multiple carbonate veins and large-scale supergene enrichment, represented by hematite, goethite, limonite and jarosite, characterise the deposit. Detailed geochronological investigation using zircon SHRIMP U-Pb dating reveals that volcanic rocks closely associated with the Dashui gold deposit were synchronous with the Ge'erkuohe Granite and pre-date mineralisation. The igneous dyke sample from the hanging wall has the same U-Pb zircon age as the footwall, ca. 213 Ma. (U-Th)/He thermochronology on dykes in the hanging wall and footwall of the Dashui Fault yields identical (U-Th)/He zircon ages of ca. 210 Ma but distinct (U-Th)/He apatite ages of ca. 136 and 211 Ma, respectively. Therefore, the hanging wall and footwall are interpreted as having distinct post-mineralisation exhumation histories. Reverse fault movement exhumed the hanging wall ~2 to 4 km since the Late Triassic with the main component of faulting taking place between the Late Triassic and Early Cretaceous. These relationships suggest a Late Triassic to Early Cretaceous age for the primary gold mineralisation at the Dashui gold deposit, with the corollary that any `missing portion' of the deposit, previously hypothesised to exist in the hanging wall of the Dashui Fault, has been eroded away. The mineralisation in the footwall may have been supergene enriched soon after the primary mineralisation was emplaced, because it has been located at shallow depth since the Late Triassic. Semi-quantitative results obtained in this study also constrain the maximum

  12. Exhumation of an eclogite terrane as a hot migmatitic nappe, Sveconorwegian orogen

    NASA Astrophysics Data System (ADS)

    Möller, Charlotte; Andersson, Jenny; Dyck, Brendan; Antal Lundin, Ildiko

    2015-06-01

    We demonstrate a case of eclogite exhumation in a partially molten, low-viscosity fold nappe within high-grade metamorphosed crust in the Eastern Segment of the Sveconorwegian orogen. The nappe formed during tectonic extrusion, melt-weakening assisted exhumation and foreland-directed translation of eclogitized crust, and stalled at 35-40 km depth within the collisional belt. The eclogites are structurally restricted to a regional recumbent fold in which stromatic orthogneiss with pods of amphibolitized eclogite make up the core. High-temperature mylonitic gneiss with remnants of kyanite eclogite (P > 15 kbar) composes a basal shear zone 50 km long and < 4 km wide. Heterogeneously sheared and partly migmatized augen gneiss forms a tectonostratigraphic marker in front of and beneath the nappe, and is in turn structurally enveloped by a composite sequence of orthogneisses and metabasites. The entire tectonostratigraphic pile underwent near-pervasive deformation and recrystallization under high-pressure granulite and upper amphibolite conditions. U-Pb SIMS metamorphic zircon ages of eclogite and stromatic orthogneiss constrain the time of eclogitization at 988 ± 6 Ma and 978 ± 7 Ma. Migmatization, concomitant deformation, and exhumation are dated at 976 ± 6 Ma, and crystallization of post-kinematic melt at 956 ± 7 Ma. Orthogneiss protoliths are dated at 1733 ± 11 and 1677 ± 10 Ma (stromatic gneiss) and 1388 ± 7 Ma (augen gneiss in footwall), demonstrating origins indigenous to the Eastern Segment. Eclogitization and exhumation were coeval with the Rigolet phase of the Grenvillian orogeny, reflecting the late stage of continental collision during construction of the supercontinent Rodinia.

  13. Tectonic, magmatic, and metallogenic evolution of the Late Cretaceous arc in the Carpathian-Balkan orogen

    NASA Astrophysics Data System (ADS)

    Gallhofer, Daniela; Quadt, Albrecht von; Peytcheva, Irena; Schmid, Stefan M.; Heinrich, Christoph A.

    2015-09-01

    The Apuseni-Banat-Timok-Srednogorie Late Cretaceous magmatic arc in the Carpathian-Balkan orogen formed on the European margin during closure of the Neotethys Ocean. It was subsequently deformed into a complex orocline by continental collisions. The Cu-Au mineralized arc consists of geologically distinct segments: the Apuseni, Banat, Timok, Panagyurishte, and Eastern Srednogorie segments. New U-Pb zircon ages and geochemical whole rock data for the Banat and Apuseni segments are combined with previously published data to reconstruct the original arc geometry and better constrain its tectonic evolution. Trace element and isotopic signatures of the arc magmas indicate a subduction-enriched source in all segments and variable contamination by continental crust. The magmatic arc was active for 25 Myr (~92-67 Ma). Across-arc age trends of progressively younger ages toward the inferred paleo-trench indicate gradual steepening of the subducting slab away from the upper plate European margin. This leads to asthenospheric corner flow in the overriding plate, which is recorded by decreasing 87Sr/86Sr (0.70577 to 0.70373) and increasing 143Nd/144Nd (0.51234 to 0.51264) ratios over time in some segments. The close spatial relationship between arc magmatism, large-scale shear zones, and related strike-slip sedimentary basins in the Timok and Pangyurishte segments indicates mild transtension in these central segments of the restored arc. In contrast, the Eastern Srednogorie segment underwent strong orthogonal intraarc extension. Segmental distribution of tectonic stress may account for the concentration of rich porphyry Cu deposits in the transtensional segments, where lower crustal magma storage and fractionation favored the evolution of volatile-rich magmas.

  14. Thin visous sheet modelling of orogen scale deformation. The Eastern Alps in plan view

    NASA Astrophysics Data System (ADS)

    Robl, J.; Stuewe, K.

    2003-04-01

    We present first results of a new numerical model to describe the dynamic evolution of the eastern Alps in plan view on the orogen scale. We investigate the influence of boundary conditions, gravitational potential energy, rheology contrast of major tectonic units and internal structures on the deformation field. We aim at estimating the Argan number of the Eastern Alps and to calculate slip rates along big lineaments that represent the northern and southern border of the Austro-Alpine extrusion corridor. A further aim is to predict the position and the activity of major faults where they disappear below quartenary sediments. All calculation are perfomed with the mechanical finite element code BASIL that allows computation on a thin visous sheet. The starting conditons are controlled by the varying crustal thickness of the region and by the rheolgy of the Adriatic indenter in the south, the Bohemian massif in the north and the Eastern Alps inbetween. We assume that the Eastern Alps are fixed to the north and the west while the southern boundary moves northward at a rate of 6-8 mm /y. The geodynamic setting in the east changed over the last 5 my. While a roll back subduction zone beneath the Carparthian belt accompanied by extension, crustal thinning and basin formation controlled the deformation of the Eastern Alps until the Miocene. Later on subduction stopped and the overall stress field changed from extension to compression resulting in uplift of many basins and the lack sediments younger than Pliocene. This well known variation of plate tectonic scenarios over the last 5 my allows us to place tight constraints for the boundary conditions of our model. Although our work is only now in progress, we can report some promising results: some of the kinematics and mechaniscs predicted by our modelling are consistent with field observations of the structural geologists and geodeticists.

  15. Orogenic structure of the Eastern Alps, Europe, from TRANSALP deep seismic reflection profiling

    NASA Astrophysics Data System (ADS)

    Lüschen, Ewald; Lammerer, Bernd; Gebrande, Helmut; Millahn, Karl; Nicolich, Rinaldo; Transalp Working Group

    2004-09-01

    The TRANSALP Group, comprising of partner institutions from Italy, Austria and Germany, acquired data on a 340 km long deep seismic reflection line crossing the Eastern Alps between Munich and Venice. Although the field work was split into four campaigns, between fall 1998 and summer 2001, the project gathered for the first time a continuous profile across the Alps using consistent field acquisition and data processing parameters. These sections span the orogen itself, at its broadest width, as well as the editor Fred Davey and the two adjacent basins. Vibroseis and explosion data, complementary in their depth penetration and resolution characteristics, were obtained along with wide-angle and teleseismic data. The profile shows a bi-vergent asymmetric structure of the crust beneath the Alpine axis which reaches a maximum thickness of 55 km, and 80-100 km long transcrustal ramps, the southward dipping 'Sub-Tauern-Ramp' and the northward-dipping 'Sub-Dolomites-Ramp'. Strongly reflective patterns of these ramps can be traced towards the north to the Inn Valley and towards the south to the Valsugana thrust belt, both of which show enhanced seismicity in the brittle upper crust. The seismic sections do not reveal any direct evidence for the presence of the Periadriatic Fault system, the presumed equivalent to the Insubric Line in the Western Alps. According to our new evolutionary model, the Sub-Tauern-Ramp is linked at depth with remnants of the subducted Penninic Ocean. The 'crocodile'-type model describes an upper/lower crustal decoupling and wedging of both the European and the Adriatic-African continents.

  16. The Ust-Belaya ophiolite terrane, West Koryak Orogen: Isotopic dating and paleotectonic interpretation

    NASA Astrophysics Data System (ADS)

    Palandzhyan, S. A.

    2015-03-01

    The Ust-Belaya ophiolite terrane in the West Koryak Orogen, which is the largest in northeastern Asia, consists of three nappe complexes. The upper Ust-Belaya Nappe is composed of a thick (>5 km) sheet of fertile peridotites and mafic rocks (remnants of the proto-Pacific lithosphere); its upper age boundary is marked by Late Neoproterozoic plagiogranites. In the middle Tolovka-Otrozhny Nappe, the Late Precambrian lherzolite-type ophiolites are supplemented by fragments of tectonically delaminated harzburgite-type ophiolites, which make up the Tolovka rock association. The isotopic age of metadacite (K-Ar method, whole-rock sample) and zircons from plagiogranite porphyry (U-Pb method, SHRIMP) determines the upper chronological limit of the Tolovka ophiolites as 262-265 Ma ago. It is suggested that igneous rocks of these ophiolites were generated in a backarc basin during the Early Carboniferous and then incorporated into the fold-nappe structure in the Mid-Permian. This was the future basement of the Koni-Taigonos arc, where the Early Carboniferous ophiolites together with Late Neoproterozoic precursors were subject to low-temperature metamorphism and intruded by plagiogranite porphyry dikes in Permian-Triassic. The polymicte serpentinite mélange, which was formed in the accretionary complex of the Koni-Taigonos arc comprises rock blocks of the upper units of Late Precambrian ophiolites (in particular, plagiogranite), the overlying Middle to Upper Devonian and Early Carboniferous deposits, as well as Early Carboniferous (?) Tolovka ophiolites and meta-ophiolites. Mélange of this type with inclusions of Late Precambrian "oceanic" granitoids also developed in the lower Utyosiki Nappe composed of Middle Jurassic-Lower Cretaceous sedimentary and volcanic sequences, the formation of which was related to the next Uda-Murgal island-arc systems.

  17. Transpressional deformation, strain partitioning and fold superimposition in the southern Chinese Altai, Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Li, Pengfei; Sun, Min; Rosenbaum, Gideon; Cai, Keda; Chen, Ming; He, Yulin

    2016-06-01

    Transpressional deformation has played an important role in the late Paleozoic evolution of the western Central Asian Orogenic Belt (CAOB), and understanding the structural evolution of such transpressional zones is crucial for tectonic reconstructions. Here we focus on the transpressional Irtysh Shear Zone with an aim at understanding amalgamation processes between the Chinese Altai and the West/East Junggar. We mapped macroscopic fold structures in the southern Chinese Altai and analyzed their relationships with the development of the adjacent Irtysh Shear Zone. Structural observations from these macroscopic folds show evidence for four generations of folding and associated fabrics. The earlier fabric (S1), is locally recognized in low strain areas, and is commonly isoclinally folded by F2 folds that have an axial plane orientation parallel to the dominant fabric (S2). S2 is associated with a shallowly plunging stretching lineation (L2), and defines ∼NW-SE tight-close upright macroscopic folds (F3) with the doubly plunging geometry. F3 folds are superimposed by ∼NNW-SSE gentle F4 folds. The F3 and F4 folds are kinematically compatible with sinistral transpressional deformation along the Irtysh Shear Zone and may represent strain partitioning during deformation. The sub-parallelism of F3 fold axis with the Irtysh Shear Zone may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation (F3) in fold zones. The strain partitioning may have become less efficient in the later stage of transpressional deformation, so that a fraction of transcurrent components was partitioned into F4 folds.

  18. Differential decay of the East-African Antarctic Orogen : an integrated examination of Northeastern Mozambique

    NASA Astrophysics Data System (ADS)

    Ueda, K.; Jacobs, J.; Emmel, B.; Thomas, R. J.; Matola, R.

    2009-04-01

    In Northeastern Mozambique, the late Proterozoic - early Paleozoic East African-Antarctic Orogen can be subdivided into two major blocks that exhibit some relevant differences. The line of divide is represented by the Lurio Belt, a kinematically poorly constrained shear zone that also marks the conceptual northern limit of frequent late-tectonic granitoid intrusions. Moreover, far-travelled granulite-facies nappes cover a much larger area north of this belt (Viola et. al, 2008), giving rise to the assumption of different exhumation and present exposure levels. U/Pb data from previous surveys (e.g., Norconsult consortium, 2007) show coeval high-grade metamorphism in the whole region between c. 610 - 550 Ma, while the block south of the Lurio Belt also shows continuing metamorphism until c. 490 Ma that can be related to extension. Geothermobarometry for samples from within the Lurio Belt (Engvik et. al, 2007) indicates rapid exhumation after high-pressure granulite facies metamorphism and is consistant with the assumption of long tectonic activity. A possible model for the outlined pattern is the delamination of the orogenic root only in the southern part, followed by rapid mechanical thinning as well as by isostatic accommodation along the Lurio Belt. A valuable marker was identified in the metasedimentary Mecuburi group that overlies the southern basement. U/Pb analysis of detrital zircons have yielded a maximum deposition age of c. 600 Ma, while metamorphism is recorded until c. 505 Ma. Investigations of the relationship between metasediments and older basement show that the basal contact is a fairly preserved depositional contact, allowing to suppose a conjoint post-depositional evolution. It is notable that the timing of deposition shortly follows the onset of the main, widespread high-grade metamorphism. Relatively high but variable degrees of migmatisation in the Mecuburi Group require a phase of burial from surface to deep levels after 600 Ma, followed by

  19. Controls on the Post-Orogenic Evolution of the Lannemezan Megafan in the Northern Pyrenean Foreland: Insights from Cosmogenic Nuclide Exposure Dating and Morphometric Analysis

    NASA Astrophysics Data System (ADS)

    Mouchene, M.; Van Der Beek, P.; Mouthereau, F.; Carcaillet, J.

    2015-12-01

    Fluvial megafans are large, low-gradient geomorphic features characterized by sandy and muddy sediments deposited by distributive sheet-flows. They hold a critical position in the source-to-sink routing of sediments and are thus particularly sensitive to changes in the erosion-deposition dynamics through tectonic and/or climatic forcing. The Mio-Pliocene Lannemezan megafan (SW France) is the most prominent feature of the Northern Pyrenean foreland and was abandoned and incised at some point in the Quaternary. However, neither the precise age of megafan abandonment nor its driving mechanism within a stable post-orogenic context has been elucidated, and either tectonic, climatic or base-level controls have been proposed. We report new cosmogenic exposure ages for the abandonment of the surface of the megafan as well as for a series of alluvial terraces produced during the incision. We show that the modern morphology of the foreland was acquired only recently, as the abandonment of the megafan happened at 253 ± 28 ky to 288 ± 12 ky, much later than what was previously assumed (Early Pleistocene). The subsequent incision of the megafan appears to have been strongly influenced by glacial/interglacial cycles, as terrace abandonment ages correlate to the MIS7, MIS5 and post-LGM cold-to-warm transitions. Geomorphic analysis of the stream network reveals that the river profile concavity increased through time and shows no indications for systematic knickpoint generation or retreat, as expected if the river network responds to active tectonics or base-level change, respectively. We do evidence river network reorganizations through river captures using chi proxy (χ) analysis. We propose that this pattern results from flexural isostatic rebound in response to increased erosion rates within the range due to Quaternary glaciations.

  20. Tracing long term tectonic evolution of accretionary orogens by U-Pb zircon geochronology: Proterozoic to Jurassic tectonics of the Santander Massif, northern Colombia

    NASA Astrophysics Data System (ADS)

    Valencia, V. A.; Cardona, A.; Gehrels, G. E.; Ruiz, J.; Ibañez, M.

    2009-12-01

    Accurate orogenic models are nedded to reconstruct complex tectonic histories of long lived convergent margins. Integrated zircon U-Pb geochronology on igneous, sedimentary and metasedimentry rocks within single crustal domains is a powerful tool, as it can be used to trace the timing of rock forming events, magmatic style and episodity, and identify crustal recycling. U-Pb detrital zircon and magmatic geochronology was carried on multiple litostratigraphic units of the Santander Massif in the northeastern Andes, in order to reconstruct its long term Late Proterozoic to Early Mesozoic tectonic evolution. Major zircon forming events includ well defined Grenvillian, Late Neoproterozoic to Ordovician, Silurian, Early Permian and Jurassic events. Major peaks of activity at ca. 197 Ma, 440-410 Ma and 470-490 Ma and 950-1052 Ma, support the existence of continental scale tectonic cycles. Older Mesoproterozoic (1.3-1.5 Ga) crustal input in metasediments and magmatic rocks link these units to crustal recycling on the margins of the Amazon Craton, whereas the older 950-1052 Ma peak indicates the link of this crustal segment with other Andean Grenvillian remnant. Previous interpretations of the Paleozoic Silgara Formation seem incorrect, as acquired dates from this study includ different metamorphic units, deposited and formed after the Silurian and Permian during final stages of Pangea's assemblage, probably as Laurentia migrated to its final Alleghanian position. Finally the presence of the NW South America Jurassic arc is also present in the region by granitoid ages. The limited input of this arc signature within the contemporaneous and overlapping Early Cretaceous sedimentary rocks suggest that this arc was developed in a back arc setting.

  1. Revisiting the Wilson Cycle in the North Atlantic: The role of inheritance

    NASA Astrophysics Data System (ADS)

    Chenin, Pauline; Manatschal, Gianreto; Müntener, Othmar; Picazo, Suzanne; Erratt, Duncan; Karner, Garry; Johnson, Christopher; Ulrich, Marc

    2016-04-01

    According to the Wilson Cycle, oceans open and close approximately parallel to ancient suture zones, suggesting a major control of inheritance in the extension and convergent process. While this paradigm is well illustrated in the northern North Atlantic where the rift follows largely the Iapetus suture between Norway and Greenland, this is not the case for the southern North Atlantic, where neither the westward, nor the northward propagating branch of the Central Atlantic rift, affected the Variscan sutures of Western Europe. These observations suggest that inheritance is not necessarily reactivated during subsequent rifting events and begs the question about what may truly control the localization and details of rift systems. One possible cause for the differing behavior of the North Atlantic rift with respect to the Caledonian and Variscan orogens may be their contrasting paleo-geographic settings. Indeed, the Scandinavian Caledonides resulted from the closure of one wide ocean between two cratonic shields, whereas the Variscides were built from the accretion of several terranes/micro-continents following the closure of a series of narrow oceans. The variability in the initial architecture of the intervening rift systems and in their subduction processes may have significantly controlled the subsequent orogenies. The aim of this presentation is twofold: first we investigate how the first-order structural and lithological characteristics of narrow/embryonic versus wide/mature oceans, as well as the processes associated with their subduction and collision, characterize orogens. Second, we study how this variable orogenic inheritance may impact subsequent rifting. Our results suggest that: (1) the margins from narrow/embryonic and wide/mature oceans are comparable, therefore the major difference between these end-members is the existence of a significant amount of normal oceanic crust; (2) subduction-induced processes significantly impact both the thermal state and

  2. Srsbnd Ndsbnd Pb isotopic compositions of Early Cretaceous granitoids from the Dabie orogen: Constraints on the recycled lower continental crust

    NASA Astrophysics Data System (ADS)

    He, Yongsheng; Li, Shuguang; Hoefs, Jochen; Kleinhanns, Ilka C.

    2013-01-01

    In order to characterize the recycled lower continental crust (LCC) in the Dabie orogen, 17 Early Cretaceous low-Mg adakitic (LMA) and 9 normal (non-adakitic) granitoids have been investigated for Srsbnd Ndsbnd Pb isotopes. Combined with literature data, LMA have low ƐNd(t) (- 27.8 to - 14.7) and 206Pb/204Pb(i) (15.69-17.16) and low to moderately high 87Sr/86Sr(i) (0.7066 to 0.7087) ratios. Normal granitoids yield isotope ratios similar to adakitic rocks, except a few with 87Sr/86Sr (i) up to 0.7105. Dabie LMA define a linear trend parallel to the North Hemisphere Reference Line (NHRL) in a 208Pb/204Pb(i)- 206Pb/204Pb(i) diagram. For a given 206Pb/204Pb(i), the 208Pb/204Pb(i) or ∆8/4 (152-217) of Dabie LMA are close to the majority of UHP gneisses and the Neoproterozoic mafic rocks from the northern margin of the South China Block (SCB), but significantly higher than adakitic rocks from the North China Block (∆8/4 < 150). Considering the commonly present Neoproterozoic inherited zircons, we suggest that the LMA in the Dabie orogen are derived from a thickened LCC that could be dominantly composed of ancient SCB lower crust. The Srsbnd Ndsbnd Pb isotopic composition of LMA are similar to those of Post-Collisional Mafic Igneous rocks (PCMI) from the Dabie orogen, but different to exhumed UHP rocks. The Srsbnd Ndsbnd Pb isotopic system of the mantle source of the PCMI could be enriched in components dominantly from delaminated LCC of the Dabie orogen instead of deeply subducted continental crust, which is in contrast to O- and C-isotope data by [Zhao, Z.F., Zheng, Y.F., Wei, C.S., Wu, Y.B., Chen, F.K., and Jahn, B.M., 2005. Zircon Usbnd Pb age, element and Csbnd O isotope geochemistry of post-collisional mafic-ultramafic rocks from the Dabie orogen in east-central China. Lithos 83(1-2), 1-28; Dai, L.Q., Zhao, Z.F., Zheng, Y.F., Li, Q.L., Yang, Y.H., and Dai, M.N., 2011. Zircon Hfsbnd O isotope evidence for crust-mantle interaction during continental deep

  3. Menu Cycles.

    ERIC Educational Resources Information Center

    Clayton, Alfred; Almony, John

    The curriculum guide for commercial foods instruction is designed to aid the teacher in communicating the importance of menu cycles in commercial food production. It also provides information about the necessary steps in getting food from the raw form to the finished product, and then to the consumer. In addition to providing information on how to…

  4. Partial melting of the South Qinling orogenic crust, China: Evidence from Triassic migmatites and diorites of the Foping dome

    NASA Astrophysics Data System (ADS)

    Zhang, He; Ye, Ri-Sheng; Liu, Bing-Xiang; Wang, Yan; Zhang, Yuan-Shuo; Siebel, Wolfgang; Chen, Fukun

    2016-09-01

    The Qinling orogen was ultimately formed by suturing of the South Qinling and Yangtze blocks, but the exact timing of the final amalgamation of the two blocks has not been well established so far. Partial melting of the Qinling orogenic continental crust resulted in the generation of migmatites, and such rocks may help to decipher the chronology of such event. In this paper, we report U-Pb ages, trace element, and Hf isotopic compositions of zircons from migmatites and diorite gneisses of the Foping dome, South Qinling. Zircons from migmatites form anhedral grains of variable sizes that are characterized by complex trace element compositions. Based on zircon U-Pb ages, the migmatites can be subdivided into two groups: Group 1 migmatites mainly retain Triassic zircons with U-Pb ages of 214-211 Ma and Hf model ages of ~ 1.46 Ga in core and rim domains; zircons from Group 2 migmatites record both Triassic (~ 210 Ma) and Neoproterozoic U-Pb ages, analogous to igneous rocks of the Wudang and Yaolinghe Groups exposed in South Qinling. Zircons from the diorite gneisses yield U-Pb ages of 216-210 Ma with Hf isotopic composition (TDM2 ages of ~ 1.46 Ga) similar to the migmatites. Evidence from whole-rock Nd isotopic analyses also points to a similar genesis between migmatites and diorite gneisses. It is proposed that Group 1 migmatites were derived by melting of Triassic diorites, while Group 2 migmatites were derived from Neoproterozoic igneous rocks, a major basement lithology of South Qinling. Partial melting of the orogenic crust took place at ~ 214-210 Ma, approximately consistent with the retrograde metamorphism of granulites exposed along the suture zone between the South Qinling and Yangtze blocks. We suggest that the collision of these two blocks occurred prior to ~ 215 Ma and that the Foping dome resulted from rapid collapse of an overthickened crust followed by partial melting enhanced by asthenospheric influx.

  5. Signature of Cenozoic orogenic movements in combustion metamorphic rocks: mineralogy and geochronology (example of the Salair-Kuznetsk Basin transition)

    SciTech Connect

    Novikov, I.S.; Sokol, E.V.; Travin, A.V.; Novikova, S.A.

    2008-06-15

    Cenozoic combustion metamorphic (CM) complexes produced by fossil natural coal fires are widespread at range-basin junctions worldwide. Large-scale fires accompany the initial orogenic phases as fresh coal-bearing strata become drawn into the aeration zone as a result of crustal deformation. In combustion metamorphism, the protolith melts to different degrees either into ferrous basic paralava or in glassy clinker. The melt rocks have a phase composition favorable for Ar-40/Ar-39 dating of ignition coeval with the onset of each episode in Late Cenozoic orogenic events. We suggest an algorithm providing correct Ar-40/Ar-39 age determination of CM rocks followed by well-grounded geological interpretation and test the new approach on melt rocks from the Kuznetsk Coal Basin. Paralava samples were dated by Ar-40/Ar-39 incremental heating and the isotope ratios were corrected for Ca-, Cl-, and K-derived Ar isotopic interferences. The interpretation of age-spectrum results was checked against internal and external criteria. The former were plateau and isochrone ages and the latter included the so-called 'couple criterion' and conventional relative ages inferred from geological and stratigraphic evidence. As a result, we distinguished two groups of dates for combustion metamorphic events bracketed between 1.2 {+-} 0.4 and 0.2 {+-} 0.3 Ma. The older ages represent rocks in the western edge of the Prokopievsk-Kiselevsk block of the Salair zone and the younger dates correspond to those in its eastern edge. The reported dates record the time when the fault boundaries of the blocks were rejuvenated during recent activity and the block accreted to the Salair orogenic area as a submontane step. The suggested approach to the choice of objects, classification of rocks, and interpretation of Ar-40/Ar-39 spectra is universal and can be practiced in any area of combustion metamorphism.

  6. Distant effects in bivergent orogenic belts - How retro-wedge erosion triggers resource formation in pro-foreland basins

    NASA Astrophysics Data System (ADS)

    Hoth, Silvan; Kukowski, Nina; Oncken, Onno

    2008-08-01

    Timeseries derived from two-dimensional sandbox simulations involving surface erosion are taken for the first time to be implemented into flexure calculations of foreland basins. Based on our results we highlight that orogenic systems are a four component system, consisting of a pro-foreland basin, a pro-wedge, a retro-wedge, and a retro-foreland basin. These four components are mechanically coupled via the load dependence of tectonic faulting [Mandl, G., 1988. Mechanics of tectonic faulting, 1st Edition. Elsevier, Amsterdam.] and the finite flexural rigidity of lithospheric plates [Beaumont, C., 1981. Foreland basins. Geophys. J. R. Astron. Soc. 5 (2), 291-329.]. We further demonstrate that the impact of pro-wedge erosion is most pronounced within the pro-wedge but also modifies the shape and size of the retro-wedge, which in turn changes the geometry and propagation velocity of the retro-foreland basin and vice versa. This suggests that one out of the four components of an orogenic system cannot be fully understood without recognition of the other three components. Thus, spatial separation between processes or observations does not necessarily imply their physical independence. This conceptual model is applied in a case study to the Pyrenean orogenic wedge and its Ebro and Aquitaine foreland basins. Our analysis suggests that the Pyrenean pro- and retro-wedge are mechanically coupled and that this coupling manifests itself in the migration of depocentres in both foreland basins. We finally explore implications for the formation of Mississippi Valley Type deposits.

  7. Contrasting Triassic ferroan granitoids from northwestern Liaoning, North China: Magmatic monitor of Mesozoic decratonization and a craton-orogen boundary

    NASA Astrophysics Data System (ADS)

    Zhang, Xiaohui; Yuan, Lingling; Xue, Fuhong; Zhang, Yanbin

    2012-07-01

    The diversity exhibited by ferroan granitoids testifies to multiple distinct crustal and mantle processes generally within extensional environments and thus holds the key to monitoring important geodynamic and crustal evolutionary processes and calibrating terrane tectonic affinity. This study documents three Middle to Late Triassic ferroan granitoid suites from northwestern Liaoning, North China, and proposes distinct petrogenetic scenarios for each. The Middle Triassic (ca. 238 Ma) Ping'andi (PAD) granites are mainly calc-alkali and peraluminous. Their radiogenic whole-rock Nd and zircon Hf isotopic signatures argue for an origin consistent with partial melting of a juvenile quartzofeldspathic crust formed by prior mantle-derived magmatic underplating and their differentiations. By contrast, the Late Triassic (ca. 220 Ma) Dashaoleng (DSL) and Sijiazi (SJZ) suites show an evolved character from alkali-calcic to alkali and from metaluminous to peraluminous. Their variably evolved whole-rock Sr-Nd and mixed zircon Hf isotopic compositions suggest that both suites were formed by variable mixing between depleted mantle-derived mafic magma and old crust-derived felsic magma, with distinctively higher input from juvenile components in the DSL suite. These contrasting ferroan granitoid suites not only provide a spatial marker for monitoring juxtaposition of the North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB) along a lithospheric-scale boundary fault in the region, but also present a temporal snapshot that records a southwardly-progressing crustal growth scenario possibly in response to lithospheric dripping within a post-orogenic extensional regime. The cratonic scale synthesis further indicates that diachronous decratonization pattern of the NCC might record episodic response of the craton to evolved plate tectonic processes with two contrasting Phanerozoic orogenic systems.

  8. Numerical investigation of the geodynamic mechanism for the late Jurassic deformation of the Ordos block and surrounding orogenic belts

    NASA Astrophysics Data System (ADS)

    Sun, Yujun; Dong, Shuwen; Zhang, Huai; Shi, Yaolin

    2015-12-01

    Orogenic belts have developed along the edges of the stable Ordos block, in northern China. Three main geodynamic models have been proposed to explain the formation of these orogenic belts. They have included the collision between the North China and South China blocks, subduction of the Pacific plate, and stresses transmitted over long distances from the closure of the Mongolia-Okhotsk ocean. However, these explanations are still controversial and not universally accepted, leaving the mechanisms that formed the orogenic belts poorly understood. To address these fundamental questions, we developed a 3D numerical model using the finite element method to explore the geodynamic mechanism for the late Jurassic deformation of the Ordos block and its surrounding orogenic belts. We investigated the effect of different dynamic regimes on the late Jurassic deformation of this region. Our primary results suggest that strong and stable Ordos block remains undeformed despite its location at the center of a region of deformation. East-west trending fold-and-thrust belts would have developed along the north and south edges of the Ordos block during the closure of the Mongolia-Okhotsk ocean or the collision between the North China and South China blocks. North-south trending fold-and-thrust belts would have developed along the east and west edges of the Ordos block due to subduction of the Pacific plate. However, the paleo-stress field in the late Jurassic indicates that the orientations of the maximum compressive principle stress were nearly perpendicular to the edge of the Ordos block and the compressive deformation around it was coeval. It is difficult to explain the distribution of belts of deformation with a single stress regime. Our numerical model reveals that multi-direction convergence pattern with time during the transformation of these three regimes can be used to interpret the formation and deformation styles of ringed mountains around the Ordos block during the late

  9. Orogenic gold mineralization at the Chah Bagh deposit, Muteh gold district, Iran

    NASA Astrophysics Data System (ADS)

    Kouhestani, Hossein; Rashidnejad-Omran, Nematollah; Rastad, Ebrahim; Mohajjel, Mohammad; Goldfarb, Richard J.; Ghaderi, Majid

    2014-09-01

    The Chah Bagh gold deposit, in the Muteh gold district, is located in the central part of the Sanandaj-Sirjan zone (SSZ), Iran. Gold mineralization at Chah Bagh is hosted by a Paleozoic sequence of rocks that is dominated by deformed schist, metarhyolite, and amphibolite that exhibits a greenschist- to lower amphibolite-facies metamorphism. Three deformation events are recognized in the Chah Bagh area, D1, D2, and D3. The major NW-trending (N280-N290) dextral strike-slip shear zone in the area was formed during D2 ductile events. Gold mineralization at Chah Bagh occurred over a prolonged deformation history, but is closely related to alteration, retrograde greenschist-facies assemblages, and ductile and brittle deformation during D2 and D3. The geometry of the Au-bearing quartz veins indicates that they are temporally related to the S2 foliation and therefore to the D2 flattening and shearing. Some veins, however, are spatially and temporally related to D3 brittle normal faults and are brecciated and boudinaged during the associated shear movement. The presence of deformed Au-bearing quartz veins, and their concordant and discordant relation with respect to the main mylonitic foliation and the shear zone, indicates continuous mineralization during the D2 and D3 episodes. The Au-hosting shear zones are characterized by extensive development of heterogeneous mylonitic rocks that enhanced the permeability within the shear zones. This gave rise to further extensive dilatancy within major dilational jogs and produced a suitable structural regime for vein-hosted Au mineralization. The epigenetic Au mineralization resulted from metamorphic hydrothermal fluids circulating through major shear zones and associated structures during the late stages of orogeny. Our investigation shows that granitic intrusions have no genetic link with gold mineralization and we propose an orogenic gold model for Chah Bagh deposit, similar to Qolqoleh and Kervian in the northwestern part of the

  10. Two modes of orogenic collapse of the Pamir plateau recorded by titanite

    NASA Astrophysics Data System (ADS)

    Stearns, M. A.; Hacker, B. R.; Ratschbacher, L.; Rutte, D.; Kylander-Clark, A. R.

    2013-12-01

    Processes that operate in the mid- to lower crust during and following continent-continent collision are important for understanding how orogenic plateaux transition from thickening to collapse. In the central and southern Pamir, mid- to lower crustal rocks crop out in two belts of extensional domes. The central Pamir domes were exhumed by symmetrical N-S extension. In contrast, the southern Pamir domes were exhumed by asymmetrical top to the south (NNW-SSE) extension via a rolling-hinge detachment. To investigate the high-temperature exhumation history, titanites were dated using LASS (laser ablation split stream-ICP-MS). A multi-collector ICP was used to collect U-Pb isotopic ratios and a single collector ICP-MS was used to measure trace-element abundances. The data indicate that the central Pamir domes began exhumation synchronously at ~17 Ma. Titanite from the southern Pamir record two periods of protracted (re)crystallization: older metamorphic dates ranging from ~35-18 Ma and younger igneous and metamorphic dates from ~15-7 Ma. Samples with single populations of titanite dates are present throughout both groups. Samples with more-complex date populations typically have distinct trace-element (e.g., Sr, Y, Zr, and Nb) groups that can be used to distinguish different date populations (e.g., older dates may have higher Zr and younger dates lower Zr). The distinct early exhumation histories of the north and south Pamir require either a diachronous single process or two semi-independent processes. The N to S sequence of exhumation, ranges of dates, and overall extension directions may be related to two important plate-tectonic events inferred from seismic data: 1) breakoff of the northward subducting Indian slab around ~20 Ma, and 2) southward subduction and northwestward rollback of the Asian lithosphere between ~15-10 Ma based on geodetic convergence rates and Benioff zone length. We interpret these two lithospheric-detachment events to have driven the

  11. Hydrothermal alteration in metasedimentary rock-hosted orogenic gold deposits, Reefton goldfield, South Island, New Zealand

    NASA Astrophysics Data System (ADS)

    Christie, Anthony B.; Brathwaite, Robert L.

    2003-01-01

    Orogenic or mesothermal quartz lodes in lower Palaeozoic Greenland Group metasedimentary rocks of the Reefton area have produced 67 tonnes (t) of gold prior to 1951, and recent exploration has identified new gold resources in several deposits, including the largest past producers, Blackwater and Globe-Progress. The metasedimentary rocks consist of alternating sandstone and mudstone beds that were metamorphosed to lower greenschist facies prior to being hydrothermally altered adjacent to the quartz lodes. The sandstones are feldspathic litharenites averaging Q65-F10-R25, with detrital grains of quartz, rock fragments, muscovite, and plagioclase and biotite that were altered to albite and chlorite, respectively, during metamorphism. Accessory minerals are graphite, apatite, zircon, tourmaline and titanite. Hydrothermal alteration of the sandstones has developed a mineral assemblage of K-mica, carbonate (dolomite, ankerite, ferroan magnesite and magnesian siderite), chlorite, pyrite and arsenopyrite. The abundance of hydrothermal chlorite is greater at Blackwater than at the other prospects studied. Hydrothermal alteration associated with the quartz lodes is marked by bleaching, magnesian siderite spots, disseminated arsenopyrite and pyrite and thin carbonate, quartz and sulphide veins. These trends are accompanied by increasing concentrations of S, As and Sb and decreasing Na, and a decrease of Fe and Mg in K-mica. The alkali alteration indices 3K/Al (representing K-mica) and Na/Al (representing albite) generally show antipathetic trends, with 3K/Al increasing near the lodes and Na/Al decreasing. These trends reflect the replacement of albite by K-mica. Carbonate alteration indices CO2/(Ca + Mg +Fe) and CO2/[Ca + Mg + Fe -0.5(S + As)] quantify the abundance of hydrothermal carbonates, but they show variable correlation with the lodes. They increase the width of the alteration halo in the hanging wall of the lodes at the Globe-Progress and General Gordon prospects

  12. Partial melting in amphibolites in a deep section of the Sveconorwegian Orogen, SW Sweden

    NASA Astrophysics Data System (ADS)

    Hansen, Edward; Johansson, Leif; Andersson, Jenny; LaBarge, Leah; Harlov, Daniel; Möller, Charlotte; Vincent, Stephanie

    2015-11-01

    Garnet amphibolite metataxites at the Steningekusten Nature Reserve in southwestern Sweden contain tonalitic patches and veins. Whole rock chemistry suggests that the protoliths were mafic igneous rocks with alkaline affinities. Orthopyroxene megacrysts are present in leucosome in parts of these garnet amphibolites but absent in others. Orthopyroxene megacrysts were formed by vapor-absent melting initiated by incongruent melting of biotite followed by the breakdown of hornblende. The net reaction was Bt + Hbl + Pl +/- Qtz ↔ Opx + Melt + Cpx + Gt. Melting occurred at pressures of approximately 1 GPa and temperatures which probably exceeded 800 °C. Pyroxenes are surrounded by hornblende-quartz symplectites, and hornblende in these coronas has distinctly lower concentrations of (Na + K) and Ti than that in adjacent mesosome. The hornblende rims formed upon cooling and reaction with crystallizing melt. This created a barrier to further reaction thus preserving the orthopyroxene megacrysts. Garnet amphibolite metatexites lacking pyroxene megacrysts have features characteristic of vapor-present melting including lack of peritectic phases predicted by vapor-absent melting reactions, larger amounts of leucosome (14 versus 7%), and less distinct melanosomes. The variation in these migmatites reflects open system behavior, either on a regional scale with the migration of aqueous fluids into the amphibolites or on a local scale with the migration of melt within the amphibolites. Zircons from all units have CL-dark core domains that are dated at 1415-1390 Ma. The core zones are cut and overgrown by CL-dark and CL-bright rims that are dated at 975-965 Ma. The zircon rims are thin in the mesosome but are thicker in the leucosome suggesting that they formed during migmatization. New growth of zircon associated with migmatization at ca. 970 Ma corresponds to the timing of crustal scale partial melting in the deep regions of the Sveconorwegian orogen, synchronous with east

  13. Late Devonian Anoxia Events in the Central Asian Orogenic Belt: a Global Phenomenon

    NASA Astrophysics Data System (ADS)

    Carmichael, S. K.; Waters, J. A.; Suttner, T. J.; Kido, E.; DeReuil, A. A.; Moore, L. M.; Batchelor, C. J.

    2013-12-01

    Atmospheric CO2 values decreased dramatically during the Middle Devonian due to the rapid rise of land plants. These changing environmental conditions resulted in widespread anoxia and extinction events throughout the Late Devonian, including the critical Kellwasser and Hangenberg anoxia events, which are associated with major mass extinctions at both the beginning and end of the Famennian Stage of the Late Devonian. Fammenian sediments in northwestern Xinjiang Province, China, represent a highly fossiliferous shallow marine setting associated with a Devonian oceanic island arc complex. Analysis of multiple geochemical proxies (such as U/Th, Ba, normalized P2O5, V/Cr, Zr), magnetic susceptibility, and mineralogical data (biogenic apatite and pyrite framboids) indicates that these Famennian sequences record not only the Upper Kellwasser Anoxic Event at the Frasnian/Famennian (F/F) boundary but also the rebound from the F/F extinction event. Preliminary evidence suggests that the Hangenberg Anoxic Event can also be recognized in the same sequence, although our biostratigraphic control is less precise. Previous studies of the Kellwasser and Hangenberg Events have been performed on continental shelf environments of Laurussia, Gondwana, Siberia, and South China. The Devonian formations of northwest Xinjiang in this study, however, are part of the Central Asian Orogenic Belt (CAOB), which is thought to have formed as part of a complex amalgamation of intra-oceanic island arcs and continental fragments prior to the end of the latest Carboniferous. These results allow us to confirm the presence of the Kellwasser and Hangenberg Events in the open oceanic part of Paleotethys, indicating that both events were global in scope. The presence of an abundant diverse Famennian fauna between these anoxia/extinction events suggests that the shallow marine ecosystems in the CAOB were somewhat protected due to their tectonic location and relative isolation within an open ocean system

  14. Petrology of metabasic and peridotitic rocks of the Songshugou ophiolite, Qinling orogen, China

    NASA Astrophysics Data System (ADS)

    Belic, Maximilian; Hauzenberger, Christoph; Dong, Yunpeng

    2013-04-01

    unclear but probably related to the closure of the Shangdan ocean during the early Paleozoic. The financial support by Eurasia-Pacific Uninet is gratefully acknowledged. Dong, Y.P., Zhou, M.F., Zhang, G.W., Zhou, D.W., Liu, L., Zhang, Q., 2008. The Grenvillian Songshugou ophiolite in the Qinling Mountains, Central China: implications for the tectonic evolution of the Qinling orogenic belt. Journal of Asian Earth Science 32 (5-6), 325-335.

  15. Gold enrichment and Bi-mineral assemblages in ores: Examples from shield and orogenic areas

    NASA Astrophysics Data System (ADS)

    Ciobanu, C. L.; Cook, N. J.; Pring, A.

    2003-04-01

    Gold deposits in metamorphic terranes, like their counterparts in hydrothermal areas, commonly contain small quantities of Bi-tellurides/selenides (BTS) or Bi-sulpho-salts (Bi-ss). These mineral groups, although eclectic in character, have the potential to offer real and hitherto-untapped information on processes of ore formation and evolution, since BTS speciation, paragenesis and assemblages lie at the heart of ideas covering mechanisms of Au-enrichment, fluid-driven infiltration and sulphide modularity. A systematic study of the distribution of BTS in Au-enriched ores from 25 deposits in the Fennoscandian/Ukrainian Shields and in European Phanerozoic orogenic belts allows comparison of telluride speciation and association and construction of a qualitative diagram for Bi-telluride stability in fS_2-fO_2 space on the basis of Bi/Te+Se+S (R{Bi/Te}). Tsumoite (BiTe) stability separates reducing environ-ments where Bi-tellurides with R{Bi/Te}>1 are associated with native Bi and maldonite, and oxidizing environments where Bi-tellurides with R{Bi/Te}<1 are associated with Au-Ag-bearing tellurides and native Te. BTS are closely associated with Au, due to incorporation within Bi{melt} above 271^oC. Such melts are powerful "scavengers" for Au, especially at the main fS_2/fO_2 buffers. Overall compositions of Bi-Te-Se-Aumelts, as seen in resultant BTS associations, reflect the reducing/oxidizing character of source fluids. Telluride speciation has implications for discriminating overprinting events, with focus on local Au-enrichment at metamorphic peaks, during retrograde stages in skarn and secondary boiling in porphyry, irrespective of age or deposit type. Unlike tellurides, Bi-ss are not necessarily genetically related to Au-enrich-ment. Instead, speciation and compositional variation relate to primary fluid sources and may be specific to regional/orefield trends. Nevertheless, as many sulphosalts form polysomatic series, they can adjust chemical variation via

  16. Pre-orogenic structural inheritances control on the Provence thrust system, SE France

    NASA Astrophysics Data System (ADS)

    Bestani, Lucie; Espurt, Nicolas; Lamarche, Juliette; Hollender, Fabrice; Bellier, Olivier

    2015-04-01

    The Provence thrust system located to the southeast of France corresponds to the foreland of the Pyrenean and Alps belts, characteristic of the late Cretaceous-Eocene and Miocene to present-day compressions respectively. Paleozoic-Mesozoic pre-orogenic substratum geometry still raise questions and the shortening of Provence and associated partitioning among these two compressional events, which are responsible for the present Provence foreland structure has never been precisely quantified. The Provence foreland is divided into two parts by the Middle-Durance and Aix-en-Provence faults system: the western and eastern Provence. The Middle Durance/Aix-en-Provence faults system is inherited from Paleozoic times and is responsible for strong thickness variations in the Mesozoic sedimentary pile ranging from 3 (easternpart) to ~10 km (western part). N-S balanced cross sections (~130 km) between the Baronnies to the north and the Mediterranean Sea to the south reveal that the Provence foreland structure results of 91% of the Pyrenean shortening. The Alpine compression led to minor reactivation of Pyrenean-Provence structures with a Miocene shortening of ~650 m (0.5%). Cross-section balancing shows an along-strike heterogeneous total horizontal shortening from ~7.6 km (6%) in the west to ~51 km (27%) in the east. These results show that during the Pyrenean-Provence compression, the Middle Durance/Aix-en-Provence faults system played the role of an oblique transfer zone between the western and eastern Provence domains and separated two contrasted structural domains. Eastward where the sedimentary pile is thin (<4 km), the inversion of deep-seated late Paleozoic-Triassic extensional structures induced a thick-skin style. On the contrary, westward where the sedimentary pile is thick (>7km), the reactivation of basement structures is not necessary involved in the accommodation of the shortening, leading to a thin-skinned tectonics above Triassic series. Paleozoic basement

  17. Orogenic-type copper-gold-arsenic-(bismuth) mineralization at Flatschach (Eastern Alps), Austria

    NASA Astrophysics Data System (ADS)

    Raith, Johann G.; Leitner, Thomas; Paar, Werner H.

    2015-10-01

    high Hg content (up to 11 mass %). The Cu-Au deposits in the Flatschach area show similarities with meso- to epizonal orogenic lode gold deposits regarding the geological setting, the structural control of mineralization, the type of alteration, the early (stage 1) sulfide assemblage and composition of gold. Unique about the Flatschach district is the lower-temperature overprint of copper arsenides (domeykite and koutekite) and copper sulfides (djurleite, yarrowite/spionkopite) on earlier formed sulfide mineralization. Based on mineralogical considerations temperature of stage 2 mineralization was between about 70 °C and 160 °C. Gold was locally mobilized during this low-temperature hydrothermal overprint as well as during stage 3 supergene oxidation and cementation processes.

  18. Numerical Modeling of Middle Crustal Flow of the Central Svecofennian Orogen

    NASA Astrophysics Data System (ADS)

    Lammi, Hannu; Korja, Annakaisa; Rey, Patrice

    2015-04-01

    This work explores the lateral spreading of thickened hot crust via a series of 2D thermo-mechanical numerical experiments using Ellipsis, a particle-in-cell finite element code. High Paleoproterozoic radiogenic heat production is assumed. The material viscosity is temperature-dependent following the Arrhenius law. The experiments use two sets of rheological parameters for the crust: wet (granite/diorite/mafic granulite) and dry (granite/felsic granulite/mafic granulite) crustal rheologies and dry olivine for the upper mantle. The first set of models, plateau-models, is based on a 480 km long section of 65 km-thick three-layer plateau crust. In the second set of models, plateau margin-models, a transition from 65 km thick plateau to 40 km thick foreland is imposed in the middle of the model. The models are studying the effect of free moving boundaries on the crustal structure. The models extended symmetrically from both ends with slow (2x0.95 mm/a) and fast (2x9.5 mm/a) velocities. Gravitational collapse is simulated with an additional set of fixed boundary plateau margin models. The models are testing the conditions for mid-crustal flow and core complex formation in high heat flow regime relevant to the Paleoproterozoic. The results of the modeling are compared to seismic reflection sections and surface geological observations from the Central Svecofennian orogen, where large volumes of granitoid intrusions as well as high-temperature low-pressure gneisses are exposed. The flow patterns are compared with seismic reflection images. The P-T-t paths of rocks at 15 km depth in the models are examined and compared to observations of peak metamorphic conditions of rocks at present-day surface level. Hot geotherm together with slow extension velocity leads to significant partial melting in the plateau models and in the plateau region of the plateau margin models. P-T-t paths of rocks at these regions best match the observations of high temperatures, but the vertical

  19. COSC-1 - drilling of a subduction-related allochthon in the Palaeozoic Caledonide orogen of Scandinavia

    NASA Astrophysics Data System (ADS)

    Lorenz, H.; Rosberg, J.-E.; Juhlin, C.; Bjelm, L.; Almqvist, B. S. G.; Berthet, T.; Conze, R.; Gee, D. G.; Klonowska, I.; Pascal, C.; Pedersen, K.; Roberts, N. M. W.; Tsang, C.-F.

    2015-05-01

    The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project focuses on mountain building processes in a major mid-Palaeozoic orogen in western Scandinavia and its comparison with modern analogues. The project investigates the subduction-generated Seve Nape Complex. These in part under ultra-high-pressure conditions metamorphosed outer continental margin and continent-ocean transition zone assemblages were emplaced onto the Baltoscandian platform and there influenced the underlying allochthons and the basement. COSC-1 is the first of two ca. 2.5 km deep, fully cored drill holes located in the vicinity of the abandoned Fröå mine, close to the town of Åre in Jämtland, central Sweden. It sampled a thick section of the lower part of the Seve Complex and was planned to penetrate its basal thrust zone into the underlying lower-grade metamorphosed allochthon. The drill hole reached a depth of 2495.8 m and nearly 100 % core recovery was achieved. Although planning was based on existing geological mapping and new high-resolution seismic surveys, the drilling resulted in some surprises: the Lower Seve Nappe proved to be composed of rather homogenous gneisses, with only subordinate mafic bodies, and its basal thrust zone was unexpectedly thick (> 800 m). The drill hole did not penetrate the bottom of the thrust zone. However, lower-grade metasedimentary rocks were encountered in the lowermost part of the drill hole together with garnetiferous mylonites tens of metres thick. The tectonostratigraphic position is still unclear, and geological and geophysical interpretations are under revision. The compact gneisses host only eight fluid conducting zones of limited transmissivity between 300 m and total depth. Downhole measurements suggest an uncorrected average geothermal gradient of ~ 20 °C km-1. This paper summarizes the operations and preliminary results from COSC-1 (ICDP 5054-1-A), drilled from early May to late August 2014, and is

  20. The Hydrologic Characteristics of an Orogenic Wedge During Burial of Pelites

    NASA Astrophysics Data System (ADS)

    Nabelek, P. I.

    2014-12-01

    The burial and exhumation of oceanic sediments in orogenic wedges is one of the most important processes that occur during continental collisions. During progressive burial, pelites lose porosity and permeability, and eventually pass into the lithostatic pressure regime where the porosity and permeability are thought to be very small. However, prograde metamorphic reactions produce fluids that must ascent though the pore space or fractures. The hydrologic characteristics of schists undergoing burial were modeled using the program SUTRAMET, which is a modified version of the program SUTRA (Voss & Provost, 2002) that allows modeling of metamorphic reactions at high pressures and temperatures and incorporates transient porosity and permeability changes due to overpressure and changing volume of the metamorphic assemblage. The initial model crustal section includes a hydrostatic pressure regime to the depth of 5 km with φ ≥ 0.007 and k ≥ 10-17 m2. Within the lithostatic pressure domain, schists have φ ≤ 0.003 and k ≤ 10-18 m2. Below the schists that are undergoing burial (3 or 6 mm/y) is an impermeable substrate with φ = 0. P and T are maintained at the top and the bottom boundaries and P is also maintained at the top of the lithostatic domain. T-dependent thermal diffusivity and heat capacity were applied. The initial T gradient is ~30°/km. The only heat sink is endothermic metamorphic reactions and the only heat source is a small amount of radioactivity. The initial schist was assumed to have 17.9 wt.% qtz, 13.2% ab, 28.7% ksp, 9.0% ms, 22.8% chl, and 8.5% law. By the time it reaches the bottom of the crustal section, metamorphic reactions produce the anhydrous assemblage 14.5% qtz, 7.8% an, 13.7% ab, 36% ksp, 0.2% sil, and 27.5% grt. Results show that fluid pressure remains slightly above lithostatic within the initially lithostatic pressure regime as metamorphic fluids are released, even at the slow burial rate of 3 mm/y, because the fluids pass to the

  1. Structural and geochronological studies on the Liba goldfield of the West Qinling Orogen, Central China

    NASA Astrophysics Data System (ADS)

    Zeng, Qingtao; McCuaig, T. Campbell; Hart, Craig J. R.; Jourdan, Fred; Muhling, Janet; Bagas, Leon

    2012-10-01

    The Liba goldfield, located to the northeast of the Zhongchuan Granite in the West Qinling Orogen (WQO) of mainland China, contains the largest known gold resource of 2.8 Moz in the Zhongchuan area. Devonian metasedimentary rocks host the structurally controlled gold mineralization, which is associated with silica-sericite-chlorite-carbonate alteration. Two major styles of mineralization occurred at the goldfield, which are disseminated sediment-hosted and quartz vein hosted types. Pyrite, arsenopyrite, and arsenian pyrite are major gold carriers and gold also occurs as native gold grains and electrum spatially associated with the sulfides. Numerous felsic/intermediate dykes have a similar structural control as the mineralization, and their contacts with host rocks are recognized as favorable zones for mineralization. Detailed fieldwork in conjunction with geochronological studies has helped to define the deformation history and gold metallogenesis of the goldfield. Three major phases of deformation have been recognized in the Zhongchuan area. The first deformation (D1) event was compressional in broadly a N-S orientation, the second (D2) event was also compressional and orientated in a NE-SW direction, and the third (D3) event was post-mineralization and was associated with the emplacement of barren calcite and anhydrite veins. Compression related to D2 is the key process that controlled the distribution of igneous dykes and gold mineralization in the Liba goldfield. Both igneous and hydrothermal fluids preferentially focused along dilational jogs under local trans-extension, which took place during the late stage of D2. Precise dating with high-resolution ion microprobe (SHRIMP) U-Pb on zircon and 40Ar/39Ar on muscovite, biotite, hornblende, and plagioclase of crosscutting pre-mineralization granitic porphyry and diorite dykes have constrained the mineralization age to after ca. 227 Ma. 40Ar/39Ar analysis of minerals formed in hydrothermal alteration zones

  2. Extension of the Mid- to Lower Crust with Orogenic Inheritance: Examples from the Death Valley Region (Western US), and the Mauleon Basin (Southwestern France).

    NASA Astrophysics Data System (ADS)

    Lima, R. D.; Hayman, N. W.; Kelly, E. D.; Lavier, L. L.

    2015-12-01

    Continental margins exhibit a range of widths and symmetries defined by the strain patterns that arise during extension and rifting. An important pattern in this respect is the early localization of extension into necking zones. The rheology of the lower crust plays a large role in this localization, and can be affected by inherited orogenic structures, fabrics, and mineral assemblages. Here, we further evaluate the role of orogenic fabrics in continental extension using microstructural observations and thermodynamic modeling of geological sections exposed in the Funeral and Black Mountains of the Death Valley region, California, and from the Mauleon Basin, France. The Death Valley region sits within the Basin-and-Range region of broadly distributed Cenozoic extension, over a relatively flat and deep moho. In contrast, in the Mauleon basin, Cretaceous extension accommodated mantle exhumation, and was strongly localized in older Hercynian orogenic crust. In both areas, mid- to lower crustal rocks are characterized by inherited migmatitic fabrics overprinted by zones of localized, extensional fabrics. Mineral assemblages that formed over a P-T cooling path define the fabrics in each field area. The high-temperature fabrics record decompression-melting due to late- to post-orogenic collapse. Yet, the two field areas show contrasting retrograde assemblages, which are hypothesized to have resulted from changes in the local effective bulk composition produced by differences in melt segregation. At subsequent extensional stages, mid- to lower crustal deformation resulted in the transposition of the inherited post-orogenic fabrics, documented with quartz fabric analysis (including EBSD). The two contrasting regions show how the rheology of inherited orogenic lower crust responds to differences in melt-segregation and metamorphic histories, potentially controlling margin structural evolution.

  3. Pan-African granulites of central Dronning Maud Land and Mozambique: A comparison within the East-African-Antarctic orogen

    USGS Publications Warehouse

    Engvik, A.K.; Elevevold, S.; Jacobs, J.; Tveten, E.; de Azevedo, S.; Njange, F.

    2007-01-01

    Granulite-facies metamorphism is extensively reported in Late Neoproterozoic/Early Palaeozoic time during formation of the East-African-Antarctic orogen (EAAO). Metamorphic data acquired from the Pan-African orogen of central Dronning Maud Land (cDML) are compared with data from northern Mozambique. The metamorphic rocks of cDML are characterised by Opx±Grt-bearing gneisses and Sil+Kfs-bearing metapelites which indicate medium-P granulite-facies metamorphism. Peak conditions, which are estimated to 800-900ºC at pressures up to 1.0 GPa, were followed by near-isothermal decompression during late Pan-African extension and exhumation. Granulite-facies lithologies are widespread in northern Mozambique, and Grt+Cpx-bearing assemblages show that high-P granulite-facies conditions with PT reaching 1.55 GPa and 900ºC were reached during the Pan-African orogeny. Garnet is replaced by symplectites of Pl+Opx+Mag indicating isothermal decompression, and the subsequent formation of Pl+amphibole-coronas suggests cooling into amphibolite facies. It is concluded that high-T metamorphism was pervasive in EAAO in Late Neoproterozoic/Early Paleozoic time, strongly overprinting evidences of earlier metamorphic assemblages.

  4. The tectonics and stages of the geological history of the Yenisei-Khatanga Basin and the conjugate Taimyr Orogen

    NASA Astrophysics Data System (ADS)

    Afanasenkov, A. P.; Nikishin, A. M.; Unger, A. V.; Bordunov, S. I.; Lugovaya, O. V.; Chikishev, A. A.; Yakovishina, E. V.

    2016-03-01

    A new interpretation of the seismic profile series for the Taimyr Orogen and the Yenisei-Khatanga Basin is given in terms of their tectonics and geological history. The tectonics and tectonostratigraphy of the Yenisei-Khatanga and the Khatanga-Lena basins are considered. In the Late Vendian and Early Paleozoic, a passive continental margin and postrift shelf basin existed in Taimyr and the Yenisei-Khatanga Basin. From the Early Carboniferous to the Mid-Permian, the North and Central Taimyr zones were involved in orogeny. The Late Paleozoic foredeep was formed in the contemporary South Taimyr Zone. In the Middle to Late Triassic, a new orogeny took place in the large territory of Taimyr and the Noril'sk district of the Siberian Platform. A synorogenic foredeep has been recognized for the first time close to the Yenisei-Khatanga Basin. In the Jurassic and Early Cretaceous, this basin was subsided under transpressional conditions. Thereby, anticlinal swells were formed from the Callovian to the Aptian. Their growth continued in the Cenozoic. The Taimyr Orogen underwent tectonic reactivation and apparently right-lateral transpression from Carboniferous to Cenozoic.

  5. Late orogenic mafic magmatism in the North Cascades, Washington: Petrology and tectonic setting of the Skymo layered intrusion

    USGS Publications Warehouse

    Whitney, D.L.; Tepper, J.H.; Hirschmann, M.M.; Hurlow, H.A.

    2008-01-01

    The Skymo Complex in the North Cascades, Washington, is a layered mafic intrusion within the Ross Lake fault zone, a major orogen-parallel structure at the eastern margin of the Cascades crystalline core. The complex is composed dominantly of troctolite and gabbro, both with inclusions of primitive olivine gabbro. Low-pressure minerals in the metasedimentary contact aureole and early crystallization of olivine + plagioclase in the mafic rocks indicate the intrusion was emplaced at shallow depths (<12 km). The Skymo rocks have trace-element characteristics of arc magmas, but the association of Mg-rich olivine (Fo88-80) with relatively sodic plagioclase (An75-60) and the Al/Ti ratios of clinopyroxene are atypical of arc gabbros and more characteristic of rift-related gabbros. A Sm-Nd isochron indicates crystallization in the early Tertiary (ca. 50 Ma), coeval with the nearby Golden Horn alkaline granite. Mantle melting to produce Skymo magma likely occurred in a mantle wedge with a long history of arc magmatism. The Skymo mafic complex and the Golden Horn granite were emplaced during regional extension and collapse of the North Cascades orogen and represent the end of large-scale magmatism in the North Cascades continental arc. ?? 2008 Geological Society of America.

  6. Correlation of the Tonga Formation and the Chiwaukum Schist, North Cascades, Washington: Implications for Late Cretaceous orogenic mechanisms

    NASA Astrophysics Data System (ADS)

    Duggan, K. M.; Brown, E. H.

    1994-12-01

    The Tonga Formation of the North Cascades, Washington, consists of weakly deformed, bedded feldspathic graywacke and shale metamorphosed at chlorite to staurolite grade. Analysis of protolith lithology, Rb-Sr isotopic signature, and plutonic, metamorphic, and deformational evolution in the Tonga Formation suggests that this unit is correlative with the Chiwaukum Schist of the Cascades crystalline core. The Tonga Formation occurs on the flank of the Late Cretaceous orogen, whereas high-grade Chiwaukum Schist is exposed in the core of the orogen. Metamorphic pressure, as reflected by the distribution of metamorphic minerals and thermobarometry, increases continuously northward in the Tonga Formation from the chlorite zone (<4 kbar) to the staurolite zone (≈ 7 kbar) and then to the kyanite zone (≈ 7 kbar) in the Chiwaukum Schist. The findings reported herein do not support the previously advanced concept that the Cascades crystalline core represents the Easton blueschist terrane that was converted into higher-temperature metamorphic rock by thermal relaxation after cessation of subduction. The results of this study suggest a relatively simple two-stage metamorphic history for the Chiwaukum Schist represented by 90-93 Ma shallow contact metamorphism, overprinted by Harrovian metamorphism that increased sharply in pressure from south-west to northeast and reached peak conditions after 90 Ma, all events occurring in a plutonic/magmatic arc setting.

  7. High-temperature geochronology constraints on the tectonic history and architecture of the ultrahigh-pressure Dabie-Sulu Orogen

    NASA Astrophysics Data System (ADS)

    Hacker, Bradley R.; Wallis, Simon R.; Ratschbacher, Lothar; Grove, Marty; Gehrels, George

    2006-10-01

    New U/Pb zircon and Th/Pb monazite ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane. Combined with Sm/Nd ages, Rb/Sr ages, inclusion relationships, and geologic relationships, they help define the timing of peak recrystallization, the timing of subsequent amphibolite-facies metamorphism, and the architecture of the Dabie-Sulu suture zone between the collided Sino-Korean and Yangtze cratons. The data indicate a ˜15 Myr record of UHP recrystallization, the first clearly documented for a giant UHP terrane; this requires that continental subduction in the Dabie-Sulu orogen involved multiple UHP tectonic or recrystallization events. A 244-236 Ma "precursor" UHP event, seen only in the Dabie Shan, was followed by a second, ˜230-220 Ma "main" UHP event, which was itself terminated by a 220-205 Ma amphibolite-facies overprint. Older eclogite-facies events seen in the Qinling segment of this orogenic belt raise the possibility that these rocks have undergone (U)HP metamorphism three or four times, but at present, there is no geochronological evidence in the Dabie-Sulu area to support this. The subduction of the lower, Yangtze plate did not proceed in a simple fashion: The ages of inherited zircon cores demonstrate that a ribbon continent of Yangtze affinity escaped subduction and became wedged against the Sino-Korean plate hanging wall.

  8. Comparative analysis of the geological evolution of the northern and southern Appalachian orogen: Late Ordovician-Permian

    USGS Publications Warehouse

    Hibbard, J.P.; van Staal, C.R.; Rankin, D.W.

    2010-01-01

    Review of the major post-Middle Ordovician lithotectonic elements of the Appalachian orogen indicates that the middle to late Paleozoic geologic evolution of the Appalachian margin was less uniform than that of the early Paleozoic. Evolutionary divergence betweenthe northern and southern segments of the orogen started in the Late Ordovician to Silurian with staggered accretion of the first peri-Gondwanan elements to reach the Laurentia margin, Carolinia in the south and Ganderia in the north. Divergence was amplified duringthe Silurian, specifi cally with respect to the nature of the Laurentian margin and the history of accretion. During this time frame, the northern margin was convergent, whereas the amagmatic southern margin may well have been a transform boundary. In terms of accretion, the Late Silurian-Early Devonian docking of Avalonia was restricted to the northern segment, whereas the southern Appalachians appear to have been largely quiescent during this interval. The evolutionary paths of the two segments of the margin converge on a commonhistory in the Late Devonian during the Famennian event; we suggest that this tectonism was related to the initial marginwide interaction of Laurentia with the peri-Gondwanan blocks of Meguma and Suwanee, providing a uniform tectonic template for margin evolution. The Laurentian-Gondwanan collision is marked by second-order divergences in history. Specifi cally, during the Carboniferous, the southern segment records a larger component of shortening than the northern Appalachians. ?? 2010 Geological Society of America.

  9. Deep Seismic Reflection Profiles Reveal The Crust Structures Beneath Xing'an-Mongolian Orogenic Belt and Its Neighboring Area

    NASA Astrophysics Data System (ADS)

    Hou, H.; Gao, R.; Keller, R. G.; Li, Q.; Li, W.; Li, H.; Xiong, X.; Guo, L.

    2012-12-01

    The Xing'an-Mongolia orogenic belt (XMOB) as the eastern part of the CAOB (Central Asian Orogenic Belt) is one of the remarkably reworked and crustal accretionary belts during the Phanerozoic in the world. It is located between the northern margin of the North China craton and the southern margin of the Siberia craton and is characterized by large-scale Mesozoic magmatism, and forms a key part of the NE-trending Mesozoic magmatic belt in East China. Therefore, it will be of great importance to study the contact relationship between these blocks, which will prodive important information on the study of mineralization and assessment of earthquake disaster. A major problem with all previous publications on Xing'an-Mongolian Orogenic belt was the lack of a seismic section which could potentially image the zone or the dominant vergence of crustal structures at depth. In the view of above, deep seismic profiling data acquisition finished in last three years and the newly processing seismic reflection profiles in this belt were possible under the support of SinoProbe project and China Geological Survey. As we not only focuse on the deep structures of crust, but also carry about the relationships between the shallow reflection fabrics with surface geology. Therefore, different with oil industrial data accquistion, we develop some deep seismic reflection techniques such as three levels of explosive shots were adopted for enough energy reflected from all crust, long offset with single side length of 15 km on symmetrical for the modeling of shallow crust by use of tomographic method, meanwhile we share the shots large than 500 kg with seismic refraction survey line providing more rays for modeling deep velocity strucutre. The seismic reflection data were mainly using the seismic processing package ProMAX and the CGG processing system, following the processing steps with special methods such as tomographic inversion static corrections, surface-consistent amplitude compensation

  10. The Ross Orogen and Lachlan Fold Belt in Marie Byrd Land, Northern Victoria Land and New Zealand: implication for the tectonic setting of the Lachlan Fold Belt in Antarctica

    USGS Publications Warehouse

    Bradshaw, J.D.

    2007-01-01

    Correlation of the Cambrian Delamerian Orogen of Australia and Ross Orogen of the Transantarctic Mountains widely accepted but the extension of the adjacent Lachlan Orogen into Antarctica is controversial. Outside the main Ross-Delamerian belt, evidence of this orogeny is preserved at Mt Murphy in Marie Byrd Land and the in Takaka Terrane of New Zealand. In all pre-break- configurations of the SW Pacific, these two areas are far removed from the Ross-Delamerian belt. Evidence from conglomerates in the Takaka Terrane, however, shows that in Late Cambrian times it was adjacent to the Ross Orogen. This indicates major tectonic displacements within Gondwana after the Cambrian and before break-up. The Lachlan Orogen formed in an extensional belt in a supra-subduction zone setting and the Cambrian rocks of Marie Byrd Land and New Zealand are interpreted as parts of a rifted continental ribbon on the outboard side of the Lachlan belt.

  11. The long-term seismic cycle in collisional margins: insights from Seismo-Thermo-Mechanical models

    NASA Astrophysics Data System (ADS)

    Dal Zilio, L.; van Dinther, Y.; Gerya, T.

    2015-12-01

    The April 25, 2015, Mw 7.8 Gorkha earthquake is the largest one in the Nepal Himalaya since 1934. Since the foreland part of it is densely populated, these events represent a considerable seismic hazard. The restricted direct observations in time and space in combination with tectonic and rheological complexities, however, pose a difficult problem for both seismic hazard assessment and modeling efforts. In this study we for the first time simulate cycles of spontaneous earthquake-like ruptures on non a-priori defined faults within a generic continental collision zone. We use the Seismo-Thermo-Mechanical (STM) numerical modeling approach, which is based on a continuum, viscoelastoplastic code I2ELVIS and is validated for seismic cycle applications against a laboratory model and natural observations (van Dinther et al., 2013a, b). The 2-D model setup consists of two continental plates separated by an oceanic plate, in which the incipient subduction phase is followed by continent-continent collision. In different collisional stages, we evaluate a non-associative Drucker-Prager plasticity yield criterion with pressure dependent yield strength and a strongly rate-dependent friction formulation. Our results show physically consistent emergence of complex rupture paths, both on- and off-main frontal thrust. Assuming different physical properties of tectonic nappes, we find that ruptures propagate following rheological or tectonic discontinuities. Our findings suggest that the interseismic coupling of the main-thrust affects the seismic cycle of the entire orogenic belt. While thrust-faulting events mainly occur on the main frontal thrust, normal-faulting events spread throughout the orogenic belt as a consequence of gravitational extension. Results from an event detection algorithm shows events in the deeper portions of the orogenic belt, both within the oceanic slab and near the bottom of the wedge where material is squeezed between stiff lithospheric mantle portions.

  12. Geological and geochemical implications of the genesis of the Qolqoleh orogenic gold mineralisation, Kurdistan Province (Iran)

    NASA Astrophysics Data System (ADS)

    Taghipour, Batoul; Ahmadnejad, Farhad

    2015-03-01

    The Qolqoleh gold deposit is located in the northwestern part of the Sanandaj-Sirjan Zone (SSZ), within the NE-SW trending Qolqoleh shear zone. Oligocene granitoids, Cretaceous meta-limestones, schists and metavolcanics are the main lithological units. Chondrite-normalised REE patterns of the ore-hosting metavolcanics indicate REE enrichment relative to hanging wall (chlorite-sericite schist) and footwall (meta-limestone) rocks. The pattern also reflects an enrichment in LREE relative to HREE. It seems that the LREE enrichment is related to the circulation of SO42- and CO2-bearing fluids and regional metamorphism in the Qolqoleh shear zone. Both positive and negative Eu anomalies are observed in shear-zone metavolcanics. These anomalies are related to the degree of plagioclase alteration during gold mineralisation and hydrothermal alteration. In progressing from a metavolcanic protomylonite to an ultramylonite, significant changes occurred in the major/trace element and REE concentration. Utilising an Al-Fe-Ti isocon for the ore-hosting metavolcanics shows that Sc, Y, K, U, P, and M-HREE (except Eu) are relatively unchanged; S, As, Ag, Au, Ca, LOI, Rb and LREE are enriched, and Sr, Ba, Eu, Cr, Co and Ni decrease with an increasing degree of deformation. Based on geochemical features and comparison with other well-known shear zones in the world, the study area is best classified as an Isovolume-Gain (IVG) type shear zone and orogenic type gold mineralisation. Based on the number of phases observed at room temperature and their microthermometric behaviour, three fluid inclusion types have been recognised in quartz-sulphide and quartz-calcite veins: Type I monophase aqueous inclusions, Type II two-phase liquid-vapour (L-V) inclusions which are subdivided into two groups based on the homogenisation temperature (Th): a) L-V inclusions with Th from 205 to 255°C and melting temperature of last ice (Tm) from -3 to -9°C. b) L-V inclusions with higher Th from 335 to 385

  13. Rapid middle Miocene collapse of the Mesozoic orogenic plateau in north-central Nevada

    USGS Publications Warehouse

    Colgan, Joseph P.; Henry, Christopher D.

    2009-01-01

    The modern Sierra Nevada and Great Basin were likely the site of a high-elevation orogenic plateau well into Cenozoic time, supported by crust thickened during Mesozoic shortening. Although crustal thickening at this scale can lead to extension, the relationship between Mesozoic shortening and subsequent formation of the Basin and Range is difficult to unravel because it is unclear which of the many documented or interpreted extensional episodes was the most significant for net widening and crustal thinning. To address this problem, we integrate geologic and geochronologic data that bear on the timing and magnitude of Cenozoic extension along an ???200km east-west transect south of Winnemucca, Battle Mountain, and Elko, Nevada. Pre-Cenozoic rocks in this region record east-west Palaeozoic and Mesozoic compression that continued into the Cretaceous. Little to no tectonism and no deposition followed until intense magmatism began in the Eocene. Eocene and Oligocene ash-flow tuffs flowed as much as 200km down palaeovalleys cut as deeply as 1.5km into underlying Palaeozoic and Mesozoic rocks in a low-relief landscape. Eocene sedimentation was otherwise limited to shallow lacustrine basins in the Elko area; extensive, thick clastic deposits are absent. Minor surface extension related to magmatism locally accompanied intense Eocene magmatism, but external drainage and little or no surface deformation apparently persisted regionally until about 16-17Ma. Major upper crustal extension began across the region ca. 16-17Ma, as determined by cross-cutting relationships, low-temperature thermochronology, and widespread deposition of clastic basin fill. Middle Miocene extension was partitioned into high-strain (50-100%) domains separated by largely unextended crustal blocks, and ended by 10-12Ma. Bimodal volcanic rocks that erupted during middle Miocene extension are present across most of the study area, but are volumetrically minor outside the northern Nevada rift. The modern

  14. Four Flavours of Orogenic Plateau Magmatism: What's Melting Beneath the Turkish-Iranian Plateau?

    NASA Astrophysics Data System (ADS)

    Neill, I.; Allen, M. B.; Kaislaniemi, L.; Van Hunen, J.

    2013-12-01

    Orogenic plateaux are first order topographic features of the continents, occurring in collision zones such as Tibet and Andean-style continental arcs. Plateaux are sites of abundant mantle-derived magmatism, but there is little understanding of its geodynamic cause in spite of assumptions that slab break-off or lithospheric thinning are controlling factors in melt production. The Turkish-Iranian Plateau formed on the Eurasian Plate after the ~30 Ma Arabia-Eurasia collision. Neogene-recent volcanoes are found up to ~800 km from the suture, and have huge compositional variation. We define four varieties of recent mafic magmatism spatially and geochemically. (1) Close to the Bitlis Suture in Eastern Turkey, slab break-off is likely to have occurred at ~10 Myr, and there is little mantle lithosphere present. Magmatism is mostly calc-alkaline, sourced from the asthenosphere or any remaining mantle lithosphere, and is affected by crustal contamination. (2) In the Lesser Caucasus up to ~500 km from the suture, magmatism is more alkaline, less crustally-contaminated and is derived from subduction-modified lithospheric mantle. (3) Close to the Zagros Suture in Iran, the lithosphere may have thickened to >200 km during collision. Magmatism is volumetrically limited and derived almost exclusively from the lithospheric mantle, with trace element-enriched alkaline or ultrapotassic compositions. Unlike the Lesser Caucasus, there is little magmatism in the Iranian desert up to ~500 km from the suture. (4) Beyond ~500 km from the Bitlis-Zagros Suture Zone, there is sparse, compositionally variable magmatism: it is OIB-like in Eastern Iran, more akin to Zagros magmatism in the Alborz, and more felsic above the ~55 km thick crust of the Greater Caucasus. Geochemical data suggest that magmatism in (1) is caused by asthenospheric upwelling following slab breakoff, whereas in (2) it is dominated by melting of the base of the still-present lithospheric mantle during convective removal

  15. Tectonics of the zagros orogenic belt of iran: new data and interpretations

    NASA Astrophysics Data System (ADS)

    Alavi, Mehdi

    1994-01-01

    The Zagros orogenic belt of Iran is the result of the opening and closure of the Neo-Tethys oceanic realm, and consists, from northeast to southwest, of three parallel tectonic subdivisions: (1) the Urumieh-Dokhtar Magmatic Assemblage; (2) the Sanandaj-Sirjan Zone; and (3) the Zagros Simply Folded Belt. Recent detailed and reconnaissance mapping and stratigraphie-structural analyses in various parts of the belt indicate that the tectonics of the Sanandaj-Sirjan Zone is dominated by either large-scale composite duplex structures or low- and high-angle NE-dipping imbricate systems. These thrust systems have transported numerous horses and sheets (nappes) of metamorphosed and non-metamorphosed Phanerozoic rocks for tens or possibly hundreds of kilometers. Stratigraphic evidence and synorogenic conglomerates indicate that thrusting was initiated in the Late Cretaceous and has continued episodically since that time. Shear sense indicators in various parts of the zone reveal a NE-SW direction of transport. Thrust systems generated in each episode are superimposed upon the previous structures, cutting across and displacing them and producing complicated stacks of thrust sheets. Southwestward transport and stacking of the thrust sheets has resulted in an increase of about 10-15 km in the thickness of the continental crust in the southwestern part of this zone. The so-called z"Main Zagros Thrust", which has been traditionally considered as the boundary between the Sanandaj-Sirjan Zone and the Zagros Simply Folded Belt, is by no means a single "high-angle reverse fault", nor is it a narrow zone of "crushed rocks" but a series of low-angle thrusts locally cut by normal faults. These thrusts are indistinguishable from those mapped in the Sanandaj-Sirjan Zone, and are geometrically part of the Sanandaj-Sirjan Zone structures. There is no single thrust among them that can be singled out and referred to as the "Main" one. The boundary of the Sanandaj-Sirjan Zone with the Urumieh

  16. Accretionary Growth in the Central Asian Orogenic Belt of Southern Siberia, Mongolia and Kasakhsthan

    NASA Astrophysics Data System (ADS)

    Kroner, A.; Windley, B. F.; Badarch, G.; Tomurtogoo, O.; Hegner, E.; Demoux, A.; Wingate, M. T.

    2005-12-01

    The Central Asian Orogenic Belt (CAOB) records a ca. 800 Ma history of arc and microcontinent accretion, from S to N, during evolution and closure of the SW Pacific-type Paleo-Asian ocean in the period ~1000 to ~300 Ma. We contest previous evolutionary models in terms of a single, large island arc. The earliest history of ocean opening is recorded by ~1020-1050 Ma ophiolites in southern Siberia. The next younger events farther S are exemplified by the evolution of ~850-570 Ma arc-ophiolite terrains, including boninites, followed by earliest Paleozoic subduction/accretion, suturing and oblique collision during which some of the arc terrains were metamorphosed to granulite-facies. Precambrian microcontinental fragments are documented by Archean to Mesoproterozoic ages, and we also found Archean to Neoproterozoic detrital zircons in Mongolian arc-derived sediments, suggesting derivation from continental sources. Voluminous intermediate to felsic island-arc magmatic rocks in Mongolia were generated at 460-417 Ma with-in a broad belt extending from northern Mongolia to the Gobi Desert. Zircon xenocrysts and Nd model ages for some of these rocks are between 600 and >1300 Ma, suggesting that older material was involved in their generation. SHRIMP detrital zircon ages for arc-derived sandstones document continental input dating back to the late Archean. Several microcontinental fragments have been identified in southern Kazakhstan with zircon ages between 2.0 and 2.7 Ga. Extensive rifting with formation of bimodal volcanic sequences occurred at ~775-820 Ma. Precambrian detrital zircons from clastic sediments resting on ocean floor supports the view that the oceanic domains between the microconti-nents and early Paleozoic arc complexes were narrow, probably marginal basins. A metadacite and granodiorite from an arc complex near Lake Balkash provided identical zircon ages of 478-480 Ma and contain zircon xenocrysts as old as 2290 Ma. One 480 Ma diorite sample from central

  17. Palaeozoic - Mesozoic tectonics of the western Transbaikalian segment of the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Gladkochub, Dmitry; Donskaya, Tatiana; Mazukabzov, Anatoly

    2014-05-01

    The western Transbaikalian segment of the Central Asian Orogenic Belt (CAOB) is a key to understand a scenario of gradual growth of continental crust in this part of the Northern Eurasia. In general this growth was directly controlled by long-living processes related to interaction of the southern margin of the Siberian craton with different units of the Palaeo-Asian and Mongol-Okhotsk oceans. The Neoproterozoic activity within this area was related to opening and development of the Palaeo-Asian Ocean. The early Palaeozoic stage was characterized by collision of numerous terranes (microcontinents, relicts of island arcs and back-arc basins, etc.) with the southern flank of the Siberian craton. These events reflect an early stage of Palaeo-Asian Ocean closure and the beginning of CAOB building. Since the late Palaeozoic the development of the Transbaikalian segment of the CAOB was related to the evolution of the Mongol-Okhotsk Ocean. Late Silurian - middle Devonian clastic and carbonate sediments were deposited along a passive margin of the Mongol-Okhotsk Ocean. The low-angle subduction of oceanic lithosphere of the Mongol-Okhotsk Ocean beneath the Siberian continent started in the middle Devonian. In the early to late Carboniferous, a steeper dip of subduction led to tectonic switching from extension to compression. This period was characterized by compression, metamorphism, deformations and thickening of continental crust. Autochthonous biotite granites of the Angara-Vitim batholith (ca. 320-300 Ma) were emplaced during this phase. Destruction of the subducted slab and roll-back toward the ocean in the late Carboniferous - early Permian caused extension of continental lithosphere and magmatic input from the mantle into the continental crust. These processes were responsible for voluminous magmatism that is spectacularly represented by allochthonous granitoids of the Angara-Vitim batholith and magmatic rocks of the western Transbaikalian belt (ca. 300-270 Ma). In

  18. A review of the tectonic evolution of the Northern Pacific and adjacent Cordilleran Orogen

    NASA Astrophysics Data System (ADS)

    Jakob, Johannes; Gaina, Carmen; Johnston, Stephen T.

    2014-05-01

    Numerous plate kinematic models for the North Pacific realm have been developed since the advent of plate tectonics in the early seventies (e.g Atwater (1970), Mammerickx and Sharman (1988)). Although published kinematic models are consistent with the broad scale features of the North Pacific, the link between plate motions and the evolution of the North American Cordillera remains poorly understood. Part of the problem lies in conflicting interpretations of geological versus paleomagnetic data sets, with the result being a lack of consensus regarding: the paleolocation of key geological units; the paleogeography of terrane formation and amalgamation; the motion, boundaries and even existence of oceanic plates; and the character (e.g. trend of subduction) and position of plate boundaries within the northern Pacific basin. Remnants of the Farallon and Kula plates, and some short-lived microplates, demonstrate the complicated tectonic evolution of the oceanic realm west of the North American margin (e.g. Rea and Dixon (1983); McCrory and Wilson (2013); Shephard et al. (2013)). The creation and destruction of major tectonic plates and microplates has presumably left a record in the Cordilleran orogen of western North America. However, working backward from the geological relationships to plate reconstructions remains difficult. Here we investigate the relationship between the plate motions of the Pacific Ocean and the terrane movements in the North American Cordillera by revising the marine magnetic and gravity anomalies of the northern Pacific. In particular, we reevaluate plate boundaries at times of major changes in plate geometry of the Pacific, Kula, Chinook and Farallon plates from C34n onward. Our focus is also on the plate geometries of the Resurrection, Eshamy and Siletz-Crescent plates during the time between anomaly C26 and C12, and the links between plate interactions and on-shore tectonic events recorded in the geological record of Vancouver Island

  19. Large landslides lie low: Vertical domains of denudation processes in the arid Himalaya-Karakoram orogen

    NASA Astrophysics Data System (ADS)

    Blöthe, Jan Henrik

    2014-05-01

    Large bedrock landslides (defined here as affecting >0.1 km2 in planform area) are thought to substantially contribute to denuding active mountain belts, and limiting the growth of topographic relief produced by concurrent tectonic uplift and fluvial or glacial incision. While most research on large landslides has focused on tectonically active, humid mountain belts with varying degrees of rainstorm and earthquake activity, lesser attention has been devoted to arid mountain belts. Especially in the Himalaya, where high denudation rates are commonly associated with high landslide activity, previous work has largely ignored landslide processes in the arid compartments of the orogen. This was motivation for us to compile a landslide inventory covering the arid Himalaya-Karakoram of NW India and N Pakistan within the Indus catchment. Our data set contains 493 rock-slope failures that we compiled from published studies and mapping from remote sensing imagery. Using an empirical volume-area scaling approach we estimate the total landslide volume at >250 km3. This is more than thousand times the contemporary annual sediment load in the Indus River. We analyse the distribution of these volumetrically significant landslides with respect to the regional hypsometry, contemporary glacier cover, and the distribution of rock glaciers. We find that large bedrock landslides in the arid Himalaya-Karakoram region preferentially detach near or from below the study area's median elevation, while glaciers and rock glaciers occupy higher elevations almost exclusively. This trend holds true for both the study area and parts thereof. The largest and highest-lying landslides occur in the Karakoram mountains, where local relief exceeds 6 km, and >90% of the landslide areas lie below the region's median elevation. Our analysis reveals a hitherto unrecognized vertical layering of denudation processes, with landslides chiefly operating below the median elevation, whereas mass transport by

  20. Early Paleozoic crustal anatexis in the intraplate Wuyi-Yunkai orogen, South China

    NASA Astrophysics Data System (ADS)

    Wang, Dan; Zheng, Jianping; Ma, Qiang; Griffin, William L.; Zhao, Huan; Wong, Jean

    2013-08-01

    Early Paleozoic amphibolite- to granulite-facies metamorphism, crustal anatexis and coeval magmatism are extensively developed in the Wuyi-Yunkai intraplate orogen in the South China block. However, the exact timing of granulite-facies partial melting and its link with orogenesis have not been well constrained. In this study, the charnockites, gneissic migmatites and Al-rich gneisses (Grt-Sil-Bt gneiss and Bt-Pl gneiss) from the Gaozhou Complex of the Yunkai uplift in the Cathaysia block were selected for the analysis of whole-rock major elements and zircon U-Pb dating, trace elements and Lu-Hf isotopes. The Gaozhou Complex experienced early Paleozoic regional high-temperature (up to 850 °C), low- to medium-pressure (4-7 kbar) metamorphism accompanied by crustal anatexis. The melts were produced through the dehydration of mica, such as biotite + quartz + plagioclase = orthopyroxene + K-feldspar + melt and biotite + quartz + plagioclase + sillimanite = garnet + K-feldspar + melt in the charnockites, and muscovite + quartz + plagioclase = sillimanite + K-feldspar + melt in the Grt-Sil-Bt gneisses. The charnockites, gneissic migmatites and gneisses are felsic with SiO2 > 64% and peraluminous with A/CNK > 1.0, reflecting protoliths with affinities to sedimentary rocks. Zircons from these rocks partly show clear core-rim structure and yield concordant ages mainly around 440-425 Ma, with minor groups at 2.8-2.4 Ga, 1.5-1.25 Ga, 1.2-0.9 Ga, 850-540 Ma and 460-450 Ma. The 440-425 Ma grains are euhedral, oscillatorily-zoned and have steep slopes from the LREE to the HREE with a positive Ce anomaly and clear negative Eu anomaly, suggesting they (re-) crystallized in the melts. These early Paleozoic zircons have negative εHf (t) (- 34.1 to - 1.5) and much older TCRUST (3.6-1.5 Ga), demonstrating they were formed by re-melting of old crustal materials (> 1.5 Ga). The zircons with ages of 2.8-2.4 Ga, 1.6-1.2 Ga and 1.2-0.9 Ga have relatively high εHf (t) values (up to + 10

  1. Seismic Tomography Reveals Breaking Crust and Lithosphere Beneath a Classic Orogen

    NASA Astrophysics Data System (ADS)

    Byrne, T. B.; Rau, R.; Kuo-Chen, H.; Lee, Y.; Ouimet, W. B.; Van Soest, M. C.; Huang, C.; Wu, F. T.

    2013-12-01

    The orogenic system in Taiwan is often considered a classic example of an accretionary prism that has grown to a steady-state size and shape above an also steady subduction zone. A new study of vertical and horizontal sections of a tomographic velocity model created by Kuo-Chen et al. (2012) show, however, both a well-developed crack in the subducted crust beneath southern Taiwan and a discontinuous lithosphere beneath northern Taiwan, suggesting that slab breakoff is actively occurring beneath Taiwan. The transition from slab breakoff to cracking crust in southern Taiwan also suggests that slab breakoff is propagating southward, consistent an oblique collision. The crack in the subducting crust is revealed by progressively deeper horizontal sections of the local-scale tomographic model. The sections show an ellipsoidal-shaped area of high velocity that plunges southeast, oblique to all of the regional trends. Taking into account the dip of the slab, however, the area of high velocity is nearly parallel to previously recognized fracture zone in the Eurasian continental margin. We interpret the area of high velocity to be a crack in the Eurasian crust that is filled high velocity Eurasian mantle. Support for this interpretation comes from: 1) new exhumation cooling data from Mt Yu, the highest peak in Taiwan; 2) a recent leveling survey along the South Cross-Island Highway that shows unusually high rates of surface uplift (up to 15 mm/yr; Ching et al., 2011); 3) Vp attenuation studies that suggest anomalously high temperatures and/or the presence of fluids; 4) earthquake focal mechanisms in the core of the southern Central Range that are dominated by NE-SW extension; and finally, 5) the core of the southern Central Range preserves anomalous areas of low topographic relief that straddle the crest of the range. The areas of low relief are fringed by stream channels with relatively high stream gradient indexes and do not appear related to weaker rock types, glacial

  2. Miocene magmatism and tectonics within the Peri-Alboran orogen (western Mediterranean)

    NASA Astrophysics Data System (ADS)

    El Azzouzi, M.; Bellon, H.; Coutelle, A.; Réhault, J.-P.

    2014-07-01

    The aim of this paper concerns Miocene igneous activity in the Alboran Sea and Peri-Alboran area (northern Morocco, western Algeria and Betic Cordilleras in Spain), considering its age and its location with regard to major tectonics structures. We have compiled previous K-Ar isotopic ages of lavas and plutonic boulders and intrusives with an error of ±1σ and completed this set by a new K-Ar isotopic age for andesitic tuffites from Alboran Island. Geochemistry of most of these samples has been considered after previous analyses completed with new data for Spain magmatism. These two sets of data allow us to place the magmatic activity within the regional stratigraphy and tectonics and their chronological framework of the three major tectonic phases of the Maghrebian orogen, at 17 Ma (Burdigalian), 15 Ma (Langhian) and 9 Ma (Tortonian). Petro-geochemical characteristics are compared through time and geographical locations. A major goal of this coupled approach is to help the elaboration of possible geodynamical processes. As an application, we present the case study of the Dellys, Djinet and Thenia region (east of Algiers) where the successive magmatic events between 19.4 ± 1 and 11.6 ± 0.5 Ma are closely related to the local tectonics and sedimentation. The Peri-Alboran igneous activity is placed in a multidisciplinary framework. Timing of activity is defined according to the ages of the neighbouring sedimentary units and the K-Ar ages of igneous rocks. In Spain, the Cabo de Gata-Carboneras magmatic province displays late Oligocene and early Miocene leucogranitic dikes, dated from 24.8 ± 1.3 to 18.1 ± 1.2 Ma; three following andesitic to rhyolitic events took place around 15.1 ± 0.8 to 14.0 ± 0.7 Ma, 11.8 ± 0.6 to 9.4 ± 0.4 Ma, 8.8 ± 0.4 to 7.9 ± 0.4 Ma; this last event displays also granitic rocks. Lamproitic magmas dated between 8.4 ± 0.4 and 6.76 ± 0.04 Ma were emplaced after the Tortonian phase. In Morocco, after the complex building of the Ras Tarf

  3. Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie-Sulu orogen in China: implications for geodynamics and fluid regime

    NASA Astrophysics Data System (ADS)

    Zheng, Yong-Fei; Fu, Bin; Gong, Bing; Li, Long

    2003-07-01

    Discovery of coesite, diamond, and extreme 18O-depletion in eclogites from the Dabie-Sulu orogen in central-east China has contributed much to our understanding of subduction of continental crust to mantle depths and its subsequent exhumation. Hydrogen, oxygen, and carbon isotope distributions were systematically investigated in the past 8 years for ultrahigh pressure (UHP) eclogites, gneisses, granulites, marbles, and peridotites from this exciting region. The available data show the following characteristic features: (1) variable δ18O values of -11‰ to +10‰ for the eclogites and gneisses, with both equilibrium and disequilibrium fractionations of oxygen isotopes among minerals; (2) disequilibrium fractionation of hydrogen isotopes between mica and epidote from both eclogites and gneisses, with low δD values up to -127‰ to -100‰ for phengite; (3) negative δ13C values of -28‰ to -21‰ for apatite as well as host-eclogites and gneisses; (4) positive δ13C values of +1‰ to +6‰ for coesite-bearing marble associated with eclogites; (5) zircons from metamorphic rocks of different grades show a large variation in δ18O from -11‰ to +9‰, with U-Pb ages of 700 to 800 Ma for the timing of low- δ18O magma crystallization. It appears that the UHP metamorphic rocks exhibit ranges of δ18O values that are typical of potential precursor protolith rocks. Preservation of the oxygen isotope equilibrium fractionations among the minerals of the UHP eclogites and gneisses suggests that these rocks acquired the low δ18O values by meteoric-hydrothermal alteration before the UHP metamorphism. Thus, the UHP metamorphic rocks largely reflect the δ18O values of their premetamorphic igneous or sedimentary precursors. The stable isotope data demonstrate that basaltic, granitic, and sedimentary protoliths of the eclogites, orthogneiss, and paragneiss in the orogen were at or near the earth's surface, and subjected to varying degrees of water-rock interaction at some

  4. Orogen-parallel extension and exhumation enhanced by denudation in the trans-Himalayan Arun River gorge, Ama Drime Massif, Tibet-Nepal

    NASA Astrophysics Data System (ADS)

    Jessup, Micah J.; Newell, Dennis L.; Cottle, John M.; Berger, Aaron L.; Spotila, James A.

    2008-07-01

    Focused denudation and mid-crustal flow are coupled in manyactive tectonic settings, including the Himalaya, where exhumationof mid-crustal rocks accommodated by thrust faults and low-angledetachment systems during crustal shortening is well documented.New structural and (U-Th)/He apatite data from the Mount Everestregion demonstrate that the trans-Himalayan Ama Drime Massifhas been exhumed at a minimum rate of ~1 mm/yr between 1.5 and3.0 Ma during orogen-parallel extension. The Ama Drime Massifoffsets the South Tibetan detachment system, and therefore theSouth Tibetan detachment system is no longer capable of accommodatingsouth-directed mid-crustal flow or coupling it with focuseddenudation. Previous investigations interpreted the NNE-SSW-strikingshear zone on the west side of the Ama Drime Massif as the MainCentral thrust zone; however, our data show that the Ama DrimeMassif is bounded on either side by 100-300-m-thick normal-senseshear zone and detachment systems that are kinematically linkedto young brittle faults that offset Quaternary deposits andrecord active orogen-parallel extension. When combined withexisting data, these results suggest that the Ama Drime Massifwas exhumed during orogen-parallel extension that was enhancedby, or potentially coupled with, denudation in the trans-HimalayanArun River gorge. This model provides important insights intothe mechanisms that exhumed trans-Himalayan antiformal structuresduring orogen-parallel extension along the southern margin ofthe Tibetan Plateau.

  5. Crustal structure in the junction of Qinling Orogen, Yangtze Craton and Tibetan Plateau: implications for the formation of the Dabashan Orocline and the growth of Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Jiang, Chengxin; Yang, Yingjie; Zheng, Yong

    2016-06-01

    The crust at the junction of Qinling Orogen, Yangtze Craton and NE Tibetan Plateau bears imprints of the Triassic collision and later intracontinental orogeny between the Qinling Orogen and the Yangtze Craton, and the Cenozoic growth of Tibetan Plateau. Investigating detailed crustal structures in this region helps to better understand these tectonic processes. In this study, we construct a 3-D crustal Vs model using seismic ambient noise data recorded at 321 seismic stations. Ambient noise tomography is performed to generate Rayleigh wave phase velocity maps at 8-50 s periods, which are then inverted for a 3-D isotropic Vs model using a Bayesian Monte Carlo method. Our 3-D model reveals deep-rooted high velocities beneath the Hannan-Micang and Shennong-Huangling Domes, which are located on the west and east sides of the Dabashan Orocline. Similar high velocities are observed in the upper/mid crust of the western Qinling Orogen. We suggest the crustal-scale bodies with high velocity beneath the two domes and the western Qinling Orogen may represent mechanically strong rocks, which not only assisted the formation of the major Dabashan Orocline during late Mesozoic intracontinental orogeny, but also have impeded the northeastward expansion of the Tibetan Plateau during the Cenozoic era.

  6. Provenance of Cretaceous trench slope sediments from the Mesozoic Wandashan Orogen, NE China: Implications for determining ancient drainage systems and tectonics of the Paleo-Pacific

    NASA Astrophysics Data System (ADS)

    Sun, Ming-Dao; Xu, Yi-Gang; Wilde, Simon A.; Chen, Han-Lin

    2015-06-01

    The Wandashan Orogen of NE China is a typical accretionary orogen related to Paleo-Pacific subduction. The Raohe Complex, as a major part of the orogen, consists of mid-Triassic to mid-Jurassic radiolarian chert and intraoceanic igneous rocks in an accretionary prism overlain by weakly sheared terrestrial-sourced clastic trench slope sediments. Sensitive high-resolution ion microprobe U-Pb dating and LA-MC-ICPMS Hf isotopic analysis of detrital zircons from the terrestrial-sourced Yongfuqiao Formation sandstone show that most zircons are Phanerozoic (90%): 140-150 Ma (10%), 180-220 Ma (25%), 240-270 Ma (15%), 300-360 Ma (15%), 391-395 Ma (3%), and 450-540 Ma (20%), whereas 10% are Precambrian in age. About 90% of the zircons have ɛHf(t) values ranging from +11.1 to -12.8. This suggests that the major provenance of the trench slope sediments was from the adjacent eastern segment of the Central Asian Orogenic Belt and the Jiamusi Block. The age of the Yongfuqiao Formation is constrained to the earliest Cretaceous, which represents the accretion time of the mid-Triassic to mid-Jurassic oceanic complexes. When compared with the Mino Complex in Japan and the Tananao Complex in Taiwan, three different provenances are identified suggesting three ancient drainage systems which transported sediments from NE China, North China, and South China to the Paleo-Pacific subduction-accretion system.

  7. Crustal structure in the junction of Qinling Orogen, Yangtze Craton and Tibetan Plateau: implications for the formation of the Dabashan Orocline and the growth of Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Jiang, Chengxin; Yang, Yingjie; Zheng, Yong

    2016-03-01

    The crust at the junction of Qinling Orogen, Yangtze Craton and NE Tibetan Plateau bears imprints of the Triassic collision and later intra-continental orogeny between the Qinling Orogen and the Yangtze Craton, and the Cenozoic growth of Tibetan Plateau. Investigating detailed crustal structures in this region helps to better understand these tectonic processes. In this study, we construct a 3-D crustal Vs model using seismic ambient noise data recorded at 321 seismic stations. Ambient noise tomography is performed to generate Rayleigh wave phase velocity maps at 8-50 s periods, which are then inverted for a 3D isotropic Vs model using a Bayesian Monte-Carlo method. Our 3D model reveals deep-rooted high velocities beneath the Hannan-Micang and Shennong-Huangling Domes, which are located on the west and east sides of the Dabashan Orocline. Similar high velocities are observed in the upper/mid crust of the western Qinling Orogen. We suggest the crustal-scale bodies with high velocity beneath the two domes and the western Qinling Orogen may represent mechanically strong rocks, which not only assisted the formation of the major Dabashan Orocline during late Mesozoic intra-continental orogeny, but also have impeded the northeastward expansion of the Tibetan Plateau during the Cenozoic era.

  8. Disclosing the Paleoarchean to Ediacaran history of the São Francisco craton basement: The Porteirinha domain (northern Araçuaí orogen, Brazil)

    NASA Astrophysics Data System (ADS)

    Silva, Luiz Carlos da; Pedrosa-Soares, Antonio Carlos; Armstrong, Richard; Pinto, Claiton Piva; Magalhães, Joana Tiago Reis; Pinheiro, Marco Aurélio Piacentini; Santos, Gabriella Galliac

    2016-07-01

    This geochronological and isotopic study focuses on one of the Archean-Paleoproterozoic basement domains of the São Francisco craton reworked in the Araçuaí orogen, the Porteirinha domain, Brazil. It also includes a thorough compilation of the U-Pb geochronological data related to the adjacent Archean and Rhyacian terranes from the São Francisco craton and Araçuaí orogen. The main target of this study is the TTG gneisses of the Porteirinha complex (Sample 1). The gneiss dated at 3371 ± 6 Ma unraveled a polycyclic evolution characterized by two metamorphic overprinting episodes, dated at 3146 ± 24 Ma (M1) and ca. 600 Ma (M2). The former (M1) is so far the most reliable evidence of the oldest metamorphic episode ever dated in Brazil. The latter (M2), in turn, is endemic in most of the exposed eastern cratonic margin within the Araçuaí orogen. Whole-rock Sm-Nd analysis from the gneiss provided a slightly negative εNd(t3370) = - 0.78 value, and a depleted mantle model (TDM) age of 3.5 Ga, indicating derivation mainly from the melting of a ca. 3.5 Ga tholeiitic source. Sample 2, a K-rich leuco-orthogneiss from the Rio Itacambiriçu Complex, was dated at 2657 ± 25 Ma and also presents a ca. 600 Ma M2 overprinting M2 age. The other two analyses were obtained from Rhyacian granitoids. Sample 3 is syn-collisional, peraluminous leucogranite from the Tingui granitic complex, showing a crystallization age of 2140 ± 14 Ma and strong post-crystallization Pb*-loss, also ascribed to the Ediacaran overprinting. Accordingly, it is interpreted as a correlative of the late Rhyacian (ca. 2150-2050 Ma) collisional stage of the Mantiqueira orogenic system/belt (ca. 2220-2000 Ma), overprinted by the Ediacaran collage. Sample 4 is a Rhyacian post-orogenic (post-collisional), mixed-source, peralkaline, A1-type suite, with a crystallization age of 2050 ± 10 Ma, presenting an important post-crystallization Pb*-loss related to Ediacaran collision. The focused region records some

  9. The Sikhote-Alin orogenic belt, Russian South East: Terranes and the formation of continental lithosphere based on geological and isotopic data

    NASA Astrophysics Data System (ADS)

    Khanchuk, A. I.; Kemkin, I. V.; Kruk, N. N.

    2016-04-01

    The Sikhote-Alin orogenic belt, Russian South East, consists of folded terranes made up of Jurassic and Early Cretaceous accretionary prisms, turbidite basins, and island arc terranes that are overlapped unconformably by undeformed upper Cenomanian to Cenozoic volcanic deposits. The Jurassic and Early Cretaceous accretionary prisms, together with the Early Cretaceous island arc, are related to subduction of the Paleo-Pacific plate. The turbidite basin, which began to form at the beginning of the Early Cretaceous, is related to left-lateral movement of the Paleo-Pacific plate along the Paleo-Asian continental margin. The collage of terranes that make up the Sikhote-Alin orogenic belt was amalgamated in two stages. The first began after Jurassic subduction beneath the Asian continent was terminated, and the second took place in the late Albian, when the Early Cretaceous island arc collided with the continental margin. Intense deformation of the terranes took place along the continental margin in the form of large-scale translations from south to north, together with oroclinal folding. The deformation resulted in rapid thickening of sediments in the upper crust, resulting in turn in the formation of granitic-metamorphic material in the continental lithosphere. In the southwestern part of the Sikhote-Alin orogen, granites were intruded during the Hauterivian-Aptian, while the entire orogenic belt was affected by intrusions in the late Albian-early Cenomanian. Synorogenic intraplate volcanic rocks and alkaline ultramafic-mafic intrusions also testify to the fact that the orogenic processes in the Sikhote-Alin were related to a transform continental margin, and not to subduction. Geochemical and Nd isotopic data indicate, the primary continental crust of the Sikhote-Alin was of a "hybrid" nature, consisting of juvenile basic components accreted from an oceanic plate and recycled sedimentary material derived from the erosion of ancient blocks.

  10. Early Cretaceous Erlangmiao Metaluminous A-type Granite in the Eastern Qinlin Orogen, central China: Geochronological and Geochemical Constraints

    NASA Astrophysics Data System (ADS)

    Zhang, J.; Ma, C.; Sher, Z.; Wang, S.; Wang, L.; Cao, Y.

    2007-12-01

    It is possible to study the Mesozoic lithospheric thinning and crust-mantle interaction due to Early Cretaceous intensive magmatism widely developed in the North China Block. As an important part of the Early Cretaceous magmatism in the North China Block, the widespread Early Cretaceous granites in the Dabieshan-Qinlin Orogen, situated in the southern margin of the North China Block, contain hornblende and/or biotite and are I-type granites, but other type granites are less reported. Early Cretaceous Erlangmiao garnet granite (EGG) with garnet and biotite found recently near Erlangmiao in Fangcheng County, Henan Province, China, has provided new constraints for evolvement of the Dabie-Qinlin Orogen. This abstract reports results of zircon U-Pb dating, elemental geochemistry, and Sr-Nd isotopic compositions of the EGG and its wall rocks in an attempt to constrain its petrogenesis and the Early Cretaceous tectonic evolution of the Dabie-Qinlin Orogen. The EGG, located in the eastern Qinlin Orogen, was emplaced in gneissic monzogranites (GG). There are abundant GG xenoliths and surmicaceous enclaves in the eastern margin of the stock. LA-ICPMS zircon U-Pb dating of the EGG suggests its crystallization at 118 ±2 Ma, whole-rock Rb-Sr isochron age, represented the cooling age of the EGG, is 110±1 Ma. The EGGs are syenogranites with almandine-Spessartine garnet and biotite. The EGGs are characterized by high SiO2 and alkali contents, high FeOt/MgO ratio, low Al2O3 and CaO contents, and display high-K calc-alkaline and metaluminous characteristics. They show low total Rare Earth Element content (ΣREE), strong negative Eu anomaly, are generally enriched in Rb, Th, Ta, Nb, Zr, Hf, Y, Yb and depleted in Sr, Ba, Ce, P, Ti, and have high Rb, Rb/Sr and Ga/Al ratios. Al2O3, Fe2O3, K2O, Zr, Rb, Pb of the EGGs decrease with increasing SiO2 content. Initial 87Sr/86Sr ratios of the whole rocks range in 0.706-0.708, while the ɛNd(110Ma) values vary from -6.6 to -9.0 and Nd

  11. The St. Elias orogen as an early stage in the development of indentor corners: Initial results from the STEEP project

    NASA Astrophysics Data System (ADS)

    Pavlis, T. L.; Bruhn, R. L.; Koons, P.; Berger, A.; Spotila, J.; Barker, A.; Chapman, J.; Doser, D.; Pavlis, G.

    2006-12-01

    The actively deforming St. Elias orogen of southern Alaska is developed at the transition between the Aleutian trench and the Queen Charlottes transform, and has produced a "one sided" indentor corner as the collided block has been carried obliquely into the subduction-transform transition. A distinctive feature generated within this evolving corner is a 90o bend in the suture at the western edge of the orogen that is associated with a refolding of earlier thin-skinned fold-thrust systems in the collided block. The refolding is recognizable in the area between the bent suture and a major glacial valley, the Bering Glacier, but is absent to the east. Bruhn et al. (2004, GSAB) used this observation to infer an active structure along the Bering Glacier, and speculated that the refolding may be active. New data collected as part of the St. Elias Erosion and tectonics Project (STEEP) provides support for this hypothesis from two observations: 1) clear evidence that many of the hundreds of surface ruptures in this region represent fault-scarps-- probably representing fold-related, bedding- plane slip during refolding; and 2) extremely young (locally <500Ka) U-Th-He apatite cooling ages just west of the Bering Glacier are consistent with the hypothesis of an active structure along the glacial valley. However, young cooling ages to the east of the Bering Glacier as well as geomorphology suggest that at least one of the fold-thrust belt structures is also active, indicating that the interplay of active faults is complex. Additional support for active deformation along the Bering Glacier comes from the spatial distribution of seismicity between the Bering Glacier and Bagley icefield and these observations should be further constrained as data are obtained from the new STEEP broadband seismic network in the area. Analog (sand) and numerical models suggest that this type of feature can originate as the indentor is driven into the corner through a variable vertical axis

  12. Late Paleozoic tectonic evolution and concentrated mineralization in Balkhash and West Junggar, western part of the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Dong, Shuwen; Chen, Xuanhua; Chen, Zhengle

    2016-04-01

    The Central Asia Orogenic Belt (CAOB) is an important area with significant growth of the crust and metallogeny in the Late Paleozoic. The Balkhash-Junggar tectono-metallogenic belt consists of the Balkhash, the West Junggar, and the East Junggar tectono-metallogenic belts in western part of the Central Asian Orogenic Belt (CAOB). According to the structural geological relationship, the East Junggar, the West Junggar, and the Balkhash belts are considered to be once a continuous E-W-trending tectono-metallogenic belt in Late Carboniferous. The West Junggar belt is featured with NE-trending left-lateral strike-slip faulting tectonic system (WJTS), while the left-lateral strike-slip faults are E-W-trending in the Balkhash belt. The WJTS consists of the Darabut, the Mayile, and the Baerluke faults, and the blocks among them. All these left-lateral strike-slip faults are forming due to the transition of tectonic settings from syn-collisional orogeny to post-collisional extension during the closure of the ocean (the Junggar Sea) in Late Carboniferous, with significant intrusion of batholiths and crust growth occurred in this period. These faults are truncated by the right-lateral strike-slip faults, such as the Chingiz-Junggar fault, and the Central Balkhash fault in Mesozoic. The Balkhash-Junggar tectono-metallogenic belt is important for the occurrence of many well-known super-large and large porphyry Cu-Mo deposits (such as the Kounrad, the Aktogai, the Borly, and the Baogutu deposits), large skarn Cu deposits (in the Sayak ore-filed), large rare metal deposits (such as the East Kounrad, the Zhanet, and the Akshatau deposits), and large gold deposits (such as the Hatu deposit). Zircon U-Pb ages, Re-Os isotopic dating of molybdenites, 40Ar/39Ar thermochronology of hornblendes, muscovites, biotites, and K-feldspars, and zircon and apatite fission track (FT) and (U-Th)/He dating and thermal history modeling, provide a multidisciplinary approach to constrain the whole

  13. Your Menstrual Cycle

    MedlinePlus

    ... during your menstrual cycle What happens during your menstrual cycle The menstrual cycle includes not just your period, but the rise ... tool is based on a sample 28-day menstrual cycle, but every woman is different in how long ...

  14. Flexural bending-induced plumelets and their seamounts in accretionary (Japanese-style) and collisional (Tethyan-style) orogenic belts

    NASA Astrophysics Data System (ADS)

    Hirano, N.; Dilek, Y.

    2015-12-01

    Seamounts and seamount chains are common in both the upper and lower plates of active subduction zones. Their OIB-type volcanic products are distinctly different from suprasubduction zone (arc, forearc and backarc) generated volcanic rocks in terms of their compositions and mantle sources. Tectonic accretion of such seamounts into the Japanese archipelago in the NW Pacific and into subduction-accretion complexes and active margins of continents/microcontinents within the Tethyan realm during the Cretaceous played a significant role in continental growth. Seamount assemblages comprise alkaline volcanic rocks intercalated with radiolarian and hemipelagic chert, and limestone, and may also include hypabyssal dolerite and gabbro intrusions. In the Tethyan orogenic belts these seamount rocks commonly occur as km-scale blocks in mélange units beneath the late Jurassic - Cretaceous ophiolites nappes, whereas on the Japanese islands they form discrete, narrow tectonic belts within the late Jurassic - Cretaceous accretionary prism complexes. We interpret some of these OIB occurrences in the Japanese and Tethyan mountain belts as asperities in downgoing oceanic plates that formed in <10 million years before their accretion. Their magmas were generated by decompressional melting of upwelling asthenosphere, without any significant mantle plume component, and were brought to the seafloor along deep-seated brittle fractures that developed in the flexed, downgoing lithosphere as it started bending near a trench. The modern occurrences of these "petit-spot volcanoes" are well established in the northwestern Pacific plate, off the coast of Japan. The proposed mechanism of the formation of these small seamounts better explains the lack of hotspot trails associated with their occurrence in the geological record. Magmatic outputs of such flexural bending-induced plumelets should be ubiquitious in the accretionary (Japanese-style) and collisional (Tethyan-style) orogenic belts.

  15. Geochronology and Geochemistry of Volcanic Suites from the Middle Sulu Orogenic Belt, Eastern China: Implication for Petrogenesis and Tectonic Setting

    NASA Astrophysics Data System (ADS)

    Wang, J.; Chang, S. C.; Lu, H.; Zhang, H.

    2014-12-01

    The widely distributed Late Mesozoic igneous events in North China have been intensively studied in the past decades and proposed to be accounted by lithospheric thinning. However, mechanism of the lithospheric thining is still in debate. In this study, we report U-Pb zircon ages, geochemical data and Sr-Nd isotopic data for the Cretaceous mafic and associated felsic intrusions from the middle Sulu orogenic belt in the Shandong Peninsula. The ICP-MS U-Pb zircon analyses show consistent crystallization ages ranging from 120.2 ± 2.1 Ma to 123.3 ± 1.6 Ma for four representative samples. These rocks are characterized by LREE and LILE enrichment, HREE depletion, high initial 87Sr/86Sr values ranging from 0.70401 to 0.70966, low negative ɛNd(t) values from -22.0 to -12.2. These features suggest that they were derived from a common enriched lithospheric mantle source. Geochemical and isotopic data indicate that the original magma of these rocks was produced by partial melting of an ancient lithospheric mantle, which was variably mixed with lower crustal eclogite derived melts. The lamprophyres, andesitic porphyrites and syenogranite may have been attributed to subsequent fractional crystallization of olivine and pyroxene, whereas the rhyolite resulted from K-feldspar and plagioclase fractionation, without any crustal contamination affection. Tectonically, the petrogenesis process of these rocks favors an intense lithospheric extensional regime beneath the Sulu orogenic belt at 120 ~ 124 Ma, which was resulted by the widespread removal of the upper mantle and lower crust that driven by the abrupt change of the subducting direction of the Pacific plate.

  16. Paleomagnetic and geochronological study of Carboniferous forearc basin rocks in the Southern New England Orogen (Eastern Australia)

    NASA Astrophysics Data System (ADS)

    Pisarevsky, Sergei A.; Rosenbaum, Gideon; Shaanan, Uri; Hoy, Derek; Speranza, Fabio; Mochales, Tania

    2016-06-01

    We present results of a paleomagnetic study from Carboniferous forearc basin rocks that occur at both limbs of the Texas Orocline (New England Orogen, eastern Australia). Using thermal and alternating field demagnetizations, two remanence components have been isolated from rocks sampled from the Emu Creek terrane, in the eastern limb of the orocline. A middle-temperature Component M is post-folding and was likely acquired during low-temperature oxidation at 65-35 Ma. A high-temperature Component H is pre-folding, but its comparison with the paleomagnetic data from coeval rocks in the northern Tamworth terrane on the other limb of Texas Orocline does not indicate rotations around a vertical axis, as expected from geological data. A likely explanation for this apparent discrepancy is that Component H postdates the oroclinal bending, but predates folding in late stages of the 265-230 Ma Hunter Bowen Orogeny. The post-Kiaman age of Component H is supported by the presence of an alternating paleomagnetic polarity in the studied rocks. A paleomagnetic study of volcanic and volcaniclastic rocks in the Boomi Creek area (northern Tamworth terrane) revealed a stable high-temperature pre-folding characteristic remanence, which is dated to c. 318 Ma using U-Pb zircon geochronology. The new paleopole (37.8°S, 182.7°E, A95 = 16.2°) is consistent with previously published poles from coeval rocks from the northern Tamworth terrane. The combination of our new paleomagnetic and geochronological data with previously published results allows us to develop a revised kinematic model of the New England Orogen from 340 Ma to 270 Ma, which compared to the previous model, incorporates a different orientation of the northern Tamworth terrane at 340 Ma.

  17. Non-volcanic tremors, low-frequency earthquake swarms, and their association with orogenic fluid flow in Taiwan

    NASA Astrophysics Data System (ADS)

    Chen, K. H.; Kim, A.; Chuang, Y. L.; Peng, W.; Leu, T.

    2012-12-01

    Taiwan is a young transpressive orogen exhibiting high uplift and exhumation. Under the southern flank of Central Range, two types of seismic activities typically observed in volcanic region are found to occur closely in space. Swarms of earthquakes showing vertical planar features occur frequently at a depth range of 0-20 km, whereas some shallow events (< 7 km) reveal the characteristics similar to volcanic low-frequency earthquakes: (1) dominant by ~ 2 Hz (2) lack of S phases (3) long coda. Below the swarm events, the deep-seated, ambient tremors are also observed. Spatial correlation between deeper tremor and shallow low-frequency swarm events indicates a common generation mechanism. These two seismic activities are confined in a small area where the localized veining, anomalous magnetic and thermal anomalies are distributed, suggesting the involvement of fluid-pressure processes within the orogen. Here we pay special attention to develop an automatic detection scheme for Taiwan tremors. Using this method we find 87 tremor episodes with duration ranging from 5 to 30 minutes. The tremors activity appears to have increased at the time of local M6 earthquakes. In March 2008, an active swarm composed of 632 events (M0.4-5.2) occurred only two days apart of the preceding tremor episode (duration 60 minutes). The possible spatial and temporal relationship between ambient tremors and earthquake swarms provides a rare opportunity for the understanding of tremor source model. Here we examine if the swarms are induced by shear stress increases due to slip events along the decollement underneath Central Range. Other than slip model, fluid-filled cracks model is also examined. Moment tensor inversion will be used to determine focal mechanism of the low-frequency earthquake swarms, to investigate the fluids related volume changes.

  18. Unsteady rock uplift and erosion in a decaying orogen in response to surface and dynamic mantle processes (Invited)

    NASA Astrophysics Data System (ADS)

    Pazzaglia, F. J.; Zeitler, P. K.; McKeon, R. E.; Idleman, B. D.; Berti, C.

    2010-12-01

    The Appalachian Mountains in the eastern United States are constructed on a lithosphere that was assembled and modified during a long period of Paleozoic collisional tectonics, and then thinned by erosion and stretching with the opening of the Atlantic Ocean. Geologic, geomorphic, and geophysical data describe the long term erosion, persistence of mountainous topography, and coupling between surface and tectonic, isostatic, or eperiogenic processes of this ancient, decaying orogen. There are several key observables that must be reconciled to explain Appalachian post-orogenic (post-Triassic) evolution. These include subsidence of the former metamorphic core of the range and topographic inversion of the foreland, a long-term record of unsteady erosion preserved in the sediments of the Atlantic margin shelf-slope basins, the steepening of the longitudinal profiles of Atlantic slope rivers forming a zone of bedrock rapids as they approach the Atlantic, river knickpoints in the foreland with a common elevation that have no apparent relation to rock-type or structure, and evidence that the divide between Atlantic slope and Ohio drainages is not a static feature. The long term rate of Appalachian rock uplift (base level fall) and erosion is ~20-30 m/my., a result consistent with AHe thermochronology and the average sediment flux to the BCT, but misleading in terms of the unsteadiness demonstrated by river incision, growth of the Atlantic slope drainage, and short-term sediment flux to the BCT. This unsteadiness is likely influenced by several factors including climate change, sediment storage in the landscape and the Coastal Plain, and eustasy; however, we propose that the most important factor has been unsteady rock uplift as the result of a lithospheric flexural response to surface loads and sub-lithospheric mantle flow driven by the foundering Farallon slab.

  19. Late Palaeozoic to early Mesozoic geodynamic evolution of the Circum-Pacific orogenic belt in South Korea and Southwest Japan

    NASA Astrophysics Data System (ADS)

    Cluzel, Dominique

    1992-02-01

    A review of the overall features of the late Palaeozoic-early Mesozoic orogen of Southwest Japan and its Korean hinterland allows a new geodynamic model for the building of the east Asian margin to be proposed. During middle Carboniferous to early Permian times, an oceanic basin formed in a passive margin environment at a moderate spreading rate as the Honshu continental block drifted away from the Sino-Korean continent. During that period, paralic platform conditions and extensional synsedimentary tectonics prevailed in Korea. In middle Permian times, the closure of the oceanic area was responsible for Yakuno ophiolite obduction and high pressure metamorphism while highly subsiding coal basins developed in the emerged Korean hinterland. Thereafter, in late Permian times, when the Honshu block collided with the Asian margin (Akiyoshi orogeny), locally derived terrestrial clastic sedimentation in Korea has succeeded coal measure deposition with local unconformity. In early Triassic times, the collision was completed and regional uplift of the thickened crust resulted in the deposition of thick distal siliciclastic rocks in rapidly subsiding hinterland troughs (Korea) and flyschoid deposition in the Tamba foreland basin (outer zone of Southwest Japan). Middle to late Triassic medium- to low-pressure metamorphism, large-scale transcurrent tectonics and associated calc-alkaline synkinematic plutonism in the Hida and Ogcheon belts represent a post-collisional feature of the Akiyoshi orogenic event. Post-tectonic intrusives and terrestrial deposits post-date intracratonic tectonics at ca. 200 Ma. Thus, the Akiyoshi orogeny of Southwest Japan is a result of continental drift and ensuing collisional rewelding of a fragment of Sino-Korea: the Honshu microcontinent.

  20. The Permian Dongfanghong island-arc gabbro of the Wandashan Orogen, NE China: Implications for Paleo-Pacific subduction

    NASA Astrophysics Data System (ADS)

    Sun, Ming-Dao; Xu, Yi-Gang; Wilde, Simon A.; Chen, Han-Lin; Yang, Shu-Feng

    2015-09-01

    The Dongfanghong hornblende gabbro is located in the western part of the Wandashan Orogen and to the east of the Jiamusi Block in NE China. It was emplaced into Early Paleozoic oceanic crust (i.e. Dongfanghong ophiolite) at ~ 275 Ma and both later collided with the eastern margin of the Jiamusi Block. The Dongfanghong gabbro is sub-alkaline with high Na2O contents and is characterized by enrichment in light rare earth elements (LREE), large ion lithosphile elements (LILE), Sr, Eu, and Ba, and depletion in high field strength elements (HFSE). The enriched isotopic signatures (87Sr/86Sri = ~ 0.7065, εNd(t) = ~- 0.5, 208Pb/204Pbi = ~ 38.05, 207Pb/204Pbi = ~ 15.56, 206Pb/204Pbi = ~ 18.20 and zircon εHf(t) = ~+ 5.8) indicate an enriched mantle (EM2) source, with some addition of continental material. It has arc geochemical affinities similar to Permian arc igneous rocks in the eastern margin of the Jiamusi Block, the Yakuno Ophiolite in SW Japan, arc rocks along the western margin of the North America Craton, and also the Gympie Group in eastern Australia. All these features, together with information from tectonic discrimination diagrams, suggest that the Dongfanghong gabbro formed in an immature island arc. The spatial configuration of ~ 290 Ma immature continental arc rocks in the eastern part of the Jiamusi Block and the ~ 275 Ma immature island arc Dongfanghong gabbro in the Wandashan Orogen to the east is best explained by eastward arc retreat and slab roll-back of the Paleo-Pacific Plate. This model is also supported by the Carboniferous-Permian stratigraphic transition in the Jiamusi Block from oceanic carbonate rocks to coal-bearing terrestrial clastic rocks and andesites. We thus suggest that both Paleo-Pacific subduction and roll-back occurred in the Early Permian along the eastern margin of Asia.

  1. Seismic Anisotropy and Upper-Mantle Velocity Structures Beneath the Qinling-Dabie Orogenic Beltand Surrounding Regions

    NASA Astrophysics Data System (ADS)

    Yu, Y.; Chen, Y. J.; Feng, Y.; An, M.; Liang, X.; Dong, S.

    2014-12-01

    The Qinling-Dabie orogenic belt was formed as the suture zone by the collision of the North China Block and the South China Block in the Late Triassic and it has been reactivated by the Indo-Asian collision since 60 million years ago. From July 2012 to May 2014 Peking University and Chinese Academy of Geological Science deployed a seismic array of 147 portable broadband seismic stations around the Qinling-Dabie orogenic belt. Here shear-wave splitting analysis of SKS data and finite-frequency tomography were applied to these data to imagine the seismic anisotropy and velocity structures in the upper mantle beneath the region. Large delay times were observed at the southwest corner of the Ordos Block and the northeast of Tibetan Plateau. Where the angles between the fast polarization directions and the GPS velocity vectors are relatively large but those in the Weihe rift and the North China plain are nearly parallel. Delay times are small, as expected, in the southeast of the Ordos Block and the north of Sichuan Basin with inconsistent fast polarization directions. Finite-frequency tomography clearly images a fast velocity anomaly under the Ordos block that extends down to 400 km depth and a slow velocity anomaly under the east of Weihe rift that can be traced down to 300 km depth. More importantly we observe a slow velocity anomaly in the upper mantle extending down to 200 km depth at the southwestern corner of the Ordos Block and the northeast of Tibetan Plateau, and the location corresponds well with the results of shear wave splitting showing large angles between the fast polarization directions and the GPS velocity vectors. These results suggest that it is possible that the mantle lithosphere of the southwestern corner of the Ordos Block could be thermally weakened by the eastward Tibetan mantle flow between the Ordos Block and Sichuan Basin.

  2. Non-tectonic base level forcings drive widespread transient incision and relief production in the waning Appalachian orogen

    NASA Astrophysics Data System (ADS)

    Prince, P. S.; Richardson, R. P.

    2012-12-01

    The development of knickpoints and steep bedrock gorges within otherwise low-relief landscapes of the Appalachian Highlands has long interested geomorphologists. While large tracts of muted Appalachian topography appear consistent with slow, steady exhumation of a thickened crustal root, active gorge development into lower-relief uplands suggests that Appalachian river systems have been re-energized to incise into bedrock and increase relief. Orogen-scale analysis of incision patterns, fluvial profiles, and surficial deposits indicate that two main sources of non-tectonic base level drop superimposed on the slowly-exhuming Appalachian landscape can explain the Cenozoic relief production observed. Along the Eastern Continental Divide on the southeast margin of the Highlands, repeated capture of headwaters of elevated, landward-draining streams of the Blue Ridge by Atlantic basin streams maintains the oversteepened slopes and bedrock gorges of the Blue Ridge Escarpment zone. Capture events induce a transient erosional response in which rapid knickpoint retreat carves deep gorges into the captured basin, ultimately adjusting topography to match the rest of the Atlantic slope. In the lower reaches of landward Appalachian rivers still following courses to the continental interior, hillslope steepening in the wake of migrating knickpoints is apparent against the backdrop of a comparatively low-relief upland. This landscape suggests that landward base level drop, possibly due to rapid, glacially-forced Plio-Pleistocene drainage rearrangement, has initiated transgressive waves of incision to adjust Highlands topography to the recently-established continental interior drainage pattern. Encroachment of both the post-rift seaward base level and the modern landward base level forces a rapid release of potential energy stored in the elevated Highlands, energizing streams to incise into bedrock without the introduction of new tectonic energy. The southern Appalachian landscape

  3. Thermal evolution of a main detachment zone during late orogenic collapse: the pilat unit case (Variscan chain).

    NASA Astrophysics Data System (ADS)

    Gardien, V.; Allemand, P.

    2003-04-01

    The high temperature uplift of deep seated nappes in different tectonic settings is a major question in orogenic domains. Detailed petrological and structural study were performed in the Pilat Unit (Eastern french Massif Central: Variscan chain) in order to discuss the thermal evolution of a main shear zone during late orogenic extension. The Pilat Unit is a 20 km long x 5 km large metamorphic unit and as a strain gradient within it perpandicular to the long axe of the unit. The mylonitic fabrics, in the strained zone are notherly directed. Geometrical and geochronological data indicate that these fabrics are associated with extesional regime that was a post-thickening event. Thermobarometrical determination supported by microstructural analyses indicate the develpoment of contrasting P-T-time evolution in the Pilat Unit. In the strongly mylonitized southern domain in which foliation and streching lineation show a top to the north normal slip, the calculated P-T conditions indicate an increase in temperature during decompression illustrated by the following mineral assemblages:1) Grt + Stau + Kya + Rut, 2) Bio + Sill + Cord, 3) And + Musco + K-feld and dated at 322 Ma. In the northern and less deformed domain, the calculated P-T conditions correspond to a cooling event during decompression illustrated by the following assemblages: 1) Grt + Stau + Kya + Rut, 2) Bio + Musco + Ilm, 3) And + Chlo + Pyro + Tour, dated at 313 Ma. The difference in T° between the southern and the northern domain is 200-250°C and Δ t is 10Ma. Based on these data, we have elaborated 1D numerical model simulating the thermal evolution of the continental crust during thinning associated or not with magma underplating.

  4. Mantle Recycling of Crustal Materials through Study of Ultrahigh-Pressure Minerals in Collisional Orogens, Ophiolites, and Xenoliths

    NASA Astrophysics Data System (ADS)

    Liou, J. G.; Tsujimori, T.; Yang, J.; Zhang, R. Y.; Ernst, W. G.

    2014-12-01

    Newly recognized ultrahigh-pressure (UHP) mineral occurrences including diamonds in ultrahigh-temperature (UHT) felsic granulites of orogenic belts, in chromitites associated with ophiolitic complexes, and in mafic/ultramafic xenoliths suggest the recycling of crustal materials through profound subduction, mantle upwelling, and return to the Earth's surface. Recycling is supported by unambiguously crust-derived mineral inclusions in deep-seated zircons, chromites, and diamonds from collision-type orogens, from eclogitic xenoliths, and from ultramafic bodies of several Alpine-Himalayan and Polar Ural ophiolites; some such phases contain low-atomic number elements typified by crustal isotopic signatures. Ophiolite-type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone. In addition to ringwoodite and stishovite, a wide variety of nanometric minerals have been identified as inclusions employing state-of-the-art analysis. Reconstitution of now-exsolved precursor UHP phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier UHP conditions. Some podiform chromitites and associated peridotites contain rare minerals of undoubted crustal origin, including Zrn, corundum, Fls, Grt, Ky, Sil, Qtz, and Rtl; the zircons possess much older U-Pb ages than the formation age of the host ophiolites. These UHP mineral-bearing chromitites had a deep-seated evolution prior to extensional mantle upwelling and its partial melting at shallow depths to form the overlying ophiolite complexes. These new findings plus stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere, recycling of biogenic carbon into the lower mantle, and ascent to the Earth's surface through deep mantle ascent.

  5. The generation, segregation, ascent and emplacement of granite magma: the migmatite-to-crustally-derived granite connection in thickened orogens

    NASA Astrophysics Data System (ADS)

    Brown, Michael

    1994-04-01

    Many granites result from anatexis of common crustal rock types and the segregation, aggregation, ascent and emplacement of the resultant magma. What then is the connection between migmatites, rocks which preserve evidence at outcrop-scale for the presence of former melt now frozen as granite, whether in situ or locally displaced with respect to the site of melting, and map-scale bodies of crustally-derived granite, clearly removed from the site of melting? Both water-rich volatile phase-present melting and volatile phase-absent dehydration melting can occur in the middle and lower crust, but dehydration melting that involves the decomposition of mica and amphibole likely is the more important process in the generation of plutonic volumes of magma with sufficient mobility to reach the upper crust. Both volatile phase-present and dehydration melting can occur in each of the two main types of orogenic belt, those that result from thickening before maximum temperatures are achieved (clockwise in P- T space) and those that result from heating prior to or concomitant with thickening (anticlockwise in P- T space). Depending upon the particular tectonic circumstances, the thermal perturbation to provide the heat necessary for crustal anatexis may be caused by internal radiogenic heat production in overthickened crust, intraplating/underplating of mantle-derived magma, an enhanced flux from the mantle, or some combination of these mechanisms. The tectonic environment to a large extent also controls the segregation, ascent and emplacement of granite magma. For example, at the present time a majority of convergent plate margins exhibit an oblique net displacement vector, and it is likely, therefore, that oblique convergence was important in the past. Retreating subduction boundaries will result in regional deformation of the overriding plate by horizontal extension or transtension in contrast to advancing subduction boundaries that will result in regional deformation of the

  6. On-, off-cratonic, orogenic and oceanic mantle roots: lithosphere unite

    NASA Astrophysics Data System (ADS)

    Wittig, Nadine; Pearson, D. Graham

    2010-05-01

    Associated with Earth's crust is the underlying lithospheric mantle, which is often categorized according its surface availability for sampling. For example, we frequently describe sub-continental lithospheric peridotite xenoliths as on- or off-cratonic to highlight the approximate lithosphere stabilization age as being Archean or post-Archean and contrast them against the lithospheric peridotites recovered from present-day oceanic basins. Orogenic peridotites and ophiolites are obducted in subduction zones and mark paleo-suture zones in amalgamated continents. Regardless of its present-day occurrence, lithospheric mantle is buoyant as a result of substantial extraction or mafic to ultramafic silicate melts. This depletion and buoyancy results in a thermal and mechanical boundary layer (lithosphere-asthenosphere boundary layer, LAB) and isolates Earth's lithosphere from the convecting mantle. The mineralogical and geochemical similarities of some of the oldest cratonic sub-continental lithosphere and highly-depleted young oceanic peridotites require common geochemical mechanisms that are capable of introducing well-correlated major element variability - until metasomatic enrichment masks the depletion signature. Efficient extraction of basaltic melts from the convecting mantle occurs at mid-ocean ridges and removes clinopyroxene and to some extent garnet leaving the olivine-rich buoyant residual lithospheric mantle. We will examine the oceanic heritage of garnet and spinel-facies sub-continental lithospheric mantle in general, but focus on xenoliths (n = 62) sampled across the North Atlantic Craton (~700km, NAC), West Greenland at c. 600 and 200 Ma. These Greenlandic samples are strongly serpentinized, harzburgitic to dunitic peridotites and generally comprise less than 15% orthopyroxene and very little clinopyroxene (<< 5%). If garnet is present it occurs in variable amounts (12 to 0.3%). Major element systematics of these peridotites are highly refractory with Al

  7. Craton Development and Stabilization: Insights from SE Canada using P and S Wave Tomography

    NASA Astrophysics Data System (ADS)

    Boyce, A.; Bastow, I. D.; Darbyshire, F. A.; Gilligan, A.; Ellwood, A.; Levin, V. L.; Menke, W. H.

    2015-12-01

    Cratons, the ancient cores of the continents, are the longest-lived parts of Earth's surface that have survived thermal and mechanical erosion during multiple Wilson cycles. They are visible in tomographic images due to their thick (>200km), seismically fast keels or roots. The Laurentian keel beneath North America is intriguing since its root is thought to extend beneath both the Archean Superior craton and the Proterozoic Grenville province thus implying that keel formation may not have been restricted to Archean times. In order to address this issue we present a P and S wave relative arrival-time tomographic study using data from seismograph networks in SE Canada and the NE US, stretching from the southern tip of Hudson Bay within the Superior craton to the coastal Phanerozoic Appalachian terranes. The tomographic images display three broad zones of increasing mantle wavespeed from globally "slow" in the Appalachian terranes, to a "fast" Grenville Province and "extremely fast" Superior craton. We observe a linear low-velocity feature resulting from modification of the Laurentian keel by the passage of the Great Meteor hotspot. This feature is progressively offset southwestward with depth, potentially due to viscous coupling with mantle flow. No major plate-scale underthrusting during the Grenville Orogeny is apparent, which contradicts the inferred results from crustal seismic reflection and refraction studies. Our results therefore may have fundamental implications for the nature of the Grenville orogenic collision and cratonic stabilization of North America. The results also support the developing consensus that keels form in two stages: a chemically depleted core of Archean age followed by a thermally developed, less-depleted lithosphere during Proterozoic times, highlighted by an abrupt wavespeed contrast in the tomographic images.

  8. Hydrological cycle.

    PubMed

    Gonçalves, H C; Mercante, M A; Santos, E T

    2011-04-01

    The Pantanal hydrological cycle holds an important meaning in the Alto Paraguay Basin, comprising two areas with considerably diverse conditions regarding natural and water resources: the Plateau and the Plains. From the perspective of the ecosystem function, the hydrological flow in the relationship between plateau and plains is important for the creation of reproductive and feeding niches for the regional biodiversity. In general, river declivity in the plateau is 0.6 m/km while declivity on the plains varies from 0.1 to 0.3 m/km. The environment in the plains is characteristically seasonal and is home to an exuberant and abundant diversity of species, including some animals threatened with extinction. When the flat surface meets the plains there is a diminished water flow on the riverbeds and, during the rainy season the rivers overflow their banks, flooding the lowlands. Average annual precipitation in the Basin is 1,396 mm, ranging from 800 mm to 1,600 mm, and the heaviest rainfall occurs in the plateau region. The low drainage capacity of the rivers and lakes that shape the Pantanal, coupled with the climate in the region, produce very high evaporation: approximately 60% of all the waters coming from the plateau are lost through evaporation. The Alto Paraguay Basin, including the Pantanal, while boasting an abundant availability of water resources, also has some spots with water scarcity in some sub-basins, at different times of the year. Climate conditions alone are not enough to explain the differences observed in the Paraguay River regime and some of its tributaries. The complexity of the hydrologic regime of the Paraguay River is due to the low declivity of the lands that comprise the Mato Grosso plains and plateau (50 to 30 cm/km from east to west and 3 to 1.5 cm/km from north to south) as well as the area's dimension, which remains periodically flooded with a large volume of water. PMID:21537597

  9. How is Silurian-Early Devonian faulting in the North America continental interior related to orogenic processes at plate boundaries? A working hypothesis from the Canadian North

    NASA Astrophysics Data System (ADS)

    Pinet, Nicolas

    2015-04-01

    The Paleozoic Appalachian/Franklinian orogen that rims the North America continent on its eastern and northern sides is comparable in size with Tethyan orogenic belts. However, the far-field effects in the continental interior of the multiple Ordovician to Carboniferous deformation phases that built the orogen were relatively minor if compared with those associated with the Himalayas and Alps, a characteristics related to the high integrated strength of the North American craton. Despite the generally little deformation of the continental interior, two regional-scale tectonic features preserved evidence of significant Paleozoic tectonism: the fault bounded Hudson Bay Central High (HBCH) and the Boothia uplift/Cornwallis fold belt (BUCF) in the Canadian Arctic. In the Hudson Bay intracratonic basin, the lower part of the sedimentary succession (Upper Ordovician to Lower Devonian) is cut by high-angle faults and overlain by a saucer-shape, essentially underformed sedimentary package (Middle to Upper Devonian). The main structural feature is the NNW-trending HBCH that extends for a minimum length of 500 km with normal faults characterized by throws up to 500 m that were mainly active during the Silurian - Early Devonian period. The >700-km long, N-trending BUCF is nearly perpendicular to the deformation front of the Franklinian mobile belt. In its southern segment (Boothia uplift), its western side is characterized by an east-dipping reverse fault zone that puts Precambrian rocks over Paleozoic strata. In its northern segment (Cornwallis fold belt), the Paleozoic succession is involved in open folds and cuts by steeply dipping reverse faults. Syn-tectonic clastic sediments constrain the age of structures to the latest Silurian-Early Devonian. Comparison of the HBCH and BUCF indicates that they are grossly parallel, partly contemporaneous but with different kinematics. This kinematic variability may be explained if they are genetically linked with different segments of

  10. Syn- and post-orogenic alkaline magmatism in a continental arc: Along-strike variations in the composition, source, and timing of igneous activity in the Ross Orogen, Antarctica

    NASA Astrophysics Data System (ADS)

    Hagen-Peter, G.; Cottle, J. M.

    2013-12-01

    Neoproterozoic-Paleozoic convergence and subduction along the margin of East Gondwana (Australia, New Zealand, Antarctica) resulted in a belt of deformed and metamorphosed sedimentary rocks and batholith-scale igneous intrusions comparable in size to the present day Andes. Mid-crustal levels of this belt, known as the Ross Orogen in Antarctica, are exposed in the basement of the Cenozoic Transantarctic Mountains, providing snapshots of the intrusive magma system of a major continental arc. Whole rock major- and trace-element geochemistry, Hf isotopes in zircon, and U-Pb geochronology have identified along-strike variations in the composition, source, and timing of magmatism along ~200 km of the southern Victoria Land segment of the orogen. There is an apparent younging of the igneous activity from south to north. New U-Pb ages for intrusive rocks from the Koettlitz Glacier Alkaline Province (KGAP) reveal that igneous activity spanned ca. 565-500 Ma (~30 m.y. longer than previously recognized), while immediately to the north in the Dry Valleys area most igneous activity was confined to a relatively short period (ca. 515-495 Ma). Alkaline and subalkaline igneous rocks occur in both the Dry Valleys area and the KGAP, but alkaline rocks in the Dry Valleys are restricted to the latest phase of magmatism. Na-alkaline rocks in the KGAP, including nepheline syenites, carbonatites, and A-type granites, range in age from ca. 545-500 Ma and overlap in age with more typical subduction/collision-related I- and S-type granites elsewhere in southern Victoria Land. Strong enrichments in the LILE and LREE and high LILE/HFSE and LREE/HREE of samples from the KGAP reveal a source enriched in aqueous-mobile elements, potentially a strongly metasomatized mantle wedge beneath the arc. In the Dry Valleys area, rocks with alkali-calcic composition constitute only the youngest intrusions (505-495 Ma), apparently reflecting a shift to post-orogenic magmatism. Zircons from Dry Valleys

  11. Possible genetic link between I-type granite and orogenic gold deposits in Egypt (metamorphic-magmatic interaction?)

    NASA Astrophysics Data System (ADS)

    Abd El Monsef, Mohamed

    2015-04-01

    The orogenic gold deposits are a distinctive type of deposits that revealed unique temporal and spatial association with an orogeny. Where, the system of gold veins and related ore minerals was confined to hydrothermal solutions formed during compressional to transpressional deformation processes at convergent plate margins in accretionary and collisional orogens, with the respect to ongoing deep-crustal, subduction-related thermal processes. In Egypt, most of vein-type and dyke-type gold mineralization are restricted to granitic rocks or at least near of granitic intrusion that seems to have had an important influence on gold mineralization. Shear zone-related, mesothermal gold deposits of Fatira and Gidami mines in the northern Eastern Desert of Egypt are found within granitic bodies or at the contact between granites and metavolcanic rocks. The hosting-granitic rocks in Fatira and Gidami areas are mainly of granodioritic composition (I-Type granite) which is related to calc-alkaline magmatic series. However, Fatira granitoids were developed within island arc tectonic settings related to mature island arc system (Late-orogenic stage), at relatively low temperature (around 660° C) and medium pressure between (5 - 10 Kbar). On the other hand, Gidami granitoids were developed during the collision stage in continental arc regime related to active continental margin (Syn-orogeny), which were crystallized at relatively high temperature (700-720° C) and low pressure (around 0.1 Kbar). The ore mineralogy includes pyrite, chalcopyrite, sphalerite, covellite, ilmenite, goethite ± pyrrhotite ± pentlandite ± galena ± molybdenite. Native gold is detected only in Gidami mineralization as small inclusions within pyrite and goethite or as tiny grains scattered within quartz vein (in close proximity to the sulfides). In Fatira deposits, it is detected only by microprobe analysis within the crystal lattice of pyrite and jarosite. Fluid inclusions study for the mineralized

  12. Proto-Pacific-margin source for the Ordovician turbidite submarine fan, Lachlan Orogen, southeast Australia: Geochemical constraints

    NASA Astrophysics Data System (ADS)

    Offler, R.; Fergusson, C. L.

    2016-04-01

    The Early Palaeozoic proto-Pacific Pacific margin of Gondwana was characterised by a huge turbidite submarine fan with abundant clastic detritus derived from unknown sources within Gondwana. These deposits are widespread in the Lachlan Orogen of southeast Australia and include the Ordovician Adaminaby Group. Here we show that the mudstones and sandstones of the Adaminaby Group have chemical compositions that indicate the detritus in them was derived from a felsic, continental source similar in composition to Post Archean Australian Shales (PAAS). Chondrite normalised REE patterns showing LREE enrichment, flat PAAS normalised patterns and elemental ratios La/Sc, Cr/Th, Cr/V, Th/Sc and Th/U, have been used to support this interpretation. The dominance of quartz, and to a lesser degree plagioclase and biotite in the sandstones, suggests that the source was mainly granodioritic to tonalitic in composition. Th/Yb and Ta/Yb ratios indicate that the source was probably calc-alkaline, continental and shoshonitic. In addition, the presence of detrital muscovite, low-grade metamorphic and felsic volcanic clasts, demonstrates that a low-grade metamorphic terrane and volcanic arc contributed to the detritus observed in the samples. The presence of well-rounded zircons and tourmalines, very high Zr contents, high Zr/Sc and higher Cr/V ratios in some samples particularly in the Shoalhaven River area, indicate that some of the detritus was recycled. SiO2 versus (Al2O3 + K2O + Na2O) plots suggest the source areas experienced conditions varying from humid/semi-humid to semi-arid. Textural features and weathering trends of samples from all locations follow a curved pathway on Al2O3 - (CaO* + Na2O) - K2O (ACNK) diagrams, and indicate that the clays formed from weathering had been K-metasomatised prior to penetrative deformation. Chemical indices of alteration (CIA) reveal that even the freshest sandstones are altered and others are moderately to strongly altered. Discrimination

  13. Drainage divide migration induced by rainfall gradient in orogenic landscape and the split of the associated drainage networks

    NASA Astrophysics Data System (ADS)

    Bonnet, S.

    2009-12-01

    Although numerical models demonstrate that topography in orogens is asymmetric when forced by orographic precipitations, drainage divide migration (the mechanism responsible for this asymmetry) has never been documented in natural orogens, even though the latter actually exhibits an asymmetric topography in most cases (e. g., Taiwan, southern Alps). Drainage divide migration is investigated here in the laboratory modelling of erosion and landscape dynamics under spatially-constant uplift forcing but with a lateral precipitation gradient. As observed numerically, precipitation gradient induces the migration of the drainage divide toward the drier side of the landscape and the development of an asymmetric topography. The divide is simultaneously uplifted such that the mean topographic slope of the wetter side of the landscape remains constant during its elongation. On the opposite, the progressive shortening of the drier side of the landscape results in a progressive steepening of its topography. This leads to a very unstable landscape and an original mechanism that splits the drainage networks is observed there: each initial drainage network progressively splits into two individual networks that become separated by the growth of a new hillcrest. Through this mechanism, the numbers of drainage basins extending to the main divide increases during divide migration such that the universal law of drainage outlet spacing is maintained. Given the pattern of rainfall, the migration of the drainage divide induces a continuous decrease in the mean runoff within the drainage basins located on the drier side of the landscape. There, the area of the drainage basins decreases because of the combination of (i) the continuous size reduction which is the direct consequence of the divide migration, and (ii) the abrupt size reduction consecutive to the split of the drainage networks and the individuation of two drainage basins from a previous single one. By continually reducing the

  14. Petrogenesis of Early Cretaceous post-collisional granitoids at Shapinggou, Dabie Orogen: Implications for crustal architecture and porphyry Mo mineralization

    NASA Astrophysics Data System (ADS)

    Wang, Guo-Guang; Ni, Pei; Yu, Wen; Chen, Hui; Jiang, Lai-Li; Wang, Bo-Hua; Zhang, Huai-Dong; Li, Peng-Fei

    2014-01-01

    The Shapinggou molybdenum deposit is located in the Qinling-Tongbai-Hong'an-Dabie Orogen, which accommodates the largest molybdenum ore belt in the world. New laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb dating, geochemical and Sr-Nd-Hf isotope data for the granitoids at Shapinggou are presented in this paper. Two stages of Early Cretaceous magmatism, 138-128 Ma and 116-114 Ma, took place at Shapinggou in a post-collisional setting. These granitoids can be divided into four groups, including the early-stage granites (134-129 Ma, SiO2 = 62.33 to 76.27%) and intermediate rocks (129-128 Ma, SiO2 = 52.41 to 60.72%) and the late-stage quartz syenite (116 Ma, SiO2 = 64.09 to 65.12%) and Mo-bearing granite porphyry (114 Ma, SiO2 = 74.95 to 77.26%). The early-stage granites have high Sr contents, Sr/Y and LaN/YbN ratios, but low Y, Yb and MgO contents, showing low-Mg adakitic chemical features. They were possibly generated by partial melting of the thickened mafic lower continental crust. The early-stage intermediate rocks have relatively high MgO contents (2.71 to 4.84%) and Mg# values (46 to 60), but low Sr/Y and LaN/YbN ratios, which were likely derived from a hybrid mantle metasomatized by melts from foundered eclogitic lower continental crust and subsequent fractional crystallization. The late-stage quartz syenite shows chemical features of metaluminous A-type granites, including high abundances of alkalis, HFSE (Zr, Nb), and trivalent REE. The late-stage Mo-bearing granite porphyry displays high SiO2, alkalis, Zr, Ce contents, and A/CNK ratios, but low Ba, Sr, P and Ti, showing affinity to highly fractionated I-type granites. The late-stage quartz syenite and granite porphyry were possibly produced by partial melting of intermediate compositions of middle crust. Our results support the conclusion that there were three-layer continental crusts in the Dabie Orogen prior to Early Cretaceous magmatism and suggest that post

  15. Sedimentary basin evolution and the link with the deformation of surrounding orogens in European case-studies

    NASA Astrophysics Data System (ADS)

    Matenco, Liviu

    2014-05-01

    Sedimentary basin evolution and the link with the deformation of surrounding orogens in European case-studies Liviu C. Matenco Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, the Netherlands Intra-plate contractional deformation and far-field transmission of strain generated at active deforming plate boundaries play a crucial role in inverting extensional basins and the kinematics of thrust belts in Mediterranean orogens (e.g., Ziegler et al., 1995; Ziegler et al., 1998). At the same time, the structural evolution of rifts is significantly controlled by the lithospheric rheology, the availability of inherited crustal weak zones with potential to be re-activated, the mode of extension and the type of crustal material being deformed (Ziegler and Cloetingh, 2004; Ziegler et al., 2006). These critical inferences has proven of major importance for the study of sedimentary basins, in particular relevant for the evolution of Central and SE European mountain chains and associated sedimentary basins. Far-field transmission of contractional strain has proven to be of important for the geometry of both foreland and back-arc basins, such as the Carpathians or Dinaridic forelands and the Pannonian Basin or Black Sea back-arcs. Crustal scale weak zones and rheological contrasts such as inherited nappe stacks or major plate boundaries have been proven recently to be of major importance in conditioning subsequent basin formation and associated footwall exhumation in extensional domains such as the Aegean, Rhodope or the transition from the Dinarides to the Pannonian basin. At a regional level, areas characterized by changes in contractional polarity have proven to efficiently transmit the strain at large distances and condition the localization of major structures, such as the change from the Alpine to the Dinaridic polarity or the change from the Carpathians to the Dinaridic polarity. The same areas were prone to the first order localization of

  16. Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation

    NASA Astrophysics Data System (ADS)

    Yin, An

    2006-05-01

    Despite a long research history over the past 150 years, the geometry, kinematics, and dynamic evolution of the Himalayan orogen remain poorly understood. This is mainly due to continued emphasis on the two-dimensionality of the Himalayan orogenic architecture and extrapolation of geologic relationships from a few well-studied but small areas to the rest of the orogen. Confusion and misconception are also widespread in the Himalayan literature in terms of the geographic, stratigraphic, and structural divisions. To clarify these issues and to provide a new platform for those who are interested in studying the geologic development of this spectacular mountain belt, I systematically review the essential observations relevant to the along-strike variation of the Himalayan geologic framework and its role in Cenozoic Himalayan exhumation, metamorphism and foreland sedimentation. A main focus of my synthesis is to elucidate the emplacement history of the high-grade Greater Himalayan Crystalline Complex (GHC) that occupies the core of the orogen. Because the north-dipping Main Central Thrust (MCT) above and South Tibet Detachment (STD) below bound the GHC in most parts of the Himalaya, it is critical to determine the relationship between them in map and cross-section views. The exposed map pattern in the central Himalaya (i.e., Nepal) indicates that the MCT has a flat-ramp geometry. The thrust flat in the south carries a 2-15-km-thick slab of the GHC over the Lesser Himalayan Sequence (LHS) and creates a large hanging-wall fault-bend fold continuing > 100 km south of the MCT ramp zone. In the western Himalayan orogen at the longitude ˜ 77°E, the MCT exhibits a major lateral ramp (the Mandi ramp). West of this ramp, the MCT places the low-grade Tethyan Himalayan Sequence (THS) over the low-grade LHS, whereas east of the ramp, the MCT places the high-grade GHC over the low-grade LHS. This along-strike change in stratigraphic juxtaposition and metamorphic grade across the

  17. Tectonic models for the Patagonian orogenic curve (southernmost Andes): An appraisal based on analog experiments from the Fuegian thrust-fold belt

    NASA Astrophysics Data System (ADS)

    Torres Carbonell, Pablo J.; Guzmán, Cecilia; Yagupsky, Daniel; Dimieri, Luis V.

    2016-03-01

    Tectonic models for the evolution of the Patagonian orogenic curve were evaluated using analog experiments that considered either a rotational or a non-rotational orogenic backstop, combined with a basement promontory on the foreland cratonic margin. Five different kinematic configurations were used, aiming to evaluate the influence of the Río Chico Arc as a rigid obstacle on the evolution of the Fuegian thrust-fold belt. Rotations, strains and displacement fields obtained from each analog experiment were compared with the structural geology known from the Fuegian thrust-fold belt, in order to appraise the tectonic models that are more consistent with the natural structure. The push of a counterclockwise rotational backstop, combined with the buttressing effect of a foreland promontory, seem of major importance in controlling the final structure and map-view shape of the thrust wedge.

  18. Collapse and Melting in a Confined Orogenic Belt: Preliminary Results From the Neoproterozoic Araçuaí Belt of Eastern Brazil

    NASA Astrophysics Data System (ADS)

    Whittington, A.; Connelly, J.; Pedrosa-Soares, A.; Marshak, S.; Alkmim, F.

    2001-12-01

    The Neoproterozoic Araçuaí belt of eastern Brazil records the closure of a Red Sea-type basin between the São Francisco and Congo cratons of West Gondwana, and is part of the Brasiliano (Pan-African) orogenic system. In its southern part this basin contained oceanic crust, so that closure involved subduction and arc magmatism prior to continental collision. In its northern part, the basin was entirely ensialic. After the cessation of subduction, a period of magmatic quiescence and possible plateau development ended with the collapse of the orogen. Extensional structures are observed across the preserved Brazilian portion of the orogen (the eastern segment of the orogen is now in West Africa). Six distinct syn- to post- kinematic granite suites were emplaced within the orogen, although the temporal relation of the different granite suites to contractional and post-collision extensional tectonism has remained enigmatic. Preliminary U-Pb ages for key granite suites are related to the structural history of the orogen, placing important constraints on the timing and duration of orogenic collapse. The age of the northernmost pluton of the subduction-related (G1) granite suite is consistent with published ages for this suite of 625 to 575 Ma. The G2 suite consists of an "anatectic sea" of variably foliated S-type migmatitic leucosomes, leucogranitic sheets and plutons, commonly associated with locally extensional mesoscopic shear zones. This migmatite-leucogranite association covers >75% of the outcrop over an area of ~20,000 km2. A preliminary U-Pb zircon age of ca. 536 Ma from a well-foliated G2 pluton is significantly younger than published ages from other G2 bodies (590 to 570 Ma). Bodies sampled from the other suites are: an I-type granite showing flow foliation but little or no tectonic fabric (G3I suite), an undeformed S-type leucogranite (G4 suite), and an undeformed I-type granite containing dioritic enclaves (G5 suite). Preliminary U-Pb zircon ages from all

  19. Early Cretaceous tectono-magmatic activity and tectonic implications along the Sulu Orogenic Belt - case study of the Dashan complex

    NASA Astrophysics Data System (ADS)

    Liu, Yanghe; Liu, Junlai; Shi, Xiaoxiao; Yuan, Fengjie; Ni, Jinlong; Wu, Wenbin; Chen, Xiaoyu

    2016-04-01

    The tectonic extension of the eastern Eurasian continent during the Early Cretaceous resulted in widespread occurrence of metamorphic core complexes, wide rifts and related magmatic emplacement, among which the Dashan complex of the Jiaonan orogenic belt is a typical example. The complex is a complex massif of several types of granitic rocks. The core of the complex is composed of massive porphry-bearing biotite-hornblende granitoid without any evidence of ductile deformation. Mylonitized augen quartz monzonite and granodiorite constitute the margin of the complex. A transition zone is composed of porphyritic biotite-hornblende monzonite with weakly orientated K-feldspar phenocryst and mafic microgranular enclave. The foliations along the northwestern margin of the complex dip to NW at with dip angles of about 38°, and along the southwestern and northeastern margins to SE with dip angles of about 45°. Stretching lineations are constantly plunging WNW-ESE with pitch angles between 10° and 40°, which is consistent with the orientation of lineations in the other regions in eastern China. The granites,porphyritic monzogranite and the mafic microgranular enclaves in monzogranite are dated of ca.126Ma. The similarities in ages of crystallization of the monzogranite and its MME's implies the existence of magmatic mixing processes. Meanwhile, the mylonitized augen quartz monzonite and granodiorite along the margins of the complex possess crystallization ages of 129.8±1.1Ma and 132.7±2.8Ma, respectively. The petrographical zonation , structural characteristics and the systematical zircon U-Pb geochronology of the granitic rocks may suggest that the Dashan complex has experienced multistage emplacement under the same tectonic extension setting. In despite of the location of the complex near the Tanlu fault zone, the remarkable consistency of the orientations of stretching lineation of the Dashan complex to those from the other parts of the eastern China area implies

  20. The Kharapeh orogenic gold deposit: Geological, structural, and geochemical controls on epizonal ore formation in West Azerbaijan Province, Northwestern Iran

    USGS Publications Warehouse

    Niroomand, Shojaeddin; Goldfarb, Richard J.; Moore, Farib; Mohajjel, Mohammad; Marsh, Erin E.

    2011-01-01

    The Kharapeh gold deposit is located along the northwestern margin of the Sanandaj–Sirjan Zone (SSZ) in the West Azerbaijan province, Iran. It is an epizonal orogenic gold deposit formed within the deformed zone between central Iran and the Arabian plate during the Cretaceous–Tertiary Zagros orogeny. The deposit area is underlain by Cretaceous schist and marble, as well as altered andesite and dacite dikes. Structural analysis indicates that the rocks underwent tight to isoclinal recumbent folding and were subsequently co-axially refolded to upright open folds during a second deformation. Late- to post-tectonic Cenozoic granites and granodiorites occur northeast of the deposit area. Mineralization mainly is recognized within NW-trending extensional structures as veins and breccia zones. Normal faults, intermediate dikes, and quartz veins, oriented subparallel to the axial surface of the Kharapeh antiform, indicate synchronous extension perpendicular to the fold axis during the second folding event. The gold-bearing quartz veins are >1 km in length and average about 6 m in width; breccia zones are 10–50 m in length and ≤1 m in width. Hydrothermal alteration mainly consists of silicification, sulfidation, chloritization, sericitization, and carbonatization. Paragenetic relationships indicate three distinct stages—replacement and silicification, brecciation and fracture filling, and cataclastic brecciation—with the latter two being gold-rich. Fluid inclusion data suggest mineral deposition at temperatures of at least 220–255°C and depths of at least 1.4–1.8 km, from a H2O–CO2±CH4 fluid of relatively high salinity (12–14 wt.% NaCl equiv.), which may reflect metamorphism of passive margin carbonate sequences. Ore fluid δ18O values between about 7‰ and 9‰ suggest no significant meteoric water input, despite gold deposition in a relatively shallow epizonal environment. Similarities to other deposits in the SSZ suggest that the deposit formed as

  1. Timing of magmatism and migmatization in the 2.0-1.8 Ga accretionary Svecokarelian orogen, south-central Sweden

    NASA Astrophysics Data System (ADS)

    Johansson, Åke; Stephens, Michael B.

    2016-07-01

    The Palaeoproterozoic (2.0-1.8 Ga) Svecokarelian orogen in central Sweden consists of a low-pressure, predominantly medium-grade metamorphic domain (central part of Bergslagen lithotectonic unit), enclosed to the north and south by low-pressure migmatite belts. Two periods of metamorphism (1.87-1.85 and 1.83-1.79 Ga) are known in the migmatite belts. In this study, new U-Th-Pb ion microprobe data on zircon and monazite from twelve samples of locally migmatized gneisses and felsic intrusive bodies determine both protolith and metamorphic ages in four sample areas north of Stockholm, inside or immediately adjacent to the medium-grade metamorphic domain. Two orthogneiss samples from the Rimbo area yield unusually old protolith ages of 1909 ± 4 and 1908 ± 4 Ma, while three orthogneisses from the Skutskär and Forsmark areas yield more typical protolith ages between 1901 ± 3 and 1888 ± 3 Ma. Migmatized paragneiss samples from this and two earlier studies contain a significant detrital component sourced from this 1.9 Ga magmatic suite. They are interpreted to be deposited contemporaneously with or shortly after this magmatism. Migmatization of the paragneiss at Rimbo was followed by intrusion of leucogranite at 1846 ± 3 Ma. Even in the other sample areas to the north (Hedesunda-Tierp, Skutskär and Forsmark), metamorphism including migmatization is constrained to the 1.87-1.85 Ga interval and penetrative ductile deformation is limited by earlier studies in the Forsmark area to 1.87-1.86 Ga. However, apart from a metamorphic monazite age of 1863 ± 1 Ma, precise ages were not possible to obtain due to the presence of only partially reset recrystallized domains in zircon, or highly discordant U-rich metamict and altered metamorphic rims. Migmatization was contemporaneous with magmatic activity at 1.87-1.84 Ga in the Bergslagen lithotectonic unit involving a mantle-derived component, and there is a spatial connection between migmatization and this magmatic phase in the

  2. Evolution and Metallogenesis of the Great Serpentinite Belt in the New England Orogen, N.S.W. Australia

    NASA Astrophysics Data System (ADS)

    Manton, R. J.; Buckman, S.; Nutman, A.

    2013-12-01

    Current tectonic interpretations of disrupted ophiolitic complexes and scattered HP/LT eclogite-blueschist blocks within the New England Orogen are still problematic due to limited or unreliable geochronological data. Ophiolitic, island-arc and accretionary complex terranes are contained or spatially associated within the 'alpine-type' Great Serpentinite Belt, which is controlled by the ~350km long, NNW-SSE orientated, Peel-Manning Fault System. The Cambro-Ordovician blocks within the mélange contrast with the Permian emplacement age for the serpentinite. Early Cambrian (530 Ma) ages of the igneous zircons extracted from plagiogranites and tonalities record the formation of the ophiolitic protolith making them the oldest rocks in eastern Australia (Aitchison et al., 1992 Geology v. 20). The timing of serpentinite emplacement is problematic. The minimum age is constrained by the stitching pluton of the Late Permian Moonbi Adamellite (256 × 4 Ma) and the presence of serpentinite clasts within adjacent Early? Permian Manning Group rocks. However, the maximum emplacement age is less well constrained and limited to the absence of serpentinite clasts in older adjacent Carboniferous sedimentary rocks, thereby suggesting the serpentinite belt was not exposed until the Early Permian. In this study we present zircon U-Pb dates and REE chemistry of an eclogite block in the serpentinite belt at Attunga. REE chemistry of the zircons shows an age of 492 × 13 Ma for high pressure metamorphism. Igneous inheritance in metamorphic zircon has an age of ~530 Ma. Titanium in zircon thermometry (zircon + rutile + quartz) indicate high pressure zircon growth at 650 - 700°C. This is higher than the previously calculated 290 - 600°C, which was based on the distribution of Fe and Mg between co-existing garnet and pyroxene (Shaw and Flood 1974, AJES v. 21). We interpret this block as being a portion of the metamorphic sole of the original ophiolite. Ongoing studies will further

  3. Temporal evolution of granitic magmas in the Luanchuan metallogenic belt, east Qinling Orogen, central China: Implications for Mo metallogenesis

    NASA Astrophysics Data System (ADS)

    Li, Dong; Han, Jiangwei; Zhang, Shouting; Yan, Changhai; Cao, Huawen; Song, Yaowu

    2015-11-01

    The Luanchuan metallogenic belt, located within the eastern part of the Qinling Orogen, central China, hosts a number of world-class Mo deposits that are closely related to small late Mesozoic granitic plutons. Zircon U-Pb dating of distinct plutons in the Luanchuan metallogenic belt has yielded ages of 153 ± 1, 154 ± 2, 152 ± 2, and 148 ± 1 Ma. Molybdenite Re-Os isotopic compositions of Yuku ore district in the southern part of Luanchuan metallogenic belt has yielded an isochron age of 146 ± 1 Ma, which is consistent with the large-scale mineralization ages in the northern part of the Luanchuan metallogenic belt. A combination of previous studies and new geochronological and isotopic data show a concordant temporal and genetic link between granitic magmatism and Mo mineralization in the Luanchuan metallogenic belt, suggesting that this mineralization episode formed the most extensive Mo mineralization belt in the east Qinling Orogen. Zircon grains from Mo-related granitic plutons show similar trace element distributions. High-precision Multi Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS) Pb isotope analysis of K-feldspar megacrysts from mineralization-related granites suggest that they were derived from the lower crust. Similarly, the Pb isotopic compositions of pyrite coprecipitated with molybdenite also suggest that the metals were derived form the lower crust, with probably minor mantle contribution. A continuum mineralization model that describes the sourcing of Mo from an evolving granitic magma over successive differentiation events, possibly in separate but connected magma chambers, could explain the remarkable Mo enrichment in the Luanchuan metallogenic belt. The volatile- and Mo-bearing granitic magmas ascended as diapirs from the deep crust, and were emplaced as dikes in the upper crust. Lithological differences between these Mo-bearing granites may relate to different stages in the evolution of individual magmas. Finally, ore

  4. Dyke Swarms in Southeastern British Columbia: Mineralogical and Geochemical Evidence for Emplacement of Multiple Magma Types During Orogenic Collapse

    NASA Astrophysics Data System (ADS)

    Freeman, M.; Owen, J. P.; Hoskin, P. W.

    2009-05-01

    Eocene dyke swarms in southeastern British Columbia provide an important record of the tectonic and magmatic history of the Cordillera following orogenic collapse. New field mapping, petrographic, and geochemical data is presented for a swarm of more than thirty dykes located near the mining town of Trail, B.C. Detailed field mapping revealed that individual dykes are highly diverse, both in composition and morphology. As a group, the dykes trend northwest (average strike of 338 degrees) and dip steeply to the southwest. Their average thickness is approximately 1.5m, with a range from 4.5m to less than 1cm. Three sub-parallel dykes were mapped for a length of 2km, and exhibit irregularities in their form such as branching and offshoots that follow fractures in the country rock. Thin-section analysis shows a wide variety of rock types within the swarm, including: micro-quartz syenite, micro-syenite, micro-monzonite, latite, basalt, basaltic andesite, and lamprophyre. Texturally, these samples are consistently porphyritic and partially altered to chlorite and sericite. This alteration commonly occurs in concentric rims around phenocrysts. The samples are typically intergranular, although some display trachytic texture. Whole-rock geochemistry shows that the dykes have a wide range in composition, with SiO2 between 76.45 wt.% and 45.15 wt.% and MgO between 0.13 wt.% and 13.16 wt.%. The results also revealed that one dyke has very high values of Ni (430 ppm), Cr (1420 ppm), and Co (50 ppm), giving it a fairly primitive composition. Harker diagrams and trace element plots show three distinct groups: mafic calc-alkaline dykes, felsic calc- alkaline dykes, and minette lamprophyres. The felsic dykes are characterized by negative Eu and Sr anomalies suggesting fractionation of plagioclase feldspar, as well as pronounced negative P and Ti anomalies. The minettes are enriched in LILE and depleted in HSFE relative to the mafic dykes. The three groups do not appear to be

  5. 40Ar/ 39Ar-ages of phlogopite in mantle xenoliths from South African kimberlites: Evidence for metasomatic mantle impregnation during the Kibaran orogenic cycle

    NASA Astrophysics Data System (ADS)

    Hopp, Jens; Trieloff, M.; Brey, G. P.; Woodland, A. B.; Simon, N. S. C.; Wijbrans, J. R.; Siebel, W.; Reitter, E.

    2008-12-01

    We applied the 40Ar/ 39Ar dating method to an extensive suite of phlogopites from kimberlite-hosted mantle xenoliths (dominantly garnet bearing) from the mines of Bultfontein (South Africa), Letseng-la-Terae and Liqhobong (Lesotho). Argon extraction was performed by conventional high resolution stepwise heating technique, laser incremental heating technique and laser spot analysis. All age spectra obtained by conventional analysis indicate various degrees of 40Ar loss during kimberlite emplacement, but never resulted in a total reset of the argon system. Most intriguingly, the sample-specific maximum apparent ages cluster between 1.0 and 1.22 Ga for the phlogopites with the least disturbed age spectra. A maximum apparent age of 1.02 Ga was observed during laser heating analysis. Individual grains tend to yield older ages in their cores, with successively younger ages at their rims. The range in age obtained via the laser fusion technique and with conventional stepwise heating technique agrees with each other, as well as with literature data. The often inferred presence of excess 40Ar in those phlogopites cannot explain the coherent age pattern in the large suite of samples. Hence, the age constraint of 1.0-1.25 Ga is regarded as geologically meaningful and assigned to metasomatism of the local cratonic mantle during the advent of Kibaran orogenesis (1.00-1.25 Ga). The major consequences of our findings are: (i) The argon system of phlogopite can remain closed for long time scales, even at ambient temperatures of 800-1200 °C within the mantle, most likely because the solid/solid partitioning behaviour of Ar between phlogopite and other major phases in the mantle strongly favours phlogopite, or because conventionally inferred diffusivity of argon in phlogopite is seriously overestimated. Thus, the 40Ar/ 39Ar phlogopite system appears to be a valuable tool for deciphering ancient metasomatic events affecting the lithospheric mantle. (ii) The cratonic lithospheric mantle below southern Africa may have been frequently influenced by different episodes of fluid or melt migration during subduction of oceanic crust at active continental margins.

  6. Aeromagnetic study of the Hengshan-Wutai-Fuping region: Unraveling a crustal profile of the Paleoproterozoic Trans-North China Orogen

    NASA Astrophysics Data System (ADS)

    Zhang, Jian; Zhao, Guochun; Shen, Wenlue; Li, Sanzhong; Sun, Min

    2015-11-01

    An integrated crustal profile of the intervening Trans-North China Orogen (TNCO) is one of the key issues to understanding the tectonic evolution of the North China Craton. However, the existing geological studies focus only on the surface-mapping based petrological, geochemical and structural analysis, but lack subsurface geophysical evidence and thus make the crustal profile interpretations ambiguous. In contrast, the current geophysical data covers a very large-scale lithospheric mantle and fails to image the detailed structural pattern of the orogenic crust. To achieve this goal, we present high-resolution aeromagnetic data for the Hengshan-Wutai-Fuping region, the largest exposure of the central TNCO. The reduced-to-pole magnetic anomaly map firstly verifies the regional tectonic subdivision that the high-grade metamorphic terranes (i.e. Hengshan and Fuping Complexes) are consistent with high-magnetic responses and long-wavelength anomalies, intervened by a low-grade terrane (Wutai Complex) characterized by low-magnetic responses and short-wavelength anomalies. 3D Euler deconvolution reveals that the tendencies of the clustered solutions show large consistence with the major structural pattern of the region which is characterized by a fan-shaped doubly-vergent orogenic wedge. Upward continuation further shows that the northwest part of the orogen yields a thicker crust and is most likely located closer to the paleosubduction zone. The new aeromagnetic data, combined with structural, petrological and metamorphic data indicate that an eastward-dipping subduction zone was most possibly active before the collision of the Western and Eastern Blocks, leading to the formation of the TNCO and the final amalgamation of the North China Craton.

  7. Influence of syn-sedimentary faults on orogenic structures in a collisional belt: Insights from the inner zone of the Northern Apennines (Italy)

    NASA Astrophysics Data System (ADS)

    Brogi, Andrea

    2016-05-01

    This paper discusses the possible influence of syn-sedimentary structures on the development of orogenic structures during positive tectonic inversion in the inner Northern Apennines (Italy). Examples from key areas located in southern Tuscany provided original cartographic, structural and kinematics data for Late Oligocene-Early Miocene thrusts, organized in duplex systems, verging in the opposite direction of the foreland propagation (back-thrusts), which affected the Late Triassic-Oligocene sedimentary succession of the Tuscan Domain, previously affected by pre-orogenic structures. These latter consist of mesoscopic-to cartographic-scale Jurassic syn-sedimentary normal faults and extensional structures, which gave rise to effective stratigraphic lateral variation and mechanical heterogeneities. Structural analysis of both syn-sedimentary faults and back-thrusts were therefore compared in order to discuss the possible role of the pre-existing anisotropies in influencing the evolution of the back-thrusts. As a result, it can be reasonably proposed that back-thrusts trajectories and stacking pattern were controlled by relevant syn-sedimentary normal faults; these latter were reactivated, in some cases, if properly oriented. Such an issue adds new inputs for discussing the potential role of structural inheritance during tectonic inversions, and helps to better understand the processes suitable for the development of back-thrusts in the inner zones of orogenic belts, as it is the case of the inner Northern Apennines.

  8. The Neoacadian orogenic core of the souther Appalachians: a Geo-traverse through the migmatitic inner Piedmont from the Brushy Mountains to Lincolnton, North Carolina

    USGS Publications Warehouse

    Merschat, Arthur J.; Hatcher, Robert D., Jr.; Byars, Heather E.; Gilliam, William G.

    2012-01-01

    The Inner Piedmont extends from North Carolina to Alabama and comprises the Neoacadian (360–345 Ma) orogenic core of the southern Appalachian orogen. Bordered to west by the Blue Ridge and the exotic Carolina superterrane to the east, the Inner Piedmont is cored by an extensive region of migmatitic, sillimanite-grade rocks. It is a composite of the peri-Laurentian Tugaloo terrane and mixed Laurentian and peri-Gondwanan affinity Cat Square terrane, which are exposed in several gentle-dipping thrust sheets (nappes). The Cat Square terrane consists of Late Silurian to Early Devonian pelitic schist and metagraywacke intruded by several Devonian to Mississippian peraluminous granitoids, and juxtaposed against the Tugaloo terrane by the Brindle Creek fault. This field trip through the North Carolina Inner Piedmont will examine the lithostratigraphies of the Tugaloo and Cat Square terranes, deformation associated with Brindle Creek fault, Devonian-Mississippian granitoids and charnockite of the Cat Square terrane, pervasive amphibolite-grade Devonian-Mississippian (Neoacadian) deformation and metamorphism throughout the Inner Piedmont, and existence of large crystalline thrust sheets in the Inner Piedmont. Consistent with field observations, geochronology and other data, we have hypothesized that the Carolina superterrane collided obliquely with Laurentia near the Pennsylvania embayment during the Devonian, overrode the Cat Square terrane and Laurentian margin, and squeezed the Inner Piedmont out to the west and southwest as an orogenic channel buttressed against the footwall of the Brevard fault zone.

  9. Neoproterozoic oceanic remnants in eastern Brazil: Further evidence and refutation of an exclusively ensialic evolution for the Araçuaí West Congo orogen

    NASA Astrophysics Data System (ADS)

    Pedrosa-Soares, Antônio Carlos; Vidal, Philippe; Leonardos, Othon Henry; Bley de Brito Neves, Benjamin

    1998-06-01

    The Araçuaí (eastern Brazil) and West Congo (southwestern Africa) belts are counterparts of the same Neoproterozoic orogen located between the São Francisco and Congo cratons. The Macaúbas Group represents a major passive margin sequence and is a key unit for interpreting the evolution of that orogen. The Salinas Formation is the distal rock assemblage of the Macaúbas Group and consists of a deep-sea sand-mud sequence, and a volcanic-sedimentary unit called the Ribeirão da Folha facies. The latter includes metamorphosed volcanic-exhalative sediments associated with ocean-floor basalts (amphibolites). The magmatic protoliths of these amphibolites crystallized at about 816 ± 72 Ma (Sm-Nd whole-rock isochron, ɛNd(t) =+3.8 ± 0.2). Regional metamorphism reached the amphibolite facies at about 630 Ma (Rb-Sr whole-rock isochron), when slabs of ultramafic rocks were tectonically emplaced over the Ribeirão da Folha facies. We consider this volcanic-sedimentary facies and the coeval slabs of ultramafic rocks to be remnants of a branch of the Adamastor-Brazilide ocean. The extensive occurrence of syntectonic to late tectonic calc-alkalic granitoids along the internal domain of the Araçuaí belt implies that a reasonably large amount of ocean crust was consumed, via an east-dipping subduction zone, during formation of the Araçuaí West Congo orogen.

  10. Solar Cycle 23: An Anomalous Cycle?

    NASA Astrophysics Data System (ADS)

    de Toma, G.; White, O. R.; Chapman, G. A.; Walton, S. R.; Preminger, D. G.; Cookson, A. M.

    2004-05-01

    We discuss the importance of solar cycle 23 as a magnetically simpler cycle and a variant from recent cycles. We see a significant decrease in sunspot activity in cycle 23 relative to cycle 22, but the strength of the total solar irradiance (TSI) cycle did not change significantly. The latest SOHO/VIRGO TSI time series is analyzed using new solar variability measures obtained from full-disk solar images made at the San Fernando Observatory and the MgII 280nm index. The TSI record for the period 1986 to the present is reproduced within about 130ppm RMS using only two indices representing photospheric and chromospheric sources of variability due to magnetic regions. This is in spite of the difference in magnetic activity between the two cycles. Our results show the continuing improvement in TSI measurements and surrogates containing information necessary to account for irradiance variability.

  11. Evolution, source and tectonic significance of Early Mesozoic granitoid magmatism in the Central Asian Orogenic Belt (central segment)

    NASA Astrophysics Data System (ADS)

    Li, Shan; Wang, Tao; Wilde, Simon A.; Tong, Ying

    2013-11-01

    Numerous Early Mesozoic granitoids have been recognized from the central segment of the Central Asian Orogenic Belt (CAOB). They can be broadly classified into two groups according to zircon U-Pb ages: an early-stage group covering the time span from Early to Middle Triassic (250-230 Ma) and a late-stage group emplaced during Late Triassic to Early Jurassic (ca. 230-190 Ma). Early-stage (250-230 Ma) granitoids are mainly distributed in the western Central Mongolia-Erguna Belt (CMEB), the western Altai Belt (AB), the South Mongolia-Xing'an Belt (SMXB) and the Beishan-Inner Mongolia-Jilin Belt (BIJB). They consist mainly of quartz-diorites, granodiorites and monzogranites, mostly of I-type, with minor mafic intrusions, with some of them showing adakite-like signatures and some with S-type features. Late-stage (230-190 Ma) granitoids mainly occur in the North Mongolia-Transbaikalia Belt (NMTB), the eastern CMEB (Erguna massif) and the eastern Altai Belt (AB). They are predominately syenogranites, monzogranites and syenites, associated with many alkaline granites and mafic intrusions and are A-type and transitional I-A type or highly fractionated I-type granites.

  12. Middle Miocene paleoaltimetry of southern Tibet: Implications for the role of mantle thickening and delamination in the Himalayan orogen

    NASA Astrophysics Data System (ADS)

    Currie, Brian S.; Rowley, David B.; Tabor, Neil J.

    2005-03-01

    The stable isotope composition of pedogenic and early diagenetic carbonates from the Oiyug Basin of southern Tibet allows model estimates of the paleoaltimetry of the Tibetan Plateau for the middle Miocene. Pedogenic calcium carbonate nodules have average δ18Occ values of -19.6‰, whereas nodular lacustrine dolomites range in composition from -7.6‰ to -5.5‰. The most negative of the carbonate isotope values can be used to constrain the oxygen isotope composition of paleoprecipitation, from which model estimates of paleoaltimetry can be made. Model results indicate that the southern Tibetan Plateau achieved elevations of ˜5200 +1370/-605 m by at least 15 Ma. Our results are identical within uncertainty to previous workers' paleoelevation estimates based on Oiyug Basin fossil floral physiognomy. This is the first time that two paleoaltimeters have been directly compared and are in accord. Collectively, these data strongly support tectonic models in which thickening of mantle lithosphere beneath the domain of crustal thickening and subsequent detachment of the mantle lithosphere plays an indiscernible role in the elevation history of this part of the Himalaya-Tibet orogenic system.

  13. Crustal structure of the south-western termination of the Alpine Pyrenean-Cantabrian Orogen (NW Iberian Peninsula)

    NASA Astrophysics Data System (ADS)

    Rodríguez Fernández, L. R.; Pedrera, A.; Pous, J.; Ayala, C.; González Menéndez, L.; Ibarra, P.; Martín-González, F.; González Cuadra, P.; Seillé, H.

    2015-11-01

    The structure of the western termination of the Alpine Pyrenean-Cantabrian Orogen (NW Iberian Peninsula) has been under debate during the last years. We present new results derived from a new magnetotelluric profile that runs through the western Cantabrian Mountains and the León Mountains, together with gravity data, structural field data and petrological modeling that allows the crustal structure of the sector to be clarified. In the Cantabrian Mountains, the overall Alpine structure is a regional monoclinal flexure that corresponds to the back-limb of E-W trending and north-dipping thrust system. It is formed by a connected thrust-fault splay that overturns the Cenozoic sediments of the Bierzo basin, probably rooted in a detachment at 15 km depth. Further to the South, in the León Mountains, we propose a "pop-up" geometry characterized by high-angle dipping reverse faults that extend toward the lower crust probably causing uplift and the erosion of the early foreland basin. Apatite fission-track and paleontological data suggests that uplift and exhumation of the León Mountains occurred later than in the Cantabrian Mountains. Uplift started in the Middle and Upper Miocene in the León Mountains linked to the activity of the highlighted "pop-up" structure whereas it took place during the Oligocene in the Cantabrian Mountains.

  14. The structure of the Temsamane fold-and-thrust stack (eastern Rif, Morocco): Evolution of a transpressional orogenic wedge

    NASA Astrophysics Data System (ADS)

    Jabaloy-Sánchez, Antonio; Azdimousa, Ali; Booth-Rea, Guillermo; Asebriy, Lahcen; Vázquez-Vílchez, Mercedes; Martínez-Martínez, José Miguel; Gabites, Janet

    2015-11-01

    The structure of the Temsamane fold-and-thrust stack corresponds to four units limited by anastomosing ductile shear zones cutting a trend of south verging recumbent folds. This ductile stack was formed in an inclined left-handed transpressional zone at the North African paleomargin during Chattian to Langhian times producing two main deformational events. The first event (Dp) produced a Sp/Lp planar linear fabric generated in a non-coaxial deformation with a top-to-the-WSW sense of movement and was associated to metamorphic P-T conditions varying from late diagenesis in the southernmost Temsamane outcrops to epizone in the north. According to the 40Ar/39Ar ages, this deformation occurred at Chattian-Aquitanian times. The second deformational event (Dc event) generated ENE-WSW trending folds with SSE vergence and a set of anastomosing shear zones with Sm/Lm planar linear fabric. The latter units were generated at around 15 Ma (Langhian), and indicate a strong localization of the simple shear component of the transpression. Moreover, this orientation is compatible with the kinematics of the Temsamane detachment, which can explain most of the uplift of the Temsamane rocks from the middle to the uppermost crust. The described evolution indicates that collision between the western Mediterranean terranes and the North African paleomargin and the formation of the Rifean orogenic wedge occurred at Chattian to Langhian times.

  15. Structural geometry of an exhumed UHP terrane in the eastern Sulu Orogen, China: Implications for continental collisional processes

    NASA Astrophysics Data System (ADS)

    Wang, Lu; Kusky, Timothy M.; Li, Sanzhong

    2010-04-01

    High-precision 1:200-1:1000 mapping of Yangkou Bay, eastern Sulu orogen, defines the structural geometry and history of one of the world's most significant UHP (ultrahigh-pressure) belts. At least four stages of folds are recognized in UHP eclogites and associated quartzo-feldspathic gneiss. UHP eclogite facies rootless F 1 and isoclinal F 2 folds are preserved locally in coesite-eclogite. Mylonitic to ultramylonitic quartzo-feldspathic and coesite-eclogite shear zones separate small-scale 5-10-m-thick nappes of ultramafic-mafic UHP rocks from banded quartzo-feldspathic gneiss. These shear zones are folded, and progressively overprinted by amphibolite-greenschist facies shear zones. The prograde to retrograde D 1-D 5 deformation sequence is explained by deep subduction of offscraped thrust slices of lower continental or oceanic crust from the down-going plate, caught between the colliding North and South China cratons in the Mesozoic. After these slices were structurally isolated along the plate interface, they were rolled in the subduction channel during exhumation and structural juxtaposition with quartzo-feldspathic gneisses, forming several generations of folds, sequentially lower-grade foliations and lineations, and intruded by in situ and exotically derived melts. Shear zones formed during different deformation generations are wider with lower grades, suggesting that deep-crustal/upper mantle deformation operates more efficiently, perhaps with more active crystallographic slip systems, than deformation at mid-upper crustal levels.

  16. STRUCTURAL GEOMETRY OF AN EXHUMED UHP TERRANE IN THE EASTERN SULU OROGEN, CHINA: IMPLICATIONS FOR CONTINENTAL COLLISIONAL PROCESSES

    NASA Astrophysics Data System (ADS)

    Wang, L.; Kusky, T.

    2009-12-01

    High-precision 1:1,000 mapping of Yangkou Bay, eastern Sulu orogen, defines the structural geometry and history of the world’s most significant UHP (Ultrahigh Pressure) rock exposures. Four stages of folds are recognized in the UHP rocks and associated quartzo-feldspathic gneiss. Eclogite facies rootless F1 and isoclinal F2 folds are preserved locally in coesite-eclogite. Mylonitic to ultramylonitic cosesit-eclogite shear zones separate 5-10-meter-thick nappes of ultramafic-mafic UHP rocks from banded quartzo-feldspathic gneiss. These shear zones are folded, and progressively overprinted by amphibolite and greenschist facies shear zones that become wider with lower grade. The deformation sequences is explained by deep subduction of offscraped thrust slices of oceanic or lower continental crust, caught between the colliding North and South China cratons in the Mesozoic. After these slices were structurally isolated along the plate interface, they were rolled like ball-bearings, in the subduction channel during their exhumation, forming several generations of folds, sequentially lower-grade foliations and lineations, and intruded by several generations of in situ and exotically derived melts. The shear zones formed during different generations of deformation are wider with lower grades, suggesting that deep-crustal/upper mantle deformation operates efficiently (perhaps with more active crystallographic slip systems) than deformation at mid to upper crustal levels.

  17. Sediment yield along the Andes: continental budget, regional variations, and comparisons with other basins from orogenic mountain belts

    NASA Astrophysics Data System (ADS)

    Latrubesse, Edgardo M.; Restrepo, Juan D.

    2014-07-01

    We assess the sediment yield at 119 gauging stations distributed from Colombia to Patagonia, covering the different morphotectonic and morphoclimatic settings of the Andes. The most productive areas are the Meta River basin within the northern Andes and the Bolivian and northern Argentina-Chaco systems, which produce an average of 3345, 4909 and 2654 t km2 y- 1 of sediment, respectively. The rivers of the northern and central Andes (excluding the Pacific watersheds of Peru, northern Chile, and central Argentina) have a weighted mean sediment yield of 2045 t km- 2 y- 1 and produce 2.25 GTy- 1 of total sediment. A major constraint estimating the Andean continental budget of sediment yield lies in the lack of gauging data for the Peruvian region. Using the available gauge stations, the regional sediment yield appears underestimated. Assuming a higher value of sediment yield for the Peruvian Andes, the total budget for the whole central Andes could range between 2.57 GT y- 1 and 3.44 GT y- 1. A minimum of ~ 0.55 GT y- 1 and a probable maximum of ~ 1.74 GT y- 1 of sediment are deposited in the intramontane and surrounding proximal sedimentary basins. The magnitude of sediment yield in the Andes is comparable to other rivers draining orogenic belts around the world.

  18. Extensional deformation structures within a convergent orogen: The Val di Lima low-angle normal fault system (Northern Apennines, Italy)

    NASA Astrophysics Data System (ADS)

    Clemenzi, Luca; Molli, Giancarlo; Storti, Fabrizio; Muchez, Philippe; Swennen, Rudy; Torelli, Luigi

    2014-09-01

    A low-angle extensional fault system affecting the non metamorphic rocks of the carbonate dominated Tuscan succession is exposed in the Lima valley (Northern Apennines, Italy). This fault system affects the right-side-up limb of a kilometric-scale recumbent isoclinal anticline and is, in turn, affected by superimposed folding and late-tectonic high-angle extensional faulting. The architecture of the low-angle fault system has been investigated through detailed structural mapping and damage zone characterization. Pressure-depth conditions and paleofluid evolution of the fault system have been studied through microstructural, mineralogical, petrographic, fluid inclusion and stable isotope analyses. Our results show that the low-angle fault system was active during exhumation of the Tuscan succession at about 180°C and 5 km depth, with the involvement of low-salinity fluids. Within this temperature - depth framework, the fault zone architecture shows important differences related to the different lithologies involved in the fault system and to the role played by the fluids during deformation. In places, footwall overpressuring influenced active deformation mechanisms and favored shear strain localization. Our observations indicate that extensional structures affected the central sector of the Northern Apennines thrust wedge during the orogenic contractional history, modifying the fluid circulation through the upper crust and influencing its mechanical behavior.

  19. Zongo-San Gaban zone: Eocene foreland boundary of the Central Andean orogen, northwes