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Taras Gerya - One of the best experts on this subject based on the ideXlab platform.
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dynamics of Exhumation and deformation of hp uhp orogens in double subduction collision systems numerical modeling and implications for the western dabie orogen
Earth-Science Reviews, 2018Co-Authors: Sanzhong Li, Zhong-hai Li, Taras Gerya, Ian D Somerville, M SantoshAbstract:Abstract The dynamics of formation and Exhumation of high-pressure (HP) and ultra-high pressure (UHP) metamorphic orogens in double subduction-collision zones remain enigmatic. Here we employ two-dimensional thermo-mechanical numerical models to gain insights on the Exhumation of HP-UHP metamorphic rocks, as well as their deformation during the collision of a micro-continent with pro- and retro-continental margins along two subduction zones. A three-stage collisional process with different convergence velocities is tested. In the initial collisional stage, a fold-and-thrust belt and locally rootless superimposed folds are developed in the micro-continent and subduction channel, respectively. In the second (Exhumation) stage of HP-UHP rocks, a faster convergence model results in upwelling of the asthenosphere, which further leads to a detachment between the crust and lithospheric mantle of the micro-continent. A slower convergence model results in rapid Exhumation of HP-UHP rocks along the north subduction channel and a typical piggy-back thrusting structure in the micro-continent. A non-convergence model produces a slab tear-off, leading to the rebound of residual lithosphere of the micro-continent. In the third and final stage, a series of back and ramp thrusts are formed in the micro-continent with the pro-continent re-subducted. Based on an analogy of our numerical results with the Western Dabie Orogen (WDO), we suggest that: (1) slab tear-off results in a rebound of residual lithosphere, which controls the two-stage syn-collisional Exhumation process of HP-UHP rocks in the WDO; and (2) in contrast to the single subduction-collision system, the Exhumation range of the partially molten rocks with lower viscosity and density is restricted to a specific region of the micro-continent by the Mianlue and Shangdan subduction zones, which generated the complex deformation features in the WDO.
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divergent plate motion drives rapid Exhumation of ultra high pressure rocks
Earth and Planetary Science Letters, 2018Co-Authors: Marco G. Malusà, Paul G. Fitzgerald, Suzanne L. Baldwin, Jie Liao, Liang Zhao, Taras GeryaAbstract:Abstract Exhumation of (ultra)high pressure [(U)HP] rocks by upper-plate divergent motion above an unbroken slab, first proposed in the Western Alps, has never been tested by numerical methods. We present 2D thermo-mechanical models incorporating subduction of a thinned continental margin beneath either a continental or oceanic upper plate, followed by upper-plate divergent motion away from the lower plate. Results demonstrate how divergent plate motion may trigger rapid Exhumation of large volumes of (U)HP rocks directly to the Earth's surface, without the need for significant overburden removal by erosion. Model Exhumation paths are fully consistent with natural examples for a wide range of upper-plate divergence rates. Exhumation rates are systematically higher than the divergent rate imposed to the upper plate, and the modeled size of exhumed (U)HP domes is invariant for different rates of upper-plate divergence. Major variations are instead predicted at depth for differing model scenarios, as larger amounts of divergent motion may allow mantle-wedge Exhumation to shallow depth under the exhuming domes. The transient temperature increase, due to ascent of mantle-wedge material in the subduction channel, has a limited effect on exhumed continental (U)HP rocks already at the surface. We test two examples, the Cenozoic (U)HP terranes of the Western Alps (continental upper plate) and eastern Papua New Guinea (oceanic upper plate). The good fit between model predictions and the geologic record in these terranes encourages the application of these models globally to pre-Cenozoic (U)HP terranes where the geologic record of Exhumation is only partly preserved.
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Formation and Exhumation of Ultrahigh-Pressure Terranes
Elements, 2013Co-Authors: Bradley R. Hacker, Taras Gerya, Jane A. GilottiAbstract:The reigning paradigm for the formation and Exhumation of continental ultrahigh-pressure (UHP) terranes is the subduction of crust to mantle depths and the return of crustal slices within the subduction channel—all at plate tectonic rates. Additional processes beyond the paradigm are needed to explain the diversity of geological observations gathered from the growing study of UHP terranes—for example, variations in the size, degree of deformation, petrologic evolution, timing of UHP metamorphism, and Exhumation rates. Numerical models that evaluate physical parameters in time and space have produced new insights into the formation and Exhumation of UHP terranes.
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Exhumation mechanisms of melt bearing ultrahigh pressure crustal rocks during collision of spontaneously moving plates
Journal of Metamorphic Geology, 2012Co-Authors: E Sizova, Taras Gerya, Michael BrownAbstract:A series of 2D petrological-thermomechanical numerical experiments was conducted to: (i) characterize the variability of Exhumation mechanisms of ultrahigh pressure metamorphic (UHPM) rocks during collision of spontaneously moving plates and (ii) study the possible geodynamic effects of melting at ultrahigh pressure conditions for the Exhumation of high-temperature-ultrahigh pressure metamorphic (HT-UHPM) rocks. To this end, the models include fluid- and melt-induced weakening of rocks. Five distinct modes of Exhumation of (U)HPM rocks associated with changes in several parameters in the models of plate collision and continent subduction are identified as follows: vertical crustal extrusion, large-scale crustal stacking, shallow crustal delamination, trans-lithospheric diapirism, and channel flow. The variation in Exhumation mechanisms for (U)HPM rocks in numerical models of collision driven by spontaneously moving plates contrasts with the domination of the channel flow mode of Exhumation in a majority of the published results from numerical models of collision that used a prescribed plate convergence velocity and/or did not include fluid- and melt-induced weakening of rocks. This difference in the range of Exhumation mechanisms suggests that the prescribed convergence velocity condition and the neglect of fluid- and melt-related weakening effects in the earlier models may inhibit development of several important collisional processes found in our experiments, such as slab breakoff, vertical crustal extrusion, large-scale stacking, shallow crustal delamination and relamination, and eduction of the continental plate. Consequently, the significance of channel flow for the Exhumation of UHPM rocks may have been overstated based on the results of the earlier numerical experiments. In addition, the results from this study extend over a larger proportion of the high-temperature range of P-T conditions documented from UHPM rocks, including those retrieved from HT-UHPM rocks, than the results of experiments from previous numerical models. In particular, the highest peak metamorphic temperatures (up to 1000 degrees C) are recorded in the case of the vertical crustal extrusion model in which subducted continental crust is subjected to a period of prolonged heating by asthenospheric mantle abutting the continental side of the vertically hanging slab. Nonetheless, some extreme temperature conditions which have been suggested for the Kokchetav and Bohemian massifs, perhaps up to 1100-1200 degrees C, are still to be achieved in experiments using numerical models.
Marco G. Malusà - One of the best experts on this subject based on the ideXlab platform.
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From Cooling to Exhumation: Setting the Reference Frame for the Interpretation of Thermochronologic Data
Fission-Track Thermochronology and its Application to Geology, 2018Co-Authors: Marco G. Malusà, Paul G. FitzgeraldAbstract:The reference frame for the interpretation of fission-track (FT) data is a thermal reference frame. Using thermochronology to constrain Exhumation largely depends on understanding the linkage between this reference frame and Earth’s surface. The thermal frame of reference is dynamic, that is it is often neither stationary nor horizontal, as it is influenced by the shape of the topography, heat advection associated with rapid Exhumation and mass redistribution across major faults. Here, we review the nomenclature and basic relationships related to cooling, uplift and Exhumation and describe strategies to independently constrain the paleogeothermal gradient at the time of Exhumation. In some cases, cooling may not be related to Exhumation, but can be used instead to constrain the thermal evolution of the upper crust and the emplacement depth of magmatic rocks. In general terms, useful constraints on Exhumation are often only directly provided by thermochronologic ages that are set during undisturbed Exhumational cooling across the closure temperature isothermal surface. Thermochronologic ages from minerals crystallised at temperatures less than the closure temperature, e.g. in volcanic rocks and shallow intrusions, provide no direct constraint on Exhumation.
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divergent plate motion drives rapid Exhumation of ultra high pressure rocks
Earth and Planetary Science Letters, 2018Co-Authors: Marco G. Malusà, Paul G. Fitzgerald, Suzanne L. Baldwin, Jie Liao, Liang Zhao, Taras GeryaAbstract:Abstract Exhumation of (ultra)high pressure [(U)HP] rocks by upper-plate divergent motion above an unbroken slab, first proposed in the Western Alps, has never been tested by numerical methods. We present 2D thermo-mechanical models incorporating subduction of a thinned continental margin beneath either a continental or oceanic upper plate, followed by upper-plate divergent motion away from the lower plate. Results demonstrate how divergent plate motion may trigger rapid Exhumation of large volumes of (U)HP rocks directly to the Earth's surface, without the need for significant overburden removal by erosion. Model Exhumation paths are fully consistent with natural examples for a wide range of upper-plate divergence rates. Exhumation rates are systematically higher than the divergent rate imposed to the upper plate, and the modeled size of exhumed (U)HP domes is invariant for different rates of upper-plate divergence. Major variations are instead predicted at depth for differing model scenarios, as larger amounts of divergent motion may allow mantle-wedge Exhumation to shallow depth under the exhuming domes. The transient temperature increase, due to ascent of mantle-wedge material in the subduction channel, has a limited effect on exhumed continental (U)HP rocks already at the surface. We test two examples, the Cenozoic (U)HP terranes of the Western Alps (continental upper plate) and eastern Papua New Guinea (oceanic upper plate). The good fit between model predictions and the geologic record in these terranes encourages the application of these models globally to pre-Cenozoic (U)HP terranes where the geologic record of Exhumation is only partly preserved.
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contrasting styles of u hp rock Exhumation along the cenozoic adria europe plate boundary western alps calabria corsica
Geochemistry Geophysics Geosystems, 2015Co-Authors: Marco G. Malusà, Martin Danišík, Paul G. Fitzgerald, Maria Laura Balestrieri, Suzanne L. Baldwin, Claudio Faccenna, Federico Rossetti, Alessandro Ellero, G Ottria, Claudia PiromalloAbstract:Since the first discovery of ultrahigh pressure (UHP) rocks 30 years ago in the Western Alps, the mechanisms for Exhumation of (U)HP terranes worldwide are still debated. In the western Mediterranean, the presently accepted model of synconvergent Exhumation (e.g., the channel-flow model) is in conflict with parts of the geologic record. We synthesize regional geologic data and present alternative Exhumation mechanisms that consider the role of divergence within subduction zones. These mechanisms, i.e., (i) the motion of the upper plate away from the trench and (ii) the rollback of the lower plate, are discussed in detail with particular reference to the Cenozoic Adria-Europe plate boundary, and along three different transects (Western Alps, Calabria-Sardinia, and Corsica-Northern Apennines). In the Western Alps, (U)HP rocks were exhumed from the greatest depth at the rear of the accretionary wedge during motion of the upper plate away from the trench. Exhumation was extremely fast, and associated with very low geothermal gradients. In Calabria, HP rocks were exhumed from shallower depths and at lower rates during rollback of the Adriatic plate, with repeated Exhumation pulses progressively younging toward the foreland. Both mechanisms were active to create boundary divergence along the Corsica-Northern Apennines transect, where European southeastward subduction was progressively replaced along strike by Adriatic northwestward subduction. The tectonic scenario depicted for the Western Alps trench during Eocene Exhumation of (U)HP rocks correlates well with present-day eastern Papua New Guinea, which is presented as a modern analog of the Paleogene Adria-Europe plate boundary.
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giant non catastrophic landslides and the long term Exhumation of the european alps
Earth and Planetary Science Letters, 2013Co-Authors: Federico Agliardi, Giovanni B Crosta, P Frattini, Marco G. MalusàAbstract:Landslides influence local slope morphology, affect sediment flux from hillslopes to rivers, and mass wasting in response to tectonics and climate forcing. However, the links between giant, non-catastrophic landslides known as Deep-Seated Gravitational Slope Deformations (DSGSDs) and the long-term evolution of orogenic landscapes are almost unknown. We explore these links in the European Alps using the first orogen-scale inventory of DSGSDs (>900 over an area >105 km2) and a dataset of published apatite fission-track ages (>1000) that provides an estimate of the long-term Exhumation patterns of the orogen. We show that DSGSDs are more widespread than previously considered, and exhibit an orogen-scale distribution not explained by well-known local lithological and structural controls. We test the hypothesis that this orogen-scale distribution correlates to the long-term evolution of the Alps by subdividing the study area into 37 square sub-areas (50×50 km), classified according to combinations of long-term Exhumation and mean annual rainfall. On each sub-area we perform a morphometric analysis of topography (hypsometry, relief, slope). Excluding local and regional controls due to rock type and structure, DSGSDs tend to cluster in areas with intermediate Exhumation rates (fission-track age between 10 and 40 Ma), where large-scale topography is less dissected and incision is localised along major valleys. Here DSGSD abundance correlates positively with the degree of valley incision and related relief. Instead, DSGSDs lack in areas which underwent either low Exhumation rates, resulting in insufficient relief production, or high Exhumation rates associated to rapid uplift or higher erosional dissection of topography. Negative correlation between DSGSD abundance and mean annual rainfall suggests that effective hydrological surface processes contribute, on the long-term timescale, to the development of large-scale topography unfavourable to DSGSDs, especially in areas of high Exhumation rates. Where DSGSDs are abundant, long-lasting slope deformations effectively adjust post-glacial relief by reducing slope inclination values, and are thus expected to significantly contribute to the long-term denudation of active orogens.
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divergence in subduction zones and Exhumation of high pressure rocks eocene western alps
Earth and Planetary Science Letters, 2011Co-Authors: Marco G. Malusà, Claudio Faccenna, Eduardo Garzanti, Riccardo PolinoAbstract:Abstract Exhumation of high-pressure rocks has long remained a controversial issue in the Earth sciences. In this article, we analyze the tectono-metamorphic, stratigraphic and plate-motion constraints from the Western Alps region, providing new insights on Exhumation mechanisms and tectonic evolution during the earliest orogenic stages. Eocene eclogites of the Western Alps form a 20–25 km wide belt on the upper-plate side of the orogen (Eclogite belt), exposed beneath extensional shear zones at the rear of a lower-pressure accretionary wedge. Units of the Eclogite belt show the youngest peak-pressure assemblages within the subduction zone, and experienced superfast tectonic Exhumation since 45–40 Ma. The role of erosion was negligible during the whole of this stage. Eocene foreland basins remained starved, and the massive arrival of axial-belt detritus began well after Exhumation was completed. Tectonic reconstructions based on fixed-boundaries Exhumation models (e.g. channel flow), and/or implying fast erosion at the surface (e.g. slab breakoff), are thus not consistent with geological evidence. In the lack of erosion, Exhumation through the overburden requires divergence within the subduction zone. We demonstrate that this was not attained by rollback of the lower plate (Europe), but was instead attained by NNEward motion of the upper plate (Adria-Africa) away from the Western Alps trench. Such motion induced localized extension within the weak portion of the upper plate, at the rear of the accretionary wedge, and allowed tectonic emplacement of the Eclogite belt in the upper crust at rates much faster than subduction rates. Tectonic Exhumation ceased in the Oligocene, when oblique-divergence along the Western Alps traverse changed into oblique-convergence. The onset of slow erosional unroofing was synchronously recorded by pressure–temperature paths in all major tectonic units of the Western Alps, and by arrival of orogenic detritus in sedimentary basins. This work demonstrates that divergence between upper plate and trench is a viable mechanism to exhume large and coherent eclogite units in continental subduction zones. Our Exhumation model can be applied to other eclogite belts showing a similar Exhumational record, including the Western Gneiss Region, the Dabie-Sulu, and eastern Papua New Guinea.
M Chichorro - One of the best experts on this subject based on the ideXlab platform.
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tectonothermal analysis of high temperature mylonitization in the coimbra cordoba shear zone sw iberian massif ouguela tectonic unit portugal evidence of intra continental transcurrent transport during the amalgamation of pangea
Tectonophysics, 2008Co-Authors: Francisco M Pereira, Arturo Apraiz, J B Silva, M ChichorroAbstract:Abstract Mylonites in pelitic and quartzofeldsphatic gneisses from the Ouguela tectonic unit (Coimbra–Cordoba shear zone, SW Iberian Massif) have been studied as an example of high-temperature ductile deformation associated with transcurrent tectonics. Detailed microstructural and P–T analysis indicates that ductile deformation evolved from a metamorphic peak at approximately 650–750 °C and 7.5–9.5 kbar (quartzofeldsphatic gneisses) and 730–790 °C and 7.5–9.5 kbar (pelitic gneisses) to retrograde conditions at 500–575 °C and 4.5/5.5–6.5/7.5 kbar (quartzofeldsphatic gneisses) and 525–600 °C and 3.5/4.5–5.5/7.5 kbar (pelitic gneisses). Following the metamorphic peak, Exhumation was very fast. The P–T trajectory, which does not reach the curve for granite melting, is distinct that of isothermal decompression. Instead, the progressive and contemporaneous decrease in pressure and temperature was a direct response to strong heat dissipation along the contacts between the ascending slice and the adjacent blocks. The horizontal component of Exhumation path, calculated for middle and shallower crustal levels, sum to ca. 57 km to 94 km (for the pressure peak). Assuming this offset acted in the Visean during a time interval of ca. 9 Ma, the estimated Exhumation horizontal slip rate is in the order of 6.3 to 10.4 mm/yr, which corresponds to an Exhumation oblique-slip Exhumation rate of 6.6 to 10.7 mm/yr (for ductile deformation). These values indicate that the transcurrent tectonic displacements accommodated by these mylonitic are similar to those of modern intra-continental shear zones, such as the still active Karakoram Fault (8.3 mm/yr) in the Himalayas. The Coimbra–Cordoba shear zone is therefore a typical intra-continental transcurrent zone with ten-to-one hundred kilometre along-strike mass movement of material that aided the Exhumation of deep crustal rocks. Study of this large-scale structure in the SW Iberian Massif is therefore central to models of orogenic deformation during the amalgamation of Pangea.
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Tectonothermal analysis of high-temperature mylonitization in the Coimbra–Córdoba shear zone (SW Iberian Massif, Ouguela tectonic unit, Portugal): Evidence of intra-continental transcurrent transport during the amalgamation of Pangea
Tectonophysics, 2008Co-Authors: M. Francisco Pereira, Arturo Apraiz, J B Silva, M ChichorroAbstract:Abstract Mylonites in pelitic and quartzofeldsphatic gneisses from the Ouguela tectonic unit (Coimbra–Cordoba shear zone, SW Iberian Massif) have been studied as an example of high-temperature ductile deformation associated with transcurrent tectonics. Detailed microstructural and P–T analysis indicates that ductile deformation evolved from a metamorphic peak at approximately 650–750 °C and 7.5–9.5 kbar (quartzofeldsphatic gneisses) and 730–790 °C and 7.5–9.5 kbar (pelitic gneisses) to retrograde conditions at 500–575 °C and 4.5/5.5–6.5/7.5 kbar (quartzofeldsphatic gneisses) and 525–600 °C and 3.5/4.5–5.5/7.5 kbar (pelitic gneisses). Following the metamorphic peak, Exhumation was very fast. The P–T trajectory, which does not reach the curve for granite melting, is distinct that of isothermal decompression. Instead, the progressive and contemporaneous decrease in pressure and temperature was a direct response to strong heat dissipation along the contacts between the ascending slice and the adjacent blocks. The horizontal component of Exhumation path, calculated for middle and shallower crustal levels, sum to ca. 57 km to 94 km (for the pressure peak). Assuming this offset acted in the Visean during a time interval of ca. 9 Ma, the estimated Exhumation horizontal slip rate is in the order of 6.3 to 10.4 mm/yr, which corresponds to an Exhumation oblique-slip Exhumation rate of 6.6 to 10.7 mm/yr (for ductile deformation). These values indicate that the transcurrent tectonic displacements accommodated by these mylonitic are similar to those of modern intra-continental shear zones, such as the still active Karakoram Fault (8.3 mm/yr) in the Himalayas. The Coimbra–Cordoba shear zone is therefore a typical intra-continental transcurrent zone with ten-to-one hundred kilometre along-strike mass movement of material that aided the Exhumation of deep crustal rocks. Study of this large-scale structure in the SW Iberian Massif is therefore central to models of orogenic deformation during the amalgamation of Pangea.
Rebecca Anne Jamieson - One of the best experts on this subject based on the ideXlab platform.
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the alps 2 controls on crustal subduction and ultra high pressure rock Exhumation in alpine type orogens
Journal of Geophysical Research, 2014Co-Authors: J P Butler, Christopher Beaumont, Rebecca Anne JamiesonAbstract:Building on our previous results, we use 2-D upper mantle-scale thermomechanical numerical models to explore key controls on the evolution of Alpine-type orogens and the Alps per se, focusing on (ultra)high-pressure ((U)HP) metamorphic rocks. The models show that UHP rocks form and exhume by burial and subsequent buoyant ascent of continental crust in the subduction conduit. Here we test the sensitivity of the models to surface erosion rate, crustal heat production, plate convergence/divergence rates, geometry of the subducting continental margin, and strength of the retrocontinent. Surface erosion affects crustal Exhumation but not early buoyant Exhumation. Metamorphic temperatures increase with crustal radioactive heat production. Maximum burial depth prior to Exhumation increases with plate convergence rates, but Exhumation rates are only weakly dependent on subduction rates. Onset of absolute plate divergence does not trigger Exhumation in these models. We conclude that contrasting peak pressures, Exhumation rates, and volumes of (U)HP crust exhumed in the Alps orogen primarily reflect along-strike contrasts in the geometry, thermal structure, and/or strength of the subducting microcontinent (Brianconnais) and continental (European margin) crust. The experiments also support the interpretation that the Western Alps (U)HP Internal Crystalline Massifs exhumed as composite, stacked plumes and that these plumes drove local crustal extension during orogen-scale shortening. For weak upper plate retrocrusts, postExhumation retrothrusting forms a retrowedge. Overall, these results are consistent with predictions using the Exhumation number (ratio of buoyancy to side traction forces in the conduit), which expresses the combined parameter control of the depth/volume of crustal subduction and the transition to buoyant Exhumation.
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crustal structure a key constraint on the mechanism of ultra high pressure rock Exhumation
Earth and Planetary Science Letters, 2009Co-Authors: Christopher Beaumont, Rebecca Anne Jamieson, J P Butler, C J WarrenAbstract:Abstract The distribution of ultra-high-pressure (UHP) metamorphic rocks demonstrates that burial (to > 100 km) and rapid Exhumation (> 1 cm a − 1 ) of continental crust is a normal part of early (∼ 10 Ma) continental collision. Currently, there is no comprehensive model for this fundamental tectonic process that also satisfactorily explains the upper-crustal structures resulting from early collisional UHP rock Exhumation. Characteristic features requiring explanation include: structural domes that are cored by UHP nappes; associated medium- to high-pressure nappes displaying a distinct “pressure gap”; overlying lower-grade rocks, including suture zone ophiolites; and, coeval foreland-directed thrust-faults and syn-Exhumation normal faults. We present a geodynamical model involving crustal burial and Exhumation in a subduction channel below an accretionary wedge. Competition between down-channel shear traction and up-channel buoyancy forces, expressed as the Exhumation number, E , controls burial and Exhumation, leading to rapid up-channel flow when E > 1. Exhuming UHP material forms a nappe stack and structural dome as it penetrates and destabilises the overlying wedge, driving thrusting and extension. This solution is compelling because it explains both the geology and the petrology of the Tso Morari and other UHP complexes, and because it demonstrates that pulse-like buoyant Exhumation from deep in the subduction channel creates observed upper crustal structures. This places constraints on the Exhumation mechanism and provides a test of alternative models. Other proposed mechanisms, such as continuous circulation in a lithospheric-scale wedge or overpressured subduction channel, predict different types of upper-crustal structures and are therefore unsatisfactory explanations for early collisional Exhumation of UHP terranes.
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deep subduction and rapid Exhumation role of crustal strength and strain weakening in continental subduction and ultrahigh pressure rock Exhumation
Tectonics, 2008Co-Authors: Christopher Beaumont, C J Warren, Rebecca Anne JamiesonAbstract:[1] The Exhumation of crustal ultra-high-pressure (UHP) material depends on temporal and spatial variations in its detachment within the subduction channel. This dependence is investigated using numerical models with variable initial crustal strengths, representing a range of initial crustal compositions, and parameterized strain weakening, representing a range of processes that reduce effective crustal viscosity during deformation. Competition between down-channel shear traction, favoring subduction, and up-channel buoyancy, favoring Exhumation, is expressed as the Exhumation number, E, which can vary with time and position along the channel. Exhumed lower strength crust, which resists subduction owing to weak down-channel traction, records peak conditions 38 kbar. Given sufficient strain weakening, Exhumation proceeds at >60 km Ma−1, indicating that buoyancy (E ≫ 1) drives Exhumation in these models. In all models, exhuming UHP material forms a deforming ductile plume, with a range of possible structural relationships predicted between exhumed UHP and HP materials.
Bradley R. Hacker - One of the best experts on this subject based on the ideXlab platform.
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Formation and Exhumation of Ultrahigh-Pressure Terranes
Elements, 2013Co-Authors: Bradley R. Hacker, Taras Gerya, Jane A. GilottiAbstract:The reigning paradigm for the formation and Exhumation of continental ultrahigh-pressure (UHP) terranes is the subduction of crust to mantle depths and the return of crustal slices within the subduction channel—all at plate tectonic rates. Additional processes beyond the paradigm are needed to explain the diversity of geological observations gathered from the growing study of UHP terranes—for example, variations in the size, degree of deformation, petrologic evolution, timing of UHP metamorphism, and Exhumation rates. Numerical models that evaluate physical parameters in time and space have produced new insights into the formation and Exhumation of UHP terranes.
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Feedback between rifting and diapirism can exhume ultrahigh-pressure rocks
Earth and Planetary Science Letters, 2011Co-Authors: Susan Ellis, Timothy A. Little, Laura M. Wallace, Bradley R. Hacker, Susanne J. H. BuiterAbstract:article i nfo The processes by which crustal rocks are buried to tremendous depths and subsequently exhumed to Earth's surface remain controversial. Rapid Exhumation of Earth's youngest (ultra-) high-pressure (UHP) rocks in the Woodlark Basin, Papua New Guinea, is occurring within an active rift, in contrast to more common scenarios of UHP Exhumation during plate convergence. We use 2D and 3D thermo-mechanical models to demonstrate that UHP Exhumation can result from feedback between rifting and the diapiric rise of a previously subducted continental fragment through the lithosphere. We infer that this feedback is responsible for the Exhumation of the UHP rocks in gneiss domes in the Woodlark Basin. Our models successfully reproduce UHP Exhumation paths and rates, and geological structures within the gneiss domes. We show that UHP Exhumation by diapirism is mechanically consistent in post-collisional rifts. Our models highlight the complex feedback between diapiric ascent and extension.
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slow Exhumation of uhp terranes titanite and rutile ages of the western gneiss region norway
Earth and Planetary Science Letters, 2008Co-Authors: Andrew R C Kylanderclark, Bradley R. Hacker, James M MattinsonAbstract:Abstract U–Pb ages of titanite and rutile were obtained from the central Western Gneiss Region, Norway, to assess the style and timing of Exhumation and cooling of the Western Gneiss UHP terrane. Approximately half of the titanite ages are concordant, the majority of which yield a limited age range from 393 to 390 Ma. The other titanite data are discordant, and define discordia arrays with upper intercept ages of either ∼ 938 Ma or ∼ 1.6 Ga, and a lower intercept of ∼ 389 Ma. Concordant rutile analyses range from 385 to 392 Ma. Both titanite and rutile ages young WNW toward the core of the orogen and are ∼ 4 Ma older than 40 Ar/ 39 Ar muscovite ages, corresponding to a cooling rate of ∼ 90 °C/Ma. A well-defined boundary between concordant and discordant titanite ages, in combination with the WNW-increasing P – T gradient and the similarity between muscovite cooling ages in the east and eclogite ages in the west, suggests that the WGR remained coherent throughout its Exhumation history, and was progressively unroofed from east to west. A 390.2 ± 0.8 Ma titanite in the Soroyane UHP domain indicates that Exhumation occurred at a vertical rate of ∼ 7 mm/yr for ∼ 12 Ma. These rates are slower than estimates from smaller UHP terranes, but similar to other large UHP terranes, suggesting that there may be fundamental differences in the mechanisms controlling the evolution of large UHP terranes that undergo protracted subduction and Exhumation, and smaller UHP terranes that undergo rapid subduction and Exhumation.
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Exhumation of ultrahigh pressure continental crust in east central china late triassic early jurassic tectonic unroofing
Journal of Geophysical Research, 2000Co-Authors: Bradley R. Hacker, Lothar Ratschbacher, Laura E Webb, Michael Mcwilliams, Trevor Ireland, Andrew J Calvert, Shuwen Dong, Hansrudolf Wenk, D ChateignerAbstract:The largest tract of ultrahigh-pressure rocks, the Dabie-Hong'an area of China, was exhumed from 125 km depth by a combination of normal-sense shear from beneath the hanging wall Sino-Korean craton, southeastward thrusting onto the footwall Yangtze craton, and orogen-parallel eastward extrusion. Prior to Exhumation the UHP slab extended into the mantle a downdip distance of 125–200 km at its eastern end, whereas it was subducted perhaps only 20–30 km at its far western end ∼200 km away. Structural reconstructions imply that the slab was >10 km thick. U/Pb zircon and 40Ar/39Ar geochronology indicate that Exhumation up to crustal depths occurred diachronously between 240 and ∼225–210 Ma, reflecting a vertical Exhumation rate of >2 mm/yr. The upper boundary of the slab is the Huwan shear zone, a normal-sense detachment that reactivated the plate suture. The lower boundary is represented by the Lower Yangtze fold-thrust belt. NW-trending stretching lineations, NE-vergent, WNW-ESE trending folds, dominant top-NW shear, and conjugate, but overall asymmetric, shear band fabrics, document that Exhumation was accomplished by updip and orogen-parallel extrusion accompanied by layer-parallel thinning. The orientation and shape of the folds, and a change from SE to SW flow directions, imply that the slab rotated clockwise about a western pivot during Exhumation; this rotation was likely caused by the eastward increasing depth of subduction mentioned above, combined with a possible marginal basin and a weak eastern plate boundary. Exhumation of the slab produced considerable shortening in the Lower Yangtze fold-thrust belt, perhaps producing the foreland orocline.
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Exhumation of Ultrahigh-Pressure Rocks: Thermal Boundary Conditions and Cooling History
When Continents Collide: Geodynamics and Geochemistry of Ultrahigh-Pressure Rocks, 1998Co-Authors: Bernhard Grasemann, Lothar Ratschbacher, Bradley R. HackerAbstract:We investigate the Exhumation of ultrahigh pressure (UHP) and high-pressure (HP) rocks in the framework of a dynamic simulation that considers heat advection, heat conduction, heat production, and consequent time-dependent changes in the geothermal gradient. In the absence of lateral heating, rocks exhuming from great depth cool or decompress isothermally and the main cooling period follows the main period of Exhumation. Even for a constant Exhumation rate, UHP rocks undergo a two-stage cooling history at the end of which the pressure-temperature (P-T) paths of all rocks approach a steady state or “final” geotherm at crustal levels; the shape of the steady-state or final geotherm is mainly a function of Exhumation rate. Reconstruction of pressure-temperature-time (P-T-t) paths permits a qualitative distinction between “fast” and “slow” UHP Exhumation: fast Exhumation is characterized by extremely rapid crustal cooling following small temperature increases or isothermal decompression, whereas slow Exhumation is characterized by steady cooling following more modest heating. Rocks exhuming from different depths (e.g., crustal and mantle levels) follow substantially different PT paths (e.g., heating and cooling during decompression), even if all rocks in an orogen are exhumed by the same orogen-scale process.