Exhumation Of Mid-crustal Rocks Research Articles

Tectonometamorphic evolution of the Himalayan metamorphic core in the Makalu–Arun region, eastern Nepal

The metamorphic core of the Himalayan orogen, once viewed as a structurally contiguous unit, is now known to be divided into multiple packages by a series of structural discontinuities. Although these structures have been identified based on distinct metamorphic and kinematic histories of the bounding packages, the number of structures identified across the exhumed, former midcrustal core varies throughout the Himalaya. The scarcity of field evidence for these structures not only impedes their identification, but also hinders our understanding of their role in the development of the Himalaya. This study characterizes the metamorphic and geochronological history of garnet-, sillimanite- and kyanite-bearing gneisses in Makalu–Arun region, Nepal. Phase equilibria modelling coupled with monazite U/Th–Pb petrochronology delineates four rock packages with distinct metamorphic histories separated by at least three thrust-sense structures. The earliest thrust activity and subsequent in-sequence thrusting lasted from c. 23 to 14 Ma, with initiation of late out-of-sequence thrusting after c. 14 Ma. These results are consistent with foreland thrust migration and juxtaposition compatible with models that incorporate underplating, metamorphism and exhumation of midcrustal rocks during orogenesis. Moreover, the demonstrated complex evolution of the metamorphic core in Makalu region is inconsistent with models that incorporate singular, orogenic-wide structures in the Himalaya. Supplementary material : Supplementary tables and figures are available at https://doi.org/10.6084/m9.figshare.c.6420244

Crustal thickening, Barrovian metamorphism, and exhumation of midcrustal rocks during doming and extrusion: Insights from the Himalaya, NW India

AbstractRocks exposed in NW India constrain the burial, partial melting, and exhumation history of the Himalayan crust. New microscale and mesoscale structural analysis, combined with pressure‐temperature estimates from the Haimanta Group exposed in the Sutlej Valley, indicate that the rocks were folded by two generations of NW trending folds. Barrovian metamorphism culminated at ~30 Ma with staurolite and kyanite overgrowths of F2 folds. Crustal thickening created a metamorphic field gradient that increases from the garnet zone (499 ± 99°C and 4.5 ± 1.4 kbar to 571 ± 92°C and 7.8 ± 1.4 kbar) to the staurolite‐kyanite zone (567 ± 105°C and 6.7 ± 1.6 kbar). These data are combined with previous studies to modify a two‐stage conceptual model for the thermal and deformation conditions of the middle and upper crust during the Eocene‐Miocene, excluding the late Miocene to recent. In the Miocene (~23 Ma), Barrovian metamorphism was overprinted by decompression during coeval south directed extrusion of the Greater Himalayan Series beneath the Sangla detachment in the foreland and doming during top‐down‐to‐the‐west displacement along the Leo Pargil shear zone in the hinterland. These data demonstrate that shear zones and detachments, such as the South Tibetan detachment that initially formed during crustal thickening (e.g., Eocene‐Oligocene), contributed to the subsequent distribution of rocks that experienced different pressure‐temperature‐time paths, degrees of partial melting, and exhumation histories during the Miocene.

Orogeny can be very short

In contrast to continent/continent collision, arc-continent collision generates very short-lived orogeny because the buoyancy-driven impedance of the subduction of continental lithosphere, accompanied by arc/suprasubduction-zone ophiolite obduction, is relieved by subduction polarity reversal (flip). This tectonic principle is illustrated by the early Ordovician Grampian Orogeny in the British and Irish Caledonides, in which a wealth of detailed sedimentologic, heavy mineral, and geochronologic data pin the Orogeny to a very short Arenig/Llanvirn event. The Orogeny, from the initial subduction of continental margin sediments to the end of postflip shortening, lasted approximately 18 million years (my). The collisional shortening, prograde-metamorphic phase of the Orogeny lasted 8 my, extensional collapse and exhumation of midcrustal rocks lasted 1.5 my, and postflip shortening lasted 4.5 my. Strain rates were a typical plate-boundary-zone 10(-15). Metamorphism, to the second sillimanite isograd, with extensive partial melting, occurred within a few my after initial collision, indicating that conductive models for metamorphic heat transfer in Barrovian terrains are incorrect and must be replaced by advective models in which large volumes of mafic/ultramafic magma are emplaced, syn-tectonically, below and into evolving nappe stacks. Arc/continent collision generates fast and very short orogeny, regional metamorphism, and exhumation.

Core complex development in central Anatolia, Turkey

Tectonic models for the central Anatolian segment of the Alpine-Himalayan orogen have not considered this region as part of the Aegean extensional province. We have found evidence, however, for Oligocene-Miocene extension and exhumation of midcrustal rocks in a metamorphic dome (Nigde massif) that is structurally and petrologically similar to Miocene core complexes >500 km to the west. Strong correspondence in the timing and kinematic development of the Menderes (western Turkey) and Nigde core complexes suggests that extensional tectonics and exhumation of mid- to lower-crustal rocks affected much of the eastern Mediterranean region. In the Nigde massif, supracrustal rocks were buried to depths of 16‐20 km at high temperatures (>700 °C) during contraction associated with closure of Neo-Tethyan seaways in late Mesozoic‐early Cenozoic time. Following cooling and decompression to <600 °C and <10 km, metasedimentary rocks underwent a second heating event at low pressures during Miocene magmatism that postdated much of the unroofing of the massif. Development of the Nigde core complex was related to exhumation of thickened and thermally weakened continental crust in the upper plate of a north-dipping subduction zone. The partial subduction of the Tauride carbonate platform in the Eocene resulted in choking of the subduction zone, followed by isostatic rebound and exhumation of the buoyant platform. This in turn caused a rapid emergence of the upper plate, resulting in erosion of upper-crustal rocks and exhumation of midcrustal rocks along the northern edge of the Inner Tauride suture zone. Although northern regions of central Anatolia contain the same protoliths as the Nigde massif and underwent extensive Tertiary magmatism, they did not undergo similar extension, suggesting that core complex development in south-central Turkey was controlled by the location of thickened and thermally weakened crust adjacent to a suture zone.

Crustal extension, the exhumation of mid-crustal rocks, and the formation of basin-and-range structure in the northern Edsel Ford Ranges, western Marie Byrd Land, West Antarctica

Crustal extension, the exhumation of mid-crustal rocks, and the formation of basin-and-range structure in the northern Edsel Ford Ranges, western Marie Byrd Land, West Antarctica