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1. Paleozoic evolution of crustal thickness and elevation in the northern Appalachian orogen, USA

   https://doi.org/10.1130/G48705.1  

   Hillenbrand, Ian W.; Williams, Michael L. (April 2021, Geology)

   null (Ed.)

   The Acadian and Neoacadian orogenies are widely recognized, yet poorly understood, tectono-thermal events in the New England Appalachian Mountains (USA). We quantified two phases of Paleozoic crustal thickening using geochemical proxies. Acadian (425–400 Ma) crustal thickening to 40 km progressed from southeast to northwest. Neoacadian (400–380 Ma) crustal thickening was widely distributed and varied by 30 km (40–70 km) from north to south. Doubly thickened crust and paleoelevations of 5 km or more support the presence of an orogenic plateau at ca. 380–330 Ma in southern New England. Neoacadian crustal thicknesses show a strong correlation with metamorphic isograds, where higher metamorphic grade corresponds to greater paleo-crustal thickness. We suggest that the present metamorphic field gradient was exposed through erosion and orogenic collapse influenced by thermal, isostatic, and gravitational properties related to Neoacadian crustal thickness. Geobarometry in southern New England underestimates crustal thickness and exhumation, suggesting the crust was thinned by tectonic as well as erosional processes. 

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2. Hydrothermal circulation cools continental crust under exhumation

   https://doi.org/https://doi.org/10.1016/j.epsl.2019.03.029  

   Cao, W.; Lee, C-T. A.; Yang, J.; Zuza, A. V. (June 2019, Earth and planetary science letters)

   The formation of continental crust in magmatic arcs involves cooling of hot magmas to a relatively colder crust enhanced by exhumation and hydrothermal circulation in the upper crust. To quantify the influence of these processes on the thermal and rheological states of the crust, we developed a one-dimensional thermal evolution model, which invokes conductive cooling, advection of crust by erosion-driven exhumation, and cooling by hydrothermal circulation. We parameterized hydrothermal cooling by adopting depth-dependent effective thermal conductivity, which is determined by the crustal permeability structure and the prescribed Nusselt number at the surface. Different combinations of erosion rate and Nusselt number were tested to study the evolution of crustal geotherms, surface heat flux, and cooling rate. Simulations and scaling analyses quantify how erosion and hydrothermal circulation promote cooling via increasing total surface heat flux compared to pure conductive cooling. Hydrothermal circulation imposes intense short-term and persistent long-term cooling effects. Thinner, warmer, fast exhuming crust, with higher permeability and more vigorous hydrothermal circulation, leads to higher steady-state total surface heat flux. Hydrothermal cooling at steady state is more effective when the Péclet number is small. Hydrothermal cooling also changes crustal rheological state and thickens the brittle crust. This in turn promotes the initiation of brittle deformation in the upper crust in magmatic arcs or in regions undergoing exhumation. Interpretation of low-temperature thermochronological data could overestimate average cooling rates if hydrothermal cooling is not considered. 

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3. Bhumichula plateau: A remnant high-elevation low-relief surface in the Himalayan thrust belt of western Nepal

   https://doi.org/10.1130/B36481.1  

   Sherpa, Tshering Z.L.; DeCelles, Peter G.; Carrapa, Barbara; Schoenbohm, Lindsay M.; Wolpert, Joshua (December 2022, GSA Bulletin)

   The Himalaya is known for dramatically rugged landscapes including the highest mountains in the world. However, there is a limited understanding of the timing of attainment of high elevation and relief formation, especially in the Nepalese Himalaya. Anomalous high-elevation low-relief (HELR) surfaces, which exhibit geomorphic antiquity and are possibly remnants of formerly widespread high-elevation paleosurfaces, provide a unique opportunity to assess the attainment of regional high elevation in the Himalaya. The Bhumichula plateau is one such HELR surface (4300−4800 m) in the western Nepalese Himalayan fold-thrust belt. The Bhumichula plateau is situated in the Dadeldhura klippe (also called the Karnali klippe), an outlier of Greater Himalayan Sequence high-grade metasedimentary/igneous rocks surrounded by structurally underlying Lesser Himalayan Sequence low-grade metasedimentary rocks. We assess the origin of the Bhumichula plateau by combining regional geological relationships and zircon and apatite (U-Th-Sm)/He and apatite fission track thermochronologic ages. The HELR surface truncates pervasive west-southwestward dipping foliations, indicating that it post-dates tilting of rocks in the hanging wall of the Main Central thrust above the Lesser Himalayan duplex. This suggests that the surface originated at high elevation by erosional beveling of thickened, uplifted crust. Exhumation through the ∼180−60 °C thermal window occurred during middle Miocene for samples on the plateau and between middle and late Miocene for rocks along the Tila River, which bounds the north flank of the Bhumichula plateau. Cooling ages along the Tila River are consistent with erosional exhumation generated by early Miocene emplacement of the Main Central (Dadeldhura) thrust sheet, middle Miocene Ramgarh thrust emplacement, and late Miocene growth of the Lesser Himalayan duplex. The most recent middle-late Miocene exhumation took place as the Tila River and its northward flowing tributaries incised upstream, such that the Bhumichula plateau is a remnant of a more extensive HELR paleolandscape. Alpine glaciation lowered relief on the Bhumichula surface, and surface preservation may owe to its relatively durable lithology, gentle structural relief, and elevation range that is above the rainier Lesser Himalaya. 

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4. Exhumation and incision of the eastern Central Andes, southern Peru: Low-temperature thermochronology observations

   https://doi.org/10.1016/j.epsl.2023.118299  

   Falkowski, Sarah; Ehlers, Todd A; McQuarrie, Nadine; Glover, Chloë O; Perez, Nicholas D; Buford_Parks, Victoria M (October 2023, Earth and Planetary Science Letters)

   Quantifying the impacts of past changes in tectonics or climate on mountain topography has proven challenging. The incision of the eastern Central Andean Plateau has been interpreted as both a result of deformation-related uplift and erosion and climate-driven erosion. Here, we contribute >100 new apatite and zircon (U-Th)/He and fission-track dates from 51 new and eight previous bedrock samples. These samples were combined with previous thermochronometer data from three ∼190-km-long and ∼200-km-apart across-strike transects along the eastern margin of the Andean Plateau in southern Peru. We discuss age-distance, age-elevation, and inverse thermal history model results along these transects to constrain the timing and extent of recent canyon incision compared to the region’s long-term (∼40 Myrs) exhumation history. Results indicate that, along the plateau flank, long-term, deformation-related exhumation is superimposed by a regional, synchronous canyon incision-related signal since ∼4–3 Ma. This incision is traceable from at least the Abancay Deflection in southern Peru to southern Bolivia along the eastern Central Andes. Based on the regional and synchronous character of canyon incision across areas with different deformation histories and exhumation magnitude, we suggest that paleoclimate change was a significant contributor to incision. However, structural processes resulting in surface uplift, erosion, and exhumation continued post-mid Miocene and contributed to the observed exhumation magnitude. 

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5. Tectonic setting of metamorphism and exhumation of eclogite-facies rocks in the South Beishan orogen, northwestern China

   https://doi.org/10.1130/GES02548.1  

   Li, Jie; Wu, Chen; Chen, Xuanhua; Yin, An; Zuza, Andrew V.; Haproff, Peter J.; Chen, Yanfei; Wang, Luojuan; Shao, Zhaogang (December 2022, Geosphere)

   Abstract High-pressure metamorphic rocks occur as distinct belts along subduction zones and collisional orogens or as isolated blocks within orogens or mélanges and represent continental materials that were subducted to deep depths and subsequently exhumed to the shallow crust. Understanding the burial and exhumation processes and the sizes and shapes of the high-pressure blocks is important for providing insight into global geodynamics and plate tectonic processes. The South Beishan orogen of northwestern China is notable for the exposure of early Paleozoic high-pressure (HP), eclogite-facies metamorphic rocks, yet the tectonism associated with the HP metamorphism and mechanism of exhumation are poorly understood despite being key to understanding the tectonic evolution of the larger Central Asian Orogenic System. To address this issue, we examined the geometries, kinematics, and overprinting relationships of structures and determined the temperatures and timings of deformation and metamorphism of the HP rocks of the South Beishan orogen. Geochronological results show that the South Beishan orogen contains ca. 1.55–1.35 Ga basement metamorphic rocks and ca. 970–866 Ma granitoids generated during a regional tectono-magmatic event. Ca. 500–450 Ma crustal thickening and HP metamorphism may have been related to regional contraction in the South Beishan orogen. Ca. 900–800 Ma protoliths experienced eclogite-facies metamorphism (~1.2–2.1 GPa and ~700–800 °C) in thickened lower crust. These HP rocks were subsequently exhumed after ca. 450 Ma to mid-crustal depths in the footwall of a regional detachment fault during southeast-northwest–oriented crustal extension, possibly as the result of rollback of a subducted oceanic slab. Prior to ca. 438 Ma, north-south–oriented contraction resulted in isoclinal folding of the detachment fault and HP rocks. Following this contractional phase in the middle Mesozoic, the South Beishan orogen experienced thrusting interpreted to be the response to the closure of the Tethyan and Paleo-Asian Ocean domains. This contractional phase was followed by late Mesozoic extension and subsequent surface erosion that controlled exhumation of the HP rocks. 

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