The Namche Barwa massif has long been identified as a region of highly localized rapid exhumation. Previous studies suggest that this region contributes ~
50% of the total sediment load of the ...Brahmaputra River, which cuts through the eastern end of the high Himalayan Range and exits into the Indian plain. This study presents new detrital zircon cooling ages from 19 sand samples collected along the Brahmaputra River and tributaries with catchments covering the Namche Barwa massif, and its surroundings, including the so-far unexplored regions in the south. The new results confirm that the Namche Barwa massif is a major source of sediment for the Brahmaputra River composing 60–70% of the entire load. Furthermore, the data from southern regions much better constrains the extent of young cooling ages at Namche Barwa, modestly extending this zone to the south. Our more robust and higher estimates of sediment yield from Namche Barwa together with the increased source area give decadal timescale denudation rates of 5–17
mm/yr. Results from thermokinematic modeling of the ages suggest million-year timescale denudation rates as high as 7–9
mm/yr.
► The rapid exhuming Namche Barwa massif composes 60–70% of the Brahmaputra sediments. ► Detrital zircon FT ages reveal long-term exhumation rates of 8
mm/yr at NB massif. ► Rapid exhumation in the NB massif extends 50
km SW than previously recognized.
The Rocky Mountain Trench (RMT) extends through the Canadian Cordillera from the Yukon to northwest Montana. The RMT's impressive length and continuous nature have been interpreted to be the result ...of faulting, and localized erosion, along a continent‐scale zone of crustal weakness located beneath the RMT, possibly associated with the ancient continental margin. Despite the continuous nature of the RMT, its kinematics vary along strike from dextral strike‐slip faults in the north to normal faults in the south. The central RMT is thought to be the transition between these two structural zones. We use low‐temperature thermochronology to compare the cooling histories across the central RMT in eastern British Columbia. We report apatite fission track ages from 23 samples and apatite (U‐Th)/He ages from 25 samples along with thermal‐history models. Our results reveal three phases of rapid cooling that followed the Cretaceous‐Paleocene cordilleran thrusting in the Eocene, the early–mid Miocene, and from the late Miocene to recent. We find that the Malton Gneiss Complex exhumed as a horst structure 20–10 Ma bounded by the North Thompson Albreda Fault and the RMT. Normal faulting along the NTAF continued in the late Miocene as well as west side down normal faulting along the RMT. Our data suggest that extends along the southern RMT continued northward to at least this portion of the central RMT in multiple episodes during the Cenozoic. We suggest transtension occurred in our study area that was driven by orogenic collapse and lithospheric mantle delamination.
Plain Language Summary
The Rocky Mountain Trench (RMT) is an impressively long and wide valley in the Canadian Cordillera. It stretches from Montana northward through entire British Columbia and Yukon. This valley follows several underlying faults that caused the displacement of rocks on either side. The southern RMT coincides with normal faults that cause stretching of earth's crust, while the northern RMT follows strike‐slip faults that cause northward motion of the western side of the fault with respect to the eastern side. We investigate the fault motion occurring in the central RMT that is thought to be the transition zone. We collected rocks from both sides of the valley along a 130 km section. The cooling history of these rocks is revealed by using radiometric dating techniques and computer modeling. In the case of extensional faulting, the cooling histories of rocks on either side of the fault are expected to be different. We found several phases of normal faulting occurred in the past 40 million years. In general, the western side of the valley is sliding down from the eastern side of the RMT and causes crustal extension much farther north than previously expected.
Key Points
We record three phases of rock exhumation along the Rocky Mountain Trench during 45–30 Ma, 20–10 Ma, and since 10 Ma
The Malton Gneiss Complex exhumed as a horst structure 20–10 Ma
Late Miocene normal faulting occurred along the Rocky Mountain Trench and North Thompson Albreda Fault
Deformation in southeastern Alaska and southwest Yukon is governed by the subduction and translation of the Pacific‐Yakutat plates relative to the North American plate in the St. Elias region. ...Despite notable historical seismicity and major regional faults, studies of the region between the Fairweather and Denali faults are complicated by glacial coverage and the remote setting. In the last decade, significant improvements have been made to the density of regional broadband seismometer networks. We relocate more than 5,000 earthquakes between 2010 and 2021 in the region of southeastern Alaska and southwestern Yukon utilizing these improved seismic networks. With reductions in catalog uncertainty, particularly in depth, we quantify the thickness of the seismogenic layer in the crust throughout the region and locate seismicity on a shallow network of upper‐crustal faults. Relocated earthquakes, combined with an updated focal‐mechanism catalog, permit estimating and classifying motion of active faults. This includes mapping the Totschunda‐Fairweather “Connector” fault, which plays an important role in explaining regional deformation, and identifying new faults like the Kathleen Lake fault. We draw similarities between our seismic observations and simplified conceptual models of regional tectonics, which describe a dominant transpressional regime and localized slip partitioning. Our results support a hypothesis where current deformation is taking place on a well‐defined and evolved network of shallow faults in the corridor between the Totschunda‐Fairweather “Connector” and Denali faults.
Plain Language Summary
Southeastern Alaska and southwest Yukon are actively deforming due to their position at the collision of multiple tectonic plates as the Pacific plate and the Yakutat microplate collide with the North American plate. The region has a large number of significant earthquakes and many mapped faults. Traditional methods of identifying faults, such as field mapping or remote satellite imagery, are complicated due to the remote setting, large glaciers, and mountainous terrain. Here, we analyzed more than 5,000 earthquakes from 2010 to 2021 to better understand how deformation occurs and where active faults are located. Our results show that earthquakes occur on a shallow network of faults, confirming models of an actively deforming and uplifting continental crust. We can map and classify motion on new faults, such as the “Connector” fault, showing that deformation transfers inland hundreds of kilometers from the colliding tectonic plates to the Denali fault in central Alaska. Our results support evidence of an evolving fault system, with deformation occurring on a well‐defined network of shallow faults in the corridor between the plate boundary and the Denali fault.
Key Points
Earthquake relocations constrain the locations and kinematics of previously unrecognized faults, including the Connector fault, in SW Yukon
Slip partitioning of thrust and strike‐slip motion is observed on these predominantly shallow faults
Active tectonics in SE Alaska and SW Yukon are characterized by a corridor of transpressional deformation bounded by regional‐scale faults
The ranges of the Eastern Sierras Pampeanas are located >600 km east of the Andean Cordillera in central Argentina and have been interpreted to be a response to shortening related to flat‐slab ...subduction of the Nazca plate. Uplift of the ranges has been broadly documented to occur during Neogene time, but many questions remain regarding the timing and style of deformation, and the subsurface structural configuration. In this study, we address these unknowns with observations at multiple scales, integrate our results into a tectonic model for the area, and discuss how our structural interpretation fits with more regional tectonic models. Our major findings are: (1) The range‐bounding faults thrust late Proterozoic to Cambrian schist and gneiss over poorly dated Pliocene to Pleistocene alluvial strata. The timing of fault displacement and age of footwall strata suggest that deformation may have been active at least by Pliocene time. (2) Apatite and zircon (U‐Th)/He thermochronometry exhibits cooling ages that range from Permian to Early Jurassic time and suggests that rock exhumation in the area is less than 2–3 km since that time. (3) Deploying a local seismic array allowed for locating seismicity and calculating receiver functions. These observations indicate that the Moho lies at a depth of 37 km and that a midcrustal discontinuity appears to correspond to a detachment zone between 15 and 20 km depth and aligns with a plane of seismicity. In our tectonic model, the craton appears to act as a rigid backstop to the eastward propagation of stresses from the shallowly subducting slab. Deformation then propagates back to the west via westward‐verging faults along a midcrustal detachment.
Key Points
The ESP is actively deforming, possibly in relation to flat‐slab subduction.Our analyses of fault outcrops and local seismicity clarify crustal structure.Results from our analyses yield insights into local intraplate deformation.
Apatite fission-track (AFT) and structural data outline the Late Cretaceous−Cenozoic history of the southern Tan-Lu fault zone (TLFZ), one of Asia's major faults, the Triassic–Jurassic Dabie orogen, ...Earth's largest track of ultrahigh-pressure rock exposure, and its foreland, the Yangtze foreland fold-thrust belt. The fission-track analyses utilized the independent (
φ-),
Z- and
ξ-methods for age determination, which yielded within error identical ages. Ages from Triassic–Jurassic syn-orogenic foreland sediments are younger than their depositional age and thus were reset. A group of ages records rapid cooling following shallow emplacement of granitoids of the widespread latest Jurassic−Early Cretaceous “Yanshanian” magmatism. Most ages are 90 to 55 Ma and document cooling following reheating at 110–90 Ma, the time when the basement units of the Dabie Shan were last at >200 °C. This cooling coincides with rifting marked by the Late Cretaceous−Eocene red-bed deposition in eastern China. During this period, the Dabie basements units exhumed in the footwall of the Tan-Lu fault with the Qianshan basin in the hanging wall; the associated stress field is transtensional (NW-trending principal extension direction). The youngest fission-track ages and temperature–time path modeling point to enhanced cooling in the footwall of the Tan-Lu and associated faults at 45±10 Ma. The related stress field is transtensional, with the principal extension direction changing trend from NW to W. It may be the far-field expression of the India–Asia collision superposed on the back-arc extension setting in eastern China. A regional unconformity at ∼25 Ma marks an upper bound for the inversion of the Late Cretaceous−Eocene rift structures. During the Neogene, further subsidence in the eastern China basins was accommodated by sub-horizontal NE–SW extension, and followed by the presently active NW–SE extension. The Tan-Lu fault along the eastern edge of the Dabie Shan had normal and then sinistral-transpressive motion during the Late Cretaceous−Eocene. Its motion changed during the Neogene from sinistral transtensive to normal and then to its present dextral transtensive activity.
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•Strike slip Fault systems at Northern margin of the Iranian Plateau experienced slip sense inversion during post Pliocene.•Post Pliocene kinematic change of the SCB is major driving ...force for the Slip sense inversion at northern Iranian Plateau margin.
Structural inversion of strike–slip faults in response to plate kinematic changes and interactions of large–scale basement blocks is a significant phenomenon in continental collision zones. In the Alborz Mountains, at the northern margin of the Iranian Plateau, Late Cenozoic kinematic of major fault systems changed in the late Pliocene from dextral to sinistral strike–slip. This kinematic change has been attributed to the clockwise rotation of the rigid South Caspian Block. However, the spatial extent and distribution of this kinematic change toward the south and into the Iranian Plateau is controversial. Here we present a detailed structural analysis of the West Saveh fault system located at the northwestern margin of the Iranian Plateau to unravel the regional distribution of this kinematic change. The West Saveh fault system is composed of the WNW–ESE trending Kushk–e Nosrat, Khalkhab, Saveh, and Nashveh faults. We identified several different sets of strike–slip related structures, which are classified into three categories based on their cross–cutting relationship and superimposition of kinematic indicators. We document dextral, dextral transpression and sinistral kinematics and their relative timing since Miocene time in the West Saveh fault system. Our structural analysis indicates recent slip sense inversion from dextral transpression to sinistral strike–slip, which possibly occurred in post Pliocene time. This kinematic change is regional and can be explained by the clockwise rotation of the South Caspian Block similarly to the kinematic change documented in the Alborz Mountains. We propose that the kinematic change due to South Caspian Block rotation is not restricted to the Alborz Mountains, but also affected the Iranian Plateau during post Pliocene time.
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