Conservation and redistribution of crust during the Indo‐Asian collision

We evaluate the mass balance of the Indo‐Asian orogen by reconstructing the Indian and Asian margins prior to collision using recently published paleomagnetic and surface shortening constraints, and subtracting modern crustal volumes derived from gravity inversions and deep seismic soundings. Results show a ~30% deficit between original and modern orogen volumes if the average global crustal thickness of 41 km is assumed prior to collision, even once eastward extrusion and crustal flow are considered. Such a large discrepancy requires crustal recycling of a magnitude that is greater than one half of the modern orogenic mass, as others have previously suggested. Proposals for extensive high elevations prior to or soon after the collision further exacerbate this mismatch and dramatically increase the volume of material necessary to be placed into the mantle. However, we show that this discrepancy can be eliminated with a 23–29 km thick crust within the orogen prior to collision along with a thick southern Tibet margin (the Lhasa and Qiangtang terranes). Because of the relatively low magnitude of surface shortening in Asia, an initially thin crust would require underplating of Indian crust in southern Tibet and displacement of a highly mobile lower crust to the north and east in order to explain modern crustal thicknesses. The contrast between a proposed thinner Asian interior and older and thicker lithosphere of the North China block may have defined the distal extent of deformation at the time of collision and since. Key Points Thick crust or high elevation in Asia prior to collision leads to mass imbalance A 23–29 km thick crust in India and Asia precollision eliminates mass imbalance Redistribution of mass at depth needed to reconcile shortening and convergencePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108001/1/tect20149.pd

Dynamic topography produced by lower crustal flow against rheological strength heterogeneities bordering the Tibetan Plateau

Dynamic stresses developed in the deep crust as a consequence of flow of weak lower crust may explain anomalously high topography and extensional structures localized along orogenic plateau margins. With lubrication equations commonly used to describe viscous flow in a thin-gap geometry, we model dynamic stresses associated with the obstruction of lower crustal channel flow due to rheological heterogeneity. Dynamic stresses depend on the mean velocity (Ū), viscosity (µ) and channel thickness (h), uniquely through the term µŪ/h^2. These stresses are then applied to the base of an elastic upper crust and the deflection of the elastic layer is computed to yield the predicted dynamic topography. We compare model calculations with observed topography of the eastern Tibetan Plateau margin where we interpret channel flow of the deep crust to be inhibited by the rigid Sichuan Basin. Model results suggest that as much 1500 m of dynamic topography across a region of several tens to a hundred kilometres wide may be produced for lower crustal material with a viscosity of 2 × 10^(18) Pa s flowing in a 15 km thick channel around a rigid cylindrical block at an average rate of 80 mm yr^(−1)

Far‐field lithospheric deformation in Tibet during continental collision

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95177/1/tect2049.pd

Sierra Nevada river incision from apatite ^4He/^3He thermochronometry

Published erosion rates suggest that acceleration of river incision beginning some time before 3 Ma initiated formation of the deep river canyons in the southern Sierra Nevada. Such acceleration signals a change in erosional efficacy but its initial timing is poorly constrained. Increased erosional efficacy caused by elevation gain is predicted by scenarios such as block faulting, mantle lithosphere removal, and passage of a slab window. The timing and magnitude of elevation gain may be used to distinguish between competing mechanisms. As in many landscapes, the small magnitude ( < 1.5 km) and antiquity of river incision in the Sierra Nevada make the timing of landscape evolution and its relation to tectonic scenarios inaccessible by most methods. Until recently, we have lacked the potential to 'see' erosional events that exhume leas than several kilometers and that occur over several to several tens of millions of years

Widespread late Cenozoic increase in erosion rates across the interior of eastern Tibet constrained by detrital low-temperature thermochronometry

New detrital low-temperature thermochronometry provides estimates of long-term erosion rates and the timing of initiation of river incision from across the interior of the Tibetan Plateau. We use the erosion history of this region to evaluate proposed models of orogenic development as well as regional climatic events. Erosion histories of the externally drained portion of the east-central Tibetan Plateau are recorded in modern river sands from major rivers across a transect that spans >750 km and covers a region with no published thermochronometric ages. Individual grains from eight catchments were analyzed for apatite (U-Th)/He and fission track thermochronometry. A wide distribution in ages that, in most cases, spans the entire Cenozoic and Late Mesozoic eras requires a long period of slow or no erosion with a relative increase in erosion rate toward the present. We apply a recently developed methodology for inversion of detrital thermochronometric data for three specified erosion scenarios: constant erosion rate, two-stage erosion history, and three-stage erosion history. Modeling results suggest that rates increase by at least an order of magnitude between 11 and 4 Ma following a period of slow erosion across the studied catchments. Synchroneity in accelerated erosion across the whole of the Tibetan Plateau rather than a spatial or temporal progression challenges the widely held notion that the plateau evolved as a steep, northward-propagating topographic front, or that south to north precipitation gradients exert a primary control on erosion rates. Instead, we suggest that accelerated river incision late in the orogen's history relates to regional-scale uplift that occurred in concert with eastern expansion of the plateau

A Cretaceous‐Eocene depositional age for the Fenghuoshan Group, Hoh Xil Basin: Implications for the tectonic evolution of the northern Tibet Plateau

The Fenghuoshan Group marks the initiation of terrestrial deposition in the Hoh Xil Basin and preserves the first evidence of uplift above sea level of northern Tibet. The depositional age of the Fenghuoshan Group is debated as are the stratigraphic relationships between the Fenghuoshan Group and other terrestrial sedimentary units in the Hoh Xil Basin. We present new radiometric dates and a compilation of published biostratigraphic data which are used to reinterpret existing magnetostratigraphic data from the Fenghuoshan Group. From these data, we infer an 85–51 Ma depositional age range for the Fenghuoshan Group. U‐Pb detrital zircon age spectra from this unit are compared to age spectra from Tibetan terranes and Mesozoic sedimentary sequences to determine a possible source terrane for Fenghuoshan Group strata. We propose that these strata were sourced from the Qiangtang Terrane and may share a common sediment source with Cretaceous sedimentary rocks in Nima Basin. Field relationships and compiled biostratigraphic data indicate that the Fenghuoshan and Tuotuohe Groups are temporally distinct units. We report late Oligocene ages for undeformed basalt flows that cap tilted Fenghuoshan Group strata. Together, our age constraints and field relationships imply exhumation of the central Qiangtang Terrane from the Late Cretaceous to earliest Eocene, followed by Eocene‐Oligocene deformation, and shortening of the northern Qiangtang and southern Songpan‐Ganzi terranes. Crustal shortening within the Hoh Xil Basin ceased by late Oligocene time as is evident from flat‐lying basaltic rocks, which cap older, deformed strata. Key Points The Fenghuoshan Group was deposited from late Cretaceous to early Eocene time The Fenghuoshan Group was likely sourced from the central Qiangtang Terrane Crustal shortening of the Hoh Xil Basin occurred from Eocene to Oligocene timePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/1/ts02.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/2/fs02.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/3/tect20113.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/4/ts06.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/5/fs06.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/6/ts03.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/7/fs03.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/8/ts07.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/9/fs07.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/10/fs04.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/11/ts04.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/12/fs01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/13/ts08.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/14/ts01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/15/fs05.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/106814/16/ts05.pd

Low-temperature thermochronometry along the Kunlun and Haiyuan Faults, NE Tibetan Plateau: Evidence for kinematic change during late-stage orogenesis

The Tibetan Plateau is a prime example of a collisional orogen with widespread strike-slip faults whose age and tectonic significance remain controversial. We present new low-temperature thermochronometry to date periods of exhumation associated with Kunlun and Haiyuan faulting, two major strike-slip faults within the northeastern margin of Tibet. Apatite and zircon (U-Th)/He and apatite fission-track ages, which record exhumation from ~2 to 6 km crustal depths, provide minimum bounds on fault timing. Results from Kunlun samples show increased exhumation rates along the western fault segment at circa 12–8 Ma with a possible earlier phase of motion from ~30–20 Ma, along the central fault segment at circa 20–15 Ma, and along the eastern fault segment at circa 8–5 Ma. Combined with previous studies, our results suggest that motion along the Haiyuan fault may have occurred as early as ~15 Ma along the western/central fault segment before initiating at least by 10–8 Ma along the eastern fault tip. We relate an ~250 km wide zone of transpressional shear to synchronous Kunlun and Haiyuan fault motion and suggest that the present-day configuration of active faults along the northeastern margin of Tibet was likely established since middle Miocene time. We interpret the onset of transpression to relate to the progressive confinement of Tibet against rigid crustal blocks to the north and expansion of crustal thickening to the east during the later stages of orogen development

Dynamic topography produced by lower crustal flow against rheological strength heterogeneities bordering the Tibetan Plateau

Dynamic stresses developed in the deep crust as a consequence of flow of weak lower crust may explain anomalously high topography and extensional structures localized along orogenic plateau margins. With lubrication equations commonly used to describe viscous flow in a thin-gap geometry, we model dynamic stresses associated with the obstruction of lower crustal channel flow due to rheological heterogeneity. Dynamic stresses depend on the mean velocity (Ū), viscosity (µ) and channel thickness (h), uniquely through the term µŪ/h^2. These stresses are then applied to the base of an elastic upper crust and the deflection of the elastic layer is computed to yield the predicted dynamic topography. We compare model calculations with observed topography of the eastern Tibetan Plateau margin where we interpret channel flow of the deep crust to be inhibited by the rigid Sichuan Basin. Model results suggest that as much 1500 m of dynamic topography across a region of several tens to a hundred kilometres wide may be produced for lower crustal material with a viscosity of 2 × 10^(18) Pa s flowing in a 15 km thick channel around a rigid cylindrical block at an average rate of 80 mm yr^(−1)

Strong variation in weathering of layered rock maintains hillslope‐scale strength under high precipitation

The evolution of volcanic landscapes and their landslide potential are both dependent upon the weathering of layered volcanic rock sequences. We characterize critical zone structure using shallow seismic Vp and Vs profiles and vertical exposures of rock across a basaltic climosequence on Kohala peninsula, Hawai’i, and exploit the dramatic gradient in mean annual precipitation (MAP) across the peninsula as a proxy for weathering intensity. Seismic velocity increases rapidly with depth and the velocity–depth gradient is uniform across three sites with 500–600 mm/yr MAP, where the transition to unaltered bedrock occurs at a depth of 4 to 10 m. In contrast, velocity increases with depth less rapidly at wetter sites, but this gradient remains constant across increasing MAP from 1000 to 3000 mm/yr and the transition to unaltered bedrock is near the maximum depth of investigation (15–25 m). In detail, the profiles of seismic velocity and of weathering at wet sites are nowhere monotonic functions of depth. The uniform average velocity gradient and the greater depths of low velocities may be explained by the averaging of velocities over intercalated highly weathered sites with less weathered layers at sites where MAP > 1000 mm/yr. Hence, the main effect of climate is not the progressive deepening of a near‐surface altered layer, but rather the rapid weathering of high permeability zones within rock subjected to precipitation greater than ~1000 mm/yr. Although weathering suggests mechanical weakening, the nearly horizontal orientation of alternating weathered and unweathered horizons with respect to topography also plays a role in the slope stability of these heterogeneous rock masses. We speculate that where steep, rapidly evolving hillslopes exist, the sub‐horizontal orientation of weak/strong horizons allows such sites to remain nearly as strong as their less weathered counterparts at drier sites, as is exemplified by the 50°–60° slopes maintained in the amphitheater canyons on the northwest flank of the island. Copyright © 2017 John Wiley & Sons, Ltd.Seismic velocity profiles across a basalt climosequence in Hawai’i reveal that above a particular precipitation threshold, rapid weathering of high‐permeability layers produces intercalated low‐velocity horizons and dramatically lowers the average seismic velocity of the rock section. However, less permeable layers remain relatively unweathered and thus still contribute significantly to the mechanical competence of the profile, which may explain maintenance of steep‐walled canyons under high precipitation rates. Such observations challenge a top‐down model of progressive weathering (i.e. weakening) of the substrate and therefore suggest that high strength can be maintained even under high precipitation rates, if horizontally layered horizons of different weathering potential exist.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143701/1/esp4290.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/143701/2/esp4290_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/143701/3/esp4290-sup-0001-SupplementaFiles_FINAL.pd

The Radio Sky at Meter Wavelengths: m-Mode Analysis Imaging with the Owens Valley Long Wavelength Array

A host of new low-frequency radio telescopes seek to measure the 21-cm transition of neutral hydrogen from the early universe. These telescopes have the potential to directly probe star and galaxy formation at redshifts $20 \gtrsim z \gtrsim 7$, but are limited by the dynamic range they can achieve against foreground sources of low-frequency radio emission. Consequently, there is a growing demand for modern, high-fidelity maps of the sky at frequencies below 200 MHz for use in foreground modeling and removal. We describe a new widefield imaging technique for drift-scanning interferometers, Tikhonov-regularized $m$-mode analysis imaging. This technique constructs images of the entire sky in a single synthesis imaging step with exact treatment of widefield effects. We describe how the CLEAN algorithm can be adapted to deconvolve maps generated by $m$-mode analysis imaging. We demonstrate Tikhonov-regularized $m$-mode analysis imaging using the Owens Valley Long Wavelength Array (OVRO-LWA) by generating 8 new maps of the sky north of $\delta=-30^\circ$ with 15 arcmin angular resolution, at frequencies evenly spaced between 36.528 MHz and 73.152 MHz, and $\sim$800 mJy/beam thermal noise. These maps are a 10-fold improvement in angular resolution over existing full-sky maps at comparable frequencies, which have angular resolutions $\ge 2^\circ$. Each map is constructed exclusively from interferometric observations and does not represent the globally averaged sky brightness. Future improvements will incorporate total power radiometry, improved thermal noise, and improved angular resolution -- due to the planned expansion of the OVRO-LWA to 2.6 km baselines. These maps serve as a first step on the path to the use of more sophisticated foreground filters in 21-cm cosmology incorporating the measured angular and frequency structure of all foreground contaminants.Comment: 27 pages, 18 figure