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On May 19, 2020, after several days of heavy precipitation, two earth dams failed (Edenville and Sanford) and two other dams were damaged (Secord and Smallwood), north of the town of Midland, MI. The failures resulted in about $100M in damages and the evacuation of about 11,000 people during the COVID-19 pandemic. In response to this destructive event, the Geo-Institute Embankment, Dams, and Slopes Technical Committee organized a team with the main objective of capturing perishable data. The Edenville Dam site was of interest because the slope failure was captured on video and occurred prior to the dam being overtopped. Based on our observations, investigation and analyses, two failure mechanisms were considered likely for Edenville Dam: (1) static liquefaction (similar to 2019 failure of Brumadinho Dam in Brazil);and (2) slope instability, where the slide debris which was contractive in nature, expelled pore water in the form of water jets and flowed as it became liquefied along the toe of the dam. © 2021 American Society of Civil Engineers (ASCE). All rights reserved.

The continental lithosphere is created and modified at convergent plate boundaries through complex, and interdependent, tectonic and magmatic processes. Evolution of the continental lithosphere modifies conditions within the mantle wedge, and therefore, the generation and composition of magmas at subduction zones. One approach to unraveling interactions between the continental lithosphere and asthenosphere in subduction zones is through the evolution of pre-, syn- and post-collisional mafic volcanism (i.e., MgO ≥ 6 wt%). Such lavas are most likely to contain characteristic geochemical fingerprints of the composition and physical condition of the mantle source at the time of melting. Here, we combine new analyses of thirty-one samples from the Hoh Xil Basin of the northern Tibetan Plateau with a compilation of published geochemical and geochronological data for late Mesozoic and Cenozoic magmas from the Indo-Eurasian collisional orogen. Geochronology of the Hoh Xil lavas reveals unrecognized early (∼27 Ma) alkali basaltic magmatism in northern Tibet that likely triggered melting of the mafic lower and metasedimentary upper continental crust, generating trachyanadesites-trachydacites and rhyolites, respectively. Based on a combined analysis of new and published compositional, age and spatial patterns of mafic lavas (MgO ≥ 6 wt%), we delineate two distinct periods of Tibetan mafic magmatism, with a major compositional change occurring at roughly 46 Ma. Early (pre-46 Ma) mafic lavas erupted only in central and southern Tibet, and were generated by melting within the mantle wedge during the northward subduction of the Neo-Tethyan oceanic slab. Late (post-46 Ma) mafic lavas were produced by the partial melting of metasomatized subcontinental lithospheric mantle. The latter group is spatially restricted to central Tibet between 46-30 Ma. After 30 Ma, lavas derived from melting of the lithospheric mantle expanded both northward and southward, eventually encompassing the entire Tibetan Plateau by 20 Ma. These observations suggest that melting of the lithospheric mantle beneath Tibet initiated earlier, and was more widespread, than previously thought. Our results also suggest that widespread melting of the mantle lithosphere across Tibet post-dates the cessation of upper crustal shortening in northern Tibet at ∼30 Ma, and precedes the initiation of crustal extension at ∼15-10 Ma. Hence, we suggest that plateau-wide melting of metasomatized subcontinental mantle lithosphere initiated following the collision of India with Eurasia, at ∼46 Ma. This melting was induced by detachment of the Neo-Tethyan slab concomitant with thinning of the continental mantle lithosphere. Such melting has continued until the present, with the exception of volcanic quiescence in south-central Tibet since ∼10 Ma. This quiescence likely arose from the underthrusting of Indian lithosphere beneath Eurasia. Although melting of the mantle lithosphere beneath Asia has previously been interpreted as an abrupt event, we propose that mantle lithosphere thinning and removal has been part of a continuum melting process during the Indo-Asian orogen that reflects an evolution from mantle wedge to mantle lithosphere melting accompanying the transition from subduction to collision.

Extreme events, such as earthquakes, hurricanes, or major storms, typically cause thousands of landslides in mountainous topography over the course of minutes to hours. In the last few years alone, we’ve witnessed more than 25,000 landslides during the 2015 Mw 7.8 Gorkha earthquake in Nepal, and more than 20,000 in the 2016 Mw 7.8 Kaikoura earthquake in New Zealand. The 2018 Mw 6.6 Hokkaido earthquake in Japan demonstrated the lethal combination of a rainy season followed by an earthquake, which led to significant parts of the landscape being scarred and stripped bare by adjacent landslides despite relatively flat topography.

We present field evidence and a kinematic study of a rock block mobilized in the Ponti area by a Mw = 6.5 earthquake near the island of Lefkada on 17 November 2015. A detailed survey was conducted using an unmanned aerial vehicle (UAV) with an ultrahigh definition (UHD) camera, which produced a high-resolution orthophoto and a digital terrain model (DTM). The sequence of impact marks from the rock trajectory on the ground surface was identified from the orthophoto and field verified. Earthquake characteristics were used to estimate the acceleration of the rock slope and the initial condition of the detached block. Using the impact points from the measured rockfall trajectory, an analytical reconstruction of the trajectory was undertaken, which led to insights on the coefficients of restitution (CORs). The measured trajectory was compared with modeled rockfall trajectories using recommended parameters. However, the actual trajectory could not be accurately predicted, revealing limitations of existing rockfall analysis software used in engineering practice.

Coseismic landslides pose immediate and prolonged hazards to mountainous communities, and provide a rare opportunity to study the effect of large earthquakes on erosion and sediment budgets. By mapping landslides using high-resolution satellite imagery, we find that the 25 April 2015 Mw7.8 Gorkha earthquake and aftershock sequence produced at least 25,000 landslides throughout the steep Himalayan Mountains in central Nepal. Despite early reports claiming lower than expected landslide activity, our results show that the total number, area, and volume of landslides associated with the Gorkha event are consistent with expectations, when compared to prior landslide-triggering earthquakes around the world. The extent of landsliding mimics the extent of fault rupture along the east-west trace of the Main Himalayan Thrust and increases eastward following the progression of rupture. In this event, maximum modeled Peak Ground Acceleration (PGA) and the steepest topographic slopes of the High Himalaya are not spatially coincident, so it is not surprising that landslide density correlates neither with PGA nor steepest slopes on their own. Instead, we find that the highest landslide density is located at the confluence of steep slopes, high mean annual precipitation, and proximity to the deepest part of the fault rupture from which 0.5–2 Hz seismic energy originated. We suggest that landslide density was determined by a combination of earthquake source characteristics, slope distributions, and the influence of precipitation on rock strength via weathering and changes in vegetation cover. Determining the relative contribution of each factor will require further modeling and better constrained seismic parameters, both of which are likely to be developed in the coming few years as post-event studies evolve. Landslide mobility, in terms of the ratio of runout distance to fall height, is comparable to small volume landslides in other settings, and landslide volume-runout scaling is consistent with compilations of data on larger slope failures. In general, the size ratios of landslide source area to full landslide area are smaller than global averages, and hillslope length seems to largely control runout distance, which we propose reflects a topographic control on landslide mobility in this setting. We find that landslide size dictates runout distance and that more than half of the landslide debris was deposited in direct connection with stream channels. Connectivity, which is defined as the spatial proximity of landslides to fluvial channels, is greatest for larger landslides in the high-relief part of the High Himalaya. Although these failures are less abundant than those at lower elevations, they may have a disproportionate impact on sediment dynamics and cascading hazards, such as landslide reactivation by monsoon rainfall and landslide dams that lead to outburst floods. The overall high fluvial connectivity of coseismic landsliding in the Gorkha event suggests coupling between the earthquake cycle and sediment/geochemical budgets of fluvial systems in the Himalaya., Geomorphology, 301, ISSN:0169-555x, ISSN:1872-695X

Thousands of landslides occurred during the April 2015 Gorkha earthquake in Nepal. Previous work using satellite imagery mapped nearly 25,000 coseismic landslides. In this study, the satellite-based mapping was analyzed in three areas where field deployment was also conducted—the Budhi Gandaki, Trishuli, and Indrawati river valleys—to better characterize the landslides. Unmanned aerial vehicles (UAVs) were deployed to map the three-dimensional (3-D) geometry of failed slopes using photogrammetry, as well as to characterize rock structure and strength. The majority of landslides were rock slides along the ridges and the steeper portions of the basins primarily involving the weathered rock zone. Additional landslides included rock falls and soil failures. Satellite imagery analysis indicated that landsliding was concentrated north of the physiographic transition, in steep areas, and in close proximity to the major rivers. The Trishuli area experienced the lowest landslide density in terms of number of landslides compared to the Budhi Gandaki and Indrawati areas, although all three areas had similar density in terms of total landslide area and other landslide statistics.

The 25 April 2015 M w 7.8 Gorkha earthquake produced strong ground motions across an approximately 250 km by 100 km swath in central Nepal. To assist disaster response activities, we modified an existing earthquake-triggered landslide model based on a Newmark sliding block analysis to estimate the extent and intensity of landsliding and landslide dam hazard. Landslide hazard maps were produced using Shuttle Radar Topography Mission (SRTM) digital topography, peak ground acceleration (PGA) information from the U.S. Geological Survey (USGS) ShakeMap program, and assumptions about the regional rock strength based on end-member values from previous studies. The instrumental record of seismicity in Nepal is poor, so PGA estimates were based on empirical Ground Motion Prediction Equations (GMPEs) constrained by teleseismic data and felt reports. We demonstrate a non-linear dependence of modeled landsliding on aggregate rock strength, where the number of landslides decreases exponentially with increasing rock strength. Model estimates are less sensitive to PGA at steep slopes (> 60°) compared to moderate slopes (30–60°). We compare forward model results to an inventory of landslides triggered by the Gorkha earthquake. We show that moderate rock strength inputs over estimate landsliding in regions beyond the main slip patch, which may in part be related to poorly constrained PGA estimates for this event at far distances from the source area. Directly above the main slip patch, however, the moderate strength model accurately estimates the total number of landslides within the resolution of the model (landslides ≥ 0.0162 km 2 ; observed n = 2214, modeled n = 2987), but the pattern of landsliding differs from observations. This discrepancy is likely due to the unaccounted for effects of variable material strength and local topographic amplification of strong ground motion, as well as other simplifying assumptions about source characteristics and their relationship to landsliding.

Combining new and published apatite (U–Th)/He and apatite ^4He/^3He data from along the Kings River canyon, California we rediscover a west-down normal fault on the western slope of the southern Sierra Nevada, one of a series of scarps initially described by Hake (1928) which we call the Western Sierra Fault System. Integrating field observations with apatite (U–Th)/He data, we infer a single fault trace 30 km long, and constrain the vertical offset across this fault to be roughly a kilometer. Thermal modeling of apatite ^4He/^3He data documents a pulse of footwall cooling near the fault and upstream in the footwall at circa 45–40 Ma, which we infer to be the timing of a kilometer-scale incision pulse resulting from the fault activity. In the context of published data from the subsurface of the Sacramento and San Joaquin Valleys, our data from the Western Sierra Fault System suggests an Eocene tectonic regime dominated by low-to-moderate magnitude extension, surface uplift, and internal structural deformation of the southern Sierra Nevada and proximal Great Valley forearc.

The timing and magnitude of deformation across the northern Tibetan Plateau are poorly constrained but feature prominently in geodynamic models of the plateau's evolution. The Fenghuoshan fold and thrust belt, located in the Hoh Xil Basin, provides a valuable record of the Cenozoic deformation history of the northern Tibetan Plateau. Here we integrate fault gouge geochronology, low-temperature thermochronology, geologic mapping, and a balanced cross section to resolve the deformation history of Hoh Xil Basin. Chronologic data suggest that deformation initiated in the mid-Eocene continued until at least 34 Ma and ceased by 27 Ma. The balanced cross section resolves 34 ± 12 km upper crustal shortening (24 ± 9%). We explore whether the observed Cenozoic shortening can account for the modern elevation and lithospheric thickness in the northern Tibetan Plateau. For a range of reasonable preshortening conditions, we conclude that the observed shortening alone cannot achieve modern crustal and mantle lithospheric thicknesses or modern elevation without either the removal of lithospheric mantle, the influx of lower crustal material, or some combination of these processes. Our results, along with previous studies, suggest that crustal shortening propagated into the northern Tibetan Plateau shortly after the onset of the Indo-Asian collision. The small magnitude of shortening and the late Oligocene cessation of deformation in the northern Tibetan Plateau raise questions of how and where the remaining Indo-Asian convergence was accommodated between Eocene to mid-Miocene time, prior to the approximately late Miocene establishment of the deformation patterns observed in the present day.

Estimation of landslide erosion rates and hazard prediction require a firm understanding of the physical controls on landslide size. In this study we seek to understand how the characteristics of different landscapes and forcing events influence the distribution of landslide size at a regional scale. We explore the parameter space of a mechanically-based landslide model through a series of simulations using digital elevation data. Consistent with previous studies, we find that large slope failures are infrequent due to the scarcity of large, steep hillslopes in typical mountainous topography. On the other hand, we find that the occurrence of small landslides is limited by the cohesive strength of hillslope material, which overcomes the weaker driving forces on short slopes. We test our model results with an empirical investigation of frequency-size distributions for eight real co-seismic landslide inventories. Although empirical data are noisy, we find a positive correlation between landslide size and hillslope relief, while the effects of PGA on landslide size are less pronounced. We conclude that landslide size distributions reflect the available distribution of hillslope geometries in a given landscape, and that external forcing (e.g., seismic ground motion) determines which subset of the hillslope distribution fails during a particular event. For a given landscape and forcing event, there is a particular length-scale over which landsliding is statistically favored, leading to the concept of characteristic landslide size distributions.

Earthquakes cause widespread landslides that can increase erosional fluxes observed over years to decades. However, the impact of earthquakes on denudation over the longer timescales relevant to orogenic evolution remains elusive. Here we assess erosion associated with earthquake-triggered landslides in the Longmen Shan range at the eastern margin of the Tibetan Plateau. We use the M w 7.9 2008 Wenchuan and M w 6.6 2013 Lushan earthquakes to evaluate how seismicity contributes to the erosional budget from short timescales (annual to decadal, as recorded by sediment fluxes) to long timescales (kyr to Myr, from cosmogenic nuclides and low temperature thermochronology). Over this wide range of timescales, the highest rates of denudation in the Longmen Shan coincide spatially with the region of most intense landsliding during the Wenchuan earthquake. Across sixteen gauged river catchments, sediment flux-derived denudation rates following the Wenchuan earthquake are closely correlated with seismic ground motion and the associated volume of Wenchuan-triggered landslides ( r 2 > 0.6 ), and to a lesser extent with the frequency of high intensity runoff events ( r 2 = 0.36 ). To assess whether earthquake-induced landsliding can contribute importantly to denudation over longer timescales, we model the total volume of landslides triggered by earthquakes of various magnitudes over multiple earthquake cycles. We combine models that predict the volumes of landslides triggered by earthquakes, calibrated against the Wenchuan and Lushan events, with an earthquake magnitude–frequency distribution. The long-term, landslide-sustained “seismic erosion rate” is similar in magnitude to regional long-term denudation rates (∼0.5–1 mm yr−1). The similar magnitude and spatial coincidence suggest that earthquake-triggered landslides are a primary mechanism of long-term denudation in the frontal Longmen Shan. We propose that the location and intensity of seismogenic faulting can contribute to focused denudation along a high-relief plateau margin.

The paper presents the field evidence and the kinematical study of the motion of a rock block mobilised by an earthquake-induced rockfall in Ponti area in the island of Lefkada during a Mw 6.5 earthquake on 17th November 2015. A detailed field survey was deployed using an Unmanned Aerial Vehicle (UAV) with an ultra-high definition (UHD) camera, which produced a high-resolution orthophoto and a Digital Surface Model (DSM) of the terrain. The sequence of impact marks from the rock trajectory on the ground surface was identified using the orthophoto and verified through a detailed field survey. Additionally, the earthquake characteristics were determined in order to define the acceleration on the rock slope and the initial conditions of the detached block. Using the impact points from the actual rockfall trajectory, an analytical approach to reconstruct the trajectory was implemented, which led to some insights on the coefficients of restitution. In order to match the actual trajectory, 2D and 3D rockfall analyses were performed using the recommended set of parameters. However, the actual trajectory could not be accurately predicted, revealing limitations of existing models.

The northeastern Tibetan Plateau constitutes a transitional region between the low-relief physiographic plateau to the south and the high-relief ranges of the Qilian Shan to the north. Cenozoic deformation across this margin of the plateau is associated with localized growth of fault-cored mountain ranges and associated basins. Herein, we combine detailed structural analysis of the geometry of range-bounding faults and deformation of foreland basin strata with geomorphic and exhumational records of erosion in hanging-wall ranges in order to investigate the magnitude, timing, and style of deformation along the two primary fault systems, the Qinghai Nan Shan and the Gonghe Nan Shan. Structural mapping shows that both ranges have developed above imbricate fans of listric thrust faults, which sole into decollements in the middle crust. Restoration of shortening along balanced cross sections suggests a minimum of 0.8–2.2 km and 5.1–6.9 km of shortening, respectively. Growth strata in the associated foreland basin record the onset of deformation on the two fault systems at ca. 6–10 Ma and ca. 7–10 Ma, respectively, and thus our analysis suggests late Cenozoic shortening rates of 0.2 +0.2/–0.1 km/m.y. and 0.7 +0.3/–0.2 km/m.y. along the north and south sides of Gonghe Basin. Along the Qinghai Nan Shan, these rates are similar to late Pleistocene slip rates of ∼0.10 ± 0.04 mm/yr, derived from restoration and dating of a deformed alluvial-fan surface. Collectively, our results imply that deformation along both flanks of the doubly vergent Qilian Shan–Nan Shan initiated by ca. 10 Ma and that subsequent shortening has been relatively steady since that time.

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.

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.

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.

The rate of India’s convergence with Eurasia has decreased exponentially since their collision, which suggests that the deforming continental lithosphere creates a type of viscous resistance that affects plate motion irrespective of how topography evolved. The forces that control plate motion are fundamental to plate-tectonic theory. It has generally been thought that deforming and thickening continental crust is responsible for slowing down the convergence between two continental plates. Marin Clark now shows, however, that the rate of penetration of the Indian subcontinent into Eurasia has decreased exponentially since the plates collided, which suggests that contractional strain across Tibet has been constant throughout the collision. She infers that this reflects deformation of the mantle lithosphere and that, under conditions of constant stress and strength, the deforming continental lithosphere creates viscous resistance that affects plate motion irrespective of how topography evolved. Because the inertia of tectonic plates is negligible, plate velocities result from the balance of forces acting at plate margins and along their base1. Observations of past plate motion derived from marine magnetic anomalies provide evidence of how continental deformation may contribute to plate driving forces2,3,4,5,6,7,8. A decrease in convergence rate at the inception of continental collision is expected because of the greater buoyancy of continental than oceanic lithosphere2,3, but post-collisional rates are less well understood. Slowing of convergence has generally been attributed to the development of high topography that further resists convergent motion7,8,9,10; however, the role of deforming continental mantle lithosphere on plate motions has not previously been considered. Here I show that the rate of India’s penetration into Eurasia has decreased exponentially since their collision. The exponential decrease in convergence rate suggests that contractional strain across Tibet has been constant throughout the collision at a rate of 7.03 × 10−16 s−1, which matches the current rate. A constant bulk strain rate of the orogen suggests that convergent motion is resisted by constant average stress (constant force) applied to a relatively uniform layer or interface at depth. This finding follows new evidence that the mantle lithosphere beneath Tibet is intact11, which supports the interpretation that the long-term strain history of Tibet reflects deformation of the mantle lithosphere. Under conditions of constant stress and strength, the deforming continental lithosphere creates a type of viscous resistance that affects plate motion irrespective of how topography evolved.

article i nfo Detrital low temperature thermochronometric data provides spatial and temporal information on catchment erosion, which is relevant to problems in climate, tectonics and geomorphology. However, direct inference of erosion rates from such data is not trivial and only the simplest inverse problems have been addressed previously. In this paper, we present a new approach that relies on the Bayesian interpretation of probability and uses a Markov chain Monte Carlo algorithm for inversion, which affords flexibility in the choice of specific model parametrization and transparent assessment of model uncertainty. We demonstrate how a single detrital sample sourced from a high relief catchment can constrain long-term (N10 6 years) changes in erosion rate that are in good agreement with published bedrock age-elevation profiles. Furthermore, we use detrital data to jointly invert for long-term exhumation history and spatial variability in short-term (b10 3 years) sediment supply, information relevant to many geomorphic studies. Where cooling histories are simple, we show that even small sample sizes (b20 grains) reliably estimate long-term rates of exhumation. We suggest that the presented approach to modeling detrital low-temperature thermochronometric data is both a powerful and efficient tool for solving tectonic and geomorphic problems.

Temporal variations in the orientation of Cenozoic range growth in northeastern Tibet define two modes by which India-Asia convergence was accommodated. Thermochronological age-elevation transects from the hanging walls of two major thrust-fault systems reveal diachronous Miocene exhumation of the Laji-Jishi Shan in northeastern Tibet. Whereas accelerated growth of the WNW-trending eastern Laji Shan began ca. 22 Ma, rapid growth of the adjacent, north-trending Jishi Shan did not commence until ca. 13 Ma. This change in thrust-fault orientation refl ects a Middle Miocene change in the kinematic style of plateau growth, from long-standing NNE-SSW contraction that mimicked the plate convergence direction to the inclusion of new structures accommodating east-west motion. This kinematic shift in northeastern Tibet coincides with expansion of the plateau margin in southeastern Tibet, the onset of normal faulting in central Tibet, and accelerated shortening in northern Tibet. Together these phenomena suggest a plateau-wide reorganization of deformation.

Models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary. Yet a growing body of evidence in the Tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen's history and thus our current mechanical understanding of orogenic plateau development is incomplete. New apatite (U–Th)/He ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures. Accelerated erosion is used as a proxy of fault timing, and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins. Helium results are synthesized with published geologic, thermochronometric, and sedimentologic data from which a growing picture of regional compressional deformation in Middle to Late Eocene time in northern Tibet emerges. We relate the early Cenozoic period of deformation to the initiation of collision between India and Eurasia, despite the fact that the plate boundary was located > 3000 km to the south. Regardless of whether or not high topography was built simultaneously as a result of this deformation, early Cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time.

Structural patterns, global positioning system (GPS) velocities, and Quaternary fault slip rates in northeastern Tibet indicate a transfer of left-lateral slip from the Kunlun fault northeast to the Haiyuan fault and minor crustal shortening and rotation within a 200-km-wide stepover zone. Related deformation also continues at least a few hundred kilometers north of the Haiyuan fault into a region of diffuse extensional(?) shear or rotation underlain by average thickness crust. Fast, localized slip along the central Kunlun fault transforms into distributed deformation across a 500-km-wide zone where the lower crust is weak. The distribution of fault-parallel GPS velocities across this region suggests a decrease in fault slip toward eastern fault tips and progressive dissipation of slip to the north rather than east of the Tibetan Plateau as previously suggested.

A common feature of convergent plate boundaries is the self-organization of strain, exhumation and topography along discrete, arcuate boundaries. Deviations from this geometry can represent first-order changes in stress applied at a plate boundary that must affect how strain is partitioned within the interior of an orogen. The simplicity of the Himalayan fold and thrust belt seen along its central portion breaks down along the eastern extremity of the arc where the 400 km-long Shillong Plateau has developed. This change in strain partitioning affects nearly 25% of the arc and has not previously been considered to be important to the orogen's development. New low-temperature thermochronometry data suggest this structure initiated in mid to late Miocene time, significantly earlier than was previously estimated from the sedimentary record alone. Development of the Shillong Plateau may be linked to a number of kinematic changes within the Himalayan and Burman collision zones that occur at the same time. These events include the onset of E–W extension in central Tibet, eastward expansion of high topography of the Tibetan Plateau, onset of rotation of crustal fragments in southeastern Tibet, and re-establishment of eastward subduction beneath the Indo-Burman ranges. We suggest that the coincidence of these tectonic events is related to the ‘dismemberment’ of the eastern Himalayan arc, signifying a change in regional stress applied along the India–Eurasia–Burma plate boundaries. Discrepancies between vertical long-term faulting rates and geodetically derived far-field convergence rates suggest that the collisional boundary in the eastern Himalayan system may be poorly coupled due to introduction of oceanic and transitional crust into the eastern plate boundary. The introduction of dense material into the plate boundary late in the orogen's history may explain regional changes in the strain field that affect not only the Himalaya, but also the deformation field more than 1000 km into the Tibetan Plateau.

Topographic change is one of the most informative measures of continental deformation, yet precise records of past elevation are scarce to non-existent in most orogenic belts. Recent advancement and new development of techniques to measure paleoelevation promise significant progress in understanding continental tectonic processes and how erosion acts on rising topography to shape the Earth’s surface. Outstanding issues in continental tectonics include understanding driving forces to deformation, how strain is vertically partitioned in the lithosphere, and how mass is redistributed or conserved in the lithosphere during deformation. Geodynamic and mechanical models that explore these topics make testable predictions of topographic evolution. This review explores key examples of how paleotopography relates to outstanding issues in continental tectonics that motivate development of robust, quantitative measures of paleoelevation.

Ocean Drilling Program sampling of the distal passive margin of South China at Sites 1147 and 1148 has yielded clay-rich hemipelagic sediments dating to 32 Ma (Oligocene), just prior to the onset of seafloor spreading in the South China Sea. The location of the drill sites offshore the Pearl River suggests that this river, or its predecessor, may have been the source of the sediment in the basin, which accounts for only ∼1.8% of the total Neogene sediment in the Asian marginal seas. A mean erosion depth of ∼1 km over the current Pearl River drainage basin is sufficient to account for the sediment volume on the margin. Two-dimensional backstripping of across-margin seismic profiles shows that sedimentation rates peaked during the middle Miocene (11–16 Ma) and the Pleistocene (since 1.8 Ma). Nd isotopic analysis of clays yielded ϵNd values of −7.7 to −11.0, consistent with the South China Block being the major source of sediment. More positive ϵNd values during and shortly after rifting compared to later sedimentation reflect preferential erosion at that time of more juvenile continental arc rocks exposed along the margin. As the drainage basin developed and erosion shifted from within the rift to the continental interior ϵNd values became more negative. A rapid change in the clay mineralogy from smectite-dominated to illite-dominated at ∼15.5 Ma, synchronous with middle Miocene rapid sedimentation, mostly reflects a change to a wetter, more erosive climate. Evidence that the elevation of the Tibetan Plateau and erosion in the western Himalaya both peaked close to this time supports the suggestion that the Asian monsoon became much more intense at that time, much earlier than the 8.5 Ma age commonly accepted.

Correlation of new multichannel seismic profiles across the upper Indus Fan and Murray Ridge with a dated industrial well on the Pakistan shelf demonstrates that ;40% of the Indus Fan predates the middle Miocene, and ;35% predates uplift of the Murray Ridge (early Miocene, ;22 Ma). The Arabian Sea, in addition to the Makran accretionary complex, was therefore an important repository of sediment from the Indus River system during the Paleogene. Channel and levee complexes are most pronounced after the early Miocene, coincident with an increase in sedimentation rates. Middle Eocene sandstones from Deep Sea Drilling Project Site 224 on the Owen Ridge yield K-feldspars whose Pb isotopic composition, measured by in situ ion microprobe methods, indicates an origin in, or north of, the Indus suture zone. This observation requires that India-Asia collision had occurred by this time and that an Indus River system, feeding material from the suture zone into the basin, was active soon after collision. Pleistocene provenance was similar to that during the Eocene, albeit with greater contribution from the Karakoram. A mass balance of the erosional record on land with deposition in the fan and associated basins suggests that only ;40% of the Neogene sediment in the fan is derived from the Indian plate.

Recent studies of the northeastern part of the Tibetan Plateau have called attention to two emerging views of how the Tibetan Plateau has grown. First, deformation in northern Tibet began essentially at the time of collision with India, not 10-20 Myr later as might be expected if the locus of activity migrated northward as India penetrated the rest of Eurasia. Thus, the north-south dimensions of the Tibetan Plateau were set mainly by differences in lithospheric strength, with strong lithosphere beneath India and the Tarim and Qaidam basins steadily encroaching on one another as the region between them, the present-day Tibetan Plateau, deformed, and its north-south dimension became narrower. Second, abundant evidence calls for acceleration of deformation, including the formation of new faults, in northeastern Tibet since ~15 Ma and a less precisely dated change in orientation of crustal shortening since ~20 Ma. This reorientation of crustal shortening and roughly concurrent outward growth of high terrain, which swings from NNE-SSW in northern Tibet to more NE-SW and even ENE-WSW in the easternmost part of northeastern Tibet, are likely to be, in part, a consequence of crustal thickening within the high Tibetan Plateau reaching a limit, and the locus of continued shortening then migrating to the northeastern and eastern flanks. These changes in rates and orientation also could result from removal of some or all mantle lithosphere and increased gravitational potential energy per unit area and from a weakening of crustal material so that it could flow in response to pressure gradients set by evolving differences in elevation. Key Points The north-south limits of Tibet were set by lateral variations in strength Roughly 15 million years ago, deformation of NE Tibet accelerated Since 20-15 million years ago, the orientation of shortening rotated eastward ©2013. American Geophysical Union. All Rights Reserved.

Field observations from Gonto La (southern Tibet), a pass through the high Himalaya, reveal a continuous, planar, ∼10°N-dipping detachment horizon (the Gonto La detachment). The detachment juxtaposes Tethyan dark slates over a footwall of extensive leucogranite of the Khula Kangri pluton, intruded into an injection complex layer regarded as an early Southern Tibet Detachment System (STDS) horizon. The leucogranite emplacement is protracted, and overlaps the STDS development. It is observed to intrude the earlier horizon of the STDS, which is deformed, partially cut by the pluton, and, in the southern part, rotated to a present south dip. Evidence for large scale folding and plutonism is also found east of Khula Kangri at Lhozag-La Kang, where the earlier STDS horizon is inferred. Here it excises the entire Palaeozoic Tethyan sedimentary sequence and is similarly folded. The Gonto La detachment, which cuts the Middle-Upper Miocene Khula Kangri pluton is, in turn, cut by the more steeply N-dipping Dzong Chu fault, demonstrating later N-S extension in this area. We interpret known outliers of Tethyan sequences in Bhutan as klippen, underlain by an early STDS horizon, which provide a more regional illustration of early extension on the STDS. In the Khula Kangri area, early extension was followed by plutonism and local relative uplift, and further N-S extension.

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.

New apatite (U-Th)/He from the northeastern margin of the Tibetan Plateau (north Qilian Shan) indicate rapid cooling began at ~10 Ma, which is attributed to the onset of faulting and topographic growth. Preservation of the paleo-PRZ in the hanging wall and growth strata in the footwall allow us to calculate vertical and horizontal fault slip rates averaged over the last 10 Myr of ~0.5 mm/yr and ~1 mm/yr respectively, which are within a factor of two consistent with Holocene slip rates and geodetic data. Low fault slip rates since the initiation of the northern Qilian Shan fault suggest that total horizontal offset did not exceed 10 km. Further, emergence of the northern Qilian Shan occurs during a period of increased aridity in northern Tibet but is associated with only a minor expansion of the northern plateau perimeter, which is well established near collision time. Outgrowth of the northern Qilian Shan at ~10 Ma could be simple propagation of the larger Qilian Shan system, occurring in response to decreased slip rates on the Altyn Tagh fault or as a result of the change in GPE of the central plateau.

[1] Crustal deformation along the present-day northern margin of the Tibetan Plateau has occurred since mid to late Eocene time, soon after India collided with Eurasia. Assuming that on these distance and time scales the lithosphere can be approximated using a thin viscous sheet, we show that far-field lithospheric deformation caused by an indenting boundary is expected to start shortly after collision begins. When a strong region analogous to cratonic lithosphere like that of the Tarim Basin is included in the calculations, shear and vertical strain rates concentrate along the southern edge of the strong region, resulting in enhanced deformation there. In calculations scaled to the dimensions of India and the Tibetan Plateau, shear and vertical strain rates on the northern edge of the plateau increase soon after collision begins. Crustal thickening accelerates later in the calculations. We propose that the apparent increased exhumation (and perhaps surface uplift) around 10 Ma may signal the arrival of the thickening front.