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It has long been hypothesized that climate can modify both the pattern and magnitude of erosion in mountainous landscapes, thereby controlling morphology, rates of deformation, and potentially modulating global carbon and nutrient cycles through weathering feedbacks. Although conceptually appealing, geologic evidence for a direct climatic control on erosion has remained ambiguous owing to a lack of high-resolution, long-term terrestrial records and suitable field sites. Here we provide direct terrestrial field evidence for long-term synchrony between erosion rates and Milankovitch-driven, 400-kyr eccentricity cycles using a Plio-Pleistocene cosmogenic radionuclide paleo-erosion rate record from the southern Central Andes. The observed climate-erosion coupling across multiple orbital cycles, when combined with results from the intermediate complexity climate model CLIMBER-2, are consistent with the hypothesis that relatively modest fluctuations in precipitation can cause synchronous and nonlinear responses in erosion rates as landscapes adjust to ever-evolving hydrologic boundary conditions imposed by oscillating climate regimes.

Understanding the evolution of continental deformation zones relies on quantifying spatial and temporal changes in deformation rates of tectonic structures. Along the eastern boundary of the Pamir-Tian Shan collision zone, we constrain secular variations of rock uplift rates for a series of five Quaternary detachment- and fault-related folds from their initiation to the modern day. When combined with GPS data, decomposition of interferometric synthetic aperture radar time series constrains the spatial pattern of surface and rock uplift on the folds deforming at decadal rates of 1–5 mm/yr. These data confirm the previously proposed basinward propagation of structures during the Quaternary. By fitting our geodetic rates and previously published geologic uplift rates with piecewise linear functions, we find that gradual rate changes over >100 kyr can explain the interferometric synthetic aperture radar observations where changes in average uplift rates are greater than ~1 mm/yr among different time intervals (~101, 104–5, and 105–6 years).

Although Earth's climate history is best known through marine records, the corresponding continental climatic conditions drive the evolution of terrestrial life. Continental conditions during the latest Miocene are of particular interest because global faunal turnover is roughly synchronous with a period of global glaciation from ∼6.2-5.5 Ma and with the Messinian Salinity Crisis from ∼6.0-5.3 Ma. Despite the climatic and ecological significance of this period, the continental climatic conditions associated with it remain unclear. We address this question using erosion rates of ancient watersheds to constrain Mio-Pliocene climatic conditions in the south-central Andes near 30° S. Our results show two slowdowns in erosion rate, one from ∼6.1-5.2 Ma and another from 3.6 to 3.3 Ma, which we attribute to periods of continental aridity. This view is supported by synchrony with other regional proxies for aridity and with the timing of glacial ‟cold" periods as recorded by marine proxies, such as the M2 isotope excursion. We thus conclude that aridity in the south-central Andes is associated with cold periods at high southern latitudes, perhaps due to a northward migration of the Southern Hemisphere westerlies, which disrupted the South American Low Level Jet that delivers moisture to southeastern South America. Colder glacial periods, and possibly associated reductions in atmospheric CO2, thus seem to be an important driver of Mio-Pliocene ecological transitions in the central Andes. Finally, this study demonstrates that paleo-erosion rates can be a powerful proxy for ancient continental climates that lie beyond the reach of most lacustrine and glacial archives.

Intermontane basins are illuminating stratigraphic archives of uplift, denudation and environmental conditions within the heart of actively growing mountain ranges. Commonly, however, it is difficult to determine from the sedimentary record of an individual basin whether basin formation, aggradation and dissection were controlled primarily by climatic, tectonic or lithological changes and whether these drivers were local or regional in nature. By comparing the onset of deposition, sediment-accumulation rates, incision, deformation, changes in fluvial connectivity and sediment provenance in two interrelated intermontane basins, we can identify diverse controls on basin evolution. Here, we focus on the Casa Grande basin and the adjacent Humahuaca basin along the eastern margin of the Puna Plateau in northwest Argentina. Underpinning this analysis is the robust temporal framework provided by U-Pb geochronology of multiple volcanic ashes and our new magnetostratigraphical record in the Humahuaca basin. Between 3.8 and 0.8 Ma, ~120 m of fluvial and lacustrine sediments accumulated in the Casa Grande basin as the rate of uplift of the Sierra Alta, the bounding range to its east, outpaced fluvial incision by the Río Yacoraite, which presently flows eastward across the range into the Humahuaca basin. Detrital zircon provenance analysis indicates a progressive loss of fluvial connectivity from the Casa Grande basin to the downstream Humahuaca basin between 3 and 2.1 Ma, resulting in the isolation of the Casa Grande basin from 2.1 Ma to

Geodetic and seismologic studies support a tectonic model for the central Himalaya wherein ~2 cm/yr of Indo-Asian convergence is accommodated along the primary d�collement under the range, the Main Himalayan thrust. A steeper midcrustal ramp in the Main Himalayan thrust is commonly invoked as driving rapid rock uplift along a range-parallel band in the Greater Himalaya. This tectonic model, developed primarily from studies in central Nepal, is commonly assumed to project along strike with little lateral variation in Main Himalayan thrust geometry or associated rock uplift patterns. Here, we synthesize multiple lines of evidence for a major discontinuity in the Main Himalayan thrust in western Nepal. Analysis of topography and seismicity indicates that west of ~82.5�E, the single band of steep topography and seismicity along the Main Himalayan thrust ramp in central Nepal bifurcates around a high-elevation, low-relief landscape, resulting in a two-step topographic front along an ~150 km segment of the central Himalaya. Although multiple models could explain this bifurcation, the full suite of data appears to be most consistent with a northward bend to the Main Himalayan thrust ramp and activation of a young duplex horse to the south. This poorly documented segmentation of the Main Himalayan thrust has important implications for the seismogenic potential of the western Nepal seismic gap and for models of the ongoing evolution of the orogen.

Fold scarps, a type of geomorphic scarp formed by folding mechanisms of hinge migration or limb rotation, serve to delineate both fault-bend characteristics and folding histories, which can, in turn, illuminate tectonic processes and seismic hazards associated with thrust systems. Because the subsurface geometry of folds is commonly difficult to determine, existing fold-scarp models, which rely on both the fold type and its causative fault geometries, remain uncertain with respect to the kinematic evolution of a given fold. In this paper, we develop a model to illustrate that, irrespective of specific fold type and subsurface geometries, fold-scarp growth in the mechanism of hinge migration can be successfully reconstructed based on analyses of bed geometry. This model reveals that the underlying bed dips and the ratio of hinge migration distance/hinge width control the fold-scarp shape and slope. During initial growth (ratio1), the slope reaches a maximum, which solely depends on underlying bed dips. The scarp height, however, is independent of the hinge width and can be used to quantify folding magnitude. Application of our model to fold scarps in the Mingyaole anticline in the southern foreland of Chinese Tian Shan indicates that the modeled fold-scarp geometry can roughly match with field observations. The Mingyaole shortening rate is estimated to be ≥5.0 mm/a since ~15ka, such that this single fold has accommodated about half of the regional convergence during the Holocene. Key Points Folding through curved hinges creates a distinctive fold-scarp geometry Scarp slope relies on ratio of hinge migration/width, but scarp height is not Shortening rate of the Mingyaole fold is estimated to be ~5.0 mm/a since ~15 ka, such that this single fold has accommodated about half of the regional convergence during the Holocene.

The flexural-slip fault (FSF), a type of secondary fault generated by bed-parallel slip, occurs commonly and plays an important role in accommodating fold growth. Although the kinematics and mechanics of FSFs are well studied, relatively few field observations or geometric models explore its geomorphic expression. In the Pamir-Tian Shan convergent zone, NW China, suites of well-preserved FSF scarps displace fluvial terraces in the Mingyaole and Wulagen folds. Integrating interpretations of Google Earth images, detailed geologic and geomorphic mapping, and differential GPS measurements of terrace surfaces, we summarize geomorphic features that typify these faults and create kinematic models of active flexural-slip faulting. Our study indicates the following: (i) FSF scarps commonly occur near synclinal hinges, irrespective of whether (a) the dip direction of beds on either side of the hinge is unidirectional or in opposite directions, (b) the hinge is migrating or fixed, or (c) the hinge shape is narrow and angular or wide and curved. (ii) Active FSFs are likely to produce higher scarps on steeper beds, whereas lower or no topographic scarps typify gentler beds. (iii) Tilt angles of the terrace surface displaced above FSFs progressively decrease farther away from the hinge, with abrupt changes in slope coinciding with FSF scarps; the changes in tilt angle and scarp height have a predictable geometric relationship. (iv) Active FSFs can accommodate a significant fraction of total slip and play a significant role in folding deformation. (v) Active FSFs may be used to assess seismic hazards associated with active folds and associated blind thrusts.

Piggyback basins on the margins of growing orogens commonly serve as sensitive recorders of the onset of thrust deformation and changes in source areas. The Bieertuokuoyi piggyback basin, located in the hanging wall of the Pamir Frontal Thrust, provides an unambiguous record of the outward growth of the northeast Pamir margin in northwest China from the Miocene through the Quaternary. To reconstruct the deformation along the margin, we synthesized structural mapping, stratigraphy, magnetostratigraphy, and cosmogenic burial dating of basin fill and growth strata. The Bieertuokuoyi basin records the initiation of the Pamir Frontal Thrust and the Takegai Thrust ~5-6 Ma, as well as clast provenance and paleocurrent changes resulting from the Pliocene-to-Recent uplift and exhumation of the Pamir to the south. Our results show that coeval deformation was accommodated on the major structures on the northeast Pamir margin throughout the Miocene to Recent. Furthermore, our data support a change in the regional kinematics around the Miocene-Pliocene boundary (~5-6 Ma). Rapid exhumation of NE Pamir extensional domes, coupled with cessation of the Kashgar-Yecheng Transfer System on the eastern margin of the Pamir, accelerated the outward propagation of the northeastern Pamir margin and the southward propagation of the Kashi-Atushi fold-and-thrust belt in the southern Tian Shan. This coeval deformation signifies the coupling of the Pamir and Tarim blocks and the transfer of shortening north to the Pamir frontal faults and across the quasi-rigid Tarim Basin to the southern Tian Shan Kashi-Atushi fold-and-thrust system.

In response to the Indo-Asian collision, deformation of the Tien Shan initiated at ∼25 Ma along the northwestern margin of the Tarim Basin. 300 km north, the Kyrgyz Range began deforming ∼15 Ma later. Although multiple intervening structures across the Tien Shan are currently active, the sequencing of initial deformation across the orogen's entire width remains poorly known. To determine whether deformation migrated sequentially northward or developed less predictably, we documented deformation patterns within the Naryn Basin in south-central Kyrgyzstan. Detailed mapping and a published balanced cross section across the Naryn Basin suggest that deep-seated, relatively steeply dipping thrust faults have disrupted the basin during late Cenozoic deformation. Dating of deformed fluvial terraces with ages between ∼10 and 250 ka constrains the rate of deformation across relatively young structures in the Tien Shan interior. Based on geodetic surveys of dated terraces, local rates of relative rock uplift span from 0.3 to 3.5 mm/yr. Folding rates and patterns are temporally persistent at a given site. Moreover, they mimic modern geodetic rates measured from interferometric synthetic aperture radar. Extrapolating these rates into the past suggests that structures within the interior of the Naryn Basin formed in the last 1 Myr, whereas the ranges surrounding the basin initiated at least 1-4 Myr earlier. Hence, within the Naryn Basin itself, deformation has migrated from margins to interior. Similarly, these new chronologies indicate that at least some deformation in the interior of the Tien Shan initiated millions of years later than along either orogenic margin. ©2014. American Geophysical Union. All Rights Reserved.

Landscape denudation in actively deforming mountain ranges is controlled by a combination of rock uplift and surface runoff induced by precipitation. Whereas the relative contribution of these factors is important to our understanding of the evolution of orogenic topography, no consensus currently exists concerning their respective infl uences. To address this question, denudation rates at centennial to millennial time scales were deduced from 10Be concentrations in detrital sediments derived from 30 small basins (10-600 km2) in an ~200-km-wide region in central Nepal. Along a northward, strike-perpendicular transect, average denudation rates sharply increase from

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.

Fold deformation in three dimensions involves shortening, uplift, and lateral growth. Fluvial terraces represent strain markers that have been widely applied to constrain a fold's shortening and uplift. For the lateral growth, however, the utility of fluvial terraces has been commonly ignored. Situated along northern margin of Chinese Pamir, the Mushi anticline preserves, along its northern flank, flights of passively deformed fluvial terraces that can be used to constrain three-dimensional folding history, especially lateral growth. The Mushi anticline is a geometrically simple fault-tip fold with a total shortening of 740 ± 110 m and rock uplift of ~1300 m. Geologic and geomorphic mapping and dGPS surveys reveal that terrace surfaces perpendicular to the fold's strike display increased rotation with age, implying the fold grows by progressive limb rotation. We use a pure-shear fault-tip fold model to estimate a uniform shortening rate of 1.5 + 1.3/-0.5 mm/a and a rock-uplift rate of 2.3 + 2.1/-0.8 mm/a. Parallel to the fold's strike, longitudinal profiles of terrace surfaces also display age-dependent increases in slopes. We present a new model to distinguish lateral growth mechanisms (lateral lengthening and/or rotation above a fixed tip). This model indicates that eastward lengthening of the Mushi anticline ceased by at least ~134 ka and its lateral growth has been dominated by rotation. Our study confirms that terrace deformation along a fold's strike not only can constrain the lateral lengthening rate but can serve to quantify the magnitude and rate of lateral rotation: attributes that are commonly difficult to define when relying on other geomorphic criteria. © 2013. American Geophysical Union. All Rights Reserved.

Whereas the existence of pronounced orbitally controlled periodicities is a major feature of Earth climate, its impact on landscape dynamics remains poorly understood. We use a Landscape Evolution Model (LEM) to systematically investigate the response of landscapes to a range of periodic oscillations in precipitation. The resulting sediment-flux evolution displays a pronounced sensitivity to the period of the input precipitation signal, such that, for a given erodibility, a specific periodicity maximizes the amplitude of the response. This optimal period of "resonance" scales as the inverse of the erodibility, but is progressively filtered out of the response when the intensity of hillslope diffusion increases. This frequency-dependent landscape behavior displayed by our model provides a mechanistic perspective on Molnar's (2004) proposition that ubiquitous changes in Late Cenozoic continental denudation could result directly from modifications in the spectral content of the climatic signal. © 2013. American Geophysical Union. All Rights Reserved.

Between June and September each year, the Indian monsoon typically delivers about 80% of the Nepalese Himalaya's annual precipitation. Topography on the windward (southern) flank of the range modulates persistent spatial variations in precipitation along the length of the range. Where topography is stepped with an initial abrupt rise as the Himalaya abut the foreland and a second rise as topography ascends toward the high peaks, two bands of high precipitation prevail, each where relief passes a threshold value. In contrast, more uniform, northward rising topography localizes a single high-rainfall band near the front of the range. A dense meteorological network that was operated in the Marsyandi catchment from 1999 to 2004 in central Nepal gives more insight on spatial variability in precipitation. Annual precipitation decreases ten-fold between the rainfall peak (of ∼4. m/yr) on the southern flank of the Himalaya and the semi-arid rain shadow on its northern flank (40-50. km farther north). Modest contrasts in rainfall between ridges versus valleys during the monsoon are replaced by strong altitude-dependent precipitation contrasts in the winter. Strikingly, above ∼4. km altitude, ∼40% of the total precipitation arrives as winter snowfall. Four years of daily discharge and suspended sediment measurements on the main-stem and on several tributaries of the Marsyandi during the monsoon document a strong north-south gradient in average erosion rates. Based on a suspended-to-bedload ratio of 2:1 (as estimated from grain-sizes in a landslide-dammed paleo-lake), erosion rates range from ∼0.1. mm/yr in the northern rain shadow to ∼2. mm/yr in the monsoon-drenched south. This strong modern spatial gradient in erosion rates mimics the precipitation gradient across the same area and broadly scales with specific discharge. In the wetter regions, nearly a meter of rain is required before significant sediment fluxes occur. After this initial meter of rain, the daily rainfall required to trigger sediment pulses (attributable to landsliding) gradually decreases during the remaining monsoon season from ∼40. mm to 10. mm. In the higher altitude rain shadow to the north, water discharge is more closely linked to temperature than to precipitation: a linkage suggesting that melting of snow and ice, rather than rainfall, modulates the runoff. The sediment flux in the rain shadow during the monsoon season displays a marked temporal hysteresis: fluxes broadly scale with discharge during the first two months of the monsoon, but decouple from discharge later in the monsoon. This behavior suggests that the sediment flux is supply limited. We interpret that much of the sediment is subglacially derived and that its transport into the river network is restricted either by limited bedrock erosion or subglacial hydrology. © 2012 Académie des sciences.

Across contractional orogens, the equivalency between decadal convergence rates from geodetic GPS data and geologic shortening rates at time scales of thousands or millions of years has rarely been documented. Here, we present an example from the northern margin of Chinese Pamir, where the Main Pamir Thrust is tectonically quiescent, and recent deformation is concentrated on the Pamir Frontal Thrust (PFT). Based on dated and faulted fluvial terraces, magnetostratigraphy, and mapping, the horizontal shortening rate of the PFT is ∼6-7 mm/a at time scales of both ∼18.4 ka and ∼0.35 Ma, comparable to the geodetic rate of ∼6-9 mm/a across the same zone, implying that modern geodetic rates are a reasonable proxy for geologic rates since ∼0.35 Ma. Comparing this example with studies in other contractional orogens, we conjecture that a match or mismatch of geologic-geodetic rates typically depends on the time scale of observation, fault geometry, and fault mechanics. © 2012 by the American Geophysical Union.

Two of the most popular mechanisms for thickening the crust beneath the Tibetan Plateau are (1) pure shear with faulting and folding in the upper crust and horizontal shortening below and (2) flow of lower or middle crust without significant shortening of the upper crust. To help discriminate between the relative contributions of these two mechanisms, well-constrained estimates of upper crustal shortening are needed. Here we document the Cenozoic shortening budget across the northeastern Tibetan Plateau margin with several balanced cross sections that exploit thermochronological and magnetostratigraphic constraints. These sections indicate 11 +2/-1% east-west shortening since middle Miocene time and ∼9 +2/-3% NNE-SSW shortening between middle Eocene and middle Miocene times with little subsequent shortening of this orientation. Shortening rates accelerate fivefold after middle Miocene time. Given the present-day crustal thickness of 56 4 km in northeastern Tibet, crustal restorations that remove Cenozoic shortening suggest that the northeastern Tibetan crust was 45±5 km thick prior to the India-Asia continental collision. This precollision thickness estimate is equivalent to average continental crustal thicknesses both adjacent to the Tibetan Plateau (44±4 km) and globally (41±6 km) and suggests that pure shear alone may account for Cenozoic crustal thickening in northeastern Tibet. In contrast to eastern Tibet where, in the absence of significant shortening structures, crustal flow has been invoked to explain the addition of crustal material since middle Miocene time, our results may obviate lower crustal flow as a necessary crustal thickening agent in northeastern Tibet. Copyright © 2012 by the American Geophysical Union.

Sedimentary rocks in Tibetan Plateau basins archive the spatiotemporal patterns of deformation, erosion, and associated climate change that resulted from Cenozoic continental collision. Despite growing understanding of basin development in northeastern Tibet during initial India-Asia collision, as well as in the late Miocene-Holocene, surprisingly little is known about the intervening period: a time when the plateau may have undergone fundamental tectonic changes. To fill this gap, we present new magnetostratigraphy from a >2300-m-thick fluviolacustrine succession that spans ca. 30-9.3 Ma. An integrated analysis of sedimentology, subsidence, and provenance from this section reveals the sequential, pulsed erosion of multiple ranges bordering the Xunhua Basin. Emergence of the WNW-trending Laji Shan is highlighted by a doubling of sediment accumulation rates between 24 and 21 Ma and a transition to coarse allu vial facies at 20.3 Ma. Detrital zircon U/Pb age spectra show that these coarse sediments came from basement terranes within the Laji Shan. Together these observations suggest accelerated growth of the Laji Shan and its coupled foreland basin at ca. 22 Ma. The most rapid accumulation rates in Xunhua Basin occur within the finest-grained strata and suggest an underfilled basin during the fastest interval of Laji Shan deformation. Growth of the Laji Shan occurred northward of the contemporaneous plateau margin, which had been defined since ca. 45-50 Ma by the West Qinling, lying ~60 km farther south. Hence, following ~20-25 m.y. of apparent stability, the deformation front in this region jumped ~60 km to the north at ca. 22 Ma. Subsequently, growth of the north-trending Jishi Shan occurred at ca. 13 Ma and is highlighted by an acceleration in Xunhua Basin accumulation rates between 12 and 9 Ma, as well as by a significant change in detrital zircon provenance of nearby Linxia Basin deposits by 11.5 Ma. Initial growth of the WNW-trending Laji Shan in the early Miocene and subsequent growth of the north-trending Jishi Shan ~10 m.y. later support interpretations of a middle Mio cene kinematic reorganization in northeastern Tibet. © 2012 Geological Society of America.

The past decade has seen a rapid increase in the application of high-resolution imagery and geographic-based information systems across every segment of society from security intelligence to product marketing to scientific research. Google Earth has positioned itself at the forefront of this spatial information wave by providing free access to high-resolution imagery through a simple, user-friendly interface. Whereas Google Earth imagery has been widely exploited across the earth sciences for spatial visualization, education, and place-based searches, few studies have utilized the highresolution imagery to yield quantitative insights about the processes and mechanisms acting at the earth's surface. In this paper, we detail the benefits of the underlying high-resolution imagery available within Google Earth, review the limited published research to date, and utilize this imagery to quantitatively illuminate previously difficult and unresolved questions within the discipline of geomorphology involving: (1) channel-width variability and scaling relations in the tectonically active Himalaya; (2) landslide characteristics related to large magnitude climatic and tectonic events in Haiti; and (3) identification and quantification of laterally offset geomorphic features within eastern California. In each example, we compare analyses using freely available Google Earth imagery with standard imagery and techniques (e.g., Landsat, ASTER, lidar) to demonstrate the potential benefits of using high-spatial resolution Google Earth imagery over established methodologies. In addition, we discuss the potential limitations and problems with using the imagery currently available in Google Earth and propose favorable future applications, namely studies in remote terrains and those requiring high-resolution imagery across a large spatial extent, where purchasing such imagery in an academic environment would be costprohibitive. Whether as a supplement, for reconnaissance, or as the primary data set, high-resolution Google Earth imagery, when properly applied, holds great promise for quantitatively tackling previously unresolved problems in the study of earth surface processes. © 2012 The Geological Society of America. All rights reserved.

Differences in the timing of glacial advances, which are commonly attributed to climatic changes, can be due to variations in valley topography. Cosmogenic 10Be dates from 24 glacial moraine boulders in 5 valleys define two age populations, late-glacial and early Holocene. Moraine ages correlate with paleoglacier valley hypsometries. Moraines in valleys with lower maximum altitudes date to the late-glacial, whereas those in valleys with higher maximum altitudes are early Holocene. Two valleys with similar equilibrium-line altitudes (ELAs), but contrasting ages, are

In Central Asia's Tien Shan, deformation is distributed across the wide orogen, a characteristic of intracontinental mountain building. Active faults are commonly found within intramontane basins that separate its constituent ranges. In order to explore the controls on this intramontane basin deformation, we study the Naryn Basin of south-central Kyrgyzstan. A series of five balanced cross-sections reveals a transition in patterns of faulting from faults confined to basin margins to faults focused within the basin center. The 20-km-wide eastern Naryn Basin displays deformation attributed to low-angle splays of the northern, basin-bounding fault. In the 40-km-wide western Naryn Basin, the pattern of deformation linked to the northern range remains, but is accompanied by steeper faults that dip both south and north without being directly linked to the basin-bounding fault. We compare these cross-sections to synthetic aperture radar interferometry (InSAR) measurements of surface deformation. Profiles of InSAR-derived surface deformation rates across the Naryn Basin reveal that in the west, deformation is distributed across the broad basin interior, whereas in the east, rapid uplift is concentrated at the margin of the narrower basin. From the geodetic and structural data, we infer that in the western Naryn Basin, deformation has migrated away from the northern basin margin and into the interior. Deformation of the eastern basin interior, however, remains linked to the basin-bounding fault. A simple mechanical model demonstrates that basin width may control basin deformation whereby basin-interior faulting in the narrow, eastern Naryn Basin is inhibited by the overburden of adjacent ranges. Copyright 2011 by the American Geophysical Union.

As soon as a fold begins to grow above base level, it becomes subject to erosion. A numerical model of simultaneous fold growth and river erosion captures the early stages of channel formation and delineates the evolution of longitudinal channel profiles. This numerical exploration suggests that the patterns of channel incision into growing folds can be used to guide kinematic interpretations of fold growth. Four types of fold limb growth are modeled: limb lengthening through (1) the fold crest or (2) the fold toe, (3) limb rotation, and (4) curved fold limbs due to trishear folding. Initially, the pattern of channel incision is sensitive to the kinematics of surface deformation. Subsequently, more mature streams evolve toward similar incision patterns with deepest incision near the midpoint of the longitudinal channel profile. We compare these model predictions to Rough Ridge on New Zealand's South Island, where high-resolution topographic data reveal the multitude of channels incised into the flanks of these folds. Because the opposing limbs of these folds have followed distinct deformation pathways, the early sensitivity of channel incision to deformation pathway can be examined. Furthermore, the lateral propagation of the fold through time allows for an approximate space-for-time substitution, such that the evolution of these incision patterns can be examined from early through more mature stages. This analysis indicates that the west limb of Rough Ridge has grown by limb lengthening through the toe and the east limb has grown primarily by lengthening through the crest. Copyright 2011 by the American Geophysical Union.

The role of bedrock fractures and rock mass strength is often considered a primary influence on the efficiency of surface processes and the morphology of landscapes. Quantifying bedrock characteristics at hillslope scales, however, has proven difficult. Here, we present a new field-based method for quantifying the depth and apparent density of bedrock fractures within the shallow subsurface based on seismic refraction surveys. We examine variations in subsurface fracture patterns in both Fiordland and the Southern Alps of New Zealand to better constrain the influence of bedrock properties in governing rates and patterns of landslides, as well as the morphology of threshold landscapes. We argue that intense tectonic deformation produces uniform bedrock fracturing with depth, whereas geomorphic processes produce strong fracture gradients focused within the shallow subsurface. Additionally, we argue that hillslope strength and stability are functions of both the intact rock strength and the density of bedrock fractures, such that for a given intact rock strength, a threshold fracture-density exists that delineates between stable and unstable rock masses. In the Southern Alps, tectonic forces have pervasively fractured intrinsically weak rock to the verge of instability, such that the entire rock mass is susceptible to failure and landslides can potentially extend to great depths. Conversely, in Fiordland, tectonic fracturing of the strong intact rock has produced fracture densities less than the regional stability threshold. Therefore, bedrock failure in Fiordland generally occurs only after geomorphic fracturing has further reduced the rock mass strength. This dependence on geomorphic fracturing limits the depths of bedrock landslides to within this geomorphically weakened zone. Copyright 2011 by the American Geophysical Union.

Bedrock fracturing and rock strength are widely believed to influence landscape morphology and erosional resistance. Yet, understanding of the quantitative relationship between rock-mass strength and landscape evolution remains limited. Here we present a new application of seismic refraction surveys that uses variations in seismic velocity to interpret differences in bedrock fracture density with depth. We use a comparative study of Fiordland and the western Southern Alps of New Zealand to examine how differences in rock type and bedrock fracturing influence landscape morphology and landslide response to rock uplift. In both regions, slopes appear invariant with differential rock-uplift rates and slope distributions reveal modal hillslope angles of ~. 32° The majority of landslides initiate on slopes steeper than the modal hillslope angle, however, landslide magnitude-frequency distributions reveal order-of-magnitude differences between the regions, with Fiordland experiencing considerably smaller and less frequent landsliding events. Landslide-driven denudation rates of ~. 9. mm/yr in the western Southern Alps and between ~. 0.1 and 0.3. mm/yr in Fiordland approximate estimates of long-term rock-uplift rates for each region. The invariance of hillslope angles, near-normal slope distributions, predominance of landslide initiation on slopes steeper than modal values, and the apparent balance between rates of uplift and landslide-driven erosion suggest that hillslopes in both regions are at threshold angles. Their similar modal slopes further suggest that both ranges are characterized by equivalent rock-mass strength, despite striking differences in lithology. Additionally, our seismic analysis reveals nearly identical surface p-wave velocities. The unexpected equivalence of both modal slopes and surface velocities between these lithologically distinct ranges is attributed to contrasting degrees of surface fracturing that have differentially affected the intact rock properties, such that they now yield equivalent surface velocities and hillslope-scale strengths. Given that surface fractures help regulate threshold angles by modulating hillslope strength; we propose that shallow seismic velocities may provide a quantitative proxy for rock-mass strength. We define two contrasting fracture and landsliding environments. In Fiordland, dense geomorphic fracturing that is focused within the shallow subsurface appears to limit the depth and magnitude of most bedrock landslides. Conversely, in the western Southern Alps, tectonic forces produce pervasive fracturing with depth that results in larger, and deeper landslides. Our data suggest that bedrock fracturing at the Earth's surface modulates threshold hillslope angles, whereas the depth of bedrock fracturing influences the magnitude and frequency of landslide response to tectonic rock uplift. © 2010 Elsevier B.V.

[1] Rarely are geologic records available to constrain the spatial and temporal evolution of thrust-fault growth as slip accumulates during repeated earthquake events. Here, we utilize multiple generations of dated and deformed fluvial terraces to explore two key aspects of the along-strike kinematic development of the Ostler fault zone in southern New Zealand over the past ~100 k.y.: accumulation of fault slip through space and time and fixed-length thrust growth that results in patterns of drainage diversion suggestive of laterally propagating faults. Along the Ostler fault, surface deformation patterns revealed by topographic surveying of terrace profiles in nine transverse drainages define systematic variations in fault geometry and suggest deformation over both listric and planar thrust ramps. Kinematic modeling of folded terrace profiles and >100 fault-scarp surveys along major fault sections reveals remarkably similar slip distributions for multiple successions of geomorphic surfaces spanning ~100 k.y. Spatially abrupt and temporally sustained displacement gradients across zones of fault section overlap suggest that either persistent barriers to fault propagation or interference between overlapping faults dominate the interactions of fault tips from the scale of individual scarps to the entire fault zone. Deformed terrace surfaces dated using optically stimulated luminescence and cosmogenic radionuclides indicate steady, maximum rates of fault slip of ~1.9 mm/yr during the Late Quaternary. Slip data synthesized along the central Ostler fault zone imply that displacement accumulated at approximately constant fault lengths over the past ~100 k.y. A northward temporal progression of abandoned wind gaps along this section thus reflects lateral tilting in response to amplification of displacement, rather than simple fault lengthening or lateral propagation. Oscillations of chmate at ~[10.sup.4]-yr time scales modulate the formation and incision of geomorphic surfaces during successive glacial stages. Superimposed on apparently steadier rates of fault slip, such climate-dependent surfaces contribute to a pattern of progressive drainage deflection along the central Ostler fault zone that is largely independent of fault propagation. Citation: Amos, C. B., D. W. Burbank, and S. A. L. Read (2010), Along-strike growth of the Ostler fault, New Zealand: Consequences for drainage deflection above active thrusts, Tectonics, 29, TC4021, doi: 10.1029/2009TC002613.

In response to the ongoing Indo-Asian collision, structural deformation has encroached into the Tian Shan foreland in western China since the early Miocene. In order to reconstruct a detailed history of foreland deformation along the southern margin of the Tian Shan, we have synthesized extensive mapping, analysis of seismic sections, magnetostratigraphy of the foreland fill and associated growth strata, apatite fission track dating, changes in sediment accumulation rates, and geodetic surveys of deformed fluvial terraces. Across an area exceeding 6500 [km.sup.2] that spans the Kashi foreland in the northwestern Tarim Basin, we use these data to place temporal constraints on individual structures that delineate at least 4 distinct stages of deformation. Stage 1 involved initial uplift of hinterland structures beginning at 20-25 Ma. Stage 2 began at ~16.3 Ma when the basement-involved deformation front stepped south to the Kashi Basin Thrust that bounds the foredeep strata. Stage 3 occurred from 13.5 to 4 Ma, as the deformation front migrated south in episodic steps above thin-skinned detachment and ramp-flat folding. Finally, stage 4 began after 4 Ma with growth of the Keketamu Anticline followed by rapid southward migration of the deformation front to the Atushi and then Kashi detachment anticlines. On the basis of the deformed Tertiary strata and available seismic data, calculations of the minimum magnitude of shortening yield totals ranging from 10 to 32 km since the early Miocene, of which 7-12 km occurred since ~4 Ma. Although average shortening rates since 16.3 Ma were 0.8-1.3 mm/a, rates were faster from 16.3 to 13.5 Ma (1.1-3.2 ram/a), after which they decreased to 0.5 mm/a between 13.5 and 4 Ma before increasing to 2.2-2.7 mm/a after 4 Ma. Similarly, the rate of southward propagation of the deformation front was initially 3-7 mm/a for the first 10 Ma of basin development, slowed to 1.5 mm/a between 13.5 and 4 Ma, and increased to more than 10 mm/a since 4 Ma. These changes in shortening and structural migration rates over time suggest that the loci of orogenic deformation shifted in and out of the foreland during the Neogene. Surveyed stream terraces suggest that, although most current deformation is located along the southern front of the foreland, modest deformation and shortening continue in the hinterland. Structural style within the foreland has also changed over time from imbricate thrust faults with hanging wall folds along the northern margin, to ramp-flat faults and detachment folds along the southern margin. Where the basement rocks are previously deformed, such as in the hinterland of the Kashi foreland, thick-skinned fault ramps and related fault propagation folds dominate the structural style. As syntectonic sedimentation within the foreland produced thick, subhorizontal stratigraphy, the structural style evolved to detachment and fault bend folding above these footwall flats within the Neogene strata.

Although detrital zircon has proven to be a powerful tool for determining provenance, past work has focused primarily on delimiting regional source terranes. Here we explore the limits of spatial resolution and stratigraphic sensitivity of detrital zircon in ascertaining provenance, and we demonstrate its ability to detect source changes for terranes separated by only a few tens of kilometers. For such an analysis to succeed for a given mountain, discrete intrarange source terranes must have unique U/Pb zircon age signatures and sediments eroded from the range must have well-defined depositional ages. Here we use ~1400 single-grain U/Pb zircon ages from northeastern Tibet to identify and analyze an area that satisfies these conditions. This analysis shows that the edges of intermontane basins are stratigraphically sensitive to discrete, punctuated changes in local source terranes. By tracking eroding rock units chronologically through the stratigraphic record, this sensitivity permits the detection of the differential rock uplift and progressive erosion that began ca. 8 Ma in the Laji Shan, a 10-25-km-wide range in northeastern Tibet with a unique U/Pb age signature. Keywords: detrital zircon, Tibet, provenance, unroofing, Miocene, U/Pb.

Despite the abundance in alpine terrain of glacially dissected landscapes, the magnitude and geometry of glacial erosion can rarely be defined. In the eastern Kyrgyz Range of central Asia, a widespread unconformity exhumed as a geomorphic surface provides a regional datum with which to calibrate erosion. As tectonically driven surface uplift has progressively pushed this surface into the zone of ice accumulation, glacial erosion has overprinted the landscape. With as little as 500 m of incision into rocks underlying the unconformity, distinctive glacial valleys display their deepest incision adjacent to cirque headwalls. The expansion of north-facing glacial cirques at the expense of south-facing valleys has driven the drainage divide southward at rates of as much as two to three times the rate of valley incision. Glacial erosion rules based on ice flux incompletely explain this dominance of cirque retreat over valley incision. Local processes that either directly sap cirque headwalls or inhibit erosion down-glacier appear to control, at least initially, alpine landscape evolution. Keywords: glacial erosion, cirque retreat, Kyrgyz Range, Tien Shan.

Comparison of rainfall data and mean hillslope angles in the Himalayas of central Nepal shows that mean hillslope angles decrease with increasing mean annual rainfall. Higher pore pressures and higher rates of chemical weathering in the wetter regions may decrease the threshold angle of hillslopes prone to landsliding. When valley spacing is held constant, the sensitivity of mean hillslope angle to climate implies that relief, in the absence of limits due to rock strength, is also dependent on climate. These results suggest that wet-to-dry climatic changes increase relief in regions with incising bedrock channels and that dry-to-wet climatic changes reduce relief. Keywords: threshold hillslopes, climatic geomorphology, relief, Himalayas, Nepal, landslides.

The steep-walled bedrock gorges of the Greater Himalayan rivers currently lack significant stored sediment, suggesting that fluvial erosion and transport capacity outpace the supply of sediment from adjacent hillsides. Despite this appearance of sustained down-cutting, such rivers can become choked with sediments and aggrade during intervals of higher precipitation. Cosmogenic dating ([sup.10]Be and [sup.26]Al) of fluvially carved bedrock surfaces indicates that sediment at least 80 m thick filled the Marsyandi River valley in central Nepal during a time of strengthened early Holocene monsoons. Despite threefold differences in height (43-124 m) above the modern river, these fluvial surfaces display strikingly similar cosmogenic exposure ages clustering around 7 [+ or -] 1 ka. We speculate that enhanced monsoonal precipitation increased pore pressure and the frequency of landsliding, thereby generating a pulse of hillslope-derived sediment that temporarily overwhelmed this alpine fluvial system's transport capacity. After the easily liberated material was exhausted ca. 7 ka, the hillslope flux dropped, and the river incised through the aggraded alluvium. It concurrently eroded adjacent rock walls, thereby removing previously accumulated [sup.10]Be and [sup.26]Al and resetting the cosmogenic clock in the bedrock. Unlike previous studies, these exposure ages cannot be used to derive river-incision rates; instead they record a coupled fluvial-hillslope response to climate change. Keywords: cosmogenic dating, aggradation, bedrock incision, precipitation, hillslope stability, Marsyandi River.

Although the large-scale stratigraphy of many terrestrial foreland basins is punctuated by major episodes of gravel progradation, the relationships of such facies to hinterland tectonism and climate change are often unclear. Structural reentrants provide windows into older and more proximal parts of the foreland than are usually exposed, and thus provide key insights to earlier phases of foreland evolution. Our magnetostratigraphic studies show that, although the major lithofacies preserved within the Himachal Pradesh structural reentrant in northwestern India resemble Neogene facies in Pakistan, they have a much greater temporal and spatial variability. From 11.5 to 7 Ma, major facies boundaries in Himachal Pradesh vary by as much as 2-3 m.y. across distances of 20-30 km and are controlled by the interference between a major southeastward-flowing axial river and a major southwestward-flowing transverse river. A thick but highly confined middle to late Miocene conglomerate facies includes the oldest extensive Siwalik conglomerates yet dated (10 Ma) and implies the development of significant erosional topography along the Main Boundary thrust prior to 11 Ma. Our studies document extensive syntectonic gravel progradation with conglomerates extending tens of kilometers into the undeformed foreland during a period of increased subsidence rate and within 1-2 m.y. of major thrust initiation. Overall, gravel progradation is modulated by the interplay among subsidence, sediment supply, and the proportion of gravels in rivers entering the foreland. Keywords: fold-and-thrust belt, basin analysis, Siwaliks, Himalaya, magnetostratigraphy.

The sediments that were deposited during the thrusting of the South Pyrenean foreland fold-and-thrust belt in Spain were analyzed to characterize the topographic and structural development of the orogen. Reconstruction of the South Pyrenean fold-and-thrust belt based on topographic slope and thrust-front advance indicated the role of internal slope deformation in maintaining the surface slope angle of the orogen. Furthermore, the flexural rigidity of the lower plate regulated the taper angle of the Pyrenean thrust wedge.