Dynamic fluvial systems and gravel progradation in the Himalayan foreland

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-flow- ing 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

Dynamic fluvial systems and gravel progradation in the Himalayan foreland

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-flow- ing 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

Channel Width Response to Differential Uplift

The role of channel width and slope adjustments to differential uplift in rivers within actively deforming terrains remains contentious. Here high‐resolution topographic surveying of formerly antecedent outwash channels demonstrates marked changes in channel width as a primary response to differential uplift. For five Late Quaternary alluvial paleochannels crossing small folds along the active Ostler fault zone of southern New Zealand, nearly continuous measurements of paleochannel width and concomitant incision reveal abrupt narrowing of widths toward minimum values at channel positions coincident with the initial uplift. When the magnitude of differential uplift is sufficiently small, narrowing alone permits these channels to remain antecedent. In the context of a unit stream power model for fluvial erosion, observed limits on the magnitude of channel narrowing suggest that above some threshold amount of differential uplift, continued incision requires concomitant changes in channel gradient. Thus when crossing small growing folds, alluvial rivers simply narrow their channels, whereas larger folds that demand greater incision prompt an initial narrowing followed by channel steepening

Quantification of three-dimensional folding using fluvial terraces: A case study from the Mushi anticline, northern margin of the Chinese Pamir

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

Along-Strike Growth of the Ostler Fault, New Zealand: Consequences for Drainage Deflection above Active Thrust

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 \u3e100 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 climate at ∼104‐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

Late Cenozoic Tectonic Evolution of the Western Nepal Himalaya: Insights from Low-Temperature Thermochronology

In the central Himalaya, an abrupt physiographic transition at the foot of the Greater Himalaya (PT2) marks the southern edge of a zone of rapid rock uplift along a ramp in the Main Himalayan Thrust (MHT). Despite being traceable along ~1500 km of the central Himalaya, PT2 is less distinct in western Nepal, reflecting along-strike changes in MHT geometry and/or a migrating locus of midcrustal deformation, the details of which have important implications for seismic hazard in western Nepal. New mineral cooling ages (apatite and zircon U-Th/He and muscovite Ar-Ar) from a series of relief transects provide constraints on exhumation rates and histories in western Nepal. Inversion of these data using Pecube and QTQt models yields results that require rapid (~1.4–2.7 mm/yr) exhumation in the rocks near the along-strike projection of PT2 until around 9–11 Ma, followed by much slower (~0.1–0.4 mm/yr) exhumation until at least the late Pliocene. In contrast, transects from ~75 km hinterlandward are best fit by rapid exhumation rates (~1.5–2.1 mm/yr) over at least the past ~4 Myr. Midcrustal deformation in western Nepal is occurring well north of the position expected from along-strike structures in central Nepal, and a growing dataset suggests that rapid exhumation has been sustained there since the late Miocene. These new constraints on the late Cenozoic exhumation history of the western Nepal Himalaya provide key insight on the active structures behind the complex seismic hazards in the region

Geomorphic Constraints on Listric Thrust Faulting: Implications for Active Deformation in the Mackenzie Basin, South Island, New Zealand

Deformed fluvial terraces preserved over active thrust-related folds record the kinematics of folding as fault slip accumulates on the underlying thrust. In the Mackenzie Basin of southern New Zealand, the kinematics revealed by folded fluvial terraces along the active Ostler and Irishman Creek fault zones are inconsistent with traditional models for thrust-related folding in which spatially uniform rock uplift typically occurs over planar fault ramps. Instead, warped and tilted terraces in the Mackenzie are characterized by broad, continuous backlimbs and abrupt forelimbs and suggest folding through progressive limb rotation. By relating this pattern of surface deformation to the underlying thrust with a newly developed, simple geometric and kinematic model, we interpret both faults as listric thrusts rooted at depth into gently dipping planar fault ramps. Constraints on the model from detailed topographic surveying of deformed terraces, ground-penetrating radar over active fault scarps, and luminescence dating of terrace surfaces suggest slip rates for the Ostler and Irishman Creek faults of ~1.1– 1.7 mm/yr and~0.5–0.7 mm/yr, respectively. The predicted depth of listric faulting for the Ostler fault (0.70 +0.1-0.2 km) and the Irishman Creek fault (1.3+0.1-0.5 km) generally agrees with geophysical estimates of basin depth in the Mackenzie and suggests control of preexisting basin architecture on the geometry of active thrusting. Despite the potential effects of changes in fault curvature and hanging wall internal deformation, the methodology presented here provides a simple tool for approximating the kinematics of surface deformation associated with slip along listric, or curviplanar, thrust faults

Growth of the South Pyrenean orogenic wedge

A six-step reconstruction of the South Pyrenean foreland fold-and-thrust belt in Spain delineates the topographic slope, basal décollement angle, internal deformation, and thrust-front advance from the Early Eocene until the end of contractional deformation in the Late Oligocene. Style of thrust-front advance, dip of the basal décollement, slope of the upper surface, and internal deformation are decoupled and not simply related. Internal deformation increased, decreased, and maintained surface slope angle at different stages. From the onset to the cessation of deformation, the basal décollement angle decreased overall suggesting translation of the thrust belt onto stronger crust with time. Taper angle of the Pyrenean thrust wedge was fundamentally controlled by the flexural rigidity of the lower plate, the relative rate of creation of structural relief in the rear versus the front of the wedge, the extent of deposition of eroded material within the deforming wedge, and the taper of the pretectonic stratigraphic wedge

Topographic control of asynchronous glacial advances: A case study from Annapurna, Nepal

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 lateglacial, whereas those in valleys with higher maximum altitudes are early Holocene. Two valleys with similar equilibrium-line altitudes (ELAs), but contrasting ages, are \u3c 5 km apart and share the same aspect, such that spatial differences in climate can be excluded. A glacial mass-balance cellular automata model of these two neighboring valleys predicts that change from a cooler-drier to warmer-wetter climate (as at the Holocene onset) would lead to the glacier in the higher altitude catchment advancing, while the lower one retreats or disappears, even though the ELA only shifted by ~120 m

Tectonic-Climate Interactions in Action Orogenic Belts: Quantification of Dynamic Topography with SRTM data

This project was undertaken to examine the approach to steady state in collisional mountain belts. Although the primary thrust of this grant was to look at larger collisional mountain belts, such as the Himalaya, the Tien Shan, and Southern Alps, we began by looking at smaller structures represented by growing and propagating folds. Like ranges that are evolving toward a topographic steady state, these folds undergo a series of morphologic changes as they are progressively uplifted and eroded. We wanted to document the nature of these changes and to try to discern some of the underlying controls on them. We initially focused on the Wheeler Ridge anticline in southern California. Subsequently, we progressed to looking at the topographic development and the effects of differential uplift and glaciation on the Kyrgyz Range in the northern Tien Shan. This range is unusual inasmuch as it is transformed along its length from a simple uplift with a largely preserved Mesozoic erosion surface arching across it to a highly dissected and heavily glaciated uplift in the region where uplift has been sustained at higher rates over longer intervals. In efforts to understand the distribution of erosion rates at 10(exp 3) - 10(exp 5) year time scales, cosmogenic radionuclide (CRN) concentrations have been gaining increasingly widespread usage (Brown et al., 1995; Riebe et al., 2004; Riebe et al., 2001; Vance et al., 2003). Most studies to date, however, have been conducted in slowly eroding ranges. In rapidly eroding mountains where landslides deliver most of the sediments to the rivers, we hypothesized that CRN concentrations could be highly perturbed by the stochastic processes of landsliding. Therefore, we undertook the development of a numerical model that simulated the effects of both landsliding and grain-by-grain attrition within fluvial catchments. This modeling effort has shown the effects of catchment size and erosion rate on CRN concentrations and allows a prediction of where to sample to obtain the optimal erosion rate estimates using CRN techniques. Finally, we developed computational techniques to operate on DEMs to extract useful information that would enable quantification of climate-erosion interactions. In particular, we worked on rapid techniques to define catchments of any given range of sizes, to extract channel gradients, to combine precipitation information to calculate discharge, and to utilize various stream-power models to determine the erosional energy within any given catchment within a transect. We briefly describe results from Wheeler Ridge, the Kyrgyz Range, the Nepal Himalaya, and our numerical modeling