In this study, we use published geologic maps and cross-sections to construct a three-dimensional geologic model of major shear zones that make up the Himalayan orogenic wedge. The model incorporates microseismicity, megathrust coupling, and various derivatives of the topography to address several questions regarding observed crustal strain patterns and how they are expressed in the landscape. These questions include: (1) How does vertical thickening vary along strike of the orogen? (2) What is the role of oblique convergence in contributing to along-strike thickness variations and the style of deformation? (3) How do variations in the coupling along the megathrust affect the overlying structural style? (4) Do lateral ramps exist along the megathrust? (5) What structural styles underlie and are possibly responsible for the generation of high-elevation, low-relief landscapes? Our model shows that the orogenic core of the western and central Himalaya displays significant along-strike variation in its thickness, from ∼25−26 km in the western Himalaya to ∼34−42 km in the central Himalaya. The thickness of the orogenic core changes abruptly across the western bounding shear zone of the Gurla Mandhata metamorphic core complex, demonstrating a change in the style of strain there. Pressure-temperature-time results indicate that the thickness of the orogenic core at 37 Ma is 17 km. Assuming this is constant along strike from 81°E to 85°E indicates that, the western and central Nepal Himalaya have been thickened by 0.5 and 1−1.5 times, respectively. West of Gurla Mandhata the orogenic core is significantly thinner and underlies a large 11,000 km2 Neogene basin (Zhada). A broad, thick orogenic core associated with thrust duplexing is collocated with an 8500 km2 high-elevation, low-relief surface in the Mugu-Dolpa region of west Nepal. We propose that these results can be explained by oblique convergence along a megathrust with an along-strike and down-dip heterogeneous coupling pattern influenced by frontal and oblique ramps along the megathrust.

The Thakkhola Graben has been a region of geologic inquiry for many decades. Although it is widely viewed to be in a class of structures that are important in accommodating the three-dimensional strain within the Himalayan thrust wedge, we still lack a detailed understanding of the total finite strain accommodated by graben-bounding faults, as well as their shape and cross-cutting relationships with structures deeper in the thrust wedge. Using geologic mapping and structural analysis, we show that a suite of pre-extensional shortening structures is offset by normal-oblique faults bounding the Thakkhola Graben that we use to define a piercing line. We calculate these faults to have accommodated 8.7 kilometers of vertical thinning, 7.2 kilometers of arc-perpendicular shear, and only 2.2 kilometers of arc-parallel extension. The magnitude of arc-parallel extension is quite low compared to extensional structures to the west in the Gurla Mandhata-Humla region. The cross-cutting relationships established in this study and timing constraints determined by previous works are consistent with a structural history of crustal thickening leading to foreland propagation of the locus of arc-perpendicular shortening contemporaneous with hinterland extension.

The Tibetan Plateau, the largest highland on Earth, formed due to the collision of India-Asia over the past 50–60 m.y., and the evolution of the Tibetan Plateau impacts our knowledge of continental tectonics. Examination of the northernmost margin of the Tibetan Plateau is key to unravelling the deformation mechanisms acting in northern Tibet. The left-slip Altyn Tagh fault system defines the northwest margin of the Tibetan Plateau, separating the Western and Eastern Kunlun Ranges in the southwest. Both Cenozoic and pre-Cenozoic crustal deformation events at this junction between the Altyn Tagh and Kunlun Ranges were responsible for the construction of northwestern Tibet, yet the relative contribution of each phase remains unconstrained. The western domain of the Eastern Kunlun Range is marked by active NE-trending, left-slip deformation of the Altyn Tagh fault and an E-striking Cenozoic thrust system developed in response India-Asia collision. To better constrain the Paleozoic Altyn Tagh and Kunlun orogens and establish the Cenozoic structural framework, we conducted an integrated investigation involving detailed geologic mapping (∼1:50,000 scale), U-Pb zircon geochronology, and synthesis of existing data sets across northwestern Tibet. Our new zircon analyses from Paleoproterozoic–Cretaceous strata constrain stratigraphic age and sediment provenance and highlight Proterozoic–Paleozoic arc activity. We propose a tectonic model for the Neoproterozoic–Mesozoic evolution of northwestern Tibet wherein restoration of an ∼56-km-long balanced cross section across the western domain of the Eastern Kunlun suggests that Cenozoic minimum shortening strain was ∼30% (∼24 km shortening). Field evidence suggests this shortening commenced after ca. 25–20 Ma, which yields an average long-term shortening rate of 1.2–0.9 mm yr–1 and strain rates of 4.7 × 10–16 s–1 to 2.3 × 10–16 s–1. Geometric considerations demonstrate that this contractional deformation did not significantly contribute to left-slip offset on the Altyn Tagh fault, which has ∼10 mm/yr slip rates.

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Between 81°30ʹE and 83°E, the Himalayan range's “perfect” arcuate shape is interrupted by an embayment. We hypothesize that thrust geometry and duplexing along the megathrust at midlower‐crustal depths play a leading role in growth of the embayment as well the southern margin of the Tibetan plateau. To test this hypothesis, we conducted thermokinematic modeling of published thermochronologic data from the topographic and structural embayment in the western Nepal Himalaya to investigate the three‐dimensional geometry and kinematics of the megathrust at midlower‐crustal depths. Models that can best reproduce observed cooling ages suggest that the megathrust in the western Nepal Himalaya is best described as two ramps connected by a long flat that extends further north than in segments to the east and west. These models suggest that the high‐slope zone along the embayment lies above the foreland limb of an antiformal crustal accretion zone on the megathrust with lateral and oblique ramps at midlower‐crustal depths. The lateral and oblique ramps may have initiated by ca. 10 Ma. This process may have controlled along‐strike variation in Himalayan‐plateau growth and therefore development of the topographic embayment. Finally, we analyze geological and morphologic features and propose an evolution model in which landscape and drainage systems across the central‐western Himalaya evolve in response to crustal accretion at depth and the three‐dimensional geometry of the megathrust. Our work highlights the importance of crustal accretion at different depths in orogenic‐wedge growth and that the midlower crustal accretion determines the location of plateau edge.

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