Himalayan mountain building has been dominantly explained by two types of models: extrusion and duplexing. To elucidate possible roles of these mechanisms during emplacement of the Himalayan crystalline core, we investigate an area speculated to contain the southern leading edge of the crystalline core: the northeastern margin of the Dadeldhura klippe, western Nepal. We found an ~700m thick, primarily top-to-the-north shear zone within the klippe; we term this as the Tila shear zone. The shear zone occurs within a right-way-up metamorphic field gradient, and separates footwall gneiss from hanging wall schist. Similarly, deformation temperatures estimated from quartz and feldspar microstructures and quartz c-axis fabrics indicate a right-way-up thermal gradient of ~77–189°C/km. U–Pb zircon dating of post-kinematic leucogranite dikes suggests that ductile shearing along the Tila shear zone occurred prior to ~17–14Ma. We correlate the Tila shear zone to the South Tibet detachment (STD) on the basis of consistent structural fabrics (shear sense), lithologies, metamorphism, and deformation timing. This interpretation, in combination with regional constraints, indicates southwards-increasing proximity of the STD (Tila shear zone) and the Main Central thrust (MCT). These two shear zones are separated by ~3km of structural section in the northern portion of our study area, and become close to within ~1km of separation, in the southern portion. Interpolation suggests that the STD (Tila shear zone) and MCT merge 15±10km southwest of our study area. The increasing-to-south proximity and potential merger of the two shear zones suggest that the STD formed as a backthrust from the MCT. This interpretation contrasts with the long-standing normal fault interpretation of the STD. Because the STD and MCT bound the Himalayan crystalline core, these findings document crystalline core emplacement at depth via tectonic wedging. This kinematic evolution is consistent with duplexing, but not extrusion to the surface.

Ongoing Himalayan growth is generally thought to be dominated by duplexing and/or extrusion processes. These models may be tested by reconstructing Himalayan fold-thrust belt growth since the middle Miocene. However, our knowledge of basic structural geometry remains too fragmentary to resolve the issue, even in areas with rich stratigraphic diversity such as the northwestern Indian Himalaya. In this region, a primary outstanding question involves the uncertain relationship of the Berinag thrust and the Tons thrust, structures with displacements of >80km and >40km, respectively. The uncertain geometry and kinematics allow for the complete range of duplexing or extrusion processes for the integrated kinematic history since the middle Miocene. To address this issue, field mapping and kinematic analysis were performed to reconstruct the deformation of the Lesser Himalayan Sequence in the northwest Indian Himalaya. Our results reveal a new discovery: a similar to 450m thick top-to-southwest shear zone, termed the Pabbar thrust. The Pabbar thrust placed the Outer Lesser Himalayan Sequence (the Tons thrust hanging wall) directly on the Berinag Group (the Berinag thrust hanging wall). This discovery requires that the Berinag thrust and Tons thrust are, in fact, the same structure, and discrete duplexing processes dominated growth of the northwest Indian Himalaya for the past similar to 10-15million years. Along-strike extension of these kinematics and corresponding geometries is consistent with the observed orogenic framework and resolves a stratigraphic continuity problem across the India-west Nepal border, where prior work suggests that structures are continuous but stratigraphy does not match.

The Himalaya is a natural laboratory for studying mountain-building processes. Concepts of extrusion and duplexing have been proposed to dominate most phases of Himalayan evolution. Here, we examine the importance of these mechanisms for the evolution of the Himalayan crystalline core via an integrated investigation across the northern Kathmandu Nappe. Results reveal that a primarily top-to-the-north shear zone, the Galchi shear zone, occurs structurally above and intersects at depth with the Main Central thrust (MCT) along the northern flank of the synformal Kathmandu Nappe. Quartz c-axis fabrics confirm top-to-the-north shearing in the Galchi shear zone and yield a right-way-up deformation temperature field gradient. U-Pb zircon dating of pre-to-syn- and post-kinematic leucogranites demonstrates that the Galchi shear zone was active between 23.1 and 18.8 Ma and ceased activity before 18.8–13.8 Ma. The Galchi shear zone is correlated to the South Tibet detachment (STD) via consistent structural fabrics, lithologies, metamorphism, and timing for four transects across the northern margin of the Kathmandu Nappe. These findings are synthesized with literature results to demonstrate (1) the broad horizontality of the STD during motion and (2) the presence of the MCT-STD branch line along the Himalayan arc. The branch line indicates that the crystalline core was emplaced at depth via tectonic wedging and/or channel tunnelling-type deformation. We proceed to consider implications for the internal development of the crystalline core, particularly in the light of discovered tectonic discontinuities therein. We demonstrate the possibility that the entire crystalline core may have been developed via duplexing without significant channel tunnelling, thereby providing a new end-member model. This concept is represented in a reconstruction showing Himalayan mountain-building via duplexing from the Oligocene to Present.

This dissertation focuses on the kinematic evolution of two major categories of contractional tectonics: collisional orogenic belts and toe structures of passive margins, which are characterized by fold-thrust belts that are hundreds of kilometer-scale and tens of kilometer-scale, respectively. The Himalayan orogen is an excellent example of collisional orogenic belts along convergent plate boundaries. It is commonly structurally defined as three stacked units separated by two fault systems: the Main Central thrust (MCT) and South Tibet detachment (STD). The development and emplacement of the middle unit, the Himalayan crystalline core, has long been debated within the extrusion framework, a process that involves exhumation of the crystalline core to the surface. Recently, the debate has expanded to two end-member regimes: extrusion versus underplating. To determine how the crystalline core evolved, an integrated investigation was conducted, involving structural mapping, microstructural, quartz c-axis fabric, and geochronological analyses across the northern margins of two frontal klippen in the Nepal Himalaya: the Dadeldhura klippe and Kathmandu Nappe. The work suggests that the STD occurs and merges with the MCT in these two klippen. The merging of the MCT and STD requires that the crystalline core was emplaced at depth via tectonic wedging kinematics, incompatible with extrusion models. By synthesizing the Himalayan evolution history from the development and emplacement of the crystalline core to ongoing deformation, a reconstruction shows that Himalayan mountain-building processes are dominated by underplating. The Perdido fold-thrust belt is a gravity-driven toe structure in the passive margin of the Gulf of Mexico. Structural models for the Perdido fold-thrust belt are highly dependent upon the interpretation of seismic images, which commonly display wipe-out zones associated with faults. Fault interpretations in seismic wipe-out zones are commonly non-unique. Trishear, a quantitative fault-propagation folding model, was applied to an anticlinal structure in the Perdido fold-thrust belt and reproduced the fold geometry. Three dimensional kinematic evolution was reconstructed by interpolating the best-fit models of the serial cross sections. The trishear modeling indicates that the Perdido fold-thrust belt underwent ~7.5-12.5 km shortening, which could balance the landward extension of the passive margin during the same period.

The Himalaya is commonly described as a three layer-two fault stack. Namely, a high-grade crystalline core featuring an inverted metamorphic field gradient, the Greater Himalayan Crystalline complex (GHC), is separated from units above and below by shear zones. The Lesser Himalayan Sequence (LHS) underlies the GHC below the Main Central thrust, and the Tethyan Himalayan Sequence (THS) overlies it along the South Tibet detachment. However, the southern Main Central thrust hanging wall consists of a lower unit dominated by a right-way-up metamorphic sequence of biotite±garnet schists (Bhimphedi Group) and an upper unit with only anchizone metamorphism (Pulchauki Group). The Bhimphedi Group is commonly equated to the GHC, while the Pulchauki Group is well correlated to the THS. However, no shear zone separates these units. We present new structural and geochronological data along the boundary between the GHC and the Bhimphedi Group in the Kathmandu region. These data reveal an ~200m thick Early–Middle Miocene top-north-northeast shear zone that we term the Galchi shear zone. We correlate this shear zone to the South Tibet detachment on the basis of lithological, metamorphic, structural, and chronological criteria. The Galchi shear zone merges with the Main Central thrust to the south, bounding the leading edge of the GHC. This result, combined with recent work in the western Himalaya, suggests that the locally preserved leading edge of the GHC is sub-parallel to the arc of the orogen and the southern Main Central thrust hanging wall is dominated by THS rocks. This orogenic architecture rules out wedge extrusion and channel flow-focused denudation kinematic models for the Himalayan orogen, but is accommodated by tectonic wedging kinematic models, including channel tunneling models with modified timing.