Geophysics and Tectonics Seminar fall-2019

Abijah Simon: A key issue in the studies of the eastern Tibetan plateau is that its high relief (>4 km) and thick crust (>50 km) cannot be adequately explained by the slow geodetic slip rates (<3 mm/yr), late Cenozoic initiation (~30 Ma), and 10s km Cenozoic shortening along its easternmost edge. The channel flow and thrust belt end-member models have been proposed to address this discrepancy, but they discount the role of deformation in the upper mantle and the upper crust in the interior of eastern Tibet. Here we address this problem by conducting field mapping, interpreting crustal-scale seismic-reflection profiles, and constructing balanced cross sections in the Longmen Shan and Min Shan of western Sichuan. Our mapping reveals a distinct contrast in the deformational styles affecting this region. In the frontal part of this margin, widespread cleavage development in Triassic-folded Silurian and Devonian strata contrast with brittle faults and discrete shear planes defined by slickensides. The parallelism, similar kinematics, and similar brittle mode of deformation between the mapped faults and the seismically active plateau-bounding structures suggest a Cenozoic age. In the northern part of eastern Tibet, Triassic E-W trending folds are cross-cut by N-S trending faults that parallel the seismically active Minjiang fault in this region, also suggesting a Cenozoic age. Our seismic interpretations consistent with regional field relationships suggest: (1) two-phase deformation first by Triassic thin-skinned folding and ductile deformation followed by later Cenozoic thick-skinned faulting and brittle deformation, (2) the presence of a west-directed thrust wedge in the Min Shan with the east-directed Minjiang thrust as its roof structure, and (3) pure-shear ductile shortening in the middle and lower crust in contrast to brittle faulting in the upper crust and the uppermost mantle. A preliminary balanced cross section across the Min Shan reveals >67% shortening accommodated by brittle thrusting. Integrating field observations and seismic data leads to a tectonic model that involves pure-shear thick-skinned crustal thickening for the Cenozoic development of eastern Tibet. If the 67% shortening from the Min Shan represents the average Cenozoic strain in eastern Tibet, this would be sufficient to explain the current crustal thickness and high relief. Valeria Jaramillo: The Himalayan Orogen is one of the largest continent-continent collision zones in the world and serves as the preeminent example of an actively evolving mountain belt. Several fundamental questions persist, however, as to how the Himalaya has tectonically evolved through time, including the origin of the Lesser Himalayan Crystallines—a series of fault bound outliers of metamorphic rocks that occur to the south of the main Himalayan structural sequence. Several tectonic models have been proposed to explain the emplacement of these thrust sheets and klippen, wherein different configurations of major faults are invoked to have resulted in the southward transport of these metamorphic rocks. As an initial attempt to answer some of these questions this study investigated the geometry and timing of deformation in one of these tectonic outliers, the Kathmandu Klippe in central Nepal. Microstructural analysis of rocks collected along four transects across the klippe allows for identification of major structures and quantification of internal strain. Microstructural data collected from several quartzites across the klippe using the Rf-φ and Fry methods yields strain ellipse ratios (Rs values) that range from 1.10 to 2.28 (YZ) and 1.67 to 2.89 (XZ). Strain magnitudes increase systematically with proximity to several major mapped and unmapped structures consistent with their occurrence as broad shear zones and not discrete faults. Among the samples displaying the largest Rs values, are those collected from the Mahabharat Thrust—a large top-south shear zone and possible analogue to the Main Central Thrust. U-Pb dating of strained titanite in these samples yields a date of ~16-18 Ma, overlapping with prior estimates for the timing of slip along the southern MCT. Petrographic study of the titanites indicates these young dates primarily reflect subgrain formation at grain tips and may thus represent the first direct determination of the timing of deformation within the klippe. When combined with information provided by the bulk strain ellipses and shear sense indicators, these results provide important insights into the various models on the development of the Himalaya and, more broadly, how high-grade metamorphic rocks are juxtaposed over low-grade metamorphic rocks in convergent orogens.