Collaborative Research: Initiation and Long Term Slip History of the Altyn Tagh Fault System, Northern Tibetan Plateau, NW China: A Tertiary Basin Piercing Point Study

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The Altyn Tagh fault system, which delineates the northern edge of the Tibetan Plateau, is one of the largest and most striking, yet also most enigmatic features attributed to collision between India and Eurasia. In spite of a generally poor understanding of the structural geology and slip history of the Altyn Tagh fault, its tectonic significance has been the subject of much speculation, and the fault figures prominently in models of the Himalayan-Tibetan orogen. Recent attempts to gain better knowledge about the Altyn Tagh fault have primarily used indirect methods to interpret age of fault initiation and magnitude of displacement, or geodetic techniques to interpret slip rate. Direct investigation of fault displacement using geologic piercing points has not been widely attempted. As a result three critical questions remain concerning the offset history of the Altyn Tagh fault: (1) What is the age of initiation of the Altyn Tagh fault? (2) How has the slip-rate on the Altyn Tagh fault varied over geologic time? (3) How does the magnitude of slip vary along the length of the Altyn Tagh fault? In order to answer these three questions it is essential to identify pre-slip and multiple syn-slip piercing points on each segment of the fault. These piercing points will allow the history of offset on the fault to be determined for each segment of the fault (western, central, and eastern) and thus demonstrate slip magnitude, slip rate, and variation in these parameters along the length of the fault. The proposed study will directly address the Tertiary slip history of the Altyn Tagh fault and the distribution of slip on the fault, by identifying multiple Eocene-Upper Pliocene sediment-source matches across the fault for ten Tertiary basins that are truncated by the Altyn Tagh fault. By identifying multiple piercing points of various ages from a single basin, the slip history for that segment of the fault ca be reconstructed throughout the Tertiary. Furthermore, by examining a series of basins along the length of the fault, along-strike changes in displacement can be documented. These results will provide a robust and unambiguous data set from which the timing and rate of faulting can be directly determined, and that will also allow ideas about systematic changes in slip magnitude along the fault to be tested. The PI's prior work on the Altyn Tagh fault (Ritts, 1998; Yue and Liou, 1999; Ritts and Biffi, 2000; Sobel et al., 2001; Yue et al., in review) has demonstrated their ability to identify and document piercing points of a variety of ages and a variety of types on the Altyn Tagh fault. The proposed research includes documentation of lithostratigraphy and chronostratigraphy, physical sedimentology and lithofacies, provenance, and paleocurrents of ten Tertiary sedimentary basins that are truncated by the Altyn Tagh fault. Two major field seasons will focus on the characterization of sedimentary basins as well as potential sediment source terranes on the opposite side of the fault. Fieldwork in the Tertiary basins will include description of measured sections, collection of paleocurrent and conglomerate composition data, and geological mapping. Fieldwork in potential sediment source areas will focus on lithologic description and verification of Chinese mapping, as well as collection of potential source units for analytical work. Analytical techniques will be applied to conglomerate clasts and potential source units in order to test sediment-source correlations that are proposed based on field data. These techniques will include petrographic examination, trace and major element geochemistry, and U-Pb and 40 Ar/ 39 Ar geochronology. Biostratigraphic analysis of mudstones will be used to confirm the ages of the Tertiary units. This study will constrain the geology of Tertiary basins in order to construct a series of time- displacement curves to characterize the history and along-strike variability of the Altyn Tagh fault. The resulting data will better define the role that the Altyn Tagh fault plays in accommodating Cenozoic convergence between India and Asia, and thereby distinguish the relative importance of mechanisms such as extrusion tectonics in continental deformation.