Extrusion vs. duplexing models of Himalayan mountain building 3: duplexing dominates from the Oligocene to Present

Dian Hea*, A. Alexander G. Webba, Kyle P. Larsonb, Aaron J. Martinc & Axel K. Schmittda Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA, USAb Earth and Environmental Sciences, University of British Columbia Okanagan, Kelowna, BC, Canadac Department of Geology, University of Maryland, College Park, MD, USAd Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, USA* Corresponding author. Email: dian.he@shell.com† Current address: Shell International Exploration and Production Inc., Houston, TX 77082, USAThis article was originally published with errors. This version has been corrected. Please see Corrigendum (http://dx.doi.org/10.1080/00206814.2015.1005906).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.