Post-collisional Evolution of the India-Asia Suture Zone: Basin Development, Paleogeography, Paleoaltimetry, and Paleoclimate

Abstract

This dissertation consists of three manuscripts that will be submitted for publication. All three of these examine various aspects of the evolution of the India-Asia suture zone in southern Tibet after the India-Asia collision. Continent-continent collision is one of the basic tectonic plate boundary types, has occurred repeatedly throughout geologic history, and represents one of the principle mechanisms responsible for the formation of high elevation plateaus and orogens. Uplift within these zones has also drastically changed the earth's climate and atmospheric circulation, and erosion from continental collision has resulted in some of the thickest accumulations of sediment in the world (Curray, 1991; Einsele et al., 1996). However, despite the global significance of continental collision, much of the fundamental geodynamic and geologic processes governing these events remain enigmatic. This is the result of several factors. First and foremost, intense deformation and uplift of rocks, often from mid crustal levels, over very short periods of time (Hodges and Silverberg, 1988; Seward and Burg, 2008; Zeitler et al., 2014) results in the erosive removal of much of the geologic record of a collision zone. Second, because the best modern example of continental collision is the Tibet-Himalayan system, the study of continental collision in general has been hampered by high elevations, remoteness, difficult working conditions, and political unrest. The work presented here represents a step toward better understanding the geology, geologic history, and geodynamic evolution of the Tibetan Plateau, the Himalaya, and the India-Asia collision. This has been accomplished through study of two of the post-collisional sedimentary basins which formed near or within the India-Asia suture zone. Appendix A addresses the structure, sedimentology, age, and provenance of the Liuqu Conglomerate. The key conclusions of this section are: 1) The Liuqu Conglomerate was deposited in north flowing, stream dominated alluvial fans. These were located situated in a wedge-top position within a system of north verging thrust faults likely associated with the Great Counter Thrust, and sediment was accommodated via burial beneath thrust structures. 2) The age of the Liuqu Conglomerate has been refined to ~20 Ma based on detrital zircon U-Pb and fission track dating, ⁴⁰Ar/³⁹Ar dating of biotite from a cross-cutting dike, re-analysis of previously published pollen data, regional structural considerations, and oxygen isotope composition of paleosol carbonates. 3) Sand-sized and finer-grained sediment eroded from the southern margin of Asia prior to collision was transported southwards across the Xigaze forearc basin, deposited within the subduction trench, and then accreted within the subduction complex mélange. After collision, this sediment was eroded from the mélange and shed northward into the India-Asia suture zone. Appendix B focuses on the abundant paleosols preserved within the Liuqu Conglomerate. This study uses major element geochemistry of these paleosols and stable isotope analyses of paleosol carbonates to constrain the degree and type of chemical weathering, and thus the paleoclimate and paleoelevation, of the Liuqu Conglomerate. The key conclusions of this paper are: 1) at ~20 Ma, the India-Asia suture zone experienced warm and wet conditions that promoted intense chemical weathering of soils exposed in the inactive portions of alluvial fans. Paleorainfall is estimated at ~1500 mm/yr, and weathering intensity was similar to soils formed in the Neogene Siwalik Group of India, Nepal, and Pakistan, which formed under wet, semitropical, and low elevation conditions. 2) The India-Asia suture zone experienced these conditions at ~20 Ma despite extensive deformation and crustal thickening which has been documented within the Tethyan Himalayan and Himalayan thrust belts. This crustal thickening should have resulted in the (surface) uplift of the entire India-Asia collision zone, and there is evidence that at least some portion of the Himalayan crest was at or near modern elevations by ~17 Ma. Our results require either that the Tethyan Himalaya and India-Asia suture zone were not uplifted despite as much as 40 million years of intense crustal shortening or that these regions attained high elevation prior to ~20 Ma, and then lost elevation around this time before being immediately re-uplifted. The viability of these two scenarios cannot be explicitly tested with the data presented in this chapter; however, based on the data presented in Appendix C, I strongly favor the second scenario. Appendix C focuses on the Kailas Formation, exposed ~20 km north of the Liuqu Conglomerate within the India-Asia suture zone. The Kailas Formation is exposed along ~1300 km of the India-Asia suture zone. For this study, I present new sedimentologic, provenance, and geochronologic data for the Kailas Formation. Key findings of this study are that 1) the Kailas Formation is younger in the center of the suture zone, near 90°E, and becomes progressively older to the west; preliminary data suggest that these rocks are older to the east as well, but additional age constraints are required. 2) The pattern of sedimentation documented for the Kailas Formation is nearly identical to the spatio-temporal pattern of adakitic and ultrapotassic rocks in southern Tibet. These rocks have been attributed to rollback and breakoff of the Indian continental slab. Sedimentation within the Kailas basin has also been attributed to rollback of the Indian slab (DeCelles et al. 2011), and this idea is corroborated by the agreement of the sedimentary and magmatic records. 3) This presents an interesting possibility for explaining the existence of low elevations within the India-Asia suture zone at ~20 Ma, as documented in Appendix B. High elevation topography produced by crustal shortening and thickening likely remained intact until slab rollback and breakoff started around 30 Ma and caused the India-Asia suture zone to experience large scale extension and subsidence. The Kailas Formation was deposited in the resulting basin, which opened first in the west, and propagated eastward. After slab breakoff occurred, contractional deformation would have resumed, and the area would have been quickly uplifted to its modern elevations.