Paleomagnetic Constraints on Tectonic Reconstructions of the India-Asia Collision Zone

Abstract

Paleomagnetically-determined paleolatitudes from the Lhasa terrane, the Yarlung-Zangbo Suture, and the Tibetan Himalaya are key to constraining how, when and where the India-Asia collision happened. This thesis aims at providing robust paleomagnetic determinations of the paleolatitudes from tectonic units in the India-Asia collision zone. Different tectonic reconstruction models are ultimately tested by a compilation of previously reported and newly collected reliable paleomagnetic data. A first focus lies on the Paleogene latitude estimates for the Lhasa terrane, which vary widely from 5°N to 30°N. We investigated Paleogene volcanic and sedimentary sequences of the Linzizong Group from southern Tibet in both the Linzhou basin and the Nanmulin Basin. Sedimentary Eocene upper Linzizong Group in the Linzhou Basin was affected by inclination shallowing. Two independent correction methods yield indistinguishable results of 40.5° [33.1°, 49.5°] (95% confidence limits) and 41.3° ± 3.3°, respectively. In the Nanmulin Basin, the Eocene volcanic rocks were remagnetized due to low-temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously. We tentatively propose a new method to recover a primary remanence with inclination of 38.1° [35.7°, 40.5°] (95% significance). In the Linzhou Basin, our analyses show that volcanic rocks of the lower Linzizong Group are thermally chemically remagnetized, whereas the upper Linzizong volcanic rocks retain a primary remanence. Regardless of the type of the rocks studied, paleomagnetic studies that have tested and corrected for all these potential pitfalls converge on a consistent Paleogene latitude of the Lhasa terrane of 20° ± 4°N. The depositional ages of sedimentary rocks unconformably overlying the Xigaze ophiolite’s mantle units overlap with the magmatic ages of the ophiolites. After correction of the compaction-related inclination ‘shallowing’ with two unrelated methods, these sedimentary rocks yield mean paleolatitudes of 16.2°N [13°N, 20.9°N] and 16.8°N [11.1°N, 23.3°N], respectively. These results are indistinguishable from recent robust paleolatitude estimates for the Gangdese arc in southern Tibet, indicating that spreading of the Xigaze ophiolite occurred in the Gangdese forearc, and formed the basement of the forearc strata. We further present a paleomagnetic investigation of a Jurassic (limestones) and Lower Cretaceous (volcaniclastic sandstones) section of the Tibetan Himalaya. Magnetic carrier of the Jurassic limestones is authigenic magnetite, whereas the dominant magnetic carrier of the Lower Cretaceous volcaniclastic sandstones is detrital magnetite. Our observations lead us to conclude that the Jurassic limestones record a secondary remagnetization, whereas the Lower Cretaceous volcaniclastic sandstones retain a primary remanence. The volcaniclastic sandstones yield an Early Cretaceous paleolatitude of 55.5°S [52.5°, 58.6°S] for the Tibetan Himalaya, suggesting it was part of the India plate at that time. Finally, we compiled the reported robust paleomagnetic data of Tibet and the Himalaya to test different tectonic reconstruction models. Our analyses confirm similar pre-collision and post-collision intra-Asia shortening of both ~600 km. Paleomagnetic data indicate an collision age of ~52 Ma, whereas collision ages of neither 34 Ma nor 65 Ma are supported. Extension must have happened within Greater India, between the Tibetan Himalaya and the cratonic Indian, from 116 Ma to 68 Ma.