Uplift of the Tibetan Plateau

Abstract

The Tibetan Plateau is the highest plateau in the world, located southwest of China. It lies between several mountain ranges: the Kunlun Mountains to the north, the Himalayas and Karakoram Range to the south and southwest, and the Daxue Mountains to the east. The Tibetan Plateau encompasses the Tethyan-Himalaya thrust belt, the Lhasa terrane, the Qiangtang terrane and the Jinsha and Kunlun sutures, all of which represent their own diverse geological history. Due to the complexity of the area, uplift history and geological evolution of the Tibetan Plateau are poorly understood. Several models have been proposed and to describe the complex history of the region, which accommodate mechanisms acting along active margins of the plateau. A popular model assumes that the thick crust and high elevation of Tibet are a direct result of the continental collision between India and Asia since the Eocene. Crustal thickening is attributed to convective removal of the lower portion of the thickened lithosphere in this model, followed by east-west extension as a result of this removal. This model is based on magmatism throughout the Tibetan Plateau where potassic lavas to the east (30-40 Ma) are compared to potassic lavas from the west (20 Ma) and are interpreted to represent diachronous uplift in the history of the plateau. Other common models for the formation of the interior involves activity along the active margins along the plateau, such as: northward underthrusting of Indian lithosphere, homogenous lithospheric shortening/thickening, upper-crustal shortening followed by passive infill of basins and oblique subduction along sutures, and lastly thickening and flow of weak crust from the continental collision zone powered by topographic gradient. More recent studies, however, have challenged these prior models with evidence showing that portions of south Asia underwent major crustal shortening and thickening prior to the Indo-Asian collision. This was indicated by carbon and oxygen isotopic studies, indicating high paleo-elevation in the late Oligocene. The conflicting views for the uplift of the Tibetan Plateau illustrate the need to readdress theories on paleo-elevation, shortening and geochemical properties of the area, to understand how the Tibetan Plateau truly became ‘the roof of the world’.

 

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The Queen Charlotte Fault

Overview

The Queen Charlotte Fault (QCF) is a transform fault that separates the Pacific plate and the North American plate. It is often referred to as the right lateral strike-slip equivalent to the San Andreas fault zone. The QCF formed off the west coast of the islands of Haida Gwaii upon the termination of margin subduction in the Eocene. To the south of the QCF is the Cascadia Subduction Zone (CSZ) where the Pacific plate is subducted below the North American plate. A complex plate triple junction between the QCF and the CSZ demonstrates slow margin convergence at the Winona basin and short spreading centers known as the Tuzo Wilson and Dellwood Knolls. To the north the QCF transitions into the Fairweather fault extending from the coast of Alaska into the continent. Along the continental margin of Haida Gwaii, the Pacific plate and North American plate motion was strike slip from the Eocene to approximately 6 Ma with a period of extensional volcanism and subsidence in the Queen Charlotte basin. At 6 Ma, a change in the Pacific-America relative plate motion resulted in 15-20° oblique convergence and caused the subduction-type underthrusting of the Pacific plate beneath Haida Gwaii. This underthrusting resulted in an offshore flexural bulge, downbowing of the oceanic lithosphere at the margin, and the uplift and erosion of the west coast of Haida Gwaii which exposed mid-Tertiary rocks. Evidence to support subduction-type underthrusting includes: an offshore forebulge, the Queen Charlotte trench, the Queen Charlotte terrace accretionary sedimentary prism and GPS vectors and small earthquake mechanisms that indicate oblique convergence. This type of underthrusting is known to cause earthquakes, such as the magnitude 7.8 earthquake off the coast of Haida Gwaii in 2012. It is important to understand complex fault systems and their structure to ensure safety in hazardous places.

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The Marcellus Shale

Overview

The Middle Devonian Marcellus Shale (or Marcellus Formation) has been investigated to determine its role in the history of the Appalachian Orogenic Belt. Numerous geological papers were analyzed to examine the impact of regional orogenic events and how the environment was shaped for sedimentary deposition. Thickness and stratigraphic trends in the Marcellus Shale indicate that two main mechanisms influenced its deposition: a) Acadian thrust loading as Avalonia collided with Laurentia, and b) base-level fluctuations due to transgressive-regressive oceanic sequences. The collision of Avalonia with Laurentia formed the Acadian Mountains and continued thrust loading led to crustal bending and the development of a foreland basin. This foreland basin became the space of accumulation for the Marcellus Shale as the continued advancement of the Acadian orogenic front produced fold-thrust sheets where Acadian Mountain rocks were uplifted, eroded, then added to the basin. Transgressive-regressive marine cycles also contributed to the sediment deposition in the basin. Transgressive systems dominantly deposited carbonate and organic-rich layers whereas regressive systems deposited mostly clay layers in the basin. High erosion and sedimentation rates during orogenesis were key to the formation of the Marcellus Shale as they generated low oxygen availability in the water and thus, anoxic water conditions aiding in the burial of organic carbon layers. Variations in thickness of this sedimentary succession can be accredited to reactivated basement faults that further shaped the foreland basin as sediment was added. It can be stated that the Marcellus Shale formed both as a result of orogenic events and eustasy in the Middle Devonian. This research on the Marcellus Shale helps paint the picture of the history of the Appalachian Orogenic Belt and reinforces how complex some depositional systems can be.

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