Depositional facies and detrital composition of the paleoproterozoic Et-Then group, NWT, Canada : signature of intracratonic indentation tectonics

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1994.Includes bibliographical references (leaves 28-33).by Bradley D. Ritts.M.S

Cenozoic multiple-phase tectonic evolution of the northern Tibetan Plateau: Constraints from sedimentary records from Qaidam basin, Hexi corridor, and Subei basin, northwest China

An integrated research of sedimentology, stratigraphy, and provenance analysis on eleven sedimentary sections from the Qaidam basin, Hexi Corridor, and Subei basin representing ∼36 km Cenozoic strata provides a detailed record of the northern Tibetan Plateau growth since the early Eocene. Sections are divided into three groups based on age, geological similarities, and geographical locations. Group One includes three early Eocene-late Miocene sections from the northern Qaidam basin; Group Two contains four sections along the Altyn Tagh fault (ATF) which preserve a complete stratigraphic record from the Oligocene to late Miocene; Group Three contains the four youngest sections investigated from the northeastern Qaidam basin and Hexi Corridor which preserve a record since the middle Miocene to Quaternary. Together, the sections reveal a multiple-phase tectonic history of the northern Tibetan Plateau. The punctuated history can be divided into four phases. (1) The Eocene Lulehe Formation from the northern Qaidam basin is interpreted as a synorogenic conglomerate deposited by high-gradient depositional systems. Strong unimodal paleocurrent towards the southwest, coarse lithology and distinct, recognizable clast types constrain the sediment source within the North Qaidam and South Qilian terranes, indicating activity on inferred thrust faults within the North Qaidam and South Qilian terranes in response to the initial India-Eurasia collision. The activity on these thrust faults continued through the late Eocene. (2) The early Oligocene conglomerate from Group Two formed in response to the sinistral transpression related to motion on the ATF, suggesting inception of substantial slip on the ATF in the early Oligocene in order to accommodate the continuing indentation of India into Eurasia. (3) Oligocene-early Miocene fine-grained fluvio-lacustrine sediments from Groups One and Two formed as the result of development of internal drainage systems in the Qaidam basin in response to the large-amplitude slip motion on the ATF. Paleocurrents collected from the Oligocene-early Miocene strata of Group One are northwest-directed, pointing towards the ATF, consistent with the pre-existing subsurface data showing the shift of depocenter from along North Qaidam and South Qilian terranes toward the ATF. (4) All eleven sections preserve a post-early Miocene upward-coarsening sequence, consistent with the extensive crustal shortening and topographic growth across the northern Tibetan Plateau

Isotopic constraints on intensified aridity in Central Asia around 12Ma

The relationship between central Asian aridification and the evolution of the Himalayan-Tibetan orogen remains elusive. New isotopic data from pedogenic and lacustrine carbonates sampled from well-dated Neogene strata (15.7-1.8Ma) in the northeastern Qaidam basin of the northern Tibetan Plateau identify a positive shift of ~2.5‰ in δ O values from 12Ma to 10.7Ma. High values were maintained until ~3.3Ma when δ O values drop by 1‰. The timing of the positive shift in δ O values is remarkably consistent with isotopic records from a vast region along and within the northern Tibetan Plateau. Isotopic, mineralogical, petrologic, and facies analyses suggest that diagenetic effects and detrital contamination are minimal. The positive shift in δ O values, as well as high δ C values from pedogenic carbonates (which are suggestive of low soil respiration rates), is interpreted to indicate intensified aridity in central Asia ca. 12Ma. The recognition of intensified aridity is critical to understanding the climatic effects of the development of the Himalayan-Tibetan orogen. We attribute this climatic change in the central Asia to: (1) retreat of Paratethys from central Asia, strengthening the seasonal contrast and resulting in the loss of a nearby moisture source; (2) attainment of high elevations in the Himalayas and south-central Tibet, blocking moisture-bearing air masses from the Indian and Pacific oceans; and (3) enhanced isolation and outward growth of the northern Tibetan Plateau. The negative shift around 3.3Ma might be related to the onset of Northern Hemisphere glaciation, intensification of the East Asia monsoon, and/or effects of orographic rain-out. © 2011 Elsevier B.V. 18 18 18 18 1

Stable isotopic constraints on the tectonic, topographic, and climatic evolution of the northern margin of the Tibetan Plateau

This study presents oxygen and carbon isotopic records from lacustrine, paleosol, alluvial, and fluvial carbonate sampled in thirteen Cenozoic sedimentary sections that span the northern margin of the Tibetan Plateau. Isotopic analyses of 1475 carbonate samples yield δ O values that range from 13.1 to 38.9‰ (SMOW), and δ C values range from - 11.5 to 3.7‰ (PDB). Based on these analyses, we see two isotopic trends regionally. We interpret a Paleogene decrease in average oxygen isotope values to result from an increase in the mean elevation of the study region\u27s drainages as they tapped waters from a growing Tibetan Plateau to the south of the study area. We interpret a Neogene increase in both carbon and oxygen isotope values to result from the initiation of local mountain building in the study area and the withdrawal of marine waters, which increased basin isolation, aridity, and temperatures. The timing of inferred initiation of uplift in this region is consistent with structural, thermochronological, and sedimentological evidence for a tectonic transition in the early and middle Miocene from accommodation of India-Eurasia convergence through extrusion along the Altyn Tagh fault to accommodation of convergence through active shortening and mountain building in this region. This study also supports tectonic models for the differential uplift of the Tibetan Plateau, in that mountain building in this study region significantly postdates estimates for the attainment of high elevation from isotopic studies of the central Tibetan Plateau. © 2009 Elsevier B.V. All rights reserved. 18 1

A high-resolution stable isotopic record from the Junggar Basin (NW China) : implications for the paleotopographic evolution of the Tianshan Mountains

This study presents high-resolution oxygen and carbon isotopic records of paleosol carbonates from fluvial sediments and lacustrine carbonates, sampled from the Jingou He and Kuitun He stratigraphic sections, located in the northern Tianshan piedmont. These sections expose remarkable outcrops of Junggar foreland basin sediments that have been previously dated by high-resolution magnetostratigraphy to between ∼23.6 and ∼1 Ma, and ∼10.5 and ∼3.1 Ma. A total of 216 samples of fluvio-lacustrine sediments were collected from which isotopic analyses yield δ18O (SMOW) values that range from 13.7‰ to 29.9‰ in the Jingou He section, and 16.3‰ to 21.0‰ in the Kuitun He section. δ13C (PDB) values range from −12.9‰ to 3.0‰ in the Jingou He section and from −7.8‰ to −4.0% in the Kuitun He section. δ18O values decrease between ∼25 and 23 Ma, and then remain relatively steady, with the exception of one period that contains samples with higher oxygen isotope values at ∼16 Ma. During the periods when there are samples that overlap in time from the Kuitun He and Jingou He sections, we observe a difference of ∼1.7% between values from the two locales. The δ13C values also decrease between ∼25 and 23 Ma, and then remain relatively steady until ∼10 Ma with, again, one short period of higher values at ∼16 Ma. Then, between ∼10 and 3.1 Ma, carbon isotope values progressively increase. We interpret that δ18O and δ13C isotopic values during lacustrine periods (∼25–23 Ma and ∼16 Ma) as largely controlled by evaporation and opening/closing of the lake to external inputs. We interpret the δ18O values of paleosol carbonate in the Junggar Basin to be influenced by the hypsometry of the high Tianshan range while the δ13C values may record the uplift history of the depositional area in the foreland basin itself as well as the isotopic composition of plants. Consequently, we conclude that the Jingou He and Kuitun He drainage basins in the Central Tianshan have remained at relatively unchanged elevations for the past ∼20 Ma. We also suggest that the elevation of the southern part of the foreland basin increased between ∼10 and ∼3.1 Ma, probably as a result of tectonic deformation in the piedmont and sedimentary filling of the sedimentary basin. The carbon isotope record remains relatively stable through time, and isotopic values suggest that there was little or no expansion of C4 plants in this region in the late Miocene