Abstract
North-trending rifts throughout south-central Tibet provide an opportunity to study the dynamics of synconvergent extension in contractional orogenic belts. In this study, we present new data from the Dajiamang Tso rift, including quantitative crustal thickness estimates calculated from trace/rare earth element zircon data, U-Pb geochronology, and zircon-He thermochronology. These data constrain the timing and rates of exhumation in the Dajiamang Tso rift and provide a basis for evaluating dynamic models of synconvergent extension. Our results also provide a semi-continuous record of Mid-Cretaceous to Miocene evolution of the Himalayan-Tibetan orogenic belt along the India-Asia suture zone. We report igneous zircon U-Pb ages of ~103 Ma and 70–42 Ma for samples collected from the Xigaze forearc basin and Gangdese Batholith/Linzizong Formation, respectively. Zircon-He cooling ages of forearc rocks in the hanging wall of the Great Counter thrust are ~28 Ma, while Gangdese arc samples in the footwalls of the Dajiamang Tso rift are 16–8 Ma. These data reveal the approximate timing of the switch from contraction to extension along the India-Asia suture zone (minimum 16 Ma). Crustal-thickness trends from zircon geochemistry reveal possible crustal thinning (to ~40 km) immediately prior to India-Eurasia collision onset (58 Ma). Following initial collision, crustal thickness increases to 50 km by 40 Ma with continued thickening until the early Miocene supported by regional data from the Tibetan Magmatism Database. Current crustal thickness estimates based on geophysical observations show no evidence for crustal thinning following the onset of E–W extension (~16 Ma), suggesting that modern crustal thickness is likely facilitated by an underthrusting Indian lithosphere balanced by upper plate extension.
Highlights
IntroductionIn the Lhasa terrane of southern Tibet, we calculate quantitative crustal thickness estimates calculated from trace/rare earth element geochemistry, U-Pb geochronology, and zircon-He (ZHe) thermochronology to examine the relationship between crustal thickness trends and the timing/dynamics of E–W extension in one of the worlds archetypal collisional orogens
Normal fault systems accommodating orogen-parallel extension provide an opportunity to study the dynamics of synconvergent extension in contractional orogenic belts [1].In the Lhasa terrane of southern Tibet, we calculate quantitative crustal thickness estimates calculated from trace/rare earth element geochemistry, U-Pb geochronology, and zircon-He (ZHe) thermochronology to examine the relationship between crustal thickness trends and the timing/dynamics of E–W extension in one of the worlds archetypal collisional orogens
To understand how ongoing east–west extension relates to these crustal thickness trends, we focus our attention on north-striking normal faults concentrated in the central and southern portions of the Lhasa terrane of southern Tibet
Summary
In the Lhasa terrane of southern Tibet, we calculate quantitative crustal thickness estimates calculated from trace/rare earth element geochemistry, U-Pb geochronology, and zircon-He (ZHe) thermochronology to examine the relationship between crustal thickness trends and the timing/dynamics of E–W extension in one of the worlds archetypal collisional orogens. Assembled by the sequential accretion of island arc terranes and continental fragments since the Paleozoic, the Tibetan Plateau is the highest and most expansive orogenic plateau on Earth with crustal thickness estimates ranging from ~65 km in the north [2,3] to. While contractional deformation has led to significant shortening and crustal thickening throughout the region [5,6,7,8], the southern and central portions of the Tibetan Plateau have experienced east–west extension since the early. A fundamental question in understanding this orogen is how crustal thickness has changed through time and how these changes relate to ongoing east–west extension [11].
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