Low‐Temperature Thermochronology of the Indus Basin in Central Ladakh, Northwest India: Implications of Miocene‐Pliocene Cooling in the India‐Asia Collision Zone

Abstract

The India-Asia collision zone in Ladakh, northwest India, records a sequence of tectonothermal events in the interior of the Himalayan orogen following the intercontinental collision between India and Asia in early Cenozoic time. We present zircon fission track, and zircon and apatite (U-Th)/He thermochronometric data from the Indus Basin sedimentary rocks that are exposed along the strike of the collision zone in central Ladakh. These data reveal a postdepositional Miocene-Pliocene (~22-4 Ma) cooling signal along the India-Asia collision zone in northwest India. Our zircon fission track cooling ages indicate that maximum basin temperatures exceeded 200°C but stayed below 280-300°C in the stratigraphically deeper marine and continental strata. Thermal modeling of zircon and apatite (U-Th)/He cooling ages suggests postdepositional basin cooling initiated in Early Miocene time by~22-20 Ma, occurred throughout the basin across zircon (U-Th)/He partial retention temperatures from~20-10 Ma, and continued in the Pliocene time until at least~4 Ma. We attribute the burial of the Indus Basin to sedimentation and movement along the regional Great Counter thrust. The ensuing Miocene-Pliocene cooling resulted from erosion by the Indus River that transects the basin. An approximately coeval cooling signal is well documented east of the study area, along the collision zone in south Tibet. Our new data provide a regional framework upon which future studies can explore the possible interrelationships between tectonic, geodynamic, and geomorphologic factors contributing to Miocene-Pliocene cooling along the India-Asia collision zone from NW India to south Tibet.

(U-Th)/He Thermochronology of the Indus Group, Ladakh, Northwest India: Is Neogene Cooling a Continental-Scale Thermal Event in the India-Asia Collision Zone?

“…3 Ma. It is difficult to explain why the ZHe cooling ages from two different locations of the Basgo Formation in this study and in the work by Bhattacharya et al [12] differ from those determined by Tripathy-Lang et al [10] from the same formation. Nevertheless, our ZHe cooling ages from the Basgo Formation are compatible with the overall trend of the ZHe cooling ages determined from the Indus Group.…”

“…20-3 Ma ZHe and AHe ages from western to eastern Ladakh overlap with the published ca. 19-8 Ma ZHe, 14-12 Ma AFT, and 7-3 Ma AHe ages from the Indus Group that infer a postdepositional Miocene-Pliocene cooling signal in central Ladakh [6,12]. Our thermal models and the ca.…”

“…The older higher temperature cluster predicts the mean ZHe age, while the younger lower temperature cluster predicts the mean AHe age. The two clusters are essentially part of a single cooling event that initiated from temperatures >180-200°C, which can be independently verified from regional paleo-temperature estimates using illite crystallinity and ZFT analyses (e.g., [12,39]). The clusters are likely a consequence of averaging single-grain ZHe or AHe ages, and this problem may be avoided if more ZHe and AHe individual grain ages from a single sample are available to analyze the distribution of ages and predict each age using the method of Fox et al [49].…”

“…Low-temperature thermochronometry in the central part of the Indus basin, using zircon fission-track (ZFT), apatite fission-track (AFT), zircon (U-Th/He) (ZHe), and apatite (U-Th)/He (AHe) data, indicates burial to >200°C and unveils a postdepositional Miocene-Pliocene cooling signal from ca. 22 to 4 Ma [6,12]. AFT ages from two samples in central Ladakh and one sample in eastern Ladakh also record postdepositional Miocene cooling in the Indus Group [6,13].…”

“…It is possible that Hemis deposition terminated earlier; however, the lack of fossils in the Hemis Formation or in the overlying Nimu Formation (ca. 32 Ma detrital zircon U-Pb maximum depositional age [7,12] makes it challenging to precisely constrain the upper depositional age limit of the Hemis Formation. The depositional age of the Basgo Formation, which contains late Oligocene ostracod fossils [4] and has a youngest single-grain detrital zircon U-Pb maximum depositional age of ca.…”