ALBERT

Description: Precise knowledge of the timing of Indo-Eurasian collision is prerequisite for understanding the subsequent evolution of the Himalayan-Tibetan orogenic system, yet the topic remains controversial despite decades of research. We present new data for the Upper Oligocene Basgo Formation of the Indus Basin of NW India that specifically address the proposal that collision initiated no earlier than the Eocene-Oligocene boundary. The Basgo Formation has been cited as the base of the Indus Group because of its previously assumed Maastrichtian age. This age has been revised to Upper Oligocene, but the stratigraphic location has not been re-evaluated. As such, it has been used as evidence of Oligocene-aged collision between India and Eurasia. Based on age constraints in the remainder of the Indus Group, we revise the stratigraphy and place it instead toward the top of the succession. We present evidence that the zircons in the Basgo sandstones originated from the Indian passive margin. Because conglomerate clasts are known to come from the Transhimalayan batholith to the north, our data support mixed provenance and require a minimum late Oligocene age for India-Eurasia collision in the NW Indian Himalaya. (U-Th)/He cooling dates for detrital zircons from the Basgo Formation range from 52.6 to 28.25 Ma, however, implying that their most probable source, the Indian passive margin, was emergent and eroding prior to Oligocene time due to collision. These data alone do not speak to whether the Basgo Formation records pre-Oligocene collision of India and Eurasia or India and the Transhimalayan Ladakh batholith, but as of the date of this publication, there is no evidence for Oligocene collision anywhere else in the Ladakh region. Thus, we interpret our data to demonstrate terminal collision between India and Eurasia prior to Oligocene time.

Description: In the central Himalaya, past researchers have identified a distinctive transition from the physiographic Lower Himalayan ranges in the south to the Higher Himalayan ranges in the north. Local relief and hillslope gradient, as well as erosion and surface uplift rates, increase abruptly across this transition to the north. In the eastern Himalaya, the same physiographic transition exists, but it is less dramatic. We describe here a previously undocumented steep, north-dipping, brittle structure that is roughly coincident with this physiographic transition in eastern Bhutan—the Lhuentse fault. Low-temperature (U-Th)/He apatite data suggest that the Lhuentse fault has been active since the Pliocene, and (U-Th)/He dates on offset hydrothermal hematite deposits from within the fault zone demonstrate a component of Quaternary slip. Although we identified no definitive evidence of fault kinematics based on field or petrographic analysis of the fault rocks, the disrupted pattern of (U-Th)/He apatite dates suggests normal-sense displacement, contrary to what was expected given previous studies of an analogous transition in the central Himalaya. We regard the existence and activity of the Lhuentse fault as evidence of (1) recent evolution in the tectonic regime of the eastern Himalaya from one of near-exclusive north-south shortening to one in which both transcurrent and normal faulting are increasingly important in the region north of the Himalayan deformation front, or (2) an active duplex south of the physiographic transition in the middle latitudes of Bhutan.

Description: A precise age for the collision of the Kohistan-Ladakh block with Eurasia along the Shyok suture zone (SSZ) is one key to understanding the accretionary history of Tibet and the tectonics of Eurasia during the India-Eurasia collision. Knowing the age of the SSZ also allows the suture to be used as a piercing line for calculating total offset along the Karakoram Fault, which effectively represents the SE border of the Tibetan Plateau and has played a major role in plateau evolution. We present a combined structural, geochemical, and geochronologic study of the SSZ as it is exposed in the Nubra region of India to test two competing hypotheses: that the SSZ is of Late Cretaceous or, alternatively, of Eocene age. Coarse-continental strata of the Saltoro Molasse, mapped in this area, contain detrital zircon populations suggestive of derivation from Eurasia despite the fact that the molasse itself is deposited unconformably onto Kohistan-Ladakh rocks, indicating that the molasse is postcollisional. The youngest population of detrital zircons in these rocks (approximately 92 Ma) and a U/Pb zircon date for a dike that cuts basal molasse outcrops (approximately 85 Ma) imply that deposition of the succession began in the Late Cretaceous. This establishes a minimum age for the SSZ and rules out the possibility of an Eocene collision between Kohistan-Ladakh and Eurasia. Our results support correlation of the SSZ with the Bangong suture zone in Tibet, which implies a total offset across the Karakoram Fault of approximately 130–190 km.

Description: The observed geomorphology and calculated thermal histories of the Bhutan Himalaya provide an excellent platform to test ideas regarding the influence of tectonics and climate on the evolution of a convergentmountain range. However, little consensus has been reached regarding the late Cenozoic history of the Bhutan Himalaya. Some researchers have argued that observed geologic relationships show slowing deformation rates, such that the range is decaying from a geomorphic perspective, while others see the range as growing and steepening. We suggest that a better understanding is possible through the integrated interpretation of geomorphic and thermochronometric data from the comparison of predictions from models of landscape evolution and thermal-kinematic models of orogenic systems. New thermochronometric data throughout Bhutan aremost consistent with a significant decrease in erosion rates, from2 to 3 km/Ma down to 0.1–0.3 km/Ma, around 6–4Ma. We interpret this pattern as a decrease in rock uplift rates due to the activation of contractional structures of the Shillong Plateau, an uplifted region approximately 100 km south of Bhutan. However, low-relief, fluvial landscapes throughout the Bhutanese hinterland record a late pulse of surface uplift likely due to a recent increase in rock uplift rates. Constraints from our youngest thermochronometers suggest that this increase in rock uplift and surface uplift occurred within the last 1.75Ma. These results imply that the dynamics of the Bhutan Himalaya and Shillong Plateau have been linked during the late Cenozoic, with structural elements of both regions active in variable ways and times over that interval.

Description: Highlights: • The central uplift of the Manicouagan impact structure has been dated by (U–Th)/He • A (U–Th)/He central age (207.1 ± 6.4 Ma, 2 standard error, n = 40) has been determined • This age reflects the rapid uplift, cooling and closure of He in ~ 1 Gyr-old titanites • This correlates with the previously determined U–Pb impact-melt age of 214 ± 1 Ma • Our new approach enables dating complex impact structures that lack impact melt rocks Abstract Forty titanite grain fragments from 9 central uplift samples of metamorphosed anorthosite from the Manicouagan impact structure were dated by the (U–Th)/He technique. A (U–Th)/He central age of 207.1 ± 6.4 Ma (2 standard error (SE), n = 40) has been determined. With 4 outlier ages removed the central age is refined to 208.9 ± 5.1 Ma (2 SE). Both of these ages are within error of the previously determined U–Pb zircon age of 214 ± 1 Ma (2σ) derived from the impact melt. Manicouagan's central uplift formed due to rapid elevation from ~ 7–10 km depth as part of the modification stage of the impact process, which has facilitated the dating of its emplacement due to resulting rapid exhumation and cooling and closure of the (U–Th)/He system in titanite. Correlation with the previous U–Pb zircon 214 ± 1 Ma impact melt crystallization age indicates that the (U–Th)/He titanite dating technique offers a new approach to dating complex impact structures in the absence of viable melt sheets, or other melt products. The youngest ca. 195 Ma (U–Th)/He dates preserved in some titanite fragments are synchronous with Early Jurassic, rift-induced lithospheric thinning and associated igneous activity that defines the Central Atlantic Magmatic Province (CAMP). The (U–Th)/He titanite data from Manicouagan indicate that the influence of this regional event may extend west of the previously proposed limit of CAMP activity.

Description: Quantitative constraints on the ages of melt-forming impact events on the Moon are based primarily on isotope geochronology of returned samples. However, interpreting the results of such studies can often be difficult because the provenance region of any sample returned from the lunar surface may have experienced multiple impact events over the course of billions of years of bombardment. We illustrate this problem with new laser microprobe 40Ar/39Ar data for two Apollo 17 impact melt breccias. Whereas one sample yields a straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document multiple impact melt–forming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga. Notably, published zircon U/Pb data indicate the existence of even older melt products in the same sample. The revelation of multiple impact events through 40Ar/39Ar geochronology is likely not to have been possible using standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe technique. Evidence for 3.83 Ga to 3.81 Ga melt components in these samples reinforces emerging interpretations that Apollo 17 impact breccia samples include a significant component of ejecta from the Imbrium basin impact. Collectively, our results underscore the need to quantitatively resolve the ages of different melt generations from multiple samples to improve our current understanding of the lunar impact record, and to establish the absolute ages of important impact structures encountered during future exploration missions in the inner Solar System.