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.

North-south-directed extension on the South Tibetan Fault System (STFS) played an important role in Himalayan tectonics of the Miocene Period, and it is generally assumed that orogen-perpendicular extension ceased in this orogenic system before the Pliocene. However, previous work in the Annapurna and Dhaulagiri Himalaya of central Nepal revealed evidence for local Pleistocene reactivation of the basal STFS structure in this area (the Annapurna Detachment). New structural mapping and (U-Th)/He apatite and zircon thermochronology in this region further document the significance of Pleistocene N-S extension in this sector of the Himalaya. Patterns of (U-Th)/He accessory-mineral ages are not disrupted across the reactivated segment of the STFS basal detachment, indicating that Pleistocene offset was limited. In contrast, the trace of a N-dipping, low-angle detachment in the hanging wall of the reactivated Annapurna Detachment - formerly linked to the STFS, but here named the Dhaulagiri Detachment - coincides with an abrupt break in the cooling-age pattern in two different drainages ∼20 km apart, juxtaposing Miocene hanging-wall dates against Pleistocene footwall dates. Our observations, combined with previous fission-track data from the region, provide direct evidence for significant N-S extension in the central Himalaya as recently as the Pleistocene.

The southern Tibetan Plateau margin between ~ 83E and 86.5E is defined by an abrupt change from the low-relief Tibetan Plateau to the rugged topography and deep gorges of the Himalaya. This physiographic transition lies well to the north of active thrusting, and thus, the mechanism responsible for the distinct topographic break remains the focus of much debate. While numerous studies have utilized thermochronology to examine the exhumation history of the Himalaya, few have done so with respect to variations across the Himalaya-Tibetan Plateau transition. In this work, we examine the nature of the transition where it is accessible and well-defined in the Nyalam valley of south-central Tibet. We employ several new and previously published thermochronologic datasets (with a closure temperature range of ~ 70C–300C) in conjunction with river incision patterns inferred by the longitudinal profile of the Bhote Kosi River.

The results reveal a sharp change in cooling rate at ~ 3.5 Ma at a location corresponding to a pronounced river knickpoint representing a sharp increase in river gradient and presumably incision rate (a proxy for rock uplift). Margin retreat models for the physiographic transition are inconsistent with the cooling pattern revealed by low-temperature thermochronologic data. Models invoking passive uplift of the upper crust over a midcrustal ramp and associated duplex to account for the physiographic transition do not explain the observed break in cooling rate there, although they may explain a suggesting in the thermochronologic data of progressively increasing exhumation rates south of the transition. The simplest model consistent with all observations is that passive uplift is augmented by contemporaneous differential uplift across a young (Pliocene-Quaternary) normal fault at the physiographic transition. Drawing on observations elsewhere, we hypothesize that similar structural relationships may be characteristic of the Tibetan Plateau-Himalaya transition from ~83E – 86.5E.