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[1] The southern Tibetan Plateau margin between~83 E and 86.5 E 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-300 C) 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~83 E -86.5 E.

Groundwater travel time is an important criterion for regulating managed aquifer recharge ͑MAR͒ operations because of its relationship to water quality. Here, three complementary methods for determining travel times are examined. Sulfur hexafluoride ͑SF 6 ͒, a gas tracer, was injected into 23 spreading basins at the Montebello Forebay MAR operation ͑Los Angeles County, United States͒ and monitored at ten monitoring and 18 production wells within 150 m. Over 2 years, SF 6 was detected at nine monitoring and 11 production wells. Travel times showed a significant relationship with depth, but not with horizontal distance or pumping rate. A pumping influence was apparent as the tracer arrived sooner at production wells then at monitoring wells of similar depth. In the unconfined aquifer, estimated hydrogeologic travel times were Ͻ0.2 years ͑Ͻ10 weeks͒ and agree with the SF 6 data. However, in the confined aquifers, estimated travel times were Ͼ4 years and the agreement with the SF 6 travel times was poor. At the seven production wells with SF 6 detections, leakage through low permeability layers leading to earlier tracer arrival provides a likely explanation. All tritium/ 3 He ages at production wells are greater than 10 years; this data combined with the SF 6 results indicate the wells produce a mixture of young and old groundwater.