David M. Whipp

Geometry and kinematics of the Main Himalayan Thrust and Neogene crustal exhumation in the Bhutanese Himalaya derived from inversion of multithermochronologic data

Both climatic and tectonic processes affect bedrock erosion and exhumation in convergent orogens, but determining their respective influence is difficult. A requisite first step is to quantify long-term (~106 year) erosion rates within an orogen. In the Himalaya, past studies suggest long-term erosion rates varied in space and time along the range front, resulting in numerous tectonic models to explain the observed erosion rate distribution. Here, we invert a large data set of new and existing thermochronological ages to determine both long-term exhumation rates and the kinematics of Neogene tectonic activity in the eastern Himalaya in Bhutan. New data include 31 apatite and five zircon (U-Th)/He ages, and 49 apatite and 16 zircon fission-track ages along two north-south oriented transects across the orogen in western and eastern Bhutan. Data inversion was performed using a modified version of the 3-D thermokinematic model Pecube, with parameter ranges defined by available geochronologic, metamorphic, structural, and geophysical data. Among several important observations, our three main conclusions are as follows: (1) Thermochronologic ages do not spatially correlate with surface traces of major fault zones but appear to reflect the geometry of the underlying Main Himalayan Thrust; (2) our data are compatible with a strong tectonic influence, involving a variably dipping Main Himalayan Thrust geometry and steady state topography; and (3) erosion rates have remained constant in western Bhutan over the last ~10 Ma, while a significant decrease occurred at ~6 Ma in eastern Bhutan, which we partially attribute to convergence partitioning into uplift of the Shillong Plateau.

Quantifying canyon incision and Andean Plateau surface uplift, southwest Peru: A thermochronometer and numerical modeling approach

incision. We quantify the timing and magnitude of incision by integrating previously published ages from the valley bottom with 19 new sample ages from four valley wall transects. Interpretation of the incision history from cooling ages is complicated by a southwest to northeast increase in temperatures at the base of the crust due to subduction and volcanism. Furthermore, the large magnitude of incision leads to additional threedimensional variations in the thermal field. We address these complications with finite element thermal and thermochronometer age prediction models to quantify the range of topographic evolution scenarios consistent with observed cooling ages. Comparison of 275 model simulations to observed cooling ages and regional heat flow determinations identify a best fit history with � 0.2 km of incision in the forearc region prior to � 14 Ma and up to 3.0 km of incision starting between 7 and 11 Ma. Incision starting at 7 Ma requires incision to end by � 5.5 to 6 Ma. However, a 2.2 Ma age on a volcanic flow on the current valley floor and 5 Ma gravels on the uplifted piedmont surface together suggest that incision ended during the time span between 2.2 and 5 Ma. These additional constraints for incision end time lead to a range of best fit incision onset times between 8 and 11 Ma, which must coincide with or postdate surface uplift.

Effects of exhumation kinematics and topographic evolution on detrital thermochronometer data

Detrital thermochronometer data collected from modern rivers or sedimentary basins have the potential to record the evolution of topographic relief, fault kinematics and erosion within drainage basins. However, few studies have addressed the effects of these different factors on detrital thermochronometer age distributions. Here we use transient 3-D thermokinematic and landform evolution models to simulate the effects of time-varying topography and fault kinematics on the thermal field through which detrital samples cool. Cooling-rate-dependent apatite (U-Th)/He (AHe), zircon fission track and muscovite 40Ar/39Ar (MAr) grain age distributions are predicted for samples collected from modern river and basin sediments. These distributions are interpreted to determine the sensitivity of different thermochronometer systems to denudation and deformation histories in drainage basins of varying size. We find that detrital thermochronometers in rapidly eroding regions have a strong sensitivity to the kinematics of exhumation, but lack sensitivity to changes in topographic relief under most conditions. In addition, we find potential for significant overestimation of denudation rates derived from conventional 1-D age-elevation relationships as compared to 3-D model-prescribed rates. At rapid (~2.5 mm/y) model-prescribed denudation rates, 1-D techniques predict rates that are ~5 and ~2 times greater than the 3-D model rate for the AHe and MAr systems, respectively. In models that explore age distributions in foreland basin sediments, we confirm that the lag time concept is a useful and reliable means for identifying denudation rate changes as no significant change in lag time occurs for changing topographic relief scenarios.

Topography, exhumation pathway, age uncertainties, and the interpretation of thermochronometer data

[1] The relationship between thermochronometer age and structural elevation is commonly used to infer long-term exhumation histories. Previous studies suggest that inferred exhumation rates from the conventional (one-dimensional, 1-D) age-elevation approach are sensitive to topography and variations in exhumation rate and pathway. Here we evaluate the magnitude of these effects by (1) using a 3-D thermalkinematic model of the central Nepalese Himalaya to predict age-elevation profiles for multiple thermochronometers as a function of exhumation rate and pathway (vertical, oblique, or thrust fault), and (2) calculating the probability that the true exhumation rate will be recovered from an age-elevation profile for sample uncertainties of different magnitudes. Results suggest that profiles oriented orthogonal to longwavelength topography and the direction of lateral transport are relatively insensitive to their influence. For profiles oriented parallel to the transport direction, horizontal transport during exhumation partly counteracts topographic effects. The difference between model imposed and 1-D exhumation rates from the slope of a best fit line through an ageelevation plot is greatest when rocks are exhumed vertically and low-temperature thermochronometers are used. The magnitude of error in 1-D exhumation rate estimates varies dramatically as a function of sample uncertainty, particularly when exhumation is rapid. The nature of this variation can be used to design sampling strategies for which 1-D interpretations of age-elevation gradients are likely to be within error of the true exhumation rate. Alternatively, if sample uncertainties can be reduced, studies that combine thermal modeling with age-elevation data can potentially provide important constraints on thermal and kinematic fields at depth. Citation: Huntington, K. W., T. A. Ehlers, K. V. Hodges, and D. M. Whipp Jr. (2007), Topography, exhumation pathway, age uncertainties, and the

Feeding the “aneurysm”: Orogen-parallel mass transport into Nanga Parbat and the western Himalayan syntaxis

The Nanga Parbat-Haramosh massif (NPHM; western Himalayan syntaxis) requires an influx of mass exceeding that in the adjacent Himalayan arc to sustain high topography and rapid erosional exhumation rates. What supplies this mass flux and feeds this “tectonic aneurysm?” We show, using a simple 3-D model of oblique orogen convergence, that velocity/strain partitioning results in horizontal orogen-parallel (OP) crustal transport, and the same behavior is inferred for the Himalaya, with OP transport diverting converging crust toward the syntaxis. Model results also show that the OP flow rate decreases in the syntaxis, thereby thickening the crust and forming a structure like the NPHM. The additional crustal thickening, over and above that elsewhere in the Himalayan arc, sustains the rapid exhumation of this “aneurysm.” Normally, velocity/strain partitioning would be minimal for the Himalayan arc where the convergence obliquity is no greater than ~40°. However, we show analytically that the Himalayan system can act both as a critical wedge and exhibit strain partitioning if both the detachment beneath the wedge and the bounding rear shear zone, which accommodates OP transport, are very weak. Corresponding numerical results confirm this requirement and demonstrate that a Nanga Parbat-type shortening structure can develop spontaneously if the orogenic wedge and bounding rear shear zone can strain rate soften while active. These results lead us to question whether the position of NPHM aneurysm is localized by river incision, as previously suggested, or by a priori focused tectonic shortening of the crust in the syntaxis region as demonstrated by our models.