Katharine W. Huntington

Influence of climate change and uplift on Colorado Plateau paleotemperatures from carbonate clumped isotope thermometry

The elevation history of Earths surface is key to understanding the geodynamic processes responsible for the rise of plateaus. We investigate the timing of Colorado Plateau uplift by estimating depositional temperatures of Tertiary lake sediments that blanket the plateau interior and adjacent lowlands using carbonate clumped isotope paleothermometry (a measure of the temperature-dependent enrichment of ^(13)C-^(18)O bonds in carbonates). Comparison of modern and ancient samples deposited near sea level provides an opportunity to quantify the influence of climate and therefore assess the contribution of changes in elevation to the variations of surface temperature on the plateau. Analysis of modern lake calcite from 350 to 3300 m elevation in the southwestern United States reveals a lake water carbonate temperature (LCT) lapse rate of 4.2 ± 0.6°C/km. Analysis of Miocene deposits from 88 to 1900 m elevation in the Colorado River drainage suggests that the ancient LCT lapse rate was 4.1 ± 0.7°C/km, and temperatures were 7.7 ± 2.0°C warmer at any one elevation than predicted by the modern trend. The inferred cooling is plausible in light of Pliocene temperature estimates off the coast of California, and the consistency of lapse rates through time supports the interpretation that there has been little or no elevation change for any of the samples since 6 Ma. Together with previous paleorelief estimates from apatite (U-Th)/He data from the Grand Canyon, our results suggest most or all of the plateaus lithospheric buoyancy was acquired ∼80–60 Ma and do not support explanations that ascribe most plateau uplift to Oligocene or younger disposal of either the Farallon or North American mantle lithosphere.

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

Sandy signs of a tsunami's onshore depth and speed

Tsunamis rank among the most devastating and unpredictable natural hazards to affect coastal areas. Just 3 years ago, in December 2004, the Indian Ocean tsunami caused more than 225,000 deaths. Like many extreme events, however, destructive tsunamis strike rarely enough that written records span too little time to quantify tsunami hazard and risk. Tsunami deposits preserved in the geologic record have been used to extend the record of tsunami occurrence but not the magnitude of past events. To quantify tsunami hazard further, we asked the following question: Can ancient deposits also provide guidance on the expectable water depths and speeds for future tsunamis?

A comparative study of detrital mineral and bedrock age-elevation methods for estimating erosion rates

traditional age-elevation method using detrital and bedrock 40 Ar/ 39 Ar muscovite data sets from a single river catchment in the Annapurna Range, Nepal Himalaya. A nominal erosion rate estimate of � 0.6 km/Myr for the 5.0–2.5 Ma period was calculated from the bedrock data, presented here for the first time. This result agrees with the � 0.7 km/Myr (maximum) estimate from the detrital data set, which was derived from the DMT variant that emphasizes the range of single-grain ages for a detrital sample, in this case, 11–2.5 Ma. However, the other DMT variant, which emphasizes the mean of the sample age distribution, yields an erosion rate estimate of � 2.3 km/Myr. The simplest explanation for this discrepancy is that erosion rate increased significantly after � 2.5 Ma, a scenario that is supported by apatite fission track data from the catchment.

Erosion of the Tsangpo Gorge by megafloods, Eastern Himalaya

At the southeastern margin of the Tibetan Plateau, the Yarlung-Tsangpo River plunges through the Himalaya to drop >2 km through the Tsangpo Gorge. Upstream, relict glacial dams and impounded lake terraces suggest that Quaternary lakes as large as 800 km 3 catastrophically drained through the gorge as megafloods. We report on new megaflood deposits downstream of the gorge and use detrital zircon U-Pb provenance data to demonstrate that these high-magnitude events originated in Tibet, and more effectively focused erosion in the gorge than both the extremely erosive modern peak flows and one of the largest landslide-dam outburst floods ever documented. Our findings support the proposition that in this steep, narrow gorge, where hillslope angles are near the threshold angle of bedrock failure, megafloods provide a mechanism to rapidly evacuate hillslope material and focus erosion on channel-adjacent hillslopes. Although megaflood frequency remains unconstrained, we demonstrate the capability of these events to contribute substantially to rapid exhumation in this region.

Rapid exhumation of the eastern Himalayan syntaxis since the late Miocene

The Himalayan syntaxes are exceptionally dynamic landscapes characterized by high-relief topography and some of the most rapid and focused crustal exhumation on Earth. In the eastern Himalayan syntaxis, it has been hypothesized that thermo-mechanical feedbacks between erosion by the Yarlung River and growth of a crustal-scale antiform may have locally sustained exhumation rates exceeding 5 km/m.y. during the late Pliocene and Pleistocene. However, young (younger than 3 Ma) cooling histories from syntaxial bedrock samples restrict interpretations of the timing and mechanism initiating feedback development. To extend this record of landscape evolution, we reconstructed an exhumation history since the late Miocene from analysis of detrital minerals in Himalayan foreland basin deposits. We combined magnetostratigraphy, detrital white mica 40Ar/39Ar thermochronology, and coupled zircon U-Pb and fission-track geothermochronology from a 4.6-km-thick stratigraphic section proximal to the eastern syntaxis. We used a simple thermal model to interpret the combined provenance and lag-time data set, concluding that rock exhumation rates in the core of the syntaxis increased by a factor of 5–10 in the late Miocene and have sustained extremely rapid exhumation rates (>5 km/m.y.) since 5 Ma. This onset significantly postdates the first appearance of Tibetan detritus in the Himalayan foreland, suggesting that thermo-mechanical feedbacks sustaining rapid exhumation are unrelated to river integration. Instead, such feedbacks may develop where large, antecedent rivers sustain elevated erosion rates across a region of enhanced rock uplift. Compilation of similar data sets across the Himalaya demonstrates extraordinary syntaxial exhumation histories, potentially resulting from peculiar geodynamics at these orogenic margins.

High late Miocene-Pliocene elevation of the Zhada Basin, southwestern Tibetan Plateau, from carbonate clumped isotope thermometry

The timing and pattern of Tibetan Pla- teau rise provide a critical test of possible mechanisms for the development and sup- port of high topography, yet views range widely on the history of surface uplift to modern elevations of ~4.5 km. To address this issue we present clumped isotope ther- mometry data from two well-studied ba- sins in central and southwestern Tibet, for which previous carbonate δ 18 O data have been used to reconstruct high paleoeleva- tions from late Oligocene to Pliocene time. Clumped isotope thermometry uses mea- surements of the 13 C- 18 O bond ordering in carbonates to constrain the temperature (T(Δ47)) and δ 18 O value of the water from which the carbonate grew. These data can be used to infer paleoelevation by exploiting the systematic decrease of surface tempera- ture and the δ 18 O-based paleoaltimetry and with paleontological and isotopic data indicating the presence of cold- adapted mammals living in a cold, high-ele- vation climate. We suggest that late Neogene elevation loss across the Zhada Basin catch- ment probably related to local expression of east-west extension across much of the south- ern Tibetan Plateau at this time.

Monsoon control of effective discharge, Yunnan and Tibet

Analysis of suspended sediment transport data for rivers in Yunnan and Tibet shows that monsoon fl ows control effective discharge. We calculate effective discharge, defi ned as the discharge that transports the most sediment, for 44 stations for which there is at least one complete year of daily suspended sediment concentration and mean daily discharge data, and fi nd that the effective discharge is approximately the mean monsoon discharge for all stations. The correspondence of the effective discharges with the mean annual fl ow and monsoon discharge for all stations demonstrates that monsoon fl ow dominates suspended sediment transport in the region, rather than storm fl ow during discrete, short-duration storm events. In this region, the monsoon lasts for 4 months (June‐September) and during that time transports 86% of the suspended sediment load. In contrast to the general observation from temperate environments that infrequent, stochastic storm events dominate sediment transport (with 90% of the suspended sediment transport occurring in 10% of the time), our fi ndings show that the mean monsoon discharge dominates sediment transport in the rivers draining the southeastern Tibetan Plateau.

Uplift of the Central Andes of NW Argentina associated with upper crustal shortening, revealed by multiproxy isotopic analyses

This study contributes to the uplift history of the Andes, which has received increasing attention in recent years because of its implications for geodynamic models and climate feedbacks. Shortening resulting in crustal thickening and removal of gravitationally unstable mantle lithosphere has been proposed to control deformation and uplift of Cordillera-type orogenic systems such as the Puna Plateau of the central Andes and its eastern margin, the Eastern Cordillera. We present new clumped isotope (Δ47), δ18O, and δ2Η data from carbonate nodules, marlstone, spring deposits, and volcanic ashes from the Puna Plateau and Eastern Cordillera of NW Argentina. When combined with other geological evidence, our data indicate that the Puna Plateau was near its present elevation since at least ~10 Ma, whereas the Eastern Cordillera rose ~1.5 km between ~14 and ~7 Ma. This history of uplift correlates with active shortening in the Eastern Cordillera and with incorporation of a regional foreland into the propagating orogenic wedge. Our study suggests that the elevation of the Puna Plateau changed little during the Miocene-Pliocene, whereas the margin experienced significant uplift associated with active deformation and crustal thickening.

TRACING PALEOFLUID SOURCES USING CLUMPED ISOTOPE THERMOMETRY OF DIAGENETIC CEMENTS ALONG THE MOAB FAULT, UTAH

Interactions among fluids, deformation structures, and chemical changes in sediments impact deformation of the shallow crust, influencing the preservation and extraction of the economic resources it contains. These interactions have been studied along the Moab Fault, in the Paradox Basin, Utah, where diagenetic cements, joints, cataclastic deformation bands and slip surfaces developed during faulting are thought to control fault permeability. Previous fluid inclusion micro-thermometry and stable isotopic data from calcite cements collected along segments of the Moab Fault suggest cements precipitated from hot basin fluids that migrated up the fault and interacted with a shallower meteoric groundwater source. In this study, we investigate the interactions of these fluids with deformation structures using clumped isotope thermometry of calcite cements along the Moab Fault. Guided by prior high-resolution mapping of deformation structures and calcite cements, we measured the growth temperature of calcite cements collected at varying distance from fault segments and fault intersections. Cement temperatures from individual segments vary greatly; cements along a relatively simple fault segment indicate temperatures ranging from 67 to 128 °C, similar to previously published fluid inclusion homogenization temperatures from a cement sample collected in the same locality, while a nearby fault intersection hosts cements with temperatures of 13 to 88 °C. The spatial pattern of cement temperatures revealed by clumped isotope thermometry suggests that intensely jointed zones associated with fault intersections enable rapid down-fault migration of cool surface waters and that deformation-band faults with their associated slip surfaces may further compartmentalize fluid flow, restricting fluid sources to warm waters thermally equilibrated with the country rock outside the jointed zone. Our data confirm that the relationship between faults and fluid flow can vary greatly over short length scales, and suggest that some fracture zones can be highly conductive to depths as great as 2 km.