Jean Braun

Quantifying the effect of recent relief changes on age–elevation relationships

The effect that recent relief changes may have on the distribution of rock ages with elevation is investigated for a range of thermochronometers. From the solution of the heat transport equation in a crustal block undergoing uplift and surface erosion, the temperature history of rock particles that are exhumed at the Earth’s surface today is computed. These T-t paths are then used to calculate apparent isotopic ages for the (U–Th)/He system in apatite, characterized by a low (≈70°C) closure temperature. The results show that recent relief changes strongly affect the distribution of ages with elevation (notably the slope of the age–elevation relationship). The calculations presented here predict that, in most situations, regions that have undergone a steady decrease in surface relief in the recent past should be characterized by an inverted age–elevation relationship, that is older ages should be found near valley bottoms and younger ages near summit tops. It is also shown how the wavelength of the topography, the geothermal gradient, the exhumation rate and the duration of the relief reduction event affect this result.

Pecube: a new finite-element code to solve the 3D heat transport equation including the effects of a time-varying, finite amplitude surface topography ☆

Abstract A robust finite-element code ( Pecube ) has been developed to solve the three-dimensional heat transport equation in a crustal/lithospheric block undergoing uplift and surface erosion, and characterized by an evolving, finite-amplitude surface topography. The time derivative of the temperature field is approximated by a second-order accurate, mid-point, implicit scheme that takes into account the changing geometry of the problem. The method is based on a mixed Eulerian–Lagrangian approach that requires frequent re-interpolation of the temperature field in the vertical direction to ensure accuracy. From the computed crustal thermal structure, the temperature history of rock particles that, following an imposed tectonic scenario, are exhumed at the Earths surface, is derived. These T − t paths can then be used to compute apparent isotopic ages for a range of geochronometers. The usefulness of the code is demonstrated by computing the predicted distribution of (U–Th)/He apatite ages in a high relief area of the Sierra Nevada, California, for a range of tectonic scenarios and comparing them to existing data.

Control of detachment geometry on lateral variations in exhumation rates in the Himalaya: Insights from low‐temperature thermochronology and numerical modeling

[1]xa0The Himalayan range is commonly presented as largely laterally uniform from west to east. However, geological structures, topography, precipitation rate, convergence rates, and low-temperature thermochronological ages all vary significantly along strike. Here, we focus on the interpretation of thermochronological data sets in terms of along-strike variations in geometry and kinematics of the main crustal detachment underlying the Himalaya: the Main Himalayan Thrust (MHT). We report new apatite fission track (AFT) ages collected along north-south transects in western and eastern central Nepal (at the latitudes of the Annapurna and Langtang massifs, respectively). AFT ages are consistently young (<3 Ma) along both N-S transects in the high-relief zone of the Higher Himalaya and increase (4 to 6 Ma) toward the south in the Lesser Himalaya. We compare our new data to published low-temperature thermochronological data sets for Nepal and the Bhutan Himalaya. We use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the Pecube code in order to constrain potential along-strike variations in the kinematics of the Himalayan range. Our results show that lateral variations in the geometry of the MHT (in particular the presence or absence of a major crustal-scale ramp) strongly control the kinematics and exhumation history of the orogen.

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.

Late Neogene exhumation and relief development of the Aar and Aiguilles Rouges massifs (Swiss Alps) from low-temperature thermochronology modeling and 4He/3He thermochronometry

[1]xa0The late Neogene–Quaternary exhumation history of the European Alps is the subject of controversial findings and interpretations, with several thermochronological studies arguing for long-term steady state exhumation rates, while others have pointed to late Miocene–Pliocene exhumation pulses associated with tectonic and/or climatic changes. Here, we perform inverse thermal-kinematic modeling on dense thermochronological data sets combining apatite fission track (AFT) data from the literature and recently published apatite (U-Th-Sm)/He (AHe) data along the upper Rhone valley (Aar and Aiguilles Rouges massifs, Swiss Alps) in order to derive precise estimates on the denudation and relief history of this region. We then apply forward numerical modeling to interpret cooling paths quantified from apatite 4He/3He thermochronometry, in terms of denudation and relief-development scenarios. Our modeling results highlight the respective benefits of using AFT/AHe thermochronology data and 4He/3He thermochronometry for extracting quantitative denudation and relief information. Modeling results suggest a late Miocene exhumation pulse lasting until ∼8–10xa0Ma, consistent with recently proposed exhumation histories for other parts of the European Alps, followed by moderate (∼0.3–0.5xa0km Myr−1) denudation rates during the late Miocene/Pliocene. Both inverse modeling and 4He/3He data reveal that the late stage exhumation of the studied massifs can be explained by a significant increase (∼85–100%) in local topographic relief through efficient glacial valley carving. Modeling results quantitatively constrain Rhone valley carving to 1–1.5xa0km since ∼1xa0Ma. We postulate that recent relief development within this part of the Swiss Alps is climatically driven by the onset of major Alpine glaciations at the mid-Pleistocene climate transition.

Recurrent internal waves in a small lake: Potential ecological consequences for metalimnetic phytoplankton populations

Worldwide, small lakes (xa0<xa01xa0km2) are numerically dominant, yet the potential for interaction between physical and ecological processes therein has been largely ignored. High-frequency time series of the thermal and current structures in a small dimictic lake (Lake Bromont, Quebec) revealed the occurrence of recurrent internal waves during the summer of 2007. Amplitudes and frequencies of the internal wave modes were characterized, along with wind and stratification conditions, during two focal periods of 5 days at the beginning and the end of the summer. Owing to a resonance with the daily wind, the second and third vertical mode seiches dominated over the first mode, which was observed only during larger wind events. Although the lake is small (0.41xa0km2) and shallow (mean depth of 4xa0m), the response of the thermal structure of the lake to wind forcing was very similar to that of alpine and other deep lakes. The phytoplankton community was controlled by the contrasting gradients of light and nutrients. Consequently, metalimnetic communities of cyanobacteria exposed to the recurrent internal waves, which occurred throughout the summer, formed the dominant phytoplankton biomass in the lake. The regular vertical excursion of the metalimnion influenced both light availability and nutrient fluxes and most likely contributed to an enhanced algal biomass.

Subducting slabs: Jellyfishes in the Earth's mantle

The constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations.

Cyclic development of sedimentary basins at convergent plate margins — 1. Structural and tectono-thermal evolution of some gondwana basins of eastern Australia

Abstract The Devono-Carboniferous Drummond Basin of eastern Australia formed by extensional tectonics, most probably in a back-arc setting, as an oceanic plate subducted westward under Gondwanas continental margin. Within this basin a syn-rift sequence and a relatively thick post-rift sequence are recognized, and the latter is separated from the overlying Galilee Basin by a mid-Carboniferous unconformity, which heralds a time of relatively mild compression, uplift and folding. The lower Galilee Basin formed as a foreland, secondary peripheral bulge, or mixed-style basin during the Late Carboniferous to Early Permian time interval. A mid-Permian unconformity separates it from the extensive Late Permian to Mid-Triassic upper Galilee Basin, here suggested to be a platform basin. Nest, the main, Mid- to Late Triassic compressional event led to reverse movement along previously normal faults, folding, uplift and erosion of up to 2 km of section from the Galilee and Bowen basins. During Jurassic and Cretaceous times, the Eromanga/Surat/Carpentaria Basin, a platform basin originally >1.7 million km 2 in extent, developed cratonward of the zone of continuing subduction. Subsequently, the culminating extensional event took place further east, where Lord Howe Rise rifted apart from the continental landmass and drifted eastward as oceanic seafloor spread in the Tasman Basin, and widespread uplift and erosion occurred over the eastern Australian coastal area. New data, re-interpretation of existing data and extensive literature support the interpretation of the Phanerozoic tectonic evolution of eastern Australia in a context of convergent plate margins. Within this context, overall eastward migration through time of the subduction zone and associated morphotectonic entities (e.g. the Warburton, Adavale, Drummond, Bowen and Tasman extensional basins) and vertical stacking of unconformity-bound extensional, foreland and platform basins (e.g. the Drummond, Galilee and Eromanga basins) occurred.

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.

Exhumation and relief development in the Pelvoux and Dora‐Maira massifs (western Alps) assessed by spectral analysis and inversion of thermochronological age transects

[1]xa0We have used dedicated sampling and analysis of apatite fission track (AFT) and apatite (U-Th)/He (AHe) thermochronological data sets in an attempt to quantify relief evolution and exhumation rates in the Pelvoux and Dora-Maira massifs (western European Alps). A dual approach comparing spectral analysis and thermal-kinematic model inversion was applied. We sampled age-elevation relationships at a range of topographic wavelengths along two north-south transects crossing these massifs. For the 40-km-long Pelvoux transect we report 35 new AFT ages that range between 3.0 ± 0.4 Ma and 12.6 ± 1.0 Ma, and 8 new AHe ages between 3.5 ± 1.5 and 4.7 ± 0.7 Ma. The Dora-Maira transect spans a distance of 60 km and includes 28 (23 new and 5 previously published) significantly older AFT ages, which vary between 13.1 ± 0.2 and 27.3 ± 0.3 Ma. Inferred exhumation rates of 0.7 km m.y.−1 since ∼8 Ma in the Pelvoux and only 0.1 km m.y.−1since ∼20 Ma in Dora-Maira are consistent between methods as well as with previous estimates, although they are associated with large uncertainties due to imperfect sampling and analytical errors. Neither massif displays clear evidence for relief change during the intervals constrained by the data. Data from the Dora-Maria massif suggest moderate recent relief increase, whereas the results from the Pelvoux are inconclusive but could imply relief decrease. Our study highlights the difficulties of applying thermochronology techniques to constrain relief changes. We show that an unreasonable number of samples is needed to perform reliable spectral analysis of age-elevation transects and discuss the overall limitations of both techniques to address relief evolution. We suggest that a more efficient approach would be to apply thermal-kinematic model inversion to relatively small study areas using spatially distributed sampling and multiple thermochronometer analyses per sample.