Thermokinematic Modeling Research Articles

Exhumation of the central Wasatch Mountains, Utah: 2. Thermokinematic model of exhumation, erosion, and thermochronometer interpretation

The Wasatch fault is a ∼370 km long normal fault in Utah that marks the boundary between the stable Colorado Plateau to the east and the extending Basin and Range to the west. Understanding the thermokinematic evolution of this fault can provide insights into intracontinental extensional tectonics and deformation processes in other rift zones (e.g., East Africa Rift, Transantarctic Mountains). We explore the thermokinematics of footwall exhumation and erosion in the Cottonwood Intrusive Belt of the central Wasatch Mountains. Emphasis is placed on using low‐temperature thermochronometers to quantify (1) the spatial and temporal variability of exhumation and erosion rates, (2) the geometry of footwall tilt, (3) the fault dip angle, and (4) the magnitude and duration of exhumation. These processes are investigated using two‐dimensional (2‐D) thermal‐kinematic models coupled with cooling‐rate‐dependent kinetic models which predict exhumed apatite fission track (AFT) and (U‐Th)/He ages. The range of model parameters considered includes footwall exhumation and erosion rates at the fault between 0.2 and 2.0 mm yr−1, footwall tilt hinge positions between 15 and 40 km distance from the fault, a single planar normal fault with dip angles of 45° and 60°, and exhumation magnitudes of up to 15 km at the fault. Simulations include the formation of a low thermal conductivity sedimentary basin and erosion of heat‐producing layers. Erosion maintains a constant topographic profile. The kinematic and exhumation history of the Wasatch Mountains is investigated by comparing model predicted thermochronometer ages to observed AFT, ZFT, and (U‐Th)/He ages. Predicted and observed ages are compared using a reduced chi‐square analysis to determine a best fit kinematic model for the Wasatch Mountains. The preferred model includes exhumation occurring on either a 45° or 60° dipping fault, a footwall hinge located a minimum of 20–25 km from the fault, and a step decrease (deceleration) in the footwall exhumation rate at the fault from 1.2 to 0.8 mm yr−1 at around 5 Ma. The model also suggests an exhumation duration of ∼12 Myr ± 2 Myr).

A site for deep ice coring in West Antarctica: results from aerogeophysical surveys and thermo-kinematic modeling

AbstractThe U.S. Science Plan for Deep Ice Coring in West Antarctica calls for two ice cores to be collected. the first of these cores, from Siple Dome, was completed during the 1997/98 field season. the second core is to be collected from a site near the divide that separates ice flowing to the Ross Sea and to the Amundsen Sea.Using high-resolution, grid-based aerogeophysical surveys of the Ross/Amundsen ice-divide region, we identify seven candidate sites and assess their suitability for deep coring. We apply ice-flow and temperature calculations to predict time-scales and annual-layer resolution, and to assess the potential for basal melting for several selected sites. We conclude that basal melting is likely for sites with very thick ice, as was observed at the Byrd core site. Nevertheless, these sites are most attractive for coring since they promise recovery of a long climate record with comparatively high time resolution during the last glacial period.

Geochronologic and thermobarometric constraints on the evolution of the Main Central Thrust, central Nepal Himalaya

The Main Central Thrust (MCT) juxtaposes the high‐grade Greater Himalayan Crystallines over the lower‐grade Lesser Himalaya Formation; an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust. Garnet‐bearing assemblages sampled along the Marysandi River and Darondi Khola in the Annapurna region of central Nepal show striking differences in garnet zoning of Mn, Ca, Mg, and Fe above and below the MCT. Thermobarometry of MCT footwall rocks yields apparent inverted temperature and pressure gradients of ∼18°C km−1 and ∼0.06 km MPa−1, respectively. Pressure‐temperature (P‐T) paths calculated for upper Lesser Himalaya samples that preserve prograde compositions show evidence of decompression during heating, whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature. In situ (i.e., analyzed in thin section) ion microprobe ages of monazites from rocks immediately beneath the Greater Himalayan Crystallines yield ages from 18 to 22 Ma, whereas late Miocene and Pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence. A Lesser Himalayan sample collected near the garnet isograd along the Marysandi River transect contains 3.3±0.1 Ma monazite ages (P ≈ 0.72 GPa, T ≈ 535°C). This remarkably young age suggests that this portion of the MCT shear zone accommodated a minimum of ∼30 km of slip over the last 3 Ma (i.e., a slip rate of >10 mm yr−1) and thus could account for nearly half of the convergence across the Himalaya in this period. The distribution of ages and P‐T histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the MCT formed by the transposition of right‐way‐up metamorphic sequences during late Miocene‐Pliocene shearing.

Rheological implications of the thermal structure of the lithosphere in the convergence zone of the Eastern Carpathians

Four different thermo-kinematic models for the tectonic evolution of the convergence zone of the Eastern Carpathians are studied, in which processes like the pre-collisional subduction of an oceanic lithosphere, the underthrusting of the European continental margin in collision and the post-collisional thermal relaxation control the long term thermal structure of the study area. The thermal field obtained for each tectonic evolution has been used to evaluate the rheological structure along a NE–SW 500 km long lithospheric cross-section perpendicular to the Eastern Carpathians structures. The thermal structure and rheological behaviour of the lithosphere rocks are discussed in correlation with characteristics of the intermediate-depth seismicity of the active Vrancea region, placed at the southern end of the Eastern Carpathians. In front of the convergence fault and in the East European Platform a mechanical decoupling between crustal and mantle layers of the lithosphere could explain the absence of intermediate-depth earthquakes along the most part of the cross-section. The crust seems to be still coupled with the mantle in a narrow volume of the convergence area in the models including an oceanic subduction process. In this volume the mechanically strong lithosphere (MSL) reaches depths of about 220 km, close to the maximum depth of the seismic activity in the Vrancea region, and the lithosphere effective elastic thickness (EET) has values as high as 80–140 km. The peculiarities of the intermediate-depth seismic activity in the Vrancea area could be interpreted in terms of the rheological behaviour of the crustal and mantle material only if an episode of pre-collisional subduction of an oceanic lithosphere is assumed.

A lithospheric cross-section through the Swiss Alps-II. Constraints on the mechanical structure of a continent-continent collision zone

SUMMARY The calculation of strength profiles along the European Geotraverse (EGT) through the Swiss Alps yields constraints on the large-scale vertical and lateral mechanical structure through the Alpine continent-continent collision zone. Strength profiles are evaluated for different assumptions on petrological stratification and strain rate and are based on temperature-depth profiles derived from transient thermo-kinematic modelling of the Neoalpine orogeny. The main contribution to the total strength results from the mantle lithosphere, which is strongly controlled by temperature. In contrast, the crustal contribution is mainly determined by variations in petrological stratification. A direct correlation between surface heat flow and the total strength of the crust, the mantle lithosphere and the entire lithosphere (crust and mantle lithosphere) is not observed. Our results demonstrate that in tectonically active areas a transient thermal model, along with detailed knowledge of the deep structure and petrology, is necessary to evaluate lithospheric strength envelopes. Inside the collision zone, strain rate has a strong control on the bottom of the mechanically strong crust, whereas outside the collision zone the effect is less significant. The cut-off depth of seismicity along the profile, which correlates largely with the bottom of the mechanically strong crust, deviates from the 300-400°C isotherm. The inferred effective elastic thickness for the Molasse Basin north of the Alps is in agreement with flexural modelling results, whereas for the Southern Alps the predictions deviate.

A Lithospheric Cross-Section Through the Swiss Alps-I. Thermokinematic Modelling of the Neoalpine Orogeny

In this paper we develop a forward 2-D thermokinematic model to investigate the Neoalpine 35-0 Ma phase of orogeny along the European Geotraverse (EGT) through the Swiss Alps on a crustal and lithospheric scale. Using a divergence-free kinematic model (div v=0), we define mass displacements, which subsequently serve as input to a transient thermal model. the thermal model uses critically assessed material prorameters and accounts for the depth dependence of the thermal properties in processes such as crustal thickening and mantle-lithospheric subduction. Based on the presentday density pattern of the deep seismic image and estimated exhumation and shortening rates, we derive, in a first modelling step, a mass-displacement field describing the Neoalpine orogeny as a uniform process in time. In a second—thermal—modelling step, this kinematic scenario is further refined by modelling the non-uniform cooling histories of the southern Lepontine in the Penninic domain. For that purpose we adopt lithospheric shortening rates—and consequently exhumation rates—to agree with total Neoalpine shortening, while keeping the geometry of the kinematic model fixed. the resultant thermokinematic model reflects the main characteristics of Neoalpine tectonics, and shows a good overall agreement with combined geological and geophysical data. the asymmetric feature of the present-day tectonic structure along the profile is strongly reflected in the thermal structure of the lithosphere. This demonstrates the need for a kinematic model to investigate the deep-temperature field in active tectonic provinces. For further refinement of the model, the amounts of shortening have to be more precisely estimated, and a higher spatial density in geochronological and metamorphic data is required. Furthermore, surface heat-flow values are, up to now, too uncertain to constrain the predicted surface heat flow. In summary, our results show that we need, in particular, data constraining the horizontal component of the tectonic and thermal evolution. the results of the Neoalpine orogeny modelling demonstrate that the presented thermokinematic procedure yields a good first-order approximation to investigate crustal-scale and lithospheric processes. We conclude. therefore, that the approach presented provides the potential for application not only to continent-continent collision zones, but also to any active tectonic province.