Advection Of Isotherms Research Articles

Tomography-based monitoring of isothermal snow metamorphism under advective conditions

Abstract. Time-lapse X-ray microtomography was used to investigate the structural dynamics of isothermal snow metamorphism exposed to an advective airflow. The effect of diffusion and advection across the snow pores on the snow microstructure were analysed in controlled laboratory experiments and possible effects on natural snowpacks discussed. The 3-D digital geometry obtained by tomographic scans was used in direct pore-level numerical simulations to determine the effective permeability. The results showed that isothermal advection with saturated air have no influence on the coarsening rate that is typical for isothermal snow metamorphism. Isothermal snow metamorphism is driven by sublimation deposition caused by the Kelvin effect and is the limiting factor independently of the transport regime in the pores.

The role of footwall deformation and denudation in controlling cooling age patterns of detachment systems: An application to the Kongur Shan extensional system in the Eastern Pamir, China

Determining the relationship between deformation and exhumation history in the footwall rocks of major low-angle normal faults (i.e., detachment faults) provides a way to quantify different models of continental extension. Although pressure–temperature–time ( P–T–t) datasets are commonly used to infer magnitude and rates of slip across detachment faults, correct use of these datasets requires a sound understanding of coupling effects between footwall deformation and advection of isotherms on the distribution of cooling ages. In this study we present thermo-kinematic modeling results which examine the relationship between different footwall kinematic models, erosion, and the spatial distribution of muscovite and biotite 40Ar/ 39Ar cooling ages. Our results show that large magnitudes of footwall erosion during normal faulting significantly affect cooling age patterns as footwall rocks at different distances from the fault surface pass through different thermal gradients. We apply our modeling results to simulating muscovite and biotite cooling-age patterns from the footwall of the Cenozoic Kongur Shan normal fault system in the Pamir. Previous studies had interpreted a documented ~5-fold increase in cooling rate at ~ 2 Ma to indicate an increase in exhumation rate during the Quaternary. However, our results show that the observed cooling age patterns and increase in cooling rate can be produced by a constant exhumation rate over the last 7 Ma due to the effect of changes in the rate of isotherm advection during exhumation.

How the Harcuvar Mountains metamorphic core complex became cool: Evidence from apatite (U-Th)/He thermochronometry

Apatite (U-Th)/He (AHe) data from ∼60 km of exposed footwall in the Harcuvar Mountains metamorphic core complex, Arizona, reveal a higher-precision late-stage cooling history than previously reported for this complex and others in the region. Progressively younger AHe ages recorded with distance along the slip direction show two distinct trends separated by a sharp inflection. At structurally shallower levels, ages decrease from ca. 23 to 15 Ma toward the northeast, defining an estimated slip rate of 2.6 +0.1/–0.2 km/ m.y. Samples from the structurally deepest ∼35 km of footwall yield concordant ages of ca. 15 Ma (±2σ error limits). The effects of mineralizing fluids, advection of isotherms toward the surface, and flexure-induced erosion are considered, but are unlikely to have influenced the ages. Therefore, it is suggested that the fault slip rate must have increased ca. 15 Ma to a rate of ∼30 km/m.y. Evidence for an increased slip rate ca. 15 Ma has also been observed in other metamorphic core complexes nearby, and this increase is attributed to the thermal effects of a slab-free window on the extending lithosphere.