Interaction of metamorphism, deformation and exhumation in large convergent orogens

Abstract

Coupled thermal‐mechanical models are used to investigate interactions between metamorphism, deformation and exhumation in large convergent orogens, and the implications of coupling and feedback between these processes for observed structural and metamorphic styles. The models involve subduction of suborogenic mantle lithosphere, large amounts of convergence (≥ 450 km) at 1 cm yr−1, and a slope‐dependent erosion rate. The model crust is layered with respect to thermal and rheological properties — the upper crust (0–20 km) follows a wet quartzite flow law, with heat production of 2.0 μW m−3, and the lower crust (20–35 km) follows a modified dry diabase flow law, with heat production of 0.75 μW m−3. After 45 Myr, the model orogens develop crustal thicknesses of the order of 60 km, with lower crustal temperatures in excess of 700 °C. In some models, an additional increment of weakening is introduced so that the effective viscosity decreases to 1019 Pa.s at 700 °C in the upper crust and 900 °C in the lower crust. In these models, a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where T≥600 °C. The channel flow zone is characterised by a reversal in velocity direction on the pro‐side of the system, and is driven by a depth‐dependent pressure gradient that is facilitated by the development of a temperature‐dependent low viscosity horizon in the mid‐crust. Different exhumation styles produce contrasting effects on models with channel flow zones. Post‐convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust‐related exhumation on the flanks. Velocities in the pro‐side channel flow zone are enhanced but the channel itself is not exhumed. In contrast, exhumation resulting from erosion that is focused on the pro‐side flank of the plateau leads to ‘ductile extrusion’ of the channel flow zone. The exhumed channel displays apparent normal‐sense offset at its upper boundary, reverse‐sense offset at its lower boundary, and an ‘inverted’ metamorphic sequence across the zone. The different styles of exhumation produce contrasting peak grade profiles across the model surfaces. However, P–T–t paths in both cases are loops where Pmax precedes Tmax, typical of regional metamorphism; individual paths are not diagnostic of either the thickening or the exhumation mechanism. Possible natural examples of the channel flow zones produced in these models include the Main Central Thrust zone of the Himalayas and the Muskoka domain of the western Grenville orogen.

Logan Medallist 2. Geophysics and Geology: An Essential Combination Illustrated by LITHOPROBE Interpretations–Part 1, Lithospheric Examples

“…However, they can be separated into four key property sets: 1) mechanical, including differing properties for upper, middle and lower crustal layers with the latter comprising blocks with decreasing strength from the exterior of the model to its core where a suture enables subduction; 2) thermal properties associated with the crustal layers; 3) velocity fields and boundary conditions, including a mantle behaviour that is only described kinematically; and 4) properties of surface processes, including erosion but only on the advancing side of the model. Since the lower and middle crust within large, hot orogen models can reach temperatures greater than 700°C, which could generate small amounts of partial melt in felsic rocks, the models also include a parameterized viscosity reduction referred to as 'melt weakening' (Beaumont et al 2001Jamieson et al 2002).…”

Abstract Volume of Sixth Nepal Geolgical Congress on Geology, Natural Resources, Infrastructures, Climate Change and Natural Disasters, 15 – 17 November 2010, Kathmandu, Nepal

“…Simulation Tectonics Laboratory, Faculty of Science, University of the Ryukyus,Okinawa,Japan, There are number of problems about the channel flow model in the Himalaya and Tibet area (Jamieson et al 2002). Those problems are separated into two kinds; one is from the viewpoint of numerical method (1, 2 and 3) and the other is from the geological side (4 and 5).…”

“…(2) Also for the boundary condition, the constraining velocity along the inclined subduction plane is too artificial comparing to the nature that should not be constrained directly by the external velocity condition but be controlled through the other internal factors. (3) When we consider the model HT-6 (Jamieson et al 2002), for example, we wonder whether the erosion condition which has connected to re-sedimentation has enough geological data supported from observation. The erosional mechanism is considered not to be unique and still unclear.…”

Geochronology: linking the isotopic record with petrology and textures — an introduction

“…These latter are key inputs to thermal and mechanical models that seek to clarify the plate tectonic settings of orogenesis (e.g. England & Thompson 1984;Huerta et al 1999;Jamieson et al 2002). A second key input to such models is time, and the constraints that chronometric information provides on the rates at which burial, heating and deformation occur.…”

“…These newly available databases provided the initial impetus for developments in the third main field which has led progress in understanding orogenic belts: the numerical simulation of the mechanical and thermal adjustments that are the consequences of orogenesis and the causes of metamorphism. The application of analytical solutions of heat transport equations to onedimensional models of metamorphic belts in the 1970s (Bickle et al 1975), was followed by more general one-dimensional models involving numerical solutions (England & Thompson 1984), and has culminated more recently in the development of two-dimensional models that more fully simulate the temperature and deformation fields in orogenic belts (Huerta et al 1999;Jamieson et al 2002). These models were initially designed as an interpretive framework for thermobarometric, chronometric and structural data, but they also make predicitions about the pressure-temperature-deformation-time histories of deeply buried rocks (and the surface development of orogenic belts) that the newly available databases can be used to test.…”