The Role of Fluid Flow in the Cooling of Metamorphic Core Complexes

The role of fluid flow in the cooling history

of metamorphic core complexes

Christian Teyssier, University of Minnesota

Mark Person, Indiana University

The aim of this project is to quantify the effects of fluid flow on the thermal history of metamorphic core complexes during exhumation using a hydrothermal model that incorporates the kinematics of crustal deformation. The numerical model is constrained by field studies based on extensive structural, metamorphic, and thermochronologic data for the Shuswap metamorphic core complex, British Columbia. The modeling allows for quantitative comparison of heat transfer with rock thermochronology and fluid-rock isotopic exchange within the Shuswap system. The modeling helps to test the sensitivity of fluid flow to fault geometry, kinematics, and permeability in the upper crust (horst and graben, domino-style blocks, listric systems), as well as ductile flow and attending heat advection in the lower crust. The modeling also evaluates the effect that fluid flow has on heat distribution and geothermal gradients below the detachment zone, which has implications for the understanding of metamorphic zoning in core complexes. Quantitative results are being compared to the large-scale thermochronology database we developed in the Shuswap metamorphic core complex, British Columbia; this type of generic modeling is directly exportable to other metamorphic core complexes as well as rift zones.

The research also sheds light on the systematics of fluid-rock interactions and thermal history in the detachment zones of the Shuswap metamorphic core complex. Towards this end, we are conducting a detailed thermochronologic study based on the fission-track and (U-Th)/He methods on two transects across the Columbia River detachment and fault system that bounds the metamorphic core complex to the east. In the same regions, we are examining fluid-rock interaction by analyzing the stable isotope signature and fluid inclusions within veins, fault rocks, and unfractured rock. This part of the study constrains the nature of the fluids, the extent of fluid pathways, the degree to which surficial fluids penetrate the metamorphic crust, the fluids paleo-temperatures, and possibly also fluid fluxes. We are constructing a suite of numerical experiments to represent isotopic fluid-rock interactions as well as thermochronologic data within this smaller-scale framework in order to better understand fluid flow in detachment/normal fault systems. The detailed field work and geochemical results also serve as ground truth against which the modeling results are tested. The quantitative analysis of fluid flow as a major heat transfer mechanism will help to refine the method in which geologists use cooling rates to determine exhumation rates.