Tectonic controls of metamorphic field gradients

Abstract

We investigate the effects of horizontal variations in tectonic factors (heat production and rate, amount, and duration of erosion) on the magnitudes and gradients of pressure ( P), temperature ( T), and cooling ages observed in mountain belts. Horizontal gradients of P, T, and cooling ages are largely controlled by two dimensionless parameters, (1) the horizontal length scale, λ′, over which erosion, erosion rate or erosion duration varies, and (2) the Peclet number, Pe, which describes the relative rates of advection and diffusion of heat. In contrast, the magnitudes of observed P and T depend strongly on the degree of thickening and original burial depth, as well as Pe, and, since horizontal heat conduction can usually be ignored, do not depend strongly on λ′. P and T gradients generally increase with decreasing λ′ and Pe. For calculations in which either, but not both, erosion rate or erosion duration is horizontally constant, P and T gradients are moderate in magnitude and increase in the direction of increasing total erosion. In contrast, calculations with horizontally constant total erosion (but varying erosion rate and erosion duration) show small temperature and pressure gradients; pressures and temperatures can increase in the same or in opposite directions, depending on the magnitude of λ′. Calculations with horizontally varying crustal heat production show small but non-zero temperature gradients and negligible pressure gradients. These results indicate that knowledge of temperature, pressure and cooling age gradients in individual mountain belts can constrain fundamental quantities such as magnitude and horizontal gradient of the exhumation rate and the average shear strain rate occurring during exhumation. On the basis of their metamorphic field gradients, most mountain belts can be divided into one of three groups. The first and largest group shows strongly positive P and T gradients. Precambrian orogenic areas comprise the second group and have small T gradients ( < ∼ 5°C/km) but moderate to large P gradients (1–100 bar/km). A third, small group, which includes the Acadian Appalachians of New England, exhibits moderate T gradients but small P gradients ( < 1 bar/km). The horizontal variations in tectonic processes responsible for the thermal development of these three groups must have been quite distinct from each other.