Exhumation, deformation, and thermochronology of experimental orogenic wedges and natural transpressional orogens: Venezuelan Paria Peninsula.

摘要

Surface processes play an important role in orogenic evolution. Mass can be transported and redistributed at the earth surface, which modifies the gravitational load and alter the stress field and kinematics within orogens. Using sandbox analog modeling, we explore the role of asymmetric erosion, indenter's dip angle, and flux steady state in determining the patterns of deformation and exhumation in doubly-sided orogenic wedges. We also investigate the implication of asymmetric erosion on the thermal structure of experimental doubly-sided orogenic wedges. Strain and exhumation were calculated using displacement fields from 2D Particle Image Velocimetry (PIV) analysis. Asymmetric erosion and the condition of flux steady state have the most noticeable role on defining exhumation, deformation (strain), and topographic patterns on doubly-sided orogenic wedges. Geothermal gradient increases in the places where erosion is concentrated. Particles velocities seem to play a more significant role than altitude in the determination of fission track (FT) ages. Our experimental FT ages are in agreement with the relative pattern of the FT ages from the retrowedge erosional orogen in the Southern Alps of New and in the Paria Peninsula in NE Venezuela, and the prowedge erosional orogen in the Olympic Mts.; The transpressional orogen of the Paria Peninsula in eastern Venezuela exposes an E-W oriented mountain belt, in which the metamorphic grade, mostly greenschist facies, decreases from north to south in a direction perpendicular to the trend of the metamorphic belt. The pattern of CPO of quartz c-axes indicates a top-to-SW or oblique-normal sense of shear consistently throughout the region. Apatite fission-track ages show a progressive decrease of the ages toward the retrowedge side. The topography of the Paria Peninsula and its current precipitation pattern are both asymmetric. Exhumation, deformation, topography, erosion, and precipitation patterns from the transpressional orogen of the Paria Peninsula are comparable to those described in the Southern Alps of New Zealand. A general model for these two-sided transpressional wedges is proposed based on geologic observations. Obliquity of the compression and erosion seems to play and important role in the evolution, exhumation, and deformation of these two naturally deformed orogens.