Lower Crustal Deformation and Vertical Coupling and Decoupling in the Continental Lithosphere During Late Orogenic Extension

University Of Vermont & State Agricultural College, Burlington VT

Investigators

Abstract

A deeply eroded mountain range in western New Zealand provides the highly unusual opportunity for direct examination of a full 50 km thick column of continental crust as it evolved over a 35 million year period. This discovery is especially significant because field observations and numerical models have led geoscientists to emphasize the role of the lower crust in controlling the behavior of deforming continental lithosphere. However, the large number of unconstrained variables in most experiments and in most exposures creates a high degree of uncertainty about the characteristics and consequences of flow in the lower continental crust. This project is determining how strain was partitioned vertically within an ancient orogen as it underwent a transition from crustal thickening and contraction to crustal thinning and extension. The direct observational approach used in this project is possible because the pre-Cenozoic configuration of western New Zealand places crust that formed at unusually deep levels of a Cretaceous orogen (25-50 km depth) next to rocks that represent the middle and upper crustal levels of this same orogen (8-27 km depth). The research involves using the original Mesozoic architecture of the belt to test the contrasting ways in which extensional deformation may be transferred vertically through a rheologically evolving crustal column. Specifically, the project involves measurement of three-dimensional characteristics and kinematics of middle, upper and lower crustal structures that formed during extension between 108 and 190 million years ago, determination if there was kinematic compatibility between simultaneously evolving extensional structures at different crustal depths, establishment of absolute and relative timing of deformation and magmatism at lower, upper and middle crustal levels using uranium-lead zircon geochronology, and detremination of pressure and temperature conditions of high-P granulite and upper amphibolite facies assemblages formed during extension. The initial results suggest that rheological transitions linked to magmatism and the partial melting of thick lower crust and their effects on the mechanical behavior of continental lithosphere are much more heterogeneous and short-lived than previously believed.