EVALUATING THRUST SHEET MODELS FOR LATE CRETACEOUS REGIONAL METAMORPHISM IN THE NASON TERRANE, CASCADES CRYSTALLINE CORE, WA

Two-dimensional geometric models presented here support previous proposals that paleobarometric data for late Cretaceous regional metamorphism in the southern Nason terrane could reflect the emplacement of a tapered SW-directed thrust sheet. Pressures in the Chiwaukum Schist decreased from NE to SW from ca. 8.5 kb to ca. 5 kb along a ca. 35 km transect. Geometric models for hypothetical thrust sheets are constructed by 1) converting pressure to estimated load thickness, 2) adjusting the load isostatically, and 3) lengthening the transect distance to account for tectonic shortening that would have steepened the pressure gradient after peak metamorphism. The surface slopes of modeled tapered thrust sheets for the observed pressure gradient are conditional on the values used for isostatic compensation and shortening and on the assumption that maximum burial is roughly synchronous across the transect. Thrust sheets modeled with no tectonic shortening, for example, might have surface slopes between 3.3 degrees (58 m/km) and 8.8 degrees (155 m/km), depending on the extent to which the thrust sheet is isostatically adjusted.

Surface slopes for thrust sheets modeled with variable amounts of isostatic adjustment and shortening are within the range of surface slopes for modern tectonic analogues. We concur with the interpretation proposed by Evans and Davidson, 1999, that paleobarometric data indicate fairly uniform loading across the Mount Stuart batholith and a steeper gradient northeast across the Chiwaukum Schist and Nason Ridge Migmatitic Gneiss. This observation and consequent paleo-surface slope predictions may indicate 1) heterogeneous loading and significant isostatic adjustment or 2) tilting and/or northeast side up shearing preferentially accommodated in the rheologically weaker Chiwaukum Schist/Nason Ridge Migmatitic Gneiss.