SYNCONVERGENT EXHUMATION OF CONTINENTAL METAMORPHIC CORE COMPLEXES IN THE NORTHERN NORTH AMERICAN CORDILLERA

Continental metamorphic core complexes in the northern North American Cordillera (MT, ID, WA, B.C.) underwent extension and exhumation during the early Paleogene, prior to the mid-Eocene decrease in Farallon-North American convergence rates. These complexes are localized in the Sevier-Laramide hinterland along the Precambrian craton margin. Thus, numerical models of the development of northern Cordilleran complexes must account for extension during far-field convergence of softer accreted terranes and supracrustal lithologies with a rigid craton.

A new three-dimensional thermal-mechanical model calculates time-dependent temperature, pressure, and velocity fields within a viscous domain that converges with a thicker, more rigid craton with an irregular margin. The irregular margin forces vertical advection of the crust, producing crustal thickening and spatially focused domains of rapid, vertical exhumation beneath an elastic lid. While the model solves only the viscous problem and maintains a flat upper surface through erosion, stress is sufficient for the removal of the elastic lid through brittle deformation. Preferred solutions have subdued cratonic relief (5–10 km) and higher average crustal viscosities (10²⁰–10²¹ Pa s). Temperature–time (T–t) paths for preferred solutions are excellent matches (e.g., exhumation rate, duration) for T–t paths recorded by deeply exhumed northern Cordilleran complexes. Paths diverge at low T, suggesting a switch in exhumation mode at mid-crustal levels from near-isothermal exhumation to brittle faulting and erosion. Preferred solutions allow for the observed variety (e.g., depth of exhumation, volume and timing of partial melting and magmatism) among northern Cordilleran complexes.

Metamorphic core complexes may form during synconvergent exhumation without imposing crustal weaknesses to localize strain. Further, northern Cordilleran complexes need not be considered a step toward development of mid-ocean ridges, as suggested by models that impose far-field extension. This model of the development of northern Cordilleran complexes may be extended to complexes in similar tectonic settings, with implications for classification of metamorphic core complexes.