INTERVAL BETWEEN REGIONAL MID-CRETACEOUS CONTRACTION AND EOCENE TRANSTENSION IN THE NORTH CASCADES SEGMENT OF THE COAST MOUNTAINS BATHOLITH: INSIGHTS AND QUESTIONS

Major mid-Cretaceous (~100-80 Ma) shortening has been widely recognized to the east and west of the crystalline core of the North Cascades (Cascades core) segment of the Coast Mountains Batholith. In the Cascades core, ductile shortening was accompanied by an ~96-87 Ma magmatic flare-up in this arc. Later dextral transtension from ~53-45 Ma was synchronous with extension to the east and a 49.5-45 Ma flare-up in the Cascades core. The interval between the two tectonic regimes is marked by a 78-60 Ma flare-up and has commonly been considered a period of regional transpression, but few ~80-60 Ma structures have been recognized. This interval is also when paleomagnetic data indicate that major dextral strike slip occurred along the continental margin (Baja B.C. hypothesis) and strike slip is commonly thought to be partitioned into an active arc.

Structures within the core that are constrained to have formed between 80-60 Ma are largely medium- to high T reverse shear zones in the margins of several 72-65 Ma plutons and ductile fabrics formed during 72-63 Ma migmatization in the Skagit Gneiss Complex (SGC). The major structure is the Gabriel Peak tectonic belt of the Ross Lake fault system, which is interpreted to have been active during emplacement of the 65 Ma Oval Peak pluton. This belt changes along strike from reverse (SW vergent) to dextral strike slip, and is perhaps the strongest evidence for transpression. Other structures of regional importance formed during inferred transpression, but their kinematics are uncertain. Incorporation of Cretaceous sediments of the Swakane Gneiss and SGC into the Cascades core, and metamorphism at pressures of up to 12 kbar, occurred from ~74-65. Sediment burial presumably involved underthrusting, but the vergence and any strike-slip component are largely obliterated by intrusion and metamorphism of orthogneisses (SGC) or obscured by structures related to Eocene exhumation. Major questions on deformation and magnitudes of strike slip during the transpressional regime clearly remain, and may be best answered by determining kinematics and timing of strain in the SGC, which contains orthogneisses and migmatites that span most of the time interval.