COLLISIONAL OROGENY FOR THE LATE CRETACEOUS–PALEOGENE LARAMIDE EVENT

The Laramide event is a Late Cretaceous–Early Cenozoic deformational event that affected rocks from Tierra del Fuego to Alaska. Here, we focus on the North American sector, which not only includes thrust and strike-slip faults within both the Cordilleran fold-and-thrust belt and Rocky Mountain foreland, but also an exhumed and collapsed metamorphic hinterland riddled with postdeformational plutons, locally rich in porphyry Cu mineralization.

In the western US, the deformation is divided into two phases: Campanian, thin-skinned deformation and a Maastrichtian-Eocene thick-skinned phase. Thin-skin thrusting in this belt was coeval with deformation and metamorphism within the orogenic hinterland and stopped by the mid-Campanian when, according to deCelles, the leading edge of the thrust belt was eroded and buried by conglomerate and gravels. The end of thin-skinned thrusting coincided with regional exhumation and emplacement of typical slab failure plutons along the length of the orogen.

The belt of metamorphism, plutons and exhumation indicates that the hinterland passes from south to north through Mexico and turns nearly east-west through Sonoran Arizona and the Mojave Desert to end abruptly in the Transverse Ranges of southern California. It reappears in the Helena salient of Idaho-Montana and continues westward into the Cascades core then northward through the Coast plutonic complex of British Columbia–Yukon Territory to the Denali fault, where it is offset dextrally before it arcs across Alaska to the Alaskan Peninsula. Using the Texas and Lewis & Clark lineaments as robust piercing points RSH honored paleomagnetic data and restored the Laramide hinterland/plutonic belt into a continuous strand with no magmatic gap.

The foredeep migrated northward from Campanian to Paleocene as plate motions became less oblique. The thick-skinned phase of the Rocky Mtn foreland started during the Maastrichtian, and this deformation was coeval with thin-skin thrusting in Canada and Mexico, with the majority of southern Canadian thrusts feeding northward into the Tintina fault system.

The orogen appears to represent the closure of a relatively small ocean basin with extremely shallow slab break-off during collision followed by ~1300 km of disruption and northward migration driven by the Kula-Farallon ridge to the south.