Thermal modelling of the Laramide orogeny: testing the flat-slab subduction hypothesis

Abstract

The Laramide orogeny is the Late Cretaceous to Palaeocene (80–55 Ma) orogenic event that gave rise to the Rocky Mountain fold and thrust belt in Canada, the Laramide block uplifts in the USA, and the Sierra Madre Oriental fold and thrust belt in Mexico. The leading model for driving Laramide orogenesis in the USA is flat-slab subduction, whereby stress coupling of a subhorizontal oceanic slab to the upper plate transmitted stresses eastwards, producing basement-cored block uplifts and arc magmatism in the foreland. The thermal models presented here indicate that arc magma generation at significant distances inboard of the trench (>600 km) during flat-slab subduction is problematic; this conclusion is consistent with the coincidence of volcanic gaps and flat-slab subduction at modern convergent margins. Lawsonite eclogite xenoliths erupted through the Colorado Plateau in Oligocene time are inferred to originate from the subducted Farallon slab, and indicate that the Laramide flat-slab subduction zone was characterised by a cold thermal regime. Thermal modelling indicates that this regime can be produced by flat-slab subduction of old (>∼50 Myr) oceanic lithosphere at high convergence rates. In the Canadian and Mexican portions of the Laramide orogen, the coeval development of a magmatic arc within 300 km of the trench refutes the existence of flat-slab subduction in these regions. It is proposed that subduction of an oceanic plateau/aseismic ridge may have overcome the negative buoyancy inherent in old oceanic lithosphere and resulted in a spatially restricted zone of flat-slab subduction in the USA. These findings cast doubt on the flat-slab model as a primary means of driving Laramide orogenesis along its entire length, and instead point to the need for an alternative mechanism for Cordilleran-wide Laramide orogenesis.