Along-strike variations in the thermal and tectonic response of the continental Ecuadorian Andes to the collision with heterogeneous oceanic crust

Abstract

Oblique to strike geological segmentation in the Andean chain has been previously recognised at various scales and is commonly attributed to changes in the convergence vectors of the oceanic and continental plates, as well as the upperplate expressions of differing along-strike subducted slab age, strength and composition. We present new white mica and biotite 40 Ar/ 39 Ar and zircon and apatite fission-track data from several traverses across the Cordillera Real of Ecuador in the northern Andes that reveal distinct along-strike differences in the timing of accelerated crustal cooling during the Cenozoic. The data record elevated cooling rates from temperatures of V380‡C during V65^55 and V43^ 30 Ma from all sampled regions of the Cordillera Real and at V15 Ma and since V9 Ma in the northern Cordillera Real. Each cooling period was probably driven by exhumation in response to the accretion and subduction of heterogeneous oceanic crust. Elevated cooling rates of up to V30^20‡C/Myr were initiated during the Palaeocene and Eocene^early Oligocene along the entire contemporaneous margin of Ecuador and were driven by the accretion of the oceanic Pallatanga Terrane and Pin ‹ on^Macuchi Block, respectively, onto northwestern South America. Both of these geological provinces originated at the southern parts of the leading and trailing boundaries of the Caribbean Plateau and accreted onto the margin during the approximately northeastward migration of the Plateau into its current position. Within Ecuador the development of higher topography and elevated cooling rates of up to 50‡C/Myr at V15 Ma and since V9 Ma are restricted to the region north of 1‡30PS and is situated above the postulated subducted flatslab section of the aseismic Carnegie Ridge. Plate convergence rate calculations suggest the Carnegie Ridge collided with the Ecuador Trench at V15 Ma, which caused the pre-existing coastal provinces to displace to the northeast, subsequently driving extension and marine ingressions in southern Ecuador and compression and uplift in northern Ecuador. fl 2001 Elsevier Science B.V. All rights reserved.