TOMOTECTONIC CONSTRAINTS ON GEODYNAMICS OF THE MESOZOIC NORTH AMERICAN CORDILLERA - TOWARDS A NEW GLOBAL "SLAB TECTONIC" PARADIGM

Inaccessibility of the vast ocean floors and deep trenches delayed development of plate tectonic theory until remote, geophysical observations were possible. Plates were defined, and their cyclic evolution was clarified. Geologic observables included oceanic plate creation at ridges, and plate loss by subduction into the inaccessible mantle as recorded by volcanic arcs and accretionary prisms. Geophysical imaging is now clarifying how subducted lithosphere is not lost but builds slabs in the mantle. Thus, a global "slab tectonic" paradigm is emerging (from Greek tekton 'builder'). Seismic tomography of ever-increasing resolution of formerly inaccessible parts of the mantle permits us to relink arcs to subducted lithosphere back to ~300 Ma. Combined with existing seafloor isochrons, surprisingly complete geometries of the earth's plate interactions back to ~190 Ma (oldest isochrons) are resolvable in some regions, such as the Americas.

Massive slab walls >1000 km high and 10,000 km long allow us to infer their construction in an intraoceanic setting where stationary trenches are possible, and by vertical sinking through the upper-lower mantle boundary at ~10 mm/a. Tomotectonic analysis, the correlation of mantle structure with crustal structure, shows that these walls must have accumulated by westward subduction of ocean lithosphere beneath an archipelago located >1000 km west of North and Central America. This scenario is consistent with the majority viewpoint that the North American Cordillera was produced by eastward subduction beneath an Andean-type continental margin in the Early Mesozoic, when arc-like magmas intruded the craton of southwest USA and volcanic detritus was carried into the continental interior to the Colorado Plateau. Also, from the Late Mesozoic to Recent, Farallon ocean lithosphere has subducted beneath parts of the margin. However, tomotectonic constraints require an interim period of westward subduction of a different plate, and along-margin diachroneity of subduction mode switching. Westward subduction requirements run counter to 'always Andean' models but support the long-standing views of a minority of Cordilleran workers and the prevailing model for early development of the Central America – Caribbean region.