Paper No. 55-8
Presentation Time: 3:45 PM
MANTLE (ALVAREZ) WAVES AND GLOBAL OROGENESIS: RESPONSES TO THE PROCESS OF PLATE CONVERGENCE, CONSUMPTION, COLLISION, COUPLING, CAPTURE, “COLLAPSE” AND CHANGES IN PLATE MOTION
Prescient ideas about plate tectonics offered by Alvarez (1982, 1990, and 2010) provide insight into processes that may link displacement of mantle to plate motion and the related distribution of global, orogenic, domains. Among principal conclusions are the following. 1) The driving mechanism of plate tectonics involves motions derived from slab pull and deeper mantle convection. 2) During some plate movements, continental roots and coupled slabs may interact with mantle in such a way that a wave of mantle may form. 3) The movement of mantle (Alvarez) waves may cause protracted continental collisions on a global scale. 4) Oceanic plates are decoupled from the lower mantle. Published hypotheses, mainly focused upon western North America, suggest that recurring processes of collision, critical coupling (CC) and capture cause abrupt changes in plate motion in response to slab pull, related to the west-directed subduction of the Pacific plate. As the Pacific plate movement drags the coupled Laurasia westward, extension takes place at the margins of the coupled Pacific and North America plates. During this post-orogenic “collapse”, extension within the coupled region of the captured plates is accommodated by regional strike-slip faults and co-genetic pull-apart or “back-arc” basins and, in ductile rocks, core complexes. Decoupling may take place after about 10 Ma, when the buoyant oceanic-ridge crust cools and resumes subduction. Capture also may result in plate rotation as shown by Mid-Jurassic and latest Silurian CC events, each of which is followed by rotation of Laurasia and Laurentia, respectively. As the captured plate moves, mantle is displaced and forms a wave. As mantle moves toward the wake of the plate, the wave crowds against plate margins and causes contraction, induces subduction, and orogenesis. Major orogenic events commonly are related to wave motion. Where the wave passes beneath continental plates, uplift coincides with shortening and inversion of rift basins at plate margins, incision and unconformities inboard. Assessment of the relationships and tempos of: 1) coupling, rifting and formation of large igneous provinces, and 2) mantle waves, orogenesis, global unconformities and glaciations should proceed from consideration of implications of proposals offered by Alvarez.