Abstract
AbstractThe analyses of the main parameters controlling the present Chile‐type and Marianas‐type tectonic settings developed along the eastern Pacific region show four different tectonic regimes: (1) a nearly neutral regime in the Oregon subduction zone; (2) major extensional regimes as the Nicaragua subduction zone developed in continental crust; (3) a Marianas setting in the Sandwich subduction zone with ocean floored back‐arc basin with a unique west‐dipping subduction zone and (4) the classic and dominant Chile‐type under compression. The magmatic, structural and sedimentary behaviours of these four settings are discussed to understand the past tectonic regimes in the Mesozoic Andes based on their present geological and tectonic characteristics. The evaluation of the different parameters that governed the past and present tectonic regimes indicates that absolute motion of the upper plate relative to the hotspot frame and the consequent trench roll‐back velocity are the first order parameters that control the deformation. Locally, the influences of the trench fill, linked to the dominant climate in the forearc, and the age of the subducted oceanic crust, have secondary roles. Ridge collisions of seismic and seismic oceanic ridges as well as fracture zone collisions have also a local outcome, and may produce an increase in coupling that reinforces compressional deformation. Local strain variations in the past and present Andes are not related with changes in the relative convergence rate, which is less important than the absolute motion relative to the Pacific hotspot frame, or changes in the thermal state of the upper plate. Changes in the slab dip, mainly those linked to steepening subduction zones, produce significant variations in the thermal state, that are important to generate extreme deformation in the foreland. Copyright © 2009 John Wiley & Sons, Ltd.
Highlights
The examination of the present tectonic settings along the eastern Pacific region, and mainly along the South American continent, shed light to some geological processes associated with subduction in an Andean-type setting
The present subduction of the eastern Pacific along the Americas shows different tectonic regimes that have been exemplified in four subduction zones: (1) Nicaragua subduction zone: an extensional regime in the upper plate associated with poorly evolved continental magmatic arc, extensional back-arc and rift basins developed in continental crust
(2) Sandwich subduction zone: an extensional regime in the upper plate associated with an island arc developed in oceanic crust and formation of an extensional oceanic floored back-arc basin controlled by the eastward mantle flow
Summary
The examination of the present tectonic settings along the eastern Pacific region, and mainly along the South American continent, shed light to some geological processes associated with subduction in an Andean-type setting. Since the early proposal of Forsyth and Uyeda (1975) that have shown that the slab pull forces are positively correlated with the donwndip length of a subduction zone, there were many studies of the processes controlling the tectonic regime. These last authors were the first to propose an eastward main stream of the mantle, responsible for a velocity gradient in the mantle flow (see recent comments in Doglioni 2009). The present analysis will be restricted to a passive mantle model, in the sense of Scholz and Campos (1995) in which motion in the mantle is taken to be stationary in a hotspot reference frame to avoid the complexity of more advanced models that could be difficult to extrapolate in Mesozoic times (Doglioni et al 2009)
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