Ridge collision at convergent margins: implications for Archean and post-Archean crustal growth

Abstract

Growth of continental crust today occurs dominantly at convergent plate margins through magmatic addition and tectonic accretion (lateral and vertical) related to 1. (1) steady-state subduction, 2. (2) non-steady-state, episodic, processes. Ridge-forearc collision is one episodic process that can cause significant, highly diachronous, thermal and tectonic effects when a buoyant, bathymetrically-high and thermally-active oceanic spreading ridge approaches, collides with and potentially subducts below a continental margin. Near-field effects (within ~ 100 km of collision site) in the forearc may include anomalous heat, anomalous magmatism, ophiolite obduction, forearc deformation and tectonic erosion of the base of the forearc. Far-field effects generally postdate collision and may include changes in the thermal structure of the continent, changes in the composition of arc magmas, a reduction or hiatus in arc magmatism, uplift and/or subsidence of the continental margin, marginal basin formation, underplating of subducted slabs and intraplate deformation. Effects of ridge collision can vary depending on the triple junction geometry. The Chile margin triple junction (CMTJ), where the actively spreading Chile ridge is colliding with the South American forearc, is the best modern example of ridge collision. Near-field effects of Pliocene-Recent ridge collisions near the CMTJ include: 1. (1) emplacement of the Pliocene Taitao ophiolite less than 15 km from the trench 2. (2) intrusion of Pliocene granitic plutons and 3. (3) forearc deformation (formation of a pull-apart basin on the continental shelf, and late Tertiary (?)-Recent movement on a major fault along the arc-forearc boundary). Far field effects include reduction in subduction-related seismicity and volcanism, chemical changes in arc magmatism, and possibly uplift and/or subsidence. Ridge-collison features in southern Chile are similar to features of Archean greenstone belts, suggesting that some greenstone sequences may have formed in a ridge-collision setting and that ridge collision may have been an important process in Archean crustal growth. These features include granitic rocks closely associated with ophiolitic rocks formed near a continental margin (mafic-ultramafic section overlain by basaltic to andesitic volcanic rocks of mixed calc-alkaline and MORB character, intercalated with volcaniclastic sedimentary rocks). Differences between Archean and Phanerozoic tectonic processes, mainly related to higher heat production in the past, probably have affected the nature and importance of ridge collision in Archean crustal growth. During the Archean, greater ridge length and smaller plates probably resulted in more ridge collisions and related processes, contributing to a higher continental growth rate. In particular, underplating of subducted slabs, and direct slab melting may have been important processes in Archean continental growth, but are of relatively minor importance today.