The plate tectonic paradigm revolutionized the way geologists and geophysicists look at the world, and produced an enormous increase in our understanding of how the Earth functions. The recognition of large, relatively rigid plates that float on the underlying asthenosphere, and interact with one another primarily along their mutual boundaries, allowed earth scientists to place tectonic processes into a contextual framework of rifting, subduction, and transform faulting that can be applied to igneous processes, metamorphism, ore deposits, and sedimentation, as well as tectonics and structural geology. However, although this paradigm has been progressively refined over the years, we still lack a fundamental understanding of how these different processes begin, and what causes them to end.
How subduction begins and how it terminates represent first-order questions in tectonics that continue to challenge geoscientists working in different disciplines around the world. The processes of subduction initiation and termination link to many other large-scale processes and problems, including the generation and emplacement of ophiolites, evolution of orogenic belts, assessments of plate tectonic processes through geologic time, as well as practical matters, such as seismic and volcanic hazards.
This special issue is an outgrowth of a topical session on subduction initiation and termination at the Annual Meeting of the Geological Society of America (Denver, 2010). The eight papers in this volume showcase the wide range of research on this topic, including studies of modern and ancient plate boundaries, field and instrumentally based studies, numerical modeling, and planetary geology. Three of the papers focus on ophiolites and subduction initiation, three on non-ophiolite settings for initiation, one on geodynamics, and one on subduction termination.
Stern et al. propose that ophiolites form during subduction initiation in what are now the fore-arc regions of island arcs, that in order to understand one, we must study the other. They present a comparison between accretionary margins (characterized by high sediment flux) and erosional margins (characterized by low sediment flux and “naked” fore-arc crust), and propose that the latter “naked” fore-arcs—which form ophiolites when emplaced on land—are the best place to study subduction initiation. They elaborate on this model with a discussion of the Izu-Bonin-Mariana fore-arc region.
Shervais and Choi present geochemical data from mafic and ultramafic rocks of the Coast Range ophiolite (CRO) and Tehama-Colusa mélange (TCM) that separates the CRO from the subjacent Franciscan subduction complex in northern California. They integrate the geochemical data with field and geochronologic relationships to propose that Franciscan subduction initiated along a large-offset oceanic transform, relics of which are locally preserved within the TCM, with formation of the CRO above the nascent subduction zone.
Osozawa et al. present geochemical and geochronologic data from the Troodos ophiolite, of Cyprus, an ophiolite long proposed to have formed over a nascent subduction zone. They suggest that extensive forearc volcanism recorded by ophiolite lavas persisted for about 15 Ma. They postulate that initial formation of the Troodos ophiolite followed subduction initiation, as previously proposed, but that an extended period of ridge subduction followed, resulting in an unusually long duration of forearc magmatism.
Leng et al. present a new geodynamic model that incorporates an algorithm to calculate melt flux and composition throughout the course of subduction initiation. They show that the first melts to form during subduction initiation are MORB (mid-ocean ridge basalt)-like followed shortly by boninites. This sequence is the same as that observed in modern fore-arcs and in ophiolites, further supporting links between fore-arcs and ophiolites. Their approach represents an important new thrust in geodynamic modeling that links melt production to the physical process of subduction.
Shimabukuro et al. evaluate new geochronologic and metamorphic data from subduction complex rocks of Calabria, southern Italy. They conclude that Alpine subduction recorded by the Calabrian rocks initiated along a continental margin in old lithosphere (>80 Ma at time of subduction), based on the nature and age of protoliths metamorphosed under high-pressure/low-temperature conditions.
Rains et al. examine Permian sedimentary rocks of the El Paso Mountains of southeastern California. They conclude that the strata record a progression of sedimentary sources consistent with subduction initiation and their facies changes reflect an uplift and subsidence history predicted by geodynamic models for subduction initiation.
Yin presents structural and geomorphic data from the Tharsis Rise region of Mars. He concludes that subduction initiation was triggered by a large impact and that Tharsis rise volcanism evolved in response to slab rollback. He proposes that subduction may have initiated by similar mechanisms in Hadean Earth and that more localized subduction systems coalesced over an extended period of time to form a globally linked system of plate boundaries.
Chi presents seismic data from the active Taiwan collision region, one of the premier sites of subduction termination. He concludes that basement highs on the downgoing plate act as local indentors resulting in map-view extrusion. He also suggests that these indentors result in segmentation of the active fault systems associated with collision, thereby controlling the magnitude and location of earthquakes in this region.
The work collected here constitutes a significant contribution to the challenging questions of how subduction zones begin and end, but it is clear that much more needs to be done. Among the outstanding topics that need further attention, we find that these issues stand out: (1) How do we distinguish between spontaneous and induced subduction initiation in the rock record, and what are specific examples of each process; (2) It is clear that many subduction zones are found along ocean-continent boundaries, but it is not clear how these form, given the inferred strength of the transitional crust that underlies passive continental margins; (3) What are the geodynamic constraints that control when and where subduction zones form, and how they evolve; and (4) A systematic examination of subduction zone termination similar to the synthesis of Stern (2004) on subduction initiation has yet to be attempted.
It is the editors hope that this volume will prove useful to researchers in a wide range of earth science disciplines, and that it will inspire further work on these important processes.