Geodynamic patterns of ophiolites and marginal basins in the Indonesian and New Guinea regions RON HARRIS Department ofGeology, Brigham Young University, Provo, UT 84602-4606, USA (e-mail: [email protected]) Abstract: Analysis of spatial, temporal, geological and geochemical patterns of ophiolites in the Indonesian and New Guinea region indicates a strong correlation with marginal basin development and closure. The spatial distribution of ophiolites is mostly linked with marginal basin producing zones of oblique convergence and collision. Strain partitioning in these zones creates a series of ephemeral plate boundaries between several independently moving lithospheric blocks. Repeated disruption of the diffuse boundaries between the blocks by changes in plate motion and collision-induced mantle extrusion creates space that is rapidly filled by new ocean basins in the upper plate of subduction zones. Suprasubduction zone (SSZ) spreading of these basins is enhanced by episodic extrusion of asthenosphere escaping collisional suture zones. Various closure events and global plate motion changes are reflected in the temporal distribution of marginal basin and ophiolite ages. Most ophiolite slabs in the Indonesian and New Guinea region represent fragments of oceanic lithosphere with a subduction zone component, as indicated by the common refractory petrochemistry of the mantle sequence and occurrence ofboninite. .Age and compositional heterogeneity may indicate that some ophiolite bodies are composite terranes. Collisions with buoyant lithosphere transfo= parts of these ocean basins into ophiolites. The connection between ophiolites and marginal basins is strongest where parts of actively spreading SSZ basins are partially represented as ophiolites in collision zones. Many of the various tectonic models proposed for rollback, subduction polarity asymmetry, astheno­ the origin and tectonic evolution of ophiolites are spheric flow and plate kinematics. supported by plate interactions in the Indonesian and New Guinea region (Dewey & Bird 1970; Silver & Smith 1983; Moores et al. 1984; Searle Tectonic evolution of the Indonesian and & Stevens 1984; Dilek & Moores 1990; Harris New Guinea region and its contribution to 1992; Dickinson et at. 1996). However, the geolo­ the ophiolite debate gical associations of these interactions remain poorly resolved, allowing different parts of the The Indonesian and New Guinea region is an region to be used as modern analogues for almost active collisional amalgamation of the complex any tectonic scenario. Simple explanations based Asian and Australian continental margins with on plate kinematics alone fail to account for the crustal heterogeneities of the western Pacific increasing levels of complexity observed in the (Hamilton. 1979; Hall & Blundell 1996; Hall Indonesian and New Guinea region. New ideas 2002). Rapid changes in plate boundary location and supporting evidence for mantle extrusion and function in this region are emblematic of mechanisms (e.g. Flower et at. 2001) also rely incipient phases of continental collision manifest heavily on descriptions ofplate interactions in this in ophiolite-bearing Cordilleran and Tethyan region (Doglioni et at. 1999), and raise new mountain systems. As in these regions, the Indo­ questions about how these processes may relate to nesian and New Guinea region has buffered the the spatial and temporal pattern of ophiolites. changing motion and boundaries of some of the This paper provides a synthesis of the temporal Earth's largest plates. Convergence of the Indo­ and spatial patterns, and tectonic associations of Australian plate from the SW has mostly been marginal basins and ophiolites throughout the absorbed along the Sunda arc-trench system, Indonesian and New Guinea region. It explores whereas convergence of the Pacific plate from the the relations between various plate boundary east has progressed by sequential movement along processes and ophiolite genesis and emplacement, an array of short-lived subduction zones and such as the role of strain partitioning, trench spreading centres. Similar plate boundary asym- From: DILEK, Y. & ROBINSON P. T. (eds) 2003. Ophiolites in Earth History. Geological SocietY, London, Special Publications, 218, 481-505.0305-8719/03/$15 © The Geological Society of London 2003. 482 R. HARRIS metry is observed in many other parts ofthe world SSZ history is superimposed onto or evolves into (Uyeda & Kanamori 1979; Doglioni et at. 1993). less refractory lithosphere or is sourced from The triple junction between the Indo-Australian, mantle previously contaminated by subduction. Pacific and Asian plates is a complex repository Geophysical and geological studies of plate of island arcs, marginal basins, continental frag­ boundaries throughout the ING region demon­ ments and ophiolites amalgamated by repeated strate, at a range of temporal and spatial scales, plate boundary reorganizations. Many of the ocea­ that plate convergence is partitioned by an intri­ nic terranes in the mix were emplaced onto the cate network of faults that separate independently edge of partially subducted continental margins moving lithospheric blocks (Hamilton 1979; that began arriving at the triple junction during Bowin et at. 1980; Cardwell et at. 1980; the mid-Tertiary. The incipient continent-conti­ McCaffrey et al. 1985; McCaffrey 1988; Curray nent collision has progressed to the stage where 1989; Audley-Charles & Harris 1990; Genrich the Sunda Shelf of Asia and the Sahul Shelf of et at. 1996; Hall & Blundell 1996; Kroenke 1996; Australia are now partially connected by a collage Pubellier & Cobbold 1996; Simons et at. 1999; of island arcs and continental fragments separated Stevens et at. 1999; Kreemer et at. 2000). The by trapped and partially obducted ocean basin rapid rates ofmovement and transitional nature of lithosphere (Fig. 1). these microplate boundaries provide a variety of An analogous stage of tectonic development to settings for SSZ ocean basin development. East­ that in the Indonesian and New Guinea region ward asthenospheric flow may also play an im­ may have existed in the Jurassic amalgamation of portant role in steepening subduction zones and Alaska between North America and Siberia, or facilitating marginal basin development (e.g. the Mediterranean region between Africa and Doglioni 1993). It is the combination of strain Europe. However, in both of these locations, partitioning, plate boundary reorganization, asthe­ ophiolites are all that remains of what once was a nospheric flow, and collisional termination of complex plate boundary system that included subduction in the Indonesian and New Guinea ocean basins that have since been closed and region that produces and transforms many newly partially obducted. The survival of many of these formed SSZ ocean basins into ophiolite-bearing features in the Indonesian and New Guinea collisional mountain systems. Each stage of this region offers a unique perspective into the con­ process is represented by the evolving nature of nection between ophiolites and marginal basins. plate boundaries in the Indonesian and New Although ocean basin closure along this plate Guinea region, where many ophiolites already boundary zone has already progressed to the abound. stage of producing several classic ophiolites, many of the original tectonic features, such as west-verging subduction zones, that produced Transitional nature of plate boundary these ophiolites remain intact. These features reorganization provide a rare glimpse of the complexity asso­ ciated with convergent triple junctions involving The interaction of the three large plates that continents. converge in the Indonesian and New Guinea From the Andaman Sea to New Guinea (Fig. 1), region produces a wide zone of plate boundary the marginal basins and suprasubduction zone deformation composed of several fault-bounded (SSZ) ophiolites in the natural laboratory of the oceanic and continental fragments (Fig. 2). Most Indonesian and New Guinea region contribute in of the fault zones between these discrete litho­ many ways to the continuing debate about the spheric blocks are ephemeral and accommodate geodynamic meaning of ophiolites worldwide. strain for only a few million years or less. The Global positioning system (GPS) and earthquake most stable and long-lived plate boundary is the studies throughout the region reveal many frag­ north-directed Sunda subduction system (Fig. 2). mentary and transitional plate boundary zones However, this boundary has been intensely mod­ capable of simultaneous opening, closure and ified by collision with India in the west and obduction of new oceanic lithosphere. The age Australia in the east. In both cases indentation of and chemistry of oceanic crust in these ocean the convergent boundary has increased its obli­ basins and the ophiolites that surround them quity and resulted in partitioning of strain away provide a context for interpreting ophiolites in from the trench into the weak parts of the upper mountain systems where the original tectonic plate. The collision of the Indo-Australian and setting is obscured. Most ophiolites of the Indo­ Asian plates is further modified by the rapid nesian and New Guinea region also have some westerly convergence of a series of Pacific micro­ component of refractory or SSZ tectonomagma­ plates linked by multiple subduction zones, tism, like most other ophiolites. In some cases the spreading centres and large strike-slip faults (Figs .. Ophiolites o and urn rocks o "0: