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A Shallow-Source Plate Model for Intraplate Volcanism in the Panthalassan And

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A Shallow-Source Plate Model for Intraplate Volcanism in the Panthalassan And A Shallow-Source Plate Model for Intraplate Volcanism in the Panthalassan and Pacific Ocean Basins Alan D. Smith Department of Geological Sciences, University of Durham, Durham, DH1 3LE, UK Email: [email protected] ABSTRACT The record of intraplate volcanism in the Panthalassan and Pacific basins since the mid Paleozoic can be explained by tapping of shallow sources with the location of volcanism controlled by localised mantle convection, the stress field acting on the plate, and lithospheric architecture. The source components include recycled subducted oceanic crust remixed into the depleted mantle by mantle convection, volatile-bearing mineral assemblages formed by fluxing of fluids into the mantle at convergent margins, and continental mantle/lower crust incorporated into the shallow mantle during the break-up of eastern Gondwana or the assembly of east Asia in the mid Paleozoic-Early Mesozoic. The latter components migrated eastwards across the basin as a result of lithospheric lag at an average rate of 3 cm yr-1 and are now associated with the South Pacific Isotope and Thermal (SOPITA) anomaly and the source of Hawaiian volcanism. In the Carboniferous to Early Jurassic, ocean island volcanism occurred in the west of the Panthalassan basin accompanying rifting of continental blocks from eastern Gondwana. The crustal fragments and islands were subsequently transported to Asia on the Izanagi plate and to North America on the Farallon plate. In the Early Jurassic to 1 mid Cretaceous, oceanic plateaus formed on ocean ridge systems around the margins of a growing Pacific plate as a result of entrainment of low-melting point heterogeneities into the ridge upwelling and focusing of melt by triple junction geometry. Seafloor spreading was curtailed by jamming of subduction in the southwest of the basin leading to formation of the Ontong Java plateau by ponding of melt over the Pacific-Phoenix ridge. Diffuse ocean island volcanism in the Marcus-Wake, Magellan and Marshall islands occurred in the centre of the Pacific plate as a result of stresses transmitted from the Pacific-Phoenix, Pacific-Farallon and Pacific-Izanagi ridges. Extensive ocean island volcanism also occurred as a result of asthenospheric shearing beneath the westerly-subducting Izanagi plate, generating many of the ocean islands fragments now preserved as terrane material in east Asia. The Pacific plate began to subduct under Asia in the Late Cretaceous, but the stress field was still dominated by the Pacific-Kula and Pacific-Antarctic ocean ridges, such that volcanism in the Marshall-Gilbert and Line Island chains may have been oblique to plate motion. The Emperor chain is ascribed to fracturing propagating from the Kula-Pacific ridge following plate re-organisations at ~82 Ma which prematurely halted triple-junction volcanism on Meiji seamount. In the mid Eocene, the cessation of spreading on the Kula-Pacific and North New Guinea-Pacific ocean ridges subjected the west of the Pacific plate to tensional stresses orthogonal to the direction of plate movement, causing generation of Hawaiian chain as a fracture propagating from the tip of the Emperor chain. Inter-chain correlations of volcanic output along the Hawaiian, Louisville and Cook-Austral-Marquesas chains with Late Oligocene and Miocene-Pliocene plate re-organisations around the margins of the basin, similarly indicate control by lithospheric architecture and the stresses acting on the plate. Three plate settings with corresponding styles of intraplate volcanism are thus recognised: (1) intra-oceanic, marked by 2 generation of oceanic plateaus and diffuse ocean island volcanism (2) plate bounded predominantly by ocean ridge systems, marked by island chains having no systematic age progression (3) plate bounded by predominantly by convergent margins, characterised by age progressive island chains. Intraplate volcanism on the Pacific plate shows a transition through the above styles, whereas the Izanagi/Kula and Farallon plates were always bounded by convergent margins and were likely characterised by the latter style of volcanism. Keywords: Pacific basin, ocean island chains, oceanic plateaus, streaky mantle, propagating fractures INTRODUCTION Before the mantle plume model was widely invoked, intraplate volcanism in the Pacific basin (Fig. 1) was attributed to shallow mantle convection or propagating fractures induced by stresses acting on the plate (Shaw, 1969, 1973; Richter, 1973; Jackson et al., 1972; Jackson and Shaw, 1975; Bonatti and Harrison, 1976). One of the most advanced explanations was the model of Jackson and Shaw (1975) who noted that volcanism along the Hawaiian chain followed a series of right-lateral en echelon loci and suggested this reflected injection of magma into the lithosphere as the plate underwent rotation due to either changing forces along its boundaries, or variations in coupling between the lithosphere and asthenosphere. Similar loci were suggested to be present on other island chains of comparable age, but at the time the bathymetry of the ocean floor was poorly known and the concepts could not be further developed. With increasing popularity of the hotspot model, emphasis shifted to interpreting 3 the volcanic record as the result of mantle plume activity. However, few examples of intraplate volcanism have been shown to conform to the predictions of the plume model (Anderson, 2005a). Only three examples in the Pacific basin (Easter-eastern Mid Pacific Mountains, Louisville-Ontong Java, Marquesas-Hess/Shatsky Rises) have been suggested to fit the plume head-tail model (Clouard and Bonneville, 2001). Doubt is cast on such plateau- hotspot correlations from paleomagnetic evidence (Rissager et al., 2003) which suggests the Ontong Java plateau formed 200 north of its inferred position in hotspot models, requiring either non-fixed hotspots or true polar wander. In the Cook-Austral-Marquesas, Marshall- Gilbert, and Line Island chains, non-linear age progressions (Bonatti et al., 1977; Jarrard and Clague, 1977; Turner and Jarrard, 1982; Schlanger et al., 1984; Duncan and Clague, 1985; Okal and Batiza, 1987; McNutt et al., 1997) have been a persistent problem, and have led to many ad hoc varieties of plume shape and plumbing arrangements (e.g. McNutt and Fischer, 1987; Sleep, 1992). Even in the Hawaiian-Emperor chain, which has been considered the product of the strongest plume (Sleep, 1990), the volcanic record conflicts with most of the predicted features of the plume model. The volcanism lacks an associated plateau, eruption rates have increased rather than declined over time (Clague and Dalrymple, 1989), and the Emperor chain lacks a swell (Davies, 1992) which is one of the principal features used as evidence that hotspots are hot. Paleomagnetic evidence also indicates formation of the Emperor seamounts as much as 23 degrees north of the present site of volcanism (Tarduno and Cottrell, 1997; Sager, 2002) such that any hotspot could only have been stationary for approximately half its history. 4 The commonly held view of Pacific intraplate volcanism with superplumes forming oceanic plateaus followed by plume tails generating ocean island chains (e.g. Larson 1991a,b; Tatsumi et al., 1998) has nonetheless been maintained on account of beliefs that other models could not explain the large volumes of melt, eruption rates, and basalt geochemistry (e.g. Mahoney, 1987, Tarduno et al., 1991; Coffin and Eldholm, 1993; Coffin and Gahagan, 1995; Neal et al., 1997). However, the plume model has been shown to be inadequate for explaining the subsidence history (Ito and Clift, 1998) and lack of uplift associated with the Ontong Java plateau (Korenaga, 2005), which as the largest oceanic plateau, is the example supposedly most requiring a plume origin. In consideration of the lack of predictive ability of the plume model, uncertainties in the number of hotspots, and the failure of the model to account for many aspects of intraplate volcanism, the question should be whether the emphasis on the plume model has been justified and whether shallow-source models alone can offer a comprehensive explanation (Anderson, 1999; 2001; Favela and Anderson, 2000; Smith 2003a; Natland and Winterer, 2005). The aim of this study is to examine concepts which could be included in a shallow-source model from present day intraplate volcanism, and then to investigate how such concepts could apply to earlier intraplate volcanism in the Panthalassan- Pacific basin from a reconstruction of the volcanic record from the mid Paleozoic through Recent. The results concur with, and build on the model of Natland and Winterer (2005) whereby volcanism on the Pacific plate is divided into three stages reflecting the tectonic setting of the developing plate, and illustrate how the history of the basin can be explained without mantle plumes. 5 CONCEPTS Distinct Reservoirs versus Sampling Effects The geochemical basis of the plume model is the belief that distinct isotopic signature in ocean island basalts (OIB) and mid ocean ridge basalts (MORB) requires derivation of these two types of magmatism from chemically distinct sources. In the most common version, the mantle has a layered structure, with a depleted reservoir which serves as the source of MORB, overlying a layer of primitive composition (e.g. Hofmann, 1997). The depleted reservoir is usually equated with the upper mantle, and is considered to be the geochemical complement of the continental crust from calculations suggesting derivation of the latter from approximately
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