Tectonics of Unusual Crustal Accretion in the Parece Vela Basin - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 150

Keywords Oceanic core complex • Intermediate-spreading ridge • Parece Vela
Basin • Transform sandwich effect

1 Introduction

Backarc basins are diverse geologic settings because they inherently involve both
divergent and convergent types of plate boundaries. They show a wide variety of
spreading styles and lithospheric compositions (Martinez et al. 2007 ), making them
important areas for studying crustal accretion processes along mid-ocean ridges.
The tectonic and magmatic evolution of mid-ocean ridges is governed by a variety
of factors, most of which influence the thermal structure of the lithosphere and the
melt productivity. These include the spreading rate, potential temperature of the
upwelling mantle and mantle composition of ridges, and the presence of large-
offset fracture zones, nearby continental crust and hotspots (Parmentier and Morgan
1990 ; Macdonald et al. 1991 ; Lin and Morgan 1992 ; Sinton and Detrick 1992 ; Niu
and Batiza 1993 ; Niu and Hekinian 1997 ; Gregg et al. 2007 ). It is widely accepted
that slow- and ultraslow-spreading ridges show characteristics of magma starvation
(Cannat et al. 2006 ; Dick et al. 2003 ) while intermediate- and fast-spreading ridges
are generally magmatically robust.
The Izu-Bonin-Mariana (IBM) arc system in the Northwest Pacific is the classic
locality for the study of oceanic convergence. At this plate boundary, the Pacific
Plate is actively subducting beneath the Philippine Sea Plate along the Izu-Bonin and
Mariana Trenches. Behind this island arc complex, the Philippine Sea is composed
of three large basins separated by the Kyushu-Palau and West Mariana Ridges (both
are remnant arcs). This region evolved through three stages of arc formation, rifting
and backarc spreading (Karig 1971 ). The Parece Vela Basin (PVB) is the southern
portion of the IBM backarc system, exemplifying backarc crustal construction
(Fig. 1a ). The basin was active during 26-12 Ma at a rapid intermediate-spreading
rate of 8.8-7.0 cm/year full-rate (Okino et al. 1998 ; Ohara et al. 2001, 2003a ). After
the extinction of the PVB at 12 Ma, backarc spreading resumed in the currently
active Mariana Trough, creating the West Mariana Ridge.
Despite its relatively faster spreading rate, the PVB shows distinct characteristics
that indicate a depressed magmatic budget, such as the occurrence of numerous
oceanic core complexes and rugged terrain, exposing abundant peridotites and gab-
bros. Many of the peridotites in the PVB are much less depleted than those exposed
at comparable spreading rates on other mid-ocean ridge systems (Ohara et al. 2001,
2003a ; Ohara 2006 ).
In this article, we will compare the morphology, tectonics and magmatic history
of the PVB to regions on other mid-ocean ridge systems that also display magma-
starved characteristics, in order to constrain the mechanisms that explain its unusual
crustal accretion processes along a backarc spreading ridge. The aim of this article
is to provide the basis for understanding these unusual characteristics to be answered
by the ongoing studies that come from a series of recent extensive expeditions to