Abstract
The Solomon Islands are a complex collage of crustal units or terrains (herein termed the `Solomon block') which have formed and accreted within an intra-oceanic environment since Cretaceous times. Predominantly Cretaceous basaltic basement sequences are divided into: (1) a plume-related Ontong Java Plateau terrain (OJPT) which includes Malaita, Ulawa, and northern Santa Isabel; (2) a `normal' ocean ridge related South Solomon MORB terrain (SSMT) which includes Choiseul and Guadalcanal; and (3) a hybrid `Makira terrain' which has both MORB and plume/plateau affinities. The OJPT formed as an integral part of the massive Ontong Java Plateau (OJP), at c. 122 Ma and 90 Ma, respectively, was subsequently affected by Eocene–Oligocene alkaline and alnoitic magmatism, and was unaffected by subsequent arc development. The SSMT initially formed within a `normal' ocean ridge environment which produced a MORB-like basaltic basement through which two stages of arc crustal growth subsequently developed from the Eocene onwards. The Makira terrain records the intermingling of basalts with plume/plateau and MORB affinities from c. 90 Ma to c. 30 Ma, and a contribution from Late Miocene–present-day arc growth. Two distinct stages of arc growth occurred within the Solomon block from the Eocene to the Early Miocene (stage 1) and from the Late Miocene to the present day (stage 2). Stage 1 arc growth created the basement of the central part of the Solomon block (the Central Solomon terrain, CST), which includes the Shortland, Florida and south Isabel islands. Stage 2 arc growth led to crustal growth in the west and south (the New Georgia terrain or NGT) which includes Savo, and the New Georgia and Russell islands. Both stages of arc growth also added new material to pre-existing crustal units within other terrains. The Solomon block terrane collage records the collision between the Alaska sized OJP and the Solomon arc. Initial contact possibly first occurred some 25–20 Ma but it is only since around 4 Ma that the OJP has more forcefully collided with the Solomon arc, and has been actively accreting since that time, continuing to the present day. We present a number of tectonic models in an attempt to understand the mechanism of plateau accretion. One model depicts the OJP as splitting in two with the upper 4–10 km forming an imbricate stack verging to the northeast, over which the Solomon arc is overthrust, whilst deeper portions of the OJP (beneath a critical detachment surface) are subducted. The subduction of young (<5 Ma), hot, oceanic lithosphere belonging to the Woodlark basin at the SSTS has resulted in a sequence of tectonic phenomena including: the production of unusual magma compositions (e.g. Na–Ti-rich basalts, and an abundance of picrites); an anomalously small arc–trench gap between the SSTS and the Quaternary–Recent arc front; calc-alkaline arc growth within the downgoing Woodlark basin lithospheric plate as a consequence of calc-alkaline magma transfer along leaky NE–SW-trending faults; rapid fore-arc uplift; and rapid infilling of intra-arc basins. The present-day highly oblique collision between the Pacific and Australian plates has resulted in the formation of rhombohedral intra- and back-arc basins.