Geological-tectonic framework of Solomon Islands, SW Pacific: crustal accretion and growth within an intra-oceanic setting

The Solomon Islands are a complex collage of crustal units or terrains ther ein termed the 'Solomon block') which have formed and accreted within an in tra-oceanic environment since Cretaceous times. Predominantly Cretaceous ba saltic basement sequences are divided into: (1) a plume-related Ontong Java Plateau terrain (OJPT) which includes Malaita, Ulawa, and northern Santa I sabel; (2) a 'normal' ocean ridge related South Solomon MORE terrain (SSMT) which includes Choiseul and Guadalcanal; and (3) a hybrid 'Makira terrain' which has both MORE and plume/plateau affinities. The OJPT formed as an in tegral part of the massive Ontong Java Plateau (OJP), at c. 122 Ma and 90 M a, respectively, was subsequently affected by Eocene-Oligocene alkaline and alnoitic magmatism, and was unaffected by subsequent are development. The SSMT initially formed within a 'normal' ocean ridge environment which produ ced a MORE-like basaltic basement through which two stages of are crustal g rowth subsequently developed from the Eocene onwards. The Makira terrain re cords the intermingling of basalts with plume/plateau and MORE affinities f rom c. 90 Ma to c. 30 Ma, and a contribution from Late Miocene-present-day are growth. Two distinct stages of are growth occurred within the Solomon b lock from the Eocene to the Early Miocene (stage 1) and from the Late Mioce ne to the present day (stage 2). Stage 1 are 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 are growth led to crustal growth in the west and south (the New Georgia terrai n or NGT) which includes Save, and the New Georgia and Russell islands. Bot h stages of are growth also added new material to pre-existing crustal unit s within other terrains. The Solomon black terrane collage records the coll ision between the Alaska sized OJP and the Solomon are. Initial contact pos sibly first occurred some 25-20 Ma but it is only since around 4 Ma that th e OJP has more forcefully collided with the Solomon are, and has been activ ely accreting since that time, continuing to the present day. We present a number of tectonic models in an attempt to understand the mechanism of plat eau 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 th e Solomon are is overthrust, whilst deeper portions of the OJP (beneath a c ritical detachment surface) are subducted. The subduction of young (<5 Ma), hot, oceanic lithosphere belonging to the Woodlark basin at the SSTS has r esulted in a sequence of tectonic phenomena including: the production of un usual magma compositions (e.g. Na-Ti-rich basalts, and an abundance of picr ites); an anomalously small are-trench gap between the SSTS and the Quatern ary-Recent are front; calc-alkaline are growth within the downgoing Woodlar k basin lithospheric plate as a consequence of calc-alkaline magma transfer along leaky NE-SW-trending faults; rapid fore-are uplift; and rapid infill ing of intra-arc basins. The present-day highly oblique collision between t he Pacific and Australian plates has resulted in the formation of rhombohed ral intra- and back-are basins. (C) 1999 Elsevier Science B.V. All rights r eserved.