Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations

Trench-arc systems (subduction zones) can be classified into two types depending on whether or not actively opening back-arc basins are associated with them. This suggests that subduction of an oceanic plate is not a sufficient condition for back-arc opening, though it may be necessary one. Mechanisms that cause the distinction between the two types have been investigated. Earthquake studies suggest that there is a significant difference in the mode of plate motion at interplate boundaries between the two types of trench-arc systems. Extreme cases are Chile, where plate motion is seismic, and the Marianas arc, where it is aseismic. This difference seems to indicate that the stress state in the back-arc area differs between the two types: compression in the Chilean type and tension in the Marianas type. This difference in the stress state is also manifested in other tectonic features, such as topography, gravity, volcanic activity, and crustal movement. Two possible mechanisms for the difference between the two types are suggested: (1) The nature of the contact zone between upper and lower plates changes from tight coupling (Chile) to decoupling (the Marianas) through the evolutionary process of subduction. The decoupling results in an oceanward retreat of the trench and back-arc opening. (2) The downgoing slab is anchored to the mantle, so that the position of a trench is also fixed with respect to the mantle. Since the motion in the mantle is slow compared to that of surface plates, it is the motion of the landward plate which controls the opening and nonopening of back-arcs.

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Back-arc opening and the mode of subduction

This paper presents a critique of the models proposed to explain the formation of back arc basins: those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related. Several marginal basins are not generically back arc, but rather are related to the initiation of subduction, continental rifting, or plate boundary readjustments. Models of back arc basin formation must be consistent with the observed characteristics of back arc basins, which include the following: (1) the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar, implying that the processes of crustal accretion in both tectonic environments are also similar; (2) arc-trench systems without currently spreading back arc basins are in the majority; (3) although petrographically similar to and within the compositional range of midocean ridge basalts (MORB), back arc basin basalts (BABB) show consistent geochemical differences from N-type MORB and are in many respects transitional toward island arc tholeiites (IAT); (4) distinct BABB and IAT mantle sources may be in extremely close proximity during the initial rifting of an island arc; (5) rifting of the volcanic arc may occur on either side of the line of active volcanoes (±50 km) and may vary from one side to the other along strike; (6) a remnant arc is not always developed; (7) back arc spreading may be initiated above subducted lithosphere, but with time the center of spreading migrates away from the arc and no longer overlies a Benioff zone; and (8) there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins. The three main classes of models proposed to explain back arc basin formation are mantle diapirism, induced aesthenospheric convection, and global plate kinematics. Because the driving mechanism for the first two classes of models is provided by the local subducting slab, these models fail to explain the temporal and spatial distribution of back arc basins. The third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line, the latter being observed to move only in a seaward direction with respect to a hot spot reference frame. However, the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions. For example, some dynamic back arc force seems required to move the New Hebrides, Tonga, and South Sandwich forearc plates seaward. Neither the local nor the global models adequately specify the necessary and/or sufficient conditions for back arc basin formation. A better understanding of the processes associated with the initiation of rifting and of subduction is required. Arc rifting and back arc spreading are both secondary phenomena to subduction, but they are also primary extensional phenomena, fundamentally analogous to continental rifting and the initial spreading of major ocean basins.

On the evolution of marginal basins

Springer, Victor G. Pacific Plate Biogeography, with Special Reference to Shorefishes. Smithsonian Contributions to Zoology, number 367, 182 pages, 65 figures, 5 tables, 1982.—It is the thesis of this study that the Pacific lithospheric plate (Pacific Plate) forms a major subunit of the Indo-Pacific biogeographic region. This thesis is defended on two primary grounds: the sharp decrease in numbers of taxa as one proceeds eastward across the western margin of the Pacific Plate, and the presence of a high degree of endemism on the Plate. The complex geotectonic history of the Plate and adjacent plate margins is discussed and the vicariance scenarios that produced the three main types of endemism (widely distributed Plate endemism, endemism restricted to a limited region on the Plate, and endemism restricted to single islands or island groups) are hypothesized. Anecdotal discussions of the distributions of 179 shorefish family groups are presented, as well as discussions of the distributions of selected terrestrial and marine taxa of other organisms. Particular attention is given to presumed erroneous or exceptional distributions of fishes on the Pacific Plate, including, but not limited to, north-south disjunct distributions, Hawaiian Islands' exceptional distributions, and distributions involving the Caroline Islands, which serve as stepping stones for dispersal onto the Pacific Plate. OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: The coral Montastrea cavemosa (Linnaeus). Library of Congress Cataloging in Publication Data Springer, Victor Gruschka, 1928Pacific Plate biogeography, with special reference to shorefishes. (Smithsonian contributions to zoology ; no. 367) Bibliography: p. Supt. of Docs, no.: SI 1.27:367 1. Fishes—Indo-Pacific Region—Geographical distribution. 2. Biogeography—Indo-Pacific Region. 3. Fishes—Pacific Ocean—Geographical distribution. 4. Geology—Pacific Ocean. I. Title. II. Title: Shorefishes. III. Series. QL1.S54 no. 367 [QL623.98] 591s [591.92'5] 82-600146 AACR2

Pacific Plate biogeography, with special reference to shorefishes

Geochemical and ^Ar—^Ar studies of the Malaita Older Series and Sigana Basalts, which form the basement of Malaita and the northern portion of Santa Isabel, confirm the existence of Ontong Java Plateau (OJP) crust on these islands. Sr, Nd, and Pb isotopic ratios of Malaita Older Series and Sigana lavas fall within limited ranges [("Sr/^Sr) T= 0-70369-0-70423, ENd(T)= + 3-7 to +6-0, and Pb/Pb = 18-25-18-64] virtually indistinguishable from those found in the three OJP basement drill sites as far as 1600 km away, indicating a uniform hotspot4ike mantle source with a slight 'Dupal' signature for the worlds largest oceanic plateau. Three chemical types of basalts are recognized, two of which are equivalent to two of the three types drilled on the plateau, and one with no counterpart, as yet, on the plateau; the chemical data indicate slightly different, but all high, degrees of melting and slight variation in source composition. All but one of the *°ArAr plateau ages determined for Malaita Older Series and Sigana Basalt lavas are identical to those found at the distant drill sites: 121 -3 ±09 Ma and 92-0 +1-6 Ma, suggesting that two short-lived, volumetrically important plateau-building episodes took place ~30 my. apart. Aside from OJP lavas, three isotopically distinct suites of alkalic rocks are present. The Sigana Alkalic Suite in Santa Isabel has an ^Ar-^Ar age of 91-7±0-4 Ma, the

Patrick J. Coleman

Papers

The Melanesian Borderlands and India — Pacific plates' boundary

On island arcs