MOST volcanic rocks in modern island and continental arcs are probably derived from melting of the mantle wedge, induced by hydrous fluids released during dehydration reactions in the subducted lithosphere1. Arc tholeiitic and calc-alkaline basaltic magmas are produced by partial melting of the mantle, and then evolve by crystal fractionation (with or without assimilation and magma mixing) to more silicic magmas2—basalt, andesite, dacite and rhyolite suites. Although most arc magmas are generated by these petrogenetic processes, rocks with the geochemical characteristics of melts derived directly from the subducted lithosphere are present in some modern arcs where relatively young and hot lithosphere is being subducted. These andesites, dacites and sodic rhyolites (dacites seem to be the most common products) or their intrusive equivalents (tonalites and trondhjemites) are usually not associated with parental basaltic magmas3. Here we show that the trace-element geochemistry of these magmas (termed 'adakites') is consistent with a derivation by partial melting of the subducted slab, and in particular that subducting lithosphere younger than 25 Myr seems to be required for slab melting to occur.

Derivation of some modern arc magmas by melting of young subducted lithosphere

A new tectonic model, postulating the growth of giant subduction-accretion complexes along a single magmatic arc now found contorted between Siberia and Baltica, shows that Asia grew by 5.3 million square kilometres during the Palaeozoic era. Half of this growth may have occurred by the addition of juvenile crust newly extracted from the mantle, supporting models of considerable continental growth continuing throughout the Phanerozoic eon.

Evolution of the Altaid tectonic collage and Palaeozoic crustal growth in Eurasia

Assembly, configuration, and break-up history of Rodinia: A synthesis

The Central Asian Orogenic Belt ( c . 1000–250 Ma) formed by accretion of island arcs, ophiolites, oceanic islands, seamounts, accretionary wedges, oceanic plateaux and microcontinents in a manner comparable with that of circum-Pacific Mesozoic–Cenozoic accretionary orogens. Palaeomagnetic and palaeofloral data indicate that early accretion (Vendian–Ordovician) took place when Baltica and Siberia were separated by a wide ocean. Island arcs and Precambrian microcontinents accreted to the active margins of the two continents or amalgamated in an oceanic setting (as in Kazakhstan) by roll-back and collision, forming a huge accretionary collage. The Palaeo-Asian Ocean closed in the Permian with formation of the Solonker suture. We evaluate contrasting tectonic models for the evolution of the orogenic belt. Current information provides little support for the main tenets of the one- or three-arc Kipchak model; current data suggest that an archipelago-type (Indonesian) model is more viable. Some diagnostic features of ridge–trench interaction are present in the Central Asian orogen (e.g. granites, adakites, boninites, near-trench magmatism, Alaskan-type mafic–ultramafic complexes, high-temperature metamorphic belts that prograde rapidly from low-grade belts, rhyolitic ash-fall tuffs). They offer a promising perspective for future investigations.

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Tectonic models for accretion of the Central Asian Orogenic Belt

We present four companion digital models of the age, age uncertainty, spreading rates, and spreading asymmetries of the world's ocean basins as geographic and Mercator grids with 2 arc min resolution. The grids include data from all the major ocean basins as well as detailed reconstructions of back-arc basins. The age, spreading rate, and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments. The age uncertainties for grid cells coinciding with marine magnetic anomaly identifications, observed or rotated to their conjugate ridge flanks, are based on the difference between gridded age and observed age. The uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities. Asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges, resulting in ridge jumps toward hot spots. We also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age-depth relationships. The new set of grids helps to investigate prominent negative depth anomalies, which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow. A combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems.

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Age, spreading rates, and spreading asymmetry of the world's ocean crust

This publication combines the interpretations of two major sets of data. One is the geophysical data that is used to interpret the position of the tectonic plates through geologic time. The other is based on a long time search of the geological literature to find, record and evaluate the lithologic descriptions of countless reports around the globe; paying careful attention to those lithologies that have climatic implications. The introduction to this volume includes a detailed discussion of the lithologies, mineralogies and biogeographies that are considered to be the most reliable in identifying the climatic conditions existing during their formation and how they are used or not used in this compilation. These include coal, cyclothems, laterite, bauxite, lateritic manganese, oolitic ironstone, kaolin, glendonite, tilites, dropstones, calcretes, evaporites, clay minerals, palms, mangroves, and crocodilians. Additionally, several others are discussed but not used for specified reasons. These include eolian sandstone, silcrete and some specific paleobotanical methodologies.

Global paleoclimatic zones based on the climatically interpreted data points are identified during twenty-eight time periods from Cambrian to Miocene using paleotectonic reconstructed maps. The paleoclimate of each time period is summarized and includes a discussion of the specific referenced data points that have been interpreted to be the most reliable available for that time period and location.

Phanerozoic Paleoclimate: An Atlas of Lithologic Indicators of Climate

The paleoclimatic maps were generated by the INTERGRAPH using PALEOCLIMATE and PALEOMAP software. The outlines of paleogeography contained in the recently generated Phanerozoic Paleogeographic maps are used as input for paleoclimatic modeling. The PALEOCLIMATE program models global atmosphere pressure, derive paleowind directions and estimate the likelihood of coastal upwelling. The program is based on the paleoclimatic models first developed by Judy Parrish, adopted by C.R. Scotese and modified by M.I. Ross. The Phanerozoic paleotemperature maps from PALEOCLIMATE contain surface temperature superimposed on paleogeographic templates (elevated areas, land masses, shallow and deep sea patterns). The paleotemperature map can be used as input for geochemical modeling software like COMPAS and for basin maturation evaluations. The paleoclimatic maps suite contains 58 maps (29 age slices, Cambrian to Present, winter and summer maps for each slice). The maps depict air pressure, wind directions, humid zones and areas favourable for upwelling conditions plotted on the paleogeographic background. The paleoclimatic modeling techniques can help evaluate the risk for source rocks in frontier exploration plays. Computer-predicted upwelling areas include major producing regions such as the Persian Gulf, Central Asia, Siberia, SE Asia, North Africa, North Sea, U.S. and Mexico. The map interpretation, integrated with traditional source rock prediction, indicates new potential exploration regions. These include the Canadian Arctic, Greenland, Barents Sea, Soviet Arctic, South America, offshore Australia and China.

Phanerozoic Paleogeographic and Paleoclimatic Modeling Maps

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Phanerozoic tectonic evolution of the Circum-North Pacific

SUMMARY Synthetic apparent polar wander (APW) paths for North America, South America, Eurasia, India, Central Africa, Australia and Antarctica for the last 200 Myr are proposed. Computation of these APW paths is based upon the latest version (4.5a) of the Global Paleomagnetic Database (GPMDB), a revised global plate tectonic model since the Early Jurassic, and a new technique for generating smoothed APW paths. The smoothing technique includes the following steps: (1) pre-selection of palaeopoles, including pre-filtering parameters (number of sites, number of samples per site, 95 per cent confidence circle about mean direction, cleaning procedure, and time uncertainty); (2) generation of palaeolatitude and declination plots for a reference site on each continent that combines palaeopoles via a global plate tectonic circuit; (3) independent spline regression analyses of the palaeolatitude and declination plots; (4) removal of palaeolatitude or declination data that deviate by more than 10 ◦ from the regression curves (post-filtering process); (5) generation of synthetic APW paths from the resulting palaeolatitude and declination plots. These synthetic APW paths are then rotated into African coordinates to determine the best-fit APW path and a global palaeomagnetic reference frame. Four representative plate tectonic reconstructions and global plate velocity fields are presented for the three time intervals that correspond to globally synchronous changes in plate motion.

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Apparent polar wander paths for the major continents (200 Ma to the present day): a palaeomagnetic reference frame for global plate tectonic reconstructions

Christopher R. Scotese

Papers

Phanerozoic Paleoclimate: An Atlas of Lithologic Indicators of Climate

Phanerozoic Paleogeographic and Paleoclimatic Modeling Maps

Phanerozoic tectonic evolution of the Circum-North Pacific

Apparent polar wander paths for the major continents (200 Ma to the present day): a palaeomagnetic reference frame for global plate tectonic reconstructions

A Revised Reconstruction of Gondwanaland