Rocks formed along magmatic arcs at convergent plate junctures are important components of eugeosyn- clinal terranes. Arc complexes include not only surficial volcanic accumulations and plutonic intrusions em- placed in the metamorphic roots of the arcs. Also important are clastic strata composed of debris dispersed from the igneous axes of the arcs by eruption or erosion and deposited in basins and troughs within or beside the arc trends. Magmatic arcs include both marginal volcanic chains along or near the edges of continental land- masses and volcanic island chains flanked on the side away from the trench either by shallow epicontinental seas or by deep oceanic areas that commonly were formed as interarc basins. Intra-arc basins and troughs are largely fault-controlled depressions, although environments of deposition range from varied terrestrial and shallow marine settings to deep marine waters where turbidites are dominant. Coarse air-fall and ash-flow pyroclastic strata, pillow breccias and aquagene tuffs, and subaqueous ash-flow deposits are distinctive facies common for intra-arc sequences. Thick prisms and wedges of clastic strata commonly accumulate along or near the flanks of magmatic arcs. On the trench side, elongate traps for sediment prisms within the arc-trench gaps include terrestrial valleys and sloping plains, subsiding shelves and slopes, and deep bathymetric benches and troughs. On the other side, in the rear of the arc, clastic wedges may be spread either as turbidite aprons built into a deep interarc basin lying beyond a rifted arc rear and bounded on the far side by a remnant arc or as riverine and deltaic plains built into a foreland basin lying beyond an arc-rear thrust belt and bounded on the far side by a cratonic mass. The history of salient arc-trench systems suggests that evolution through time typically involves coordinate widening of the arc-trench gap, the magmatic arc structure itself, and the interarc basin or thrust belt behind the arc.
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Modern and Ancient Geosynclinal Sedimentation
The Kay Conference was held in Madison, Wisconsin, November 1972. This symposium volume contains the texts of papers presented at Madison. It is organized in a topical manner, and in most areas of discussion, modern analogues and ancient examples together provide a comparative basis for evaluating sedimentary models for geosynclines. In the 1970s students of both modern and ancient sediments have compiled an immense body of knowledge relevant to the geosynclinals concept. Moreover, the new theory of plate tectonics has required a complete reassessment of the geosynclines as well as orogenesis. The purpose of this volume is to evaluate by comparison of modern and ancient sediments a number of depositional models applicable to the great variety of strata seen in orogenic belts also called geosynclinal.