| Abstract: | ![]()
The Andean geosynclinal pair in Peru consists of a eugeosynclinal marine andesitic volcanic trough and a miogeosynclinal sedimentary trough. Both troughs developed on a block-faulted basement of Precambrian crystalline rocks in which movement occurred along faults parallel to the continental margin. Subsidence in the eugeosyncline was most rapid during the Albian during which period about 7000 m of marine volcanics were deposited. In the miogeosyncline the greatest subsidence took place during the Tithonian but continued throughout the Cretaceous to accumulate a total thickness of about 6000 m. From the Late Cretaceous to the mid-Tertiary the Andean granitoid batholith was emplaced, mainly in the eugeosynclinal zone. A Benioff zone has been active at the continental margin from the Early Jurassic until the present so that the entire geotectonic cycle from the geosynclinal stage through to orogeny and uplift has taken place under a regime of active subduction.During the geosynclinal stage the sedimentary troughs developed in fault-bounded blocks which subsided under tension. It is possible to envisage crustal thinning within the subsident blocks by rotational movement on extensional faults or by plastic stretching of the lower crust. Stretching would be facilitated by the high geothermal gradient resulting from the emplacement of plutonics and volcanics and might be analogous to the formtion of marginal basins of western Pacific type. It is more difficult to account for the subsequent uplift for although the granites contributed to the crustal thickening, the main uplift did not occur in the granitic sector but further inland, and was moreover delayed for at least 20 m.y. after the last granites were emplaced. |
