MODELLO

In southwest Puerto Rico, discontinuous bodies up to several km long form an olistostrome consisting of: 1) Maastrichtian limestone lenses interlayered with intermediate to mafic flows among conglomeratic beds (El Rayo Fm.); 2) thin-bedded, calcareous, sandy, shaly, and cherty Campanian strata (Yauco Fm.) that enclose lenses of Cretaceous limestone; and 3) mafic volcanics. The blocks are commonly surrounded by coarse, poorly bedded debris, rich in andesitic material, some of which compose flows (Sabana Grande Fm.). Along the margins of the block-like masses, steep dips are common as are diverse orientations of strike. Steeply tilted layers within blocks that terminate at block margins coincide with abrupt changes in lithology. Some blocks are bounded by faults.
The faunas and sedimentology of the blocks and surrounding units are interpreted as having formed in a Late Cretaceous backarc depositional setting including: 1) a carbonate platform to the southwest, 2) an intervening marginal basin (Yauco Fm.) and, 3) a volcanic edifice to the northeast (Río Blanco-Lago Garzas). Blocks entrained within debris composing even larger blocks within the olistostrome, indicate a complex sedimentary history that includes: 1) accumulation of Campanian shallow water carbonate beds; 2) an episode of volcanism, erosion, and development of a Maastrichtian carbonate platform; 3) uplift followed by disruption of volcanic and carbonate layers, and formation of coarse, layered debris; and 4) a second disruption during the early Tertiary when large blocks that comprise the olistostrome, were formed from the slightly older debris.
Cretaceous strata probably accumulated upon ultramafic basement, as shown by exposures of Late Cretaceous conglomerates resting unconformably upon Jurassic serpentinite. Recent mapping corroborates the interpretation that serpentinite has been thrust onto the Tertiary olistostrome. The olistostrome and thrust sheets were tilted and folded together during late Eocene contraction. This event may be related to left-lateral transpression, based upon contractional structures recognized where east-striking left-lateral faults bend south at the Great Southern Puerto Rico Fault Zone or to other contractional processes. The previous interpretation that the serpentinite was emplaced in the middle Cretaceous due to a collision or change in subduction polarity is not precluded. However, it must have been distinct from the younger tectonic history.