Tectonic wedging in the forearc basin: Accretionary prism transition, Lesser Antilles forearc
Authors: Torrini Rudolph , Speed Robert C. 
Published: February 2008

The southern Lesser Antilles forearc contains an extensive 50‐ to 70‐km‐wide belt of active deformation at an arcward (inner) locus, between the structural high of the Barbados accretionary prism and undeformed forearc basin (FAB) of the Tobago Trough. This inner forearc deformation belt (IFDB) extends at least 400 km on strike between about 12°N and an undefined northern limit. The IFDB developed above crystalline basement of the FAB close to and perhaps above its probable subduction trace with Atlantic lithosphere. It is a thin‐skinned thrust and fold belt developed in thick FAB strata by arcward (westward) wedging of the accretionary prism, which may be translating obliquely with respect to it. Its western boundary is the inner deformation front (IDF), a bathymetric scarp underlain by fault propagation or detachment type frontal folds. The IFDB is mostly erosionally truncated south of 12°N and north of 13.5°N by an unconformity of probable late Miocene age. Seismic reflection studies permit subdivision of the IFDB into western and eastern divisions, each containing an upper and lower zone. Barbados Island exposes the eastern IFDB and provides calibration of nearby seismics and dating of strata and structural events. The eastern zones contain the inner flank of the accretionary prism and its allochthonous stratal cover, whereas the western zones are composed entirely of FAB strata that deformed ahead of the translating prism tip. Early displacements in the IFDB were taken up on a long (40–90 km) deep detachment into the FAB that issued rapidly from the prism floor and on a conjugate backthrust up the prism's inner (western) slope. The forward detachment's tip locked at the IDF, and progressive early contraction occurred by folding in the hanging wall of the conjugate fault system and by thrusting of FAB strata eastward across the prism's crest. The generation of early structures was followed by late deformation in which the major thickening of the IFDB has occurred. North of Barbados, such late deformation was modest. From near Barbados south, however, late deformation was accompanied by upramping of part of the accretionary prism to a high wedging level in the FAB strata. This mainly out‐of‐sequence wedging caused FAB beds in the ramped prism's footwall to imbricate as a duplex between the prism wedge and the early detachment and to propagate westward toward the IDF. Finally, after development of the FAB duplex, out‐of‐sequence thrusting of the high level prism wedge occurred at the back of the duplex. Balanced section models suggest arcward prism translation between 20 and 60 km and horizontal contraction of FAB strata of the western zone between 4% and 45%, both increasing southward. Development of the early long forward detachment and conjugate back thrust probably began in the early Miocene from 12.5° to 14°N but earlier (probably Oligocene) south of 12.5°N. The out‐of‐sequence upramping of the prism and duplexing of FAB beds probably occurred in late Miocene or Pliocene time. Wide areas of the IFDB became emergent in the Miocene. Tectonic loading of the lithosphere beneath the IFDB caused large Pliocene and Quaternary subsidence of the belt and eastward shift of the FAB depoaxis to the toe of the IDF. The Miocene and younger features of the IFDB and the Tobago Trough probably reflect a superposition of tectonism associated with the development of the Neogene Lesser Antilles arc on an older arc system. The IFDB and Neogene forearc basin were superposed on extant forearc elements and represent a major change in subduction or accretion phenomena. The IFDB is not the product of steady convergence and accretion.