Contrasting structural styles in siliciclastic and carbonate rocks of an offscraped sequence: The Peralta accretionary prism, Hispaniola

Abstract

The Peralta accretionary prism of Hispaniola, which developed in the late Eocene epoch during a period of convergence or oblique convergence, constitutes an off-scraped sequence containing both siliciclastic and carbonate rocks. These different rock types responded to accretionary deformation in markedly different ways, with most strain being accommodated in the mudstone-rich siliciclastic unit. Deformation in the siliciclastic unit is concentrated in thick zones (1 to 450 m) of intense stratal disruption, which we interpret as major accretionary thrust faults. The disrupted zones are characterized by boudinaged sandstone beds, common tight to isoclinal folds, numerous thrust faults, and a pervasive scaly clay fabric in mudstone interbeds. Microstructural studies of the sand-stones indicate that deformation began when these sediments were unlithified, and it continued through complete lithification. The sandstones are characterized by a microstructural progression from possible particulate flow, to distributed cataclastic grain breakage, to grain breakage confined to zones (web structure), to discrete planes of slip. Pressure solution occurred concurrently with development of the web structure and the discrete faults. In contrast, the carbonate units are generally stratally coherent. Microfabrics in the carbonate units indicate that they were relatively well lithified during deformation, with strain being accommodated primarily by brittle faulting and pressure solution. In addition to enhanced lithification of the limestones, we suggest that the presence or absence of highly ductile mudstone interbeds represented the dominant control on the location of the stratally disrupted thrust fault zones within the accretionary prism. The presence of these mechanically weak layers in the siliciclastic unit facilitated the development of the pervasive scaly clay fabric in the stratally disrupted zones, and slip along the numerous scaly fault planes probably accommodated a large component of the total strain during accretion. The absence of such mudstone layers in the limestone outcrops resulted in more coherent behavior of these rocks during accretion.