Abstract Subduction under South America has been active for the past 550 m.y. but large porphyry copper deposits were essentially emplaced during the Paleocene (60–50 Ma) in southern Peru, and mid-Eocene-early Oligocene (43–32 Ma) and late Miocene-Pliocene (10–6 Ma) in north and central Chile. Although the tectonic setting of the Paleocene porphyry deposits is still poorly understood, those of the northern Chile Eocene-Oligocene belt were emplaced along the margin-parallel Domeyko fault system, where active compressional and/ortranspressional deformation and block rotations took place during the formation of the Bolivian orocline. Eocene-early Oligocene oroclinal bending was a consequence of differential tectonic shortening focused along a mechanically weak zone of the Central Andean crust inherited from the Paleozoic. Deformation occurred during an episode of accelerated westward absolute motion of the South American plate, which coincided with very high rates of oceanic crust production in the eastern Pacific. The slow South American-Farallon convergence rates recorded for the Eocene-Oligocene suggest, however, that strong interplate coupling existed during that time. This permitted the transfer of horizontal stresses and large-scale deformation of the Andean margin, creating a favorable scenario for the generation and emplacement of porphyry copper magmas along the Domeyko fault system. The younger, Miocene-Pliocene porphyry copper deposits of central Chile-Argentina were emplaced in a different setting, after the initiation of compressional deformation within a volcano-tectonic depression (Abanico basin) that evolved during another, late Oligocene to early Miocene, period of increased East Pacific oceanic crust production. Nevertheless, in contrast to the Eocene-Oligocene situation in northern Chile, the relatively stationary position of the South American plate compared to the mantle reference frame and weak interplate coupling that permitted rapid subduction, increased volcanism, and overriding plate extension. Tectonic inversion of the basin and compressional deformation along with crustal thickening and mountain building began at around 20 m.y. ago as interplate coupling increased when the westward motion of South America accelerated and the Nazca-South America convergence velocity decreased in the mid-Miocene. Compression was accompanied, as during the Eocene-Oligocene in northern Chile, by slab shallowing and increased fore-arc subduction erosion. In both cases, the largely structurally controlled, syn- to post-tectonic porphyry copper deposits are associated with long-lived magmatic systems that were active for more than 10 m.y. In northern Chile, the deposits occur as parts of discrete intrusive clusters that comprise a suite of precursor plutons emplaced during multiple events since the Cretaceous. Porphyry copper mineralization is linked to multistage, amphibole-bearing intrusions of intermediate composition derived from hydrous, oxidized magmas with adakitic geochemical signatures. These intrusions appeared when crustal thickness increased to a critical threshold in the course of deformation. Production of magmas with high metal-carrying capacity was fostered as fluids were liberated when amphibole became unstable and was destroyed as the crust thickened. At the same time, source regions within the mantle were contaminated by hydrated fragments of fore-arc continental crust, as the result of enhanced subduction erosion during peaks of compressional deformation.

Abstract The wide acceptance of plate tectonics two decades ago caused earth scientists interested in neotectonics and seismicity to focus their attention along plate boundaries with resulting great advances in our understanding of interplate deformation. In contrast, crustal deformation and earthquake hazard in the interior of presumably rigid plates have been considered second-order phenomena and have received relatively little attention. Although the concept of a tectonically quiescent intraplate environment may be justified for some areas, in others, as in parts of eastern North America, the historic record of damaging earthquakes indicates that neotectonics is quite significant, at least from the seismological viewpoint, and needs to be better understood. Certainly more earthquakes and, particularly, a greater number of damaging earthquakes occur in interplate western North America than in the intraplate rest of the continent. However, the largest known earthquakes in the conterminous U.S. have occurred in the East: the three 1811 to 1812 events in New Madrid, Missouri, and the 1886 event in Charleston, South Carolina. In this discussion the “size” of an earthquake refers to its damage potential, or the size of the area with damaging intensities (which is here arbitrarily taken as the area with Modified Mer- calli intensity VIII or greater). Damage potential depends on the spectrum and attenuation of seismic waves and is not a reliable measure of the total radiated seismic energy or the tectonic strain, which are expressed by the moment or magnitude. Thus, the 300-km-long 1906 rupture of the San Andreas fault produced a smaller area of damaging intensities (Fig. 1), but probably a much larger moment than the 1811 to 1812 earthquakes in the East. Such a systematic difference between eastern and western earthquakes has been attributed in part to differences in seismic attenuation properties (e.g., Nuttli, 1973). High stress-drops may also contribute to this difference.

Abstract We describe the regional fault pattern, geological setting and active fault kinematics of Jamaica, from published geological maps, earthquakes and GPS-based geodesy, to support a simple tectonic model for both the initial stage of restraining-bend formation and the subsequent stage of bend bypassing. Restraining-bend formation and widespread uplift in Jamaica began in the Late Miocene, and were probably controlled by the interaction of roughly east–west-trending strike-slip faults with two NNW-trending rifts oriented obliquely to the direction of ENE-trending, Late Neogene interplate shear. The interaction of the interplate strike-slip fault system (Enriquillo- Plantain Garden fault zone) and the oblique rifts has shifted the strike-slip fault trace c . 50 km to the north and created the 150-km-long by 80-km-wide restraining bend that is now morphologically expressed as the island of Jamaica. Recorded earthquakes and recent GPS results from Jamaica illustrate continued bend evolution during the most recent phase of strike-slip displacement, at a minimum GPS-measured rate of 8±1 mm/a. GPS results show a gradient in left-lateral interplate strain from north to south, probably extending south of the island, and a likely gradient along a ENE–WSW cross-island profile. The observed GPS velocity field suggests that left-lateral shear continues to be transmitted across the Jamaican restraining bend by a series of intervening bend structures, including the Blue Mountain uplift of eastern Jamaica.