Ages and cooling history of the Early Cretaceous Caleu pluton: testimony of a switch from a rifted to a compressional continental margin in central Chile

Journal of the Geological Society, Mar 2005 by Parada, Miguel A, Féraud, Gilbert, Fuentes, Francisco, Aguirre, Luis, Et al

Discussion: plutonism during a shift from an extensional to a compressional regime

Extension during Caleii magma generation

An extensional tectonic regime and associated crustal attenuation during the Early Cretaceous has been documented along the Pacific margin of South America (e.g. Dalziel 1981; Atherton et al. 1983), probably related to an episode of slow sea-floor spreading (Larson & Pitman 1972). which, in turn, conditioned a Mariana-type subduction (e.g. Pankhurst et al. 1988) associated with a trench retreat (roll-back effect). Along the central Chilean margin, the crustal attenuation developed progressively since the early Mesozoic (Vergara et al. 1995; Parada et al. 1999) and culminated with an aborted marginal basin where the thickest volcanic and sedimentary sucessions were deposited during the Early Cretaceous (Åberg et al. 1984). The progressive extension in the segment studied would be reflected in magmas exhibiting isotopic (Sr-Nd) secular variations from an enriched lithospheric-dominated mantle source of the late Palaeozoic plutonism (Parada et al. 1999), to a depleted asthenospheric-dominated source of the mid-Cretaceous Caleu-type magmatism (Fig. 8). The increasing isotopic depletion of the Mesozoic magmatism with decreasing age can be explained by a progressive removal of the mantle lithosphere as a result of a continuous asthenospheric upwelling that accompanied extension. Thus, the Caleu magma, the most depleted of all, would have been formed when extension reached its maximum.

Exhumation during compressional deformation

The exhumation of the Caleu pluton is part of a more regionalscale mid-Cretaceous exhumation in the Coastal Batholith of central Chile (32°30'S-33°30'S). In fact, an exhumation event of the late Palaeozoic, Mid-Jurassic and Early Cretaceous granitoids of this batholith during the interval 106-90 Ma (Gana & Zentilli 2000; this study) has also been identified by means of fissiontrack dating. The magnitude of this exhumation was stronger along the present-day coastline where the deep-seated (c. 3.0-5.5 kbar or c. 12-20 km depth based on Al-in-hornblende geobarometry; Gana & Tosdal 1996; Sial et al. 1999) late Palaeozoic to Mid-Jurassic plutono-metamorphic belt is now exposed.

It is well known that deep-seated erogenic terranes can be exhumed during extension associated with collapse of a thickened crust (see Dewey 1988; Liu & Shen 1998), or during convergent deformation and erosion (see Stiiwe & Barr 1998). In the studied case, the crustal thickness prior to the Early Cretaceous would have been 50 km needed to produce dynamic instability capable of driving collapse of the crust (see Liu & Shen 1998). It is then possible to infer that the cause of the exhumation of the studied coastal segment can be attributed to a change in the tectonic regime identified at these latitudes in the Coastal Range and along the Chile-Argentina Andes, during the mid-Cretaceous. At this time, extensional tectonics ended and a high-stress compressional regime started, giving rise to: (1) the roughly north-south Silla del Gobernador contractional shear zone (Arancibia 2002), recently dated at 97.8 ±1.5 Ma (Arancibia, pers. commun.), in the western boundary of the Coastal Range, c. 150km north of the Caleu pluton; (2) the shortening that generated the mid- to Late Cretaceous Agrio and Aconcagua fold-thrust belts located in the central High Andes of Argentina and Chile (Mpodozis & Ramos 1989; Ramos & Aleman 2000). Mid-Cretaceous basin inversion and folding (Mochica event) have also been recognized in western Peru (Atherton 1990).