Mesozoic fault systems, deformation and fault block rotation in the Andeanforearc: a crustal scale strike-slip duplex in the Coastal Cordillera of northern Chile

In this paper we discuss the evolution and tectonic significance of the Mes ozoic trench-parallel fault systems which affected the Coastal Cordillera a nd their relation to magmatism and crustal rotation. The oldest, extensiona l, fault system separates basement from rift-related Late Triassic and youn ger sedimentary units. This system [I] subsequently developed into a wider extensional fault system which acted as the locus of magma ascent and empla cement of the Coastal Batholith during much of the Jurassic to earliest Cre taceous period. This extensional fault system defined the forearc sliver du ring this period and was the consequence of a retreating subduction boundar y, During the Early Cretaceous (c. 132-125 Ma) the kinematics of this fault system changed to transtension [II] and accommodated a major component of left-lateral strike-slip motion, the principal fault being the Atacama Faul t Zone along which plutons continued to be emplaced. The final phase of plu ton emplacement within the Coastal Cordillera appears to be c, 106 Ma, afte r which this magmatic are and fault system was abandoned. An Late Cretaceou s are and fault system [III] developed some 20 Ma later and located some 50 km to the east in what is now the Central Valley of northern Chile. This p aper seeks to show that the Coastal Cordillera was deformed as a whole by t his Late Cretaceous fault system [III] which formed a crustal-scale left-la teral transpressional duplex. During this deformation the thermally weakene d crust was dissected into a series of large-scale blocks bounded by NW-tre nding left-lateral strike-slip faults which merge into a NNE-SSW fault zone which forms the eastern boundary to the duplex. We term this eastern bound ary zone the Central Valley Fault Zone (CVFZ) and this together with the NW -trending faults defines the duplex system which we refer to as a whole as the Coastal Cordillera Fault System (CCFS) [III], we have traced the CCFS d uplex between 25 degrees S and 29 degrees S and suspect that it continues n orthward. The timing of the deformation is constrained to be post 106 Ma, t he age of Coastal Cordillera are abandonment, and pre-Tertiary based on the deformation and pluton emplacement in and along the Central Valley Fault Z one, Palaeomagnetic data from the fault bound blocks within the CCFS duplex indicate 35 degrees-45 degrees of post-Early Cretaceous clockwise rotation with no substantial latitudinal motion. We suggest that the observed fault kinematics of the CCFS are consistent with this crustal-scale duplex model where rotations would have occurred in response to left-lateral transpress ion. (C) 1998 Elsevier Science B.V. All rights reserved.