Tectonics of the Jurassic- Early Cretaceous magmatic arc of the north Chilean Coastal Cordillera (22 degrees-26 degrees S): A story of crustal deformation along a convergent plate boundary

The tectonic evolution of a continental magmatic are that was active in the north Chilean Coastal Cordillera in Jurassic-Early Cretaceous times is des cribed in order to show the relationship between are deformation and plate convergence. During stage I (circa 195-155 Ma) a variety of structures form ed at deep to shallow crustal levels, indicating sinistral are-parallel str ike-slip movements. From deep crustal levels a sequence of structures is de scribed, starting with the formation of a broad belt of plutonic rocks whic h were sheared under granulite to amphibolite facies conditions (Bolfin Com plex). The high-grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike-slip and thrust kinematics with a NW-SE directed maximum horizontal shortening, i.e., para llel to the probable Late Jurassic vector of plate convergence. A kinematic pattern compatible to this plate convergence is displayed by nonmetamorphi c folds, thrusts, and high-angle normal faults which formed during the same time interval as the discrete shear zones. During stage II (160-150 Ma), s trong are-normal extension is revealed by brittle low-angle normal faults a t shallow levels and some ductile normal faults and the intrusion of extend ed plutons at deeper levels. During stage III (155-147 Ma), two reversals i n the stress regime took place indicated by two generations of dikes, an ol der one trending NE-SW and a younger one trending NW-SE. Sinistral strike-s lip movements also prevailed during stage IV (until similar to 125 Ma) when the Atacama Fault Zone originated as a sinistral trench-linked strike-slip fault. The tectonic evolution of the magmatic are is interpreted in terms of coupling and decoupling between the down-going and overriding plates. Th e structures of stages I and IV suggest that stress transmission due to sei smic coupling between the plates was probably responsible for these deforma tions. However, decoupling of the plates occurred possibly due to a decreas e in convergence rate resulting in extension and the reversals of stages II and III.