1996/01/01 by R. D. Dallmeyer, R. David Dallmeyer, Michael Brown +3 · 2 citations
Earth and Planetary Sciences · #Geological and Geochemical Analysis #High-pressure geophysics and materials #earthquake and tectonic studies
paper · doi:10.1086/629799
In North Chile, Jurassic-Cretaceous arc magmatism is represented by several narrow belts of plutons and two separate volcanic sequences. New 40Ar/39Ar mineral ages confirm five distinct episodes of magma emplacement as plutonic complexes (c. 202-188 Ma, c. 160-153 Ma, c. 142-138 Ma, c. 130-127 Ma, and c. 106-103 Ma). Conjugate dike sets were emplaced immediately before periods of plutonism, and each distinct episode of magmatism prior to c. 127 Ma was located successively eastward, inboard from the subduction boundary, features interpreted to result from regional extension within the magmatic arc. Steeply dipping, ductile mylonitic shear zones were developed in wall-rocks along western contacts of Late Jurassic and Early Cretaceous plutons. Steeply-plunging mineral stretching lineations and a variety of kinematic indicators record an east-side-down extension. Along eastern faults of the Atacama Fault System steeply dipping, ductile mylonitic shear zones display subhorizontal mineral stretching lineations, and a variety of kinematic indicators record sinistral strike-slip displacement. Hornblende separated from mylonitic am-phibolite yields 40Ar/39Ar isotope correlation ages similar to those of adjacent plutons, indicating contemporaneous deformation and magma emplacement. Further, these ages suggest that a change from extensional dip-slip displacement to sinistral strike-slip displacement occurred at c. 126 Ma within the Atacama Fault System. Global plate reconstructions indicate that South America was essentially static in a mantle reference frame during the Jurassic-Early Cretaceous prior to c. 127 Ma, and the Andean plate boundary zone is interpreted to have been a retreating subduction boundary during this period of time. As a consequence, extensional deformation characterized the overriding plate, and magma emplacement appears to have been localized at ramps within a hinterland-propagating extensional duplex. Alternating episodes of predominantly volcanism or plutonism are interpreted to have reflected changes in the rate of subduction, whereas the change from a dip-slip extensional to a sinistral strike-slip tectonic regime is suggested to have resulted from a change in the convergence vector.