Pampean deformation in the Sierra Norte de Cordoba, Argentina: Implications for the collisional history at the western pre-Andean Gondwana margin

摘要

In the Sierra Norte de Cordoba, Argentina, at the inferred western pre-Andean Gondwana margin, structural analyses show that Upper Proterozoic-Lower Cambrian strips of metamorphic clastic rocks underwent two stages of a compressive deformational history. The D_1 deformation led to the formation of an S_1 cleavage/foliation and affected an older granite. It is related to F_1 folding around ～N-S to NE-SW trending axes that partly record a steep plunge. Reconstructed fold structures display maximum wavelengths and amplitudes on a scale of hundreds of meters to kilometers. A steep to overturned dip of thick packages of metaconglomerates and metasandstones is related to this folding event. Also, because of variably oriented L_1 lineations and dextral shearing, structures of the D_1 deformation are assigned to different structural domains interpreted as the result of strain partitioning during dextral transpression. Strips of post-D_1 granite and clastic mylonites with a dextral sense of shear are attributed to the second-stage deformational event. The late stage dextral shearing and mylonitization locally affected intrusions of granites and dacitic porphyries. In the country rocks it took place along preexisting S_1 planes or locally developed S_2/C_2 planes related to F_2 fold structures with steeply plunging axes. The regional metamorphism accompanying the deformation was in the range of the subgreenschist facies-greenschist facies transition and greenschist facies. The emplacement of synkinematic intrusions presumably followed active dextral shear zones. It cannot be excluded that during compressive deformation, older (normal) fault lines were reactivated and caused strain partitioning and later dextral shearing. After cessation of compressive deformation, granites and grano-diorites intruded the deformed clastic succession and led to contact metamorphism of variable extent. Younger high-level intrusions record chilled margins also at angular xenoliths. Isotopic dates from granitoids,rndacites, and metaclastic rocks in the Sierra Norte area and surroundings suggest that compressive deformations and metamorphism took place during the Early Cambrian Pampean orogeny. The deformational and metamorphic history can be attributed to either ridge subduction/collision or collision of the Pampean ter-rane with the western pre-Andean Gondwana margin. The two-stage kinematics suggests that (oblique?) collision was followed by a change to dextral displacements in the upper plate during the final state of and after collisional tectonics. The Sierra Norte area represented the upper (eastern) plate as supposed part of the Rio de la Plata craton and its cover, overriding the lower plate that consisted of an accretionary complex in the west.