The Structure and Evolution of Volcanic Plumbing Systems in Fold-and-Thrust Belts: a Case Study of a Miocene Intrusive Complex in the Neuquén Andes, Argentina

Abstract:

Magma ascent and emplacement in compressional tectonic settings remain poorly understood. Geophysical studies show that volcanic plumbing systems in compressional environments are vertically partitioned into a deep level subject to regional compression and a shallow level subject to local extension. Such vertical partitioning has also been documented for the plumbing systems of mud volcanoes, implying common, yet unresolved, underlying processes. In order to better constrain the mechanisms governing this depth partitioning of magma emplacement mechanisms, we studied the structure and evolution of the Cerro Negro Intrusive Complex (CNIC) emplaced in the Chos Malal Fold-and-Thrust Belt (CMFTB) in the foothills of the Neuquén Andes, northern Neuquén Province, Argentina. The CNIC consists of sills and N-S-striking dikes, which crosscut the sills. The most prominent structures in the study area are N-S-trending folds, and both E and W verging thrusts. We provide new U-Pb ages of 11.63 ± 0.20 Ma and 11.58 ± 0.18 Ma for sills and 11.55 ± 0.06 Ma for a dike, which show that the CNIC was emplaced in a short period of time. Our ages and field observations demonstrate that the emplacement of the CNIC was coeval with the tectonic shortening and associated development of the CMFTB. This implies that the dikes were emplaced perpendicular to the main shortening direction. The systematic location of the dikes at the anticlinal hinges suggest that their emplacement was controlled by local, shallow stresses related to outer-arc stretching at the anticlinal hinge. We conclude that folding-related outer-arc stretching is one mechanism responsible for the vertical partitioning of igneous plumbing systems in compressional tectonic settings.