Abstract

Structures associated with the forcibly emplaced Hanover-Fierro pluton were studied to determine its mechanisms of intrusion. The pluton has been extensively examined, and good three-dimensional information is available from drill holes and mines. The funnel-shaped body plunges northward, and approximately 700 m of structural relief appears along the length of the present outcrop. At higher structural levels around the southern end, the country rocks were deformed by folding and thrusting; at deeper levels near the center and north end, the country rocks were deformed largely by rotation and tectonic thinning.

Joints in the pluton indicate that maximum principal compressive stress (σ1) rotated from a vertical orientation at depth to a horizontal east-west orientation near the surface, forming a pair of vertical conjugate shear (transverse) joints of moderate dihedral angle (36°). Along the main axis of the pluton, a set of vertical north-trending (longitudinal) joints formed; plumose structure on the joints indicates that they propagated upward. Apparently, upward-directed maximum principal compressive stress governed the direction of fracture propagation, and tensile stresses caused by outward rotation of σ1 governed their orientation. The intermediate principal compressive stress (σ2) rotated with σ1, forming a set of joints dipping gently inward toward the center of the pluton with dip progressively decreasing from the center to the margins.

Structures around the pluton indicate that the early stage of injection produced arching and deformation by simple shear in the sedimentary overburden. With continued intrusion, an entire plug of overburden was uplifted, and lateral pressure of the magma locally exceeded the coefficient of friction along pre-existing regional joints causing minor shearing. Strata near the pluton were significantly thinned tectonically. During the final stage, lithostatic pressure of 200 to 640 m of overburden was no longer sufficient to prevent large-scale deformation, and folds and thrusts developed around the pluton.

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