Abstract

Conspicuous faults in the New Red Sandstone (NRS, mainly Permian-Triassic) of Arran are shown to be kinematically related to the intrusion of the Northern Granite in Paleocene time. Early inflation of the granite pluton in the upper crust induced radial stretching in the overlying sedimentary cover, recorded in predominantly normal offsets on the faults in the NRS. Continued ballooning of the pluton superimposed strong concentric dip-slip displacement over its flanks. Both stretching and uplift were heterogeneous, strongly controlled by a nonuniform stratigraphy and by reactivation of older faults striking predominantly north-northwest and northeast and dating back at least to Permian time. The Paleocene displacements on these discrete weak sub-NRS faults diffused upward through NRS sandstones on numerous minor faults, each with only small offset. This behavior resulted from slip-hardening on each fault in the poorly cemented NRS rocks. Late in the intrusion history, the major faults on the southeast side of the granite formed a conjugate strike-slip system that accommodated radial expansion of the pluton, mainly during intrusion of a discrete magma batch that now forms the inner fine-grained granite.

Other structures around and within the Northern Granite are shown to be more compatible with this two-stage ballooning model than with a vertical plug emplacement model. Previous assessments of the ballooning process in general have been made on deep-level lateral margins of plutons where radial shortening dominates in the envelope. North Arran is an important example of the complexities of ballooning at shallow levels in a heterogeneous crust.

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