Interactions between low-angle normal faults and plutonism in the upper crust; insights from the island of Elba, Italy

The late Miocene to early Pliocene Zuccale low-angle normal fault on the Island of Elba, Italy, was one of the principal structures active during extensional collapse of the Apennine fold-and-thrust belt. We investigate the relationships among the Zuccale fault, subsidiary footwall fault networks, and igneous bodies that were intruded into the immediate footwall of the Zuccale fault. Both brittle and ductile kinematic indicators found in association with fault zones and igneous bodies yield a consistent WNW-ESE extension direction, suggesting that faulting and intrusion overlapped in time. Structure contour analysis indicates that the Zuccale fault has a regional domal morphology. The dimensions and spatial location of the dome correlate with the likely subsurface position of the Porto Azzurro pluton, originally intruded at approximately 6 km depth. We propose that doming of the Zuccale fault may have been related in part to emplacement of the Porto Azzurro pluton as a tabular intrusion, involving some component of vertical inflation and roof uplift. The immediate footwall of the Zuccale fault is everywhere crosscut by a complex, linked network of high- and low-angle extensional faults with observed displacements of <10 m. Mutual crosscutting relationships suggest that low- and high-angle faults were active broadly contemporaneously. The final geometry of the footwall fault networks is adequately explained by their position with respect to the regional domal structure, and they suggest that certain sections of the Zuccale fault were back-rotated--during doming--out of an orientation capable of accommodating continued regional extension.