Elongated domes in extended orogens: A mode of mountain uplift in the Betics (southeast Spain)

Abstract

The Sierra Nevada elongated dome in the Betic hinterland (westernmost Mediterranean region) formed by polymetamorphic, non-melted rocks involving crustal thickening and subsequent exhumation via extensional denudation including both normal faulting and vertical ductile thinning. Core rocks record a clockwise P-T-t path with segments of quasi-isothermal decompression that do not cross the melting solidi. Doming was caused by the interference of two orthogonal sets of Miocene-Pliocene, large-scale open folds (trending roughly E-W and N-S) that warp both WSW-directed extensional detachments and the footwall regional foliation. N-S folds were generated by a rolling hinge mechanism while E-W folds formed due to shortening perpendicular to the direction of extension. Strike-slip faults striking subparallel to the direction of extension laterally bound the domes, adjoining highly extended domains to less extended blocks.

Using a three-dimensional model of the crustal structure of the Sierra Nevada elongated dome constrained by surface geological data, the relationships with present-day topography, and the deep crustal structure, this paper explores the role of crustal flow in the origin and evolution of the dome. Collectively, the crustal structure, the rheological considerations, and other geophysical data suggest the occurrence of flow channels at two levels: mid-crustal depths and the deep crust. Flow in the upper channel is closely related to the mode of footwall denudation by detachment unroofing. The flowing channel in the deep crust is probably induced by the NW-SE crustal thinning pattern inferred for the region, with a relatively thick crust at the NW, and is likely to be oblique to the direction of extension in the upper crust.

A geometric model assuming footwall deformation by subvertical simple shear examines the possible exhumation paths of the lower-plate rocks and the evolution of the dome core in the upper crust during extension. In this model, the dome width measured parallel to the direction of extension can be used to estimate the amount of horizontal extension, once the dip of the non-readjusted segment of the detachment is well constrained. Finally, we also discuss two interesting associated problems common in extensional tectonics; namely, (1) what causes mountain uplift in recently extended continental terrains? and (2) what holds up high mountain belts in these regions where the Moho is often subhorizontal? A rolling hinge model and simultaneous transverse shortening can explain the high values of extension, the orthogonal folding, and the high mountains in the Sierra Nevada elongated dome. Flow beneath the dome of a relatively low-velocity, highly conductive, and probably low-density crustal material can support the high topography.