Repeated parallel deformation in part of the eastern Sierra Nevada, California and its implications for dating structural events

Abstract

Study of a thick section of late Paleozoic to mid-Cretaceous sedimentary and volcanogenic rocks in eastcentral Sierra Nevada has revealed an involved structural succession not readily apparent when analysed under the traditional assumptions of structural analysis (e.g. parallel structures are of the same age). Earliest structures in the area occur as sparse folds in late Paleozoic rocks, whereas in Triassic to mid-Cretaceous rocks earliest structures occur as penecontemporaneous slumps. Upon these earliest structures are superimposed slaty cleavage with associated lineations and subsequent crenulations. The slaty cleavage across the area is statistically parallel, as are the axial planes of crenulations which fold the slaty cleavage. Such a succession would traditionally be interpreted as representing two periods of deformation, the first forming the slaty cleavage and the second the crenulation of the slaty cleavage. There is evidence, however, to indicate that the slaty cleavage itself was formed during more than one period of deformation and the same may be true for the crenulations. Dykes emplaced in Jurassic rocks have been dated (U/Pb) as mid-Cretaceous and lie parallel to what is probably an early slaty cleavage direction. The dykes, however, also bear a slaty cleavage, albeit weaker than in the host rock. In addition, quantitative strain determinations of rocks in the area show that the older units are more strongly deformed than the younger units. These and other data suggest that the statistically parallel slaty cleavage and related structures (folds, lineations, etc.) found in the Jurassic and older rocks have formed during at least two, and possibly three, increments of strain, each increment separated by a lengthy period of geologic time, possibly as much as 45 Ma or more. Crenulations of the slaty cleavage at any point (subsequently formed after each period of slaty cleavage formation) may even predate slaty cleavage formed later at another nearby point. While it is possible to set up a chronology between earlier (tectonic and/or penecontemporaneous slumps) and later structures (slaty cleavage, folds, lineations, etc.), it is not valid to designate for the entire area a relative time sequence of formation of slaty cleavage and crenulations in the Jurassic or older rocks by the usual methods (e.g. S 2, S 3, F 2, F 3, etc.). These later structures can only be designated as Only in the youngest stratigraphic unit in the area, which has been subjected to one deformation (mid-Cretaceous), can a valid structural succession be applied areally. We suggest that multiphase, parallel structures, comparable to those we have described, may be a relatively common phenomenon in orogenic belts. Until one arrives at a thorough understanding of the detailed stratigraphy and the absolute ages of units in key relationships to the structures, it may only be possible to delineate the broadest of time sequences for the structures concerned.