Potassium-argon cooling ages in the northern part of the Peninsular Ranges Batholith and offsets on the Elsinore and San Jacinto fault zones

Conventional potassium-argon (K-Ar) ages were obtained on biotite from samples of granitic rocks collected at as regular spacing as outcrop and sample suitability permitted across the entire northern exposed part of the Peninsular Ranges batholith. Uranium-lead (U-Pb) ages on zircons range from 6 to 24 m.y. older than roughly corresponding conventional K-Ar biotite ages. The U-Pb zircon ages are considered to be emplacement or near-emplacement ages and provide a basis for using the conventional biotite ages to approximate variations in cooling history. Contouring of the biotite cooling ages shows the same west-to-east younging trend that earlier regional dating studies have shown. Contours generated by these earlier regional studies produced a relatively smooth, even age gradient across the batholith. Biotite cooling age contours generated by the much more closely spaced data set used here suggest a more complicated cooling history and show strong digressions from the smooth, even regional-scale gradient. Along much of their respective traces, the right-lateral strike-slip Elsinore and San Jacinto fault zones cut granitic rocks of the batholith; we have used the age contours as datums to estimate maximum offset across the Elsinore fault and to support a proposed offset based on geologic mapping evidence on the San Jacinto fault. These estimates, based on offsets of the age contours, may differ from true offsets because of uncertainties related to (1) inability to establish the dip of the age contours, (2) possible vertical components of offsets on faults, and (3) paucity of samples in some areas. Cooling age contours are offset 12 km across the Elsinore fault zone, a figure in keeping with offsets of 10-15 km based on detailed geologic mapping. Likewise, alignment of contour features at the north end of the Perris block with those at the north end of the San Jacinto block requires restoration of 29 km of right-lateral displacement, i.e., essentially the same as documented offsets based on geologic mapping.