Potassium-argon geochronology of the eastern Transverse Ranges and southern Mojave Desert, southern California

Abstract

More than 200 potassium-argon apparent ages on minerals from crystalline rocks, chiefly from the San Bernardino and eastern San Gabriel Mountains and the southern Mojave Desert, define an area greater than 10,000 km 2 in which the potassium-argon isotopic systematics have been highly disturbed. The disturbance or disturbances appear to have culminated at different times in different parts of the region, ranging from 57 m.y. ago in the eastern San Gabriel Mountains to about 70 m.y. ago in the southern Mojave Desert. The region can be subdivided into three parts on the basis of potassium-argon dating: (1) An inner area of anomalous ages in which the rocks yield apparent potassium-argon ages that indicate complete or nearly complete resetting of the isotopic system. (2) An outer area in which the rocks yield apparent ages that are, or approach, emplacement ages. (3) A zone separating these two areas from which rocks yield discordant apparent ages on coexisting mineral pairs. This discordant zone varies in width from about 6 to 12 km and grades inward to rocks reset to the degree that they yield concordant potassium-argon apparent ages on coexisting mineral pairs and outward toward rocks that yield near-concordant apparent ages. Rocks from the center and the inner parts of the discordant zone yield the most discordant apparent ages. Contouring of the apparent ages defines the extent of the reset region that occurs on both sides of the San Andreas fault. The apparent ages can be contoured across the fault, although the position of the fault is well defined by abrupt deflection of the contours parallel to the fault. The reverse fault bounding the north side of the San Bernardino Mountains mayor may not be reflected by offset contours; correlation of possible offset features across the fault is uncertain. Several northwest-trending faults on the Mojave Desert strongly disrupt the contours but do not show the right-lateral displacements that have been attributed to them on the basis of apparent offsets of geologic features. These faults may have a component of vertical movement, and it is not known what effect this might have on the contours; even a small amount could be profound. The cause of the isotopic disturbance is not well understood, as the area of most complete resetting does not appear to be coincident with any single batholithic mass. The apparent-age contours cross the boundaries of individual plutons, and the configuration of the contours shows no apparent relation to the shapes of plutons or groups of plutons. Even though there does appear to be a lack of correlation between individual plutons or batholith-size collections of plutons, this lack of correlation may be more apparent than real. It is possible that some of the plutons within the area of anomalous ages are part of an extensive batholithic mass of which only the uppermost part is exposed. An alternative interpretation of the anomalous ages is that a continuing post-emplacement heat source generated by continued under thrusting of the Pacific plate beneath North America caused the region to remain at elevated temperatures such that argon retention by minerals datable by potassium-argon methods was not possible for some time after pluton emplacement. As suggested by Coney and Reynolds (1977), possibly the angle of underthrusting was shallower than when the Transverse Range plutons were emplaced. Locally, the youngest ages show a spatial relation to the Vincent thrust fault and its correlatives. If this fault is the cause of the region-wide resetting, it has to have extended under most of the western part of southern California. The configuration of the anomalous-age zone is roughly coincident with the position of the Transverse Ranges, the recently discovered southern California uplift, a relatively shallow high-velocity zone in southern California, and the projection of the Murray fracture zone. The eastward extent coincides roughly with the eastern limit of high seismicity in southern California.