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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 km2 in which the potassium-argon isotopic systematics have been highly disturbed. The disturbance or disturb ances 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 ap proach, emplacement ages. (3) A zone separating these two areas from which rocks yield discordant apparent ages on coexisting min eral 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 concor dant 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 appar ent 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 may or may not be reflected by offset con tours; correlation of possible offset features across the fault is uncer tain. Several northwest-trending faults on the Mojave Desert strongly disrupt the contours but do not show the right-lateral dis placements that have been attributed to them on the basis of appar ent 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 con tours 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 con tinuing postemplacement heat source generated by continued under-thrusting of the Pac...
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