GEOLOGY OF JOSHUA TREE NATIONAL PARK

Joshua Tree National Park includes probably the largest semi-contiguous exposure of Precambrian basement in the far western United States, and originally lay at the southwestern edge of Laurentia during formation of the Cordilleran miogeocline. The park extends more than 100 km across the strike of the Mesozoic Cordilleran orogen, and its Cenozoic landscape was shaped at the intersection of the eastern California shear zone and the southern San Andreas fault system. Previous work by Miller (1946), Rogers (1954), Dibblee (1967, 1968), Hope (1969), Powell (1981), Calzia (1982), Stone and Pelka (1989), and Howard (2002) and our new mapping and geochronology are incorporated into a bedrock map of the park. The oldest exposed rocks are 1.8 to 1.69 Ga Paleoproterozoic metasedimentary and metaigneous rocks metamorphosed as late as 1.65 Ga – evidence from detrital and xenocrystic zircons indicates these rocks formed by recycling of nearby older Archean and Proterozoic crust. Basement rocks are overlain by a shallow water sedimentary cover sequence deposited between 1.63 and 1.45 Ga, and intruded by granodiorite, mangerite and granite about 1.20 Ga. Precambrian rocks were intruded by discordant Permo-Triassic, Middle Jurassic, Late Jurassic, and Cretaceous plutons. In the eastern part of the park, a sequence of conglomerate, sandstone, phyllitic siltstone and minor limestone was intruded by Late Cretaceous granodiorite, and has been correlated with the mid-Cretaceous McCoy Mountains Formation. A large portion of the western part of the park is underlain by a sheeted complex consisting of 1000s of Mesozoic intrusive sheets, ranging in composition from gabbro to two-mica-garnet granite, and separated by rafts of Proterozoic gneiss. The distribution of Mesozoic rocks across the park is consistent with an east to west increase in exposure level, probably resulting from differential Laramide exhumation. Continued evolution of the park landscape in the Cenozoic included widespread alkali basalt volcanism and sinistral faulting associated with initiation of the southern San Andreas fault system. Seismicity in the last 25 years has been focused in the western part of the park and is not associated with major mapped faults – this observation suggests a modern change in type of faulting, with depth and/or with time.