Petrology and structure of the Ivanpah Mountains area, California
Weisenberg, Charles William
Burchfiel, B. C.
Master of Arts
tn the Ivanpah Mountains area of southeastern California may be seen the intersection of the north west-southeast trend of the late Mesozoic Sevier orogenic belt of westward directed thrust faults and a similar unnamed belt of early Mesozoic age, generally found from fifty to one hundred miles apart. In the same area is found the eastern limit of Sierra Nevadan age Mesozoic plutonism in the Mojave desert. This study examines the nature of Mesozoic igneous activity in the Ivanpah Mountains area and its relation to the extensive development of at least two superimposed systems of Mesozoic thrust faulting in the area. A field and petrologic study of the Ivanpah Mountains suggests that there are three and possibly four Mesozoic plutonic episodes in the area. The Oro Wash Quartz Diorite is a small pluton petrologically similar to plutons in nearby areas that have yielded K/Ar mineral ages of 19 to 2 million years. The Striped Hills granodiorite is a relatively mafic rock which appears to be a more mafic hybrid border phase of the Teutonia Quartz Monzonite. K/Ar mineral ages suggest it is 161-167 million years in age. The Teutonia Quartz Monzonite is a tabular body, inclined west, several miles thick. it forms what is probably a gradational contact with the Striped Hills Granodiorite, and has a minimum K/Ar mineral age of 135138 million years. The Kessler Springs Quartz Monzonite is a relatively fresh, porphyritic rock which, has an isotopic age of 91-94 million years. Non-plutonic igneous rocks include the Piute Valley Plug, a small body of rhyolite which intrudes along the contact of the Teutonia Quartz Monzonite, and paleozoic carbonates. Since it is within a few miles of, and petrographically similar to, the Mesozoic Delfonte Volcanics, it may have been part of the feeder dike system of these volcanics, suggesting they are younger than the Teutonia Quartz Monzonite. The Cima Road Dikes are a group of widespread dacite dikes of probable Cenozoic age. Chemically, all the igneous rocks are calcalkaline, and most of the plutonic rocks are chemically similar to the central Sierra Nevada Batholith. Silica values for the plutonic rocks vary from 51% for the Oro Wash body to 11% for some samples of the Teutonia Quartz Monzonite. Both K2O and Na2Û vary from near 2% in the Oro Wash body to near 5% in the Teutonia Quartz Monzonite, which is chemically similar to rocks of the Eastern Sierra Nevada Batholith. The Mesozoic structures related to the plutonic rocks include cataclastic and mylonitic zones associated with Mesozoic thrusting. The tabular west-dipping form of the pluton in the Ivanpah Mountains is probably related to Mesozoic structures and may intrude the core of a large eastward overturned anticline. The pluton may be pretectonic but is probably syntectonic. Cenozoic structures include joints and numerous types of shear zones. One breccia zone appears to have incorporated alluvium along the fault zone. Tectonism appears to have occurred in this area during the Triassic prior to emplacement of the Oro Wash pluton and again after emplacement of the Teutonia Quartz Monzonite in the Jurassic-Cretaceous. Some folding may have occurred synchronously with emplacement of the Teutonia Quartz Monzonite. The last period of major thrusting (probably middle Cretaceous) does not appear to have affected the Kessler Springs Quartz Monzonite.