RESULTS OF GEOLOGIC MAPPING OF THE PINE VALLEY MOUNTAINS, SOUTHWEST UTAH

Recently completed geologic mapping of 1:24,000 quadrangles (Page Ranch, Central East, Grass Valley, Saddle Mtn, and Signal Peak) within the Pine Valley Mountains (PVM) has provided a wealth of new data on the stratigraphy and structure of this area of the Basin and Range-Colorado Plateau Transition Zone. Cover rocks of the PVM include mostly Tertiary volcanic and hypabyssal intrusive rocks. The older (Oligocene and Miocene) volcanics include mostly regional calc-alkaline ash-flow tuffs from caldera sources (Indian Peak and Caliente) outside the PVM. These rocks overlie Paleocene-Oligocene Claron Fm fluvial and lacustrine rocks that in turn unconformably overlie clastic rocks of the Upper Cretaceous Iron Springs Fm. The Iron Springs and underlying Mesozoic rocks were folded during the Sevier orogeny (Cretaceous to Paleocene), producing a NE-trending open fold (Big Hollow syncline) with its axis aligned parallel to the Virgin anticline. During the Early Miocene (22-20 Ma), magmatism in the PVM produced a series of shallow, calc-alkaline laccoliths and associated volcanics and gravity-slide structures. The plutons are part of the NE-trending (140 km long) Iron Axis magmatic province of mostly quartz monzonite plutons. The gigantic 30 km by 11 km Pine Valley laccolith (20.5 Ma) caps a large portion of the PVM and has a remaining thickness of about 900 m. The laccolith intruded beneath a thin cover (<200 m) of Claron Fm and Tertiary volcanics. Gravity-slide structures associated with several of the laccoliths consists of allochthonous masses of brecciated Tertiary volcanic and sedimentary strata detached along low-angle faults from the flanks of the Pinto Peak, Stoddard Mtn, and Pine Valley intrusions as well as the Bull Valley-Big Mtn (BV-BM) intrusion to the west. The largest slide mass (from BV-BM) covers 170 km2 and is as much as 670 m thick. Venting of several ash flows and/or lava flows from the laccoliths followed each sliding episode and partially or totally covered individual slide masses. The PVM later (post-20 Ma) received regional ash flows from the west followed by local bimodal volcanism of basalt lava and minor dacitic domes. Post-8 Ma NS-trending high-angle faulting produced an extensional related fragmentation of the PVM that controlled the location and alignment of younger volcanic centers.