Abstract

Tertiary volcanic and hypabyssal intrusive rocks cover most of the Pine Valley Mountains in Washington and Iron Counties, Utah. The older (Oligocene and Miocene) part of the volcanic sequence consists mostly of regional calc-alkaline ashflow tuffs derived from caldera sources (Indian Peak and Caliente caldera complexes) outside the area. These volcanic rocks rest on, or are interbedded near the top of, fluvial and lacustrine rocks of the Paleocene-Oligocene Claron Fm. The Claron rests unconformably on fluvial rocks of the Upper Cretaceous Iron Springs Fm. Beginning in the latest Cretaceous and ending in the Paleocene, the Iron Springs and underlying older Mesozoic rocks were folded during the Sevier orogeny, producing a NE-trending open fold, named here the Big Hollow syncline. The axial trend of the syncline is aligned parallel to the Virgin anticline to the east. The Virgin-Big Hollow fold system is interpreted to be younger than thrusting in the Iron Springs district to the north and therefore may represent the youngest structural feature of the orogenic belt in this part of southwest Utah.

During the early Miocene (22 to 20 Ma), an episode of igneous activity in the Pine Valley Mountains produced a series of shallow, calc-alkaline laccolithic intrusions with associated volcanics and gravity-slide structures. The intrusions of the Pine Valley Mountains are part of the larger (140 km long) NE-trending Iron Axis magmatic province, which includes more than a dozen-exposed intrusions consisting mostly of quartz monzonite. The gigantic 30 km long by 11 km wide Pine Valley laccolith (20.5 Ma) caps a large portion of the Pine Valley Mountains and has a remaining thickness of as much as 900 meters. The laccolith intruded beneath a thin cover (<200m of Claron and Tertiary volcanics) and most likely occupied an area of 600+km2, as delineated by erosional outliers to the south and subsurface extensions beneath domed country rocks to the north. Gravity-slide structures associated with intrusive doming of several laccoliths consist of allochthonous masses of brecciated Tertiary volcanic and sedimentary strata detached along low-angle faults from the growing uplifted flanks of the Pinto Peak, Stoddard Mountain, and Pine Valley intrusions as well as the Bull Valley-Big Mountain (BV-BM) intrusion to the west. The largest slide mass (from BV-BM) covers 170 km2, is as much as 670 m thick, and extends more than 20 km from its intrusive arch. Immediately following each sliding episode, each intrusion erupted ash flows and (or) lava flows that partially or totally covered the slide masses. Thus, the laccoliths of the Pine Valley Mountains each show continuous growth stages from (1) initial rapid sill emplacement to its full lateral extent within the Iron Springs or Claron Fms, (2) vertical growth and bending of the overburden as the sill thickened into a laccolith, (3) gravity sliding from the upturned roof as the intrusion continued its vertical growth, and (4) eruption of ash flows and (or) lava flows as a result of pressure release due to gravity sliding.

Following intrusive activity (post-20 Ma), the area again received regional ash-flow deposits from the Caliente caldera complex (Racer Canyon Tuff) followed by local bi-modal magmatism that produced abundant basalt lava flows and minor dacitic domes. Numerous post-8 Ma NS-trending high-angle normal faults produced an overall extensional related fragmentation of the Pine Valley Mountains at this time related to Basin and Range tectonism. The location and alignment of the youngest volcanic centers are highly controlled by the presence of these faults.