Previous section Volume contents Cenozoic Igneous and Tectonic Setting of the Marysvale Volcanic Field and Its Relation to Other Igneous Centers in Utah and Nevada By Peter D. Rowley, Charles G. Cunningham, Thomas A. Steven, Harald H. Mehnert, and Charles W. Naeser CONTENTS Abstract........................................................................................................................... 168 Introduction..................................................................................................................... 168 Acknowledgments .......................................................................................................... 172 Igneous Rocks................................................................................................................. 172 Middle Cenozoic Calc-alkaline Rocks ................................................................... 172 Upper Cenozoic Basalt and Alkali Rhyolite........................................................... 174 Regional Versus Local Extensional Deformation........................................................... 176 Middle Cenozoic Faults .......................................................................................... 176 Late Cenozoic Faults and Basin-fill Deposits......................................................... 179 Igneous Belts and Mineral Belts..................................................................................... 180 Transverse Structures...................................................................................................... 181 Plate-tectonic History ..................................................................................................... 184 Spreading ........................................................................................................................ 185 Conclusions..................................................................................................................... 188 References Cited ............................................................................................................. 189 FIGURES 1. Map showing major igneous and tectonic features of Nevada and Utah................... 169 2. Plot of volumes of volcanic rocks versus age, Marysvale volcanic field................... 170 3. Maps showing the Marysvale volcanic field and other features of interest in southwestern Utah ..................................................................................................... 171 4. Plot of total alkalies versus silica of calc-alkaline rocks from the Marysvale volcanic field, the Henry, La Sal, and Abajo Mountains, the Iron axis, and the Spry intrusion....................................................................................................... 174 167 168 LACCOLITH COMPLEXES OF SOUTHEASTERN UTAH ABSTRACT in age from at least 23 Ma to Quaternary and intertongued in part with coeval clastic basin-fill sedimentary rocks. The The Marysvale volcanic field of southwestern Utah, bimodal association is dominated by high-silica rhyolite ash- largely in the High Plateaus transition zone of the Colorado flow tuff and volcanic domes, potassium-rich mafic rocks, Plateau, lies at the east-northeastern end of the Pioche- and basalt cinder cones. Rocks of the bimodal association Marysvale igneous belt. This belt consists of exposures of are much less voluminous (about 5 percent of the total mostly Cenozoic volcanic rocks and is underlain by a volume of the Marysvale field) than the older calc-alkaline batholith complex of even greater volume. The volume of rocks, but they host many metallic mineral deposits, mostly volcanic rocks in the Marysvale field totals at least 12,000 of gold, silver, and lithophile elements. Bimodal rocks are km3. The field consists mostly of middle Cenozoic, interme- also irregularly distributed elsewhere in the Pioche- diate-composition, fundamentally calc-alkaline rocks erupt- Marysvale and Delamar–Iron Springs igneous belts. The ed at 34(?)–22 Ma; associated mineral deposits are mostly of late Cenozoic volcanism and extension began after subduc- chalcophile elements. Stratovolcano deposits, especially tion had started to diminish, as transform motion became volcanic mudflow breccia and lava flows, dominate; ash- significant on the San Andreas transform fault zone. flow tuffs derived from several calderas make up less than Regional oblique extension, referred to as the basin-range 10 percent of the volume of the volcanic rocks. Most of the episode and involving mostly faults under a normal-fault central and northern part of the field consists of the Bullion stress regime, was oriented generally east-west, and it began Canyon Volcanics of relatively crystal-rich dacite and in the Marysvale area and the southeastern Great Basin at andesite; source cupolas reached shallow levels, and many about 10 Ma, as faults formed the present topography. intrusions have associated mineral deposits. The southern Transverse structures continued to be active in the Great part of the field is dominated by crystal-poor andesite of the Basin but most were oriented east-west, parallel to the new Mount Dutton Formation; postulated source cupolas are extension direction. The eastern Snake River Plain is a deep and unexposed, and associated mineral deposits are youthful model for volcanism and extension along probable sparse. Shallow laccoliths that are unrelated to and south of underlying transverse structures oriented parallel to its the deep Mount Dutton sources were emplaced into lower current northeastern extension direction. Tertiary sedimentary rocks at the same time. These lacco- The overall effect of extension and volcanism in the liths are comagmatic with laccoliths and stocks of the “Iron Great Basin during the middle and late Cenozoic has been Axis,” in the Great Basin southwest of the Marysvale field. one of east-west spreading (widening) of the Great Basin, Large underlying batholiths fed both the southern Marysvale creating a bilateral symmetry. Any single axis of spreading field and Iron Axis laccoliths. is unlikely; probably there were many north-northwest- to The calc-alkaline igneous rocks of the Pioche- north-northeast-trending axes perpendicular to the then- Marysvale igneous belt and the slightly younger active extension direction. Extension related to these axes Delamar–Iron Springs igneous belt to the south are part of a was expressed by faulting, igneous intrusions, or both. generally middle Cenozoic igneous sequence that spans Under each axis, heat flow increased and the brittle-ductile much of the Western United States. The igneous rocks prob- transition zone rose. Extensional stress migrated with time, ably originated by oblique convergence during subduction parallel to the extension direction, toward the cooler, more of oceanic lithosphere beneath western North America. The brittle margins of the Great Basin. The transverse structures, overall area underlain largely by the igneous rocks is anom- oriented parallel to the extension direction of the time, alously wide when compared with igneous areas in other accommodated differing amounts of extension and magma- parts of the Pacific rim, apparently because the subducted tism north and south of them; they are fundamental features slab continued at a shallow depth as far to the east as the probably passing down to the brittle-ductile transition. longitude of the Rocky Mountains. The Pioche-Marysvale and Delamar–Iron Springs igneous belts formed under extension parallel to the subduction direction and were INTRODUCTION bounded by transverse structures (“lineaments”) of the same trend that separate areas of different amounts and types of The Marysvale volcanic field of southwestern Utah is extension. The generally east-northeast-trending crustal one of the largest Cenozoic volcanic fields in the Western extension of the same age as calc-alkaline magmatism was United States; it largely straddles the High Plateaus Section especially profound in the Basin and Range province, but of the Colorado Plateaus province, but the western part of extensional faults were dominant only in some areas, where- the field is in the Great Basin Section of the Basin and Range as elsewhere extension was accomplished by passive province (fig. 1). The High Plateaus is geologically a transi- emplacement of shallow intrusions. tion zone between the Great Basin and the main, much less The middle Cenozoic calc-alkaline rocks at Marysvale deformed part of the Colorado Plateau. The volcanic field are overlain by an upper Cenozoic, fundamentally bimodal overlies Paleozoic to lower Tertiary sedimentary rocks (rhyolitic and basaltic) volcanic association of rocks ranging mostly east of the northeast-trending Cordilleran hingeline; rocks northofthe field(R.E.AndersonandBarnhard, 1992). clastic materialthatisnowincorporated inlowerTertiary of thefield,andapparentLaramide upwarpsshedcoarse directed thrustsheetsthatunderlie atleastthenorthernpart Sevier compressionaldeformation formedsoutheast- 120° 42° 118° 116° 114° 112° NEVADA UTAH Northern Nevada Rift 110° QUIET ZONE IGNEOUS ANDTECTONICSETTING,MARYSVALEFIELD 40° U A R E T SIN A L P O AT BA R D GRE WALKER Marysvale P COLORA Volcanic Field LaSal Snake Range Mountains Tertiary rocks in theMarysvalefieldconsist ofthreemain mineralization aboveamajor batholithcomplex(fig.1). extensive, mostlyOligocene andMiocene,volcanism east-trending Pioche-Marysvale igneousbelt,thesiteof The volcanicfieldliesatthe easternendoftheeast-north- Pioche-Marysvale