Geologic Map of the Welcome Quadrangle and an Adjacent Part of the Wells Quadrangle, Elko County, Nevada

Located in central Elko County, the Welcome and adjacent part of the Wells quadrangles expose a remarkable array of critical relationships for understanding the geologic history of the State of Nevada and the interior of the southwestern U.S. Cordillera. Covering the northern end of the East Humboldt Range and adjacent Clover Valley and Clover Hill, this map includes the northern terminus of the Ruby Mountains-East Humboldt Range metamorphic core complex. The oldest rocks in the State of Nevada (the gneiss complex of Angel Lake), and Nevada’s only exposures of Archean rock, form the core of a multikilometer scale, southward-closing recumbent foldnappe, the Winchell Lake nappe (WLN). Although intensely metamorphosed and profoundly ductilely attenuated, the WLN folds a series of pre-metamorphic thrust allochthons that collectively form an essentially complete sequence of Paleoproterozoic to Mississippian metasedimentary rocks. The WLN transported what may be Nevada’s most deeply exhumed rocks, with peak pressures ranging to 10 kb, peak temperatures in excess of 750 ºC, and widespread partial melting and stromatic migmatization, all related to Late Cretaceous to Paleocene tectonism. Overprinting the metamorphic core is a WNW-directed kilometer-scale shear zone that, together with the detachment fault that forms its roof, accommodated tens of kilometers of extensional displacement in mid- to late Cenozoic time, diachronously exhuming the terrain from mid-crustal depths by late Miocene time. In addition, the high-grade rocks are extensively intruded by one of the Nevada’s most diverse suites of magmatic rocks, ranging in age from Archean to Miocene and in composition from mafic to felsic. On the west flank of Clover Hill, a westdipping detachment-fault system separates the highgrade metamorphic core from an overlying plexus of brittlely deformed, partly correlative but lower grade to non-metamorphosed Paleozoic rocks. In turn, a sequence of partly syntectonic volcanic and sedimentary rocks ranging in age from Eocene to Miocene structurally overlie the fault-bounded Paleozoic units. The Cenozoic sequence includes late Eocene and Oligocene ignimbrites and volcaniclastic rocks, Miocene sedimentary rocks and megabreccias, a Miocene rhyolite complex, and younger sedimentary rocks and vitric tuffs. The presence of the most distal northeasterly exposure of a key Oligocene volcanic marker, the 29 Ma tuff of Campbell Creek, suggests that a broad, low-relief (unfaulted) terrain was dissected by paleovalleys that extended at least 200 km to the west. Bracketed between the tuff of Campbell Creek and a 15.5 Ma tuffaceous sandstone at the base of the Miocene Humboldt Formation is a proximal sedimentary sequence known as the sedimentary sequence of Clover Creek that includes conglomerate, sedimentary breccia, sandstone, and megabreccia as well as intercalations of fossiliferous lacustrine strata. The megabreccias consist of unmetamorphosed mid- Paleozoic rocks (chiefly Upper Devonian Guilmette Formation) interpreted as rock-avalanche deposits shed from evolving normal-fault scarps inferred to have bounded the basin to the east. Disconformably overlying the sedimentary sequence of Clover Creek is a thick sequence of Miocene Humboldt Formation that is tilted steeply down against the detachment fault system, documenting large-scale displacement on the detachment system extending to at least as young ca. 9 Ma. Finally, bounding the range today on both east and west are large, normal-fault systems that were active in Quaternary time, including the Clover Hill fault, which may represent a southerly extension of the blind fault that caused the 2008 Mw 6.0 Wells earthquake

Time relations and structural-stratigraphic patterns in ophiolite accretion, west central Klamath Mountains, California

New geochronological data and published structural and stratigraphic data show that two distinctly different ophiolitic assemblages formed in general proximity to one another at nearly the same time and were subsequently imbricated along a regional thrust zone. The Josephine ophiolite constitutes a complete oceanic crust and upper mantle sequence which lies within the western Jurassic belt of the Klamath province. Within the study area the Josephine ophiolite was formed by seafloor spreading at about 157 m.y. before present. It was immediately covered by a thin pelagic and hemipelagic sequence which grades into a thick flysch sequence, both of which comprise the Galice Formation. The Galice flysch was derived from volcanic arc and uplifted continental margin orogenic assemblages. A major nonvolcanic source for the Galice flysch appears to have been the western Paleozoic and Triassic belt of the Klamath province exposed to the east. Proximal volcanic arc activity migrated to the site of the Josephine-Galice section by 151 m.y. and is represented by numerous dikes and sills which intrude the ophiolite and Galice Formation. The Preston Peak ophiolite is a polygenetic assemblage consisting of (1) a pre-mid-Jurassic tectonitic peridotite-amphibolite substrate which represents disrupted and unroofed basement of the western Paleozoic and Triassic belt and (2) an upper mafic complex which was intruded through and constructed above the tectonite substrate at about 160 m.y. The mafic complex consists primarily of diabase hypabyssal rocks that are overlain by diabase-clast breccia and hemipelagic deposits. A major arc-plutonic complex was emplaced into the Preston Peak ophiolite in at least two pulses at 153 and 149 m.y. Major phases of this complex consist of wehrlite, gabbro, pyroxene diorite, and hornblende diorite. The Josephine ophiolite is interpreted as the remnants of interarc basin crust. The Preston Peak ophiolite is interpreted as either a primitive remnant arc complex or a rift edge facies for the Josephine interarc basin. The Galice Formation represents a submarine fan complex that was built on juvenile crust of the Josephine basin floor. During the time interval of 153 to 149 m.y. the locus or arc magmatism migrated to an area which included the interarc basin floor and the remnant arc or basin edge. The basin shortly thereafter closed by convergent tectonics during the Nevadan orogeny resulting in the imbrication of the Josephine and Preston Peak ophiolites and their superimposed arc assemblages. The transition from seafloor spreading generation of Josephine ophiolite to its tectonic accretion by convergence and basin closure occurred within 5 to 10 m.y. The process of rifting and ophiolite formation in series with convergence and ophiolite accretion is considered an important mechanism for generating and displacing allocthonous terranes in the Klamath Mountains-Sierra Nevada region, and perhaps throughout the western cordillera

The Grand Tour of the Ruby-East Humboldt Metamorphic Core Complex, Northeastern Nevada: Part 1-Introduction & Road Log

The purpose of this geological excursion is to provide an overview of the multiphase developmental history of the Ruby Mountains and East Humboldt Range, northeastern Nevada. Although these mountain ranges are commonly cited as a classic example of a Cordilleran metamorphic core complex developed through large-magnitude, mid-Tertiary crustal extension, a preceding polyphase Mesozoic contractional history is also well preserved in the ranges. An early phase of this history involved Late Jurassic two-mica granitic magmatism, high-temperature but relatively low-pressure metamorphism, and polyphase deformation in the central Ruby Mountains. In the northern Ruby Mountains and East Humboldt Range, a Late Cretaceous history of crustal shortening, metamorphism, and magmatism is manifested by fold-nappes (involving Archean basement rocks in the northern East Humboldt Range), widespread migmatization, injection of monzogranitic and leucogranitic magmas, all coupled with sillimanite-grade metamorphism. Following Late Cretaceous contraction, a protracted extensional deformation partially overprinted these areas during the Cenozoic. This extensional history may have begun as early as the Late Cretaceous or as late as the mid-Eocene. Late Eocene and Oligocene magmatism occurred at various levels in the crust yielding mafic to felsic orthogneisses in the deep crust, a composite granitic pluton in the upper crust, and volcanic rocks at the surface. Movement along a west-rooted, extensional shear zone in the Oligocene and early Miocene led to core-complex exhumation. The shear zone produced mylonitic rocks about 1 km thick at deep crustal levels, and an overprint of brittle detachment faulting at shallower levels as unroofing proceeded. Megabreccias and other synextensional sedimentary deposits are locally preserved in a tilted, upper Eocene through Miocene stratigraphic sequence. Neogene magmatism included the emplacement of basalt dikes and eruption of rhyolitic rocks. Subsequent Basin and Range normal faulting, as young as Holocene, records continued tectonic extension

SHRIMP-RG U-Pb Isotopic Systematics of Zircon from the Angel Lake Orthogneiss, East Humboldt Range, Nevada: Is This Really Archean Crust?

In this article the authors cite the work of W. R. Premo and colleagues on new SHRIMPRG U-Pb zircon from the Angel Lake orthogneiss in the East Humboldt Range, Nevada. They cite that their work reinterprets the age and origin of the rock body that formerly had been interpreted as delimiting the southwestern extent of the Archean Wyoming province based on integrated field study. They state that their work represents an important advance and offers a valid interpretation of the rock unit

Significance of mid-Mesozoic peridotitic to dioritic intrusive complexes, Klamath Mountains–western Sierra Nevada, California

A mid-Mesozoic plutonic suite ranging from ultramafic-gabbroic rocks to dioritic rocks and commonly intruded by younger granitoids is widespread in the Klamath Mountains–western Sierra Nevada, California. The ultramafic-gabbroic rocks are clinopyroxene-rich and commonly vary from wehrlite to olivine-hornblende clinopyroxenite and melagabbro. Associated dioritic rocks include biotite–two-pyroxene diorite–monzodiorite, and the granitic rocks typically range from tonalite to granodiorite. These plutonic complexes are coextensive and broadly coeval with a suite of weakly metamorphosed volcanic rocks ranging in composition from basalt to basaltic andesite to andesite. The volcanic rocks are chiefly volcaniclastic and are characterized by several distinct phenocryst assemblages: clinopyroxene + plagioclase (± olivine or hornblende). The plutonic complexes and the associated volcanic rocks are restricted to terranes that lack continental (sialic) crust and that, at least locally, contain juvenile ophiolitic crust generated adjacent to the locus of magmatism. Thus, their petrogenesis may be related to recurrent fracturing and magmatism within a rifted ensimatic arc

A Crustal Cross-Section for a Terrain of Superimposed Shortening and Extension: Ruby Mountains-East Humboldt Range Metamorphic Core Complex, Nevada

Geologic mapping coupled with geochronological, thermobarometric, and seismic reflection studies provide the data for constructing a crustal cross-section through a Tertiary extensional orogen in the eastern Great Basin, western U.S. Cordillera. Oligocene-Miocene sedimentary and volcanic rocks were deposited during brittle, upper-level crustal extension dominated by high-angle normal faults, rotation of the strata and the faults themselves, and the progressive evolution of a low-angle detachment fault system. Together with nonmetamorphosed to very low-grade rocks of the Cordilleran miogeocline, the synextensional strata comprise an upper crustal suprastructure that was attenuated during Tertiary crustal extension. Structurally below the suprastructure and commonly separated from it by a regional detachment fault is a transitional metasedimentary/granitoid zone which preserves a principally Mesozoic magmatic, metamorphic, and deformational history. In turn, this zone grades downward abruptly into a 1.5- to 2.0-km-thick, upper amphibolite facies Tertiary shear zone that forms the top of a mylonitic to nonmylonitic migmatitic infrastructure of variable age. This infrastructural zone clearly records a complex Mesozoic history, but is in part characterized by a Tertiary magmatic-metamorphic-deformational history that appears to increase in intensity with depth. Seismic reflection data collected across the Ruby-East Humboldt complex indicate a highly reflective character throughout the crust. Nonetheless, distinct zones of strong seismic reflectivity occur at 4 and 6 s and are interpreted to be zones of intense plastic deformation that probably acted as middle crustal decoupling zones. Middle and lower crustal velocities derived from wide-angle seismic reflection data suggest the presence of mafic rocks interpreted as additions during crustal extension; and these underplated mafic intrusions together with their wallrocks probably experienced granulite facies metamorphism during Cenozoic crustal extension. Taken as a whole, this crustal cross-section suggests that noncoaxial strain was principally partitioned along distinct km-scale extensional shear zones, but when viewed on the scale of the entire middle and lower crust this deformation probably constituted a bulk coaxial strain regime that generated a distinctive crustal extensional metamorphic fabric