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Wayne R. Premo

Publications and source records attributed to Wayne R. Premo.

At least 37 records · Page 2Linked to original sources
USGS2014

SHRIMP-RG U-Pb ages of provenance and metamorphism from detrital zircon populations and Pb-Sr-Nd signatures of prebatholithic metasedimentary rocks at Searl Ridge, northern Peninsular Ranges batholith, southern California: Implications for their age, origin, and tectonic setting

Twenty-four samples were collected from prebatholithic metasedimentary rocks along Searl Ridge, the north rim of the Diamond Valley Reservoir, Domenigoni Valley, centrally located in the northern Peninsular Ranges of southern California. These rocks exhibit progressive metamorphism from west to east across fundamental structural discontinuities now referred to as a “transition zone.” Documented structural and mineralogical changes occur across this metamorphic gradient. Sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) U-Pb ages were obtained from detrital zircons from metasedimentary rocks through the transition zone. To the west, metapelitic and minor metasandstone units yielded numerous concordant 206 Pb/ 238 U ages between 210 and 240 Ma, and concordant 207 Pb/ 206 Pb ages at 1075–1125 Ma, 1375–1430 Ma, and 1615–1735 Ma, although distinct differences in provenance were noted between units. A few older 207 Pb/ 206 Pb ages obtained were ca. 2250 Ma and ca. 2800 Ma. Rocks of the eastern part of the transition zone include high-grade paragneisses that yielded numerous concordant 206 Pb/ 238 U ages between 103 and 123 Ma and between 200 and 255 Ma, and concordant 207 Pb/ 206 Pb ages at 1060–1150 Ma, 1375–1435 Ma, and 1595–1710 Ma. Some zircon results from these high-grade gneisses are marked by distinct Pb-loss discordia with lower-intercept ages of ca. 215 Ma and Paleoproterozoic upper-intercept ages. Younger ages between 100 and 105 Ma are mainly obtained from rims of some zircon grains that are characterized by low Th/U values (<0.1) and high U contents (>1000 ppm), indicating the likelihood of metamorphic zircon growth at that time. The similarity of zircon age populations between western and eastern units through the transition zone indicates that this fundamental structure probably dissects sediments of the same basin. This supposition is further supported by initial whole-rock Pb-Sr-Nd isotopic data that show similar average initial 206 Pb/ 204 Pb (18.65 to 18.9), 87 Sr/ 86 Sr (0.713 to 0.718), and ε Nd (−7 to −12) values for both the western and eastern units—values that also indicate the presence of significantly older crustal material in their provenance. Magmatic zircons from a diorite dike that crosscuts the foliation, but is itself subsequently metamorphosed, yielded a SHRIMP-RG concordia age of 103.3 ± 0.73 Ma, which is within agreement of an isotope dilution–thermal ionization mass spectrometry (ID-TIMS) U-Pb age of 103.37 ± 0.25 Ma. A postmetamorphic, cross-cutting pegmatite yielded discordant U-Pb zircon age data, but euhedral, glassy monazite from the pegmatite yielded a slightly discordant 207 Pb/ 235 U age of 101.85 ± 0.35 Ma and a Th-Pb age of 97.53 ± 0.18 Ma, suggesting that this pegmatite was injected during or just after deformation ceased. The age and initial Pb-Sr-Nd signature for the dioritic dike indicate it was produced during the transition zone plutonism elsewhere in the northern Peninsular Ranges batholith, whereas the pegmatitic dike was derived from crustal anatexis. Collectively, these results indicate that this sequence of metasedimentary rocks was derived from mainly a Late Permian to Early Triassic igneous provenance that probably intruded Proterozoic crust. The sequence was subsequently metamorphosed during deformation of the Cretaceous continental margin at ca. 105 to 97 Ma.

Book chapter↗
USGS2013

Geologic map of the Harvard Lakes 7.5' quadrangle, Park and Chaffee Counties, Colorado

The Harvard Lakes 1:24,000-scale quadrangle spans the Arkansas River Valley in central Colorado, and includes the foothills of the Sawatch Range on the west and Mosquito Range on the east. The Arkansas River valley lies in the northern end of the Rio Grande rift and is structurally controlled by Oligocene and younger normal faults mostly along the west side of the valley. Five separate pediment surfaces were mapped, and distinctions were made between terraces formed by the Arkansas River and surfaces that formed from erosion and alluviation that emanated from the Sawatch Range. Three flood deposits containing boulders as long as 15 m were deposited from glacial breakouts just north of the quadrangle. Miocene and Pliocene basin-fill deposits of the Dry Union Formation are exposed beneath terrace or pediment deposits in several places. The southwestern part of the late Eocene Buffalo Peaks volcanic center, mostly andesitic breccias and flows and ash-flow tuffs, occupy the northeastern corner of the map. Dated Tertiary intrusive rocks include Late Cretaceous or early Paleocene hornblende gabbro and hornblende monzonite. Numerous rhyolite and dacite dikes of inferred early Tertiary or Late Cretaceous age also intrude the basement rocks. Basement rocks are predominantly Mesoproterozoic granites, and subordinately Paleoproterozoic biotite gneiss and granitic gneiss.

Colorado↗
USGS2012

Reactivation of the Archean-Proterozoic suture along the southern margin of Laurentia during the Mazatzal orogeny: Petrogenesis and tectonic implications of ca. 1.63 Ga granite in southeastern Wyoming

The presence of ca. 1.63 Ga monzogranite (the “white quartz monzonite”) in the southern Sierra Madre, southeastern Wyoming, is anomalous given its distance from the nearest documented plutons of similar age (central Colorado) and the nearest contemporaneous tectonic margin (New Mexico). It is located immediately south of the Cheyenne belt—a ca. 1.75 Ga Archean-Proterozoic tectonic suture. New geochronological, isotopic, and geochemical data suggest that emplacement of the white quartz monzonite occurred between ca. 1645 and 1628 Ma (main pulse ca. 1628 Ma) and that the white quartz monzonite originated primarily by partial melting of the Big Creek Gneiss, a modified arc complex. There is no evidence that mafic magmas were involved. Open folds of the ca. 1750 Ma regional foliation are cut by undeformed white quartz monzonite. On a regional scale, rocks intruded by the white quartz monzonite have experienced higher pressure and temperature conditions and are migmatitic as compared to the surrounding rocks, suggesting a genetic relationship between the white quartz monzonite and tectonic exhumation. We propose that regional shortening imbricated the Big Creek Gneiss, uplifting the now-exposed high-grade rocks of the Big Creek Gneiss (hanging wall of the thrust and wall rock to the white quartz monzonite) and burying correlative rocks, which partially melted to form the white quartz monzonite. This tectonism is attributed to the ca. 1.65 Ga Mazatzal orogeny, as foreland shortening spread progressively into the Yavapai Province. Mazatzal foreland effects have also been described in the Great Lakes region and have been inferred in the Black Hills of South Dakota. We suggest that the crustal-scale rheologic contrast across the Archean-Proterozoic suture, originally developed along the southern margin of Laurentia, and including the Cheyenne belt, facilitated widespread reactivation of that boundary during the Mazatzal orogeny. This finding emphasizes the degree to which crustal heterogeneities can localize subsequent deformation in accretionary orogens, producing significant crustal melting in the distal foreland—a region not typically associated with orogenic magmatism.

Colorado;Wyoming↗
USGS2012

Isotopic signatures of black tektites from the K‐T boundary on Haiti: Implications for the age and type of source material

U‐Th‐Pb, Rb‐Sr, and Sm‐Nd isotopic signatures of corroded, but unaltered, black glassy tektites from Cretaceous‐Tertiary (K‐T) boundary rock on Haiti are not consistent with their derivation from an impact on MOR‐derived oceanic crust or continental regions involving middle Proterozoic or older crustal material. Two single‐grain and two batches of these tektites yielded present‐day ∍ Nd = −3.0 to −3.4, ∍ Sr = +55 to 56, 206 Pb/ 204 Pb = 18.97; 207 Pb/ 204 Pb = 15.74; 208 Pb/ 204 Pb = 38.91 values, and Pb, Rb, Sr, Sm, and Nd concentrations of ∼6, ∼45, ∼535, ∼4.7, and ∼22 ppm, respectively. Initial ∍ Nd and ∍ Sr values for the tektites are different from time‐integrated Nd‐Sr isotopic signatures for almost all oceanic crustal types. Age‐corrected Pb isotopic values are similar to those for pelagic sediments with distinctly higher 207 Pb/ 204 Pb values compared to MORB. However, these results do not exclude the possibility of an oceanic impact site, if the tektites were derived from fine‐grained sediments that typically overlie such regions, although other mineralogic and chemical evidence from K‐T boundary debris suggests otherwise. Moreover, the Nd average crustal residence age of ∼ 1080 Ma (T DM ) for the black tektites eliminates impact sites on continental crustal regions involving middle Proterozoic or older rocks, or sedimentary rocks largely derived from them. Previously reported major and trace element data from the black tektites suggest that the source material was possibly sedimentary with a composition similar to average shale or graywacke. If this is the case, then the Nd isotopic data suggest that the source rocks were not older than Silurian (T CHUR = 400 Ma) in age, and were composed largely of young (< 1080 Ma) crustal material. Of the suspected K‐T boundary impact sites, both the Manson (Iowa) and Chicxulub (Yucatan) structures occur in suitable lithologies to yield the Haitian black tektites, although neither structure has as yet proven to be the tektite source.

Meteoritics↗
USGS2011

The geochemistry and petrogenesis of the Paleoproterozoic Green Mountain arc: A composite(?), bimodal, oceanic, fringing arc

The inferred subduction affinity of the ∼1780-Ma Green Mountain arc, a dominantly bimodal igneous terrane (together with immature marine and volcaniclastic sedimentary rocks) accreted to the southern margin of the Wyoming province, is integral to arc-accretion models of the Paleoproterozoic growth of southern Laurentia. Conversely, the dominantly bimodal nature of many putative arc-related igneous suites throughout southern Laurentia, including the Green Mountain arc, has also been used to support models of growth by extension of pre-existing crust. We report new geochemical and isotopic data from ∼1780-Ma gabbroic and granodioritic to tonalitic rocks of the Big Creek Gneiss, interpreted as consanguineous with previously studied metavolcanic rocks of the Green Mountain Formation. The ∼1780-Ma Big Creek Gneiss mafic rocks show clear geochemical signatures of a subduction origin and provide no supporting evidence for extensional tectonism. The ∼1780-Ma Big Creek Gneiss felsic rocks are attributed to partial melting of mafic and/or mixed lower-crustal material. The bimodal nature of the suite results from the combination of arc basalts and felsic crustal melts. The lack of andesite is consistent with the observed tholeiitic differentiation trend of the mafic magmas. The lower ɛ Nd (1780 Ma) values for the felsic rocks vs. the mafic rocks suggest that the unexposed lower crust of the arc may be older than the arc and that Trans-Hudson- or Penokean-aged rocks possibly form the substratum of the arc. Our results reinforce previous interpretations that arc-related magmatism played a key role in the Paleoproterozoic crustal growth of southern Laurentia, but also support the possibility of unexposed older crust as basement to the arcs.

Wyoming, Colorado↗
USGS2011

Geologic map of the Bailey 30' x 60' quadrangle, North-Central Colorado

The Bailey, Colo. 1:100,000-scale quadrangle lies within two physiographic and geologic provinces in central Colorado: 1) the Front Range and 2) South Park. Most of the Front Range is composed of Proterozoic rocks ranging in age from 1,790 Ma to 1,074 Ma. Along the eastern flanks and within the Denver Basin, sedimentary rocks ranging from Pennsylvanian to Cretaceous are deformed and steeply tilted to the east. Upper Cretaceous through Paleocene rocks were deposited in the foreland (that is, the Front Range eastern flank) and hinterland (that is, South Park) of this thrust and reverse fault system developed during the Late Cretaceous to Paleocene Laramide orogeny. Within South Park, rocks range in age from Pennsylvanian to Miocene with Quaternary deposits indicating tectonic subsidence of the basin. These rocks record five major geologic episodes: 1) the Paleozoic Anasazi uplift that formed the Ancestral Rockies, 2) the Late Cretaceous to Paleocene Laramide orogeny, 3) widespread Eocene to Oligocene volcanism, 4) Oligocene-Quaternary tectonics, and 5) Quaternary glacial episodes.

Colorado↗
USGS2011

Geologic map of the eastern half of the Vail 30' x 60' quadrangle, Eagle, Summit, and Grand Counties, Colorado

Recent mapping and geochronologic studies for the eastern half of the Vail 1:100,000-scale quadrangle have significantly improved our understanding of (1) Paleoproterozoic history of the basement rocks of the Gore Range and Williams Fork Mountains (western margin of the Front Range), (2) the Late Paleozoic history of the Gore fault system, (3) Laramide contractional tectonism, including deformation along the Gore fault and Williams Range thrust, (4) Oligocene and younger extensional history of the Blue River half graben (The northern extent of the Rio Grande rift), and (5) late Neogene and Quaternary surficial history. The recently active Gilman mining district, a major producer of zinc and lead, is in the southwestern corner of the map area. Marine sediments and mafic to felsic volcanic rocks deposited between about 1,740 and 1,780 m.y. were generally metamorphosed to amphibolite grade and intruded and deformed by mostly calc-alkalic granitic rocks during an orogenic episode that lasted about 110 m.y. The distribution of well-studied Upper Cambrian to thick Upper Cretaceous platform sediments is now greatly improved, which allows a better definition of the late Paleozoic uplift, erosion, and flanking sedimentation of the ancestral Front Range. Detailed mapping has also better defined the geometry of Late Cretaceous to early Tertiary Laramide deformation along both the Gore fault system and Williams Range thrust, as well as increased understanding of the details of mostly Neogene extension along the Blue River normal fault system (the western margin of the Blue River half graben). Scarps along the latter fault system indicate movement may be as young as Holocene. Detailed mapping of surficial deposits has defined and described (1) six ages of terrace alluvium, (2) three general ages of landslides, (3) glacial and periglacial deposits, and (4) fan, pediment, talus, and debris-flow deposits. The map is intended as a database for a variety of land-use and scientific purposes, including (1) assessment of geologically stable building sites, (2) planning for road and highway construction, (3) assessment of groundwater resources, (4) assessment of mineral resources, (5) determining geologic-hazard potential (flooding, landslide, rockfall, and seismic risk), (6) evaluating the structure of the northern Rio Grande rift in the Blue River valley, (7) improvement in understanding of the sedimentary section, which spans the period from the Cambrian to the Holocene, and (8) new insights into the geologic history of the Proterozoic basement rocks, including a number of new radiometric dates.

Scientific Investigations Map↗
USGS2008

Geologic map of the Denver West 30' x 60' quadrangle, north-central Colorado

The Denver West quadrangle extends east-west across the entire axis of the Front Range, one of numerous uplifts in the Rocky Mountain region in which Precambrian rocks are exposed. The history of the basement rocks in the Denver West quadrangle is as old as 1,790 Ma. Along the east side of the range, a sequence of sedimentary rocks as old as Pennsylvanian, but dominated by Cretaceous-age rocks, overlies these ancient basement rocks and was upturned and locally faulted during Laramide (Late Cretaceous to early Tertiary) uplift of the range. The increasingly coarser grained sediments up section in rocks of latest Cretaceous to early Tertiary age record in remarkable detail this Laramide period of mountain building. On the west side of the range, a major Laramide fault (Williams Range thrust) places Precambrian rocks over Cretaceous sedimentary rocks. The geologic history of the quadrangle, therefore, can be divided into four major periods: (1) Proterozoic history, (2) Pennsylvanian to pre-Laramide, Late Cretaceous history, (3) Late Cretaceous to early Tertiary Laramide mountain building, and (4) post-Laramide history. In particular, the Quaternary history of the Denver West quadrangle is described in detail, based largely on extensive new mapping.

Scientific Investigations Map↗
USGS2007

Tectonic setting and metallogenesis of volcanogenic massive sulfide deposits in the Bonnifield Mining District, Northern Alaska Range: Chapter B in Recent U.S. Geological Survey studies in the Tintina Gold Province, Alaska, United States, and Yukon, Canada--results of a 5-year project

This paper summarizes the results of field and laboratory investigations, including whole-rock geochemistry and radiogenic isotopes, of outcrop and drill core samples from volcanogenic massive sulfide (VMS) deposits and associated metaigneous rocks in the Wood River area of the Bonnifield mining district, northern Alaska Range (see fig. 1 of Editors’ Preface and Overview). U-Pb zircon igneous crystallization ages from felsic rocks indicate a prolonged period of Late Devonian to Early Mississippian (373&plusmn;3 to 357&plusmn;4 million years before present, or Ma) magmatism. This magmatism occurred in a basinal setting along the ancient Pacific margin of North America. The siliceous and carbonaceous compositions of metasedimentary rocks, Precambrian model ages based on U-Pb dating of zircon and neodymium ages, and for some units, radiogenic neodymium isotopic compositions and whole-rock trace-element ratios similar to those of continental crust are evidence for this setting. Red Mountain (also known as Dry Creek) and WTF, two of the largest VMS deposits, are hosted in peralkaline metarhyolite of the Mystic Creek Member of the Totatlanika Schist. The Mystic Creek Member is distinctive in having high concentrations of high-field-strength elements (HFSE) and rare-earth elements (REE), indicative of formation in a within-plate (extensional) setting. Mystic Creek metarhyolite is associated with alkalic, within-plate basalt of the Chute Creek Member; neodymium isotopic data indicate an enriched mantle component for both members of this bimodal (rhyolite-basalt) suite. Anderson Mountain, the other significant VMS deposit, is hosted by the Wood River assemblage. Metaigneous rocks in the Wood River assemblage span a wide compositional range, including andesitic rocks, which are characteristic of arc volcanism. Our data suggest that the Mystic Creek Member likely formed in an extensional, back-arc basin that was associated with an outboard continental-margin volcanic arc that included rocks of the Wood River assemblage. We suggest that elevated HFSE and REE trace-element contents of metavolcanic rocks, whose major-element composition may have been altered, are an important prospecting tool for rocks of VMS deposit potential in east-central Alaska.

Alaska;Yukon↗
USGS2006

New insights into the history and origin of the southern Maya block, SE Mexico: U-Pb-SHRIMP zircon geochronology from metamorphic rocks of the Chiapas massif

The histories of the pre-Mesozoic landmasses in southern M&eacute;xico and their connections with Laurentia, Gondwana, and among themselves are crucial for the understanding of the Late Paleozoic assembly of Pangea . The Permian igneous and metamorphic rocks from the Chiapas massif as part of the southern Maya block, M&eacute;xico, were dated by U&ndash;Pb zircon geochronology employing the SHRIMP (sensitive high resolution ion microprobe) facility at Stanford University. The Chiapas massif is composed of deformed granitoids and orthogneisses with inliers of metasedimentary rocks. SHRIMP data from an anatectic orthogneiss demonstrate that the Chiapas massif was part of a Permian (&sim; 272 Ma) active continental margin established on the Pacific margin of Gondwana after the Ouachita orogeny. Latest Permian (252&ndash;254 Ma) medium- to high-grade metamorphism and deformation affected the entire Chiapas massif, resulting in anatexis and intrusion of syntectonic granitoids. This unique orogenic event is interpreted as the result of compression due to flat subduction and accretionary tectonics. SHRIMP data of zircon cores from a metapelite from the NE Chiapas massif yielded a single Grenvillian source for sediments. The majority of the zircon cores from a para-amphibolite from the SE part of the massif yielded either 1.0&ndash;1.2 or 1.4&ndash;1.5 Ga sources, indicating provenance from South American Suns&aacute;s and Rondonian-San Ignacio provinces.

Maya block↗
USGS2005

Isotopic dating of Meso- and Neoproterozoic mafic magmatism in the southern Tobacco Root Mountains, Southwestern Montana

New isotopic dating of mafic dikes in the southern Tobacco Root Mountains of southwestern Montana provides evidence for two distinct episodes of subparallel Proterozoic mafic magmatism separated by about 700 m.y. Previously published geochemical data from dikes in the southern Tobacco Root Mountains had identified three geochemical groups (termed groups A, B, and C) with apparent Rb–Sr ages of 1455 Ma (group A) and 1100–1120 Ma (groups B and C). Sm–Nd dating of a geochemical group A dike yields an age of 1448 ± 49 Ma that is interpreted to represent the emplacement age of the dikes. This age is similar to published U–Pb dates from mafic sills that intrude the nearby Mesoproterozoic Belt Supergroup and Laramide basement-cored uplifts of the Archean Wyoming Province. Paleomagnetic results from groups A and B dikes are similar, suggesting that the were emplaced at about the same time. We suggest that the mafic magmatism recorded by the southern Tobacco Root Mountains group A/C dikes at about 1450 Ma probably corresponds to continental extension that accompanied development of the Mesoproterozoic Belt Basin. U–Pb dating of baddeleyite from the group B dikes provides evidence for a younger magmatic event with an age of 782.4 ± 4.9 Ma (95% confidence; ±7.1 Ma incorporating uranium decay constant errors). This date is similar to isotopic dates from Neoproterozoic mafic dikes and sills exposed elsewhere in uplifts of the Rocky Mountain foreland and that intrude the Mesoproterozoic strata of the Belt Supergroup. Geochronologic data from elsewhere in the northern Cordillera and Canadian Shield indicate that the group B dikes in the southern Tobacco Root Mountains are part of a regional 780 Ma magmatic event that extended for more than 2400 km along the western margin of Laurentia. The origin of the magmatic event is not clear, but may be related to a mantle plume and crustal extension accompanying initial breakup of the supercontinent Rodinia and development of the proto-Pacific Ocean. No evidence of ca. 1100 Ma mafic magmatism in this part of the northern Cordillera was found.

Montana↗
USGS2005

Contrasting Proterozoic basement complexes near the truncated margin of Laurentia, northwestern Sonora–Arizona international border region

We utilize new geological mapping, conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses, and whole-rock radiogenic isotope characteristics to distinguish two contrasting Proterozoic basement complexes in the international border region southeast of Yuma, Arizona. Strategically located near the truncated southwest margin of Laurentia, these Proterozoic exposures are separated by a northwest-striking Late Cretaceous batholith. Although both complexes contain strongly deformed Paleoproterozoic granitoids (augen gneisses) intruded into fine-grained host rocks, our work demonstrates marked differences in age, host rock composition, and structure between the two areas. The Western Complex reveals a >5-km-thick tilted section of finely banded felsic, intermediate, and mafic orthogneiss interspersed with tabular intrusive bodies of medium-grained leucocratic biotite granite (1696 ± 11 Ma; deepest level), medium-grained hornblende-biotite granodiorite (1722 ± 12 Ma), and coarse-grained porphyritic biotite granite (1725 ± 19 Ma; shallowest level). Penetrative ductile deformation has converted the granites to augen gneisses and caused isoclinal folding and transposition of primary contacts. Exposed in a belt of northwest-trending folds, these rocks preserve southwest-vergent shear fabric annealed during amphibolite facies metamorphism, when crystalloblastic textures developed. Deformation and regional metamorphism occurred before emplacement of 1.1 Ga(?) mafic dikes. Throughout the Eastern Complex, meta-arkose, quartzite, biotite schist, and possible felsic metavolcanic rocks comprise the country rocks of strongly foliated medium- and coarse-grained biotite granite augen gneisses that yield mean 207 Pb/ 206 Pb ages of 1646 ± 10 Ma, 1642 ± 19 Ma, and 1639 ± 15 Ma. Detrital zircons from four samples of host sandstone are isotopically disturbed; nevertheless, the data indicate a restricted provenance (ca. 1665 Ma to 1650 Ma), with two older grains (1697 and 1681 Ma). The pervasively recrystallized Paleoproterozoic map units strike parallel to foliation and are repeated in south-trending folds that are locally refolded about easterly hinges. Southeasterly lineation developed in augen gneiss and host strata becomes penetrative in local domains of L-tectonite. Regional metamorphism associated with this tectonism persisted until ca. 1590 Ma, as recorded by metamorphic growths within some zircon grains. Mesoproterozoic intrusions that crosscut the Paleoproterozoic metasediments and augen gneisses include coarsely porphyritic biotite granite (1432 ± 6 Ma) and diabase dikes (1.1 Ga?). Emplacement of the granite was accompanied by secondary high-U overgrowths, dated at 1433 ± 8 Ma, on some of the Paleoproterozoic detrital zircons, and apparently was also responsible for resetting the whole-rock Pb isotopic systematics (1441 ± 39 Ma) within these Eastern Complex augen gneisses. Younger plutons emplaced into both Proterozoic basement complexes include medium-grained quartz diorite (73.4 ± 3.3 Ma and 72.8 ± 1.7 Ma), Late Cretaceous hornblende-biotite granodiorite, and Paleogene leucocratic biotite granite. Neogene sedimentary and volcanic strata overlie basement along unconformities that are tilted to the northeast, southeast, or southwest. A brittle normal fault, dipping gently northeast, juxtaposes Tertiary andesite with Paleoproterozoic metasandstone. These relationships suggest that the area shares a common history of mid-Tertiary extension with southwestern Arizona. Later influence of the southern San Andreas fault system is implied by multiple dextral offsets of pre-Tertiary units across northwest-trending valleys. Our structural, geochronologic, and isotopic data provide new information to constrain pre–750 Ma Rodinia reconstructions involving southwestern Laurentia. Whole-rock U-Th-Pb and Rb-Sr isotopic systematics in both Paleoproterozoic gneiss complexes are disturbed, however, well-behaved Sm-Nd analyses preserve depleted initial ε Nd values (+2 to +4) that are distinct from the Mojave crustal province, but overlapping with the Yavapai and Mazatzal Provinces of Arizona. The Eastern Complex has the appropriate age and Nd isotopic signature to be part of the Mazatzal Province, but records major tectonism and metamorphism at ca. 1.6 Ga that postdates the Mazatzal orogeny. Deformed granitoids of the Western Complex have “Yavapai-type” ages and ε Nd but display structures discordant to the southwesterly Yavapai trend in central Arizona. The Western Complex lies along-strike with similar-age rocks (1.77 Ga to 1.69 Ga) of the “Caborca block” that have only been studied in detail near Quitovac and south of Caborca. Collectively, these rocks form a northwest-trending strip of basement situated at the truncated edge of Laurentia. The present-day basement geography may reflect an original oroclinal bend in the Yavapai orogenic belt. Alternatively, the western Proterozoic belt of Sonora may represent displaced fragments of basement juxtaposed against the Yavapai-Mazatzal Provinces along a younger sinistral transform fault (e.g., the Late Jurassic Mojave-Sonora megashear or the Permian Coahuila transform). Crustal blocks with these specific petrologic, geochronologic, and isotopic characteristics can be found in south-central and northeastern portions of the Australian Proterozoic basement, further supporting a connection between the two continents prior to breakup of the Rodinian supercontinent.

Arizona, California, Nevada, Sonora↗
USGS2005

Northward laramide thrusting in the quitovac region, northwestern sonora, mexico: Implications for the juxtaposition of paleoproterozoic basement blocks and the mojave-sonora megashear hypothesis

Restoration of 12%–30% Basin and Range extension allows direct interpretation of ductile fabrics associated with a stack of Laramide thrust faults in the Quitovac region in northwestern Sonora. The inferred direction of displacement of these thrusts varies gradually from N63°W to N23°E and is interpreted to represent a clockwise rotation of the direction of Laramide thrusting through time. The thrust faults represent a piggy-back sequence of thrusting propagating north, toward the foreland. The average direction and sense of displacement of the thrusts is N18°W, and the cumulative 45 km of estimated northward-directed displacement corresponds to ∼86% of shortening. Based on geochronological constraints, onset of thrusting in Quitovac occurred sometime between 75 and 61 Ma, whereas cessation occurred at ca. 39 Ma. The presence of Paleocene-Eocene orogenic gold mineralization, spatially associated with thrusting, strengthens our idea that compressional tectonism associated with the Laramide orogeny is a very important and widespread dynamometamorphic event in the region. Similarities in age, kinematics, and structural stratigraphy indicate that the thrusting in the Quitovac region may be equivalent to the Laramide Quitobaquito Thrust in southwestern Arizona. In both areas, thrust faults juxtapose the Paleoproterozoic Caborca and “North America” basement blocks. This juxtaposition was previously proposed as exclusively related to movements along the hypothetical Upper Jurassic Mojave-Sonora megashear. The Laramide northward displacements and clockwise rotations recorded in the Caborca block rocks in Quitovac contradict the southward displacements (∼800 km) and counterclockwise rotations inherent in the left-lateral Upper Jurassic Mojave-Sonora megashear hypothesis. We conclude that if this megashear exists in northwestern Sonora, its trace should be to the southwest of the Quitovac region.

Special Papers of the Geological Society of Americ↗
USGS2004

Reply to discussion on “Tectonic controls of Mississippi Valley-type lead–zinc mineralization in orogenic forelands”

Bradley and Leach (2003) focused on the tectonics of a subset of Mississippi Valley-type (MVT) lead–zinc deposits. Age assignments followed those given in a review of MVT deposits worldwide (Leach et al. 2001)—a paper that provoked a discussion by Kesler and Carrigan (2002) and reply by Leach et al. (2002), similar to the present exchange. While the new discussion by Kesler et al. (2004) does address tectonic issues, it also continues the old debate about the pros and cons of Rb–Sr isotopic and paleomagnetic dating of MVT deposits. We address these dating techniques first.

Mineralium Deposita↗