Geology topics

James V. Jones

Publications and source records attributed to James V. Jones.

32 records · Page 2Linked to original sources

USGS2020

Exploring regional scale metamorphic fabrics in the Yukon Tanana terrane and environs using quantitative domain analyses

Metamorphic rock fabrics such as foliations and lineations provide a rock record of numerous deformational characteristics in the Earth’s crust. When spatial information is combined with fabric data collected at points on geologic maps, the nature and consistency of metamorphic fabrics can be explored through structural domain analysis. This is particularly useful in large regions where there is not well-established stratigraphy and where bedrock exposures are limited. Domains that contain distinctive orientations and patterns of fabrics can be constructed on the basis of several different parameters, but in folded, polydeformational regions cylindricity can be particularly useful. Distinct domains of cylindrical folding can sometimes be determined where poles to foliations show characteristic patterns on equal-area projections and lie perpendicular to a single axis in space. Additionally, the patterns of elements such as fold axes and mineral lineations can be used in conjunction with foliation data to refine domains and confirm parameters such as coaxiality.

Alaska, Yukon↗

USGS2019

Detrital zircon geochronology along a structural transect across the Kahiltna assemblage in the western Alaska Range: Implications for emplacement of the Alexander-Wrangellia-Peninsular terrane against North America

The Kahiltna assemblage in the western Alaska Range consists of deformed Upper Jurassic and Cretaceous clastic strata that lie between the Alexander-Wrangellia-Peninsular (AWP) terrane to the south, and the Farewell and other peri-cratonic terranes to the north. Differences in detrital zircon populations and sandstone petrography allow geographic separation of the strata into two different successions, each consisting of multiple units, or petrofacies, with distinct provenance and lithologic characteristics. The northwestern succession was largely derived from older, inboard peri-cratonic terranes and correlates along strike to the southwest with the Kuskokwim Group. The southeastern succession is characterized by volcanic and plutonic rock detritus derived from Late Jurassic igneous rocks of the AWP terrane and mid to Late Cretaceous arc related igneous rocks and is part of a longer belt to the southwest and northeast, here named the Koksetna-Clearwater belt. The two successions remained separate depositional systems until the Late Cretaceous, when the northwestern succession overlapped the southeastern succession at about 81 Ma and they were deformed together by about 80 Ma by southeast-verging fold-and-thrust style deformation interpreted to represent final accretion of the AWP terrane along the southern Alaska margin. We interpret the tectonic evolution of the Kahiltna successions as a progression from forearc sedimentation and accretion in a south-facing continental magmatic arc to arrival and partial underthrusting of the backarc flank of an active, south-facing island arc system (AWP terrane). A modern analogue is the ongoing collision and partial underthrusting of the Izu-Bonin-Marianas island arc beneath the Japan Trench-Nankai Trough on the east side of central Japan.

Alaska↗

USGS2019

Links between tectonics, magmatism, and mineralization in the formation of Late Cretaceous porphyry systems in the Yukon-Tanana upland, eastern Alaska, USA

Cretaceous-Paleocene porphyry Cu(±Mo±Au) occurrences are scattered throughout the Yukon-Tanana upland in eastern Alaska. Known occurrences in eastern Alaska are poorly characterized, despite a resurgence in exploration. Porphyry deposits in the upland are emplaced into structurally complex metamorphic rocks representing a variety of tectonic environments, resulting in diverse alteration and mineralization assemblages. New mapping, drill core logging, petrography, geochemistry, geochronology, and structural analysis allow improved characterization of the parameters of porphyry systems and identify key linkages to regional tectonic and magmatic events. New sericite 40 Ar/ 39 Ar and zircon U/Pb dates constrain porphyry systems to the Late Cretaceous-earliest Paleocene (ca. 71-63 Ma). Zircon Hf-isotope ratios and Ce and Eu concentrations indicate that Late Cretaceous-Paleocene intrusions emplaced into basement dominated by Triassic and Jurassic plutons are more isotopically juvenile, reflecting more oxidized conditions. In contrast, those emplaced into basement dominated by mid-Cretaceous plutons are more reduced crustal geochemical-affinity. Diversity in mineral assemblages in contrasting systems may reflect emplacement into crustal domains of varying compositions and oxidation states. Those formed within a domain containing more-oxidized Triassic and Jurassic plutons are molybdenite-rich and apparently lack gold. In contrast, systems formed within domains dominated by more reduced mid-Cretaceous plutons contain lower-sulfidation state mineral assemblages with reported gold.

Alaska, Yukon↗

USGS2019

The US Geological Survey’s Earth Mapping Resources Initiative (Earth MRI)—Providing framework geologic, geophysical, and elevation data to the nation’s critical mineral-bearing regions

New detailed mapping of the geologic resources of the Nation has the potential to significantly close the gap in the essential data needed to fuel a modern era of economic development and technological innovation, while at the same time dramatically enhancing our understanding of the fundamental way geology impacts everyday life, from the domestic critical mineral resources that are necessary for modern technology and the economy, to domestic energy and water resources, geologic hazards, agriculture, and other pressing needs. The U.S. Geological Survey established the Earth Resources Mapping Initiative (Earth MRI) to address the shortfall in geologic, geophysical, and elevation data with sufficient detail to support evaluation of regions in the United States that have potential to host critical mineral resources. The new effort is a collaboration with the Association of American State Geologists, who are providing new detailed geologic maps and making available online archived data and information related to critical mineral resources. The geophysical and lidar surveys are being contracted through industry specialists to assure that high-quality data are available to the public. This article provides an overview of the Earth MRI effort with discussions on the initial geophysical surveys funded for areas that have known potential for rare earth element resources. Subsequent projects are being designed to address areas that may host other critical mineral resources.

Fast Times↗

USGS2018

The Mystic subterrane (partly) demystified: New data from the Farewell terrane and adjacent rocks, interior Alaska

The youngest part of the Farewell terrane in interior Alaska (USA) is the enigmatic Devonian–Cretaceous Mystic subterrane. New U-Pb detrital zircon, fossil, geochemical, neodymium isotopic, and petrographic data illuminate the origin of the rocks of this subterrane. The Devonian–Permian Sheep Creek Formation yielded youngest detrital zircons of Devonian age, major detrital zircon age probability peaks between ca. 460 and 405 Ma, and overall age spectra like those from the underlying Dillinger subterrane. Samples are sandstones rich in sedimentary lithic clasts, and differ from approximately coeval strata to the east that have abundant volcanic lithic clasts and late Paleozoic detrital zircons. The Permian Mount Dall conglomerate has mainly carbonate and chert clasts and yielded youngest detrital zircons of latest Pennsylvanian age. Permian quartz-carbonate sandstone in the northern Farewell terrane yielded abundant middle to late Permian detrital zircons. Late Triassic–Early Jurassic mafic igneous rocks occur in the central and eastern Mystic subterrane. New whole-rock geochemical and isotopic data indicate that magmas were rift related and derived from subcontinental mantle. Triassic and Jurassic strata have detrital zircon age spectra much like those of the Sheep Creek Formation, with major age populations between ca. 430 and 410 Ma. These rocks include conglomerate with clasts of carbonate ± chert and youngest detrital zircons of Late Triassic age and quartz-carbonate sandstone with youngest detrital zircons of Early Jurassic age. Lithofacies indicating highly productive oceanographic conditions (upwelling?) bracket the main part of the Mystic succession: Upper Devonian bedded barite and phosphatic Upper Devonian and Lower Jurassic rocks. The youngest part of the Mystic subterrane consists of Lower Cretaceous (Valanginian–Aptian) limestone, calcareous sandstone, and related strata. These rocks are partly coeval with the oldest parts of the Kahiltna assemblage, an overlap succession exposed along the southern margin of the Farewell terrane. Our findings support previous models suggesting that the Farewell terrane was proximal to the Alexander-Wrangellia-Peninsular composite terrane during the late Paleozoic, and further suggest that such proximity continued into (or recurred during) the Late Triassic–Early Jurassic. But middle to late Permian detrital zircons in northern Farewell require another source; the Yukon-Tanana terrane is one possibility.

Alaska↗

USGS2018

Neoproterozoic–early Paleozoic provenance evolution of sedimentary rocks in and adjacent to the Farewell terrane (interior Alaska)

New detrital zircon U-Pb data from the Farewell terrane of interior Alaska illuminate its early provenance evolution and connections with other Alaskan terranes. Five samples come from Neoproterozoic units in the central Farewell terrane. Basal “ferruginous beds” and the overlying Windy Fork Formation have prominent detrital zircon age populations between 2000 and 1800 Ma, with the Windy Fork Formation also having major age peaks between 700 and 600 Ma. Younger (Lone Formation) samples yield grains mainly between 750 and 550 Ma, with fewer older Proterozoic grains. Eleven samples come from deep-water early Paleozoic rocks (southeastern Farewell terrane). Ordovician sandstone (Post River Formation) has a major age population at ca. 490 Ma and subordinate 785–550 Ma populations that overlap age peaks in the Lone Formation. Turbidites in the overlying Terra Cotta Mountains Sandstone (Silurian) yield distinctly different spectra, with major ca. 450–420 Ma age populations and numerous grains between 2000 and 900 Ma. Devonian Barren Ridge Limestone samples have spectra like those of the Terra Cotta Mountains Sandstone, plus some Early Devonian grains. The Silurian shift in detrital zircon age spectra coincides with a major influx of siliciclastic sediment suggestive of a tectonic (collisional?) event involving the Farewell terrane. Neoproterozoic through Devonian successions in the Arctic Alaska–Chukotka and Alexander terranes show a similar up-section shift in detrital zircon spectra, supporting links between these terranes and the Farewell terrane during the early Paleozoic. Detrital zircon ages from the White Mountains and Livengood terranes, adjacent to the northern Farewell terrane, include major early Paleozoic populations that overlap those seen in partly coeval Farewell strata.

Alaska↗

USGS2017

Zirconium and hafnium

Zirconium and hafnium are corrosion-resistant metals that are widely used in the chemical and nuclear industries. Most zirconium is consumed in the form of the main ore mineral zircon (ZrSiO 4 , or as zirconium oxide or other zirconium chemicals. Zirconium and hafnium are both refractory lithophile elements that have nearly identical charge, ionic radii, and ionic potentials. As a result, their geochemical behavior is generally similar. Both elements are classified as incompatible because they have physical and crystallochemical properties that exclude them from the crystal lattices of most rock-forming minerals. Zircon and another, less common, ore mineral, baddeleyite (ZrO 2 ), form primarily as accessory minerals in igneous rocks. The presence and abundance of these ore minerals in igneous rocks are largely controlled by the element concentrations in the magma source and by the processes of melt generation and evolution. The world’s largest primary deposits of zirconium and hafnium are associated with alkaline igneous rocks, and, in one locality on the Kola Peninsula of Murmanskaya Oblast, Russia, baddeleyite is recovered as a byproduct of apatite and magnetite mining. Otherwise, there are few primary igneous deposits of zirconium- and hafnium-bearing minerals with economic value at present. The main ore deposits worldwide are heavy-mineral sands produced by the weathering and erosion of preexisting rocks and the concentration of zircon and other economically important heavy minerals, such as ilmenite and rutile (for titanium), chromite (for chromium), and monazite (for rare-earth elements) in sedimentary systems, particularly in coastal environments. In coastal deposits, heavy-mineral enrichment occurs where sediment is repeatedly reworked by wind, waves, currents, and tidal processes. The resulting heavy-mineral-sand deposits, called placers or paleoplacers, preferentially form at relatively low latitudes on passive continental margins and supply 100 percent of the world’s zircon. Zircon makes up a relatively small percentage of the economic heavy minerals in most deposits and is produced primarily as a byproduct of heavy-mineral-sand mining for titanium minerals. From 2003 to 2012, world zirconium mineral concentrates production increased by more than 40 percent, and Australia and South Africa were the leading producers. Global consumption of zirconium mineral concentrates generally increased during the same time period, largely as a result of increased demand in developing economies in Asia and the Middle East. Global demand weakened in 2012, causing a decrease in world production of zirconium mineral concentrates and delaying the development of several new mining projects. Global consumption is expected to increase in the future, however, as demand from the ceramics, chemicals, and metals industries increases (driven by renewed growth in developing economies) and demand for zirconium and hafnium metal increases (driven by the construction and operation of new nuclear powerplants). The behaviors of zirconium and hafnium in the environment are very similar to one another in that most zirconium- and hafnium-bearing minerals have limited solubility and reactivity. Anthropogenic sources of zirconium, and likely hafnium, are from industrial zirconium-containing byproducts and emissions from the processing of sponge zirconium, and exposure to the general population from these sources is small. Zirconium and hafnium are likely not essential to human health and generally are considered to be of low toxicity to humans. The main exposure risks are associated with industrial inhalation and dermal exposure. Because of the low solubility of zirconium and hafnium, ecological health concerns in the aquatic environment and in soils are minimal. Heavy-mineral-sand mining may lead to increased erosion rates when the mining is managed improperly. In addition, surface mining requires removal of the overlying organic soil layer and produces waste material that includes tailings and slimes. The soil removal and mining activity disturbs the surrounding ecosystem and alters the character of the landscape. Dry mineral separation processes create high amounts of airborne dust, whereas wet mineral separation processes do not. In operations that restore the landscape to pre-mining conditions, the volume of waste and the impact on the landscape may be relatively temporary.

Professional Paper↗

USGS2017

Detrital zircon geochronology of quartzose metasedimentary rocks from parautochthonous North America, east-central Alaska

We report eight new U-Pb detrital zircon ages for quartzose metasedimentary rocks from four lithotectonic units of parautochthonous North America in east-central Alaska: the Healy schist, Keevy Peak Formation, and Sheep Creek Member of the Totatlanika Schist in the northern Alaska Range, and the Butte assemblage in the northwestern Yukon-Tanana Upland. Excepting 1 of 3 samples from the Healy schist, all have dominant detrital zircon populations of 1.9–1.8 Ga and a subordinate population of 2.7–2.6 Ga. Three zircons from Totatlanika Schist yield the youngest age of ca. 780 Ma. The anomalous Healy schist sample has abundant 1.6–0.9 Ga detrital zircon, as well as populations at 2.0–1.8 Ga and 2.7–2.5 Ga that overlap the ages from the rest of our samples; it has a minimum age population of ca. 1007 Ma. Detrital zircon age populations from all but the anomalous sample are statistically similar to those from (1) other peri-Laurentian units in east-central Alaska; (2) the Snowcap assemblage in Yukon, basement of the allochthonous Yukon-Tanana terrane; (3) Neoproterozoic to Ordovician Laurentian passive margin strata in southern British Columbia, Canada; and (4) Proterozoic Laurentian Sequence C strata of northwestern Canada. Recycling of zircon from the Paleoproterozoic Great Bear magmatic zone in the Wopmay orogen and its Archean precursors could explain both the Precambrian zircon populations and arc trace element signatures of our samples. Zircon from the anomalous Healy schist sample resembles that in Nation River Formation and Adams Argillite in eastern Alaska, suggesting recycling of detritus in those units.

Alaska, British Columbia, Yukon Territory↗

USGS2017

Regional patterns of Mesozoic-Cenozoic magmatism in western Alaska revealed by new U-Pb and 40Ar/39Ar ages

In support of regional geologic framework studies, we obtained 50 new argon-40/argon-39 ( 40 Ar/ 39 Ar) ages and 33 new uranium-lead (U-Pb) ages from igneous rocks of southwestern Alaska. Most of the samples are from the Sleetmute and Taylor Mountains quadrangles; smaller collections or individual samples are from the Bethel, Candle, Dillingham, Goodnews Bay, Holy Cross, Iditarod, Kantishna River, Lake Clark, Lime Hills, McGrath, Medfra, Talkeetna, and Tanana quadrangles. A U-Pb zircon age of 317.7±0.6 million years (Ma) reveals the presence of Pennsylvanian intermediate igneous (probably volcanic) rocks in the Tikchik terrane, Bethel quadrangle. A U-Pb zircon age of 229.5±0.2 Ma from gabbro intruding the Rampart Group of the Angayucham-Tozitna terrane, Tanana quadrangle, confirms and tightens a previously cited Triassic age for this intrusive suite. A fresh mafic dike in Goodnews Bay quadrangle yielded a 40 Ar/ 39 Ar whole rock age of 155.0±1.9 Ma; this establishes a Jurassic or older age for the previously unconstrained (Paleozoic? to Mesozoic?) sandstone unit that it intrudes. A thick felsic tuff in the Gemuk Group in Taylor Mountains quadrangle yielded a U-Pb zircon age of 153.0±2.0 Ma, extending the age of magmatism in this part of the Togiak terrane back into the Late Jurassic. We report three new U-Pb zircon ages between 120 and 110 Ma—112.0±0.9 Ma from syenite in the Candle quadrangle, 114.9±0.3 Ma from orthogneiss assigned to the Ruby terrane in Iditarod quadrangle, and 116.6±0.1 Ma from a gabbro of the Dishna River mafic-ultramafic complex in Iditarod quadrangle. The latter result requires a substantial age revision, from Triassic to Cretaceous, for at least some rocks that have been mapped as the Dishna River mafic-ultramafic complex. A tuff in the Upper Cretaceous Kuskokwim Group yielded a U-Pb zircon (sensitive high-resolution ion microprobe, SHRIMP) age of 88.3±1.0 Ma; we speculate that the eruptive source was an arc along the trend of the Pebble porphyry copper deposit along the Gulf of Alaska continental margin. More than half of the new ages fall between 75 and 65 Ma, confirming the existence, based on conventional potassium-argon (K-Ar) ages, of a 70-Ma igneous flare-up across southwestern Alaska. Our new ages hint that during this pulse, the locus of magmatism shifted toward the Gulf of Alaska, that is, toward a more outboard position. This shift is consistent with the hypothesis that magmatism was the product of rollback of a subducted slab, which at that time would have been the Resurrection Plate. Intrusive rocks in the Taylor Mountains and Sleetmute quadrangles in the age range of 63 to 59 Ma were emplaced shortly before the onset of ridge subduction as dated by near-trench plutons in the adjacent part of the Chugach accretionary complex. Southwestern Alaska at this time would have been positioned above a very young subducted slab belonging to the Resurrection Plate; magmas, in this scenario, were generated near the edge of the slab window related to ridge subduction. A 56.3±0.2 Ma granite in Taylor Mountains quadrangle and a 54.7±0.7 Ma ashfall tuff in McGrath quadrangle were likely emplaced above the Resurrection-Kula slab window, which by this time is inferred to have entered the region. Another ashfall tuff in McGrath quadrangle, at 42.8±0.5 Ma, likely belongs to the Meshik Arc, the product of renewed subduction after inferred passage of the slab window. A 49.0±0.3-Ma rhyolite in Taylor Mountains quadrangle is about the age of the transition from slab window to renewed subduction. Two plutons in the western Alaska Range, at 31.8±0.4 and 30.9±0.6 Ma, belong to a suite of gabbro to peralkaline granite of unknown origin. Finally, a 4.6±0.1-Ma basalt from a flow in Taylor Mountains quadrangle belongs to the Neogene basaltic province of western Alaska. These rocks were erupted in a distal retroarc setting; the cause of magmatism is unknown.

Alaska↗

USGS2016

Late Oligocene to present contractional structure in and around the Susitna basin, Alaska—Geophysical evidence and geological implications

The Cenozoic Susitna basin lies within an enigmatic lowland surrounded by the Central Alaska Range, Western Alaska Range (including the Tordrillo Mountains), and Talkeetna Mountains in south-central Alaska. Some previous interpretations show normal faults as the defining structures of the basin (e.g., Kirschner, 1994). However, analysis of new and existing geophysical data shows predominantly (Late Oligocene to present) thrust and reverse fault geometries in the region, as previously proposed by Hackett (1978). A key example is the Beluga Mountain fault where a 50-mGal gravity gradient, caused by the density transition from the igneous bedrock of Beluga Mountain to the >4-km-thick Cenozoic sedimentary section of Susitna basin, spans a horizontal distance of ∼40 km and straddles the topographic front. The location and shape of the gravity gradient preclude a normal fault geometry; instead, it is best explained by a southwest-dipping thrust fault, with its leading edge located several kilometers to the northeast of the mountain front, concealed beneath the shallow glacial and fluvial cover deposits. Similar contractional fault relationships are observed for other basin-bounding and regional faults as well. Contractional structures are consistent with a regional shortening strain field inferred from differential offsets on the Denali and Castle Mountain right-lateral strike-slip fault systems.

Alaska↗

USGS2015

Tectonic and sedimentary linkages between the Belt-Purcell basin and southwestern Laurentia during the Mesoproterozoic ca. 1.60-1.40 Ga

Mesoproterozoic sedimentary basins in western North America provide key constraints on pre-Rodinia craton positions and interactions along the western rifted margin of Laurentia. One such basin, the Belt-Purcell basin, extends from southern Idaho into southern British Columbia and contains a >18-km-thick succession of siliciclastic sediment deposited ca. 1.47–1.40 Ga. The ca. 1.47–1.45 Ga lower part of the succession contains abundant distinctive non-Laurentian 1.61–1.50 Ga detrital zircon populations derived from exotic cratonic sources. Contemporaneous metasedimentary successions in the southwestern United States–the Trampas and Yankee Joe basins in Arizona and New Mexico–also contain abundant 1.61–1.50 Ga detrital zircons. Similarities in depositional age and distinctive non-Laurentian detrital zircon populations suggest that both the Belt-Purcell and southwestern successions record sedimentary and tectonic linkages between western Laurentia and one or more cratons including North Australia, South Australia, and (or) East Antarctica. At ca. 1.45 Ga, both the Belt-Purcell and southwest successions underwent major sedimentological changes, with a pronounced shift to Laurentian provenance and the disappearance of the 1.61–1.50 Ga detrital zircon. Upper Belt-Purcell strata contain strongly unimodal ca. 1.73 Ga detrital zircon age populations that match the detrital zircon signature of Paleoproterozoic metasedimentary rocks of the Yavapai province to the south and southeast. We propose that the shift at ca. 1.45 Ga records the onset of orogenesis in southern Laurentia coeval with rifting along its northwestern margin. Bedrock uplift associated with orogenesis and widespread, coeval magmatism caused extensive exhumation and erosion of the Yavapai province ca. 1.45–1.36 Ga, providing a voluminous and areally extensive sediment source–with suitable zircon ages–during upper Belt deposition. This model provides a comprehensive and integrated view of the Mesoproterozoic tectonic evolution of western Laurentia and its position within the supercontinent Columbia as it evolved into Rodinia.

Lithosphere↗

USGS2015

GIS-Based Identification of Areas with Mineral Resource Potential for Six Selected Deposit Groups, Bureau of Land Management Central Yukon Planning Area, Alaska

This study, covering the Bureau of Land Management (BLM) Central Yukon Planning Area (CYPA), Alaska, was prepared to aid BLM mineral resource management planning. Estimated mineral resource potential and certainty are mapped for six selected mineral deposit groups: (1) rare earth element (REE) deposits associated with peralkaline to carbonatitic intrusive igneous rocks, (2) placer and paleoplacer gold, (3) platinum group element (PGE) deposits associated with mafic and ultramafic intrusive igneous rocks, (4) carbonate-hosted copper deposits, (5) sandstone uranium deposits, and (6) tin-tungsten-molybdenum-fluorspar deposits associated with specialized granites. These six deposit groups include most of the strategic and critical elements of greatest interest in current exploration. This study has used a data-driven, geographic information system (GIS)-based method for evaluating the mineral resource potential across the large region of the CYPA. This method systematically and simultaneously analyzes geoscience data from multiple geospatially referenced datasets and uses individual subwatersheds (12-digit hydrologic unit codes or HUCs) as the spatial unit of classification. The final map output indicates an estimated potential (high, medium, low) for a given mineral deposit group and indicates the certainty (high, medium, low) of that estimate for any given subwatershed (HUC). Accompanying tables describe the data layers used in each analysis, the values assigned for specific analysis parameters, and the relative weighting of each data layer that contributes to the estimated potential and certainty determinations. Core datasets used include the U.S. Geological Survey (USGS) Alaska Geochemical Database (AGDB2), the Alaska Division of Geologic and Geophysical Surveys Web-based geochemical database, data from an anticipated USGS geologic map of Alaska, and the USGS Alaska Resource Data File. Map plates accompanying this report illustrate the mineral prospectivity for the six deposit groups across the CYPA and estimates of mineral resource potential. There are numerous areas, some of them large, rated with high potential for one or more of the selected deposit groups within the CYPA.

Alaska↗

USGS2015

Synthesis of geological, structural, and geochronologic data (phase V, deliverable 53)

This report is a companion to the new Geologic Map of Mauritania (Bradley and others, 2015; referred to herein as “Deliverable 51”) and the new Structural Geologic Map of Mauritania (Bradley and others, 2015a; referred to herein as “Deliverable 52”). Section 1 contains explanatory information for these two digital maps. Section 2 covers the analytical methods used in obtaining new U-Pb ages from 9 igneous rock samples, new detrital zircon ages from 40 sedimentary or metasedimentary rock samples, and new 40 Ar/ 39 Ar ages from 12 samples of metamorphic rocks and veins. Sections 3 through 6 present the new geochronological results, organized by region. In Section 7, we discuss implications of the new ages for the regional geology and discuss problematic results. Finally, in Section 8, we summarize the geology and tectonic evolution of Mauritania in narrative form, drawing on new and published information, in the context of global tectonics. The report is being released in both English and French. In both versions, we use the French-language names for formal stratigraphic units.

Open-File Report↗

USGS2013

Making the case for the Picuris orogeny: Evidence for a 1500 to 1400 Ma orogenic event in the southwestern United States

The early Mesoproterozoic (ca. 1400 Ma) is an enigmatic time in the tectonic evolution of southern Laurentia. Circa 1400 Ma granites within Laurentia and multiple other continents have distinctive geochemistry consistent with crustal extension or mantle upwelling. In the southwestern United States, these granites are commonly foliated and are often spatially associated with km-scale ductile shear zones. Deformation is attributed to intracontinental tectonism driven by active convergence along the distal southern margin of Laurentia. The recent discovery of deformed and metamorphosed, ca. 1450 Ma sedimentary rocks in northern New Mexico has strengthened the case for regional deformation and orogenesis. However, important questions remain about the tectonic significance of these events and how to reconcile tectonic models with granite petrology at the regional to global scale. This trip focuses on the protolith age of Proterozoic metasedimentary rocks and the kinematics, timing, and tectonic significance of deformation, magmatism, and metamorphism for the Mesoproterozoic across different crustal levels in the southern Rocky Mountains to highlight the ongoing questions and controversies regarding the Mesoproterozoic tectonic setting of Laurentia. This field trip will examine some of the diverse and most recently discovered evidence for ca. 1400 Ma orogenesis in the southern Rocky Mountains. We hope this trip will promote new interest and discussion about the Mesoproterozoic tectonic evolution of Laurentia. We will visit multiple outcrops in the Wet Mountains of southern Colorado and the Picuris Mountains of northern New Mexico. Stops in the Wet Mountains are arranged from north to south to examine contrasting styles of ca. 1400 Ma deformation with increasing paleodepth across the tilted Proterozoic crustal section. In the Picuris Mountains, we focus on detrital zircon geochronology and revisions to the lithostratigraphy of Paleoproterozoic and recently documented Mesoproterozoic metasedimentary rocks, the nature of regional metamorphism, and the style of deformation, ca. 1450–1400 Ma.

Geological Society of America Field Guides↗