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USGS2010

The Timan-Pechora oil-gas basin: A geological review

At the base of the sedimentary section of the Timan-Pechora basin are Late Precambrian rifts, which are filled by unmetamorphosed sedimentary rocks 10 km and more than 10 km thick. The upper parts of these rift deposits have potential for gas. Structurally the Phanerozoic sedimen tary fill of the basin is mildly deformed in the west and central parts into several broad depressions and more narrow intervening highs. The eastern part is strongly deformed by overthrusts that were emplaced in the Late Paleozoic in connection with development of the Ural fold belt. Eight oil-gas plays are recognized in the basin: 1—Ordovician-Lower Devonian, 2—Middle Devonian-lower Frasnian, 3—upper Frasnian-Tournaisian, 4—Lower Carboniferous, 5—upper Visean-Lower Permian, 6—Lower Permian, 7—Upper Permian, and 8—Triassic. The promising oil-gas basin is of great interest for global stratigraphy and tectonics as well as for petroleum exploration.

International Geology Review↗

USGS2010

Advances in the hydrogeochemistry and microbiology of acid mine waters

The last decade has witnessed a plethora of research related to the hydrogeochemistry and microbiology of acid mine waters and associated tailings and waste-rock waters. Numerous books, reviews, technical papers, and proceedings have been published that examine the complex bio-geochemical process of sulfide mineral oxidation, develop and apply geochemical models to site characterization, and characterize the microbial ecology of these environments. This review summarizes many of these recent works, and provides references for those investigating this field. Comparisons of measured versus calculated Eh and measured versus calculated pH for water samples from several field sites demonstrate the reliability of some current geochemical models for aqueous speciation and mass balances. Geochemical models are not, however, used to predict accurately time-dependent processes but to improve our understanding of these systems and to constrain possible processes that contribute to actual or potential water quality issues. Microbiological studies are demonstrating that there is much we have yet to learn about the types of different microorganisms and their function and ecology in mine-waste environments. A broad diversity of green algae, bacteria, archaea, yeasts, and fungi are encountered in acid mine waters, and a better understanding of their ecology and function may potentially enhance remediation possibilities as well as our understanding of the evolution of life.

International Geology Review↗

USGS2010

Comparative deformation behavior of minerals in serpentinized ultramafic rock: Application to the slab-mantle interface in subduction zones

The layer-structure minerals serpentine, brucite, and talc are postulated to form in the mantle wedge above a subducting slab as a result of progressive hydration and silica metasomatism. Tectonic mixing at the slab-mantle interface generates serpentinite mélanges that contain blocks of high-pressure (HP) or ultrahigh-pressure (UHP) metamorphic rock derived from the subducting slab. Such serpentinite mélanges may provide a means of exhumation of HP/UHP metamorphic rocks, and may define the lower limit of locked regions on the subduction interface that fail in large earthquakes. We review recently obtained frictional strength data for brucite and talc over the temperature range 25-400°C at 100 MPa effective normal stress and compare them with new data for antigorite. These minerals respond to heating in different ways, causing their frictional strengths to diverge. Water-saturated antigorite strength increases toward the fixed dry value of μ ≈ 0.75-0.80 with heating: μ ≈ 0.50 at 25°C and μ > 0.60 at 400°C. The difference in μ between dry and watersaturated talc gouge also decreases with increasing temperature, but both the dry and watersaturated values of μ are lower at elevated temperatures. For dry talc, μ decreases from 0.35 to 0.25 between 25° and 300°C, whereas for water-saturated talc, μ is approximately 0.20 at 25°C and 0.10-0.15 at elevated temperatures. Weakening of the interlayer bond of talc with heating may be responsible for the overall reduction in its frictional strength. The strength of dry brucite also is fixed at μ = 0.45-0.50, but the water-saturated value of μ decreases from ≈0.30 at 25°C to 0.20-0.25 at 200°-400°C. The water-saturated brucite gouge has extensively recrystallized along the shear surfaces, and its weakening may be attributable to solution-transfer processes.

International Geology Review↗

USGS2025

Did the Aleutian Basin form by plate capture or backarc basin opening?

The origin of the Aleutian Basin is unresolved because its crust is deeply buried beneath sediments. It has been interpreted as forming in the Eocene when the Beringian convergent margin jumped seaward to south of the Aleutian arc, thereby capturing a large sector of Cretaceous Pacific crust. Alternatively, it may have formed by backarc spreading. We present new magnetic and seismic reflection data compilations and review other pertinent data to evaluate these two possibilities. Arguments for entrapment are: 1) Palaeomagnetic and geologic data document that the Aleutian arc formed approximately in place and as a seaward, on-strike continuation of the Alaska Peninsula; 2) basin-central spreading anomalies trend N-S, normal to the Aleutian arc and exhibit amplitudes and lengths typically formed at mid-ocean ridges; 3) seismic reflection profiles document that cross-basin depositional sequences are like those expected of filling a fixed-width basin. Arguments for a backarc spreading origin include: 1) The perpendicular orientation of magnetic anomalies in the Aleutian Basin and the northern Pacific Plate suggest different origins; 2) the sub-parallel orientation of Aleutian Basin spreading magnetic fabric and Palaeogene rift basins on the Bering Shelf suggest a common extensional regime; 3) thinner continental crust beneath outer-shelf basins is consistent with extension; 4) lineated magnetic fabrics on the margins of the basin are similar to early rifting magnetic fabrics found in other backarc basins; 5) basin heat flow is consistent with Palaeogene seafloor spreading; 6) its marginal basin setting is analogous to marginal basins elsewhere that formed by extension and seafloor spreading, not entrapment; and 7) entrapment requires formation of the Aleutian subduction zone by transference, a mode of subduction initiation that has not yet been documented. We present evidence for both interpretations and underscore the need for continued data collection (e.g. scientific drilling) to test competing hypotheses. Similar approaches may be useful for understanding thickly sedimented marginal basins elsewhere.

Alaska↗

USGS2010

Application of organic geochemistry to coastal tar residues from central California

Tar residues are common on the coastline of the Monterey Bay National Marine Sanctuary. These coastal tar residues have been washed ashore and usually occur on headlands near the high-tide line. In this study, 18 coastal tar residues were collected and analyzed to determine their carbon isotopic compositions and the values of selected biomarker ratios. All of the residues have very heavy ( 13 C-enriched) carbon isotopic compositions spanning a narrow range (δ 13 C = −22.2 to −23.4‰), and 28,30-bisnorhopane is present in all samples. These same geochemical characteristics are found in Monterey Formation oils from which the coastal tar residues were likely derived. These coastal residues could result from natural seeps or from accidental spills. Statistically the coastal tar residues can be organized into three groups, each of which may represent different spill or seep events. Seven samples of potential local representative sources for the tar residues were examined, but none could account for the coastal tars.

California↗

USGS2010

Legacy of the California Gold Rush: Environmental geochemistry of arsenic in the southern Mother Lode Gold District

Gold mining activity in the Sierra Nevada foothills, both recently and during the California Gold Rush, has exposed arsenic-rich pyritic rocks to weathering and erosion. This study describes arsenic concentration and speciation in three hydrogeologic settings in the southern Mother Lode Gold District: mineralized outcrops and mine waste rock (overburden); mill tailings submerged in a water reservoir; and lake waters in this monomictic reservoir and in a monomictic lake developing within a recent open-pit mine. These environments are characterized by distinct modes of rock-water interaction that influence the local transport and fate of arsenic. Arsenic in outcrops and waste rock occurs in arsenian pyrite containing an average of 2 wt% arsenic. Arsenic is concentrated up to 1300 ppm in fine-grained, friable, iron-rich weathering products of the arsenian pyrite (goethite, jarosite, copiapite), which develop as efflorescences and crusts on weathering outcrops. Arsenic is sorbed as a bidentate complex on goethite, and substitutes for sulfate in jarosite. Submerged mill tailings obtained by gravity core at Don Pedro Reservoir contain arsenic up to 300 ppm in coarse sand layers. Overlying surface muds have less arsenic in the solid fraction but higher concentrations in porewaters (up to 500 μg/L) than the sands. Fine quartz tailings also contain up to 3.5 ppm mercury related to the ore processing. The pH values in sediment porewaters range from 3.7 in buried gypsum-bearing sands and tailings to 7 in the overlying lake sediments. Reservoir waters immediately above the cores contain up to 3.5 μg/L arsenic; lake waters away from the submerged tailings typically contain less than 1 μg/L arsenic. Dewatering during excavation of the Harvard open-pit mine produced a hydrologic cone of depression that has been recovering toward the pre-mining groundwater configuration since mining ended in 1994. Aqueous arsenic concentrations in the 80 m deep pit lake are up to 1000 μg/L. Redistribution of the arsenic occurs during summer stratification, with highest concentrations at middle depths. The total mass of arsenic in the pit lake increases coinciding with early winter rains that erode, partially dissolve, and transport arsenic-bearing salts into the pit lake. Arsenic concentration, speciation, and distribution in the Sierra Nevada foothills depend on many factors, including the lithologic sources of arsenic, climatic influences on weathering of host minerals, and geochemical characteristics of waters with which source and secondary minerals react. Oxidation of arsenian pyrite to goethite, jarosite, and copiapite causes temporary attenuation of arsenic during summer, when these secondary minerals accumulate; subsequent rapid dissemination of arsenic into the aqueous environment is caused by annual winter storms. As the population of the Mother Lode area grows, it is increasingly important to consider these effects during planning and development of land and groundwater resources.

California↗

USGS2010

A portfolio approach to evaluating natural hazard mitigation policies: An application to lateral-spread ground failure in coastal California

In the past, efforts to prevent catastrophic losses from natural hazards have largely been undertaken by individual property owners based on site—specific evaluations of risks to particular buildings. Public efforts to assess community vulnerability and encourage mitigation have focused on either aggregating site—specific estimates or adopting standards based upon broad assumptions about regional risks. This paper develops an alternative, intermediate—scale approach to regional risk assessment and the evaluation of community mitigation policies. Properties are grouped into types with similar land uses and levels of hazard, and hypothetical community mitigation strategies for protecting these properties are modeled like investment portfolios. The portfolios consist of investments in mitigation against the risk to a community posed by a specific natural hazard. and are defined by a community's mitigation budget and the proportion of the budget invested in locations of each type. The usefulness of this approach is demonstrated through an integrated assessment of earthquake—induced lateral—spread ground failure risk in the Watsonville, California area. Data from the magnitude 6.9 Loma Prieta earthquake of 1989 are used to model lateral—spread ground failure susceptibility. Earth science and economic data are combined and analyzed in a Geographic Information System (CIS). The portfolio model is then used to evaluate the benefits of mitigating the risk in different locations. Two mitigation policies, one that prioritizes mitigation by land use type and the other by hazard zone, are compared with a status quo policy of doing no further mitigation beyond that which already exists. The portfolio representing the hazard zone rule yields a higher expected return than the land use portfolio does; however, the hazard zone portfolio experiences a higher standard deviation. Therefore, neither portfolio is clearly preferred. The two mitigation policies both reduce expected losses and increase overall expected community wealth compared to the status quo policy.

California↗

USGS2009

Age, geochemical composition, and distribution of Oligocene ignimbrites in the northern Sierra Nevada, California: Implications for landscape morphology, elevation, and drainage divide geography of the Nevadaplano

To gain a better understanding of the topographic and landscape evolution of the Cenozoic Sierra Nevada and Basin and Range, we combine geochemical and isotopic age correlations with palaeoaltimetry data from widely distributed ignimbrites in the northern Sierra Nevada, California. A sequence of Oligocene rhyolitic ignimbrites is preserved across the modern crest of the range and into the western foothills. Using trace and rare earth element geochemical analyses of volcanic glass, these deposits have been correlated to ignimbrites described and isotopically dated in the Walker Lane fault zone and in central Nevada (Henry et al. , 2004, Geologic map of the Dogskin mountain quadrangle; Washoe County, Nevada; Faulds et al. , 2005, Geology, v. 33, p. 505–508). Ignimbrite deposits were sampled within the northern Sierra Nevada and western Nevada, and four distinct geochemical compositions were identified. The majority of samples from within the northern Sierra Nevada have compositions similar to the tuffs of Axehandle Canyon or Rattlesnake Canyon, both likely sourced from the same caldera complex in either the Clan Alpine Mountains or the Stillwater Range, or to the tuff of Campbell Creek, sourced from the Desatoya Mountains caldera. New 40 Ar/ 39 Ar age determinations from these samples of 31.2, 30.9, and 28.7 Ma, respectively, support these correlations. Based on an Oligocene palinspastic reconstruction of the region, our results show that ignimbrites travelled over 200 km from their source calderas across what is now the crest of the Sierra Nevada, and that during that time, no drainage divide existed between the ignimbrite source calderas in central Nevada and sample locations 200 km to the west. Palaeoaltimetry data from Sierra Nevada ignimbrites, based on the hydrogen isotopic composition of hydration water in glass, reflect the effect of a steep western slope on precipitation and indicate that the area had elevations similar to the present-day range. These combined results suggest that source calderas were likely located in a region of high elevation to the east of the Oligocene Sierra Nevada, which had a steep western slope that allowed for the large extent and broad distribution of the ignimbrites.

International Geology Review↗

USGS2009

Provenance and palaeogeographic implications of Eocene-Oligocene sedimentary rocks in the northwestern Basin and Range

A thick sequence of uppermost Eocene to lower Oligocene volcaniclastic and sedimentary rocks is exposed at the base of the Warner Range in northeastern California. This isolated exposure provides insight into the palaeogeographic setting of the northwestern Basin and Range during this time period. Significant thinning of the unit over 35km of lateral exposure and predominantly volcanic clast compositions suggest that the sequence was deposited in an alluvial plain adjacent to a volcanic arc. Palaeocurrent indicators in the conglomerates define a NNE transport direction. Detrital zircon analysis on coarse sandstones and dating of individual granite cobbles show a range of ages consistent with a local, volcanic source area primarily from the SSW with some far-travelled input from northern Nevada; the far-travelled component increases in influence as the unit thins to the north. Comparison with other sedimentary sequences of Eocene age and integration with palaeofloral and geophysical data help to define drainage divides, and suggest that this sequence accumulated in a relatively isolated, intra-arc basin. This localized accumulation differs markedly from contemporaneous drainages to the south that transported material westwards from central Nevada to the palaeoshoreline, and suggests that ongoing volcanism had a strong influence on palaeogeography in this region during the Eocene and Oligocene.

International Geology Review↗

USGS2009

Can footwall unloading explain late Cenozoic uplift of the Sierra Nevada crest?

Globally, normal-fault displacement bends and warps rift flanks upwards, as adjoining basins drop downwards. Perhaps the most evident manifestations are the flanks of the East African Rift, which cuts across the otherwise minimally deformed continent. Flank uplift was explained by Vening Meinesz (1950, Institut Royal Colonial Belge, Bulletin des Seances, v. 21, p. 539-552), who recognized that isostasy should cause uplift of a normal-faulted footwall and subsidence of its hanging wall. Uplift occurs because slip on a dipping normal fault creates a broader root of less-dense material beneath the footwall, and a narrowed one beneath the hanging wall. In this paper, we investigate the potential influence of this process on the latest stages of Sierra Nevada uplift. Through theoretical calculations and 3D finite element modelling, we find that cumulative slip of about 4km on range-front faults would have produced about 1.3km peak isostatic uplift at the ridge crest. Numerical models suggest that the zone of uplift is narrow, with the width controlled by bending resistance of the seismogenic crust. We conclude that footwall unloading cannot account for the entire elevation of the Sierran crest above sea level, but if range-front faulting initiated in an already elevated plateau like the adjacent Basin and Range Province, then a hybrid model of pre-existing regional uplift and localized footwall unloading can account for the older and newer uplift phases suggested by the geologic record.

International Geology Review↗

USGS2010

Pre-eruptive storage conditions of the Holocene dacite erupted from Kizimen Volcano, Kamchatka

This study describes an investigation of the pre-eruptive conditions (T, P and fO 2 ) of dacite magma erupted during the KZI cycle (12,000-8400 years ago) of Kizimen Volcano, Kamchatka, the earliest, most voluminous and most explosive eruption cycle in the Kizimen record. Hydrothermal, water-saturated experiments on KZI dacite pumice coupled with titanomagnetite-ilmenite geothermometry calculations require that the KZI dacite existed at a temperature of 823 ?? 20??C and pressures of 125-150 MPa immediately prior to eruption. This estimate corresponds to a lithologic contact between Miocene volcaniclastic rocks and Pliocene-Pleistocene volcanic rocks located at a depth of 5-6 km beneath the Kizimen edifice, which may have facilitated the accumulation of atypically large volumes of gas-rich dacite during the KZI cycle.

International Geology Review↗

USGS2014

Mantle peridotite in newly discovered far-inland subduction complex, southwest Arizona: Initial report

The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generally considered a low-angle subduction complex that underlies much of southern California and Arizona. A recently discovered exposure of Orocopia Schist at Cemetery Ridge west of Phoenix, Arizona, lies exceptionally far inland from the continental margin. Unexpectedly, this body of Orocopia Schist contains numerous blocks, as large as ~300 m, of variably serpentinized mantle peridotite. These are unique; elsewhere in the Orocopia and related schists, peridotite is rare and completely serpentinized. Peridotite and metaperidotite at Cemetery Ridge are of three principal types: (1) serpentinite and tremolite serpentinite, derived from dunite; (2) partially serpentinized harzburgite and olivine orthopyroxenite (collectively, harzburgite); and (3) granoblastic or schistose metasomatic rocks, derived from serpentinite, made largely of actinolite, calcic plagioclase, hercynite, and chlorite. In the serpentinite, paucity of relict olivine, relatively abundant magnetite (5%), and elevated Fe 3 + /Fe indicate advanced serpentinization. Harzburgite contains abundant orthopyroxene, only slightly serpentinized, and minor to moderate (1–15%) relict olivine. Mantle tectonite fabric is locally preserved. Several petrographic and geochemical characteristics of the peridotite at Cemetery Ridge are ambiguously similar to either abyssal or mantle-wedge (suprasubduction) peridotites and serpentinites. Least ambiguous are orthopyroxene compositions. Orthopyroxene is distinctively depleted in Al 2 O 3 , Cr 2 O 3 , and CaO, indicating mantle-wedge affinities. Initial interpretation of field and petrologic data suggests that the peridotite blocks in the Orocopia Schist subduction complex at Cemetery Ridge may be derived from the leading corner or edge of a mantle wedge, presumably in (pre-San Andreas fault) southwest California. However, derivation from a subducting plate is not precluded.

Arizona↗

USGS2015

Detrital zircon U-Pb reconnaissance of the Franciscan subduction complex in northwestern California

In northwestern California, the Franciscan subduction complex has been subdivided into seven major tectonostratigraphic units. We report U-Pb ages of ≈2400 detrital zircon grains from 26 sandstone samples from 5 of these units. Here, we tabulate each unit's interpreted predominant sediment source areas and depositional age range, ordered from the oldest to the youngest unit. (1) Yolla Bolly terrane: nearby Sierra Nevada batholith (SNB); ca. 118 to 98 Ma. Rare fossils had indicated that this unit was mostly 151-137 Ma, but it is mostly much younger. (2) Central Belt: SND; ca. 103 too 53 Ma (but poorly constrained), again mostly younger than previously thought. (3) Yager terrane: distant Idaho batholith (IB); ca. 52 to 50 Ma. Much of the Yager's detritus was shed during major core complex extension and erosion in Idaho that started 53 Ma. An eocene Princeton River-Princeton submarine canyon system transported this detritus to the Great Valley forearc basin and thence to the Franciscan trench. (4) Coastal terrane: mostly IB, ±SNB, ±nearby Cascade arc, ±Nevada Cenozoic ignimbrite belt; 52 to <32 Ma. (5) King Range terrane: dominated by IB and SNB zircons; parts 16-14 Ma based on microfossils. Overall, some Franciscan units are younger than previously thought, making them more compatible with models for the growth of subduction complexes by positive accretion. From ca. 118 to 70 Ma, Franciscan sediments were sourced mainly from the nearby Sierra Nevada region and were isolated from southwestern US and Mexican sources. From 53 to 49 Ma, the Franciscan was sourced from both Idaho and the Sierra Nevada. By 37-32 Ma, input from Idaho had ceased. The influx from Idaho probably reflects major tectonism in Idaho, Oregon, and Washington, plus development of a through-going Princeton River to California, rather than radical changes in the subduction system at the Franciscan trench itself.

California↗

USGS2015

High influx of carbon in walls of agglutinated foraminifers during the Permian-Triassic transition in global oceans

The Permian – Triassic mass extinction is postulated to be related to the rapid volcanism that produced the Siberian flood basalt (Traps). Unrelated volcanic eruptions producing several episodes of ash falls synchronous with the Siberian Traps are found in South China and Australia. Such regional eruptions could have caused wildfires, burning of coal deposits, and the dispersion of coal fly ash. These eruptions introduced a major influx of carbon into the atmosphere and oceans that can be recognized in the wall structure of foraminiferal tests present in survival populations in the boundary interval strata. Analysis of free specimens of foraminifers recovered from residues of conodont samples taken at a Permian – Triassic boundary section at Lung Cam in northern Vietnam has revealed the presence of a significant amount of elemental carbon , along with oxygen and silica, in their test wall structure, but an absence of calcium carbonate . These foraminifers , identified as Rectocornuspira kalhori , Cornuspira mahajeri , and Earlandia spp. and whose tests previously were considered to be calcareous, are confirmed to be agglutinated , and are now referred to as Ammodiscus kalhori and Hyperammina deformis . Measurement of the 207 Pb/ 204 Pb ratios in pyrite clusters attached to the foraminiferal tests confirmed that these tests inherited the Pb in their outer layer from carbon -contaminated seawater. We conclude that the source of the carbon could have been either global coal fly ash or forest fire-dispersed carbon , or a combination of both, that was dispersed into the Palaeo-Tethys Ocean immediately after the end- Permian extinction event.

Ha Giang Province↗

USGS2016

Metaultramafic schists and dismembered ophiolites of the Ashe Metamorphic Suite of northwestern North Carolina, USA

Metaultramafic rocks (MUR) in the Ashe Metamorphic Suite (AMS) of northwestern North Carolina include quartz ± feldspar-bearing QF-amphibolites and quartz-deficient, locally talc-, chlorite-, and/or Mg-amphibole-bearing TC-amphibolites. Some workers divide TC-amphibolites into Todd and Edmonds types, based on mineral and geochemical differences, and we provisionally add a third type – olivine ± pyroxene-rich, Rich Mountain-type rocks. Regionally, MUR bodies range from equant, Rich Mountain- to highly elongate, Todd-TC-amphibolite-type bodies. The MURs exhibit three to five mineral associations containing assemblages with olivine, anthophyllitic amphibole, Mg-hornblende, Mg-actinolite, cummingtonite, and serpentine representing decreasing eclogite to greenschist facies grades of metamorphism over time. MUR protoliths are difficult to determine. Southwestern MUR bodies have remnant olivine ± pyroxene-rich assemblages representing ultrabasic-basic, dunite-peridotite-pyroxenite protoliths. Northeastern TC-amphibolite MURs contain hornblende and actinolitic amphiboles plus chlorites – aluminous and calcic assemblages suggesting to some that metasomatism of basic, QF-amphibolites yields all TC-amphibolites. Yet MgO-CaO-Al 2 O 3 and trace element chemistries of many TC-amphibolites resemble compositions of plagioclase peridotites. We show that a few AMS TC-amphibolites had basaltic/gabbroic protoliths, while presenting arguments opposing application of the metasomatic hypothesis to all TC-amphibolites. We establish that MUR bodies are petrologically heterolithic and that TC-amphibolites are in contact with many rock types; that those with high Cr, Ni, and Mg have olivine- or pyroxene-dominated protoliths; that most exhibit three or more metamorphic mineral associations; and that contacts thought to be metasomatic are structural. Clearly, different MUR bodies have different chemistries representing various protoliths, and have different mineral assemblages, reflecting both chemical composition and metamorphic history. Spot sampling of heterolithic MUR bodies does not reveal MUR body character or history or allow ‘type’ designations. We recommend that the subdivision of MUR bodies into ‘types’ be abandoned and that the metasomatic hypothesis be carefully applied. AMS MURs and associated metamafic rocks likely represent fragments of dismembered ophiolites from various ophiolite types.

North Carolina↗

USGS2021

Comment on ‘New insights on Franciscan Complex geology, architecture, depositional age, and provenance for the western Mt. Tamalpais area, Marin County, California’ by Bero et al. (2020)

Serious errors and inconsistencies in the article undermine many of its interpretations to the point that principal conclusions are not valid. Much dependence is placed on the maximum depositional age (Dmax) of sandstone units based on zircon analysis of 10 samples, but calculation of those Dmax values is flawed, and their use confuses maximum with actual depositional ages and makes age distinctions finer than the resolution of the data. Conclusions that are compromised include the concept of a westward/downward-younging tectonostratigraphic stack of accretionary units, comparison of mapped sandstones with other California Coast Ranges sandstones, comparison of timing of events recorded in the map area with regional events, identification of mélange only in a narrow band in the study area, and detrital provenance of some of the sandstone units.

California↗

USGS2023

U-Pb zircon ages and Lu-Hf isotope systematics across northwestern Mexico: Implications for Cretaceous to Paleocene tectonomagmatic evolution during Farallon subduction

This study presents U-Pb zircon ages and Lu-Hf zircon isotope data for Cretaceous-Paleocene plutonic rocks along a W-E transect in northwestern Mexico. These data are combined with tectonic reconstruction that restores Late Cenozoic extensional deformation and shows the position of magmatism at 36 Ma. Zircon U-Pb ages results span from 142 to 58 Ma and demonstrate that the continental arc migrated northeastward at 1–2.5 km/Myr. These rates are slower than previously interpreted, but consistent with landward arc migration rates observed in the Andes. Weighted mean initial epsilon hafnium εHf( t ) values of plutonic rocks along the transect range from + 8.8 to −9.1. The heterogeneity in the zircon εHf( t ) is spatially related to the pre-Cretaceous basement provinces that the intrusive rocks were emplaced into. Zircon εHf( t ) values of western Baja California display positive values ranging from + 8.8 to + 2.6 suggesting they were formed from a moderately depleted mantle and were emplaced into the Guerrero-Alisitos-Vizcaino terrane. Zircon εHf( t ) values in the eastern part of Baja California and most of Sonora are heterogeneous ranging between −0.7 and −9.1 and may be formed from a relatively slightly more evolved mantle source and end up more evolved after crustal assimilation of metasediments. Zircon εHf( t ) values ranging from + 8.7 to + 2.9 in Chihuahua are consistent with a depleted-mantle derived melt and assimilation of Grenville lithospheric province. Our results highlight how Hf isotopic signatures help to constrain the pre-Cretaceous basement configuration in northwestern Mexico despite the few exposed outcrops along the transect.

northwestern Mexico↗

USGS2009

Geobotanical map of the U.S.S.R.

A newly compiled geobotanical map of the U.S.S.R. scale 1:4,000,000 is described. Two hundred and ten categories of vegetation are mapped, classified under 15 major groups. The relation of the mapped vegetation units to climate, soil, ground condition (permafrost), watersheds, and major geomorphic features is emphasized.

International Geology Review↗