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At least 397 records · Page 22Linked to original sources

USGS2009

Carbon isotope chemostratigraphy and precise dating of middle Frasnian (lower Upper Devonian) Alamo Breccia, Nevada, USA

At Hancock Summit West, Nevada, western USA, uppermost Givetian (upper Middle Devonian) and lower and middle Frasnian (lower Upper Devonian) rocks of the lower Guilmette Formation include, in stratigraphic sequence, carbonate-platform facies of the conodont falsiovalis , transitans , and punctata Zones; the type Alamo Breccia Member of the middle punctata Zone; and slope facies of the punctata and hassi Zones. The catastrophically deposited Alamo Breccia and related phenomena record the ~ 382 Ma Alamo event, produced by a km-scale bolide impact into a marine setting seaward of an extensive carbonate platform fringing western North America. Re-evaluation of conodonts from the lower Guilmette Formation and Alamo Breccia Member, together with regional sedimentologic and conodont biofacies comparisons, now firmly locates the onset of the Johnson et al. (1985) transgressive–regressive (T–R) cycle IIc, which occurred after the start of the punctata Zone, within a parautochthonous megablock low in the Alamo Breccia. Whole-rock carbon isotope analyses through the lower Guilmette Formation and Alamo Breccia Member reveal two positive δ 13 C carb excursions: (1) a small, 3‰ excursion, which is possibly correlative with the falsiovalis Event previously identified from sections in Western Europe and Australia, occurs below the breccia in the Upper falsiovalis Zone to early part of the transitans Zone; and (2) a large, multi-part excursion, dominated by a 6‰ positive shift, begins above the start of the punctata Zone and onset of T–R cycle IIc and continues above the Alamo Breccia, ending near the punctata – hassi zonal boundary. This large excursion correlates with the punctata Event, a major positive δ 13 C excursion previously recognized in eastern Laurussia and northern Gondwana. Consistent with previous studies, at Hancock Summit West the punctata Event is apparently not associated with any regional extinctions or ecosystem reorganizations. In the study area, onset of the main punctata Event began after the start of both the punctata Zone and T–R cycle IIc, and preceded the Alamo impact by less than 650 k.y., as inferred from conodont biochronologic and regional rock-accumulation rate estimates. Although complicated by the heterolithic, high-energy deposits of the Alamo Breccia, the carbon isotope record of the breccia and post-breccia beds does not indicate a major impact-correlative perturbation to the carbon cycle. This study extends recognition of the punctata Event to western Laurussia, further reinforcing the potential global scale of the event and its potential importance to understanding early to middle Frasnian marine geochemistry and palaeoenvironments. Based on previous models and our observations, increased tectonic activity, increased nutrient flux to oceans, increased marine bioproductivity, widespread anoxia, and increased organic carbon burial were all likely key factors in driving the punctata Event excursion. Furthermore, periodic eustatic and regional relative sea-level rises may have played an important role in promoting organic carbon burial and in maintaining a link between the primary open-marine geochemical signal and that recorded on the shallow-marine, lower Guilmette carbonate platform.

Nevada↗

USGS2012

Neoproterozoic tectonic evolution of the Jebel Saghro and Bou Azzer - El Graara inliers, eastern and central Anti-Atlas, Morocco

New mapping, geochemistry, and 17 U–Pb SHRIMP zircon ages from rocks of the Sirwa, Bou Azzer–El Graara, and Jebel Saghro inliers constrain the Neoproterozoic evolution of the eastern Anti-Atlas during Pan-African orogenesis. In the Sirwa inlier, Tonian quartzite from the pre Pan-African passive margin deposits of the Mimount Formation contains detrital zircon derived entirely from the West African Craton (WAC), with most grains yielding Eburnean Paleoproterozoic ages of about 2050 Ma. Cryogenian Pan-African orogenic activity (PA1) from about 760 to 660 Ma included northward-dipping subduction to produce a volcanic arc, followed by ophiolite obduction onto the WAC. In the Bou Azzer–El Graara inlier, calc-alkaline granodiorite and quartz diorite, dated at 650–646 Ma, are syn- to post-tectonic with respect to the second period of Pan-African orogenesis (PA2), arc-continent accretion, and related greenschist facies metamorphism. Slab break-off and lithospheric delimination may have provided the source for the supra-subduction calc-alkaline plutons. At about 646 Ma, quartz diorite intruded the Tiddiline formation placing an upper limit on molassic deposition. Widespread Ediacaran high-K calc-alkaline to shoshonitic plutonism and volcanism during the final stage of Pan-African orogenesis (PA3) occurred in a setting related to either modification of the margin of the WAC or formation of a continental volcanic arc above a short-lived southward-dipping subduction zone. In the Saghro inlier, eight plutonic rocks yield ages ranging from about 588 to 556 Ma. Sampled plutonic rocks previously considered to be Cryogenian yielded Ediacaran ages. Peraluminous rhyolitic volcanic rocks in the lower part of the Ouarzazate Supergroup, including ash-flow tuffs of the Oued Dar’a caldera, yield ages between about 574 and 571 Ma. The Oued Dar’a caldera developed in a pull-apart graben produced by a left-step in a northeast-trending, left-lateral strike-slip fault zone, and much of the lower Ouarzazate Supergroup volcanic rocks in the area are probably related to caldera out-flow facies and collapse. Late stage PA3 intrusive rocks include the Bouskour–Sidi Flah and Timijt rhyolitic dike swarms at about 563 Ma, the voluminous pink Isk-n-Alla granite (559 ± 5 Ma), and volumetrically minor gabbro of Tagmout (556 ± 5 Ma). Rhyolite flows from the upper part of the Ouarzazate Supergroup, above a regional angular unconformity, yielded ages of 558 ± 4 and 556 ± 4 Ma. The youngest ages place an upper limit on block faulting and weak folding during latest Pan-African tectonic activity (PA3), coincident with the departure of the Cadomian crustal fragment from the northern margin of the WAC.

Precambrian Research↗

USGS2024

Re-evaluating Marine Isotope Stage 5a paleo-sea-level trends from across the Florida Keys reef tract

Unraveling how Global Mean Sea Level (GMSL) fluctuated during past warm periods can improve our understanding of linkages between sea-level fluctuations, orbital forcing, and ice-sheet dynamics. Current estimates of GMSL for Marine Isotope Stages (MIS) 5a and 5c — two warm intervals following the relatively well-documented MIS 5e — contain meters of uncertainty and fewer data due to several challenges. These challenges include concealment of datable in-situ coral facies by MIS 1 deposits and inaccessibility due to submergence by modern sea level. We present a comprehensive dataset based on U–Th dating and stratigraphic correlation of 23 cores totaling over 170 m of recovered coral-reef deposits across the tectonically stable Florida Keys Reef Tract (FKRT). Following detailed facies descriptions, 34 in-situ, minimally altered aragonitic coral samples (≤2.7% calcite) below the Holocene-Pleistocene boundary were targeted for U–Th geochronology. Fourteen closed-system coral U–Th ages from MIS 5a include the commonly used sea-level indicator Acropora palmata , but also the massive coral taxa Pseudodiploria strigosa, Siderastrea siderea, Orbicella spp., and Porites astreoides . Dating yielded ages in the range of 88–81 ka (average 2σ uncertainty of less than 200 years). These ages suggest MIS 5a reef initiation at ∼88 ka BP, a peak near 83 ka with minimum elevations between −6.0 ± 0.5 and −5.6 ± 0.5 m MSL (2σ uncertainty and subsidence-corrected), and reef termination and sea-level fall by ∼81 ka BP. Notably, the range of peak MIS 5a relative sea-level estimates of −6.5 to −5.1 m MSL are more than 2 m shallower (higher) than previous estimates of −11 to −9 m. Our higher resolution regional sea-level reconstruction across four subregions of the Florida Keys reef tract aligns with changes in July insolation at 65° N: a trend that most other records, such as deep-sea sediments, do not have the accuracy and precision to resolve. Three massive coral samples from MIS 5c, consisting of Pseudodiploria clivosa , and Orbicella spp., yielded ages in the range of 104 to 99 ka (average 2σ uncertainty less than 200 years); however, because only one sample met the closed-system criteria, our ability to estimate MIS 5c sea level is relatively limited. More empirical estimates of sea-level from the MIS 5a and MIS 5c intervals based on numerical dating of reliable local sea-level constraints are critical for GMSL calculations and relating changes in sea-level amplitude and timing to global ice volume modeling and glacio-isostatic effects, all of which can improve predictions of future sea-level changes in coastal regions.

Florida↗

USGS2006

Influence of depositional setting and sedimentary fabric on mechanical layer evolution in carbonate aquifers

Carbonate aquifers in fold-thrust belt settings often have low-matrix porosity and permeability, and thus groundwater flow pathways depend on high porosity and permeability fracture and fault zones. Methods from sedimentology and structural geology are combined to understand the evolution of fracture controlled flow pathways and determine their spatial distribution. Through this process bed-parallel pressure-solution surfaces (PS1) are identified as a fracture type which influences fragmentation in peritidal and basinal carbonate, and upon shearing provides a major flow pathway in fold - thrust belt carbonate aquifers. Through stratigraphic analysis and fracture mapping, depositional setting is determined to play a critical role in PS1 localization and spacing where peritidal strata have closer spaced and less laterally continuous PS1 than basinal strata. In the peritidal platform facies, units with planar lamination have bed-parallel pressure-solution seams along mudstone laminae. In contrast, burrowed units of peritidal strata have solution seams with irregular and anastamosing geometries. Laminated units with closely spaced bed-parallel solution seams are more fragmented than bioturbated units with anastamosing solution seams. In the deeper-water depositional environment, pelagic settling and turbidity currents are the dominant sedimentation processes, resulting in laterally continuous deposits relative to the peritidal platform environment. To quantify the fracture patterns in the basinal environment, mechanical layer thickness values were measured from regions of low to high bed dip. The results define a trend in which mechanical layer thickness decreases as layer dip increases. A conceptual model is presented that emphasizes the link between sedimentary and structural fabric for the peritidal and basinal environments, where solution seams localize in mud-rich intervals, and the resulting pressure-solution surface geometry is influenced by sedimentary geometry (i.e., stacked fining upward cycles, burrows, planar laminations). In both facies types, laterally continuous PS1 can behave as mechanical layer boundaries. As layer-parallel slip increases to accommodate shear strain in the fold - thrust belt, more PS1 behave as mechanical layer boundaries. ?? 2005 Elsevier B.V. All rights reserved.

Sedimentary Geology↗

USGS2017

Quaternary tephrochronology and deposition in the subsurface Sacramento–San Joaquin Delta, California, U.S.A.

We document characteristics of tephra, including facies and geochemistry, from 27 subsurface sites in the Sacramento-San Joaquin Delta, California, to obtain stratigraphic constraints in a complex setting. Analyzed discrete tephra deposits are correlative with: 1) an unnamed tephra from the Carlotta Formation near Ferndale, California, herein informally named the ash of Wildcat Grade (<~1.450 - >~0.780 Ma), 2) the Rockland ash bed (~0.575 Ma), 3) the Loleta ash bed (~0.390 Ma), and 4) a middle Pleistocene tephra resembling volcanic ash deposits at Tulelake, California, and Pringle Falls, Bend, and Summer Lake, Oregon, herein informally named the dacitic ash of Hood (<~0.211 to >~0.180 Ma, correlated age). All four tephra are derived from Cascades volcanic sources. The Rockland ash bed erupted from the southern Cascades near Lassen Peak, California, and occurs in deposits up to >7 m thick as observed in core samples taken from ~40 m depth below land surface. Tephra facies and tephra age constraints suggest rapid tephra deposition within fluvial channel and overbank settings, likely related to flood events shortly following the volcanic eruption. Such rapidly deposited tephra are important chronostratigraphic markers that suggest varying sediment accumulation rates (~0.07-0.29 m/1000 yr) in Quaternary deposits below the modern Sacramento-San Joaquin Delta. This study provides the first steps in developing a subsurface Quaternary stratigraphic framework necessary for future hazard assessment.

California↗

USGS1998

Fluid-deposited graphitic inclusions in quartz: Comparison between KTB (German Continental Deep-Drilling) core samples and artificially reequilibrated natural inclusions

We used Raman microsampling spectroscopy (RMS) to determine the degree of crystallinity of minute (2–15 μm) graphite inclusions in quartz in two sets of samples: experimentally reequilibrated fluid inclusions in a natural quartz grain and biotite-bearing paragneisses from the KTB deep drillhole in SE Germany. Our sequential reequilibration experiments at 725°C on initially pure CO 2 inclusions in a quartz wafer and the J. Krautheim 1993 experiments at 900–1100°C on organic compounds heated in gold or platinum capsules suggest that, at a given temperature, (1) fluid-deposited graphite will have a lower crystallinity than metamorphosed organic matter and (2) that the crystallinity of fluid-deposited graphite is affected by the composition of the fluid from which it was deposited. We determined that the precipitation of more-crystalline graphite is favored by lower f H 2 (higher f O 2 ), and that the crystallinity of graphite is established by the conditions (including gas fugacities) that pertain as the fluid first reaches graphite saturation. Graphite inclusions within quartz grains in the KTB rocks show a wide range in crystallinity index, reflecting three episodes of carbon entrapment under different metamorphic conditions. Isolated graphite inclusions have the spectral properties of totally ordered, completely crystalline graphite. Such crystallinity suggests that the graphite was incorporated from the surrounding metasedimentary rocks, which underwent metamorphism at upper amphibolite-facies conditions. Much of the fluid-deposited graphite in fluid inclusions, however, shows some spectral disorder. The properties of that graphite resemble those of experimental precipitates at temperatures in excess of 700°C and at elevated pressures, suggesting that the inclusions represent precipitates from C-O-H fluids trapped under conditions near those of peak metamorphism at the KTB site. In contrast, graphite that is intimately associated with chlorite and other (presumably low-temperature) silicates in inclusions is highly disordered and spectrally resembles kerogens. This graphite probably was deposited during later greenschist-facies retrograde metamorphism at about 400–500°C. The degree of crystallinity of fluid-deposited graphite is shown to be a much more complex function of temperature than is the crystallinity of metamorphic graphite. To some extent, experiments can provide temperature-calibration of the crystallinity index. However, the difference in time scales between experimental runs and geologic processes makes it difficult to infer specific temperatures for naturally precipitated graphite.

Bohemian Massif↗

USGS2000

Correlation of offshore seismic profiles with onshore New Jersey Miocene sediments

The New Jersey passive continental margin records the interaction of sequences and sea-level, although previous studies linking seismically defined sequences, borehole control, and global ??18O records were hindered by a seismic data gap on the inner-shelf. We describe new seismic data from the innermost New Jersey shelf that tie offshore seismic stratigraphy directly to onshore boreholes. These data link the onshore boreholes to existing seismic grids across the outer margin and to boreholes on the continental slope. Surfaces defined by age; facies, and log signature in the onshore boreholes at the base of sequences Kw2b, Kw2a, Kw1c, and Kw0 are now tied to seismic sequence boundaries m5s, m5.2s, m5.4s, and m6s, respectively, defined beneath the inner shelf. Sequence boundaries recognized in onshore boreholes and inner shelf seismic profiles apparently correlate with reflections m5, m5.2, m5.4, and m6, respectively, that were dated at slope boreholes during ODP Leg 150. We now recognize an additional sequence boundary beneath the shelf that we name m5.5s and correlate to the base of the onshore sequence Kw1b. The new seismic data image prograding Oligocene clinoforms beneath the inner shelf, consistent with the results from onshore boreholes. A land-based seismic profile crossing the Island Beach borehole reveals reflector geometries that we tie to Lower Miocene litho- and bio-facies in this borehole. These land-based seismic profiles image well-defined sequence boundaries, onlap and downlap truncations that correlate to Transgressive Systems Tracts (TST) and Highstand Systems Tracts (HST) identified in boreholes. Preliminary analysis of CH0698 data continues these system tract delineations across the inner shelf The CH0698 seismic profiles tie seismically defined sequence boundaries with sequences identified by lithiologic and paleontologic criteria. Both can now be related to global ??18O increases and attendant glacioeustatic lowerings. This integration of core, log, and seismic character of mid-Tertiary sediments across the width of the New Jersey margin is a major step in the long-standing effort to evaluate the impact of glaciouestasy on siliciclastic sediments of a passive continental margin. (C) 2000 Elsevier Science B.V. All rights reserved.

Conference Paper↗

USGS2000

Lithofacies identification using multiple adaptive resonance theory neural networks and group decision expert system

Lithofacies identification supplies qualitative information about rocks. Lithofacies represent rock textures and are important components of hydrocarbon reservoir description. Traditional techniques of lithofacies identification from core data are costly and different geologists may provide different interpretations. In this paper, we present a low-cost intelligent system consisting of three adaptive resonance theory neural networks and a rule-based expert system to consistently and objectively identify lithofacies from well-log data. The input data are altered into different forms representing different perspectives of observation of lithofacies. Each form of input is processed by a different adaptive resonance theory neural network. Among these three adaptive resonance theory neural networks, one neural network processes the raw continuous data, another processes categorial data, and the third processes fuzzy-set data. Outputs from these three networks are then combined by the expert system using fuzzy inference to determine to which facies the input data should be assigned. Rules are prioritized to emphasize the importance of firing order. This new approach combines the learning ability of neural networks, the adaptability of fuzzy logic, and the expertise of geologists to infer facies of the rocks. This approach is applied to the Appleton Field, an oil field located in Escambia County, Alabama. The hybrid intelligence system predicts lithofacies identity from log data with 87.6% accuracy. This prediction is more accurate than those of single adaptive resonance theory networks, 79.3%, 68.0% and 66.0%, using raw, fuzzy-set, and categorical data, respectively, and by an error-backpropagation neural network, 57.3%. (C) 2000 Published by Elsevier Science Ltd. All rights reserved.

Computers & Geosciences↗

USGS1997

Organic geochemical study of sequences overlying coal seams; example from the Mansfield Formation (Lower Pennsylvanian), Indiana

Roof successions above two coal seams from the Mansfield Formation (Lower Pennsylvanian) in the Indiana portion of the Illinois Basin have been studied with regard to sedimentary structures, organic petrology and organic geochemistry. The succession above the Blue Creek Member of the Mansfield Formation is typical of the lithologies covering low-sulphur coals (< 1%) in the area studied, whereas the succession above the unnamed Mansfield coal is typical of high-sulphur coals (>2.%). The transgressive-regressive packages above both seams reflect the periodic inundation of coastal mires by tidal flats and creeks as inferred from bioturbation and sedimentary structures such as tidal rhythmites and clay-draped ripple bedforms. Geochemistry and petrology of organic facies above the Blue Creek coal suggest that tidal flats formed inland in fresh-water environments. These overlying fresh water sediments prevented saline waters from invading the peat, contributing to low-sulphur content in the coal. Above the unnamed coal, trace fossils and geochemical and petrological characteristics of organic facies suggest more unrestricted seaward depositional environment. The absence of saline or typically marine biomarkers above this coal is interpreted as evidence of very short periods of marine transgression, as there was not enough time for establishment of the precursor organisms for marine biomarkers. However, sufficient time passed to raise SO 4 2− concentration in pore waters, resulting in the formation of authigenic pyrite and sulphur incorparation into organic matter.

International Journal of Coal Geology↗

USGS2015

Enigmatic eight-meter trace fossils in the Lower Pennsylvanian Lee sandstone, central Appalachian basin, Tennessee

Enigmatic tubular trace fossils up to eight meters long occur in the Lower Pennsylvanian Middlesboro Member of the Lee Formation. Two morphotypes occur: type 1 trace fossils are plain, smooth, vertical, nonbranching, parallel-walled, tubular structures; type 2 trace fossils branch, have walls with faint vertical striations, regularly or irregularly spaced nodes, and funnel-shaped terminations. Sandstone casts filling type 2 structures have helical spiral morphology, and, in rare individuals, faint meniscate fills have been observed. Both trace-fossil morphotypes have poorly cemented wall linings containing framboidal pyrite, amorphous carbon, quartz sand, and poorly preserved fecal material. The trace fossils occur in a massive, structureless, channel-form sandstone, originating at the contact between a channel lag and the overlying massive fill. The stratigraphic sequence is interpreted to represent a barrier island transgressing an estuarine facies. A tidal inlet within the barrier facies scoured into the underlying estuarine sediments. Subsequent rapid filling of the inlet led to the deposition of the massive sandstone. Origin of these structures is uncertain. The preponderance of evidence favors the hypothesis that the structures are escape burrows of animals that had colonized, or were concentrated in, the lag and were suddenly buried by the deposition of the massive sand. However, no likely burrower has been identified, and several characteristics of the structures and the enclosing sediments indicate that they may be completely inorganic in origin.

Journal of Paleontology↗

USGS2006

Satellite-derived, melt-season surface temperature of the Greenland Ice Sheet (2000-2005) and its relationship to mass balance

Mean, clear-sky surface temperature of the Greenland Ice Sheet was measured for each melt season from 2000 to 2005 using Moderate-Resolution Imaging Spectroradiometer (MODIS)–derived land-surface temperature (LST) data-product maps. During the period of most-active melt, the mean, clear-sky surface temperature of the ice sheet was highest in 2002 (−8.29 ± 5.29°C) and 2005 (−8.29 ± 5.43°C), compared to a 6-year mean of −9.04 ± 5.59°C, in agreement with recent work by other investigators showing unusually extensive melt in 2002 and 2005. Surface-temperature variability shows a correspondence with the dry-snow facies of the ice sheet; a reduction in area of the dry-snow facies would indicate a more-negative mass balance. Surface-temperature variability generally increased during the study period and is most pronounced in the 2005 melt season; this is consistent with surface instability caused by air-temperature fluctuations.

Geophysical Research Letters↗

USGS2009

Contemporaneous deposition of phyllosilicates and sulfates: Using Australian acidic saline lake deposits to describe geochemical variability on Mars

Studies of the origin of the Martian sulfate and phyllosilicate deposits have led to the hypothesis that there was a marked, global-scale change in the Mars environment from circum-neutral pH aqueous alteration in the Noachian to an acidic evaporitic system in the late Noachian to Hesperian. However, terrestrial studies suggest that two different geochemical systems need not be invoked to explain such geochemical variation.Western Australian acidic playa lakes have large pH differences separated vertically and laterally by only a few tens of meters, demonstrating how highly variable chemistries can coexist over short distances in natural environments. We suggest diverse and variable Martian aqueous environments where the coetaneous formation of phyllosilicates and sulfates at the Australian sites are analogs for regions where phyllosilicates and sulfates coexist on Mars. In these systems, Fe and alkali earth phyllosilicates represent deep facies associated with upwelling neutral to alkaline groundwater, whereas aluminous phyllosilicates and sulfates represent near-surface evaporitic facies formed from more acidic brines. Copyright 2009 by the American Geophysical Union.

Geophysical Research Letters↗

USGS2011

Hydrostratigraphic analysis of the MADE site with full-resolution GPR and direct-push hydraulic profiling

Full‐resolution 3D Ground‐Penetrating Radar (GPR) data were combined with high‐resolution hydraulic conductivity ( K ) data from vertical Direct‐Push (DP) profiles to characterize a portion of the highly heterogeneous MAcro Dispersion Experiment (MADE) site. This is an important first step to better understand the influence of aquifer heterogeneities on observed anomalous transport. Statistical evaluation of DP data indicates non‐normal distributions that have much higher similarity within each GPR facies than between facies. The analysis of GPR and DP data provides high‐resolution estimates of the 3D geometry of hydrostratigraphic zones, which can then be populated with stochastic K fields. The lack of such estimates has been a significant limitation for testing and parameterizing a range of novel transport theories at sites where the traditional advection‐dispersion model has proven inadequate.

Mississippi↗

USGS2018

Kinematic, deformational, and thermochronologic conditions along the Gossan Lead and Fries shear zones: Constraining the western-eastern Blue Ridge boundary in northwestern North Carolina

The fault boundary between the western and eastern Blue Ridge (WBR-EBR) in the southern Appalachians separates Mesoproterozoic basement rocks and their cover from Neoproterozoic to Paleozoic accreted rocks. Several northeast striking faults delineate the boundary, including the Gossan Lead shear zone in northwestern North Carolina. Varying tectonic interpretations of WBR-EBR boundary include a premetamorphic fault, an Acadian dextral strike-slip fault, or an Alleghanian fault. We use field-based, microstructural, and theromochronometric analyses to determine the conditions, kinematics, and timing of deformation, in order to distinguish among competing hypotheses for the Gossan Lead shear zone. This comprehensive approach has allowed us to attribute a number of new and previously observed tectonic fabrics to specific orogenic events; key relationships necessary to the study of multiply deformed tectonic margins. Detailed mapping and microstructural analysis of the Gossan Lead shear zone document a several kilometer-wide mylonitic zone with kinematic indicators that record dominantly top-to-the-NW thrust motion, with local strike-slip and normal sense indicators. Dynamically recrystallized quartz and feldspar constrain a range of deformation conditions from amphibolite to greenschist facies. Two unaltered lineation-forming amphiboles from mylonitic amphibolites record 40 Ar/ 39 Ar cooling ages of 347–345 Ma, and a mylonitized metagraywacke records a muscovite 40 Ar/ 39 Ar cooling age of 336 Ma. These data are consistent with dominantly NW directed thrusting along the Gossan Lead shear zone at amphibolite to greenschist facies conditions, and rapid cooling in the Middle Mississippian. We suggest these data support overprinting and/or reactivation of an earlier structure along this complexly deformed boundary by 336 Ma.

North Carolina↗

USGS2010

Mid‐Cretaceous extensional tectonics of the Yukon‐Tanana Terrane, Trans‐Alaska Crustal Transect (TACT), east‐central Alaska

Mid‐Cretaceous crustal extension played a fundamental role in the structural evolution of the Yukon‐Tanana terrane (YTT) in the northern Cordilleran interior. In the central portion of the YTT northwest of Delta Junction, Alaska, a mylonitic shear zone juxtaposes greenschist facies rocks in the upper plate against middle to upper amphibolite facies metamorphic rocks in the lower plate, a juxtaposition suggesting elimination of as much as 10 km of crustal section. The mylonites form a partial sheath enveloping a domal footwall structure and kinematic analysis of the mylonite zone yields a uniform transport direction of hanging wall to ESE. These relations suggest analogies to the metamorphic core complexes of the southern Cordillera. However, the YTT structures are entirely ductile, suggesting either a relatively deep erosional level or relatively high geothermal gradients during extension. In the study area remnants of an older preextensional thrusting event are preserved at the highest structural levels at the base of the Seventymile terrane and the leading edge of YTT in the Wickersham terrane. However, most areas display a regional, subhorizontal fabric that is superimposed on older fabrics, and in the study area this latest fabric is subparallel to the mylonitic sheath of the apparent extensional structure. Thus the conventional viewpoint that this latest fabric is related to thrusting needs to be reevaluated and this fabric may be entirely extensional in origin. Further evidence for extension is provided by clear similarities between YTT and characteristic features of other extensional terranes. Thus we suggest that the YTT is a deeply eroded view of highly extended continental crust. The tectonic mechanism for the extensional event and the magnitude of the extension is uncertain because of complications in regional timing relationships and in alternative interpretations of the reconstruction of the crustal section. Three end‐member models based on analogies with Neogene extensional systems are presented as working models to accommodate the alternative interpretations: (1) a Jurassic collision and Cretaceous extension model based on comparisons with the Neogene history of the Mediterranean region; (2) an Early to mid‐Cretaceous syncollisional model analogous to the Carpathian Mountains of eastern Europe; and (3) a syncollisional plateau uplift model with extension driven by gravity spreading.

Alaska↗

USGS2010

Structure, metamorphism, and geochronology of the Cosmos Hills and Ruby Ridge, Brooks Range schist belt, Alaska

The boundary of the internal zones of the Brooks Range orogenic belt (the schist belt) is a fault contact that dips toward the hinterland (the Yukon-Koyukuk province). This fault, here referred to as the Cosmos Hills fault zone, juxtaposes oceanic rocks and unmetamorphosed sedimentary rocks structurally above blueschist-to-greenschist facies metamorphic rocks of the schist belt. Near the fault contact, schist belt rocks are increasingly affected by a prominent, subhorizontal transposition foliation that is locally mylonitic in the fault zone. Structural and petrologic observations combined with 40 Ar/ 39 Ar incremental-release geochronology give evidence for a polyphase metamorphic and deformational history beginning in the Middle Jurassic and continuing until the Late Cretaceous. Our 40 Ar/ 39 Ar cooling age for Jurassic metamorphism is consistent with stratigraphic and other evidence for the onset of Brooks Range orogenesis. Jurassic metamorphism is nearly everywhere overprinted by a regional greenschist-facies event dated at 130–125 Ma. Near the contact with the Cosmos Hills fault zone, the schist belt is increasingly affected by a younger greenschist metamorphism that is texturally related to a prominent foliation that folds and transposes an older fabric. The 40 Ar/ 39 Ar results on phengite and fuchsite that define this younger fabric give recrystallization ages ranging from 103 to less than 90 Ma. We conclude that metamorphism that formed the transposition fabric peaked around 100 Ma and may have continued until well after 90 Ma. This age for greenschist metamorphism is broadly synchronous with the depositional age of locally derived, shallow-marine clastic sedimentary strata in the hanging wall of the fault zone and thus substantiates the interpretation that the fault zone accommodated extension in the Late Cretaceous. This extension unroofed and exhumed the schist belt during relative subsidence of the Yukon-Koyukuk province.

Tectonics↗

USGS2012

Rapid extension recorded by cooling‐age patterns and brittle deformation, Naxos, Greece

The metamorphic core complex exposed as the island of Naxos in the Aegean Sea records an unusually complete sequence of structures developed as a result of continental extension. The structures formed during Miocene rise and cooling from ductile, upper amphibolite facies and anatectic conditions to brittle near‐surface conditions beneath the Naxos detachment fault. Top‐to‐the‐north ductile fabrics in the footwall, which initially developed during amphibolite facies prograde metamorphism, were overprinted by a succession of north directed, normal sense lower‐temperature brittle structures as the footwall was tectonically unloaded and unroofed. Pseudotachylite and cataclasite formation, brittle faulting, alteration, and erosion of the footwall occurred during continued slip and tectonic denudation. Neogene conglomerate and megabreccia, in part derived from exhumation of this footwall, lie structurally above the peripheral Naxos fault. Published K‐Ar and 40 Ar/ 39 Ar ages for hornblende, white mica, and biotite in the footwall decrease northwestward; apparent ages 17–50 Ma in the southeast correspond to areas of low metamorphic grade where preextension argon was partially retained. Published ages 16–10 Ma in higher‐grade rocks of the domal core in the north are cooling ages that for each of the three minerals show a component of younging in the NNE direction of extension. Assuming this is the direction of unroofing, we interpret the rate of this younging as the fault slip rate as the footwall rocks moved >20 km SSW relative to their hanging wall along the base of the Naxos detachment fault. The calculated rates of slip average 5–8 mm/yr, comparable to maximum rates reported in the Basin and Range province.

Naxos↗

USGS1998

Paleozoic orogens in New England, USA

Stratigraphy and isotope geochronology in the crystalline core of the Appalachians suggest revised interpretations of the extent, nature and timing of Paleozoic orogens in New England. Five major episodes of magmatism, deformation, and high-grade regional metamorphism are recognized: Taconian (455-442 Ma), Acadian (423-385 Ma), Neo-Acadian (366-350 Ma), Late Pennsylvanian (300-290 Ma) and Alleghanian (280-260 Ma). In the Taconian, the passive margin of Laurentia was subducted below a complex magmatic arc lasting from 480 to 442 Ma, founded in part on continental crust of a Medial New England terrane with possible affinities with Amazonia. Questions about Medial New England involve its coherence as a single plate, and the nature of its underlying crust. The Acadian began in Late Silurian as a collision between the amalgamated Laurentia-Medial New England and outer belts of Composite Avalon along a cryptic suture in coastal Maine, and progressed northwestward to the Connecticut Valley basin by mid-Devonian. Tonalitic-granitic magmatism and up to granulite-facies metamorphism culminated in Early Devonian, possibly tied to lithospheric detachment below the subducting northwestern plate and consequent asthenosphere upwelling. Newly discovered Neo-Acadian Late Devonian to Early Mississippian tonalitic-granitic magmatism, up to granulite-facies metamorphism, and severe deformation in central Massachusetts took place in a plate context poorly understood. Late Pennsylvanian effects include magmatism, metamorphism, and deformation near south New England gneiss domes and the Sebago batholith, and development of the right-lateral Norumbega fault system. Permian Alleghanian effects include penetrative deformation, granitic intrusions and up to sillimanite-grade metamorphism of Pennsylvanian beds in southeastern New England. These last two episodes relate to the arrival of Africa.

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