North American Continental Margin Research Articles

A Complex Interplay Between Pluton Emplacement, Tectonic Deformation, and Plate Kinematics in the Cretaceous Sierra Nevada Magmatic Arc, California

AbstractThe relation of plate kinematics to the structural record of arc plutons and their host rocks is complex and still not fully understood. We address this issue through a combination of field mapping, structural analysis, anisotropy of magnetic susceptibility analysis, and fabric modeling in the Late Cretaceous Tuolumne Intrusive Complex, Sierra Nevada, California. A pattern of anti‐clockwise rotation from ∼NNW–SSE to WNW–ESE steep foliations and change in fabric ellipsoid shape from oblate to prolate was revealed in successively emplaced Kuna Crest (∼95–92 Ma), Half Dome (∼92–89 Ma), and Cathedral Peak (∼89–84 Ma) granodiorites. The numerical model indicates that the Kuna Crest was emplaced in a transpressional setting with an angle of convergence α = 60–40°, whereas the Half Dome and Cathedral Peak required simultaneous vertical constriction overprinted by transpression with α = 35–15°. This transition, which occurred at ∼92 Ma, is accompanied by a shallowing of the lineation plunge observed also in other ∼88–84 Ma central Sierra Nevada plutons. Provided that the Cretaceous Sierra Nevada arc was constructed during overall dextral transpression, these transitions reflect significant changes in kinematics, where ∼107–92 Ma plutons were emplaced during pure shear‐dominated transpression, which was followed by a transition to wrench‐dominated transpression recorded in ∼92–84 Ma plutons. Such a transition in kinematics is explained as a result of progressively increasing obliquity of the relative convergence of the Farallon plate subducting beneath the North American continental margin, in agreement with most paleogeographic reconstructions.

A common magma source for plutonic and volcanic rocks of The Geysers geothermal field, California: Volume and intrusive history derived from zircon

The Geysers Plutonic Complex (GPC) in the California Coast Ranges underlies one of the world's premier geothermal resources. The GPC consists of three major intrusive phases, orthopyroxene-biotite microgranite porphyry, orthopyroxene-biotite granite, and hornblende-biotite-orthopyroxene granodiorite (in sequence of emplacement). These are nearly coeval and compositionally equivalent to lavas of the overlying Cobb Mountain Volcanic Center which includes (from older to younger) rhyolite of Alder Creek, dacite of Cobb Mountain, and dacite of Cobb Valley. Zircon from GPC plutonic and associated volcanic rocks were analyzed at high spatial resolution for their trace element abundances along with oxygen and hafnium isotopic compositions. Pronounced negative Eu anomalies and high incompatible trace element (e.g., Y, Hf, and U) abundances in zircon along with low Ti-in-zircon temperatures reveal that the GPC microgranite porphyry magma was more evolved than GPC granite and granodiorite. Isotopically, GPC microgranite porphyry, granite, and granodiorite zircon crystals closely overlap (δ18O = +4.76 to +9.18; εHf = +1.4 to +10.7), but a subpopulation with elevated δ18O (∼8.05) and lower εHf (∼4.4) is only present in GPC granite and granodiorite. Zircon from coeval volcanic units share this dichotomy, with a dominant population at (δ18O = +4.92 to +9.38; εHf = +3.4 to +11.3) and a generally minor population with elevated δ18O and low εHf, which is particularly prominent in dacite of Cobb Valley. Anticorrelated δ18O and εHf values indicate progressive assimilation of Franciscan Complex basement rocks, but fractional crystallization was decoupled from crustal assimilation. Together with published U-Th-Pb geochronology, these data reveal distinct degrees of crust-mantle interaction and constrain the thermochemical conditions during different stages in the evolution of a composite intrusive body that fed the GPC and contemporaneous volcanoes. A thermal model for such a body emplaced at ∼7 km depth quantitatively matches the GPC zircon age distribution when magma accumulation started at low recharge rates (0.1 km3/ka), intermittently peaked during a brief flare-up (4 km3/ka for 50 ka), and then returned to a low recharge flux (0.1 km3/ka). This model also qualitatively explains the initial presence of small-volume, highly evolved melts forming the microgranite porphyry, followed by massive emplacement of less evolved magmas forming the GPC granite-granodiorite complex. During this second stage, crustal assimilation locally intensified due to prior thermal priming of the country rock. The total injected magma volume into the upper-crustal reservoir between c. 2.1 and 1.1 Ma amounts to ∼300 km3, which is about three to four times the known volume from geothermal well penetration into the GPC. The accumulation of large volumes of silicic magma along the western North American continental margin in the wake of the northward migrating Gorda slab edge thus appears limited to a brief (c. 50 ka) pulse of high magma influx from the mantle that was pre- and post-dated by protracted low-flux magmatism.

Cretaceous Changes of Strike-Slip Tectonics on the North Pacific Margins: Implications for the Earth’s Rotation

This study reviews the Meso–Cenozoic tectonic paleo-reconstructions for the East Asian and western North American continental margins, focusing on strike-slip tectonics. It follows previous studies by the present and other authors, which investigated the Cretaceous turn of geological evolution (CTGE). They largely studied significant changes in the Earth’s mineralization, magmatism and climate. The present study focuses on significant changes related to the Earth’s rotation velocity. This question is significant not only for fundamental science, but also for applied geology, because CTGE is marked by abundant ore and energetic resources. The results show domination of sinistral shearing on the NE-oriented Asian margin during the pre-early Cretaceous time that turned to significant development of dextral movements in the mid Cretaceous–Cenozoic time. On the NW-oriented American margin, significant development of sinistral movements in the pre-early Cretaceous time turned to domination of dextral shearing during late Cretaceous and Cenozoic. These tectonic changes indicate the transition of the Earth’s rotation from the accelerating towards decelerating regime after CTGE (135–120 Ma). This change may be caused by the transition of the Earth’ mass to, and then, away from the polar regions, the processes being related to melting and freezing of the ice caps.

Abrupt Quaternary Ocean-ice Events in the Arctic: Evidence from the Ostracode Rabilimis

The Arctic Ocean has experienced orbital and millennial-scale climate oscillations over the last 500 kilo-annum (ka) involving massive changes in global sea level and components of the Arctic cryosphere, including sea-ice cover, land-based ice sheets and ice shelves. Although these climate events are only partially understood, micropaleontological studies utilizing ostracodes and benthic foraminifera have demonstrated that major changes in faunas have occurred at different timescales that signify ecosystem regime changes linked to sea-ice cover, surface productivity, bottom temperature and other factors. In addition to faunal changes characterizing glacial-interglacial cycles, Arctic sediments contain several unusual faunal events that cannot be explained by orbital-scale sea level and cryospheric changes. One indicator of such events involves the ostracode Rabilimis mirabilis (Brady 1868), a shallow-water species that inhabits continental shelves in the modern Arctic. We conducted studies of the stratigraphic distribution of R. mirabilis in cores from the Northwind, Mendeleev, Lomonosov, and Alpha Ridges; the Siberian and North American (Beaufort Sea) continental margins; and the Lincoln Sea off North Greenland and in the northern Greenland Sherard Osborn Fjord. Evidence from these records suggests that this species occurs as a fossil in deeper water sediment cores on the upper parts of submarine ridges (mainly 700-900 meters water depth, mwd), in significant numbers (from 1%to 50% of total ostracodes) during Marine Isotope Stages (MIS) 5a (125-109 ka), MIS 4 (71-57 ka), and MIS 3 (57-29 ka). Furthermore, it occurs in cores from various depths on the Siberian margin, the Beaufort and Lincoln Seas during MIS 1 (the Holocene, approx. 11-0 ka). These occurrences involve well-preserved, stratigraphically consistent adult and juvenile populations, which are autochthonous in nature and not caused by downslope transport or ice rafting. Based on their age and associated paleoceanographic conditions in the Arctic, we interpret these R. mirabilis events as signifying basin-ward migration during abrupt changes in growth and decay of massive ice shelves and may be useful as biostratigraphic markers.

A crucial geologic test of Late Jurassic exotic collision versus endemic re-accretion in the Klamath Mountains Province, western United States, with implications for the assembly of western North America

Abstract Differing interpretations of geophysical and geologic data have led to debate regarding continent-scale plate configuration, subduction polarity, and timing of collisional events on the western North American plate margin in pre–mid-Cretaceous time. One set of models involves collision and accretion of far-traveled “exotic” terranes against the continental margin along a west-dipping subduction zone, whereas a second set of models involves long-lived, east-dipping subduction under the continental margin and a fringing or “endemic” origin for many Mesozoic terranes on the western North American plate margin. Here, we present new detrital zircon U-Pb ages from clastic rocks of the Rattlesnake Creek and Western Klamath terranes in the Klamath Mountains of northern California and southern Oregon that provide a test of these contrasting models. Our data show that portions of the Rattlesnake Creek terrane cover sequence (Salt Creek assemblage) are no older than ca. 170–161 Ma (Middle–early Late Jurassic) and contain 62–83% Precambrian detrital zircon grains. Turbidite sandstone samples of the Galice Formation are no older than ca. 158–153 Ma (middle Late Jurassic) and contain 15–55% Precambrian detrital zircon grains. Based on a comparison of our data to published magmatic and detrital ages representing provenance scenarios predicted by the exotic and endemic models (a crucial geologic test), we show that our samples were likely sourced from the previously accreted, older terranes of the Klamath Mountains and Sierra Nevada, as well as active-arc sources, with some degree of contribution from recycled sources in the continental interior. Our observations are inconsistent with paleogeographic reconstructions that are based on exotic, intra-oceanic arcs formed far offshore of North America. In contrast, the incorporation of recycled detritus from older terranes of the Klamath Mountains and Sierra Nevada, as well as North America, into the Rattlesnake Creek and Western Klamath terranes prior to Late Jurassic deformation adds substantial support to endemic models. Our results suggest that during long-lived, east-dipping subduction, the opening and subsequent closing of the marginal Galice/Josephine basin occurred as a result of in situ extension and subsequent contraction. Our results show that tectonic models invoking exotic, intra-oceanic archipelagos composed of Cordilleran arc terranes fail a crucial geologic test of the terranes’ proposed exotic origin and support the occurrence of east-dipping, pre–mid-Cretaceous subduction beneath the North American continental margin.

A Numerical Model to Estimate the Effects of Variable Sedimentation Rates on Methane Hydrate Formation—Application to the ODP Site 997 on Blake Ridge, Southeastern North American Continental Margin

AbstractSedimentation and burial rate can vary considerably at continental slopes, due to constantly changing tectonic processes on the geological timescale. This variation may affect the transport of methane in sediment, as manifested in the change of pore water flux, methanogenesis, methane diffusion and hydrate removal from hydrate stability zone (HSZ) and further affect the accumulation of hydrate in submarine sediments. Most of previous models assumed a constant sedimentation rate and thus may result in inaccurate estimates of total amount of hydrate within the HSZ. In this study, we developed a hydrate accumulation model that is capable of handling multiple sedimentation stages. Site 997 of ODP Leg 164, with data indicating its recent history of four sedimentation stages of different rates, is chosen to investigate the significance of sedimentation rate variation experienced at this location. We examined the effect of varying sedimentation rates on hydrate accumulation by taking in consideration sediment compaction, in situ methanogenesis, and composition and component transport processes. Simulation results suggest that the history of hydrate accumulation at Site 997 is characterized by a sequence of increase, decrease, and then increase till the present day. At present, the hydrate deposit has accumulated to 8.30 × 104 mol/m2, and the average hydrate saturation near the base of the HSZ is 6.3%, which is in general agreement with published estimates in the literature. The accumulation of hydrate at Site 997 was significantly affected by variable sedimentation rates, and nearly one half of the hydrate deposit was accumulated during the last 2.5 Myr.

Syn-accretionary multistage assembly of an Early Jurassic Alaskan-type intrusion in the Canadian Cordillera: U–Pb and 40Ar/39Ar geochronology of the Turnagain ultramafic–mafic intrusive complex, Yukon–Tanana terrane

The magmatic evolution of the Early Jurassic Turnagain Alaskan-type intrusion is inextricably linked to terrane accretionary events at the North American continental margin. We have calibrated the multistage assembly of the Turnagain ultramafic–mafic complex using high-precision air and chemical abrasion ID-TIMS U–Pb and 40Ar/39Ar geochronology. Our results are the first published geochronological calibration of zoning in an Alaskan-type intrusion. Three of four intrusive phases (Stages 1–4, oldest to youngest) were successfully dated. Cooling ages obtained on Stage 2 dunite–wehrlite include statistically identical 40Ar/39Ar plateau dates of 187.4 ± 1.5 (2σ) Ma on hornblende and 188.6 ± 1.2 Ma on phlogopite, and a concordant 206Pb/238U date of 190.3 ± 4.6 Ma on titanite. Stage 3 diorite has a 206Pb/238U zircon crystallization age of 188.11 ± 0.13 Ma. Stage 4 wehrlite and leucodiorite yield 206Pb/238U zircon crystallization ages of 185.68 ± 0.19 and 185.30 ± 0.12 Ma, respectively. The ca. 189 Ma phlogopite date represents a minimum crystallization age for Stage 2, indicating that assembly of the Turnagain intrusion spanned at least ∼4 million years from ca. 189 Ma to 185 Ma. Regional progressive contractional deformation in the Early Jurassic is constrained by the geochronological results (>189 to <185 Ma) and initially generated northeast-vergent folds in the country rocks (>189 Ma), deformed Stage 1 wehrlite–clinopyroxenite prior to emplacement of Stages 2–4, and subsequently thrust the intrusion and its host rocks onto the ancient North American continental margin (<185 Ma). Contrary to previous interpretations, new geophysical data support an allochthonous origin for the host rocks of the Turnagain intrusion. The Turnagain host succession is correlated with late Paleozoic arc-related and basinal lithostratigraphic units developed at the inboard margin of the accreted Yukon–Tanana terrane and peripheral to the Slide Mountain marginal ocean basin. The geochronological results rigorously constrain the timing and style of Early Jurassic deformation attending the initial accretion of allochthonous arc terranes in the northern Cordillera.

A Global Probabilistic Prediction of Cold Seeps and Associated SEAfloor FLuid Expulsion Anomalies (SEAFLEAs)

AbstractA newly compiled, open‐source database of focused fluid flow sites (e.g., cold seeps) and associated SEAfloor FLuid Expulsion Anomalies (SEAFLEAs) reveals a variable distribution of anomalies across global continental margins. The SEAFLEA distribution is heavily biased toward North American continental margins, with most observations between 100‐ and 200‐m water depth globally, and with an equal distribution between active and passive margins. Using a machine learning classification methodology based on outlier detection algorithms, we predict the probability of encountering a SEAFLEA globally. Results show the highest probability in regions with multiple SEAFLEA observations and parametrically similar regions concentrated on continental margins. In general, geologic, biologic, and chemical predictors are the best predictors of SEAFLEAs. We validate our results using a random and geospatial validation technique that reveals our methods are robust to random variations in observations, but that certain margins, such as the Svalbard Margin, represent parametrically distinct locals. These distinct regions have control over the global distribution of predicted anomalies due to their unique features. Our final prediction on a global 5 × 5 arc minute grid reveals that the average probability of encountering a SEAFLEA is 33.1 ± 17.7% on active margins and 31.2 ± 18.9% on passive margins, showing equal likelihood of encountering fluid expulsion between passive and active margins. Therefore, the lateral compaction on active margins does not increase the likelihood of fluid expulsion relative to the predominantly vertical compaction on passive margins. These results however say nothing of the fluid flux rates or density of expulsion features.

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Evaluating tectonic models for the formation of the North American Cordillera using multivariate statistical analysis of Late Triassic conodont faunas

The tectonic processes that led to the current configuration of the Quesnel and Stikine terranes within the North American Cordillera are contentious, with competing models favouring either northward, strike-slip movement or southward, rotational movement of the Stikine terrane. Multivariate statistical analysis of Late Triassic conodonts faunas from Canada and the USA has been used to evaluate these competing models and to constrain the movement of terranes relative to the North American continent. Late Carnian to early Norian conodonts are present in both the Stikine and Quesnel terranes, as well as at high and low latitudes on the North American continental margin, in northeastern British Columbia and Nevada respectively. The conodont faunas from each of these four geographic areas have been compared using the Dice, Jaccard, Ochiai and Simpson similarity indices. The highest values of similarity were found to be between the faunas of the Stikine and Quesnel terranes, supporting previous conclusions that these belonged to a single volcanic arc system. The conodont faunas of both of these terranes were more similar to that of northeastern British Columbia than either were to that of Nevada, suggesting that during the Late Triassic the terranes were more likely located at higher latitudes than lower latitudes. Finally, the conodont fauna of the Stikine terrane was significantly more similar to that of Nevada than the fauna of the Quesnel terrane was, which indicates that of the two terranes, the Stikine terrane lay farther south than the Quesnel terrane during the Late Triassic. This interpretation of the orientation of these terranes supports tectonic models that favour northward strike-slip translation of the Stikine terrane, rather than those models that require southward rotation of the terrane.

Influence of Ocean Circulation and Benthic Exchange on Deep Northwest Atlantic Nd Isotope Records During the Past 30,000 Years

AbstractNeodymium (Nd) isotopes extracted from authigenic sediment phases are increasingly used as a proxy for past variations in water mass provenance. To better constrain the controls of water mass provenance and nonconservative effects on the archived Nd isotope signal, we present a new depth transect of Nd isotope reconstructions from the Blake Bahama Outer Ridge along the North American continental margin covering the past 30 ka. We investigated five sediment cores that lie directly within the main flow path of the Deep Western Boundary Current, a major advection route of North Atlantic Deep Water. We found offsets between core tops and seawater Nd isotopic compositions that are observed elsewhere in the Northwest Atlantic. A possible explanation for this is the earlier suggested redistribution of sediment by nepheloid layers at intermediate as well as abyssal depths, transporting material downslope and along the continental margin. These processes potentially contributed to Nd isotope excursions recorded in Northwest Atlantic sediment cores during the Bølling‐Allerød and early Holocene. An Atlantic‐wide comparison of Nd isotope records shows that the early Holocene excursions had an additional contribution from conservative advection of unradiogenic dissolved Nd. Nevertheless, the trends of the Nd isotope records are in general agreement with previous reconstructions of water mass provenance from the entire Atlantic and also reveal millennial‐scale changes during the last deglaciation in temporal high resolution, which have rarely been reported before. Further, the new records confirm that during cold periods the Northwest Atlantic was bathed by an increased contribution of southern sourced water.

The Dynamic History of 220 Million Years of Subduction Below Mexico: A Correlation Between Slab Geometry and Overriding Plate Deformation Based on Geology, Paleomagnetism, and Seismic Tomography

AbstractGlobal tectonic reconstructions of pre‐Cenozoic plate motions rely primarily on paleomagnetic and geological data from the continents, and uncertainties increase significantly with deepening geological time. In attempting to improve such deep‐time plate kinematic reconstructions, restoring lost oceanic plates through the use of geological and seismic tomographical evidence for past subduction is key. The North American Cordillera holds a record of subduction of oceanic plates that composed the northeastern Panthalassa Ocean, the large oceanic realm surrounding Pangea in Mesozoic times. Here we present new paleomagnetic data from subduction‐related rock assemblages of the Vizcaíno Peninsula of Baja California, Mexico, which yield a paleolatitudinal plate motion history equal to that of the North American continent since Late Triassic time. This indicates that the basement rocks of the Vizcaíno Peninsula formed in the forearc of the North American Plate, adjacent to long‐lived eastward dipping subduction at the southern part of the western North American continental margin. Tomographic images confirm long‐lived, uninterrupted eastward subduction. We correlate episodes of overriding plate shortening and extension to flat and steep segments of the imaged slab. By integrating paleomagnetic, geological, and tomographic evidence, we provide a first‐order model that reconciles absolute North American plate motion and the deformation history of Mexico since Late Triassic time with modern slab structure.

Environmental influence on growth history in marine benthic foraminifera

AbstractEnergy availability influences natural selection on the ontogenetic histories of organisms. However, it remains unclear whether physiological controls on size remain constant throughout ontogeny or instead shift as organisms grow larger. Benthic foraminifera provide an opportunity to quantify and interpret the physicochemical controls on both initial (proloculus) and adult volumes across broad environmental gradients using first principles of cell physiology. Here, we measured proloculus and adult test dimensions of 129 modern rotaliid species from published images of holotype specimens, using holotype size to represent the maximum size of all species’ occurrences across the North American continental margin. We merged size data with mean annual temperature, dissolved oxygen concentration, particulate organic carbon flux, and seawater calcite saturation for 718 unique localities to quantify the relationship between physicochemical variables and among-species adult/proloculus size ratios. We find that correlation of community mean adult/proloculus size ratios with environmental parameters reflects covariation of adult test volume with environmental conditions. Among-species proloculus sizes do not covary identifiably with environmental conditions, consistent with the expectation that environmental constraints on organism size impose stronger selective pressures on adult forms due to lower surface area-to-volume ratios at larger sizes. Among-species adult/proloculus size ratios of foraminifera occurring in resource-limited environments are constrained by the limiting resource in addition to temperature. Identified limiting resources are food in oligotrophic waters and oxygen in oxygen minimum zones. Because among-species variations in adult/proloculus size ratios from the North American continental margin are primarily driven by the local environment’s influence on adult sizes, the evolution of foraminiferal sizes over the Phanerozoic may have been strongly influenced by changing oceanographic conditions. Furthermore, lack of correspondence between among-species proloculus sizes and environmental conditions suggests that offspring sizes in foraminifera are rarely limited by physiological constraints and are more susceptible to selection related to other aspects of fitness.

Paleomagnetic constraints on the kinematic relationship between the Guerrero terrane (Mexico) and North America since Early Cretaceous time

The North American Cordillera has been shaped by a long history of accretion of arcs and other buoyant crustal fragments to the western margin of the North American plate since early Mesozoic time. The southernmost accreted terrane is the terrane of southwestern Mexico, a latest Jurassic-Cretaceous volcanic arc built on a Triassic accretionary prism. Interpretations of the origin of the terrane vary: Some authors consider it a far-traveled, exotic intra-oceanic island arc, while others view it as the (par)autochthonous, extended North American continental margin. We present new paleomagnetic and U-Pb zircon data from Lower Cretaceous sedimentary rocks of the terrane. These data show that the terrane has a latitudinal plate-motion history equal to that of the North America plate, both before and after accretion. This confirms paleogeographic models in which the arc successions formed on North American crust that rifted away from the Mexican mainland by approximately east-west opening of a back-arc basin above an eastward-dipping subduction zone. Additionally, it renders alternative paleogeographical models in which the terrane is considered to be exotic to the North American continent unlikely. The phase of back-arc spreading resulted in the short-lived existence of an additional Guerrero tectonic plate between the North American and Farallon plates and, upon closure of the back-arc basin, the growth of the North American continent.

Detrital zircon evidence for Paleoproterozoic West African crust along the eastern North American continental margin, Georges Bank, offshore Massachusetts, USA

Research Article| July 10, 2017 Detrital zircon evidence for Paleoproterozoic West African crust along the eastern North American continental margin, Georges Bank, offshore Massachusetts, USA Yvette D. Kuiper; Yvette D. Kuiper * 1Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, Colorado 80401, USA *E-mail: ykuiper@mines.edu Search for other works by this author on: GSW Google Scholar Margaret D. Thompson; Margaret D. Thompson 2Department of Geosciences, Wellesley College, 106 Central Street, Wellesley, Massachusetts 02481, USA Search for other works by this author on: GSW Google Scholar Sandra M. Barr; Sandra M. Barr 3Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Search for other works by this author on: GSW Google Scholar Chris E. White; Chris E. White 4Department of Natural Resources, P.O. Box 698, Halifax, Nova Scotia B3J 2T9, Canada Search for other works by this author on: GSW Google Scholar J. Christopher Hepburn; J. Christopher Hepburn 5Department of Earth and Environmental Sciences, Boston College, 140 Commonwealth Avenue, Chestnut Hill, Massachusetts 02467, USA Search for other works by this author on: GSW Google Scholar James L. Crowley James L. Crowley 6Department of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Yvette D. Kuiper * 1Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, Colorado 80401, USA Margaret D. Thompson 2Department of Geosciences, Wellesley College, 106 Central Street, Wellesley, Massachusetts 02481, USA Sandra M. Barr 3Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Chris E. White 4Department of Natural Resources, P.O. Box 698, Halifax, Nova Scotia B3J 2T9, Canada J. Christopher Hepburn 5Department of Earth and Environmental Sciences, Boston College, 140 Commonwealth Avenue, Chestnut Hill, Massachusetts 02467, USA James L. Crowley 6Department of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA *E-mail: ykuiper@mines.edu Publisher: Geological Society of America Received: 04 Apr 2017 Revision Received: 11 May 2017 Accepted: 15 May 2017 First Online: 10 Jul 2017 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2017 Geological Society of America Geology (2017) 45 (9): 811–814. https://doi.org/10.1130/G39203.1 Article history Received: 04 Apr 2017 Revision Received: 11 May 2017 Accepted: 15 May 2017 First Online: 10 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Yvette D. Kuiper, Margaret D. Thompson, Sandra M. Barr, Chris E. White, J. Christopher Hepburn, James L. Crowley; Detrital zircon evidence for Paleoproterozoic West African crust along the eastern North American continental margin, Georges Bank, offshore Massachusetts, USA. Geology 2017;; 45 (9): 811–814. doi: https://doi.org/10.1130/G39203.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Basement rocks from the Continental Offshore Stratigraphic Test (COST) No. G-1 petroleum exploration well, Georges Bank, offshore Massachusetts, USA, show the first direct evidence for a fragment of Paleoproterozoic West African crust preserved along the eastern North American continental margin. Detrital zircon from two metawacke samples near the base of the core yielded ca. 3.05–1.90 Ga laser ablation–inductively coupled plasma–mass spectrometry dates, with a predominant 2.2–2.0 Ga population. This detrital zircon signature is consistent with that in the Taghdout Group of the Anti-Atlas region, which is part of the Paleoproterozoic passive margin rocks deposited directly on the Archean–Paleoproterozoic WAC shortly after its formation. African provenance has previously been recognized for the Suwannee and Meguma terranes in the southern and northern Appalachians, respectively. However, these areas represent Neoproterozoic–Paleozoic sedimentary basins, deposited well over a billion years after formation of the Paleoproterozoic WAC and cover rocks, including those in the COST No. G-1 well. The Paleoproterozoic rocks in the COST No. G-1 well were likely amalgamated with the North American margin during final closure of the Rheic Ocean and assembly of Pangea near the end of the Paleozoic. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structural transect through Ellesmere Island (Canadian Arctic): superimposed Palaeozoic Ellesmerian and Cenozoic Eurekan deformation

Abstract The 400 km long transect through Ellesmere Island is located perpendicular to the North American continental margin between the Arctic Ocean in the NNW and the Greenland–Canadian Shield in the SSE. It provides an insight into the structural architecture and tectonic history of the upper parts of the continental crust. The northernmost segment of the transect is dominated by the composite Pearya Terrane, which amalgamated with the Laurentian margin during the Late Devonian–Early Carboniferous Ellesmerian Orogeny. The Neoproterozoic to Devonian Franklinian Basin is exposed south of the terrane boundary and most probably overlies the crystalline basement of the Greenland–Canadian Shield. The structures along the transect in this area are dominated by kilometre-scale Ellesmerian folding of the Franklinian Basin deposits above a deep-seated detachment, which is suggested to be located at the boundary between the basement of the Canadian Shield and the overlying >8 km thick Franklinian Basin. Following the development of the Late Mississippian–Palaeogene Sverdrup Basin, the complex Eurekan deformation reactivated Ellesmerian thrust faults and probably parts of the associated deep-seated detachment. In addition, large Eurekan strike-slip faults affected and displaced pre-Eocene deposits and tectonic structures, particularly in the northern part of the transect.

Physicochemical controls on biogeographic variation of benthic foraminiferal test size and shape

AbstractThe sizes and shapes of marine organisms often vary systematically across latitude and water depth, but the environmental factors that mediate these gradients in morphology remain incompletely understood. A key challenge is isolating the individual contributions of many, often correlated, environmental variables of potential biological significance. Benthic foraminifera, a diverse group of rhizarian protists that inhabit nearly all marine environments, provide an unparalleled opportunity to test statistically among the various potential controls on size and volume–to–surface area ratio. Here, we use 7035 occurrences of 541 species of Rotallid foraminifera across 946 localities spanning more than 60 degrees of latitude and 1600 m of water depth around the North American continental margin to assess the relative influences of temperature, oxygen availability, carbonate saturation, and particulate organic carbon flux on their test volume and volume–to–surface area ratio. For the North American data set as a whole, the best model includes temperature and dissolved oxygen concentration as predictors. This model also applies to data from the Pacific continental margin in isolation, but only temperature is included in the best model for the Atlantic. Because these findings are consistent with predictions from the first principles of cell physiology, we interpret these statistical associations as the expressions of physiological selective pressures on test size and shape from the physical environment. Regarding existing records of temporal variation in foraminiferal test size across geological time in light of these findings suggests that the importance of temperature variation on the evolution of test volume and volume–to–surface area ratio may be underappreciated. In particular, warming may have played as important a role as reduced oxygen availability in causing test size reduction during past episodes of environmental crisis and is expected to inflict metabolic stress on benthic foraminifera over the next century due to anthropogenic climate change.

An orogenic wedge model for diachronous deformation, metamorphism, and exhumation in the hinterland of the northern Canadian Cordillera

Development of amphibolite-facies transposition fabrics in the northern Canadian Cordilleran hinterland occurred diachronously in the Permian–Triassic, Early Jurassic, Middle Jurassic to Early Cretaceous, and Early to mid-Cretaceous. Rocks tectonized in the Permian–Triassic and Early Jurassic were exhumed in the Early Jurassic, while rocks immediately to the northeast (toward the foreland) were not buried and heated until the Middle Jurassic to mid-Cretaceous. Early Jurassic to mid-Cretaceous emplacement of the Yukon-Tanana terrane on the North American continental margin, together with the imbrication of parautochthonous rocks, formed a foreland-propagating orogenic wedge. Cooler rocks in front of the wedge were progressively buried and metamorphosed to amphibolite facies from the Jurassic to mid-Cretaceous as they were underthrust into a spatially and temporally transient distributed ductile shear zone near the base of the overriding wedge. Rocks previously incorporated into this zone were displaced upward and exhumed through the combined effects of renewed underplating at depth and compensating extensional and erosional denudation above to maintain a critically tapered wedge. Extensional exhumation of the metamorphic hinterland in the mid-Cretaceous marked the collapse and end of orogen-perpendicular wedge dynamics in operation since the Early Jurassic. Rocks incorporated into the midcrustal shear zone in the Middle Jurassic to mid-Cretaceous were exhumed in the mid-Cretaceous along southeast-directed (orogen-parallel) extensional faults from beneath a supracrustal “lid” tectonized in the Permian–Triassic and Early Jurassic. Like the Himalayan orogen and eastern Alps, orogen-parallel extension developed in an orthogonal plate-convergent setting, simultaneous with, and bounded by, orogen-parallel strike-slip faulting that facilitated northwestward lateral extrusion of rocks normal to the direction of convergence.

Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Alaska: Zircon U-Pb geochronology, whole-rock geochemistry, and Pb isotopes

The Mesozoic magmatic history of the North American margin records the evolution from a more segmented assemblage of parautochthonous and allochthonous terranes to the more cohesive northern Cordilleran orogenic belt. We characterize the setting of magmatism, tectonism, and epigenetic mineralization in the western Fortymile mining district, east-central Alaska, where parautochthonous and allochthonous Paleozoic tectonic assemblages are juxtaposed, using sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon geochronology, whole-rock geochemistry, and feldspar Pb isotopes of Mesozoic intrusions and spatially associated mineral prospects. New SHRIMP U-Pb zircon ages and published U-Pb and 40 Ar/ 39 Ar ages indicate four episodes of plutonism in the western Fortymile district: Late Triassic (216–208 Ma), Early Jurassic (199–181 Ma), mid-Cretaceous (112–94 Ma), and Late Cretaceous (70–66 Ma). All age groups have calc-alkalic arc compositions that became more evolved through time. Pb isotope compositions of feldspars from Late Triassic, Early Jurassic, and Late Cretaceous igneous rocks similarly became more radiogenic with time and are consistent with the magmas being mantle derived but extensively contaminated by upper crustal components with evolving Pb isotopic compositions. Feldspar Pb isotopes from mid-Cretaceous rocks have isotopic ratios that indicate magma derivation from upper crustal sources, probably thickened mid-Paleozoic basement. The origin of the mantle component in Late Cretaceous granitoids suggested by Pb isotopic ratios is uncertain, but we propose that it reflects asthenospheric upwelling following slab breakoff and sinking of an inactive inner subduction zone that delivered the previously accreted Wrangellia composite terrane to the North American continental margin, after the outer Farallon subduction zone was established. Epigenetic Pb-Zn-Ag ± Cu prospects in the western Fortymile district are spatially associated with splays of the northeast-trending Kechumstuk sinistral-normal fault zone and with ca. 68–66 Ma felsic intrusions and dikes. The similarity between Pb isotope compositions of feldspars from the Late Cretaceous igneous bodies and sulfides from the epithermal prospects suggests a Late Cretaceous age for most of the mineralization. Fluid flow along the faults undoubtedly played a major role in mineralization. We interpret displacement on the northeast-trending faults to be a far-field effect of dextral translation along Late Cretaceous plate-scale boundaries and faults that were roughly parallel to the subsequently developed Denali and Tintina fault systems, which currently bound the region.

Detrital zircon geochronology of Neoproterozoic–Lower Cambrian passive-margin strata of the White-Inyo Range, east-central California: Implications for the Mojave–Snow Lake fault hypothesis

Correlation of lithotectonic packages across major transcurrent structures is critical to understanding the tectonic evolution of the North American continental margin. Detrital zircon geochronology of uppermost Proterozoic to Lower Paleozoic miogeoclinal strata from the White-Inyo Mountains permits evaluation of: (1) the age and provenance of these metasediments and (2) a model for truncation of the passive margin along a postulated large-magnitude Cretaceous dextral shear zone, i.e., the Mojave–Snow Lake fault. U-Pb ages of detrital zircons from the Neoproterozoic Wyman Formation, the uppermost Proterozoic Reed Dolomite (Hines Tongue Member), and clastic strata of the Lower Cambrian Deep Springs, Campito (Montenegro Member), Poleta, and Harkless formations reflect ultimate derivation from the adjacent 1.7–pre-1.8 Ga Mojavia terrane and/or 1.7–1.8 Ga Yavapai continental basement, with subsidiary sources in both the ca. 1.4 Ga Yavapai-Mazatzal anorogenic granitoids and the >2.5 Ga North American craton, and a small proportion of 1.0–1.3 Ga grains, most likely reworked from Grenville clastic wedge deposits. Detrital zircon age spectra from the Lower Cambrian Andrews Mountain Member of the Campito Formation are unique in comparison with the remainder of the studied section, containing a major age peak centered at ca. 1.1 Ga and a subsidiary Lower Cambrian age peak, permitting calculation of a ca. 527 ± 12 (2σ) Ma maximum depositional age. These features, in addition to abundant detrital magnetite-ilmenite grains, reflect a distal source for these rocks, most likely from the ca. 1.1 Ga Pikes Peak batholith and/or the Midcontinent rift and ca. 0.53 Ga bimodal intrusions of the Oklahoma-Colorado aulacogen. In terms of zircon age distribution, allochthonous metamorphic pendants in the Snow Lake terrane of the central Sierra Nevada batholith are most similar to those of stratigraphically equivalent units in the Death Valley region and, to lesser degrees, the White-Inyo section, and the Mojave Desert region. Given the similarity and relative proximity of Death Valley facies assemblages to the Snow Lake terrane, we suggest that the latter was not transported northward from the Mojave Desert region and instead represents footwall assemblages of a late Early to early Middle Jurassic low-angle normal fault system, probably along the outer transform-truncated margin of the Last Chance thrust stack. This model implies a few tens of kilometers of offset, in contrast to the hundreds of kilometers required by the Mojave–Snow Lake fault hypothesis.