Crystalline Terrane Research Articles

A Cascadia Slab Model From Receiver Functions

AbstractWe map the characteristic signature of the subducting Juan de Fuca and Gorda plates along the entire Cascadia forearc from northern Vancouver Island, Canada, to Cape Mendocino in northern California, USA, using teleseismic receiver functions. The subducting oceanic crustal complex, possibly including subcreted material, is parameterized by three horizons capable of generating mode‐converted waves: a negative velocity contrast at the top of a low velocity zone underlain by two horizons representing positive contrasts. The amplitude of the conversions varies likely due to differences in composition and/or fluid content. We analyzed the slab signature for 298 long‐running land seismic stations, estimated the depth of the three interfaces through inverse modeling and fitted regularized spline surfaces through the station control points to construct a margin‐wide, double‐layered slab model. Crystalline terranes that act as the static backstop appear to form the major structural barrier that controls the slab morphology. Where the backstop recedes landward beneath the Olympic Peninsula and Cape Mendocino, the slab subducts sub‐horizontally, while the seaward‐protruding and thickened Siletz terrane beneath central Oregon causes steepening of the slab. A tight bend in slab morphology south of the Olympic Peninsula coincides with the location of recurring large intermediate depth earthquakes. The top‐to‐Moho thickness of the slab generally exceeds the thickness of the oceanic crust by 2–12 km, suggesting thickening of the slab or underplating of slab material to the overriding North American plate.

Structural controls on bedrock weathering in crystalline basement terranes and its implications on groundwater resources

Crystalline basement rock aquifers underlie more than 20% of the earth’s surface. However, owing to an inadequate understanding of geological structures, it is challenging to locate the groundwater resources in crystalline hard rock terranes. In these terranes, faults, fractures, and shear zones play an important role in bedrock weathering and ultimately groundwater storage. This study integrates important geological structures with 2D high-resolution subsurface resistivity images in understanding the factors that influenced bedrock weathering and groundwater. The results reveal the variability of weathered zone depth in different structural zones (Zone-I to Zone-IV). This is due to the presence of foliations, fractures, and faults. A thicker weathered zone develops when a fracture/fault overprints a pre-existing planar pervasive structure like foliations (Zone-II) as compared to zones only with faults/fractures (Zone-III). Further, the transmissivity of boreholes also shows relatively higher in Zone-II than Zone-III, which implies a good pact between different structural features and possible groundwater storage. The study also demonstrates the role of paleostress and different tectonic structures influencing the depth of the “Critical Zone”. While the geology may vary for different structural terranes, the approach presented in this paper can be readily adopted in mapping bedrock weathering and groundwater resources in crystalline basement terranes globally.

Machine Learning Applied to a Modern‐Pleistocene Petrographic Data Set: The Global Prediction of Sand Modal Composition (GloPrSM) Model

AbstractPetrography has long been used as a tool to decipher the sedimentary provenance of sand and sandstone based on relative proportions of detrital grain types. However, there is not yet a predictive framework that constrains the relationship between sand grain composition and the factors that influence it, including bedrock lithology, elevation, slope, basin area, precipitation, and temperature. We used a globally extensive modal point‐count data set of 3,522 Pleistocene to modern sand samples to train an inline series of random forest meta‐estimators to predict relative abundances of quartz (Q), feldspar (F), and lithic (L) grain types and eight subcompositions. The trained random forest models were fit to a global data set of fluvial watersheds to produce the global prediction of sand modal composition (GloPrSM) model which predicts Q‐F‐L of fluvially transported sediment. GloPrSM predicts quartz enrichment in low latitudes (35°N to 35°S), feldspar enrichment near plutonic and metamorphic crystalline terranes in middle to high latitudes, and lithic enrichment near modern and ancient collisional plate boundaries and flood basalts. Our analyses reveal that slope, temperature, and the relative abundance of exposed metamorphic and felsic to intermediate plutonic rocks are the most important predictors of Q‐F‐L composition. In addition, temperatures higher than 15°C and large drainage areas appear to promote quartz enrichment, while steeply sloping environments favor lithic enrichment. The GloPrSM model represents the first global‐scale estimate of fluvial sand compositional modes and illustrates that the spatial distribution of Q‐F‐L at Earth's surface can be predicted from the first‐order factors that generate sediment.

40Ar/39Ar dating of Paleoproterozoic shear zones in the Ellesmere–Devon crystalline terrane, Nunavut, Canadian Arctic

Paleoproterozoic gneisses of the Ellesmere–Devon crystalline terrane on southeastern Ellesmere Island are deformed by metre-scale, east-striking mylonite zones. The shear zones commonly offset pegmatitic dikes and represent the last episode of ductile deformation. Samples were dated by the 40Ar/39Ar step-heating method to put an upper limit on the time of deformation. Biotite from one tonalitic protolith and five shear zones give geologically meaningful results. Clusters of unoriented biotite grains pseudomorph granulite-facies orthopyroxene in some of the weakly deformed gneisses, whereas the shape-preferred orientation of biotite defines the mylonitic fabric. The intrusive age of the tonalitic protolith is 1958 ± 12 Ma, based on previous U–Pb dating of zircon. 40Ar/39Ar analysis of biotite from the same sample gave a plateau age of 1929 ± 23 Ma, which is interpreted as cooling from regional granulite facies metamorphism. Three nearby samples of mylonitic tonalite have 40Ar/39Ar ages in the range of ≈1870–1840 Ma. Biotite from two granitic mylonites over 80 km away return high-resolution Ar spectra in the same range, implying that widespread ductile shearing occurred at ≈1870–1840 Ma, or ≈90 million years after cooling from regional metamorphism. Although the 2.0–1.9 Ga gneisses of southeastern Ellesmere Island correlate with the Inglefield Mobile Belt in North-West Greenland and the Thelon Tectonic Zone, the late shear zones are superimposed on that juvenile arc long after the 1.97 Ga Thelon orogeny.

The deep structure of the Nepa‐Peleduy arch of the Nepa‐Botuoba anteclise in the development of B.A. Sokolov's hypothesis

The studies were conducted in the territory of the Nepa‐Peleduy arch (NPA) of the Nepa‐Botuoba anteclise (NBA), which is strategically important for the Russian Federation as it contains large and unique hydrocarbon fields, including those essential for the ESPO oil pipeline and the presently under‐construction Power of Siberia gas pipeline. Because the hydrocarbon potential of the sedimentary cover is largely exhausted, the aim is now to increase the regional stocks. Verifying the hypothesis of the Corresponding Member of the Russian Academy of Sciences B.A. Sokolov, stating that the allochthonous block of the crystalline basement, overlapping the aulacogen Riphean sediments, lies at the base of the NPA, would open the possibility of increasing the hydrocarbon reserves in the ‘subbasement’ formations. It would also clarify the controversial nature of the NPA deposits, which have no sediments with a high organic carbon content. In this study, we present a Hilbert transform of a time section of the common midpoint method (CDP) from a fragment of the Batholith‐1 geotraverse passing through the Katanga saddle, the NPA and the Pre‐Patom trough, as well as geophysical materials and well data from the same profile. Supporting B.A. Sokolov’s hypothesis, we confirm the existence of a relict crystalline terrane crosscut by the wells at the base of the NPA and forming the roof of the underlying lower Vendian and Riphean sediment units. Our data clearly document the position of the western side of the inverted paleorift, the thickness of the terrane and the physical characteristics of the underlying sediments. These results are justifying hydrocarbon exploration of the ‘subbasement’ sediments.

The inception of the Maliac Ocean: Regional geological constraints on the western Circum-Rhodope belt (northern Greece)

Abstract Considerable debate surrounds the early-Alpine paleogeographic significance of the Circum-Rhodope belt (CRB, Greece), in particular, whether it was a rifted section of the southern Euroasian foreland, or of the late Mesozoic back-arc system of the Vardar Ocean or of both. In the attempt to clarify the complex nature of the CRB, nine new lithostratigraphic formations (with its members) in consideration of the spatiotemporal and regional geological context are constrained by tracing the changes in regional plate interactions and linked magmato-sedimentary record. The analysis of the stratigraphic data in combination with the highly complex, yet poorly constrained regional geology is employed to reconstruct the late Permian, Triassic, and latest Jurassic developments of this part of the southern European foreland. A continuum between the rifting and Triassic opening of the Maliac Ocean, the Carnian Eo-Cimmerian docking, is supported by the consolidated new data embedded into a collection of the nine distinct lithostratigraphic formations. The terminal Vardar plate approach during the latest Jurassic - earliest Cretaceous and the convergence with the investigated portion positioned to the south of the Balkan-Hellenic foreland thrust belt configured this regional Neotethyan flysch trough, whereas the Paleocene - Eocene east-west shortening and nappe stacking ejected the incoherent early Alpine system referred to as the CRB. The ejected Permo-Triassic, including the early Jurassic ophiolite-bearing formations of the CRB, are matched to be in line with those of the regional Southeast European importance. In fact, the results place the western CRB or eastern Maliac Ocean to lie at the distal southern margin (promontory) of the Eurasian craton aligned with the early- and late Paleozoic crystalline terranes: Paleozoic to Lower Triassic Variscan basement (segment of the Pelagonian Unit), “Veles Series” and Lower Complex of the Serbo-Macedonian Unit (Vertiskos Unit).

Tectono-stratigraphic evolution of the Southern Campania Margin: a key area for the evolution of the Tyrrhenian-Apennine system

The Southern Campania Margin (SCM) represents a key area of the Central Mediterranean because it records all the rifting stages of the Tyrrhenian Sea. The interpretation of a seismic dataset calibrated with deep wells and outcrops, using seismic stratigraphy and structural geology methods in a dedicated Geographic Information System (GIS) environment, the seismic depth conversion, the generation of 2-D and 3-D models led to the reconstruction of a polyphased tectono-stratigraphic evolution of the SCM. During the early stage of Tyrrhenian opening a terrigenous transtensional Basin (Langhian-Tortonian Cilento Basin) formed on the Liguride accretionary prism adjacent to the Calabria crystalline terrane. In the SCM the Liguride thrust sheets tectonically overly the Apennine Platform units and both these nappes have been dismembered by Quaternary faults. Three rifting stages, not homogeneously distributed, affected the region since the Lower Pleistocene. They are associated to the deposition of a thick Quaternary succession (A, B and C units). During these Pleistocene stages there was an abrupt change of the extension direction (from NE-SW to NW-SE) accompanying a change of the nappe transport direction of the Southern Apennines. The construction of balanced sections using dedicated software, permitted us to recognize the true geometry of the faults and compute the amount of Quaternary extension of the SCM that results comparable to those calculated for other sectors of the Tyrrhenian margin and further extensional regions worldwide.

Glacial erosion of East Antarctica in the Pliocene: A comparative study of multiple marine sediment provenance tracers

The history of the East Antarctic ice sheet provides important understanding of its potential future behaviour in a warming world. The provenance of glaciomarine sediments can provide insights into this history, if the underlying continent eroded by the ice sheet is made of distinct geological terranes that can be distinguished by the mineralogy, petrology and/or geochemistry of the eroded sediment. We here present a multi-proxy provenance investigation on Pliocene sediments from Integrated Ocean Drilling Program (IODP) Site U1361, located offshore of the Wilkes Subglacial Basin, East Antarctica. We compare Nd and Sr isotopic compositions of <63μm detrital fractions, clay mineralogy of <2μm fractions, 40Ar/39Ar ages of >150μm ice-rafted hornblende grains, and petrography of >2mm ice-rafted clasts and >150μm mineral grains. Pliocene fine-grained marine sediments have Nd and Sr isotopic compositions, clay mineralogy, and clast characteristics that can be explained by mixing of sediments eroded from predominantly proximal crystalline terranes with material derived from inland sources from within the currently glaciated Wilkes Subglacial Basin. Conversely, evidence for such an inland source is absent from ice-rafted hornblende ages. We render a lithological bias against hornblende grains in the doleritic and sedimentary units within the basin the most likely explanation for this observation. 40Ar/39Ar hornblende ages however record additional provenance from the distal margins of the Ross Sea, and possibly even the West Antarctic area of Marie Byrd Land. The latter lies >2000km to the east and hints at significant iceberg release from the West Antarctic ice sheet during warm intervals of the Pliocene. Together our results make a strong case for combining geochemical and mineralogical signatures of coarse- and fine-grained glaciomarine sediment fractions in order to derive robust provenance interpretations in ice covered areas.

Petro-structural geology of the Eastern Aspromonte Massif crystalline basement (southern Italy-Calabria): an example of interoperable geo-data management from thin section – to field scale

The presented maps provide an example of the synoptic display of multi-scale geological features characterizing the tectono-metamorphic evolution of the crystalline basement terranes in a poly-orogenic-multistadial evolutionary scenario. The petro-structural map of the eastern Aspromonte Massif (southern Calabrian Peloritani Orogen) is characterized by a nappe-like edifice composed by the superimposition of three crystalline basement tectonic units which, sharing the same Alpine-Apennine reworking, underwent a different metamorphic evolution, mostly derived from the Hercynian orogenic cycle. This geological framework is completed by the suture deposition of a Oligocene-Miocene syn-orogenic clastic formation, partly roofed by the back-thrusting of a clay-rich mélange. In order to understand the potential relationships within the complex dataset deriving from the geological investigations of this crystalline basement area, a geo-database, capable of handling multi-scale information from field-derived structural data (i.e. foliation, lineation, fold- and fault-related structures) to micro-scale derived ones (i.e. thin-section analysis, electron microscope and microprobe investigations, thermodynamic modeling outputs) has been constructed, according to the international standards using the Geo-Scientific Markup Language developed by the Commission for the Management and Application of Geo-science Information. Proposed outputs will show the relationships between field-related geological features, showing collected samples and the subsequent laboratory investigations. These are fundamental to achieving reliable results in geological contexts, such as those for reconstructing the tectono-metamorphic evolution of crystalline basement terrains.

Salinia revisited: a crystalline nappe sequence lying above the Nacimiento fault and dispersed along the San Andreas fault system, central California

Salinia revisited: a crystalline nappe sequence lying above the Nacimiento fault and dispersed along the San Andreas fault system, central California

A two-stage exhumation of the Variscan crust: U-Pb LA-ICP-MS and Rb-Sr ages from Greater Kabylia, Maghrebides

The significance and role of major shear zones are paramount to understanding continental deformation and the exhumation of deep crustal levels. LA-ICPMS U–Pb dating of monazites, combined with Rb–Sr analyses of biotites, from an anatectic metapelite from Greater Kabylia (Algeria) highlights the history of shear zone development and the subsequent exhumation of deep crustal levels in the internal zones of the Maghrebides. Monazites give an age of 275.4 ± 4.1 Ma (2σ) dating the thermal peak coeval with anatexis. This age is identical to those recorded in other crystalline terranes from south-easternmost Europe (i.e. South Alpine and Austro-Alpine domains) that suffered crustal thinning during the continental rifting predating the Tethys opening. Rb–Sr analyses of biotites yield a cooling age of 23.7 ± 1.1 Ma related to the exhumation of the buried Variscan crust during the Miocene as an extrusive slice, roughly coeval with the emplacement of nappes, and shortly followed by lithospheric extension leading to the opening of the Alboran sea.

Cretaceous and Cenozoic cooling history across the ultrahigh pressure Tongbai–Dabie belt, central China, from apatite fission-track thermochronology

The crystalline terrane of the Tongbai–Dabie region, central China, comprising the Earth's largest ultrahigh-pressure (UHP) exposure was formed during Triassic collision between the Sino–Korean and Yangtze cratons. New apatite fission-track (AFT) data presented here from the UHP terrane, extends over a significantly greater area than reported in previous studies, and includes the (eastern) Dabie, the Hong'an (northwestern Dabie) and Tongbai regions. The new data yield ages ranging from 44 ± 3 to 142 ± 36 Ma and mean track lengths between ∼10 and 14.4 μm. Thermal history models based on the AFT data taken together with published 40Ar/ 39Ar, K–Ar, apatite and zircon (U–Th)/He and U–Pb data, exhibit a three-stage cooling pattern that is similar across the study region, commencing with an Early Cretaceous rapid cooling event, followed by a period of relative thermal stability during which rocks remained at temperatures within the AFT partial annealing zone (∼60–110 °C) and ending with a possible renewed phase of accelerated cooling during Pliocene to Recent time. The first cooling phase followed large-scale transtensional deformation between ∼140 and 110 Ma and is related to Early Cretaceous eastward tectonic escape and Pacific back arc extension. Between this phase and the subsequent slow cooling phase, a transition period from ∼120 to 80 Ma (to ∼70 to 45 Ma along the Tan–Lu fault) was characterised by a relatively low cooling rate (∼3–5 °C/Ma). This transition is likely related to a tectonic response associated with the mid-Cretaceous subduction of the Izanagi–Pacific plate as well as lithospheric extension and thinning in eastern Asia. The present regional AFT age pattern is therefore basically controlled by the Early Cretaceous rapid cooling event, but finally shaped through active Cenozoic faulting. Following the transition phase the subsequent slow cooling phase pattern implies a net reduction in horizontal compressional stress corresponding to increased extension rates along the continental margin due to the decrease in plate convergence. Modelling of the AFT data suggests a possible Pliocene–Recent cooling episode, which may be supported by increased rates of sedimentation observed in adjacent basins. This cooling phase may be interpreted as a response to the far-field effects of the frontal India–Eurasia collision to the west. Approximate estimates suggest that the total amount of post ∼120 Ma denudation across the UHP orogen ranged from ∼2.4 to 13.2 km for different tectonic blocks and ranged from ∼0.8 to 9.7 km during the Cretaceous to between ∼1.7 and 3.8 km during the Cenozoic.

Shaping the Australian crust over the last 300 million years: Insights from fission track thermotectonic imaging and denudation studies of key terranes

Apatite fission track thermochronology is a well‐established tool for reconstructing the low‐temperature thermal and tectonic evolution of continental crust. The variation of fission track ages and distribution of fission track lengths are primarily controlled by cooling, which may be initiated by earth movements and consequent denudation at the Earth's surface and/or by changes in the thermal regime. Using numerical forward‐modelling procedures these parameters can be matched with time‐temperature paths that enable thermal and tectonic processes to be mapped out in considerable detail. This study describes extensive Australian regional fission track datasets that have been modelled sequentially and inverted into time‐temperature solutions for visualisation as a series of time‐slice images depicting the cooling history of present‐day surface rocks during their passage through the upper crust. The data have also been combined with other datasets, including digital elevation and heat flow, to image the denudation history and the evolution of palaeotopography. These images provide an important new perspective on crustal processes and landscape evolution and show how important tectonic and denudation events over the last 300 million years can be visualised in time and space. The application of spatially integrated denudation‐rate chronology is also demonstrated for some key Australian terranes including the Lachlan and southern New England Orogens of southeastern Australia, Tasmania, the Gawler Craton, the Mt Isa Inlier, southwestern Australian crystalline terranes (including the Yilgarn Craton) and the Kimberley Block. This approach provides a readily accessible framework for quantifying the otherwise undetectable, timing and magnitude of long‐term crustal denudation in these terranes, for a part of the geological record previously largely unconstrained. Discrete episodes of enhanced denudation occurred principally in response to changes in drainage, base‐level changes and/or uplift/denudation related to far‐field effects resulting from intraplate stress or tectonism at plate margins. The tectonism was mainly associated with the history of continental breakup of the Gondwana Supercontinent from Late Palaeozoic time, although effects related to compression are also recorded in eastern Australia. The results also suggest that the magnitude of denudation of cratonic blocks has been significantly underestimated in previous studies, and that burial and exhumation are significant factors in the preservation of apparent ‘ancient’ features in the Australian landscape.

Early Cretaceous Normal Faulting in Southern New England: Evidence from Apatite and Zircon Fission‐Track Ages

New apatite and zircon fission‐track (AFT, ZFT) ages from Mesozoic sediments and adjacent crystalline rocks from southern New England reveal age gradients from Middle Jurassic to Early Cretaceous. These gradients reflect the rotation of crustal blocks after the setting of the youngest AFT ages (∼140 Ma). The AFT ages of 168–98 Ma for 32 samples of Paleozoic metamorphic rocks east and west and within the Hartford Basin of Massachusetts and Connecticut indicate that unroofing in these regions occurred from Late Jurassic through Early Cretaceous. In both the Hartford Basin and rocks from crystalline terranes east of the basin, AFT ages show a regional trend of increasing age to the east, suggesting a down‐to‐the‐east rotation of ∼10° in each area. The ZFT ages from the Hartford Basin arkoses (167–238 Ma) and northern Bronson Hill (147–196 Ma) rocks support this gradient of eastward increase in AFT age. A south‐to‐north gradient of decreasing AFT age (139–107 Ma) for the sedimentary rocks in the Hartford and Deerfield Basins of Connecticut and Massachusetts suggests a 1°‐hinged uplift to the north that postdates the youngest AFT age of ∼100 Ma. Final juxtaposition of AFT ages across the eastern Border fault between the Hartford Basin and Bronson Hill terrane in Massachusetts and Connecticut indicates displacement younger than the youngest AFT ages of ≤100 Ma (Late Cretaceous). Thus, the age of the graben structure of the Hartford Basin is Cretaceous, and this structure cannot be cited as evidence that these basins are Early Mesozoic “rift” basins.

Fracture chronology and neotectonic control of water wells location in crystalline terranes: an example from the Equador region, northeasternmost Brazil

The brittle deformation in the Equador area, in the center-south domain of the Rio Grande do Norte State, NE Brazil, was analyzed to establish the chronological sequence of the different tectonic episodes and their stress fields, emphasizing the neotectonic activity by structural and morphotectonic criteria. The aim of this study was to test the relationship between local and/or regional neotectonic stress field and the yield of water wells drilled in the region. The local neotectonic structures and morphotectonic features agree with the regional stress field, defined by E-W compression (S 1 or S H ) and N-S extension (s 3 or S h .). The current kinematics of the fracture sets, with an open or closed behavior, seems to be related to the neotectonic stresses and to be highly influential upon the water-yielding potential of the fissural aquifer. Besides other controls, water wells located along roughly E-W trending fractures that act as extension joints have a higher hydrological potential than those along N-S fractures, which tend to be closed. NE-or NW-trending old fractures tend to reactivate by shearing. In this region, EW tension joints generated at end of the Brasiliano Cycle (filled with pegmatite or quartz veins), were reactivated under the N-S to NNW extensional deformation related to rifting and development of the Cretaceous basins. The same N-S extension direction is once again active as part of the Holocene stress field. In the Caicara area, 2 km NW of Equador, the wells drilled in quartzite and metaconglomerate, located along E-W (± 20°) extension fractures and NE or NW shear fractures have been pumped in a full-time regime since the last 20 years, without major variations in their dynamic levels. On the other hand, wells located along longitudinal, N-S fractures or in poorly fractured sites frequently have very low yields.

Structural and neotectonic criteria for location of water wells in semi-arid crystalline terrains: a preliminary approach in the eastern domain of Rio Grande do Norte state, northeast Brazil

The traditional methods to locate water wells in crystalline terranes are being re-evaluated aiming to improve the success rate of the procedures. We describe the preliminary approach being used in the eastern crystalline domain of Rio Grande do Norte (NE Brazil),encompassing areas around Santo Antonio and Santa Cruz cities. Emphasis is given to re-evaluate the classical structural criteria in the light of modern concepts on rock deformation and the regional tectonic framework, especially in the context of the neotectonic stress regime. At a first step, the structural analysis is applied to better understand the formation and/or reactivation of brittle structures, their chronology, and past and present kinematics. Then, these informations are used to understand water flow and accumulation in fracture aquifers, and the reasons for success or failure in the location of water wells already drilled in the region, in the hope to establish a more consistent and practical routine methods. Lineament maps of the studied areas were generated to determine the main fracture sets for well location. Structural data were collected near selected water wells, locally combined with'geophysical data. Neotectonic reactivation is evaluated both from regional and local information, as available. In some cases, as in Tararaca Farm near Santo Antonio, a correlation was found between a fracture zone and a photointerpreted lineament following a drainage; the favorable orientation of the fractures, at a high angle to the present (neotectonic) extension direction, appears to convey an important control in water accumulation and well production. In other cases, as in the Santa Rita Farm near Santa Cruz, the photolineament and the drainage do not correspond to a fracture zone; and the accumulation of water is related to the opening of foliation surfaces by weathering, its intersection with different fracture sets and favorable recharge conditions provided by locally thicker, alluvial covers, forming a through-like structure.

STRUCTURAL AND NEOTECTONIC CRITERIA FOR LOCATION OF WATER WELLS IN SEMI-ARID CRYSTALLINE TERRAINS: A PRELIMINARY APPROACH IN THE EASTERN DOMAIN OF RIO GRANDE DO NORTE STATE, NORTHEAST BRAZIL

The traditional methods to locate water wells in crystalline terranes are being re-evaluated aiming to improve the success rate of the procedures. We describe the preliminary approach being used in the eastern crystalline domain of Rio Grande do Norte (NE Brazil), encompassing areas around Santo Antônio and Santa Cruz cities. Emphasis is given to re-evaluate the classical structural criteria in the light of modern concepts on rock deformation and the regional tectonic framework, especially in the context of the neotectonic stress regime. At a first step, the structural analysis is applied to better understand the formation and/or reactivation of brittle structures, their chronology, and past and present kinematics. Then, these informations are used to understand water flow and accumulation in fracture aquifers, and the reasons for success or failure in the location of water wells already drilled in the region, in the hope to establish a more consistent and practical routine methods. Lineament maps of the studied areas were generated to determine the main fracture sets for well location. Structural data were collected near selected water wells, locally combined with geophysical data. Neotectonic reactivation is evaluated both from regional and local information, as available. In some cases, as in Tararaca Farm near Santo Antônio, a correlation was found between a fracture zone and a photointerpreted lineament following a drainage; the favorable orientation of the fractures, at a high angle to the present (neotectonic) extension direction, appears to convey an important control in water accumulation and well production. In other cases, as in the Santa Rita Farm near Santa Cruz, the photolineament and the drainage do not correspond to a fracture zone; and the accumulation of water is related to the opening of foliation surfaces by weathering, its intersection with different fracture sets and favorable recharge conditions provided by locally thicker, alluvial covers, forming a through-like structure.

Anhydride Theory - A Theory of Petroleum And Coal Generation

Abstract Methane pervades deep mines in crystalline, volcanic, and sedimentary terranes. It appears to effuse ubiquitously from the earth's interior. The author advances the theory that this methane provides hydrogen and energy for hyperthermophyllic bacteria and archaea, thus incidentally generating petroleum and adding carbon in the coalification of peat. Alkane and other hydrocarbon mixtures that make up petroleum from the stripped methane are "anhydrides." The "terminal anhydride," pure carbon, is the additional carbon that changes peat to coal in the coalification process or deposits as asphaltite. The paper addresses and answers the following question in the negative, "Does coal generate methane?" It concludes, on the contrary, that methane generates coal. It addresses the question of whether petroleum is "abiogenic" or "biogenic" with the conclusion that petroleum is largely a biogenic derivation from abiogenic methane. Introduction The idea that higher carbon numbered hydrocarbons might be generated from methane or that carbon might have been added to peat in coalification, have not been generally considered in geology. Let us review the evolution of thinking that has led to the present outlook and the options for redirection of current thinking. In antiquity the Greeks named the oil they found in rocks petroleum to distinguish it from oil obtained by compressing olives or rendering fish livers or animal fat. To them petroleum was evidently "abiogenic," a seemingly simple observation, and coal was "the stone that burns," also a simple declaration of observed fact. Nineteenth century theorists, understanding fossils and, knowing one cannot compress or render rock to obtain oil, took biogenesis from fossil biomass to be the most likely origin of petroleum. Their idea of "fossil fuel" was a theory, no more, and it was not accepted by all, in particular, Mendeleev(1), the most capable biochemist of the day. Plant macerals in coal led coal miners to regard coal as compressed plant material, thus another "fossil fuel." These two simplistic ideas have prevailed to our day despite objections from many prominent scientists. Canadian geologist and engineer, Eugene Coste, was one such scientist. In 1900 he based a successful oil exploration program on his deduction that petroleum was rising through great fractures in the earth under Ontario,(2,3) and in 1903 he published a 56-page paper in which he made the point that natural gas had a volcanic origin. He was roundly opposed and the fossil origin for petroleum left to prevail unopposed until our day. Changes in thinking started when hydrocarbons were identified in the gaseous envelopes of the outer planets and on the solid surfaces of their satellites. Kerogen in a martian meteorite completed the picture of an abundance of cosmic hydrocarbons. Immediate consensus followed that the very abundance must reflect an abiogenetic origin(4, 5) for cosmic hydrocarbons, another seeming observation of fact. Pointing to the now well known fact that hydrocarbons are present in meteorites and abundant on some satellites of the outer planets, 20th century astrophysicist, Thomas Gold in 1987(6) drew the conclusion that petroleum on earth must also be produced abiogenically.

Isotopic and elemental chemistry of subsurface Precambrian igneous rocks, west Texas and eastern New Mexico

We present major element, trace element, and Nd isotopic analyses from cuttings and core samples for three subsurface terranes in west Texas and eastern New Mexico. The most northerly is the Panhandle volcanic terrane, which represents a large part of the Mesoproterozoic southern granite-rhyolite province. This terrane is comprised of undeformed rhyolite, ignimbritic tuff, granite, and diabase. The Panhandle terrane is split by the Debaca terrane, which consists of intercalated metasedimentary and metavolcanic rocks intruded by olivine gabbro, ferrogabbro, and diabase. Mildly to strongly deformed intermediate and felsic intrusive rocks of unknown affinity make up the third terrane, called here the crystalline terrane; it is located south and southeast of the Panhandle and Debaca terranes. Intermediate-to-felsic rocks of the terranes can be subdivided on the basis of their geochemistry into those with: (1) K 2 O/Na 2 O > 1 and A-type trace element characteristics; and (2) K 2 O/Na 2 O 1.7-Ga crust. The southern edge of Laurentia, therefore, is farther south than previously inferred. A diabase from the Panhandle terrane has a T DM of 1.44 Ga. If this model age is close to the crystallization age, then diabase in the Panhandle terrane is approximately coeval with the granite and rhyolite. The model age for the gabbro from the Debaca terrane is distinctly younger at 1.26 Ga, and is the same as crystallization ages of felsic tuffs associated with shelf carbonates in the Franklin Mountains and Van Horn area. In the crystalline terrane, both A- and I-type granites are present. Model ages for the I-type granites are 1.40–1.47 Ga. These are distinctly younger than the model age for the Panhandle terrane, and an A-type granite has a T DM of 1.35 Ga. These data indicate that granites in the crystalline terrane are not part of the granite-rhyolite province; rather, they constitute a separate group.

Global Geoscience Transect 20. Central Appalachians: Cratonic North America to the Atlantic Abyssal Plain

The Central Appalachian Geoscience Transect (GGT20) extends from the Atlantic coast across the Appalachian orogen to the Grenvillian structural front in the cratonic interior of North America. The character of the continental crust along the transect results from two Wilson cycles during Proterozoic and Phanerozoic time, comprised of the Grenvillian orogeny, Eocambrian rifting, Paleozoic orogeny, and Mesozoic rifting. The purpose of the transect is to address questions regarding the Paleozoic development of the eastern margin of North America related to: (1) structural development of the middle and lower crust during rifting and orogeny; (2) sources of major potential field anomalies, including the New York–Alabama magnetic lineament; (3) displacement termination of the Appalachian foreland thrust; and (4) thrust displacement transfer from the crystalline terranes into the foreland.