Tectonic Grain Research Articles

This volume, Sedimentary Successions of the Arctic Region and their Hydrocarbon Prospectivity , developed around maps of the sedimentary successions of the Arctic Region, and contains a brief, but comprehensive compilation of geological and geophysical data characterizing all significant sedimentary successions in the Arctic, which cover 57% of the polar area north of 64° N. Its two main goals are to provide, based on present-day knowledge and data, a characterization of all Arctic sedimentary successions (or sedimentary accumulations) and to supply a snapshot of hydrocarbon-related exploration in the Arctic at the end of the first quarter of this century. To achieve these goals, we represent sedimentary successions as consisting of one or several ‘tectono-sedimentary elements’ (TSEs) based on the main tectonic regimes that formed accommodation space for accumulation of sediments. A TSE characterization template has been developed as an efficient method of organizing and presenting the most important information about the stratigraphy, structure and petroleum geology of a TSE, including the most significant exploration facts. This organizational architecture is the backbone of the volume and is a key feature that distinguishes it from other studies of Arctic sedimentary basins. The online volume includes six large-size foldout maps portraying the mapped TSEs in the Circum-Arctic context, including tectonic grain of the consolidated basement, anomalous gravity and magnetic fields, location of the Arctic sampling sites and seismic profiles.

This study establishes a robust methodology using raw, desurveyed. uninterpolated drill-sampled 3D-grade data to identify superposed structural symmetries and determine mineralisation timing at the deposit-scale. This methodology has transformative implications for exploration and resource estimation of gold and other structurally controlled commodities. We introduce the concept of Perkins Discontinuities—planar boundaries that abruptly terminate mineralisation continuity, creating asymmetric grade distributions. These features , including fractures, low-displacement faults, veins, dykes or intrusions—define asymmetric mineralisation patterns unrelated to fault displacement. Such asymmetry arises from minor permeability contrasts across fractures intersecting the primary tectonic grains defined by fold axes. Our ‘outside-in’, deposit-scale structural analysis of drill-sampled assay-grade data from five well-established epigenetic gold deposits—Ballarat East (Victorian goldfields), Exodus (Carlin Trend), Bronzewing, Aphrodite from the Bardoc Tectonic Zone and Coolgardie (Yilgarn Craton)—reveals two key structural symmetry axes. A quasi-cylindrical axial symmetry (Axis 1) that parallels early shear zones and folds, while oblique Perkins Discontinuities define a later superposed axial symmetry (Axis 2). If Axis 1 and Axis 2 are parallel, gold mineralisation timing coincides at earliest with the first ductile deformation (syn-folding), although a later timing remains possible. Conversely, if Axis 2 is oblique to Axis 1, mineralisation must post-date folding and fracturing, ruling out syngenetic or syn-fold origins proposed by orogenic shear-zone gold models. Applying this approach to three deposits classified as syngenetic—Haile (South Carolina), Chelopech (Bulgaria) and Vaal Reef (Witwatersrand Basin, South Africa)—demonstrates their epigenetic nature. Using Fast Lagrangian Analysis of Continua finite difference modelling of Darcy fluid flow, we show upward-flowing fluids converge towards antiform apices and diverge from synforms, explaining the prevalence of orogenic gold in antiformal saddle reefs and scarcity in synformal trough reefs. A similar process may occur where fracture networks converge in an A-shaped geometry, as at Chelopech. Recognising Perkins Discontinuities and integrating their geometry with deposit-scale grade data redefines gold emplacement timing as late-orogenic in many goldfields, challenging both crustal-scale shear-zone models of orogenic gold emplacement and syngenetic classifications. This refined structural and fluid-flow framework impacts exploration targeting and resource estimation by highlighting post-fold and post-fracture controls on gold localisation, reshaping our understanding of major gold deposit formation.

Abstract This study integrates geophysical‐geochemical data to investigate the thermochemical structure of the lithosphere and sublithospheric mantle, along the Southern Tyrrhenian Basin, Apennines, Adriatic Sea, Dinarides, and Carpathians‐Balkanides. We present the lithospheric structure of the Adria microplate and the two opposing mantle slabs along its NE and SW margins. The modeling shows the presence of two asthenospheric mantle wedges aligning with the Apenninic and Dinaric continental mantle slab rollback, along with cold (−200°C) sublithospheric anomalies beneath Adria's NE and SW margins. In the northern Adria region, the lithosphere undergoes synchronous thinning in the Tyrrhenian domain and thickening toward the forefront of the northern Apennines. This is associated with the northeastward rollback of the SW Adriatic slab, leading to subsequent delamination of the continental mantle. In the southern Adria region, the complex deep structure results from the variably oriented lithospheric slabs, and nearly 90‐degree shift of the tectonic grain between the southern Apennines and the Calabrian Arc. At the SW Adria margin, beneath the northern Apennines, the thermal sublithospheric anomaly is attached to the shallower lithosphere, while a slab gap is modeled in the southern Apennines. One possibility is that the gap is due to a recent horizontal slab tear. Along the NE margin of Adria, the thermal anomaly penetrates to depths of about 200 km in the northern Dinarides and 280 km in the southern Dinarides, shallower than the SW Adria anomaly, which extends to at least 400 km depth.

ABSTRACT A new geological mapping of Sierra Calderona (SE Iberian Cordillera) has confirmed the widespread presence of the Muschelkalk M2 unit overlying a ubiquitous Muschelkalk M1 carbonate unit. Thus, the allocation of this range in the Levantine-Balearic Triassic Domain is not justified and a previous assignment of the entire Muschelkalk carbonate succession of this region to the M3 unit is untenable. Evidence of Permo-Triassic extensional tectonics by local unconformities separating Permian and Triassic units and a more widespread unconformity at the base of the Jurassic succession is recorded. Paleogene-Early Neogene compression-related structures associated with the formation of the NW-SE trending Sierra Calderona anticlinorium are scarce, but numerous detachment surfaces within the stratigraphic pile suggest a complex thick-skinned tectonics. The dominant tectonic grain in Sierra Calderona is a Neogene extensional structure with a preferential pattern of blocks stepping down to the coast, which is related to the opening of the Valencia Trough.

Abstract As North America collided with Africa to form Pangea during the Alleghanian orogeny, crystalline and sedimentary rocks in the southeastern United States were thrust forelandward along the Appalachian décollement. We examined Ps receiver functions to better constrain the kinematics of this prominent subsurface structure. From Southeastern Suture of the Appalachian Margin Experiment (SESAME) and other EarthScope stations on the Blue Ridge–Piedmont crystalline megathrust, we find large arrivals from a 5–10-km-deep converter. We argue that a strong contrast in dipping anisotropic foliation occurs at the subhorizontal Appalachian décollement, and propose that such a geometry may be typical for décollement structures. Conversion polarity flips can be explained by an east-dipping foliation, but this orientation is at odds with the overlying northeast-trending surface tectonic grain. We suggest that prior to late Alleghanian northwest-directed head-on collision, the Appalachian décollement accommodated early Alleghanian west-vergence, independent of the overlying Blue Ridge–Piedmont structural inheritance. The geophysical expression of dipping anisotropic foliation provides a powerful tool for investigating subsurface kinematics, especially where they are obscured by overlying fabric, to disentangle the tectonic complexities that embody oblique collisional orogens.

Abstract Deep continental crustal structures are enigmatic due to lack of direct exposures and limited tools to investigate them remotely. Seismic waves can sample these rocks, but most seismic methods focus on coarse crustal structures while laboratory measurements concentrate on crystal‐scale rock properties, and little work has been conducted to bridge this interpretation gap. In some places, geologic maps of crystalline basement provide samples of the intermediate‐scale fabrics and structures that may represent in situ deep crust. However, previous research has not considered natural geometric variations from map data, nor is this heterogeneity typically included in map‐scale seismic property calculations. Here, we test how map‐scale fabrics influence crustal seismic anisotropy in Colorado by analyzing structural data from geologic maps, combining those data with bulk rock elastic tensors to calculate map‐scale seismic properties, and evaluating the resulting comparisons with observed receiver function A1 (360° periodic) arrivals. Crystalline fabrics, predicted seismic properties, and tectonic structures positively correlate with shallow and deep crustal A1 arrivals. Additionally, widespread correlations occur between mapped fault traces and regional foliations, implying that preexisting mechanical heterogeneity may have strongly influenced subsequent reactivation. We interpret that various mapped geologic contact types (e.g., lithologic and structural) generate A1 arrivals and that multiple parallel features (e.g., faults, foliations, and intrusions) contribute to a seismically visible tectonic grain. Therefore, Colorado's exhumed basement, as expressed in outcrops and maps, offers insight into modern deep crustal geological and geophysical structure.

The Gold District of Korhogo, in the northern region of C?te d’Ivoire, holds enormous potential for gold mineralizations, some of which are under exploration phase and others in exploitation phase (example of Tongon Gold Mine). Dormant since 1998, Mapping Services of most of the West African countries hardly provide geological maps at the scales of: 1/200,000, 1/50,000 and 1/25,000. This situation of unavailability of detailed geological maps does not help mining operators in the selection of prolific areas and also in the interpretation of in soil geochem anomies or gold mineralizations. Consequently, during the years 2020 and 2021, we have undertaken a campaign of geological mapping and petro-structural study of the northern sector of Komborodougou, located in the southern extension of Banfora Birimian Belt. This work, which allowed the realization of a geological map at 1/20,000 scale, reveals that: the mapped area includes three (3) major lithological units: 1) a volcano-sedimentary unit made up of metasediments (metaarenites, metasiltites and metaflyschs) and metavolcanites (metabasalts), which are metamorphosed and outcrop in the form of schists; 2) a metaplutonic and intrusive unit composed on the one hand of quartz-diorites and metagabbros, and on the other hand of granites and granodiorites; 3) and finally, a unit of dikes formed by microgranites, microgabbros, aplites and quartzites. The volcano-sedimentary complex is affected by a regional fold with an axis subparallel to the regional tectonic grain and an S1 schistosity oriented NE-SW to NNE-SSW with steep dips (>60o), except for those of the metaflyschs which are weak (o); a regional NW-SE compression would be at the origin of the setting of this schistosity. The volcano-sediment and metaplutonite complex is crossed in a NNE-SSW direction by a senetral shear-zone or main shear-zone and secondary shear-zones oriented sometimes NE-SW or N-S. These shear-zones are intersected by more or less dextral or senestral strike-slip faults of NW-SE and E-W trends. N-S to NNE-SSW (N0o - N20o) and NW-SE (N130o - N160o) vein systems associated with the various shear-zones are the hosts of the gold mineralization in the region. These veins have been mined by orpaillors for more than two decades. The northern sector of Komborodougou is in the NNE continuity of the gold mineralization highlighted by the mining company Mako Gold Sarl on its Gogbala and Tchaga prospects, Napie project.

Middle Jurassic orogeny in the northern North China block

Structural diversity within a thrust complex reflecting progressive overprinting during a protracted orogenic process of terrane accretion, Natal belt, South Africa

Abstract Azimuthal variations in receiver function conversions can image lithospheric structural contrasts and anisotropic fabrics that together compose tectonic grain. We apply this method to data from EarthScope Transportable Array in Alaska and additional stations across the northern Cordillera. The best-resolved quantities are the strike and depth of dipping fabric contrasts or interfaces. We find a strong geographic gradient in such anomalies, with large amplitudes extending inboard from the present-day subduction margin, the Aleutian arc, and an influence of flat-slab subduction of the Yakutat microplate north of the Denali fault. An east–west band across interior Alaska shows low-amplitude crustal anomalies. Anomaly amplitudes correlate with structural intensity (density of aligned geological elements), but are the highest in areas of strong Cenozoic deformation, raising the question of an influence of current stress state. Imaged subsurface strikes show alignment with surface structures. We see concentric strikes around arc volcanoes implying dipping magmatic structures and fabric into the middle crust. Regions with present-day weaker deformation show lower anomaly amplitudes but structurally aligned strikes, suggesting pre-Cenozoic fabrics may have been overprinted or otherwise modified. We observe general coherence of the signal across the brittle-plastic transition. Imaged crustal fabrics are aligned with major faults and shear zones, whereas intrafault blocks show imaged strikes both parallel to and at high angles to major block-bounding faults. High-angle strikes are subparallel to neotectonic deformation, seismicity, fault lineaments, and prominent metallogenic belts, possibly due to overprinting and/or co-evolution with fault-parallel fabrics. We suggest that the underlying tectonic grain in the northern Cordillera is broadly distributed rather than strongly localized. Receiver functions thus reveal key information about the nature and continuity of tectonic fabrics at depth and can provide unique insights into the deformation history and distribution of regional strain in complex orogenic belts.

Indian peninsular region comprises several Archean cratonic blocks (Dharwar, Bastar, Singhbhum, Aravalli – Bundelkhand), bordered by Proterozoic mobile belts. Therefore, this region is considered as tectonically stable and designated as the least vulnerable region to earthquake hazard except the still active Central Indian Tectonic Zone (CITZ). The latter is a major suture between southern and northern Indian blocks. Seismicity in India is common along its northern and northeastern (Himalayan) region defining the Indian Plate margin, in collision with the Eurasian (Tibetan) Plate. Being tectonically active, this region has a documented record of frequent earthquakes including some high magnitude and devastating ones. The northwestern Indian block is amongst the relatively stable Precambrian regions of India and categorized under Zone – II by the Indian Meteorology Department, one of the seismically least vulnerable regions. However, the region has been a site of recurring low to moderate magnitude earthquakes. Compilation of the earthquake data from the Rajasthan State in NW India documents at least 45 earthquakes in this region during the last one and a half decades. Several of these have remained unnoticed because of their low magnitude. The NE –SW trending Aravalli Mountain Region (AMR), running across the eastern half of Rajasthan State represents the most significant tectonomorphic feature of NW India. The AMR is an ensemble of Proterozoic age Aravalli and Delhi Supergroup rocks (metasediments, volcanics and intrusives) that overlie an Archean basement (Banded Gneissic Complex – BGC). These Proterozoic mobile belts have evolved through several episodes of deformation that have shaped its present day geomorphology. The western part of the State, the Marwar Block, is relatively younger in age and was accreted to the AMR during the 1 Ga subduction event. The western margin of AMR, also called as the Western Margin Fault, represents a major suture between the two terranes. However, the entire region was cratonized by end Proterozoic and has remained tectonically stable during the Phanerozoic Eon. The Archean basement and overlying Proterozoic cover rocks are infested with several major and minor faults and shear zones. The most prominent ones are the NE-SW trending major lineaments corresponding with the regional tectonic grain and several minor ones across. The earthquake epicenter distribution pattern shows a close spatial association with these lineaments. In the absence of any significant tectonic activity in the region and rise in pore pressure either due to magmatism or excessive rainfall, we attribute recurrence of earthquakes in the region to reactivation of such old sutures/weaker zones as a response to stress build-up along the northern margin of the Indian Plate resulting from ongoing northward convergence of the Indian Plate. Some of the faults in western part are traceable into the Cambay Basin active faults in the south that may have triggered seismic activity in western Rajasthan.

Located in the southwestern of the Oume-Fettekro greenstone belt, the Agbahou gold deposit is controlled by structural factors. Geophysics, teledetection and core data suggest the presence of NE and NW faults. However, the NE-faults define two major shear zones (ATZ: Agbahou Tectonic Zone and WTZ: West Tectonic Zone) that control the Agbahou gold mineralization. These first order structures are subparallel to the regional tectonic grain mostly north-east oriented. They seem to respectively develop on the both arms (eastern and western) of an anticline moderately plunging ~25° towards northeast. Each shear-zone contains several second-order shear-zones or lenses of variable direction and of 50° - 80° dip. NW-faults however correspond to strike-slip faults and their development should be related to transcurrent tectonics. They acted as control channels on the distribution of gold mineralizations. The ductile-brittle character of shear-zones favored the ascent of hydrothermal fluids and the formation of multiple auriferous quartz veins: veins Type IIa and veins Type IIb relating respectively to the shear-veins and extensional veins. However, Agbahou also shows the existence of a disseminated sulphides-bearing mineralization within host-rocks. At Agbahou, the precipitation of gold probably occurred in a post to late ductile-brittle deformation period.

It is now accepted that the active tectonic grain comprises a self-organized complex system, therefore its expression (seismicity) should be manifested in the temporal and spatial statistics of energy release rates, and exhibit memory due to long-range interactions in a fractal-like space-time. Such attributes can be properly understood in terms of Non-Extensive Statistical Physics. In addition to energy release rates expressed by the magnitude M, measures of the temporal and spatial interactions are the time (Δt) and hypocentral distance (Δd) between consecutive events. Recent work indicated that if the distributions of M, Δt and Δd are independent so that the joint probability p(M, Δ t, Δd) factorizes as p(M) p( Δt) p( Δd), earthquake frequency is related to M, Δt and Δd by well defined power-laws consistent with NESP. The present work applies these concepts to investigate the self-organization and temporal/spatial dynamics of North Californian seismicity. The results indicate that the statistical behaviour of seismicity in this area is consistent with NESP predictions and has attributes of universality, as its holds for a very broad range of spatial, temporal and magnitude scales. They also indicate that the expression of the regional active tectonic grain comprises a mixture of processes significantly dependent on Δd, which include near (<100km) and far (>400km) field interactions.

The effects of plate margin inhomogeneity on the deformation pattern within west-Central Zagros Fold-and-Thrust Belt

Integrated geological mapping approach and gold mineralization in Buhweju area, Uganda

The volcanic, magmatic and tectonic setting of the Taupo Volcanic Zone, New Zealand, reviewed from a geothermal perspective

Abstract The Gulcheru Quartzite (30–200 m thick) overlies Archaean basement rocks, and comprises an impersistent conglomerate at the base of more widespread quartzites, with shale and ferruginous interbeds. The overlying Vempalle Formation (≤1500 m) comprises mainly stromatolitic carbonates, lesser cherts and mudrocks. Together, these two units make up the Palaeoproterozoic Papaghni Group, which displays an arcuate outcrop belt in the SW Cuddapah Basin, bearing no obvious relationship to the tectonic grain in the basement lithologies. Facies defined in the Gulcheru Quartzite are ascribed to initial alluvial fans, which transitioned into a shallow-marine coastline and shelf, as transgression occurred under overall thermal subsidence, possibly related to post-plume thermal relaxation. The shallow siliciclastic Gulcheru shelf is inferred to have evolved to a stromatolitic ramp that accommodated deposition of the Vempalle chemical sediments. The abrupt vertical transition from clastic to carbonate deposits is ascribed to rapid sea-level rise. Late-stage shoreline progradation during uppermost Vempalle times and concomitant clastic inflows terminated the carbonate factory.

Walden's Shore: Henry David Thoreau and Nineteenth-Century Science. By Robert M. Thorson. (Cambridge, MA: Harvard University Press, 2014. Pp. 421. Cloth, $29.95.)Reviewed by John HayWith the rise of ecocriticism, many recent studies of Thoreau's writings have favorably reconsidered the author's strong relationship with science; this trend received much of its impetus from Laura Dassow Walls's Seeing New Worlds: Henry David Thoreau and Nineteenth-Century Natural Science (Madison, WI, 1995). Similarly subtided, Walden's Shore begins by explaining such scholarship still lacks an engagement with hard science and a solid understanding of Thoreau's work, and especially of Walden (1854), requires more intimate knowledge of geological phenomena. Robert Thorson is a professor of geology at the University of Connecticut whose last book, Beyond Walden: The Hidden History of America's Kettle Lakes and Ponds (New York, 2009), was a general account of small lakes in the Midwest and Northeast; he now restricts his view to Walden's immediate environs in order to establish Thoreau's reputation as a pioneering geoscientist (16). While countless books and articles have promoted Thoreau's love of nature, this nature is often characterized as organic: flowers, trees, birds, fish, etc. Many overlook the fact Thoreau, as Thorson insists, was just as strongly attuned to the ?^organic: minerals, mountains, rivers, and lakes.Walden's Shore begins by describing the physical history of Walden Pond (or Lake which Thorson explains is a more accurate name), a kettle lake formed by deglaciation. Thorson carefully guides the reader through a detailed description informed by modem geological data and theory-a story of plate movement, erosion, glaciation, and hydrological processes-while also explaining the scientific accounts prevalent in America during Thoreau's lifetime. Against this background, he presents Thoreau as a meticulous observer whose precise descriptions of the environment often seem to anticipate the discoveries of twentieth-century geologists.Thorson's literary interest lies in the entirety of Walden but only in that part of Walden dealing with material Nature (15), a selection of text sometimes referred to as geo-Walden. Claims regarding this text are substantially buttressed by citations of the massive Journal from which Thoreau culled material for Walden and other publications. Regarding the choice to emphasize only the scientific elements, Thorson baldly admits this is not a fair and balanced treatment (16). Thus while Walden's Shore has much to say about one aspect of Thoreau's masterpiece, it does attempt to offer a new reading of the text as a whole. Nevertheless, the perspective offered is useful. As Thorson explains, Thoreau scholars have erred in stressing his ??ophilia at the expense of his ggophilia. The very shape and texture of the land, he argues, is the heart of Walden, and this simple substitution-geocritical for ecocritical-deserves the serious attention of those interested in literature and science.Thorson's focus on geology is serious, and at times his prose features daunting professional jargon. Readers should be prepared for explanations such as the following: Stratigraphically, the eastern edge of the paleo-valley below Smith's Hill and Pine Hill would have shunted meltwater sediment above the block, speeding up the rate at which the kame delta would have advanced (154). But these scientific details regarding the physical context of Thoreau's work occasionally prove revelatory. For example, in his essay Walking Thoreau mysteriously declared his inner compass often directed him toward the southwest, an inclination Thorson explains as possibly due to the tectonic grain of the Nashoba Terrane (53), a piece of the Earth's crust shaped to steer pedestrians in very direction. …

Heterogeneous deformation recorded by magnetic fabrics in the Pyrenean Axial Zone

Abstract The southern part of the Indian Peninsular has started witnessing fast recurring seismicities and earth tremors with the magnitude of 3-5 in the recent years. In this context, the earthquake occurred, with a magnitude of 3.5, east of Ariyalur, Central Tamil Nadu, on 12th August, 2011 is significant. It is because, followed by the earthquake, tremors were felt in over 35-40 villages in an area of 8000 sq km along with the development of cracks and dislocations in buildings in several places. The remote sensing observations showed that while the Ariyalur epicenter is in the eastern Tertiary formations, the tremor felt villages lie in the western crystalline formations. Again the correlation of remote sensing based lineaments with Ariyalur earthquake, the related tremors and also a few past seismic events of the area showed that these epicenters mostly lie along NE-SW lineaments. The tremors related to Ariyalur event were also felt mostly in villages located again in the vicinity of NE-SW lineaments. The lineaments appear to be post collision tectonic grains related to the still on going northerly compressive force due to which only the Indian plate is buckling and fracturing now. Hence the 12th August, 2011 seismic event of Ariyalur cannot be taken as an isolated phenomenon and appears to be related to the Indian plate deformation as a whole. So it calls for in depth studies in the context of fast relapsing seismicities in peninsular India.