Basement Uplift Research Articles

Neoproterozoic evolution of the eastern Arabian basement based on a refined geochronology of the Marbat region, Sultanate of Oman

Abstract New high spatial resolution secondary ion mass spectrometry (SIMS) U–Pb zircon data from the Sadh gneiss complex and the intruding Marbat granodiorite of the Marbat region, southern Sultanate of Oman, yield Cryogenian magmatic protolith ages for gneisses ranging from c. 850 to 830 Ma. Zircon ages record a c. 815–820 Ma period of deformation and migmatization, followed by intrusion of a hornblende gabbro/diorite and the undeformed Marbat granodiorite at c. 795 Ma. Following break-up and rifting of Rodinia at c. 870 Ma, crustal growth in the Marbat region occurred via arc accretion at c. 850–790 Ma, possibly in the easternmost part of the Mozambique Ocean based on earlier cessation of accretion here compared to the Arabian–Nubian Shield. Similarity of the new zircon geochronology to peaks of detrital zircon ages in the unconformably overlying Ediacaran Marbat sandstone suggests relatively local derivation from uplifted basement for the latter. Supplementary material: Detailed petrographic descriptions and photographs of hand specimens and thin-sections are available at http://www.geolsoc.org.uk/SUP18685 .

The Talya Conglomerate: an Archean (∼2.7 Ga) Glaciomarine formation, Western Dharwar Craton, Southern India

The ∼2700 Ma Talya Conglomerate is Comprised of 15 Diamictites (i.e. Matrix-supported Conglomerates) Interbedded with Mudstone and Sandstone Units, and is Interpreted as a Glaciomarine Deposit. the Entire Thickness of This Conglomeratic Member within the Vanivilas formation, the Lowest formation of the Chitradurga Group of the Neoarchean Dharwar Supe Rgroup, is Exposed in a 543 M-thick Measured Section. it is in a Sub-vertical Attitude, is Highly Sheared, and has Undergone Greenschist Facies Metamorphism. the Diamictites had an Original Matrix of Laminated Mud/Silt and Fine Sand. while Including Diamictites Throughout, the Talya is a Fining-upward Sequence with Intercalated Sandstones Dominant in its Lower Portion and Mudstones Dominant in its Upper Portion. we Interpret that the Talya Conglomerate was Deposited in a Marine Environment, with Diamictites Composed of Ice-rafted Detritus (ird) Deposited from Icebergs Calved from Tidewater Glacier Tongues And/or possibly from Ice Shelves. in these 'rainout Diamictites' the Larger Clasts were Dropped into Finegrained Bottom Sediment Deposited by Sediment Plumes and Currents. the Source Ice Sheet was Located to the West and Southwest on a Land Mass that Included the Older than 2720 Ma Bababudan Group of Quartzites and Mafic Volcanics and Older than 3000 Ma Basement of Granitic/gneissic Rocks. Application of Walther's Law Indicates that the Mudstonebearing Portion of the Talya was Deposited upon the Sandstone-bearing Portion as the Sea further Inundated the Land Mass Due to Glacial Retreat and a Decrease in Glacial Mass, thereby Resulting in the Fining- upward Nature of the Talya Conglome Rate. we also Interpret the Lower Portion of the Kaldurga Conglomerate, Located 50-75 Km to the Southwest of the Talya, to be Equivalent with the Talya. the Kaldurga Contains mostly Granitic Basement Detritus, perhaps Exposed Due to Basement Uplift Related to Isostatic Rebound Caused by Glacial Melting or Due to Tectonism Related to Westward Subduction.

Abrupt and high-magnitude changes in atmospheric circulation recorded in the Permian Maroon Formation, tropical Pangaea

Geochemical, magnetic, sedimentologic, and U-Pb detrital zircon data from loessitepaleosol couplets within the Maroon Formation of Colorado (western U.S.) record abrupt and high-magnitude changes in atmospheric circulation in western tropical Pangaea during Early Permian (Wolfcampian) time. The relative quartz grain size increases by 50%–100% in loessite compared to superjacent paleo sols. Quartz grain size correlates inversely with bulk magnetic susceptibility (r 2 = 0.89); susceptibility values from paleosols are two times higher than from underlying loessite. This pattern is interpreted to refl ect high-frequency shifts in dust-transporting wind intensity. Loessite represents semi-arid times when dusttransporting winds were stronger, depositing coarser sediment, whereas paleosols record accumulation of fisediment marked by pedogenic enhancement of magnetic susceptibility within a wetter, less windy climate. Both major- and trace-element geochemistry differ between the loessite and superjacent paleosols. The paleosols are more phyllosilicate rich, likely tied to differences in grain size. However, a multivariate analysis of log-transformed trace-element data suggests that the thicker loessite units exhibit a shift in provenance compared to paleosols and thinner loessite units. U-Pb ages of detrital zircons also differ between loessite and paleosols. Four couplets were analyzed; all loessite samples contain at least one grain with U-Pb ages (1) <300 Ma and (2) 760–940 Ma, whereas no paleosol sample contains these ages. The fi rst population refl ects coeval arcs fringing western Pangaea and eastward transport in a westerly wind regime. The second population is more enigmatic but may refl ect erosion of Proterozoic rift-related volcanics, located along the western edge of Pangaea or located in northern Canada. In addition, the loessite samples contain a lower ratio (0.69) of Neoproterozoic grains (760–570 Ma) to Paleozoic grains (500–300 Ma) compared to paleosols (1.32). Both loessite and paleosol samples contain zircons with U-Pb ages between 1360–925 Ma and 1800–1610 Ma. The Neoproterozoic, Paleozoic, and 1360–925 Ma (Mesoproterozoic) grains were sourced from the Appalachian-Ouachita orogen to the east to southeast, whereas the 1800–1610 Ma (Paleo proterozoic) grains refl ect local basement uplifts (Ancestral Rocky Mountains). Overall, the combined geochemical and detrital zircon data indicate changes in provenance tied to abrupt and repeated changes in the direction of dust-transporting winds. The loessite-paleosol pairs exhibit a nested stratigraphy with a thick paleosol capping a thick loessite followed by thinner loessites grading into thinner, and less well-developed, paleosols. Our data suggest that thick loessite intervals accumulated in a monsoonal climate with strong, seasonal westerly winds coupled with semi-arid conditions, whereas the superjacent thick paleosol records more humid climate and a provenance derived primarily from the east. The muted sedimentologic, magnetic, and geochemical differences exhibited in the series of thinner loessitepaleosol couplets likely refl ect muted climate change. In this model, the thick loessitepaleosol couplet represents a period of major glaciation and the subsequent interglacial, respectively, in which a more elliptical Earth orbit accentuates climatic extremes. The thinner couplets likely represent more circular Earth orbits during times of advancing glacial conditions when climatic extremes were muted. The inferred depositional duration of the Maroon Formation indicates these

Petrography and geochemistry of Jumara Dome sediments, Kachchh Basin: Implications for provenance, tectonic setting and weathering intensity

In the Kachchh Mainland, the Jumara Dome mixed carbonate-siliciclastic succession is represented by the Jhurio and Patcham formations and siliciclastic-dominating Chari Formation (Bathonian to Oxfordian). The Jumara Dome sediments were deposited during sea-level fluctuating, and were interrupted by storms in the shallow marine environment. The sandstones are generally medium-grained, moderately sorted, subangular to subrounded and of low sphericity. The sandstones are mineralogically mature and mainly composed of quartzarenite and subarkose. The plots of petrofacies in the Qt-F-L, Qm-F-Lt, Qp-Lv-Ls and Qm-P-K ternary diagrams suggest mainly the basement uplift source (craton interior) in rifted continental margin basin setting. The sandstones were cemented by carbonate, iron oxide and silica overgrowth. The Chemical Index of Alteration values (73% sandstone and 81% shale) indicate high weathering conditions in the source area. Overall study suggests that such strong chemical weathering conditions are of unconformity with worldwide humid and warm climates during the Jurassic period. Positive correlations between Al2O3 and Fe2O3, TiO2, Na2O, MgO, K2O are evident. A high correlation coefficient between Al2O3 and K2O in shale samples suggests that clay minerals control the major oxides. The analogous contents of Si, Al, Ti, LREE and TTE in the shale to PAAS with slightly depleted values of other elements ascribe a PAAS like source (granitic gneiss and minor mafics) to the present study. The petrographic and geochemical data strongly suggest that the studied sandstones/shales were deposited on a passive margin of the stable intracratonic basin.

Sandstone composition of the Upper Jurassic Emuerhe Formation from the Mohe Basin, China

AbstractThe sandstone composition analysis carried out on the samples from three different sections (western section, middle section and eastern section) of the Upper Jurassic Emuerhe Formation in the Mohe Basin, northeastern China, reveals that the Okhotsk orogenic belt and the Ergun massif basement are the source areas for the Emuerhe Formation. The source area of the western section and the A segment of the middle section is the Okhotsk orogenic belt. The tectonic attributes of the Okhotsk orogenic belt are those of a dissected arc. The transportation distance of clastic rocks from these sections gradually lengthened, and the uplift rate of the Okhotsk orogenic belt gradually increased. The source area of the eastern section and the B segment of the middle section is the Ergun massif basement. Compared with the Okhotsk orogenic belt, the tectonic attributes and uplift rate of the Ergun massif basement are different from the former. The attributes of the Ergun massif basement are those of basement uplift. The transportation distance of clastic rocks from the eastern section and the B segment of the middle section gradually shortened, and the uplift rate of the Ergun massif basement gradually decreased. These conclusions are consistent with the characteristics of a foreland basin. Thus, we can draw the conclusion that the Mohe Basin belongs to the foreland basin.

Late Miocene shortening of the Northern Apennines back-arc

The inner Northern Apennines (western Tuscany and Tyrrhenian basin) is characterized by a relatively thin continental crust (∼20–25km), high heat flow (>100mWm−2), and the presence of relevant tectonic elision of stratigraphic sequences, a setting known as Serie Ridotta. These features are normally ascribed to an extensional deformation that affected the back-arc area above the subducting Adria plate since the Early-Middle Miocene (∼16Ma). However, various geophysical studies image the continental crust to be currently affected by W-dipping thrust faults (and associated basement uplifts) that have not been obliterated by this claimed long-lasting extensional process. These observations raise the question whether the thrusts are older or younger than the continental extension. To address this question we have reprocessed and interpreted the deep seismic reflection profile CROP03/c that crosses the onshore hinterland sector, and investigated the structural setting of some of the Late Miocene-Pliocene hinterland basins (Cinigiano-Baccinello, Siena-Radicofani, Tafone, Albegna and Radicondoli basins) that are situated at the front or in-between the basement uplifts. The analysis of field structures and commercial seismic profiles has allowed the recognition that both substratum and basins’ infill have been intensely shortened. These findings and the architecture of the basins suggest that the latter developed under a contractional regime, which would have started around 8.5Ma with the onset of the continental sedimentation. This compressive stress state followed an earlier phase of continental extension that presumably started at ∼16Ma (with the blocking of the Corsica-Sardinia rotation), and thinned both the continental crust and sedimentary cover producing most of the Serie Ridotta. The main phases of basin shortening are bracketed between 7.5 and 3.5Ma, and thus overlap with the increase in the exhumation rate of the metamorphic cores at ∼6–4Ma determined through thermochronological data. We therefore propose a correlation between the basin deformation and the activity of the nearby basement thrusts, which would have thus shortened a previously thinned continental crust. This chronology of deformation may suggest a geodynamic model in which the back-arc and hinterland sector of the Northern Apennines was recompressed during Late Miocene-Early Pliocene times. This evolution may be explained through different speculative scenarios involving a blockage of the subduction process, which may vary between end members of complete slab detachment and stalled subduction.

Remote sensing and geophysical investigations of Moghra Lake in the Qattara Depression, Western Desert, Egypt

The Western Desert covers two-thirds of the land area of Egypt and occupies one of the driest regions of the Sahara. Seven depressions within the desert – Siwa, Qattara, Fayum, Bahariya, Farafra, Dakhla, and Kharga – may represent parts of old drainage systems with deflation, extensive erosion, and possibly, some tectonic activity. Oases with freshwater exist in these depressions. Geological and geophysical investigations in the Qattara Depression indicate the presence of buried fluvial channels with southeast to northwest flow directions from the highland areas. The origin of these fluvial systems, as well as the origin of the depressions themselves, is still unresolved, and many ideas have been suggested. Moghra Lake at the northeastern tip of the Qattara basin may be a remnant of a larger paleolake, including the mouth of a paleo-river.We present here the results of our recent work in this area using ALOS PALSAR radar remote sensing data, which indicated the presence of buried channels that may have fed the larger Moghra paleolake. Ground penetrating radar (GPR) data along 2D profiles were acquired; the migrated GPR sections identified a major paleochannel with numerous minor channels at its margins. GPR interpretations are verified by field observations, trenching, and stratigraphic information from outcrop data. Potential field analyses identify possible aquifers that are controlled by regional structures. Density contrasts within the sedimentary units, physical boundaries of uplifted basement blocks and depths to causative sources were also identified. This work contributes to the reconstruction of paleodrainage of this region and helps in understanding processes involved in the formation of the Qattara Depression.

Passive obduction and gravity-driven emplacement of large ophiolitic sheets: The New Caledonia ophiolite (SW Pacific) as a case study?

AbstractThe 300 km long allochthonous sheet of oceanic mantle forming the New Caledonia ophiolite displays three specific characters: 1) the ophiolite pile lacks concordant sheeted dykes and pillow basalt layers; 2) the ophiolite, refered to as the Peridotite nappe, is thrusted over the basaltic formations of the Poya terrane which are classicaly thought to originate from a different oceanic environment; 3) The basal contact of the ultramafic sheet is remarkably flat all along New-Caledonia and the Peridotite nappe has not been thickened during obduction, rather it experienced significant extension. This suggests that the peridotites have not been emplaced by a tectonic force applied to the rear. New petrological and geochemical results obtained from mantle rocks finally show that the Poya terrane may originate from the same oceanic basin as the peridotites. In this article, we consider such possible cogenetic links and we propose a simple model for the obduction of the New Caledonia ophiolite in which the Poya basalts represent the original cover of the Peridotite nappe. We infer that continuous uplift of the subducted units buried beneath the oceanic lithosphere in the northern part of New Caledonia drove passive uplift of the ophiolite and led to erosion and to initiation of sliding of the basaltic layer. During the Priabonian (latest Eocene), products of the erosion of the basaltic layer were deposited together with sediments derived from the Norfolk passive oceanic margin. These sediments are involved as tectonic slices into an accretionary wedge formed in response to plate convergence. The volcaniclastic sedimentation ends up with the emplacement of large slided blocks of basalts and rafted mafic units that progressively filled up the basin. Obduction process ended with the gravity sliding of the oceanic mantle sheet, previously scalped from its mafic cover. This process is contemporaneous with the exhumation of the HP-LT units of Pouebo and Diahot. Gravity sliding was facilitated by the occurrence of a continuous serpentine sole resulting from metasomatic hydratation of mantle rocks, which developed during the uplift of the Norfolk basement and overlying Diahot and Pouébo units. Progressive emersion of the obducted lithosphere allowed subsequent weathering under subaerial, tropical conditions.

Initiation of the intra-cratonic Cuddapah basin: Evidence from Paleoproterozoic (1995 Ma) anorogenic porphyritic granite in Eastern Dharwar Craton basement

Initiation of large intra-cratonic sedimentary basins has been constrained by dating the magmatic rocks intercalated in the sedimentary sequence; one such major basin in southeastern India is the Paleoproterozoic Cuddapah Basin. Near-basal Cuddapah sediments, deposited over the Eastern Dharwar Craton basement, are interlayered with mafic lavas and intruded mafic–ultramafic sills (1885±3Ma) being evidence for mantle melting. One of the expected major consequences of the asthenospheric upwelling on the lithosphere, apart from crustal doming, would have been partial melting of the cratonic tonalite–granodiorite crust and generation of granitic melts with anorogenic geochemical characteristics, the precise dating of which would constrain the timing of the basin initiation.From the southeastern part of the eroded basin, being the only exposed part into the basement below the sediment cover, we report presence of exactly such intrusive porphyritic granite. Petrological and geochemical characteristics of these anorogenic metaluminous granites strongly support partial melting (∼10–12%) of the cratonic tonalite–diorite crust most likely due to basaltic underplating. The crystallization time of this granite, sampled from SE of SriKalahasti, was determined on magmatic zircons and yielded a precise 207Pb/206Pb weighted mean age of 1995±11Ma. As the felsic crustal melting phase significantly predated the 1885±3Ma-aged mafic sills emplacement, early events include significant erosion of the previously uplifted basement followed by sedimentation (>1500m) before mafic volcanism and sill emplacement. This history broadly suggests active rift basin evolution from a non-extensional setting progressing to passive extension.

Evolution, paleoecology and sequence architecture of an Eocene carbonate ramp, southeast Zagros Basin, Iran

ABSTRACTWe document and discuss the results of detailed fieldwork, facies analysis and the subsequent integration of paleoecological evidence from the Middle Eocene carbonate ramp succession in the southeast Zagros Basin (Jahrum Formation). A combination of a sea-level fall and tectonic and diapiric basement uplift favored the initiation of the Jahrum carbonate platform. The lower portions are affected by pervasive, probably early diagenetic dolomitization, whilst the upper Jahrum consists mainly of limestone. Here, the focus is on the limestone portions of the Jahrum Formation. Based on the abundance, diversity and rapid evolutionary turnover of the alveolinids and nummulitids, the limestone intervals of the Jahrum Formation are interpreted for the upper Middle Eocene (Bartonian). The Jahrum Formation is capped by a major unconformity and overlain by the Lower Oligocene mixed clastic/carbonate Razak Formation.Based on data from field sections, eight facies associations and a series of sub-types have been established, which correspond to inner-, middle-and outer-ramp depositional environments. In their overall context, these data show a southward-dipping inner-ramp-to-basin transect. Towards the Coastal Fars (e.g. Hulur-01 Well) the Jahrum grades laterally into deep-marine Pabdeh foredeep shale units. Based on facies analysis and paleoecological evidence from larger benthic foraminifera, a major transgressive-regressive pattern is recognized in all outcrop sections of the Jahrum. The lowermost stratigraphic units of the formation are here interpreted as a distally steepened ramp. Evidence comes from abundant allochthonous shallow-water facies in the distal, deeper-ramp setting. Shallow-water carbonate clasts were exported towards the basin, a feature that is probably linked to relative sea-level fall control. Furthermore, local to regional basement instabilities by salt diapir-related basement reorganization was arguably of significance. Upsection, evidence is found that the ramp system evolved from a distally steepened to a homoclinal geometry with an overall very gentle slope geometry during the Late Bartonian.The data shown here are significant for those concerned with the Paleogene evolution of the southeast Zagros Basin and provide a well-exposed case example of a Middle Eocene carbonate ramp factory.

Thermal‐mechanical modeling of salt‐based mountain belts with pre‐existing basement faults: Application to the Zagros fold and thrust belt, southwest Iran

Two‐dimensional thermal‐mechanical models of thick‐skinned, salt‐based fold and thrust belts, such as the Zagros, SW Iran, are used to address: (1) the degree of deformation and decoupling between cover and basement rocks due to the presence of a weak salt detachment, (2) the reactivation potential of pre‐existing basement normal faults due to brittle or ductile behavior of the lower crust (as related to cold or hot geothermal gradients), and (3) variations in deformation style and strain distribution. The geometry and kinematics of the orogenic wedge and the activity of pre‐existing basement faults are strongly influenced by the geothermal gradient (defined by the Moho temperature, MT) and basement rheology. We infer that the MT plays a major role in how the lower and upper crust transfer deformation toward the foreland. In relatively hot geotherm models (MT = 600°C at 36 km depth), the lowermost basement deforms in a ductile fashion while the uppermost basement underlying the sedimentary cover deforms by folding, thrusting, and displacements along pre‐existing basement faults. In these models, cover units above the salt detachment occur within a less deformed, wide plateau in the hinterland. In relatively cold geotherm models (MT = 400°C at 36 km depth), deformation is mainly restricted to basement imbricate thrusts that form within the orogenic hinterland. Detachment folding, thrusting, and gravity gliding occur within cover sediments above uplifted basement blocks. Gravity gliding contributes to a larger amount of shortening in the cover compared to the basement.

Tracing Clastic Delivery To the Permian Delaware Basin, U.S.A.: Implications For Paleogeography and Circulation In Westernmost Equatorial Pangea

Abstract The Guadalupe Mountains of the southwestern U.S. (New Mexico–West Texas) expose the shelf edge of the Permian Delaware Basin, which formed near the western edge of the Central Pangean suture in equatorial Pangea. Voluminous siliciclastic strata filled this basin in mid-Permian time, and compose the Delaware Mountain Group (Brushy Canyon, Cherry Canyon, and Bell Canyon formations). These units have long been recognized to record ultimate deepwater deposition of siliciclastic sediments that were pre-sorted via eolian transport as they progressed toward the shelf edge of the Delaware Basin in Permian time. The generally arkosic mineralogy led most previous researchers to identify crystalline basement uplifts of the Ancestral Rocky Mountains (ARM) of the western U.S. as the primary source for the large volumes of Permian siliciclastic material that fill this basin. New provenance data combining modal mineralogy with detrital-zircon geochronology, however, refute this interpretation. Rather than an ARM basement source, indicated by Paleoproterozoic ages, the predominant age populations, comprising 70–80% of the grains, reflect contributions from Paleozoic, Neoproterozoic, and Mesoproterozoic (Grenvillian) age basement. The combined (arkosic–subarkosic) mineralogy and provenance spectra of the siliciclastic strata from the Delaware Mountain Group, together with comparisons with coeval strata and available paleocurrent data indicate sources in the Ouachita system, including recycled Appalachian detritus, and terranes accreted and uplifted south of the Ouachita suture, now in Mexico and Central America. Dispersal pathways likely involved a component of fluvial transport draining the piedmont region of the Ouachita orogenic belt, and ultimate eolian deflation of these fluvial systems within an easterly to southeasterly and seasonal westerly to northwesterly atmospheric circulation. Monsoonal transport, especially northern excursions of the ITCZ during northern hemisphere summers, would have facilitated transport from terranes south of the Ouachita–Marathon suture in present-day Mexico and Central America. Source locations predominantly south-southeast of the study region potentially obviate the need to call upon transcontinental fluvial transport directly from the Appalachian orogen to the southwestern edge of Pangea, at least for this time period. Furthermore, these results place new constrains on the final docking, major uplift, and consequent erosion of the Yucatan–Maya terrane south of the Ouachita orogenic belt, which provided significant siliciclastic detritus to depocenters in the western U.S. during middle Permian time. In contrast, the uplifts of the ARM were either covered, or greatly diminished as sources by this time.

Paleogene sedimentation, volcanism, and deformation in eastern Tibet: Evidence from structures, geochemistry, and zircon U–Pb dating in the Jianchuan Basin, SW China

The Cenozoic tectonics of southeastern Tibet feature several large-scale strike-slip ductile shear zones along block boundaries and complex rotation and internal deformation of the blocks that are widely thought to be the result of India–Eurasia collision. The way in which we can match the rotation and internal deformation with the strike-slip movement remains an open issue. The easternmost of these shear zones, the 1000-km-long Ailao Shan/Red River sinistral shear zone, consists of three segments, and the gaps between the segments are occupied by the Chuxiong and Jianchuan Basins. Geologic data from these basins may provide positive constraints on the nature of Cenozoic deformation of continental crust of southeastern Tibet, but few data are available. This paper documents structural, geochronological, and geochemical results to constrain the tectonics of the Cenozoic Jianchuan Basin, eastern Tibet. Detailed field observations along a transect across the Jianchuan Basin revealed a horizon of volcanic rocks that overlies thick massive Eocene mudstones and siltstones, and which in turn is overlapped by coarse-grained Oligocene sandstones and conglomerates. The Late Eocene volcanics are shoshonitic and high in magnesium, and they consist of trachybasalts, trachyandesites, and trachydacites. In situ zircon U–Pb dating demonstrates that the volcanism was short-lived from 35 to 36Ma. Structural data reveal that some of the basement of the Jianchuan Basin has been uplifted, and then thrust over the Cenozoic sediments. Mudstone dikes developed along the main fault plane, and diapirs developed along other minor fractures of the hanging wall. Except for a few gentle folds, no solid-state deformation-induced structures are found in the Cenozoic strata, whereas syndepositional structures are common. These observations suggest that the basement uplift, thrusting, and sedimentation were coeval. Widespread pervasive NW–SE striking vertical fracture-cleavages in the basement suggest that the syndepositional uplift and thrusting were caused by NE–SW directed compression. These structures are distinct from ductile deformational features associated with the sinistral Ailao Shan shear zone. Available data revealed that the ductile shearing lagged ca. 10Ma behind the NE–SW compression. Our data thus suggest that deformation was widely distributed within continental block(s) at an early stage of the India–Eurasia collision, and then localized into some ductile shear zones likely due to block rotation. New zircon U–Pb geochronology and geochemistry of the igneous rocks in the basement show that these igneous rocks formed during the Early Triassic and the earliest Middle Triassic, and not as previously thought during the Late Triassic; they are therefore probably part of the Jomda–Weixi arc rather than the Yidun arc.

Detrital zircon geochronology from the Bighorn Basin, Wyoming, USA: Implications for tectonostratigraphic evolution and paleogeography

Tectonostratigraphic assemblages record phases of basin history during which the fundamental controls of tectonic setting, subsidence style, and basin geometry are relatively similar. Because these fundamental controls, in combination with climate and eustasy, influence paleogeography and sediment-dispersal patterns, they should also yield similar patterns, or facies, of detrital zircon age spectra. Such age-distribution patterns should be documented on the craton in order to make meaningful comparisons to sedimentary rocks from suspect terranes along continental margins. The Rocky Mountains of western North America provide excellent outcrops of sedimentary rocks that record >500 m.y. of tectonostratigraphic evolution. One such Phanerozoic section is exposed along the margins of the Bighorn Basin in northwest Wyoming, from which we report over 4000 U/Th/Pb detrital zircon ages from 48 samples that span a stratigraphic interval from the Middle Cambrian Flathead Sandstone through the Eocene Willwood Formation. These data provide one of the most complete records of detrital zircon age patterns from this part of cratonic North America. The stratigraphic record of the Bighorn Basin is subdivided into four tectonostratigraphic assemblages (TSA1–TSA4). These assemblages record an initial passive margin, followed by a transition to a convergent margin, followed by a marine-dominated retroarc foreland basin, followed by a retroarc foreland segmented by local basement uplifts. This tectonostratigraphic architecture is expressed as four, first-order patterns within the detrital zircon age distributions. TSA1 represents a Paleozoic–Triassic proximal continental margin assemblage dominated by Proterozoic zircons with abundant grains in the 1600–1950 Ma range, a Grenville population at ca. 1100 Ma, and a Phanerozoic population at ca. 420 Ma. TSA2 is a transitional assemblage associated with the Jurassic–Early Cretaceous organization of a west-facing convergent margin and Cordilleran orogen. The TSA2 detrital zircon age distribution is characterized by the appearance of Mesozoic grains, age peaks at ca. 420 and 600 Ma, and a dominant population of Grenville (1.0–1.1 Ga) grains with a suite of Proterozoic grains diminishing in abundance as age increases to 1.9 Ga. TSA3 sedimentary rocks were deposited in the Cretaceous Interior Seaway in a retroarc foreland basin and are dominated by zircons for which ages are close to the depositional age of the strata, reflecting input from the active Idaho Batholith and Sierran segments of the Cordilleran magmatic arc. The older zircon fractions from TSA3 sedimentary rocks are characterized by a dominant detrital zircon age peak at 1.7–1.8 Ga, which probably reflects reworking of Belt Supergroup metasedimentary rocks from the northwest into the Cretaceous foreland, based on regional paleogeographic patterns. TSA4 reflects the phase of basin fill associated with Paleogene structural segmentation of the retroarc foreland during the Laramide orogeny. Detrital zircon age spectra from this assemblage record erosion and redeposition of all previous sedimentary rocks from surrounding basement uplifts. Patterns of detrital zircon ages reflect fundamental changes in paleogeography and sediment dispersal at the 10–100 m.y. time scale and are clearly related to major tectonic events or phases. Detrital zircon ages also provide evidence for linkages between convergent margin processes such as arc magmatism and sedimentation in the retroarc foreland. During these times of strong arc-retroarc linkage, detrital zircon geochronology provides a potentially useful tool for high-resolution chronostratigraphy.

The Waqf as Suwwan crater, Eastern Desert of Jordan: aspects of the deep structure of an oblique impact from reflection seismic and gravity data

The deeply eroded Waqf as Suwwan ring structure was recently discovered to be a large impact, the first identified in the near east. Large-scale reflection seismic structure shows the impact situated high on the northeastern flank of the Jordan Uplift sloping into Wadi Sirhan Basin. If exhumation is linked to the Arabia–Eurasia collision, a likely time window for the impact event may be latest Eocene to Late Oligocene. Impact into a shallow sea seems an optional scenario. Old reflection seismic lines offer limited insight into the deep structure of the rim and part of the central uplift of the complex crater. An important structural clue is provided by a well-resolved seismic horizon of a yet tentative correlation with a Paleozoic black shale. The central gravity high is compatible with a mass surplus by the uplift of denser Paleozoic basement below the central uplift. The gravity model further indicates a ring of dense Paleozoic sediments rising from below into the ring syncline. Seismics show presumably radial synclines in the central uplift which are interpreted by centripetal constrictional flow during crater collapse. Beneath the final crater’s outer boundary, a shallow-dip normal fault zone, subtle seismic structure in uncollapsed footwall segments reveal an asymmetry of strain. The asymmetry is attributed to the cratering flow by an oblique impact directed toward NE. The finding provides independent support to an earlier suggestion of impact obliquity based on vergency of folds exposed on the central uplift.

Vendian sedimentation and low-rate syn-sedimentary tectonic activity in SE Lithuania

The Vendian deposits represent the basal part of the sedimentary cover of the Baltic sedimentary basin. The sedimentation was restricted to the eastern part of Lithuania that represented the western margin of the vast Moscow basin. The thickness of the Vendian deposits reaches 170 m in the east. Data of more than 100 wells were inspected to map lithofacies distribution of different stratigraphical units of Vendian in the southeastern Lithuania. An emphasis has been made to standard logging data because stratigraphy of the Vendian deposits is based on lithological criteria. Various authors presented different interpretations of the lithofacies variations that resulted in a non-unique stratigraphical subdivision of well sections. The presented study allowed unification of the stratigraphical subdivision of the southeastern Lithuanian wells. Five formations, i. e. the Merkys, Jasiūnai, Rūdninkai, Skynimai, and Vilkiskės Fms, are defined up the Vendian succession of Lithuania. They compose individual sedimentation cycles. The sedimentation trend indicates a gradual levelling of the relief and an increasing degree of the chemical weathering of the provenance rocks (exposed crystalline basement). The lithofacies and isopach maps of the Vendian formation were compiled that revealed sourcing and sedimentation trends as well as the syn-sedimentary tectonic activity of local structures. Three sub-regional structures were defined in the study area, i. e. the Žiežmariai–Maisiagala and Merkys depressions separated by the South Lithuanian elevation striking west–east. Against this background structural pattern smaller-scale structures were recognised that also affected thickness and lithofacies variations, the latter pointing to syn-depositional activity of structures. The South Lithuanian uplift is characterised by deposition of coarser grained sediments compared to the adjacent Žiežmariai–Maisiagala and Merkys depressions. The local structures Salcininkėliai, Salcininkai, Pabarė, Kalesninkai, Barciai, Varėna were defined within the Merkys depression in the south. The lithofacies distribution was sensitive to the structural grain of the basin bottom. The lithofacies distribution reflects not only local features, but also it is indicative of activity of the uplifts located beyond the basin that were eroded during the Vendian time. A discordant trend of lithofacies belts with respect to thickness changes pursued the presence of the uplifted structure further south that can be related to the Grodno–Mosty basement uplift suggested in the northwest Belarus, that shaded the clastic material to the adjacent basin in southwest Lithuania.

Morphometric analysis of a reactivated Variscan fault in the southern Bohemian Massif (Budějovice basin, Czech Republic)

Quantitative geomorphic parameters are used to assess active vertical displacements at the NW–SE striking Hluboká fault and the NNE–SSW striking Rudolfov fault in the southern Bohemian Massif. The faults are part of a late Variscan fault system that was repeatedly reactivated in Mesozoic, Miocene, and Pliocene times forming the margins of the Budějovice basin. This basin is filled with up to 340-m-thick Cretaceous to Quaternary sediments and forms morphological lowland surrounded by hill country.We compared the basin-facing hillslopes along the faults with other slopes that are not fault-controlled. All creeks and drainage basins share similar geological and hydrological settings and the common base level of the Vltava River. Morphological differences in valley shapes, stream profiles, and drainage basin geometry therefore are likely to reflect different uplift of the crystalline basement with respect to the Budějovice basin.All calculated geomorphic parameters characterise the hillslope along the Hluboká fault as a very straight mountain–piedmont junction with a morphology that is influenced by uplift along the fault. Differential uplift is indicated by extremely low values of mountain front sinuosity (Smf 1.01 to 1.06), high stream length gradients (SL up to 200), and very low valley floor width to height ratios (Vf 0.05 to 0.26). The values are clearly distinct from the values observed at the other hillslopes. Streams showing convex-up thalweg sections with marked single knickpoints close to the Hluboká fault and previously published geodetic data (Vyskočil, 1973) support the interpretation of active vertical fault displacement. Values observed at the Hillslope crossing the Rudolfov fault (Smf 1.17 to 2.20, SL up to 130, Vf 0.29 to 2.5) and convex-up stream profiles of creeks crossing the fault may classify the slope along the Rudolfov fault as moderately active. The interpretation is again corroborated by published geodetic data.The analysed part of the margin of the Budějovice basin that is not fault controlled revealed geomorphic parameters that are not indicative for differential vertical displacements (Smf 1.30 to 4,16, SL up to 57, Vf 2.10 to 4.80). Concave-up stream profiles of creeks draining into the basin are characteristic for equilibrated streams.

Origin of the central magnetic anomaly at the Haughton impact structure, Canada

The 23km-diameter well-preserved Haughton impact structure shows a rather unique combination of a positive magnetic anomaly with a negative gravity anomaly over the center of its central uplift. Using a new ground magnetic dataset and several modeling approaches, we investigate the properties and geometry of its central magnetized source. Our results confirm that a km-sized magnetic body with a narrow near-surface extension is necessary to account for the anomaly. Additional measurements of rock magnetic properties of samples of all lithologies encountered in and outside the crater show that the target sedimentary rocks and the vast majority of the Precambrian basement rocks cannot be the source of the magnetic anomaly. While in larger impact structures such magnetic anomalies are often explained by magmatic mafic intrusions or highly magnetic glass lenses in the impact melt rocks, we propose that impact-generated hydrothermal activity enhanced the magnetization of the highly-porous unmelted uplifted basement rocks. Such a process may be considered for the interpretation of the geophysical signature of planetary impact craters.

Evidence for Late Devonian vertical movements and extensional deformation in northern Africa and Arabia: Integration in the geodynamics of the Devonian world

AbstractThe Upper Paleozoic geodynamic evolution is discussed at the scale of a wide part of Gondwana from North Africa to Arabia. With the aim of giving an integrated tectonic scenario for the study domain, we revisit six key areas, namely, the Anti‐Atlas Belt (Morocco), the Bechar Basin (west Algeria), the Hassi R'Mel High (central Algeria), the Talemezane Arch (south Tunisia), the Western Desert (Egypt), and, finally, the High Zagros Belt (Iran). Below the so‐called “Hercynian unconformity,” which is in reality a highly composite discontinuity, surface and subsurface data display a well‐known arch‐and‐basin geometry, with basement highs and intervening Paleozoic basins. We show that this major feature results mainly from a Late Devonian event and can no longer be interpreted as a far effect of the Variscan Orogeny. This event is characterized by a more or less diffuse extensional deformation and accompanied either by subsidence, in the western part of the system, or by an important uplift of probable thermal origin followed by erosion and peneplanation. By the end of the Devonian, the whole region suffered a general subsidence governed by the progressive cooling of the lithosphere. Such a primary configuration is preserved in Arabia with typical sag geometry of the Carboniferous and Permian deposits but strongly disturbed elsewhere by the conjugated effects of the Variscan Orogeny during the Carboniferous and/or by subsequent uplifts linked to the central Atlantic and Neo‐Tethys rifting episodes. In conclusion, we try to integrate this new understanding in the geodynamics of the Late Devonian, which at world scale is characterized by the onset of the Variscan Orogeny on the one hand and by magmatism, rifting, and basement uplift on the other hand.

Geodynamics of NW India: Subduction, Lithospheric Flexure, Ridges and Seismicity

Abstract Spectral analysis of digital data of the Bouguer anomaly map of NW India suggests maximum depth of causative sources as 134 km that represents the regional field and coincides with the upwarped lithosphere – asthenosphere boundary as inferred from seismic tomography. This upwarping of the Indian plate in this section is related to the lithospheric flexure due to its down thrusting along the Himalayan front. The other causative layers are located at depths of 33, 17, and 6 km indicating depth to the sources along the Moho, lower crust and the basement under Ganga foredeep, the former two also appear to be upwarped as crustal bulge with respect to their depths in adjoining sections. The gravity and the geoid anomaly maps of the NW India provide two specific trends, NW-SE and NE-SW oriented highs due to the lithospheric flexure along the NW Himalayan fold belt in the north and the Western fold belt (Kirthar – Sulaiman ranges, Pakistan) and the Aravalli Delhi Fold Belt (ADFB) in the west, respectively. The lithospheric flexures also manifest them self as crustal bulge and shallow basement ridges such as Delhi – Lahore – Sagodha ridge and Jaisalmer – Ganganagar ridge. There are other NE-SW oriented gravity and geoid highs that may be related to thermal events such as plumes that affected this region. The ADFB and its margin faults extend through Ganga basin and intersect the NW Himalayan front in the Nahan salient and the Dehradun reentrant that are more seismogenic. Similarly, the extension of NE-SW oriented gravity highs associated with Jaisalmer – Ganganagar flexure and ridge towards the Himalayan front meets the gravity highs of the Kangra reentrant that is also seismogenic and experienced a 7.8 magnitude earthquake in 1905. Even parts of the lithospheric flexure and related basement ridge of Delhi – Lahore – Sargodha show more seismic activity in its western part and around Delhi as compared to other parts. The geoid highs over the Jaisalmer – Ganganagar ridge passes through Kachchh rift and connects it to plate boundaries towards the SW (Murray ridge) and NW (Kirthar range) that makes the Kachchh as a part of a diffused plate boundary, which, is one of the most seismogenic regions with large scale mafic intrusive that is supported from 3-D seismic tomography. The modeling of regional gravity field along a profile, Ganganagar – Chandigarh extended beyond the Main Central Thrust (MCT) constrained from the various seismic studies across different parts of the Himalaya suggests crustal thickening from 35-36 km under plains up to ~56 km under the MCT for a density of 3.1 g/cm3 and 3.25 g/cm3 of the lower most crust and the upper mantle, respectively. An upwarping of ~3 km in the Moho, crust and basement south of the Himalayan frontal thrusts is noticed due to the lithospheric flexure. High density for the lower most crust indicates partial eclogitization that releases copious fluid that may cause reduction of density in the upper mantle due to sepentinization (3.25 g/cm3). It has also been reported from some other sections of Himalaya. Modeling of the residual gravity and magnetic fields along the same profile suggest gravity highs and lows of NW India to be caused by basement ridges and depressions, respectively. Basement also shows high susceptibility indicating their association with mafic rocks. High density and high magnetization rocks in the basement north of Chandigarh may represent part of the ADFB extending to the Himalayan front primarily in the Nahan salient. The Nahan salient shows a basement uplift of ~ 2 km that appears to have diverted courses of major rivers on either sides of it. The shallow crustal model has also delineated major Himalayan thrusts that merge subsurface into the Main Himalayan Thrust (MHT), which, is a decollment plane.