Tectonic Quiescence Research Articles

Archean crust and metallogenic zones in the Amazonian Craton sensed by satellite gravity data

The formation of ore deposits has been extensively studied from a shallow crust perspective. In contrast, the association of mineral systems with deep crustal structure of their host terranes remains relatively undisclosed, and there is evidence that processes throughout the lithosphere are coupled for their evolution. The current debate centers on the control of the regional deep crustal architecture in focusing and transferring fluids between geochemical reservoirs. Defining such architecture is not unequivocal, and involves combining indirect information in order to constrain its physical properties and evolution. Herein, based on evidence from satellite gravity, constrained by airborne potential field data (gravity and magnetics), we provide an example on how the lithosphere geometry controlled the location of copper and gold systems in the world-class Archean Carajás Mineral Province (Amazonian Craton, South America). Validation with information from passive seismic (wave speeds, crustal and lithospheric thickness) and geochronologic data (model, crystallization ages, and Neodymium isotope ratio determinations) portrays a significantly enlarged, poly-phase, Archean crust that exerted geometric control on the location of the mineral systems within and adjacent to the province during tectonic quiescence and switches. This new geologic scenario impacts the understanding of the Amazonian Craton. Synergy between multi-source data, as experimented here, can provide robust models efficiently and, conceivably, help to unveil similar terrains elsewhere.

Cenozoic cooling history of the North Qilian Shan, northern Tibetan Plateau, and the initiation of the Haiyuan fault: Constraints from apatite- and zircon-fission track thermochronology

The growth of the Tibetan Plateau resulted primarily from India-Asia convergence since initial collision at 65–55 Ma. The Cenozoic Qilian Shan thrust belt and the left-slip strike Haiyuan fault system together define the northeastern margin of the plateau. Although these thrust and strike-slip fault systems play an important role in accommodating continental convergence, our knowledge of the temporal and spatial distribution of upward and outward growth of the northeastern Tibetan Plateau is still lacking. In this study, we integrate new geologic mapping and low-temperature themochronometry (apatite- and zircon-fission-track ages) to provide constraints on the uplift and cooling history of the North Qilian Shan and the initiation of the Haiyuan fault. Cooling ages and thermal history modeling from a traverse across a prominent restraining bend on the Haiyuan fault suggest the North Qilian Shan experienced a three-phase cooling history, including: (1) broad cooling during the Late Triassic to early Cenozoic, (2) long-term tectonic quiescence from late Cretaceous to middle Miocene, and (3) rapid cooling and exhumation from ~15–10 Ma to present. We relate this most recent local cooling (~15–10 Ma) to the initiation of strike-slip faulting along the central Haiyuan fault, which corroborates other recent studies, suggesting a middle Miocene activation of strike-slip deformation along the Haiyuan fault in northern Tibet.

U-Pb detrital zircon geochronology, petrography, and synthesis of the middle Neoproterozoic Visingsö Group, Southern Sweden

The middle Neoproterozoic Visingsö Group, Southern Sweden, is a fossiliferous mixed carbonate-siliciclastic unit that provides an environmental and biological record of southeast (present day coordinates) Baltica. Extensive work on microfossil assemblages has allowed correlation with geochronologically constrained middle Neoproterozoic units elsewhere and a proposed depositional age between ∼780 and 730 Ma. The stratigraphic succession comprises three informal formations that record a shift in depositional environment from fluviomarine to a retrograding marine fan delta system. Petrographic and U-Pb LA-ICPMS detrital zircon (DZ) analyses are reported on six sandstone samples spanning the three formations. DZ ages range from 2840 to 919 Ma, with significant populations at 1870, 1800, 1320, and 1020 Ma. Each formation is characterized by a distinct age distribution. In the lower formation, ages are dominated by a peak at 1020 Ma, in the middle formation, however, ages are skewed toward 1800–1870 Ma. DZs from the upper formation display a more bell-curve shaped distribution, with dominant age peaks at 1550 and 1420 Ma. Provenance analysis indicates that the lower formation was likely sourced from the Feda-Sirdal magmatic belt in the distal Telemarkia lithotectonic unit to the west. DZ populations in the middle formation, combined with sedimentology, indicate tectonic activity and sourcing of the older Transscandinavian Igneous Belt east of the Sveconorwegian Front. DZs from the upper member show a predominance of Gothian and Hallandian magmatic rocks sourced from the west and, coupled with sedimentology, indicate tectonic quiescence. The maximum depositional age (919 ± 25 Ma, youngest single grain; 941 ± 12 Ma, average of five youngest grains) is more than 100 m.y. older than the purported age of the strata based on previous paleontological studies, and therefore necessitates that other dating methods be utilized to generate an age model for the Visingsö Group.

Influence of basement heterogeneity on the architecture of low subsidence rate Paleozoic intracratonic basins (Reggane, Ahnet, Mouydir and Illizi basins, Hoggar Massif)

Abstract. The Paleozoic intracratonic North African Platform is characterized by an association of arches (ridges, domes, swells, or paleo-highs) and low subsidence rate syncline basins of different wavelengths (75–620 km). The Reggane, Ahnet, Mouydir and Illizi basins are successively delimited from east to west by the Amguid El Biod, Arak-Foum Belrem, and Azzel Matti arches. Through the analysis of new unpublished geological data (i.e., satellite images, well logs, seismic lines), the deposits associated with these arches and syncline basins exhibit thickness variations and facies changes ranging from continental to marine environments. The arches are characterized by thin amalgamated deposits with condensed and erosional surfaces, whereas the syncline basins exhibit thicker and well-preserved successions. In addition, the vertical facies succession evolves from thin Silurian to Givetian deposits into thick Upper Devonian sediments. Synsedimentary structures and major unconformities are related to several tectonic events such as the Cambrian–Ordovician extension, the Ordovician–Silurian glacial rebound, the Silurian–Devonian Caledonian extension/compression, the late Devonian extension/compression, and the Hercynian compression. Locally, deformation is characterized by near-vertical planar normal faults responsible for horst and graben structuring associated with folding during the Cambrian–Ordovician–Silurian period. These structures may have been inverted or reactivated during the Devonian (i.e., Caledonian, Mid–Late Devonian) compression and the Carboniferous (i.e., pre-Hercynian to Hercynian). Additionally, basement characterization from geological and geophysics data (aeromagnetic and gravity maps), shows an interesting age-dependent zonation of the terranes which are bounded by mega-shear zones within the arches–basins framework. The old terranes are situated under arches while the young terranes are located under the basins depocenter. This structural framework results from the accretion of Archean and Proterozoic terranes inherited from former orogeny (e.g., Pan-African orogeny 900–520 Ma). Therefore, the sedimentary infilling pattern and the nature of deformation result from the repeated slow Paleozoic reactivation of Precambrian terranes bounded by subvertical lithospheric fault systems. Alternating periods of tectonic quiescence and low-rate subsidence acceleration associated with extension and local inversion tectonics correspond to a succession of Paleozoic geodynamic events (i.e., far-field orogenic belt, glaciation).

Neoproterozoic evolution and Cambrian reworking of ultrahigh temperature granulites in the Eastern Ghats Province, India

AbstractThe time‐scales and P–T conditions recorded by granulite facies metamorphic rocks permit inferences about the geodynamic regime in which they formed. Two compositionally heterogeneous cordierite–spinel‐bearing granulites from Vizianagaram, Eastern Ghats Province (EGP), India, were investigated to provide P–T–time constraints using petrography, phase equilibrium modelling, U–Pb geochronology, the rare earth element composition of zircon and monazite, and Ti‐in‐zircon thermometry. These ultrahigh temperature (UHT) granulites preserve discrete compositional layering in which different inferred peak assemblages are developed, including layers bearing garnet–sillimanite–spinel, and others bearing orthopyroxene–sillimanite–spinel. These mineral associations cannot be reproduced by phase equilibrium modelling of whole‐rock compositions, indicating that the samples became domainal on a scale less than that of a thin section, even at UHT conditions. Calculation of the P–T stability fields for six compositional domains within which the main rock‐forming minerals are considered to have attained equilibrium suggests peak metamorphic conditions of ~6.8–8.3 kbar at ~1,000°C. In most of these domains, the subsequent evolution resulted in the growth of cordierite and final crystallization of melt at an elevated (residual) H2O‐undersaturated solidus, consistent with <1 kbar of decompression. Concordant U–Pb ages obtained by SHRIMP from zircon (spread 1,050–800 Ma) and monazite (spread 950–800 Ma) demonstrate that crystallization of these minerals occurred during an interval of c. 250 Ma. By combining LA‐ICP‐MS U–Pb zircon ages with Ti‐in‐zircon temperatures from the same analysis sites, we show that the crust may have remained above 900°C for a minimum of c. 120 Ma between c. 1,000 and c. 880 Ma. Overall, the results suggest that, in the interval 1,050 to 800 Ma, the evolution of the Vizianagaram granulites culminated with UHT conditions from c. 1,000 Ma to c. 880 Ma, associated with minor decompression, before further zircon crystallization at c. 880–800 Ma during cooling to the solidus. However, these rocks are adjacent to the Paderu–Anantagiri–Salur crustal block to the NW that experienced counterclockwise P–T–t paths, and records similar UHT peak metamorphic conditions (7–8 kbar, ~950°C) followed by near‐isobaric cooling, and has a similar chronology during the Neoproterozoic. The limited decompression inferred at Vizianagaram may be explained by partial exhumation due to thrusting of this crustal block over the adjacent Paderu–Anantagiri–Salur crustal block. The residual granulites in both blocks have high concentrations of heat‐producing elements and likely remained hot at mid‐crustal depths throughout a period of relative tectonic quiescence in the interval 800–550 Ma. During the Cambrian Period, the EGP was located in the hinterland of the Denman–Pinjarra–Prydz orogen. A later concordant population of zircon dated at 511 ± 6 Ma records crystallization at temperatures of ~810°C. This age may record a low‐degree of melting due to limited influx of fluid into hot, weak crust in response to convergence of the Crohn craton with a composite orogenic hinterland comprising the Rayner terrane, EGP, and cratonic India.

Mesozoic to Cenozoic retroarc basin evolution during changes in tectonic regime, southern Central Andes (31–33°S): Insights from zircon U-Pb geochronology

The southern Central Andes of Argentina and Chile (27–40°S) are the product of deformation, arc magmatism, and basin evolution above a long-lived subduction system. With sufficient timing and provenance constraints, Andean stratigraphic and structural records enable delineation of Mesozoic-Cenozoic variations in subsidence and tectonic regime. For the La Ramada Basin in the High Andes at ∼31–33°S, new assessments of provenance and depositional age provided by detrital zircon U-Pb geochronology help resolve deformational patterns and subsidence mechanisms over the past ∼200 Myr. Marine and nonmarine clastic deposits recorded the unroofing of basin margins and sediment contributions from the Andean magmatic arc during Late Triassic to Early Cretaceous extension, thermal subsidence, and possible slab rollback. Subsequent sediment delivery from the Coastal Cordillera corresponded with initial flexural accommodation in the La Ramada Basin during Andean shortening of late Early Cretaceous to Late Cretaceous age. The architecture of the foreland basin was influenced by the distribution of precursor extensional depocenters, suggesting that inherited basin geometries provided important controls on later flexural subsidence and basin evolution. Following latest Cretaceous to early Paleogene tectonic quiescence and a depositional hiatus, newly dated deposits in the western La Ramada Basin provide evidence for a late Paleogene episode of intra-arc and proximal retroarc extension (development of the Abanico Basin, principally in Chile, at ∼28–44°S). Inversion of this late Paleogene extensional basin system during Neogene compression indicates the southern Central Andes were produced by at least two punctuated episodes of shortening and uplift of Late Cretaceous and Neogene age.

Geodynamic conditions for Cenozoic activation of tectonic structures in Southeastern Mongolia

The knowledge of the neotectonic structures inSoutheastern Mongolia, that is considerably distant from the active plate boundaries, is important for determining a source of tectonic deformation and regular features of activation in the intracontinental setting. Our research was focused on the East Gobi and South Gobi depressions located inSoutheastern Mongolia, which developed since the Mesozoic and were activated to various degrees in the neotectonic stage. The study aimed to assess the paleostress state of the crust inSoutheastern Mongolia, identify the stages, factors and mechanisms of the Cenozoic activation of the regional structures of different strike, and determine the sources of activation. The analysis of the available literature suggests a similar history of their development in the Late Jurassic – Early Cretaceous (rifting) and Late Cretaceous – Paleogene (tectonic quiescence). In the Cenozoic stage, the depressions experienced activation of completely different styles. In theEast Gobidepression, left-lateral strike-slip faults were activated in the Tertiary, and the post-Late Cretaceous thrusting took place along the northeastern faults on the northern slope of the Totoshan uplift. In the Early Cenozoic, the N-S and N-W compression was dominant as evidenced by the deformed Late Cretaceous sediments and the reconstructed stress tensors typical of the compression and transpression regimes. An overview of the published data suggests that the most probable cause of such deformation was the impact of the Western Pacific zone of plate interaction. However, a potential influence of compression at the early stages of the Indo-Asian collision cannot be completely excluded. TheEast Gobidepression was low active in the second half of the Cenozoic. In contrast to the East Gobi depression, theSouth Gobiactivation began in the Late Cenozoic (Late Miocene – Early Pliocene). Young uplifts and forbergs (Gobi Altai eastern termination) developed actively and ‘cut’ the sediments of the basins originating from the Mesozoic. The W-E and N-W strike-slip and thrust faults were active in the Pliocene–Quaternary. The stress field reconstructions show compression, transpression and strike-slip regimes with the NE-trending axis of compression. Deformation in the East Goby Altay (as well as in Western andSouthwestern Mongolia) is driven by the India-Eurasia collision.

Petrogenesis, provenance, and rare earth element geochemistry, southeast desert phosphorite, Jordan

Representative samples from the Al-Hisa Phosphorite Formation (AHP) in the SE desert of Jordan, are selected from three sections: Batn El-Ghoul, Nagb Etayyeg, and Zgaimat Al-Hasah. The samples are investigated geochemically and petrographically. Geochemistry is discussed through the analysis of the major, trace and rare earth elements (REEs), loss on ignition (LOI), sulphur, and organic matter. The studied sections differ from the AHP in central Jordan by having highly reduced thicknesses, omission of underlying formations, abundant detrital quartz sand and age. The phosphate particles are of authigenic and biogenic origin. Early reworking from phosphate mud is evident from the shape and sorting of the phosphate particles. Major and trace elements are distributed into five factors: upwelling, provenance, redox potential, sea level changes, and resistates. The studied sediments were formed under oxic marine conditions because their REEs patterns preserve the REEs seawater linear pattern: depletion of LREEs, a negative Ce anomaly, and an enrichment of HREEs. This signature seems to have persisted through reworking and late diagenetic cementation, dolomitization and silicification. The SE desert phosphorite-siliciclastic sequence is better correlated with the Paleocene Jalamid Formation of northwestern Saudi Arabia because of similarities in age and lithology due to being both deposited on the western flanks of the Sirhan Paleohigh and the Rutba Paleohigh, respectively. The siliciclastics were delivered from a mixed mafic-felsic-older cycle siliciclastic provenance, and subjected to intensive chemical weathering under tectonic quiescence giving rise to mature quartzarenites.

Incremental Emplacement of the Late Jurassic Midcrustal, Lopolith‐Like Qitianling Pluton, South China, Revealed by AMS and Bouguer Gravity Data

AbstractModes of magma emplacement may provide key information on geodynamic‐tectonic settings of magmatism, when the threshold of magma volume and duration of emplacement process is crossed, brittle/ductile crustal deformation behavior, and volumes and rates of magma transfer in the crust. The majority of upper‐crustal granitic intrusions are incrementally emplaced as laccoliths or sills, while midcrustal or deeper‐level granitic intrusions appear to be predominantly emplaced as sills or lopoliths, but relatively few examples have been studied. We present a case study on the Jurassic Qitianling pluton from South China, which was constructed in the upper part of the middle crust (at a depth of ~10–15 km). To characterize its emplacement mechanism and evolution, we have combined field and microscopic observations, Anisotropy of Magnetic Susceptibility measurements and Bouguer gravity anomaly modeling. The main findings are (1) the generally undeformed granite‐country rock contacts with a narrow thermal aureole, (2) irregularly wavy and gradational contacts between the main granite facies, (3) dominant subhorizontal magnetic fabrics in the granites with a scattered magnetic foliation, (4) rare occurrence of dykes, and (5) a subcircular surface and lopolith‐like pluton geometry. Accordingly, we suggest that the Qitianling pluton forms an ~25‐km wide and ~5‐km thick lopolith that was incrementally emplaced during a period of tectonic quiescence, most likely by under‐accretion of multiple magma sheets deflected from magma pulses that ascended through pipe‐like channels. The mineralogical and compositional variation of the pluton may be the result of in situ fractionation, fractionation at deeper crustal level, or variation in source‐derived melt compositions.

CONTROLE ESTRUTURAL DA DEPOSIÇÃO MESOZOICA NAS BACIAS DA MARGEM CONTINENTAL BRASILEIRA

The sedimentary fill of the Brazilian marginal basins is made up, when complete, of three suites reflecting the evolutionary stages of the continental margin: (1) the lower clastic non-marine suite was deposited in two stages, the basal section in a pre-rift stage, and the upper section in a rift-valley stage: (2) the middle suite, including the evaporitic section and the overlying shelf carbonates, was deposited in a stage of tectonic quiescence; (3) the upper marine suite, including some paralic and continental sediments, was deposited in a stage of vertical differential movements along flexure lines. The Upper Jurassic non-marine sediments of the basal section in Brazil (Sergipe-Alagoas, Reconcavo, and Almada Basins), correlate well with those of West Africa (Gabon and Congo Basins) showing a symmetric areal distribution. The absence of this Upper Jurassic section, south of the Jequitinhonha Basin in Brazil and the Cuanza Basin in West Africa is probably due to non-deposition in uplifted areas coinciding with sites of Early Cretaceous volcanic activity. The juxtaposition of the uplifted-volcanic areas in Brazil and Africa in a pre-drift arrangement suggests the existence of a hot-spot, active in Late Jurassic-Early Cretaceous times. An earlier rifting in the uplifted areas is suggested by the presence of Wealdian evaporites in the Santos and of Aptian marine sediments in the Pelotas Basin. Both sediment types are older than their lithological - environmental counterparts along the continental margin from the Campos to the Sergipe Basin. Along the northern margin, from the Amazon delta to the Potiguar Basin, the pre-rift and rift-valley stages are not well known. There is evidence of Early Cretaceous rifting in aborted rift-valleys [v.g. the Sao Luiz Basin) and also of Late Cretaceous compressional forces. The relationship between, and the duration of the tensional and compressional forces are still unclear.

1.39 Ga mafic dyke swarm in southwestern Yilgarn Craton marks Nuna to Rodinia transition in the West Australian Craton

The Archean Yilgarn Craton in Western Australia hosts at least five generations of mafic dykes ranging from Archean to Neoproterozoic in age, including the craton-wide ca. 2408 Ma Widgiemooltha and the 1210 Ma Marnda Moorn Large Igneous Provinces (LIP), the 1888 Ma Boonadgin dykes in the southwest and the 1075 Ma Warakurna LIP in the northern part of the craton. We report here a newly identified NNW-trending mafic dyke swarm, here named the Biberkine dyke swarm, in the southwestern Yilgarn Craton dated at 1390 ± 3 Ma by ID-TIMS U-Pb geochronology of baddeleyite. The regional extent of the dyke swarm is uncertain but aeromagnetic data suggest that the dykes are part of a linear swarm several hundred kilometers long, truncated by the Mesoproterozoic Albany-Fraser Orogen to the south. Geochemical data indicate that the dykes have tholeiitic compositions with a significant contribution from metasomatically enriched subcontinental lithospheric mantle and/or lower continental crust. Paleogeographic reconstructions suggest that a prolonged tectonic quiescence in the Yilgarn Craton from ca. 1600 Ma was interrupted by renewed subduction along the southern and southeastern margin at ca. 1400 Ma, reflecting a transition from Nuna to Rodinia configuration. The 1390 Ma Biberkine dykes may be a direct consequence of this transition and mark the change from a passive to active tectonic setting, which culminated in the Albany-Fraser Orogeny at ca. 1330 Ma. The Biberkine dykes are coeval with a number of other mafic dyke swarms worldwide and provide an important target for paleomagnetic studies.

Source-to-sink analysis of continental rift sedimentation: Triassic Cuyo basin, Precordillera Argentina

The northernmost outcrops of the Triassic extensional Cuyo basin in western Argentina reveal a basal coarse conglomerate overlain by a thinning-fining upward siliciclastic red-bedded section with subordinated tuffs and microbial-rich carbonates (Cerro Puntudo Formation), unconformably covered by a second coarse conglomerate (El Relincho Formation), which were previously interpreted as two rifting events. An integrated analysis of facies associations, stacking patterns, paleocurrent and provenance was carried out in order to reveal the rift sedimentation using a source-to-sink approach. Besides the traditional controls in continental basins (e.g.,sedimentation, tectonics and climate), this study considers possible drainage pattern evolution in order to differentiate isolated versus integrated depocenter stages. U-Pb zircon data corroborate our analysis and reveal a volcanic basement source of 256.8±3.5Ma (Permian/Lopingian) and a sedimentation age of 249.8±2.5Ma (Early Triassic) for a tuff interbedded within the Cerro Puntudo Formation, complementing previous age of 243.8±1.9Ma (Middle Triassic) at the top of this unit. Stratigraphic stacking allows the interpretation of three evolutionary stages. A first stage, represented by thick massive boulder-volcanic-rich conglomerates characterizing alluvial fan deposition, indicates strong tectonic activity of border faults (first rifting event). These features suggest high surface gradient and high available space controlling accommodation and dispersal of gravity and debris-flow deposits. Paleocurrents to the east reveal largely north-south normal faults located to the west and sediment supply from a proximal volcanic source area. A second stage, represented by a notable fining-thinning upward facies, including fluvial and palustrine (stromatolites with pedogenetic features) deposits, indicates a transition to tectonic quiescence. These facies associations developed in a context of low sedimentation rate, limited tectonic relief and low accommodation space. A third stage is represented by greenish well-roundedcoarse-grained polymictic conglomerates of El Relincho Formation that unconformably truncate the Cerro Puntudo Formation. These conglomerates are interpreted as perennial braided fluvial systems (with paleocurrents to the northwest) indicating drainage reorganization primarily influenced by climate change and combined tectonic reactivation, previously considered the second rifting event. Climate change affected sediment delivery and allowed regionally connectivity and overfilling of the depocenters. In this last stage, northwest axial transport from variable sources within the Precordillera (to the southeast) is inferred from provenance analysis indicating open linked-depocenters. This study suggests a new stratigraphic correlation for the northern part of the Cuyo basin, implying that during the first rifting event a semi-arid climate prevailed with development of isolated and independent depocenters (first and second stages), whereas above the unconformity initiating the second rifting event (third stage) depocenters were interconnected. In the light of our source-to-sink analysis, such paleogeographic change allowed overfilling and laterally linkage of separate half-grabens along the Cuyo basin. Whereas tectonic reactivation may better explain renewed conglomerate deposition, climate change toward more humid conditions allowed previously separated depocenters to be connected through axial drainages.

The late Mesozoic evolution of the Central High Atlas domain (Morocco): Evidence from the paleo-drainage record of the Adrar Aglagal syncline

This paper illustrates the results of a stratigraphic, sedimentologic and structural study of the continental succession exposed at the core of the Adrar Aglagal syncline, located at the southern front of the Central High Atlas (Morocco). This structure deforms a Lower Jurassic-Upper Cretaceous succession, resting unconformably on Triassic clastic and volcanic deposits. Marine-transitional Lower Jurassic limestone, dolomite and gypsum crop out at the syncline's limbs whereas the core is characterized by a continental clastic succession which is the local expression of a sedimentary complex regionally known as Couches Rouges (red beds). These continental successions, attesting to a Middle Jurassic-Lower Cretaceous regressive phase, have different interpretations in the frame of the tectono-sedimentary evolution of the Central High Atlas. According to some authors these red beds record localized early compressive-transpressive stages of deformation, others refer them to a period of tectonic quiescence. Facies analysis and recognition of unconformities have allowed a stratigraphic revision and a new subdivision of these, mostly fluvial, deposits in five units. Paleocurrent analysis, compositional data and the evidence of syn-depositional growth of the syncline support a picture of crustal shortening that affected this axial portion of the Central High Atlas in a period considered tectonically quiescent. This case study supports the hypothesis that the Moroccan Atlas system experienced significant relief formation, as the consequence of the peculiar geodynamic setting in which the northern margin of the Africa plate was located during the Mesozoic.

Late Miocene stratigraphy, palaeoclimate and evolution of the Sandanski Basin (Bulgaria) and the chronology of the Pikermian faunal changes

Abstract We study the sedimentology, palaeontology and palaeomagnetism of the 500 m thick sedimentary sequence in the Gorna Sushitsa gorge in the Sandanski Basin (southwest Bulgaria), which is exceptionally rich in fossil mammals of Pikermian type. Magnetostratigraphy indicates that this section was deposited within 2 myrs, between 8.5 Ma and 6.5 Ma (late Tortonian to early Messinian). Our investigations reveal that the Sandanski Basin infill represents two stacking synrift phases (Delcevo and Kalimantsi formations), with an intervening tectonic quiescence package (Sandanski Formation). Kalimantsi Formation deposition ends with the cessation of slip on basin-bounding faults at 6.9 Ma, probably caused by a switch from regional E-W to N S extension between 7 and 6.5 Ma. High slip rates compared to hanging wall sedimentation rates induced the development of mega-breccia dominated substantial fault scarps during the younger synrift phase (8.2 to 6.9 Ma). Tectonic changes are superimposed by several climatic changes. Moderately humid conditions prevail from 8.5–8.2 Ma, followed by relatively dry period from 8.2 to 7.8 Ma. A second humid interval from 7.74–7.56 Ma, which may be responsible for the Maeotian transgression in the Eastern Paratethys lake, is followed by a significant trend in local and supra-regional aridization, indicated by the onset of orbital driven aeolian dust deposition at 7.42 Ma. Our new data expand the Northern Mediterranean Sahara-born dust record at the Tortonian-Messinian transition to at least 600 kyrs (7.42 to 6.85–6.8 Ma). Coeval with the onset of dust deposition we observed evolutionary changes in the large mammal record, leading to the classical Pikermian association dated to between 7.42 and 7.27 Ma. We interpret faunal changes at the Tortonian-Messinian transition in the Eastern Mediterranean as driven by global cooling and mid-latitudinal desertification.

Neoproterozoic and post-Caledonian exhumation and shallow faulting in NW Finnmark from K–Ar dating and <i>p</i>∕<i>T</i> analysis of fault rocks

Abstract. Well-preserved fault gouge along brittle faults in Paleoproterozoic, volcano-sedimentary rocks of the Raipas Supergroup exposed in the Alta–Kvænangen tectonic window in northern Norway yielded latest Mesoproterozoic (approximately 1050 ± 15 Ma) to mid-Neoproterozoic (approximately 825–810 ± 18 Ma) K–Ar ages. Pressure–temperature estimates from microtextural and mineralogy analyses of fault rocks indicate that brittle faulting may have initiated at a depth of 5–10 km during the opening of the Asgard Sea in the latest Mesoproterozoic–early Neoproterozoic (approximately 1050–945 Ma) and continued with a phase of shallow faulting to the opening of the Iapetus Ocean–Ægir Sea and the initial breakup of Rodinia in the mid-Neoproterozoic (approximately 825–810 Ma). The predominance and preservation of synkinematic smectite and subsidiary illite in cohesive and non-cohesive fault rocks indicate that Paleoproterozoic basement rocks of the Alta–Kvænangen tectonic window remained at shallow crustal levels (< 3.5 km) and were not reactivated since mid-Neoproterozoic times. Slow exhumation rate estimates for the early–mid-Neoproterozoic (approximately 10–75 m Myr−1) suggest a period of tectonic quiescence between the opening of the Asgard Sea and the breakup of Rodinia. In the Paleozoic, basement rocks in NW Finnmark were overthrusted by Caledonian nappes along low-angle thrust detachments during the closing of the Iapetus Ocean–Ægir Sea. K–Ar dating of non-cohesive fault rocks and microtexture mineralogy of cohesive fault rock truncating Caledonian nappe units show that brittle (reverse) faulting potentially initiated along low-angle Caledonian thrusts during the latest stages of the Caledonian Orogeny in the Silurian (approximately 425 Ma) and was accompanied by epidote–chlorite-rich, stilpnomelane-bearing cataclasite (type 1) indicative of a faulting depth of 10–16 km. Caledonian thrusts were inverted (e.g., Talvik fault) and later truncated by high-angle normal faults (e.g., Langfjorden–Vargsundet fault) during subsequent, late Paleozoic, collapse-related widespread extension in the Late Devonian–early Carboniferous (approximately 375–325 Ma). This faulting period was accompanied by quartz- (type 2), calcite- (type 3) and laumontite-rich cataclasites (type 4), whose cross-cutting relationships indicate a progressive exhumation of Caledonian rocks to zeolite-facies conditions (i.e., depth of 2–8 km). An ultimate period of minor faulting occurred in the late Carboniferous–mid-Permian (315–265 Ma) and exhumed Caledonian rocks to shallow depth at 1–3.5 km. Alternatively, late Carboniferous (?) to early–mid-Permian K–Ar ages may reflect late Paleozoic weathering of the margin. Exhumation rates estimates indicate rapid Silurian–early Carboniferous exhumation and slow exhumation in the late Carboniferous–mid-Permian, supporting decreasing faulting activity from the mid-Carboniferous. NW Finnmark remained tectonically quiet in the Mesozoic–Cenozoic.

The development of a regional-scale intraplate strike-slip fault system; Alpine deformation in the north of Ireland

A paucity of displacement markers can make constraining the kinematics and interaction of regional-scale strike-slip faults difficult. In this study, a high-resolution aeromagnetic survey allows quantitative analysis of kilometre-scale Paleogene faulting in the north of Ireland. Mapping offset dykes and igneous centres reveals the presence of two orientations of Cenozoic strike-slip faults attributed to broadly N-S Alpine shortening arising from the convergence of Africa and Europe. Reactivated NE-SW striking faults accommodated up to 2.3 km of sinistral displacement mainly during the Paleocene. Following tectonic quiescence, up to 2.5 km of Oligocene-aged dextral movement was accommodated on newly-formed NNW-SSE striking faults. Displacement analysis suggests that the faults were kinematically linked and experienced a switch in polarity of the predominantly active fault orientation during the Paleogene, forming an apparent conjugate system with asymmetric displacement distributions. The switch in fault polarity is attributed to a rotation in the regional maximum horizontal stress orientation and the locus of deformation moving from pre-existing reactivated structures to newly formed faults. The analysis also illustrates that, despite low strain rates, the interaction of the principal components of the fault system controlled the formation of sedimentary basins and defined the present-day geography of the region, far from an active plate boundary.

The Lonfearn Member, Lealt Shale Formation, (Middle Jurassic) of the Inner Hebrides, Scotland

The Lonfearn Member, Lealt Shale Formation, is a distinctive unit within the Great Estuarine Group in the Middle Jurassic of the Inner Hebrides, Scotland. Its dominant lithology of fine-grained, partly laminated mudstone marks it out as a time of minimum input of coarse clastic material into the Hebridean Basin, when consistent facies extended over some 90 km between north Skye and the Isle of Eigg. Interbedded with the mudstones are thin (decimetre) shelly and partly ferruginous and oolitic limestones, the combination giving a unique facies association in the British Bathonian. The predominantly low-salinity biota is dominated by the bivalve Neomiodon , ostracods and spinicaudatans. Episodes of inferred higher salinity aid in correlating sections from the type locality in North Trotternish, Skye to other localities, principally Strathaird in south Skye and the Isle of Eigg. We describe the principal outcrops of the Lonfearn Member and the remarkable facies continuity within the Great Estuarine Group indicating tectonic quiescence within the basin and hinterland at this time. Supplementary material: The detailed measured stratigraphical logs discussed in this paper are available at https://doi.org/10.6084/m9.figshare.c.4093991

The near steady state landscape of western Namibia

Quantitative geomorphic field studies and modeling efforts have focused on the margins of southwestern Africa as an example for landscape evolution in prolonged aridity conditions and tectonic quiescence of passive margins. These efforts concluded that this region is a prime example of a steady state landscape, in which relief changes extremely slowly. Using cosmogenic isotopes, these studies suggested overall landscape exhumation rates of 5–10 m Ma−1 over the past 105–106 yrs. Slightly slower rates on flat-lying exposed bedrock surfaces and faster exhumation rates along the Namibian Great Escarpment as well as on steep slopes of granitic inselbergs, such as the Gross Spitzkoppe are also documented. Here we explore the state of “steady state” in central Namibia. Concentrations of 10Be were measured in bedrock and sediment samples collected throughout the watershed of the Ugab River (~29,000 km2), which drains the highlands of central Namibia and flows to the Atlantic Ocean. Samples were collected from the main stem of the ephemeral Ugab River, from the slopes and streams draining the Brandberg, which is the largest inselberg in the Namib, and from smaller inselbergs around it. We also sampled several other formerly large, but currently subdued, inselbergs such as the Messum Crater. 10Be concentrations in sediment transported along the axial Ugab River indicate that its drainage basin erodes uniformly at 5–6 m Ma−1 and sediment transport from its headwaters source to the ocean is rapid. 10Be concentrations measured in sediment transported in ephemeral streams draining the Brandberg indicate its erosion at ~4 m Ma−1. However, slower rates of 1–3 m Ma−1 were measured for bedrock samples collected from (a) flat lying bedrock surfaces within the Brandberg, (b) top of small tors that rise only a few meters above their surroundings, and (c) exhumed and denuded large magmatic complexes such as the Messum Crater. Furthermore, we found that bedrock buried under grus in the hyperarid zone of Namib (<100 mm yr−1) erodes at similar rates as the exposed bedrock. This difference between the rate of bedrock erosion and the overall average erosion rate of drainage basins has been previously attributed to the contribution of sediment weathered from underneath transported sediment and soil on the pediments. Our results do not fully support this explanation. Results from this and earlier studies point to two possible sources of relatively low dosed (i.e. more rapidly eroding) sediment: (a) the steep slopes and cliffs of the large inselbergs and the Great Escarpment, and (b) rock buried under soil in the upper, semi-arid, parts of the drainage systems, where soil and vegetation can promote weathering of plagioclase and biotite and the disintegration of granitic bedrock.We therefore suggest that the “steady state” landscape along the Namibian passive margin be viewed as follows: The entire landscape erodes slowly, generally at ~5 m Ma−1 and this maintains the view of steady state. Small differences in erosion rates between the landscape elements result in very slow and only small changes in relief over time scales ≥106 yrs. We find that the large inselbergs and the Great Escarpment erode primarily by retreat of steep slopes and cliffs within the drainage basins while preserving relief over considerable timescales. In the wetter upper reaches of the Namibian drainage systems, erosion of buried rock is most likely increased by the vegetation-covered soil.

Provenance and depositional environments of Quaternary sediments in the southern Kalahari Basin

The vast Kalahari Basin is characterized by uplifted margins, terrestrial sedimentation within semi endorheic sub-basins, subdued morphology and tectonic quiescence. This intracratonic basin has been subjected to a prolonged period of subsidence affecting its sedimentary fill by plate motion and climatic cycles. Provenance studies of Kalahari Group sediments mainly focused on the surficial deposits that represent only the last phase of sedimentation, leaving unresolved questions related to the rest of the strata.The southern Kalahari Group succession exposed along the walls of the Mamatwan Mine, Northern Cape, South Africa, reveals three main depositional environments: a bottom low-energy water-body, a middle fluvial, high-energy environment and an upper aeolian sandy unit. The entire section, which was deposited during the early to middle Quaternary, records significant environmental and depositional changes suggesting a highly dynamic landscape.The fully exposed section (55m) of the Kalahari Group at Mamatwan Mine was analysed for its mineralogy, elemental composition, Sr, Nd and Pb isotopic ratios and iron species. Isotopic fingerprints were used to determine the source rocks of the sediments. The basal part of the section was derived from the distant Angolan highlands supporting previous suggestions for the existence of a paleo, trans-Kalahari drainage system. Weathering sensitive indices show that these sediments underwent considerable chemical weathering typical to humid areas. Mineralogical assemblage and iron phases imply that a brackish and alkaline shallow water-body existed during the early-middle Pleistocene contemporaneous with relative dense hominin occupation of the area.During the Early-Middle Pleistocene transition, the lacustrine environment was rapidly filled with volcano-sedimentary Archean clasts that were derived mainly from the surrounding hills and experienced limited degree of source rock chemical weathering, but underwent subsequent groundwater alteration by iron, silica, and calcium-rich solutions and precipitation of calcrete and silcrete duricrusts. These changes in provenance and depositional environments may be related to tectonic adjustments along the southern margins of the Kalahari basin. Ultimately, a shift into an aeolian-transport dominated landscape occurred in the region. This was accompanied by reduced surface water availability and a supply of sediments from Paleoproterozoic western and north-western source areas following the establishment of the modern wind regime.

Elemental and isotopic behaviour of Zn in Deccan basalt weathering profiles: Chemical weathering from bedrock to laterite and links to Zn deficiency in tropical soils

Zinc (Zn) is a micronutrient for organisms and essential for plant growth, therefore knowledge of its elemental cycling in the surface environment is important regarding wider aspects of human nutrition and health. To explore the nature of Zn cycling, we compared its weathering behaviour in a sub-recent regolith versus an ancient laterite profile of the Deccan Traps, India – an area of known soil Zn deficiency. We demonstrate that progressive breakdown of primary minerals and the associated formation of phyllosilicates and iron oxides leads to a depletion in Zn, ultimately resulting in a loss of 80% in lateritic residues. This residue is mainly composed of resistant iron oxides and hydroxides ultimately delivering insufficient amounts of bio-available Zn. Moreover, (sub)-tropical weathering in regions experiencing extended tectonic quiescence (e.g., cratons) further enhance the development of old and deep soil profiles that become deficient in Zn. This situation is clearly revealed by the spatial correlation of the global distribution of laterites, cratons (Africa, India, South America and Australia) and known regions of Zn deficient soils that result in health problems for humans whose diet is derived from such land.We also investigate whether this elemental depletion of Zn is accompanied by isotope fractionation. In the saprolitic horizons of both weathering profiles, compositions of δ66ZnJMC-Lyon lie within the “crustal average” of +0.27±0.07‰ δ66ZnJMC-Lyon. By contrast, soil horizons enriched in secondary oxides show lighter isotope compositions. The isotopic signature of Zn (Δ66Znsample-protolith up to ~ −0.65‰) during the formation of the ferruginous-lateritic weathering profile likely resulted from a combination of biotically- and kinetically-controlled sorption reactions on Fe-oxyhydroxides. Our findings suggest that oxide rich soil types/horizons in (sub)-tropical regions likely exert a control on riverine Zn isotope compositions such that these become heavier than the crustal average. This isotopic behaviour invites a broader study of global soils to test whether light isotope composition alone could serve as an indicator for reduced bioavailability of Zn.