Basement-cover Interface Research Articles

Contractional salt-tectonic system in the south Dezful embayment, Zagros

The Dezful embayment in Zagros has traditionally been interpreted as a typical fold belt with multiple detachment levels, including a lower detachment at the basement-cover interface and an upper detachment along the Miocene Gachsaran halite-bearing evaporites. This paper constrains diapiric processes in the Gachsaran detachment coeval with regional shortening that resulted in complex and hitherto unstudied salt structures and flanking synclinal minibasins. In this paper, well-tied seismic interpretation is integrated with field and remote-sensing mapping, cross-section construction, and stepwise tectono-sedimentary restoration. The results indicate that after the deposition of Mishan–lower Aghajari pre-kinematic overburden sediments, detachment folding initiated above the Gachsaran evaporites during late Tortonian times as a result of regional shortening. The salt-cored anticlines later actively pierced their roof to subsequently rise as passive diapirs and evolve into salt walls. Synclinal minibasins developed due to salt evacuation into flanking walls during the deposition of the Lahbari Mb. in Messinian–Pliocene. Remarkable sedimentary thickening and lateral shifts in minibasin depocenter towards the salt walls, accompanied by overturned collars and hook halokinetic sequences, are diagnostics of near-surface, syndepositional passive growth of diapirs during this stage. From Pliocene to recent, the major episode of regional shortening has squeezed these precursor salt walls to form thrust-welds. In conclusion, the well-exposed structures of south Dezful show an early shortening stage, then a transition to a combination of shortening and passive diapirism, and final thrusting along diapirs. This evolution model may be more common in other salt-influenced fold-thrust belts than is currently recognized.

Tectonic inheritance controls nappe detachment, transport and stacking in the Helvetic nappe system, Switzerland: insights from thermomechanical simulations

Abstract. Tectonic nappes have been investigated for more than a hundred years. Although geological studies often refer to a “nappe theory”, the physical mechanisms of nappe formation are still disputed. We apply two-dimensional numerical simulations of shortening of a passive margin to investigate the thermomechanical processes of detachment (or shearing off), transport and stacking of nappes. We use a visco-elasto-plastic model with standard creep flow laws, Drucker–Prager and von Mises yield criteria. We consider tectonic inheritance with two initial mechanical heterogeneities: (1) lateral heterogeneity of the basement–cover interface due to half-grabens and horsts and (2) vertical heterogeneities due to layering of mechanically strong and weak sedimentary units. The model shows detachment and horizontal transport of a thrust nappe that gets stacked on a fold nappe. The detachment of the thrust sheet is triggered by stress concentrations around the sediment–basement contact and the resulting brittle–plastic shear band that shears off the sedimentary units from the sediment–basement contact. Horizontal transport is facilitated by a basal shear zone just above the basement–cover contact, composed of thin, weak sediments that act as a décollement. Fold nappe formation occurs by a dominantly ductile closure of a half-graben and the associated extrusion of the half-graben fill. We apply our model to the Helvetic nappe system in western Switzerland, which is characterized by stacking of the Wildhorn thrust nappe above the Morcles fold nappe. The modeled structures, the deformation rates and the temperature field agree with data from the Helvetic nappe system. Mechanical heterogeneities must locally generate effective viscosity (i.e., ratio of stress to viscoplastic strain rate) contrast of about 3 orders of magnitude to model nappe structures similar to the ones of the Helvetic nappe system. Our results indicate that the structural evolution of the Helvetic nappe system was controlled by tectonic inheritance and that material softening mechanisms are not essential to reproduce the first-order nappe structures.

Utilising geochemical data for the identification and characterisation of mineral exploration sample media within cover sequence materials

A first pass workflow for placing cover sequence materials into four broad lithological groups using A–CNK–FM (Al2O3–CaO + Na2O + K2O–Fe2O3+MgO) and A–CN–K (Al2O3–CaO + Na2O–K2O) ternary diagrams in conjunction with the SiO2 vs Al2O3, Fe2O3 and CaO plus the Ca versus Sr XY diagrams has been derived from lithological logging and laboratory whole-rock geochemistry of 2346 samples. These lithological groups include siliciclastics (quartz sands and clays), weathered material, carbonate lithologies (marine limestone, pedogenic carbonate and compositions intermediate between the two end members) and ferruginous lithologies (including ferricrete and ferruginous sediments). Depositional, weathering and groundwater processes influence sample geochemistry during or after the formation of the material and will influence where the sample will plot on the geochemical diagrams used in this study. Geochemical data for discrete lithologies targeted during sampling (e.g. carbonate lithologies) clusters on diagrams used in the geochemical workflow, whereas data from targeted stratigraphic horizons (e.g. basement–cover interface) are more variable. The ability to characterise rock types that are texturally defined using the geochemical workflow is limited. For instance, the geochemistry of diamictite/conglomerate samples is dependent on the matrix versus clast content and composition as well as the source rock and catchment area. This workflow has the potential to be applied in a mineral exploration environment for rapid interpretation of real-time geochemical (e.g. portable XRF) data to gain an understanding of background geochemistry and geochemical signatures that may be related to underlying mineralisation. Key points A workflow for broad lithogeochemcial characterisation of cover sequence materials has been developed. Uses commonly collected major element plus strontium data acquired by laboratory or portable techniques. Categorises materials into siliciclastics, weathered material, carbonate lithologies and ferruginous lithologies. Applications include a mineral exploration environment for interpretation of large geochemical datasets to understand geology and mineralisation vectors.

Estimating Cover Thickness in the Southern Thomson Orogen – A Comparison of Applied Geophysics Estimates with Borehole Results

Potential sites for up to sixteen stratigraphic boreholes were selected to intersect the basement geology throughout the southern Thomson Orogen as part of the Southern Thomson Project. At each of these sites high resolution estimates of cover thickness were derived by applying refraction seismic, Audio-MagnetoTelluric, Targeted Magnetic Inversion and passive seismic geophysical techniques. Estimating cover thickness in this way reduced the technical risk associated with drilling and allows for the various geophysical techniques to be compared at each site. A comparison of the estimates derived from the applied geophysical techniques with the actual cover thicknesses determined from borehole geological and geophysical logs, together with an analysis of the uncertainties for each method, has highlighted the effectiveness of each geophysical technique. These new data and interpretations contribute to an Explorers’ Toolkit of techniques to help reduce the technical risk to the mineral exploration industry in searching for new mineral deposits in covered terrains in general, and in particular in the underexplored terrain of the southern Thomson Orogen. The various geophysical estimates highlight that the basement-cover interface throughout the southern Thomson Orogen can be recognised by its seismic velocity, electrical conductivity and magnetic susceptibility contrasts. However, the cover thickness estimates determined by the geophysical techniques shown here provide non-coincident estimates in some cases and, as such, it is important to take into account their unique limitations and uncertainties. When comparing results from the GSQ Eulo 1 and GSQ Eulo 2 boreholes it is clear that the refraction technique produced the most accurate estimates due to a strong contrast between the velocity structure of the Eromanga Basin sediments and the basement geology throughout the study area.

Application of AEM for cover thickness mapping in the southern Thomson Orogen

Regional airborne electromagnetic (AEM) surveying in the southern Thomson Orogen of southwest Queensland and northwest New South Wales highlights the efficacy of AEM for mapping near-surface Paleozoic basement rocks, the basement-cover interface, and the hydrostratigraphy of cover rocks of the Eromanga and Lake Eyre basins. Interpretations of AEM data are applied to model the basement-cover interface and answer the important question “how thick is cover” in this underexplored region, and encourage the mineral exploration industry to explore more widely. This paper shows some of the interpretation highlights from regional AEM surveys conducted in 2014 and 2016. The interpretations, when considered with regional borehole basement intersections, reduce exploration risk by demonstrating that large portions of the basement are within reach of exploration drilling.

AusAEM; acquisition of AEM at an unprecedented scale

Exploring for the Future is a four-year programme of the Commonwealth Government that aims to boost Australia’s attractiveness as a destination for investment in resource exploration. A significant component of the data acquisition component of the programme is the AusAEM Airborne Electromagnetic (AEM) Survey. This survey will focus on wide line-spaced acquisition as a regional mapping tool to gather new pre-competitive data and information, on extents never previously attempted. The objectives are to map, at a reconnaissance scale: • trends in regolith thickness, character, and variability • variations in bedrock conductivity • the continuity of key conductive bedrock (lithology-related) conductive units under cover • the groundwater resource potential of the region. The first AEM survey of this programme, when completed, will cover an area of over one million square kilometres (more than the areas of France and Spain together), at a nominal line spacing of 20 km with infill surveys in selected areas for subscription companies. In order to have the greatest impact the survey targets greenfield areas where the resource potential is unknown. The AEM data will contribute to estimating features and character of the cover material. To maximize industry collaboration on the project GA sought, and received, expressions of interest from industry subscribers for infill flying on the regional survey for lines spaced at 200 or 400 meters. Regional AEM surveys improve geological understanding in areas with little or no outcrop. The processed data enables informed interpolations of conductivity between sparse drill-holes and estimates on the depth of the basement-cover interface from geophysical modelling. The modelling results are used to inform follow-up exploration activities thereby reducing exploration risk. The new data will be released to the public domain at regular intervals to promote future activity by the government, exploration and research sectors.

Application of two-dimensional gravity models as input parameters to balanced cross-sections across the margin of the East European Craton in SE Poland

Our paper demonstrates how two-dimensional density models can deliver critical constraints that permit construction and validation of geological models in an underconstrained subcropped fold-and-thrust belt. As a case study, we use a newly discovered thrust-and-fold belt developed above the margin of the East European Craton (EEC) in SE Poland. A geological model constructed using gravity data resolved the basement-cover interface and a top-Cambrian horizon. This model was used as a framework for cross-section construction employing at this step also borehole and seismic evidence. Two cross-sections were created to address a source of uncertainty related to non-uniqueness of gravity modelling with different emphasis on gravimetric vs geological evidence. Model 1 strictly obeys the top-Cambrian derived from gravity modelling that was treated as an envelope of deeper structures. Model 2 allowed for local departures from the gravity model, especially in the case of conflicts with formation thicknesses. The comparison between the present structure and restorations allowed for separating effects of Caledonian and Variscan shortening. Although the fit of both models to the gravity data is satisfactory, model 2 better complies to geological constraints, resolving the long-discussed problem concerning the distribution and intensity of Caledonian tectonics across the SW periphery of the EEC.

Applied geophysics for cover thickness mapping in the southern Thomson Orogen

Potentially mineralised Paleozoic basement rocks in the southern Thomson Orogen region of southern Queensland and northern New South Wales are covered by varying thicknesses of Mesozoic to Cenozoic sediments. To assess cover thickness and methods for estimating depth to basement, we collected new airborne electromagnetic (AEM), seismic refraction, seismic reflection and audio-frequency magnetotelluric data and combined these with new depth to magnetic basement models from airborne magnetic line data and ground gravity data along selected transects. The results of these investigations over two borehole sites, GSQ Eulo 1 and GSQ Eulo 2, show that cover thickness can be reliably assessed to within the confidence limits of the various techniques, but that caveats exist regarding the application of each of the disciplines. These techniques are part of a rapid-deployment explorers’ toolbox of geophysical techniques that have been tested at two sites in Australia, the Stavely region of western Victoria, and now the southern Thomson Orogen in northern New South Wales and southern Queensland. The results shown here demonstrate that AEM and ground geophysics, and to a lesser extent depth to magnetic source modelling, can produce reliable results when applied to the common exploration problem of determining cover thickness. The results demonstrate that portable seismic systems, designed for geotechnical site investigations, are capable of imaging basement below 300 m of unlithified Eromanga Basin cover as refraction and reflection data. The results of all methods provide much information about the nature of the basement–cover interface and basement at borehole sites in the southern Thomson Orogen, in that the basement is usually weathered, the interface has paleotopography, and it can be recognised by its density, natural gamma, magnetic susceptibility and electrical conductivity contrasts.

Thermal profiles inferred from fluid inclusion and illite geothermometry from sandstones of the Athabasca basin: Implications for fluid flow and unconformity-related uranium mineralization

The Proterozoic Athabasca basin and underlying basement host numerous unconformity-related uranium deposits that were formed from extensive fluid circulation near the basement-cover interface. Although it is generally agreed that the mineralizing fluids were basinal brines, it is still unclear what driving forces were responsible for the circulation of the basinal fluids. Because different fluid flow driving forces are associated with different thermal profiles, knowing the basin-scale distribution of paleo-fluid temperatures can help constrain the fluid flow mechanism. This study uses fluid inclusions entrapped in quartz overgrowths and authigenic illite in sandstones from three drill cores (WC-79-1, BL-08-01, and DV10-001) in the central part of the Athabasca basin as thermal indicators of paleo-fluids in the basin. A total of 342 fluid inclusions in quartz overgrowths were studied for microthermometry. The homogenization temperatures (Th) range from 50° to 235°C, recording the minimum temperatures in various diagenetic stages. Temperatures estimated from illite geothermometry (121 points) range from 212° to 298°C, which are systematically higher than (partly overlapping) the Th values, suggesting that illite was precipitated in hotter fluids following the formation of quartz overgrowths. Neither the fluid inclusion Th values nor the illite temperatures show systematic increase with depth in individual drill cores. This, together with the high illite temperatures that cannot be explained by burial at a normal geothermal gradient (35°C/km), is interpreted to indicate that basin-scale fluid convection took place during the diagenetic history of the basin. Prolonged fluid convection is inferred to be responsible for delivering uranium (extracted from the basin or the upper part of the basement) to the unconformity, where uranium mineralization took place due to redox reactions associated with fluid-rock interaction or structurally controlled fluid mixing.

The deep structure of south-central Taiwan illuminated by seismic tomography and earthquake hypocenter data

The Taiwan mountain belt is generally thought to develop above a through-going basal thrust confined to within the sedimentary cover of the Eurasian continental margin. Surface geology, magnetotelluric, earthquake hypocenter, and seismic tomography data suggest, however, that crustal levels below this basal thrust are also currently being involved in the deformation. Here, we combine seismic tomography and earthquake hypocenter data to investigate the deformation that is taking place at depth beneath south-central Taiwan. In this paper, we define the basement as any pre-Eocene rifting rocks, and use a P-wave velocity of 5.2km/s as a reference for the interface between these rocks and their sedimentary cover. We found that beneath the Coastal Plain and the Western Foothills clustering of hypocenters near the basement-cover interface suggests that this interface is acting as a detachment. This detachment is located below the basal thrust proposed from surface geology for this part of the mountain belt. Inherited basement faults appear to determine the geometry of this detachment, and their inversion in the Alishan area result in the development of a basement uplift and a lateral structure in the thrust system above them. However, across the Shuilikeng and the Chaochou faults, earthquake hypocenters define steeply dipping clusters that extend to greater than 20km depth, above which higher velocity basement rocks are uplifted beneath the Hsuehshan and Central ranges. We interpret these clusters to form a deeply penetrating, east-dipping ramp that joins westward with the detachment at the basement-cover interface. It is not possible to define a basal thrust to the east, beneath the Central Range.

Constraints on plateau architecture and assembly from deep crustal xenoliths, northern Altiplano (SE Peru)

Newly discovered xenoliths within Pliocene and Quaternary intermediate volcanic rocks from southern Peru permit examination of lithospheric processes by which thick crust (60–70 km) and high average elevations (3–4 km) resulted within the Altiplano, the second most extensive orogenic plateau on Earth. The most common petrographic groups of xenoliths studied here are igneous or meta-igneous rocks with radiogenic isotopic ratios consistent with recent derivation from asthenospheric mantle ( 87 Sr/ 86 Sr = 0.704–0.709, 143 Nd/ 144 Nd = 0.5126–0.5129). A second group, consisting of felsic granulite xenoliths exhibiting more radiogenic compositions ( 87 Sr/ 86 Sr = 0.711–0.782, 143 Nd/ 144 Nd = 0.5121–0.5126), is interpreted as supracrustal rocks that underwent metamorphism at ~9 kbar (~30–35 km paleodepth, assuming a mean crustal density of 2.8 g/cm 3 ) and ~750 °C. These rocks are correlated with nonmetamorphosed rocks of the Mitu Group and assigned a Mesozoic (Upper Triassic or younger) age based on detrital zircon U-Pb ages. A felsic granulite Sm-Nd garnet whole-rock isochron of 42 ± 2 Ma demonstrates that garnet growth took place in Eocene time. Monazite grains associated with quenched anatectic melt networks in the same rocks yield ion microprobe U-Pb ages ranging from 3.2 ± 0.2 to 4.4 ± 0.3 Ma (2σ). These disparate geochronologic data sets are reconciled by a model wherein Mesozoic cover rocks were transferred to >30 km depth beneath the plateau in the Eocene and progressively heated until at least Pliocene time. Isothermal decompression and partial melting ensued as these rocks were entrained as xenoliths in volcanic host magmas and transported toward the surface. Mafic granulites and peridotites from the same xenolith suite comprise the basement of the metasedimentary sequence, exhibiting isotopic characteristics of Central Andean crust. Calculated equilibrium pressures for these basement rocks are >11 kbar, suggesting that the basement-cover interface lies beneath the northernmost Altiplano at ~30–40 km below the surface. Together, these results indicate that crustal thickening under the northernmost Altiplano started earlier than major latest Oligocene and Miocene uplift episodes affecting the region and was coeval with a flat slab–related regional episode of deformation. Total shortening must have been at least 20% more than previous estimates in order to satisfy the basement to cover depth constraints provided by the xenolith data. Sedimentary rocks at >30 km paleodepth require that Andean basement thrusts decapitated earlier Triassic normal faults, trapping Paleozoic and Mesozoic rocks below the main decollement. Magma loading from intense Cenozoic plutonism within the plateau probably played an additional role in transporting Mesozoic cover rocks to >30 km and thickening the crust beneath the northern Altiplano.

Pangea rifting and onward pre-Central Atlantic opening as the main ore-forming processes for the genesis of the Aouli REE-rich fluorite–barite vein system, Upper Moulouya District, Morocco

The Aouli fluorite–barite±sulphides vein system in the Upper Moulouya District of Central Morocco is hosted in a folded and low to medium grade sedimentary and volcanic rocks, unconformably overlaid by Permo-Triassic to Cretaceous red beds and limestones. Intrusion of the hydrothermally altered multiphase ca. ∼330–319Ma Aouli granite batholith has contact metamorphosed the host rocks to a metamorphic assemblage of cordierite, andalusite, chlorite, muscovite, and biotite±sillimanite±garnet.The mineralized structures that consist mostly of quartz, fluorite, and barite occur principally as ENE–WSW, WNW–ESE, and E–W-trending trans-tensional steeply dipping veins, veinlets and en echelon tension gash fillings. Irrespective of colour, location, paragenesis and textural position within the mineralized vein structure, the fluorite is characterized by high total REY contents ranging from 250 to 662ppm, distinctive positive Eu and Y anomalies, and middle rare-earth element enrichment.Fluid inclusion data indicate that the ore-forming fluids correspond to evolved NaCl–CaCl2+other cations sedimentary (94–174°C), saline (14–24wt% NaCl equiv.) brines. The strontium isotopic compositions of fluorite (87Sr/86Sr=0.710155–0.712293) and barite (0.710215–0.701401), along with the Liassic dolomitized limestones (0.707867–0.708140) are more radiogenic than the Cambro-Ordovician and Triassic–Early Jurassic seawater values, with the Aouli Late Variscan granite (0.70814±12) and the Triassic arkoses (0.709839–0.712313) displaying the highest 87Sr/86Sr ratios. Barite separates show uniform δ34S ratios of +11‰ to +13.4‰ consistent with Permian–Triassic seawater sulphate.The calculated REY fluid compositions along with fluid inclusion, strontium and sulphur isotope data point to the role of hot sedimentary brines with fluid–rock interaction at high fluid/rock ratios. The fluid system is likely related to the Pangea rifting and subsequent Central Atlantic opening during Permian–Triassic time. The fluorite–barite mineralization is likely due to mixing at the basement–cover interface of an ascending deep-seated fluid that equilibrated with Variscan crystalline basement rocks and cooler more dilute formation water.

Shortening of the European Dauphinois margin (Oisans Massif, Western Alps): New insights from RSCM maximum temperature estimates and 40Ar/39Ar in situ dating

The Oisans Massif, located in the external zones of the western Alps, experienced significant shortening during the Alpine collision. While a series of major top-to-the west shear zones was recently described, the general low grade of the metamorphism has not attracted much petrological and geochronological studies. This paper provides combined temperature and age constraints on the evolution of the Oisans Massif. Temperature was estimated with the Raman Spectrometry of Carbonaceous Material (RSCM) method and chlorite geothermometry. Maximum temperature reached by the Mesozoic cover (Tmax) from Grenoble to the Galibier pass (E-W) and from Saint Jean de Maurienne to Embrun (N-S) yielded almost constant ca. 330°C temperatures all through the Massif. Temperatures however strongly decrease either westward towards the top of the Vercors sedimentary sequence or eastward towards the Penninic Frontal Thrust. Age constraints were retrieved using 40Ar/39Ar in situ analyses performed on variously strained samples from Alpine shear zones. Over strain gradients, incipient Alpine recrystallizations progressively develop at the expense of former Variscan parageneses. A combined textural and EPMA approach permitted to identify newly formed chlorite and phengite that unequivocally grew in response to deformation or to low grade metamorphism. Chlorites recorded temperatures from ca. 350–150°C during the activity of shear zones. In parallel, 40Ar/39Ar in situ experiments enabled dating deformation using both synkinematic phengites crystallized below the closure temperature of white-micas and former Variscan muscovite whose isotopic system have been, at least partially re-opened. Activity of top-to-the-west shear zones responsible for the shortening and thickening of the Oisans massif thus occurred between 34 and 33 and 25Ma. Integrating these new age-constraints, Tmax estimates, and published geological data on the Oisans Massif and neighbouring areas allow proposing a new shortening scenario for the external zones. From ca. 34–33Ma, the Oisans Massif was buried as a rigid block below more internal units and reached a temperature of ca. 330°C. Shearing progressively localized along a series of top-to-the-W shear zones in the basement until 25Ma. Deformation then localized along a major thrust at the base of the ECM massif and propagated along the basement-cover interface below the Vercors massif after 16Ma.

Red bed and basement sourced fluids recorded in hydrothermal Mn–Fe–As veins, Sailauf (Germany): A LA-ICPMS fluid inclusion study

The hydrothermal Mn–Fe–As vein mineralization at Sailauf (Germany) hosts a complex sequence of oxide and carbonate minerals that record a protracted fluid history. The mineralization is related to a major unconformity that separates Permian (Zechstein) sedimentary rocks from underlying Variscan crystalline basement. The hydrothermal veins contain two principal mineralization stages, which are Mn oxides associated with calcite and hematite associated with Mn-calcite. The fluid evolution of the hydrothermal system has been reconstructed from fluid inclusion petrography, microthermometry, and LA-ICPMS microanalysis, coupled with stable isotope geochemistry of carbonate and oxide minerals. The fluid inclusions are high salinity sodic–calcic brines and the bulk fluid properties show no major differences between the Mn oxide and the hematite stage. LA-ICPMS analysis of major and trace elements demonstrates that the mineralization formed from chemically distinct fluid pulses characterized by variations in their K, Li, B, Pb and Zn concentrations. The fluid that precipitated the Mn oxide stage has anomalous Pb/Zn and Li/B ratios, which closely resemble fluids found in fracture fillings in red beds of the Permian Rotliegend basin. By contrast, the fluids associated with the hematite stage have Pb/Zn and Li/B ratios typical of crustal fluids that were derived from interaction with crystalline basement. Both fluids possess characteristic element ratios including Cl/Br, but variable absolute concentrations of most metals. This suggests that both fluids were modified by mixing with a common metal-depleted brine that had a similar Cl/Br ratio, most likely a formation water from the overlying Zechstein sedimentary rocks. The Sailauf mineralization provides insight into the protracted post-Variscan fluid evolution at the basement–cover interface. The compositionally anomalous fluid that precipitated the Mn oxides is comparable to brines derived from interaction with red beds and likely represents the ore fluid of Kupferschiefer-type sediment-hosted Cu deposits. Conversely, the fluid that deposited the hematite mineralization resembles fluids that typically form basement- and sediment-hosted Pb–Zn deposits.

Fabric development in a weathering profile at a basement–cover interface, the sub-Cambrian peneplain, Israel: Implications for decollement tectonics

The present study examines a fossil saprock–saprolite–laterite-profile beneath the sub-Cambrian peneplain in the Pan-African Roded Granite, Israel, with regard to structure and magnetic fabrics (anisotropy of magnetic susceptibility, AMS), and image analysis of compaction. The deformed granite shows two pre-weathering foliations, S1m (magmatic) and S2g (gneissic). Pre-Early Cambrian weathering comprised weathering-brecciation in saprock and saprolite, and chemical weathering with clay-formation in saprolite and laterite. During subsequent Phanerozoic burial the laterite was vertically compacted to 73% of its original thickness. In the laterite, compaction produced an unconformity-parallel cleavage (S3d) with increasing intensity towards the unconformity. Bulk susceptibility (κbulk) and anisotropy (P′) decrease from the unweathered granite into the saprolite, as a result of progressive magnetite breakdown, martitization and weathering-brecciation. In the laterite, an enrichment of haematite and relic Fe–Mg–mica lead to increased κbulk. Here, magnetic fabrics trace the compaction fabrics. The subhorizontal, compactional clay–/mica-fabric S3d defines a structurally weak and impermeable layer. The mechanical weakness of a clay-enriched weathering horizon with an unconformity-parallel, planar shape-preferred orientation, combined with the potentially overpressured state due to the sealing character of such a zone provides a viable explanation for the abundant localization of decollement horizons at or beneath basement-cover interfaces.

The role of the Kupferschiefer in the formation of hydrothermal base metal mineralization in the Spessart ore district, Germany: insight from detailed sulfur isotope studies

The Spessart district (SW Germany), located at the southwestern margin of the Permian Kupferschiefer basin in Central Europe, hosts abundant stratabound and structurally controlled base metal mineralization. The mineralization styles identified are (1) stratabound Cu-Pb-Zn-(Ag) ores in Zechstein sedimentary rocks, (2) structurally controlled Cu-As-(Ag) ores in Zechstein sedimentary rocks, (3) crosscutting Co-Ni-(Bi)-As and Cu-Fe-As veins, (4) stratabound metasomatic Fe-Mn carbonate ores in Zechstein dolomite, (5) barren barite veins, and (6) Fe-Mn-As veins in Permian rhyolites. Building on previous work that involved mineralogical, textural, and chemical characterization of the major mineralization types, we have performed a comprehensive sulfur isotope study that applied both conventional and novel laser-ablation multi-collector inductively coupled plasma mass spectrometry techniques. The δ34S values of sulfide minerals from the different ore types are consistently negative and highly variable, in the range between −44.5‰ and −3.9‰, whereas the δ34S values of barite are all positive in the range between 4.7‰ and 18.9‰. Remarkably, stratabound and structurally controlled mineralization in Zechstein sedimentary rocks has the least negative δ34S values, whereas vein-type deposits have consistently more negative δ34S values. The observed pattern of sulfide δ34S values can be best interpreted in terms of fluid mixing at the basement-cover interface. Hydrothermal fluids originating from the crystalline basement migrated upward along subvertical fault zones and were periodically injected into groundwaters that were flowing in the post-Variscan sedimentary cover. These groundwaters had interacted with the Zechstein sedimentary rocks, resulting in fluids characterized by elevated concentrations of reduced sulfur (with negative δ34S values) and alkaline pH. Repeated mixing between both chemically contrasting fluids caused rapid and efficient precipitation of sulfide ore minerals in hydrothermal veins with highly variable but distinctly negative δ34S values.

Kinematic consequences of an angular unconformity in simple shear: an example from the southern border of the Lower Palaeozoic Rocroi inlier (Naux, France)

AbstractThe presence of an angular unconformity in combination with complex structures in the basement, lacking in the cover, is commonly seen as an indication for an orogenic event pre-dating the unconformity. The recognition of such an older orogenic event becomes, however, less evident in areas where both cover and basement were deformed together during an orogen post-dating the angular unconformity.The validity of this common interpretation has been evaluated at the southern border of the Lower Palaeozoic Rocroi basement inlier (Naux, northern France), where the basement-cover interface is very well exposed. This basement-cover interface, showing an angular unconformity, has classically been interpreted as evidence for an early Palaeozoic tectonometamorphic event, called the Ardennian orogeny, though only one penetrative cleavage, co-genetic with the structures present in both cover and basement, can be observed.A detailed geometrical study shows, however, that the presence of a tilted basement, involving the angular unconformity, provokes a rheological heterogeneity that causes a contrasting response of basement and cover with respect to the Variscan shortening. While Variscan progressive deformation gave rise to a rather regular cleavage refraction pattern in the subhorizontal multilayer cover sequence, a complex deformation, expressed by non-cylindrical folds, boudinage and shearing developed in the basement. The basement-cover interface itself played no rheological role, but has been passively sheared and folded as a consequence of the deformation of the basement. This study proves that the deformed basement-cover interface, allowing to link deformation in basement and cover, is a necessary tool to properly interpret complex deformation in the basement. With respect to the regional geodynamic evolution of the northern parts of the Central European Variscides, our kinematic model indeed demonstrates that this classical outcrop area bears no evidence for an early Palaeozoic orogenic event, and that the angular unconformity reflects the late Silurian – early Devonian onset of the Ardenne-Eifel basin development, rather than a middle Ordovician Ardennian orogeny.

The Mesoproterozoic Irumide belt of Zambia

The Mesoproterozoic Irumide belt is a northeast-trending structural province stretching from central Zambia to the Zambia–Tanzania border and northern Malawi. Mesoproterozoic and Neoproterozoic transcurrent shear zones within reactivated parts of the Palaeoproterozoic Ubendian belt define its northeastern limit. The northwestern margin is defined by the largely undeformed basement lithologies of the Bangweulu block. An intensely folded and sheared zone at the southeastern margin of the Mporokoso Group sedimentary depocentre on the Bangweulu block, interpreted to have developed above a thrust at the basement-cover interface, indicates that far-field effects of the Irumide Orogen also affected the southeastern part of the Bangweulu block sedimentary cover. To the west and southwest, Irumide and basement lithologies were reworked by the Damara–Lufilian–Zambezi Orogen within the Neoproterozoic Zambezi and Lufilian belts. The Choma–Kalomo block, previously regarded as the southwesterly continuation of the Irumide belt, is a distinct Mesoproterozoic province, while a succession of structurally juxtaposed tectonic terranes in eastern Zambia record a deformation event related to the Irumide Orogen. The lithological units identified in the Irumide belt include: (1) limited Neoarchaean rocks emplaced between 2.73 and 2.61 Ga and representing the oldest rocks in the Bangweulu block; (2) ca. 2.05–1.85 Ga volcano-plutonic complexes and gneisses representing the most important components in the Bangweulu block; (3) an extensive quartzite–metapelite succession with minor carbonate forming the Muva Supergroup, and deposited at ca. 1.85 Ga; (4) granitoids emplaced between 1.65 and 1.55 Ga; (5) a minor suite of anorogenic plutons (nepheline syenite and biotite granite) restricted to the far northeastern Irumide belt and emplaced between 1.36 and 1.33 Ga; (6) voluminous syn- to post-kinematic Irumide granitoids emplaced between 1.05 and 0.95 Ga. Crustal shortening and thickening in the Irumide belt are shown by northwestward-directed thrusts and related folds and metamorphic parageneses recording a clockwise medium-pressure/medium-temperature P– T– t path. Metamorphic grades range from greenschist facies in the foreland to the northwest to upper amphibolite facies in the southeast, with local granulites. Peak metamorphism is diachronous across the belt and bracketed between 1.05 in the southeast and 1.02 Ga in the northwest.

Evidence and mechanisms for folding of granite, Sierra de Hualfín basement-cored uplift, northwest Argentina

Sierra de Hualfn is a 22-km (14-mi)-long basement-cored uplift with an approximately 66-km2 (26-mi2) exposure of the basement-cover interface. The uplift is composed of Ordovician granite, overlain by and locally thrust over Tertiary sedimentary rocks. The geometry of the uplift is that of a faulted anticlinorium in granite basement that is delineated by the basement unconformity. Detailed mapping of the basement unconformity documents large-scale folding of granite basement, the result of fault-propagation folding associated with a thrust-fault tip initially located deep in the basement. Our findings at Sierra de Hualfn indicate that homogeneous granite can fold as a deformable body by stress-induced development and/or exploitation of joints, faults and fractures, and an unconformity-parallel fracture foliation in the uppermost basement. Specifically, folding is achieved through systematic coordinated movements involving (1) reactivation of joints as faults and mode I fissures, (2) fracturing and faulting near large displacement faults, and (3) flexural shear of the uppermost basement. The extent to which joints, microfractures, and fracture foliation are reactivated determines the deformability of granite. At Sierra de Hualfn, the deformability of granite is such that the folding of the basement is consistent with trishear kinematics. Our observations contradict standard models of basement-cored uplifts that assume that the fault tip is located at the basement-cover interface. We postulate that the folded shape of Sierra de Hualfn and of some uplifts in the Rocky Mountain foreland can be attributed to basement distortion taking place in advance of a propagating fault tip below the basement-cover unconformity.

Geological boundary conditions of the 1909 Lambesc (Provence, France) earthquake : structure and evolution of the Trévaresse ridge anticline

Abstract The 06/11/1909, Lambesc, M=6 earthquake is the strongest instrumental seismic event recorded in French history. A review of the geology of the epicentral area combined with detailed field mapping and structural analysis allows constraining the tectonic and geomorphic evolution of the Trévaresse ridge anticline and associated fault that ruptured during the 1909 event. The Trévaresse fold is a WNW-trending, southerly-verging forced ramp anticline developed above the Trévaresse reverse fault during late Miocene and Pliocene times, and possibly after the early Pleistocene. Over the last 11 Ma, the Trévaresse reverse fault records an average reverse slip rate of 0.03 ± 0.02 mm/yr consistent with a homogeneous N005 – N010 trending shortening direction. The fault is segmented into two major segments (the eastern one being linked to a possibly active frontal blind thrust fault) separated by a relay fault zone associated with an en-échelon fold pattern consistent with a slight left-lateral component on the easternmost segment. Our structural analysis combined with recent estimates of the source parameters of the Lambesc earthquake indicates that the fault has to reach at least the basement-cover interface, at a minimum depth of 6 km. The westward longitudinal decrease in morphological maturity of the relief of the Trévaresse ridge anticline and its associated fault scarp suggests a westward propagation of the fold during partly diachronous activation of the two segments of the fault.