Mesozoic Cover Rocks Research Articles

Tectonics of the Neuch\xe2tel Jura Mountains: insights from mapping and forward modelling

This study focuses on a geological section in the Jura Mountains across the villages of Travers, La Brévine in Switzerland, and Morteau in France. Field mapping was conducted to complement and densify existing data. A kinematically and geometrically consistent forward model has been developed to understand and interpret the observed surface structures. The proposed solution features a low-angle thrust fault with a multiple ramp-flat or staircase trajectory on which several hinterland-verging thrusts nucleate. The main décollement level is located in the Triassic evaporites of the Keuper and Muschelkalk Groups. Our model implies secondary detachments in the Opalinus Clay and the Cretaceous layers leading to repetitions in the Mesozoic cover rocks over large distances. This in turn explains the high topographic position of exposed sediments. The proposed solution is an alternative to models showing overthickening of Triassic evaporites associated with a single detachment level. Along the investigated profile, the Jura Mountains accommodate a shortening of 8.5 km. The kinematic forward model suggests an oscillating sequence of thrusting, rather than a simple, in sequence, forward propagation succession of thrusts.

The Neogene and Quaternary of England: landscape evolution, tectonics, climate change and their expression in the geological record

During the Neogene and Quaternary, tectonic and climatic processes have had a profound impact upon landscape evolution in England and, perhaps as far back as 0.9Ma, patterns of early human occupation. Until the Late Miocene, large-scale plate tectonic processes were the principal drivers of landscape evolution causing localised basin inversion and widespread exhumation. This drove, in places, the erosion of several kilometres of Mesozoic cover rocks and the development of a regional unconformity across England and the North Sea Basin. By the Pliocene, the relative influence of tectonics on landscape evolution waned as the background tectonic stress regime evolved and climatic influences became more prominent. Global-scale climate-forcing increased step-wise during the Plio-Pleistocene amplifying erosional and depositional processes that operated within the landscape. These processes caused differential unloading (uplift) and loading (subsidence) of the crust (‘denudational isostasy’) in areas undergoing net erosion (upland areas and slopes) and deposition (basins). Denudational isostasy amplified during the Mid-Pleistocene Transition (c.0.9Ma) as landscapes become progressively synchronised to large-scale 100ka ‘eccentricity’ climate forcing. Over the past 0.5Ma, this has led to the establishment of a robust climate record of individual glacial/interglacial cycles enabling comparison to other regional and global records. During the Last Glacial-Interglacial Transition and early Holocene (c.16–7ka), evidence for more abrupt (millennial/centennial) scale climatic events has been discovered. This indicates that superimposed upon the longer-term pattern of landscape evolution is a more dynamic response of the landscape to local and regional drivers.

Glacial sediment and landform record offshore NW Scotland: a fjord–shelf–slope transect through a Late Quaternary mid-latitude ice-stream system

Current estimates of ice-mass loss from ice sheets vary, but there is consensus that the rate of loss has increased over the last two decades from c. 100 to c. 400 Gt a−1 with the great majority occurring within ice-stream systems (e.g. Shepherd et al. 2012; Hanna et al. 2013). Where ice streams terminate in open-marine settings large volumes of ice are discharged into the oceans via calving, as currently seen in Pine Island Bay, West Antarctica or at Jakobshavn Isbrae, West Greenland. At the Last Glacial Maximum (LGM), an ice sheet covered more than two-thirds of the British Isles (50–61° N) and reached onto the continental shelf, in places extending to the shelf edge in the west (Gibbard & Clark 2011; Clark et al. 2012). In this contribution we summarize the Pleistocene sediment and landform record of a large mid-latitude ice stream that once drained the NW sector of the British–Irish Ice Sheet (BIIS). Drawing on marine geophysical and geological data from the NW UK continental shelf collected over four decades, we describe the main elements of the system along a transect stretching for >300 km from the heads of the fjords to beyond the shelf slope. The work draws largely on previously published research (Stoker et al. 1993, 2006, 2009, 2010; Stoker & Bradwell 2005; Bradwell et al. 2007, 2008 a , b ; Bradwell & Stoker 2015 a , b ), but includes some new insights and interpretations. The bedrock geology of NW Scotland and the Hebrides Shelf is structurally complex and lithologically diverse. Unsurprisingly, the geological structure imposes a general first-order control on the topography of the region with resistant upstanding Precambrian rocks typically forming the extant landmasses and bathymetric highs, and the weaker Mesozoic cover rocks flooring …

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.

Structure, geometry and kinematics of the northern Adula nappe (Central Alps)

The eclogitic Adula nappe of the Central Alps (cantons Graubunden and Ticino, Switzerland) displays an exceptionally complex internal structure with the particularity of enclosing numerous slices of Mesozoic cover rocks (Internal Mesozoic) within the Palaeozoic gneiss basement. This study is principally based on detailed lithological and structural mapping of selected areas of the northern Adula nappe. Specific focus was placed on the Mesozoic slivers embedded in pre-Mesozoic basement (Internal Mesozoic). The most pervasive structures are related to the Zapport deformation phase that is responsible for the development of a fold-nappe and ubiquitous north-directed shear. Locally, the structures in the upper and frontal part of the nappe can be assigned to the older ductile Ursprung phase. These earlier structures are only compatible with top-to-S shear movement. The superposition of the Ursprung and Zapport phases is responsible for the north-dipping internal duplex-like structure and the sliced aspect of the Northern and Central Adula nappe. We conclude that the Adula nappe represents a major shear zone involving the entire nappe and responsible for the emplacement of the Lower Penninic sediments and the Middle Penninic nappes in the eastern part of the Lepontine Dome.

Lu–Hf garnet systematics of a polymetamorphic basement unit: new evidence for coherent exhumation of the Adula Nappe (Central Alps) from eclogite-facies conditions

The Adula Nappe in the Central Alps is a mixture of various pre-Mesozoic continental basement rocks, metabasics, ultrabasics, and Mesozoic cover rocks, which were pervasively deformed during Alpine orogeny. Metabasics, ultrabasics, and locally garnet–mica schists preserve eclogite-facies assemblages while the bulk of the nappe lacks such evidence. We provide garnet major-element data, Lu profiles, and Lu–Hf garnet geochronology from eclogites sampled along a north–south traverse. A southward increasing Alpine overprint over pre-Alpine garnets is observed throughout the nappe. Garnets in a sample from the northern Adula Nappe display a single growth cycle and yield a Variscan age of 323.8 ± 6.9 Ma. In contrast, a sample from Alpe Arami in the southernmost part contains unzoned garnets that fully equilibrated to Alpine high-pressure (HP) metamorphic conditions with temperatures exceeding 800 °C. We suggest that the respective Eocene Lu–Hf age of 34.1 ± 2.8 Ma is affected by partial re-equilibration after the Alpine pressure peak. A third sample from the central part of the nappe contains separable Alpine and Variscan garnet populations. The Alpine population yields a maximum age of 38.8 ± 4.3 Ma in line with a previously published garnet maximum age from the central nappe of 37.1 ± 0.9 Ma. The Adula Nappe represents a coherent basement unit, which preserves a continuous Alpine high-pressure metamorphic gradient. It was subducted as a whole in a single, short-lived event in the upper Eocene. Controversial HP ages and conditions in the Adula Nappe may result from partly preserved Variscan assemblages in Alpine metamorphic rocks.

Structural analysis of superposed fault systems of the Bornholm horst block, Tornquist Zone, Denmark.

The Bornholm horst block is composed of Precambrian crystalline basement overlain by Palaeozoic and Mesozoic cover rocks. The cover intervals are separated by an angular unconformity and a hiatus spanning the Devonian through Middle Triassic interval. Late Palaeozoic faulting of the Early Palaeozoic Baltica platform is correlated with early-middle Carboniferous deformation in the Variscan foreland and with faulting associated with dolerite dyke injection in Skåne in the Late Carboniferous – Early Permian. The Palaeozoic fault systems are striking NW-SE and WNW-ESE and the platform series are dipping towards the SE and ESE respectively. The Mesozoic faulting was associated with the development of a horst-graben framework in the Bornholm-Skåne segment of the Sorgenfrei-Tornquist Zone. Mesozoic fault subsidence started in the Rønne Graben in the Triassic. In the Jurassic the Arnager-Sose block became active, cut off from the Bornholm block; in addition the Læså Graben (new) and the Øle Å fault block complex (new) were cut into the central Bornholm block from the south. In the Late Cretaceous the central Bornholm block was perforated by isolated fault blocks, i.e. the Nyker block, the Bøsthøj block and the Lobbæk block (new) along with subsidence of the Holsterhus block and renewed subsidence of the Arnager-Sose block. The Mesozoic series are dipping towards the southwest. The Palaeozoic fault systems were associated with two-dimensional plane strain during ENE-WSW and NNE-SSW extension. By contrast the Jurassic and Cretaceous fault systems were associated with three-dimensional strain with maximum extension striking NE-SW and secondary extension striking NW-SE. The Mesozoic palaeostress fields were associated with the break down of the Pangea supercontinent.

MINERALOGY, MINERAL COMPOSITIONS AND FLUID EVOLUTION AT THE WENZEL HYDROTHERMAL DEPOSIT, SOUTHERN GERMANY: IMPLICATIONS FOR THE FORMATION OF KONGSBERG-TYPE SILVER DEPOSITS

The post-Variscan Wenzel vein-type deposit near Wolfach, in the Black Forest, Germany, the type locality of the Ag–Sb alloy dyscrasite, was investigated by ore microscopy, electron-microprobe analysis, stable isotope and fluid-inclusion analysis. Three stages of mineralization could be distinguished. Whereas the first stage is a typical sulfide mineralization including galena and tetrahedrite, the second and third stage show a sulfide-poor association of Ag–Sb alloys, as well as Fe, Co and Ni arsenides and sulfarsenides, in a calcite matrix. The main ore minerals of this stage are allargentum and dyscrasite. The microprobe data for the diarsenides show extensive, and partly hitherto undocumented, solid solution in Fe–Co–Ni space. Seven distinct generations of calcite were distinguished. The δ13C (V–PDB) and δ18O (V–SMOW) values of these generations show a positively correlated trend that evolves from −13.0 to −4.0‰ and from 12.3 to 23.6‰, respectively. Fluid-inclusion data of stage I show homogenization temperatures of 100–180°C at salinities of 17–26 wt.% NaCl eq. Fluid inclusions in stage-II calcite display similar, but more restricted values, 110–150°C and 25–28 wt.% NaCl eq., respectively. The stage-III fluid inclusions show similar temperatures of homogenization, but different salinities. Earlier calcite of this stage contains inclusions with salinities of 27–30 wt.% NaCl eq., whereas later calcite has lower salinities, 3–10 wt.% NaCl eq. The initial temperatures of ice melting of most fluid inclusions range between −45 and −60°C and are typical of an H2O–NaCl–CaCl2 fluid. On the basis of all available geochemical data and phase-equilibrium constraints, we favor a model in which basement-derived near-neutral-pH hydrothermal fluids remobilized older products of mineralization. Mixing of these fluids with more alkaline formation-waters from the Mesozoic cover rocks resulted in the precipitation of the silver alloys in an enrichment zone at P–T conditions of 120–150°C and approximately 200 bars. A significant shift in pH from near-neutral to alkaline can explain the abundant association of silver alloys with calcite gangue and the general absence of quartz in the enriched ore zone. This conceptual model can be applied to similar ore deposits worldwide, where rich silver ores are hosted by calcite-rich and quartz-poor assemblages of gangue minerals.

Impact of backstop thickness lateral variations on the tectonic architecture of orogens: Insights from sandbox analogue modeling and application to the Pyrenees

We performed sandbox analogue models to investigate the influence of along‐strike tapering of the backstop on the three‐dimensional architecture of natural doubly vergent thrust‐fold belts, applying the results to a natural example as the Pyrenees. Available geological and geophysical data indicate that the European crust, which provided the backstop in the Pyrenean orogen, has a westward tapering on both the upper (westward thickening of the Mesozoic cover rocks) and the lower (westward rising of the Moho) envelope surfaces. Accordingly, we performed laboratory models characterized by backstops with an along‐strike tapering of either the upper or the lower surface. Our results indicate a significant impact of backstop lateral tapering in thrust wedge architecture in terms of the outward propagation of the deformation fronts, the width and elevation of the axial zone, and the timing of thrust polarity reversal along strike. When compared to the Pyrenees, our results support the inferred westward tapering of the European basement and its commonly accepted backstop role in the Pyrenean orogen. Moreover, comparison between the map pattern of the Pyrenees and of laboratory experiments suggests that the along‐strike tapering of the bottom envelope surface of the backstop played a most important role on the three‐dimensional thrust wedge architecture than the tapering of the upper envelope surface.

Mesozoic cover over northern England: interpretation of apatite fission track data

Whether a significant thickness of Mesozoic cover rocks once rested upon the Palaeozoic rocks, which form the structural highs of the English Lake District and the Northern Pennines, is a subject of long debate which has been revived recently by apatite fission track analysis (AFTA) studies. Evidence from adjacent basins suggests that 1200–1750 m of Mesozoic cover was probably formerly present on the highs, prior to Cenozoic erosion. Higher estimates ( c . 3000 m) of cover thickness, based on such fission track-derived palaeotemperatures, result from underestimation of early Cenozoic surface temperatures and from not allowing for the relatively low thermal conductivity of the eroded cover rocks. For similar reasons, apatite fission track estimates of eroded cover rocks elsewhere in Britain may also prove to be overestimates.

Crustal architecture across Phanerozoic Australia along the Eromanga-Brisbane Geoscience Transect: evolution and analogues

The crustal architecture across Phanerozoic Australia has been established from seismic data along an 1100-km-long east-west transect, the Eromanga-Brisbane Geoscience Transect. This has enabled a better understanding of deep structures and processes that have controlled the development of major sedimentary basin systems. It has shown that crustal dynamics throughout geological history have played an important role in the development of these sedimentary basins and that structures developed during the early Palaeozoic have influenced, and continue to influence, basin systems. The transect crosses three major basement provinces of the Tasman Orogenic System in eastern Australia - the Thomson, northern Lachlan, and New England Orogens. The basement geology in the transect region has, until now, been only poorly understood because it is largely obscured by the Mesozoic cover rocks of the Eromanga, Surat and Clarence-Moreton Basins. The transect interpretation has firmly identified crustal-scale ramp structures, multiple intra-crustal detachment surfaces, strike-slip fault architecture, lower-crustal magmatism/underplating, Mono remobilisation, and intra-crustal terranes in the geological reconstructions of southern Queensland. The boundaries between the orogens, the Foyleview and Burunga—Mooki Geosutures, have been identified as lithospheric-scale structures that have influenced the evolution of the Tasman Orogenic System as a whole.

Kinematic history of a foreland uplift from Paleocene synorogenic conglomerate, Beartooth Range, Wyoming and Montana

An integrated structural, sedimentological, and provenance study of the upper Paleocene Beartooth Conglomerate along the eastern flank of the Beartooth Range in Montana and Wyoming yields new information about Laramide tectonics in the northern Rocky Mountain foreland and has implications for models of fault-propagation folding and the development of synorogenic basins in thrust-faulted terranes. During the Laramide orogeny, the Beartooth Conglomerate was deposited by alluvial fans and a coarse braided fluvial system along the eastern flank of the range in response to displacement on the Beartooth fault, a 30°-35° west-dipping thrust fault that places Precambrian rocks on Paleozoic and Mesozoic cover rocks. Displacement along the Beartooth fault produced a large fault-propagation fold (Berg's fold-thrust) that eventually was transected by the fault along the entire eastern flank of the range. Lithologic provenance modeling indicates that the Beartooth Conglomerate was produced by unroofing of the Upper Cretaceous through Precambrian section of the eastern Beartooth Range. The model results indicate that source-area relief during deposition of the lower part of the Beartooth Conglomerate was relatively low (∼600 m) while the soft, mudstone-dominated Upper Cretaceous part of the source section was being eroded from the crest of the uplift. Coarse, alluvial-fan accumulation commenced when resistant Paleozoic carbonate and quartz-arenitic rocks were exposed and source-area relief increased to ∼1,400 m. Relief steadily diminished thenceforth to ∼600-800 m, as the upper part of the source section was eroded from the Beartooth Range, leaving only a resistant carapace of Paleozoic carbonate rocks and Precambrian crystalline rocks. Source-area relief was controlled both by bedrock composition of the source terrane and by individual episodes of uplift. Like many thrust-derived conglomerates, the Beartooth Conglomerate contains intraformational, syndepositional folds, faults, and angular unconformities that can be used with provenance modeling to define the kinematics of uplift during conglomerate deposition. By utilizing the results of our provenance modeling and the orientations of the basal and intraformational angular unconformities, we have been able to retrodeform stepwise an area-balanced cross section through the eastern Beartooth uplift. Retrodeformation of the cross section indicates that 3.8 km of uplift occurred before deposition of the oldest part of the proximal Beartooth Conglomerate. Further uplift of ∼1.7 km and total horizontal shortening of ∼3.7 km accompanied ∼1.3 km of displacement on the Beartooth fault. Cross sections indicate that additional uplift of ∼1 km and horizontal shortening of ∼2 km accompanied displacement on the Line Creek fault. Our data indicate that during the early stages of uplift by fault-propagation folding, sediment derived from the uplift bypassed the proximal realm and was deposited in distal settings. Bypassing occurred because the eventual footwall of the Beartooth fault was uplifted during fault-propagation folding. Viewed from this perspective, the early fine-grained fills of many Laramide basins may owe their existence to a combination of fine-grained source material and sediment bypassing during the early stages of Laramide uplifts.

Denudation Surfaces of a Shield Area in South Sweden

Precambrian basement rocks form a domelike uplift in southern Sweden that emerges from Cambrian cover rocks in the north and east and from Mesozoic cover rocks in the south and west. Its morphology is analysed with the aid of contour maps and topographic profiles constructed by the Land Survey of Sweden from the elevation data base. Two sets of denudation surfaces have been delimited, viz. the exhumed and the epigene ones. The sub-Cambrian peneplain is still of importance for the highest surface, and tilted tectonic blocks can be defined with reference to it. Its limit to the south and west is a more or less pronounced erosional scarp. The analysis provides and explanation for the present river patterns and leads to conclusions concerning Phanerozoic tectonics in the area.

Alpine metamorphism of pelitic schists in the Nufenen Pass area, Lepontine Alps

Abstract The effects of Tertiary Alpine metamorphism on pelitic Mesozoic cover rocks have been studied along a cross‐section in the central Lepontine Alps in the Nufenen Pass area, Switzerland.Greenschist facies to amphibolite facies conditions are indicated by the formation of the index minerals chloritoid, garnet, staurolite and kyanite in pelitic rocks. Regional metamorphism reached maximum conditions during the interkinematic period between a main Alpine penetrative (D2) and a late Alpine (D3) crenulation type deformation phase or synchronous with the late Alpine deformation. Based on AFM phase relationships four different metamorphic zones can be distinguished: (1) chloritoid zone; (2) staurolite + chlorite zone; (3) staurolite + biotite zone; and, (4) kyanite zone.The isograds that separate these zones can be modelled by univariant reactions in the KFMASH system. The conditions of metamorphism calculated from geological ther‐mobarometers for the maximum post‐D2 por‐phyroblast stage are from North to South: 500° C at 5‐6 kbar and 600° C at 7‐8 kbar.Detailed thermobarometry of garnet por‐phyroblasts with complex textures suggests that maximum temperature was reached later than maximum pressure. Early garnet growth occurred along a prograde P‐T‐path, post‐D2 rims grew with increasing temperature but decreasing pressure, and finally post‐D3 garnet formed along a retrograde P‐T‐path.It may be concluded from the calculated pressure and temperature difference over a short distance (3 km) across the mapped area that the isogradic surfaces of the post‐D2 metamorphism are steeply oriented. The data also suggest that isobaric and isothermal surfaces are parallel.Much of the observed metamorphic pattern can be explained as the result of a significant post‐D2 differential uplift of the hot Pennine area relative to the Helvetic area along a tectonic contact zone. The closely spaced isograds (isotherms) in the North may then be interpreted as a thermal effect owing to the emplacement of the hot Pennine rocks against the Got‐thard massif with its cover. Whereas, in the Pennine metasediments, post‐D2 porphyroblast formation can be related to the decompression path which was steep enough for dehydration reactions to proceed. It is also remarkable that late kyanite porphyroblasts probably formed with decreasing pressure.The interpretation given here for the Nufenen Pass area may also apply to the Luk‐manier Pass area where similar metamorphic patterns have been reported by Fox (1975). The formation of the ‘Northern Steep Belt’;, as denned by Milnes (1974b), and the associated late Alpine fold zones may, therefore, have significantly modified the metamorphic pattern of the Helvetic‐Penninic contact zone.

Saudi Arabian seismic-refraction profile: A traveltime interpretation of crustal and upper mantle structure

The crustal and upper mantle compressional-wave velocity structure across the southwestern Arabian Shield has been investigated by a 1000-km-long seismic refraction profile. The profile begins in Mesozoic cover rocks near Riyadh on the Arabian Platform, trends southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan, and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, and six shot points were used, including one in the Red Sea. Two-dimensional ray-tracing techniques, used to analyze amplitude-normalized record sections indicate that the Arabian Shield is composed, to first order, of two layers, each about 20 km thick, with average velocities of about 6.3 km/s and 7.0 km/s, respectively. West of the Shield-Red Sea margin, the crust thins to a total thickness of less than 20 km, beyond which the Red Sea shelf and coastal plain are interpreted to be underlain by oceanic crust. A major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition. Elsewhere along the profile, several high-velocity anomalies in the upper crust correlate with mapped gneiss domes, the most prominent of which is the Khamis Mushayt gneiss. Based on their velocities, these domes may constitute areas where lower crustal rocks have been raised some 20 km. Two intracrustal reflectors in the center of the Shield at 13 km depth probably represent the tops of mafic intrusives. The Mohorovičić discontinuity beneath the Shield varies from a depth of 43 km and mantle velocity of 8.2 km/s in the northeast to a depth of 38 km and mantle velocity of 8.0 km/s depth in the southwest near the Shield-Red Sea transition. Two velocity discontinuities occur in the upper mantle, at 59 and 70 km depth. The crustal and upper mantle velocity structure of the Arabian Shield is interpreted as revealing a complex crust derived from the suturing of island arcs in the Precarnbrian. The Shield is currently flanked by the active spreading boundary in the Red Sea.

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During the Neogene and Quaternary, tectonic and climatic processes have had a profound impact upon landscape evolution in England and, perhaps as far back as 0.9 Ma, patterns of early human occupation. Until the Late Miocene, large-scale plate tectonic processes were the principal drivers of landscape evolution causing localised basin inversion and widespread exhumation. This drove, in places, the erosion of several kilometres of Mesozoic cover rocks and the development of a regional unconformity across England and the North Sea Basin. By the Pliocene, the relative influence of tectonics on landscape evolution waned as the background tectonic stress regime evolved and climatic influences became more prominent. Global-scale climate-forcing increased step-wise during the Plio-Pleistocene amplifying erosional and depositional processes that operated within the landscape. These processes caused differential unloading (uplift) and loading (subsidence) of the crust (‘denudational isostasy’) in areas undergoing net erosion (upland areas and slopes) and deposition (basins). Denudational isostasy amplified during the Mid-Pleistocene Transition (c.0.9 Ma) as landscapes become progressively synchronised to large-scale 100 ka ‘eccentricity’ climate forcing. Over the past 0.5 Ma, this has led to the establishment of a robust climate record of individual glacial/interglacial cycles enabling comparison to other regional and global records. During the Last Glacial-Interglacial Transition and early Holocene (c.16–7 ka), evidence for more abrupt (millennial/centennial) scale climatic events has been discovered. This indicates that superimposed upon the longer-term pattern of landscape evolution is a more dynamic response of the landscape to local and regional drivers.

Shear band foliation as an indicator of sense of shear: Field observations in central Spain

The Macizo de Nevera, a Paleozoic basement inlier surrounded by essentially flat-lying Mesozoic cover rocks of central Spain, exhibits an extremely consistent system of steep, dextral strike-slip faults. Different faults have a variable amount of displacement, but their en echelon arrangement and similar sense of displacement imply a cogenetic origin. A maximum displacement of 1.0 km, corresponding to an averaged angular shear strain of 1.5 and peak values of 6.6, occurs along the central part of the 6-km-long Truchas ductile shear zone. An early axial-plane cleavage is locally deflected into the Truchas shear zone and superposed by coarse shear band foliation. The consistent angular relationship between the new foliation and the shear zone boundary confirms the megascopic sense of shear.