Deep Crustal Root Research Articles

Seismic reflections from a lithospheric suture zone below the Archaean Yilgarn Craton

Seismic reflectors in the uppermost mantle, which can indicate past plate tectonic subduction, are exceedingly rare below Archaean cratons, and restricted to the Neoarchaean. Here we present reprocessed seismic reflection profiles from the northwest Archaean Yilgarn Craton and the Palaeoproterozoic Capricorn Orogen of western Australia that reveal the existence of a ~4 km thick south-dipping band of seismic reflectors that extends from the base of the Archaean crust to at least 60 km depth. We interpret these reflectors, which lie south of a ~50 km deep crustal root, as a relict suture zone within the lithosphere. We suggest that the mantle reflectors were created either by subduction of an oceanic plate along the northern edge of the Yilgarn Craton, which started in the Mesoarchaean and produced the rocks in northern Yilgarn greenstone belts that formed in a supra-subduction zone setting, or, alternatively, by underthrusting of continental crust deep into the lithosphere during the Palaeoproterozoic.

Accretion kinematics and driving mechanism of the eastern Central Asian Orogenic Belt: Insights from seismic tomography and middle Permian–Middle Triassic magmatism in central Jilin Province

The amalgamation of accretionary terranes plays an important role in the evolution of ancient accretionary orogens, but the kinematic process and its driving mechanism are commonly ambiguous. Here we provide new constraints on the accretion kinematics and associated driving mechanism of the eastern Central Asian Orogenic Belt (CAOB), using seismic tomography and magmatism. Permian to Middle Triassic magmatism was linked to two distinct accretionary orogens, that existed along the northern margin of the North China Craton. The geochemistry of middle Permian to Middle Triassic igneous rocks indicates a magmatic arc origin at an active continental margin. Based on the horizontal zoning of magmatism and south-dipping stagnant oceanic slab imaged by seismic tomography, we conclude that southward subduction of the oceanic slab resulted in the accretion and collision of multiple arc systems along the continental margin. Our seismic tomographic models indicate that formation and growth of the eastern CAOB occurred as the result of the continuous accretion and amalgamation of arc assemblages with addition of crustal material. Tomographic models and previous Re-Os geochronological studies indicate the existence of a deep crustal root and ancient sub-continental lithospheric mantle in most regions of the eastern CAOB. Taking into account the magmatic evolution and sedimentary sequences, we conclude that the terrane accretion should be the driving mechanism of accretionary orogenesis in the eastern CAOB.

Deep crustal structure in the Taiwan–Ryukyu arc–trench system junction area: New constraints from gravity modelling

AbstractThe Taiwan–Ryukyu arc–trench junction area is a seismic active region and has high potential to generate large earthquakes. However, the detailed deep structures are not fully revealed because of data resolution problems. The Taiwan Integrated Geodynamic Research (TAIGER) project collected two wide‐angle and multichannel seismic profiles across the Taiwan mountain belt (T5) and the western end of the Ryukyu arc–trench system (T6). In this study, we perform 2‐D gravity modelling for T5 and T6 by taking advantage of well‐constrained P‐wave velocity models, which helps to reduce the nonunique problem. To better fit the observed gravity anomaly, a block with relatively high‐density must exist beneath eastern Taiwan. This high‐density material could be serpentinised mantle, which may be resulted from the dehydration of the subducting Eurasian plate. Also, our result indicates that the subducting Gagua Ridge has a deep crustal root and extends northward to the Ryukyu forearc area.

Strain and retrogression partitioning explain long-term stability of crustal roots in stable continents

Abstract Away from tectonically active regions, the continental crust has an average thickness of 40 ± 1 km. Yet, it shows a remarkable variability from 25 to 65 km, comparable to that of the most tectonically active regions. Here, we consider the problem of the formation and preservation of anomalous deep crustal roots in stable intracontinental regions. Using two-dimensional thermomechanical experiments, we show that the interplay between partial melting, the formation of garnet-pyroxene-bearing rocks, and their strain rate–dependent retrogression result in the preservation of thick and strong crustal roots. We argue that it is the partitioning into narrow regions of strain, retrogression, and weakening coupled into a positive feedback loop that explains why strong high-grade crustal roots remain largely immune to gravitational stresses and are able to persist over hundreds of millions of years.

Growth of the northeastern margin of the Tibetan Plateau by squeezing up of the crust at the boundaries

In classic orogenic models, the mountain range is underlain by a deep crustal root. Here we present the crustal and upper mantle structures along two receiver function profiles across Qilian, an orogen experiencing recent growth at the northern margin of the Tibetan plateau. Opposite to an expected crustal root beneath the orogen, the Moho beneath Qilian is arch-like, shallower beneath the center and deepens by up to 10 km beneath its southern and northern boundaries. Additional velocity interfaces sub-parallel to the Moho are observed in the lower crust of the basins south of Qilian, which we interpret as the top of a mechanically strong lower crust thrusting several tens of kilometers underneath Qilian. In the north, the small lateral offset between the surface and mantle traces of the thrust system reveals a steep boundary, indicating that the North China cratonic crust acts as a strong resistance to the northward growth of the plateau, forcing the development of the left-lateral strike-slip Haiyuan fault south of the northern Qilian suture. The young Qilian orogen thus has been rising and growing progressively from the boundaries to the center, squeezed up by more rigid tectonic blocks in the north and south.

Crustal structure beneath the Rif Cordillera, North Morocco, from the RIFSIS wide‐angle reflection seismic experiment

AbstractThe different geodynamic models proposed since the late 1990s to account for the complex evolution of the Gibraltar Arc System lack definite constraints on the crustal structure of the Rif orogen. Here we present the first well‐resolved P‐wave velocity crustal models of the Rif Cordillera and its southern continuation toward the Atlas made using controlled‐source seismic data. Two 300+ km‐long wide‐angle reflection profiles crossed the Rif along NS and EW trends. The profiles recorded simultaneously five land explosions of 1Tn each using ∼850 high frequency seismometers. The crustal structure revealed from 2‐D forward modeling delineates a complex, laterally varying crustal structure below the Rif domains. The most surprising feature, seen on both profiles, is a ∼50 km deep crustal root localized beneath the External Rif. To the east, the crust thins rapidly by 20 km across the Nekkor fault, indicating that the fault is a crustal scale feature. On the NS profile the crust thins more gradually to 40 km thickness beneath Middle Atlas and 42 km beneath the Betics. These new seismic results are in overall agreement with regional trends of Bouguer gravity and are consistent with recent receiver function estimates of crustal thickness. The complex crustal structure of the Rif orogen in the Gibraltar Arc is a consequence of the Miocene collision between the Iberian and African plates. Both the abrupt change in crustal thickness at the Nekkor fault and the unexpectedly deep Rif crustal root can be attributed to interaction of the subducting Alboran slab with the North African passive margin at late Oligocene‐early Miocene times.

Hot upwelling conduit beneath the Atlas Mountains, Morocco

AbstractThe Atlas Mountains of Morocco display high topography, no deep crustal root, and regions of localized Cenozoic alkaline volcanism. Previous seismic imaging and geophysical studies have implied a hot mantle upwelling as the source of the volcanism and high elevation. However, the existence, shape, and physical properties of an associated mantle anomaly are debated. Here we use seismic waveform analysis from a broadband deployment and geodynamic modeling to define the physical properties and morphology of the anomaly. The imaged low‐velocity structure extends to ~200 km beneath the Atlas and appears ~350 K hotter than the ambient mantle with possible partial melting. It includes a lateral conduit, which suggests that the Quaternary volcanism arises from the upper mantle. Moreover, the shape and temperature of the imaged anomaly indicate that the unusually high topography of the Atlas Mountains is due to active mantle support.

Crustal structure beneath the Paleozoic Parnaíba Basin revealed by airborne gravity and magnetic data, Brazil

Abstract The Parnaiba Basin is a large Paleozoic syneclise in northeastern Brazil underlain by Precambrian crystalline basement, which comprises a complex lithostructural and tectonic framework formed during the Neoproterozoic–Eopaleozoic Brasiliano–Pan African orogenic collage. A sag basin up to 3.5 km thick and 1000 km long formed after the collage. The lithologic composition, structure, and role in the basin evolution of the underlying basement are the focus of this study. Airborne gravity and magnetic data were modeled to reveal the general crustal structure underneath the Parnaiba Basin. Results indicate that gravity and magnetic signatures delineate the main boundaries and structural trends of three cratonic areas and surrounding Neoproterozoic fold belts in the basement. Triangular-shaped basement inliers are geophysically defined in the central region of this continental-scale Neoproterozoic convergence zone. A 3-D gravity inversion constrained by seismological data reveals that basement inliers exhibit a 36–40.5 km deep crustal root, with borders defined by a high-density and thinner crust. Forward modeling of gravity and magnetic data indicates that lateral boundaries between crustal units are limited by Brasiliano shear zones, representing lithospheric sutures of the Amazonian and Sao Francisco Cratons, Tocantins Province and Parnaiba Block. In addition, coincident residual gravity, residual magnetic, and pseudo-gravity lows indicate two complex systems of Eopaleozoic rifts related to the initial phase of the sag deposition, which follow basement trends in several directions.

Heterogeneous extrusion and exhumation of deep-crustal Variscan assembly: Geochronology of the Western Tatra Mountains, northern Slovakia

The nature and style of mid-crustal assembly and exhumation during continental collision has been investigated in the Tatra Mountains of the Western Carpathians. The pre-Alpine basement of the Western Carpathians represents the easternmost exposure of the Variscan orogen in Europe, which marks the collision of Laurasia with Gondwanian-affiliated terranes during the Palaeozoic. The Tatric crystalline unit of the Western Tatra in northern Slovakia displays an inverted metamorphic sequence where a high-grade unit comprising migmatites with relicts of eclogite has been thrust over a lower-grade mica schist unit. New geochronological and thermochronological data together with published thermobarometry illuminate the metamorphic history of the Western Tatra. The Upper Unit eclogites with occasionally preserved omphacite record near isothermal decompression from 1.6GPa to 1.0–1.2GPa at 750–800°C which lead to intensive re-equilibration at high-pressure granulite facies conditions, comparable to the peak metamorphic conditions of the host migmatite. Both eclogite and migmatite shared a retrograde P–T path following the insertion of the eclogite assemblage into the migmatites. The metamorphic evolution of the Lower Unit mica schist is constrained to peak P–T conditions of 0.6–0.8GPa and 640 and 660°C followed by retrogression. This suggests that different rock types of the Western Tatra metamorphic core shared only their exhumation path from mid-crustal levels. ID-TIMS Sm–Nd dating of garnet from eclogite yields a whole rock-garnet isochron age of 337±10Ma, with an initial εNd isotopic composition of +8.3. In situ U–Pb dating of monazite from a migmatite surrounding the eclogite shows one age population of c. 380Ma whereas monazite from a migmatite away from the eclogite preserves a robust 340±11Ma age which is indistinguishable from Sm–Nd garnet age and U–Pb age of zircons in the anatectic leucosome of the migmatite (347±7Ma). A younger monazite age population from the migmatite of 300±16Ma is consistent with 40Ar/39Ar mica ages of c. 310Ma. This argues for a contemporaneous, and likely shared, exhumation path of the assemblage pair. In situ monazite total-Pb analyses from the Lower Unit mica schists yields xenocrystic and c. 370Ma ages, but no geochronologic evidence for peak Variscan tectonism. Exhumation of the deep crustal root occurred most probably in a two-stage process. The timing of the high-pressure, eclogite facies metamorphism before the onset of exhumation into the mid crust, was likely between c. 380Ma and 360Ma. Subsequent exhumation into the middle crust was coeval with migmatite generation at c. 340Ma and garnet diffusion modeling suggest ~30°C/Ma cooling rates. The exhumation was likely tectonically forced by the action of a rigid indentor, which prompted the weak lower crust to be heterogeneously extruded to mid-crustal levels at a time coeval with anatexis and subsequently extruded with mid-crustal material to the upper crust.

Diamond and coesite discovered in Saxony-type granulite: Solution to the Variscan garnet peridotite enigma

AbstractThe pressures required for diamond and coesite formation far exceed conditions reached by even the deepest present-day orogenic crustal roots. Therefore the occurrence of metamorphosed continental crust containing these minerals requires processes other than crustal thickening to have operated in the past. Here we report the first in situ finding of diamond and coesite, characterized by micro-Raman spectroscopy, in high-pressure granulites otherwise indistinguishable from granulites found associated with garnet peridotite throughout the European Variscides. Our discovery confirms the provenance of Europe's first reliable diamond, the “Bohemian diamond,” found in A.D. 1870, and also represents the first robust evidence for ultrahigh-pressure conditions in a major Variscan crustal rock type. A process of deep continental subduction is required to explain the metamorphic pressures and the granulite–garnet peridotite association, and thus tectonometamorphic models for these rocks involving a deep orogenic crustal root need to be significantly modified.

Insights into the Crustal Structure and Geodynamic Evolution of the Southern Granulite Terrain, India, from Isostatic Considerations

The Southern Granulite Terrain of India, formed through an ancient continental collision and uplift of the earth’s surface, was accompanied by thickening of the crust. Once the active tectonism ceased, the buoyancy of these deep crustal roots must have supported the Nilgiri and Palani-Cardamom hills. Here, the gravity field has been utilized to provide new constraints on how the force of buoyancy maintains the state of isostasy in the Southern Granulite Terrain. Isostatic calculations show that the seismically derived crustal thickness of 43–44 km in the Southern Granulite Terrain is on average 7–8 km more than that required to isostatically balance the present-day topography. This difference cannot be solely explained applying a constant shift in the mean sea level crustal thickness of 32 km. The isostatic analysis thus indicates that the current topography of the Southern Granulite Terrain is overcompensated, and about 1.0 km of the topographic load must have been eroded from this region without any isostatic readjustment. The observed gravity anomaly, an order of magnitude lower than that expected (−125 mGal), however, shows that there is no such overcompensation. Thermal perturbations up to Pan-African, present-day high mantle heat flow and low Te together negate the possible resistance of the lithosphere to rebound in response to erosional unloading. To isostatically compensate the crustal root, compatible to seismic Moho, a band of high density (2,930 kg m−3) in the lower crust and low density (3,210 kg m−3) in the lithospheric mantle below the Southern Granulite Terrain is needed. A relatively denser crust due to two distinct episodes of metamorphic phase transitions at 2.5 Ga and 550 Ma and highly mobilized upper mantle during Pan-African thermal perturbation reduced significantly the root buoyancy that kept the crust pulled downward in response to the eroded topography.

Magmatic evolution of the eastern Coast Plutonic Complex, Bella Coola region, west-central British Columbia

The eastern Coast Plutonic Complex in west-central British Columbia (51–54°N) consists of a broad belt of Jurassic to Eocene granitic to dioritic intrusive rocks that collectively represent more than 140 m.y. of nearly continuous, subduction-related magmatism. Seven distinct intrusive suites have been identified in the Bella Coola region, including, from oldest to youngest, the Howe Lake, Stick Pass, Firvale, Desire, Fougner, Big Snow, and Four Mile suites. Pre–Late Cretaceous rocks were produced by episodic subduction-related magmatism characterized by partial melting of preexisting lower-arc crust, with minimal incorporation of evolved continental material. The eastern Coast Plutonic Complex experienced a magmatic lull between ca. 110 and 95 Ma, coincident with sinistral transpression potentially linked to translational displacement of the Wrangellia-Alexander composite terranes. Crustal thickening in Late Cretaceous time (ca. 100–90 Ma), possibly due to underplating of the Wrangellia-Alexander terrane, produced a deep crustal root (>40 km) that likely extended into the eclogite transition zone. Adakitic magmas (ca. 95–67 Ma) were produced by partial melting of an amphibolitic protolith in this thickened lower-arc crust. Adakitic plutons are pre-, syn-, and postkinematic with respect to Late Cretaceous high-angle shear zones, and they are coeval and spatially associated with postkinematic two-mica granites (ca. 70–75 Ma) having a hybrid A-type geochemistry. We suggest that delamination of this dense crustal root led to voluminous magmatism, extension, and crustal exhumation in Paleocene-Eocene time. The successive intrusive events in the eastern Coast Plutonic Complex may reflect a predictable evolutionary progression common to all continental arc systems.

Reflection of seismic surface waves at the northern Apennines

Seismic surface waves with periods of 15 to 20 s are reflected laterally at the northern Apennines. For earthquakes originated and recorded in the wider Alpine area, a few hundred kilometres north of the Apennines, we can directly observe late arrivals of reflected regional surface waves. These have a characteristic polarization of particle motion and often dominate the intermediate-period surface wave coda. Love waves are the most prominent coda arrivals, while reflected Rayleigh waves show smaller amplitudes, and become clear only after rotation of the recordings to the reflection incidence direction. We can track the development of the Love wave reflection along a temporary broad-band transect across the northern Apennines, indicating the location of the reflector near to the highest topography. From spectral element simulations of three-dimensional wave propagation, we attribute the reflection to a continuous, ∼ 270 km long offset in the crust-mantle boundary under the mountain belt with vertical throw of ∼ 20 km, thus supporting a deep crustal root under the outer side of the Apennines fold and thrust belt, and significant crustal thinning on the inner side. Amplitude reflection coefficients for near-normal incidence can be estimated as ∼ 7% for both Love and Rayleigh waves.

Geophysical evidence for a deep crustal root beneath the Yilgarn Craton and the Albany-Fraser Orogen

Using a marine seismic reflection survey as a source for onshore and offshore seismic refraction recorders, high quality refraction data were recorded to a maximum offset of 400 km. A two-dimensional velocity model for Line 19 of Geoscience Australia survey 280 in the Bremer Sub-basin and its onshore extension was derived by forward modelling using iterative ray tracing. Our velocity model indicates the presence of a deep crustal root located under the southern margin of the Yilgarn Craton and the Albany-Fraser Orogen. Comparison of the gravity response derived from this model with the gravity profile extracted from regional grids shows reasonable correlation. However, fine tuning of the seismic model is required to better define the complex geometry of the crustal root. Deep crustal roots are common under Proterozoic mobile belts that surround Archaean cratons. However, these results also indicate the presence of a crustal root under the southern margin of the Yilgarn Craton. This is consistent with the view that Proterozoic taphrogenesis penetrated tens of kilometres into the Yilgarn Craton.

Structure and evolution of the Baikal rift: A synthesis

Active continental rifts are spectacular manifestations of the deformation of continents but are not very numerous at the surface of the Earth. Among them, the Baikal rift has been extensively studied during the last decades. Yet no simple scenario explains its origin and development because the style of rifting has changed throughout its ∼30 Myr history. In this paper, we use forward and inverse models of gravity data to map the Moho and lithosphere‐asthenosphere boundary in three dimensions. We then integrate these new results with existing geophysical and geological data on the Baikal rift structure and dynamics, and propose a scenario of its evolution. Earthquake depths, mantle xenoliths, heat flow, and seismic and gravity models advocate for a normal to moderately thinned continental lithosphere and crust, except beneath the Siberian craton, which exhibits a >100‐km‐thick lithosphere. Relatively thin lithosphere (70–80 km) is found east and south of the rift system and is in spatial connection with the Hangai‐Hövsgöl region of anomalous mantle in Mongolia. From top to bottom, the rift structure is asymmetric and appears strongly controlled by the geometry of the suture zone bounding the Siberian craton. Moreover, the mode of topography support changes significantly along the length of the rift: mountain ranges south and north of the rift are underlain by negative Bouguer anomalies, suggesting deep crustal roots and/or anomalous mantle; rift shoulders in the center of the rift seem to result from flexural uplift. The commonly assumed “two‐stage” rift evolution is not corroborated by all stratigraphic and seismic data; however, it seems clear that during the Oligocene, an “early stage,” which might be dominated by strike‐slip tectonics instead of pure extension, created primitive basins much different from the present ones. Most of the “true” rift basins seem to have initiated later, during the Late Miocene or Pliocene. This kinematic change from strike‐slip to extensional tectonics in the Baikal rift is part of a more general kinematic reorganization of Asia and can be associated with the rapid growth of the Tibetan plateau and the end of marginal basins opening along the Pacific boundary.

Post-Variscan evolution of the lithosphere in the area of the European Cenozoic Rift System

Abstract The European Cenozoic Rift System (ECRIS) transects Variscan basement, Permo-Carboniferous troughs and Late Permian to Mesozoic series, deposited in thermal sag basins, which are exposed on rift-related arches. We have analysed processes governing the transformation of the orogenically destabilized Variscan lithosphere into end-Cretaceous stabilized cratonic lithosphere, prior to its renewed Cenozoic rift-related destabilization. In the ECRIS area, crustal and lithospheric thicknesses at present are in the range of 24-35 km and 60-120 km, respectively. The Variscan orogen was characterized at the time of its end-Westphalian consolidation by 45-60 km deep crustal roots, marking major sutures. During the Stephanian-Early Permian wrench-induced collapse of the Variscan orogen, subducted lithospheric slabs were detached, causing upwelling of the asthenosphere, thermal thinning and/or partial delamina-tion of the lithospheric mantle, and regional uplift. By mid-Permian times, the crust was thinned to 28-35 km owing to its regional erosional unroofing, localized mechanical stretching and the interaction of mantle-derived melts with its basal parts. By mid-Permian times, when the temperature of the asthenosphere returned to ambient levels, thermal subsidence of the lithosphere commenced, controlling development of a system of Late Permian and Mesozoic intracratonic basins. These experienced repeated minor subsidence accelerations, related to the build-up of far-field stresses, which did not involve renewed lithospheric destabilization. Modelling of observed subsidence curves indicates that during the mid-Permian lithospheric thicknesses ranged in the ECRIS area between 40 and 80 km, but had increased by the end of the Cretaceous to 100-120 km. Cenozoic rifting and mantle-plume activity caused renewed lithospheric thinning.

Crustal evolution of Western and Central Europe

Abstract A new Moho depth map has been assembled for Western and Central Europe and the Western Mediterranean area that is exclusively based on published regional Moho depth maps. Tectonic overlays summarize Caledonian and Variscan tectonic units, Permo-Carboniferous fault systems and magmatic provinces, Mesozoic and Cenozoic rift-wrench systems, areas of intraplate compression, the outlines of Alpine orogens and the distribution of oceanic crust. Based on a comparison of these overlays with the Moho depth map we assess processes that controlled the evolution of the crust in the various parts of Europe through time. The present-day crustal configuration of Western and Central Europe results from polyphase Late Palaeozoic to recent lithospheric deformation that overprinted the margin of the Proterozoic East European Craton and particularly the Caledonian and Variscan crustal domains. Following consolidation of the Caledonides, their crustal roots were destroyed in conjunction with Devonian wrench tectonics and back-arc rifting. During the Permo-Carboniferous tectonomagmatic cycle, wrench faulting disrupted the crust of the Variscan Orogen and its foreland and the lithosphere of these regions was thermally destabilized. Late Permian and Mesozoic re-equilibration of the lithosphere-asthenosphere system was interrupted by the development of the Arctic-North Atlantic, Tethyan and associated rift systems. During the Alpine orogenic cycle, intraplate compressional stresses controlled basin inversion-related crustal thickening and lithospheric folding, as well as the evolution of the Rhine-Rhone rift system. Variably deep crustal roots characterize the Alpine orogenic chains. Neogene back-arc extension disrupted the eastern Pyrenees, Betic-Balearic, Apennine and Dinarides orogens.

Evolution of the lithosphere in the area of the Rhine Rift System

The Rhine Rift System (RRS) forms part of the European Cenozoic Rift System (ECRIS) and transects the Variscan Orogen, Permo-Carboniferous troughs and Late Permian to Mesozoic thermal sag basins. Crustal and lithospheric thicknesses range in the RRS area between 24–36 km and 50–120 km, respectively. We discuss processes controlling the transformation of the orogenically destabilised Variscan lithosphere into an end-Mesozoic stabilised cratonic lithosphere, as well as its renewed destabilisation during the Cenozoic development of ECRIS. By end-Westphalian times, the major sutures of the Variscan Orogen were associated with 45–60 km deep crustal roots. During the Stephanian-Early Permian, regional exhumation of the Variscides was controlled by their wrench deformation, detachment of subducted lithospheric slabs, asthenospheric upwelling and thermal thinning of the mantle-lithosphere. By late Early Permian times, when asthenospheric temperatures returned to ambient levels, lithospheric thicknesses ranged between 40 km and 80 km, whilst the thickness of the crust was reduced to 28–35 km in response to its regional erosional and local tectonic unroofing and the interaction of mantle-derived melts with its basal parts. Re-equilibration of the lithosphere-asthenosphere system governed the subsidence of Late Permian-Mesozoic thermal sag basins that covered much of the RRS area. By end-Cretaceous times, lithospheric thicknesses had increased to 100–120 km. Paleocene mantle plumes caused renewed thermal weakening of the lithosphere. Starting in the late Eocene, ECRIS evolved in the Pyrenean and Alpine foreland by passive rifting under a collision-related north-directed compressional stress field. Following end-Oligocene consolidation of the Pyrenees, west- and northwest-directed stresses originating in the Alps controlled further development of ECRIS. The RRS remained active until the Present, whilst the southern branch of ECRIS aborted in the early Miocene. Extensional strain across ECRIS amounts to some 7 km. Plume-related thermal thinning of the lithosphere underlies uplift of the Rhenish Massif and Massif Central. Lithospheric folding controlled uplift of the Vosges-Black Forest Arch.

The northern Walker Lane refraction experiment: Pn arrivals and the northern Sierra Nevada root

In May 2002, we collected a new crustal refraction profile from Battle Mountain, Nevada across western Nevada, the Reno area, Lake Tahoe, and the northern Sierra Nevada Mountains to Auburn, CA. Mine blasts and earthquakes were recorded by 199 Texan instruments extending across this more than 450-km-long transect. The use of large mine blasts and the ultra-portable Texan recorders kept the field costs of this profile to less than US$10,000. The seismic sources at the eastern end were mining blasts at Barrick's GoldStrike mine. The GoldStrike mine produced several ripple-fired blasts using 8000–44,000 kg of ANFO each, a daily occurrence. First arrivals from the larger GoldStrike blasts are obvious to distances of 300 km in the raw records. First arrivals from a quarry blast west of the survey near Watsonville, CA, located by the Northern California Seismic Network with a magnitude of 2.2, can be picked across the recording array to distances of 600 km. The Watsonville blast provides a western source, nearly reversing the GoldStrike blasts. A small earthquake near Bridgeport, CA. also produced pickable P-wave arrivals across the transect, providing fan-shot data. Arrivals from M5 events in the Mariana and Kuril Islands also appear in the records. This refraction survey observes an unexpectedly deep crustal root under the northern Sierra Nevada range, over 50 km in thickness and possibly centered west of the topographic crest. Pn delays of 4–6 s support this interpretation. At Battle Mountain, Nevada, we observe anomalously thin crust over a limited region perhaps only 150 km wide, with a Moho depth of 19–23 km. Pn crossover distances of less than 80 km support this anomaly, which is surrounded by observations of more normal, 30-km-thick crust. A 10-km-thick and high-velocity lower-crustal “pillow” is an alternative hypothesis, but unlikely due to the lack of volcanics west of Battle Mountain. Large mine and quarry blasts prove very effective crustal refraction sources when recorded with a dense receiver array, even over distances exceeding 600 km. New elastic synthetic seismogram modeling suggests that Pn can be strong as a first arrival, easing the modeling and interpretation of crustal refraction data. Fast eikonal computations of first-arrival time can match pickable Pn arrival times.

Post-Variscan evolution of the lithosphere in the Rhine Graben area: constraints from subsidence modelling

Abstract In the area of the Cenozoic Rhine rift system, crustal and lithospheric thicknesses range between 24 and 35 km, and 60 and 120 km, respectively. This rift system transects the deeply truncated Variscan Orogen and superimposed Permo-Carboniferous wrench-induced troughs, and Late Permian and Mesozoic thermal sag basins. At the time of its Westphalian consolidation, the Variscan Orogen was probably characterized by 45–60 km deep-crustal roots that were associated with its Rheno-Hercynian-Saxo-Thuringian, Saxo-Thuringian-Bohemian and Bohemian-Moldanubian sutures, all of which are transected by the Cenozoic Rhine rift system. During the Stephanian-Early Permian wrench-induced disruption of the Variscan Orogen, subducted lithospheric slabs were detached causing upwelling of hot mantle material. During the resulting thermal surge, partial delamination and/or thermal thinning of the continental mantle-lithosphere induced regional uplift. At the same time the Variscan orogenic roots were destroyed and crustal thicknesses reduced to 28–35 km in response to the combined effects of mantle-derived melts interacting with the lower crust, regional erosional unroofing of the crust and, on a more local scale, by its mechanical stretching. Towards the end of the Early Permian, the potential temperature of the asthenosphere returned to ambient levels. With this, regional, long-term thermal subsidence of the lithosphere commenced, controlling the development of a new system of Late Permian and Mesozoic thermal sag basins. However, the evolution of these basins was repeatedly overprinted by minor short-term subsidence accelerations that reflect the build-up of far-field stresses related to rifting in the Tethyan and Atlantic domains. Comparison of observed and modelled subsidence curves suggests that in the area of the Rhine rift system the lithosphere had equilibrated with the asthenosphere at the end of the Cretaceous at depths of 100–120 km, before it became thermally destabilized again by Cenozoic rifting and plume-related magmatism. Modelled subsidence curves indicate that by the end of Early Permian times the thermal thickness of the remnant mantle-lithosphere ranged between 10 and 50 km in areas that were later incorporated into Mesozoic thermal sag basins; this corresponds to mid-Permian thermal lithosphere thicknesses of 40–80 km.