Thick Lower Crust Research Articles

Fault Networks in Triaxial Tectonic Settings: Analog Modeling of Distributed Continental Extension With Lateral Shortening

AbstractTriaxial deformation is a general feature of continental tectonics, but its controls and the systematics of associated fault networks remain poorly understood. We present triaxial analog experiments mimicking crustal thinning resulting from distributed longitudinal extension and lateral shortening. Contemporary longitudinal extension and lateral shortening are related by the principal horizontal strain ratio (PHSR). We investigate the effect of crustal geometry, rheology and strain rate on deformation localization, faulting regime and pattern, and PHSR in brittle and brittle‐viscous crustal‐scale models. We find that in brittle models the fault networks reflect the basal boundary condition and fault‐density scales inversely with brittle layer thickness. In brittle‐viscous models, as strain rate (ė) decreases, (a) Three fault patterns emerge: conjugate sets of strike‐slip faults (ė > 3 × 10−4 s−1, PHSR > 0.31), sets of parallel oblique normal faults (ė = 0.3–3 × 10−4 s−1, PHSR = 0.15–0.25), horst‐and‐graben system (ė < 0.3 × 10−4 s−1, PHSR < 0.1). (b) The strain localization increases systematically and gradually. We interpret the strain rate dependent of faulting regimes to be controlled by vertical coupling between the model upper mantle and model upper crust resulting in spontaneous permutation of principal stress axes. Rate‐dependency of strain localization can be related to mechanical coupling between the upper and lower crust. We identify the following parameters controlling triaxial tectonic deformation: upper crustal thickness and friction coefficient, lower crustal thickness and viscosity, as well as strain rate. We test our models and predictions against natural prototypes (Tibet, Anatolia, Apennines, and Basin and Range Province) thus providing new perspectives on triaxial deformation.

Crustal structure of the central and southern Kyushu-Palau Ridge: Implications for intra-oceanic arc evolution

As an intra-oceanic arc, the Kyushu–Palau Ridge (KPR) is an exceptional place for studying intra-oceanic arc evolution and related magmatism using its interior velocity structure. Two wide-angle seismic profiles along the KPR provide an opportunity to unveil the crustal structure of its central and southern parts. Strong along-ridge variations are found either in P-wave velocity or crustal thickness of the central and southern KPR. The crustal thickness is between 6 and 12 km, with the velocity increasing from 4.0 km/s to 7.0 km/s from top to bottom. The upper crust has a large velocity gradient of ∼1 s−1 and a thickness between 2 and 4 km, while the lower crust has a lower velocity gradient of <0.2 s−1 and is characterized by inhomogeneous velocity anomalies and a substantial lateral variation in thickness. By comparing with the mature arc in the Izu-Bonin-Mariana arc in the east, we suggest the central and southern KPR is a thickened oceanic crust rather than a mature arc crust. Partly thick crust, velocity anomalies in the lower crust and upper mantle imply the crust is in a transition stage from oceanic crust to mature arc one. Compared with the quantitatively calculated seamount volumes, the lower crustal thickness is generally correlated well with the distribution of the seamount volumes along the central and southern KPR. These observations imply that the central and southern parts of the KPR are at the juvenile stage of arc evolution, with crustal growth primarily attributed to the lower crust thickening.

High velocity lower crust with anomalous rheological parameters under the Red Sea Passive Margin, SW Saudi Arabia – insight into the evolution of the Hijaz-Asir Escarpment Zone

SUMMARY The Red Sea Passive Margin (RSPM) in SW Saudi Arabia is characterized by two morphological provinces; the Red Sea Coastal Plains (RSCP) and the Hijaz-Asir Escarpment Zone (HAEZ) at the Arabian Shield Edge (ASE). Of these, the HAEZ is conspicuous by its seaward-facing heights of &amp;gt; 3 km. Based on the available receiver function analysis (REF) results from 25 seismic stations, we first present the Moho relief map underlying the RSPM to demonstrate that: (i) the transitional crust under the coastal region is 20–25 km thick which steadily increases landward to attain a thickness of ∼40 km below ASE, across an intervening region of depressed Moho with depths of 45–52 km. The latter exhibits significant regional extent and displays a close correlation with the topography of the HAEZ, suggesting a tectonic control on its formation. Eight rheological parameters for the upper as well as the lower crust are computed from the REF data, namely; Vp, Vp/Vs, density (ρ), Lame's first constant (λ), Lame's second constant (μ), Poisson's ratio (σ), bulk modulus (K) and acoustic impedance (AI). Their 2-D distribution for the lower crust unravels, for the first time, a high velocity lower crust (HVLC) underneath the HAEZ. The rheological character of the HVLC is further elucidated by investigating the relations between six rheological parameters: Vp–Vp/Vs, Vp/Vs–lower crustal thickness (hLC), Vs/ρ–hLC, λ–ρ, σ–ρ and λ–μ. The results indicate that lithology of the lower crust varies from diorite–felsic gneiss under RSCP to gabbro–diabase and mafic gneiss under HAEZ. We conjecture that the HVLC is an outcome of the exchange of materials from the subcrust at the hinterland of the RSPM directly underlying the HAEZ. Deep seismic control can only resolve the finer details of such crustal transformation.

Heterogeneous mantle associated with asthenosphere and Indian slab metasomatism: Constraints on fertilization of porphyry Cu mineralization in Tibetan orogen

Previous models related to ultrapotassic mantle and post-collisional porphyry Cu mineralization have been challenged by the newly identified ultrapotassic lamprophyres (14.31 ± 0.61 Ma), diorite porphyries (14.82 ± 0.21 Ma) and high Sr/Y monzogranite porphyries (14.19 ± 0.23 Ma) from Dabu deposit in the Gangdese, Tibet. We found that the continental crust and lithospheric mantle beneath Gangdese were highly heterogeneous. The western Gangdese (80°E to 89°E) are mainly characterized by enriched lithospheric mantle with εNd(t) ≤ -10 and 87Sr/86Sr ≥ 0.71, and relatively normal crustal thickness and maturation (∼55–65 km), whereas eastern Gangdese (89°E to 95°E) are dominated by relatively depleted lithospheric mantle with εNd(t) ≥ -10 and 87Sr/86Sr ≤ 0.71, and much more thickened and mature crust (∼62–75 km). Based on three-component modeling, the geodynamic mechanism in the western Gangdese is subducted Indian slab metasomatism induced by its roll-back and subsequent break-off, whereas the eastern Gangdese is upwelling asthenosphere metasomatism resulted from convective delamination of thickened lithospheric mantle root. The lamprophyres at Dabu with εNd(t) values of −11.34 to −10.59 represent the very rare and well-preserved ancient lithospheric mantle without significant metasomatism and modification. Instead of previous model that focused on connection with Indian continental plate, we prefer to the generation of porphyry Cu deposits in Tibetan orogen were optimal alignments and combinations of multiple magmatic and dynamic processes, involving metasomatism of subducted Neo-Tethyan slab, metamorphic dehydration and thickening of lower crust, long-lived melting-assimilation-storage-homogenization (MASH) hot zone in the lower crust, and further fertilization of multiple magma chambers (∼30 km, ∼10 km, 1–6 km) in the upper crust.

The crustal structure of the transition from the East Black Sea Basin to the Shatsky Ridge from the reinterpretation of deep seismic sounding data on profiles 14-15-16

The paper represents the results of velocity modeling (ray-tracing modeling) performed for three deep seismic sounding (DSS) profiles 14, 15 and 16 acquired in the eastern part of the Black Sea more than 40 years ago. These profiles represent a system of radial profiles diverging from one common shot point in the East Black Sea Basin (EBSB) and crossing the Shatsky Ridge. The performed modeling showed that thin (~10 km) crystalline EBSB crust, with velocities increasing from 6,5 km/s in the basement to 7,0 km/s on the Moho (20—22 km), is overlain by sediments as thick as ~10 km. The continental crust of the Shatsky Ridge of ~30 km thickness comprises two layers — the upper crust (Vp=6,0÷6,5 km/s) as thick as 15 km and the 10 km thick lower crust (Vp =6,5÷7,0 km/s). The transition from thin EBSB suboceanic crust to the Shatsky Ridge continental crust occurs rather sharply, over an interval of ~25 km, where changes are observed in all crustal layers — from sediments to the Moho. The transition of the two types of the crust is ascribed by a lineament, parallel to the coastline of the eastern part of the Black Sea, and is associated with a collinear Alushta-Batumi magnetic anomaly of the same (NW) strike. These features may testify in favor of the tectonic nature of the transition zone, the formation and activation of which took place during the main stages of evolution of the study region — at the closure of Mesozoic Tethys ocean, during the riftogenic opening of the EBSB in Cretaceous, and during the Alpine orogeny in the compression setting. The wedge-like shape of the EBSB, expanding southeastward up to 160—180 km width, is consistent with the concept of riftogenic opening of the EBSB in the Early Cretaceous as a result of clockwise rotation of the Mid Black Sea Ridge.

Crustal structure of the Mendeleev Rise and the Chukchi Plateau (Arctic Ocean) along the Russian wide-angle and multichannel seismic reflection experiment “Arctic-2012”

We present a seismic and density model for the crust and the uppermost mantle of the Arctic Ocean off-shore Chukotka down to a 40 km depth along a 740-km long latitudinal (at ca. 77°N) “Arctic-2012” wide-angle/MCS profile. Joint seismic and gravity modeling indicates significant differences in the crustal velocity and density structure of the northeastern Vilkitsky Trough, the Mendeleev Rise, the Chukchi Basin, and the Chukchi Plateau.The Vilkitsky Trough and the Chukchi Basin have a thin crust (23 km and 18 km, correspondingly), 6–8 km thick sedimentary cover, 3–6 km thick upper/middle crust (with the smallest thickness of 3–4 km beneath the Chukchi Basin), and 9–10 km thick lower crust. The uppermost mantle of the Chukchi Basin has a high density (3.27–3.31 g/cm3) and a low velocity (Vp ∼ 7.8 km/s), which we explain by 5–10% serpentinization of mantle peridotite at a 22–35 km depth as a result of crustal hyperextension and seawater penetration.The Chukchi Plateau and the Mendeleev Rise have a thick crust (28–29 km and 33–34 km, correspondingly), underlain by a normal mantle (Vp ∼ 8.0 km/s). The Chukchi Plateau has a 2‐4 km thick sedimentary cover, a thick (15–18 km) upper/middle crust with low-Vp, low-density lenses interpreted as magmatic intrusions, and a 9–12 km thick lower crust. The Mendeleev Rise has a 3–7 km thick sedimentary cover (most of which is formed by metasediments with a possible presence of volcanic rocks), a 7–8 km thick upper/middle crust, and a thick (20 km) lower crust which includes a 3–4 km thick high-velocity (Vp ∼ 7.3 km/s) underplated magmatic material. The high density anomaly (at depths >35 km) below the Mendeleev Rise is interpreted as an eclogitic body in the upper mantle lithosphere.Seismic Vp and Vp/Vs structure of the crust along the “Arctic-2012” profile indicates its continental nature: a 3–18 km thick upper/middle crustal layer with Vp ∼ 6.0–6.8 km/s and Vp/Vs ∼ 1.70–1.73 typical of felsic-intermediate continental upper crust is present along the entire profile. Strong variability of the crustal structure along the profile reflects its significant modification by metamorphism and magmatism, possibly related to the High-Arctic Large Igneous Province and localized lithosphere extension beneath the Chukchi Basin.

Deep crustal architecture of the Parnaíba basin of NE Brazil from receiver function analysis: implications for basin subsidence

Abstract We investigated the crustal architecture of the Parnaíba basin of NE Brazil by analysing receiver functions along a c. 600 km long transect crossing the central portion of the basin. The transect consisted of nine broadband stations interspaced at c. 70 km distance recording continuously for a period of 15 months, with the goal of improving our understanding of the origin and evolution of this large cratonic basin. Our results show that crustal thickness varies between 39 and 45 km along the transect, gradually thickening towards the depocentre, and that bulk V p / V s ratios vary between 1.70 and 1.78. The crust can be divided into a 2.0–3.5 km thick layer of low-velocity sediments, a 15–20 km thick upper crust (3.5 &lt; V s &lt; 3.6 km s −1 ) and a 18–22 km thick lower crust (3.7 &lt; V s &lt; 3.8 km s −1 ). Near the depocentre, where the crust is thickest, the bottom 10–12 km of the crust are characterized by fast S-velocities (4.0–4.2 km s −1 ). Our findings confirm that stretching of the lithosphere is minimal and compatible with flexural subsidence. However, loading from a thick, high-density layer of mafic intrusive rocks pervading the lower crust – as recently proposed for the basin – is found to be inconsistent with our bulk V p / V s ratios and lower crustal S-velocities. Flexural bending by a deeper load, perhaps related to deep mantle convection, seems more plausible.

Analisa Receiver Function Teleseismic untuk Mendeteksi Moho pada Stasiun Bkb Data Meramex

&lt;span&gt;It has been done a research to determine internal earth using receiver function teleseismic analysis &lt;span&gt;method. This method have been done by using MERAMEX (MErapi Amphibious Experiment) data from &lt;span&gt;broadband seismometer BKB. Event of teleseismic is chosen from Honshu Japan with radius 30&lt;span&gt;o &lt;span&gt;and &lt;span&gt;magnitude 7.2. This research begun by analysing radial and vertical characteristic of teleseismic event&lt;br /&gt;&lt;span&gt;and using bandpass filter with range 0.003 Hz – 0.5 Hz. Then Iteractive Deconvolution is used to get &lt;span&gt;velocity model. The result of this model shows crustal model that has 4 Km thick upper crust, a 26 Km &lt;span&gt;thick lower crust and 10 Km thick Moho transition zone, with velocity increasing gradually.&lt;/span&gt;&lt;/span&gt;&lt;/span&gt;&lt;/span&gt;&lt;/span&gt;&lt;/span&gt;&lt;br /&gt;&lt;/span&gt;&lt;/span&gt;&lt;/span&gt;

Crustal velocity structure of the Deccan Volcanic Province, Indian Peninsula, from observed surface wave dispersion

&lt;p&gt;Through inversion of fundamental mode group velocities of Love and Rayleigh waves, we study the crustal and subcrustal structure across the central Deccan Volcanic Province (DVP), which is one of the world’s largest terrestrial flood basalts. Our analysis is based on broadband seismograms recorded at seismological station Bhopal (BHPL) in the central India from earthquakes located near west coast of India, with an average epicentral distance about 768 km. The recording station and epicentral zone are situated respectively on the northern and southern edges of DVP with wave paths across central DVP. The period of group velocity data ranges from 5 to 60 s for Rayleigh waves and 5 to 45 s for Love waves. Using the genetic algorithm, the observed data have been inverted to obtain the crust and subcrustal velocity structure along the wavepaths. Using this procedure, a similar velocity structure was also obtained earlier for the northwestern DVP, which is in the west of the present study region. Comparison of results show that the crustal thickness decreases westward from central DVP (39.6 km) to northwestern DVP (37.8 km) along with the decrease of thickness of upper crust; while the thickness of lower crust remains nearly same. From east to west S-wave velocity in the upper crust decreases by 2 to 3 per cent, while P-wave velocity in the whole crust and subcrust decreases by 3 to 6 per cent. The P- and S-wave velocities are positively correlated with crustal thickness and negatively correlated with earth’s heat flow. It appears that the elevated crustal and subcrustal temperature in the western side is the main factor for low velocities on this side.&lt;/p&gt;

Seismic imaging of crust beneath the Dharwar Craton, India, from ambient noise and teleseismic receiver function modelling

We use cross-correlation of continuous 18 months (2009 February to 2010 August) ambient noise data recorded over 35 broad-band seismographs in the Archean Dharwar Craton and the adjoining granulite terrain to generate Rayleigh-wave group velocity maps in the period 5–28 s. This is supplemented with longer period data (40–70 s) from earthquake source. Combined group velocity measurement was inverted jointly with the teleseismic receiver functions obtained at 50 stations (includes 15 stations operated during 1998–2002) to produce shear velocity image of the crust. The velocity image reveals thinner crust (34–38 km) in the late Archean (∼2.7 Ga) Eastern Dharwar Craton (EDC), while all other terrains (mid-Archean and Proterozoic) have crustal thickness from 40 to over 50 km. The mid-Archean (3.36 Ga) greenstone belt of the Western Dharwar Craton (WDC) has the thickest crust (∼50 km). The average crustal Vs beneath the EDC is ∼3.70–3.78 km s−1 as compared to 3.80–3.95 km s−1 beneath the WDC. We observe significant lateral variation in the thickness of lower crust (Vs ∼ 3.8–4.2 km s−1): ∼10–15 km in the EDC compared to ∼20–30 km in the WDC. The lowermost part of the crust (Vs ≥ 4.0 km s−1) is thin (<5 km) beneath the EDC in contrast to more thickness (10–27 km) beneath the WDC. Our analysis suggests intermediate composition for the crust beneath the EDC similar to those for other cratons. In contrast, the mid-Archean exposed WDC crust has more mafic composition and exceptional thickness—a scenario at variance with the global observations. We interpret this thick mafic crust to represent undeformed geological segment of 3.36 Ga. The EDC with a nearly flat Moho, felsic to intermediate composition of crust and thin basal layer may represent a reworked terrain during the late-Archean.

Crustal Structure of the Northeastern Tibetan Plateau from the Southern Tarim Basin to the Sichuan Basin, China

We present a new crustal cross section of the northeastern Tibetan plateau based on active-source seismic data recorded along a 1600-km-long profile crossing the southern Tarim basin, the western flank of the South-Qilian Shan, the northeastern margin of Qaidam basin, East-Kunlun Shan, Songpan-Ganzi terrane, and Sichuan basin. The crustal P- and S-wave velocity structure and Poisson's ratio outline the seismic characteristics of the crust and provide constraints on the crustal composition. The derived crustal cross section shows several significant features. (1) The crustal thickness varies considerably along this profile, from 48km to 70km. (2) North of the Kunlun fault variations in total crustal thickness are mainly caused by variations in lower crustal thickness, whereas south of the Kunlun fault they are caused by thickness variations throughout the crust. (3) North of the Kunlun fault we detect a mid-crustal low-velocity zone that is not apparent south of the fault. (4) The Kunlun fault seems to act as a compositional boundary for the lower crust, with a Poisson's ratio of 0.29 north of the fault (Kunlun-Qaidam terrane) and 0.26 south of the fault (Songpan-Ganzi terrane). Measured Poisson's ratio and P-wave velocity values suggest that the lower crust throughout the Tibetan plateau (South-Qilian Shan, margins of the Qaidam Basin, East-Kunlun Shan, Songpan-Ganzi terrane) is of intermediate composition. Thus the NE Tibetan plateau along our profile is missing a mafic lower crustal layer.

Crustal structure beneath the Archean–Proterozoic terrain of north India from receiver function modeling

The shear velocity structure of the north Indian crust comprising of the Archean cratons (Aravalli–Bundelkhand) and Proterozoic Vindhyan basin is modeled and compared with the eastern Dharwar craton as well the global models for Archean and Proterozoic terrains. The velocity structure is deduced from joint inversion of teleseismic receiver function time series and surface wave group velocity measurements. The north Indian Archean crust is a simple two layered: an average Vs∼3.3±0.1km/s in the upper 10km increasing to 3.7±0.1km/s in the 26–28km thick lower crust. This upper crustal velocity is significantly less compared to those of either eastern Dharwar craton (3.5–3.6km/s) or the global average. These results suggest presence of a highly silica rich upper crust and an intermediate composition of the lower crust. The Moho is modeled at a depth of 36–38km with thin transition (∼2km) similar to those in eastern Dharwar craton. In contrast, the Proterozoic Vindhyan basin has significantly deeper Moho (44–46km) with a 4–10km thick underplated layer (Vs⩾4.0km/s) at the base of the crust. The sedimentary layer thickness is ∼2–4km. The results support the view that Proterozoic crust is thicker and underlain by a mafic layer at its base in contrast to a thin Archean crust with near absence of the basal layer as observed globally.

DEEP CRUSTAL PROFILE ACROSS THE SOUTHERN KAROO BASIN AND BEATTIE MAGNETIC ANOMALY, SOUTH AFRICA: AN INTEGRATED INTERPRETATION WITH TECTONIC IMPLICATIONS

Two geophysical onshore-offshore lines on the southern margin of Africa form the Agulhas-Karoo Geophysical Transect (AKGT) and cross prominent geological features such as the Karoo Basin, Cape Fold Belt (CFB) and the Beattie Magnetic Anomaly (BMA). Geophysical data acquired along this AKGTransect between 2004 and 2007 within the Inkaba yeAfrica (lyA) framework, provide the platform for constructing a deep crustal section (IyA-200501) for the centre 100 km of the western AKGT-transect in order to resolve these features at depth. We present a detailed deep crustal model constructed from the joint interpretation of: i. archive data comprising surface geology, aeromagnetic data, nearby deep boreholes, teleseismic receiver functions and regional seismic reflection profiles, and ii. line coincident newly acquired high-resolution geophysical data consisting of near vertical seismic reflection data, shallow P- and S-wave velocity data, wide-angle refraction data, high resolution magnetotelluric data and impedance spectroscopy measurements on borehole samples. Our model differentiates four components in the up to 45 km thick crust: 1. a ~2 to 5 km thick folded Karoo Supergroup, disrupted by low-angle thrust faults rooted in a zone of local decollements in the lower Ecca Group and resting paraconformably on 2. a continuous undeformed sub-horizontal ~1.5 to 10 km thick wedge of the Cape Supergroup (CSG). This CSG wedge stretches from the Escarpment in the north to the tectonic front of the CFB in the south, and rests on an unconformity that dips about three degrees to the south. The angular unconformity is interpreted as an erosional peneplain that separates the CSG wedge from component 3. the ~13 to 21 km mid-crust basement below. The mid-crust contains a distinct north-dipping seismic fabric, here interpreted as ~1.4 to 1.0 Ga Mesoproterozoic Namaqua-Natal Metamorphic Belt (NNMB) crust. A south-dipping mid-crustal detachment, interpreted as a ductile thrust zone, separates the mid-crust from component 4. a highly reflective ~10 to 24 km thick lower crust. The latter is interpreted as an older Palaeoproterozoic section of the NNMB (or even Archean cratonic basement), and bounded by a ~2 to 5 km thick, highly reflective bottom layer below that lies sub- parallel to a clear Moho. This bottom layer is interpreted as a mafic underplate, metasomatic reaction zone, or lower-crust to mantle transition zone. Collectively the seismic reflection and wide-angle refraction data support an interpretation that the NNMB mid-crustal layer contains the BMA source, possibly connected to two zones of strong reflectivity: a ~10 to 12 km wide northern zone and a ~5 to 7 km wide southern zone, both about 5 km thick and 7 to 8 km below surface. We interpret the BMA source to be at least in part, a Namaqua-like massive to disseminated, deformed/metamorphosed stratiform sulphide-magnetite ore body with metasomatic overprint. The seismic reflection and -refraction data support an interpretation that a Pan-African suture zone at the BMA is absent and that instead, the NNMB continues below the CFB tectonic front, probably up to the continental margin and the Agulhas Fracture Zone. The seismic reflection data also supports a thin-skinned tectonic thrust model for the evolution of the CFB without significant fore-deep stratigraphic thickening of the Karoo Basin strata. A compatible tectonic model implies a Palaeozoic collision orogen setting, coupled to a south verging subduction zone much farther south of the CFB. Similarly, the geophysical data support a south dipping subduction zone during the amalgamation of the NNMB in the Mesoproterozoic. Current reconstructions of the Rodinia supercontinent link the NNMB and the Grenville Province of North America across the Grenville-Kibaran orogen. Our seismic section tests this reconstruction through a direct comparison with seismic profiles on the opposite flank of the orogen. Although the once adjacent continental blocks are now 1000s of kilometres apart, the seismic images show a good correlation and support the reconstruction.

Geochemistry and zircon U–Pb ages of adakitic rocks from the Dulan area of the North Qaidam UHP terrane, north Tibet: Constraints on the timing and nature of regional tectonothermal events associated with collisional orogeny

Geochemistry and U–Pb ages of leucosomes and tonalites in a high pressure granulite unit of the Dulan area have been determined to constrain the tectonothermal evolution related to collision and thickening of lower crust in the North Qaidam Mountains (NQD). Leucosomes and tonalites show a marked chemical resemblance to adakites: (1) high La/Yb and Sr/Y, and low Y and HREE; (2) high Al 2O 3 and low Mg # values with obvious positive Eu anomalies; and (3) slightly positive ε Nd(t) values. Zircon U–Pb analysis of leucosomes and tonalites yielded 206Pb/ 238U ages of 428–437 Ma and 436–437 Ma respectively, which constrain the emplacement ages of the adakitic rocks. Petrological, geochronological and geochemical characters indicate that the adakitic rocks may have been derived from partial melting of a thickened mafic lower crust (> 50 km), suggesting a dominating source regime of synchronous high-pressure mafic granulites. Contemporary magmatism in other units of the NQD shows evidence of a widespread tectonothermal event during early Silurian (420–450 Ma) that includes metamorphism, magmatism and anatexis related to collision and thickening of lower crust.

Initial pedogenesis in a topsoil crust 3 years after construction of an artificial catchment in Brandenburg, NE Germany

Cyanobacteria and green algae present in biological soil crusts are able to colonize mineral substrates even under extreme environmental conditions. As pioneer organisms, they play a key role during the first phases of habitat colonization. A characteristic crust was sampled 3 years after installation of the artificial water catchment “Chicken creek”, thus representing an early successional stage of ecosystem development. Mean annual rainfall and temperature were 559 mm and 9.3°C, respectively. We combined scanning electron microscopy (SEM/EDX) and infrared (FTIR) microscopy to study the contact zone of algal and cyanobacterial mucilage with soil minerals in an undisturbed biological soil crust and in the subjacent sandy substrate. The crust was characterized by an approximately 50 μm thick surface layer, where microorganisms resided and where mineral deposition was trapped, and by an approximately 2.5 mm thick lower crust where mineral particles were stabilized by organo-mineral structures. SEM/EDX microscopy was used to determine the spatial distribution of elements, organic compounds and minerals were identified using FTIR microscopy and X-ray diffraction (XRD). The concentration of organic carbon in the crust was about twice as much as in the parent material. Depletion of Fe, Al and Mn in the lower crust and in the subjacent 5 mm compared to the geological substrate was observed. This could be interpreted as the initial phase of podzolization. Existence of bridging structures between mineral particles of the lower crust, containing phyllosilicates, Fe compounds and organic matter (OM), may indicate the formation of organo-mineral associations. pH decreased from 8.1 in the original substrate to 5.1 on the crust surface 3 years after construction, pointing to rapid weathering of carbonates. Weathering of silicates could not be detected.

Crustal Structure of the Western Bengal Basin from Joint Analysis of Teleseismic Receiver Functions and Rayleigh-Wave Dispersion

Teleseismic data recorded at the broadband seismological observatory of the Indian Institute of Technology (IIT) Kharagpur are analyzed to determine the seismic characteristics of the crust beneath the western Bengal basin. Receiver functions calculated from the teleseismic P waveform for a range of back azimuths show little variation in the Moho Ps arrival time, indicating a nearly one-dimensional crustal structure beneath Kharagpur. Transforming the time domain receiver functions into the H - V p / V s domain for a stack of all receiver functions reveals a 38±2 km thick crust with a V p / V s of 1.73±0.01. Receiver functions stacked in narrow bins of back azimuth and great-circle-arc distance were inverted jointly with group velocity dispersion data for periods from 15 to 45 sec. Joint inversion results reveal a near surface sedimentary layer of ∼1.5 km thickness, a 13 km thick upper crust, and a 23 km thick lower crust with the Moho at a depth of ∼37.5 km. The average crustal S -wave velocity is 3.7 km/sec. The top 0.5 km of the sedimentary layer has very low S -wave velocity and a large impedance contrast with the underlying layer. This represents the thin layer of unconsolidated sediments and weathered laterites near the surface as seen from deep seismic sounding (DSS) studies across the region. The S -wave velocity structure of the crust beneath Kharagpur is the first such result for the western Bengal basin and is broadly similar to that of the Indian Shield crust. A comparison of the crustal thickness and lower crustal velocities between Kharagpur and Agartala, seperated by the Eocene hinge zone, shows remarkable differences. Crustal models beneath Agartala reveal ∼36 km thick crust with the presence of a high-velocity lower crust (∼6 km thick), possibly oceanic in nature, which is absent beneath Kharagpur. The hinge zone, therefore, separates the continental crust of the western Bengal basin from the eastern deep basinal oceanic crust, both being overlain by sediments derived from the Himalayas.

Lithospheric structure of the Bohemian Massif and adjacent Variscan belt in central Europe based on profile S01 from the SUDETES 2003 experiment

The SUDETES 2003 seismic experiment investigated the lithospheric structure of the eastern part of the Variscan belt of central Europe. The key profile of this experiment (S01) was 630 km long and extended southwestward from the margin of the East European craton, across the Trans‐European suture zone (TESZ) and Sudetes, and across the Bohemian Massif that contains the active Eger (Ohře) rift, which is an element of the European Cenozoic rift system. Good quality first arrivals and later phases of refracted/reflected P and S waves were interpreted using 2‐D ray‐tracing techniques. The derived seismic model shows large variations in the internal structure of the crust, while the depth to the Moho varies in the relatively narrow depth interval of 28–35 km. Except for the Polish basin on the northeast end of the profile, the sedimentary cover is thin. The crystalline upper and middle crust with velocities of 5.9–6.4 km s−1 is about 20 km thick, and the 7–10 km thick lower crust can be divided into three regions based on P wave velocities: a low‐velocity region (6.5–6.6 km s−1 beneath Eger rift and Sudetes) that is bounded on the southwest and northeast by regions of significantly higher velocity (6.8–7.1 km s−1 beneath the Saxothuringian and Moldanubian in the southwest and Fore‐Sudetic Monocline and Polish Basin in the northeast). High‐velocity bodies (Vp &gt; 6.5 km s−1) were delineated in the upper crust of the Eger rift region. The seismic structure along the S01 profile images a Variscan orogenic wedge resting on the down warped margin of the plate margin containing the TESZ. This situation implies the northerly directed subduction of the Rheic Ocean that existed between the southern margin of the Old Red Continent and the Armorican terranes presently accreted into the Variscan belt. Closure of this ocean produced the Rheic suture between low‐velocity crust of the Variscan orogenic wedge and higher‐velocity crust of the TESZ.

Formation and evolution of the chaotic terrains by subsidence and magmatism: Hydraotes Chaos, Mars

The origin of the martian chaotic terrains is still uncertain; and a variety of geologic scenarios have been proposed. We provide topographic profiles of different chaos landscapes, notably Aureum and Hydraotes Chaos, showing that an initial shallow ground subsidence occurred at the first step of the chaos formation. We infer that the subsidence was caused by intrusion of a volcanic sill; which could have produced consequent melting as well as release of ground water from disrupted aquifer. Signs of a volcanic activity are observed on the floor of Hydraotes Chaos, a complex and deep depression located at the junction of three channels. The volcanic activity is represented by small, 0.5 to 1.5 km diameter, rounded cones with summit pits. The cone's size and morphology, as well as the presence of possible surrounding lava flows, suggest that they are primary volcanic cones similar to terrestrial cinder cones. The identification of volcanic activity on the deepest chaos, where the lower crustal thickness and the faults/fractures system contributed to the magma rising, reveals that magmatic activity, proved by the cones, and possibly help by structural activity, has been a major factor in the formation of chaotic terrains.

Formation of the superdeep South Caspian basin: subsidence driven by phase change in continental crust

The large hydrocarbon basin of South Caspian is filled with sediments reaching a thickness of 20–25 km. The sediments overlie a 10–18 km thick high-velocity basement which is often interpreted as oceanic crust. This interpretation is, however, inconsistent with rapid major subsidence in Pliocene-Pleistocene time and deposition of 10 km of sediments because the subsidence of crust produced in spreading ridges normally occurs at decreasing rates. Furthermore, filling a basin upon a 10–18 km thick oceanic crust would require twice less sediments. Subsidence as in the South Caspian, of ≥20 km, can be provided by phase change of gabbro to dense eclogite in a 25–30 km thick lower crust. Eclogites which are denser than the mantle and have nearly mantle P velocities but a chemistry of continental crust may occur beneath the Moho in the South Caspian where consolidated crust totals a thickness of 40–50 km. The high subsidence rates in the Pliocene-Pleistocene may be attributed to the effect of active fluids infiltrated from the asthenosphere to catalyze the gabbro-eclogite transition. Subsidence of this kind is typical of large petroleum provinces. According to some interpretations, historic seismicity with 30–70 km focal depths in a 100 km wide zone (beneath the Apsheron-Balkhan sill and north of it) has been associated with the initiation of subduction under the Middle Caspian. The consolidated lithosphere of deep continental sedimentary basins being denser than the asthenosphere, can, in principle, subduct into the latter, while the overlying sediments can be delaminated and folded. Yet, subduction in the South Caspian basin is incompatible with the only 5–10 km shortening of sediments in the Apsheron-Balkhan sill and south of it and with the patterns of earthquake foci that show no alignment like in a Benioff zone and have mostly extension mechanisms.

Crust restoration for the western Lachlan Orogen using the strain-reversal, area-balancing technique: implications for crustal components and original thicknesses

Strain reversal of structural/stratigraphic profiles at different scales in the western Lachlan Orogen provides a perspective on original crustal thickness estimates, the former depositional basin width of the proto-western Lachlan Orogen, the original sedimentary-fan thickness, and the possible length extent of lower crust lost by subduction. Retrodeformation using strain-reversal techniques allows basin reconstruction giving an original width of the western Lachlan Orogen basin receptor of between 800 km (minimum) and ∼1150 km (maximum), depending on the amount of stratal duplication allowed in the turbidites. Crude area balancing of the regional cross-section, adding in sectional volume lost by erosion and assuming strain compatibility between the upper and lower crust, suggests that the predeformation crustal thickness ranges between 15 km and ∼21 km, with a lower crustal thickness (oceanic lithosphere) of ∼9 km and a turbidite fan thickness of ∼6 km (minimum) and ∼12 km (maximum allowable), respectively. Disparity between the calculated fan thickness and that derived from measured stratigraphic sections adjusted for strain (∼6 km) indicates that some form of crustal stacking must be important in structural thickening of the turbidite crustal component. By varying shortening due to fault stacking, mass balance dictates the mismatch of the upper crustal (uc) and lower crustal (lc) retrodeformed lengths, and therefore provides an estimate of lower crustal loss by subduction. End members range from: (i) a 12 km-thick fan without fault duplication, a basin width of ∼800 km where uc = lc giving no lower crustal loss by subduction; to (ii) a ∼6 km fan, requiring duplication by faulting, a basin of ∼1150 km where uc > lc, and ∼360 km of lower crust length (∼30%) lost by subduction. This suggests that the total thickness of underplated igneous material in the western Lachlan Orogen is low, probably < ∼2 km.