Thinning Of Continental Lithosphere Research Articles

Petrogenesis of alkaline magmas across a continent to ocean transect, northern Ross Sea, Antarctica

The West Antarctic Rift System (WARS) is one of the world's largest active rift systems and alkaline magmatic provinces. The alkaline magmatism crosscuts oceanic and continental lithosphere but maintains generally similar Sr-Nd-Pb isotope signatures. Here we present new major-, trace- and highly siderophile-element (Os, Ir, Ru, Pt, Pd, Re) abundances and Sr-Nd-Pb-Os isotope data for a Miocene to recent WARS alkaline basalt suite from a continental-oceanic transect in the northwest Ross Sea. The samples are geochemically enriched similar to ocean island basalts, with oceanic lavas having higher absolute incompatible trace element abundances than continental lavas. The suite exhibits a large range in measured 187Re/188Os (1.7 to 1305) and age-corrected 187Os/188Os (0.1254 to 1.055) that has a positive, highly curved relationship with measured 87Sr/86Sr (0.70287–0.70331), but a negative, highly curved relationship with both measured 143Nd/144Nd (0.51280–0.51300) and 206Pb/204Pb (19.237–20.237). These data suggest that contamination by a mafic continental lithospheric component has occurred to varying degrees and exerts some control on the geochemistry of the continental lavas. Primitive lavas from the oceanic Adare Basin that have experienced negligible contamination are used to constrain the primary source of northwest Ross Sea alkaline lavas and to assess previously proposed hypotheses regarding the origin of magmatism. Explanations for the origin of WARS alkaline lavas range from plume magmatism to partial melting of continental lithospheric mantle metasomatized by subduction fluids prior to cessation of subduction of the Phoenix Plate underneath the Gondwana margin. Samples from the continental-oceanic transect, however, neither exhibit classic arc-lava geochemical signatures nor provide any evidence to support an active mantle plume source. Rather, they are geochemically enriched magmas generated through partial melting of previously subducted oceanic lithospheric mantle concomitant with continental lithospheric thinning during the Cenozoic.

The early Late Triassic Yumen mafic rocks in the eastern Tethyan Himalaya: A record of the progressive opening of the Yarlung Zangbo Neo-Tethyan Ocean

The timing of the opening of the Yarlung Zangbo Neo–Tethyan Ocean (YZNTO) is debated. In this contribution, geochemical and geochronological analyses of diabase, pillow basalt and layered basalt from the Upper Triassic Yumen tectonic mélange (T3Y) of the eastern Tethyan Himalaya in South Tibet are used to elucidate the timing and the magmatic evolution preceding the development of the YZNTO. Zircon U-Pb geochronology indicates that the Yumen mafic rocks crystallized in the Late Triassic (diabase, c. 225 Ma). These rocks display alkaline basalt affinities and have variable enrichment in Ba, U, Pb, Sr and Ti and depletion in Rb, Th and Nb. The diabase and pillow basalt samples have compositions transitional between enriched mid–ocean ridge basalt and ocean island basalt (OIB); layered basalt is more compatible with an OIB affinity. Whole–rock εNd (t) ratios of the mafic rocks range from +5.5 to +11.5 and zircons from a diabase have εHf(t) values varying from – 1.1 to +10.0. Whole–rock trace element concentrations indicate that the parental magmas of the Yumen mafic rocks were derived from the mantle at variable depths extending from the garnet to spinel stability fields. The Yumen mafic rocks originated from partial melting of a heterogeneous magma source composed of dominant depleted asthenospheric magmas and the addition of some components of an ancient and enriched lithospheric mantle; negligible crustal contamination occurred. Combined with previous studies, we conclude that the Yumen mafic rocks were probably produced in an intercontinental rift setting with extremely thin continental lithosphere in the early Late Triassic prior to the opening of the eastern YZNTO. The YZNTO progressively opened along the northern edge of the Indian plate from west to east in the Phanerozoic.

Lithospheric structure of the eastern Mediterranean Sea: Inferences from surface wave tomography and stochastic inversions constrained by wide-angle refraction measurements

The tectonic plate under the eastern Mediterranean Sea shows a remarkable variability as it comprises Earth's oldest oceanic lithosphere as well as the transition towards continental lithosphere beneath the Levant Basin. Its thickness and other properties offer essential information on the lithospheric evolution but have been difficult to determine seismically due to the high heterogeneity of the region and its complex crustal structure. Here, we combine a large, new surface wave dataset with published wide-angle data in order to determine lithospheric properties in the eastern Mediterranean. Our stochastic inversions of broad-band, phase-velocity dispersion measurements resolve the crust-mantle structural trade-offs and yield robust, 1-D shear-wave velocity models down to 300 km depth beneath the Ionian and Levant Basins. The thickness of the crust beneath the two locations is 16.4 ± 3 km and 22.3 ± 2 km, respectively. The Poisson's ratio (σ) of 0.32 and Vp/Vs of 1.93 in the crystalline crust confirm the presence of serpentinized oceanic crust beneath the Ionian Basin. Beneath the Levant Basin, low crustal Vp/Vs (∼ 1.7) and Poisson's (∼ 0.24) ratios indicate continental crust. Beneath the Ionian Basin, the lithosphere is about 180 km thick. By contrast, thin, 75 km thick lithosphere is found beneath the Levant Basin. S-velocity tomography based on surface wave data also shows thick, spatially variable oceanic mantle lithosphere beneath the eastern Mediterranean. Thickness of the oceanic lithosphere increases eastwards from the Triassic Ionian towards the Permo-Carboniferous lithosphere in the Central Eastern Mediterranean. These results demonstrate that oceanic lithosphere can thicken by cooling substantially beyond the limits suggested by the plate cooling model. Beneath the eastern Herodotus oceanic Basin, lithospheric thickness is decreasing to about 180 km. Thin continental lithosphere and shallow asthenosphere are present beneath the Dead Sea Fault, demonstrating that the localization of the lithospheric deformation and crustal seismicity along the fault correlates spatially with the thinning of the underlying continental lithosphere.

Origin, Accretion, and Reworking of Continents

AbstractThe continental crust is unique to the Earth in the solar system, and controversies remain regarding its origin, accretion and reworking of continents. The plate tectonics theory has been significantly challenged in explaining the origin of Archean (especially pre‐3.0 Ga) continents as they rarely preserve hallmarks of plate tectonics. In contrast, growing evidence emerges to support oceanic plateau models that better explain characteristics of Archean continents, including the bimodal volcanics and nearly coeval emplacement of tonalite‐trondjhemite‐granodiorite (TTG) rocks, presence of ∼1600°C komatiites and dominant dome structures, and lack of ultra‐high‐pressure rocks, paired metamorphic belts and ophiolites. On the other hand, the theory of plate tectonics has been successfully applied to interpret the accretion of continents along subduction zones since the late Archean (3.0–2.5 Ga). During subduction processes, the new mafic crust is generated at the base of continents through partial melting of mantle wedge with the addition of H2O‐dominant fluids from subducted oceanic slabs and partial melting of the juvenile mafic crust results in the generation of new felsic crusts. This eventually leads to the outgrowth of continents. Subduction processes also cause softening, thinning, and recycling of continental lithosphere due to the vigorous infiltration of volatile‐rich fluids and melts, especially along weak belts/layers, leading to widespread continental reworking and even craton destruction. Reworking of continents also occurs in continental interiors due to either plate boundary processes or plume‐lithosphere interactions. The effects of plumes have proven to be less significant and cause lower degrees of lithospheric modification than subduction‐induced craton destruction.

Understanding the 3D Formation of a Wide Rift: The Central South China Sea Rift System

AbstractRifted margins result from continental lithosphere extension, breakup, and transition to seafloor spreading by modes typically described by end‐member conceptual models. However, current geophysical data challenge these archetypes showing a more complex spectrum of rifting styles. Here, we integrate geophysical and geological observations to constrain the time and space evolution of the central South China Sea (SCS) rift system. We provide new insights into the continental extension mechanisms and continent‐ocean transition (COT) formation from a seismic transect parallel to the extensional direction of the central SCS rift system. We present 850‐km‐wide region of continental crust where distributed extensional deformation formed a pattern of lithospheric boudinage defined by seven 100‐ to 200‐km‐long structural segments. Each segment is formed by an 18‐ to 24‐km‐thick crust sector laterally thinning into an interboudin neck with ≤10‐km ultrathin crust (ß factors ~3.3–6.5). Drill and stratigraphic information support that extension continued at all six interboudin necks in the central SCS until ~23 Ma, when breakup by seafloor spreading propagation reached one of them. We propose that the seven segments evolved as discrete subsystems from early rifting to breakup. Structurally equivalent ultrathin crust grabens occur NE and SW of both SCS conjugate margins, supporting a 3D wide rift mode of deformation across the SCS rift system. The six ultrathin crust contemporaneous necks and the abrupt nature and location of the COT support that the breakup was not controlled by further continental lithospheric thinning but rather was determined by the seafloor spreading propagation toward the SW.

Measurements of the extension required for crustal breakup on the magma-poor Iberia-Newfoundland conjugate margins

Rifted margins have been extensively studied, however there are still many important questions related to their formation processes. We use the Iberia-Newfoundland conjugate rifted margins as a natural laboratory to investigate how much extension is required to produce crustal rupture and separation. To achieve our aim we use; (i) gravity anomaly inversion to measure continental crustal thinning, (ii) subsidence analysis to measure continental lithosphere thinning and (iii) fault heave summation from seismic observations to obtain brittle continental crust extension. Integration of thinning from gravity anomaly inversion and subsidence analysis is used to determine continental crust extension and continental lithosphere extension respectively. These measurements have been made between the proximal continental crust and the distal Limit of Contiguous Continental Crust (LCCC) on the northern (SCREECH1 and ISE01) and the southern (SCREECH2 and TGS/LG12) conjugate seismic profiles. For the Iberia-Newfoundland conjugate rifted margins, extension values determined from the integration of crustal and lithosphere thinning are similar and suggest that on average 172 km of extension is required to produce crustal breakup. In contrast, measured extension from fault heave summation is on average 128 km which indicates an apparent extension discrepancy at the scale of the whole conjugate margin system when observed brittle continental crust extension is compared with lithosphere and crust extension. Fault population analysis shows that sub-seismic resolution faulting significantly contributes to an under-estimate of brittle crustal extension by up to 25%. Allowing for fault extension at sub-seismic resolution indicates that an extension discrepancy at the scale of the whole conjugate margin system does not exist. Fault geometries and the location of the distalmost contiguous continental crust are also important issues that should be carefully considered when comparing fault heave summation and extension from crust and lithosphere thinning. The absence of an extension discrepancy between fault extension, crustal extension and lithosphere extension at the scale of a whole conjugate margin system should not be confused with the presence of depth-dependent stretching and thinning at a given location on the margin which is both expected and observed at rifted continental margins. • Continental breakup and formation of the conjugate Iberia-Newfoundland rifted margins. • Extension required for rupture and separation of continental crust during breakup. • Measure extension from continental crust and lithosphere thinning and fault heaves. • No overall extension discrepancy but local depth-dependent stretching and thinning.

Numerical Modeling of Weathering, Erosion, Sedimentation, and Uplift in a Triple Junction Divergent Margin

AbstractThe majority of numerical models of landscape evolution in divergent margins are focused on the simulation of margins with simplified lithological control on landscape erosion. However, this approach is insufficient to study the evolution of margins where chemical weathering is an important element increasing rock resistance to physical erosion. One example of this margin is the Borborema Province, northeastern Brazil, where postrift marine sediments are now preserved at elevations ∼700–800 m in up to 1‐km‐high plateaus capped by duricrust layers. The landscape evolution of these uplifted sedimentary basins still eludes explanation. Here we use numerical models that couple weathering, erosion, sedimentation, sea level changes, flexural isostasy, and thermal effects due to lithospheric stretching to simulate the tectonosedimentary evolution of the Borborema Province since the onset of continental stretching during the Lower Cretaceous. These numerical experiments reveal that nearly 70% of the postrift regional uplift observed in the Borborema Province can be explained by differential denudation of the continent and flexural rebound of the lithosphere. The remaining ∼250 m of uplift can be explained by thermal uplift induced by partial erosion of the base of the continental lithosphere under the Borborema Province due to edge‐driven convection, in accordance with the anomalously thin continental lithosphere observed under the Borborema Province. Additionally, the numerical results can explain the regional pattern of fission track ages by the combined effect of differential denudation and flexural rebound in this geometrically complex margin.

Geochemistry and geochronology of the Dongyang gold deposit in southeast China: Constraints on ore genesis

Mesozoic magmatism and lithospheric destruction in East China exerted significant control on gold metallogeny. Here, we investigate the mineralogy, geochemistry, geochronology, and isotopic systematics of the Dongyang gold deposit in southeast China to constrain its origin and geodynamics. This is a newly discovered epithermal gold deposit along the Circum–Pacific metallogenic belt and is characterized by intense silicification and sericitization. The C–H–O isotope data suggest deep magmatic sources of the ores and indicate that the ore‐forming fluids comprised a mixture of magmatic water and meteoric water. The S–Pb values of the pyrite indicate magmatic origin for the sulfur, and deposit was derived mostly from crustal sources, mixed with mantle materials. The Rb–Sr dating of the pyrite from the ore deposit shows an age of 164.2 ± 9.9 Ma, whereas the zircon U–Pb SHRIMP ages of the rhyolite porphyry is 160.9 ± 0.5 Ma, suggesting that the timing of mineralization is close to that of the crystallization of the dykes. In conjunction with previous studies on similar deposits, we suggest that the gold mineralization was the response of continental lithospheric thinning in southeast China triggered by the subduction of the Palaeo‐Pacific Ocean Plate.

Late Cenozoic lithosphere dynamics in Svalbard: Interplay of glaciation, seafloor spreading and mantle convection

In the last 10 Ma, the northwest Barents Sea passive margin and Svalbard archipelago experienced tectonic reactivation including regional uplift, volcanism and fault movements. Previous studies have variously attributed this reactivation to the establishment of continuous seafloor spreading between the North Atlantic and Arctic oceans within the Fram Strait, mantle convection, or a lithospheric response to climate-driven processes such as fluvial and glacial erosion in a late Pliocene-Pleistocene time (∼1-3 Ma). The relative magnitudes, and the physical relationship between surface processes, plate boundaries and mantle flow, has remained unclear. In this work, I jointy interpret seismological models of the upper mantle, gravity anomalies, rates of glacial isostatic adjustment (GIA) and the spatiotemporal characteristics of earthquakes with the aim to i) constrain the thermal state and rheological structure of the upper mantle beneath Svalbard, ii) infer the magnitude of vertical motion in the late Cenozoic time related to various processes, iii) better understand the likely causes of the tectonic and volcanic activity.Most part of the northwest Barents Sea passive margin including Svalbard is underlain by a thin continental lithosphere (∼50–80 km) and a normal thickness of crust (30–35 km). The geophysical data indicate an average excess mantle temperature of ∼40–100°C beneath western Svalbard compared to the central Barents Sea. The peak of the mantle temperature anomaly coincides with the location of Quaternary basalt eruptions. The predicted long-wavelength (>420 km) non-isostatic uplift of ∼600-800 m correlates with the distribution of pre-glacial uplift previously inferred from geological data in the northwest Barents Sea. The gravity anomalies within an intermediate range of wavelengths (30–420 km) allows me to estimate topography changes due to erosion by ice streams to be ∼200–500 m. I conclude that a transient temperature anomaly in the asthenosphere northeast of Jan Mayen, combined with the northwest absolute plate motion of Eurasia, is the likely primary cause of the late Cenozoic tectonic reactivation in Svalbard.

South China Sea crustal thickness and oceanic lithosphere distribution from satellite gravity inversion

Inversion of satellite-derived free-air gravity-anomaly data has been used to map crustal thickness and continental lithosphere thinning in the South China Sea. Using this, we determine the ocean–continent transition zone structure, the distal extent of continental crust, and the distribution of oceanic lithosphere and continental fragments in the South China Sea. We construct a set of regional crustal cross-sections, with Moho depth from gravity inversion, spanning the South China Sea from offshore China and Vietnam to offshore Malaysia, Brunei and the Philippines to examine variations in ocean–continent transition structure and ocean-basin width. Our analysis shows a highly asymmetrical conjugate margin structure. The Palawan margin shows a narrow transition from continental to oceanic crust. In contrast, the conjugate northern margin of the South China Sea shows a wide region of thinned continental crust and an isolated block of continental crust (the Macclesfield Bank) separated from the Chinese margin by a failed oceanic rift. The Dangerous Grounds are predicted to be underlain by fragmented blocks of thinned continental crust. We use maps of crustal thickness and continental lithosphere thinning from gravity inversion together with free-air gravity- and magnetic-anomaly data to identify structural trends and to show that rifting and the early seafloor-spreading axis had an ENE–WSW trend while the later seafloor-spreading axis had a NE–SW trend.

New 40Ar–39Ar dating of Lower Cretaceous basalts at the southern front of the Central High Atlas, Morocco: insights on late Mesozoic tectonics, sedimentation and magmatism

This study is based upon a stratigraphic and structural revision of a Middle Jurassic–Upper Cretaceous mostly continental succession exposed between Boumalne Dades and Tinghir (Southern Morocco), and aims at reconstructing the relation among sedimentary, tectonic and magmatic processes that affected a portion of the Central High Atlas domains. Basalts interbedded in the continental deposits have been sampled in the two studied sites for petrographic, geochemical and radiogenic isotope analyses. The results of this study provide: (1) a robust support to the local stratigraphic revision and to a regional lithostratigraphic correlation based on new 40Ar–39Ar ages (ca. 120 Ma) of the intervening basalts; (2) clues for reconstructing the relation between magma emplacement in a structural setting characterized by syn-depositional crustal shortening pre-dating the convergent tectonic inversion of the Atlasic rifted basins; (3) a new and intriguing scenario indicating that the Middle Jurassic–Lower Cretaceous basalts of the Central High Atlas could represent the first signal of the present-day Canary Islands mantle plume impinging, flattening, and delaminating the base of the Moroccan continental lithosphere since the Jurassic, and successively dragged passively by the Africa plate motion to NE. The tectono-sedimentary and magmatic events discussed in this paper are preliminarily extended from their local scale into a peculiar geodynamic setting of a continental plate margin flanked by the opening and spreading Central Atlantic and NW Tethys oceans. It is suggested that during the late Mesozoic this setting created an unprecedented condition of intraplate stress for concurrent crustal shortening, related mountain uplift, and thinning of continental lithosphere.

Petrogenesis of Cretaceous shoshonitic rocks in the northern Wuyi Mountains, South China: A result of the roll-back of a flat-slab?

Potassic magmatism is commonly linked to post-/late-orogenic environments, such as foundering or convection thinning of continental lithosphere. Their petrogenesis is crucial for constraining the chemical and physical properties of the remnant sub-continental lithospheric mantle. Here we report new SHRIMP zircon U–Pb ages, whole rock geochemical results and Sr–Nd and zircon Hf isotope data from four potassic plutons (the Da'an, Yingcheng, Zixi and Honggong plutons) in the northern Wuyi Mountains, South China. SHRIMP U–Pb zircon analyses indicate that these potassic rocks formed at 139–126Ma. They are characterized by high SiO2 (56–73%) and K2O (3.8–6.7%), with a K2O/Na2O ratio of 2.18–2.04, plotting within the field of high-SiO2 shoshonites. Their ISr and εNd(t) values vary from 0.7077 to 0.7162 and −5.66 to −10.52, respectively. The initial zircon εHf(t) values range from 2.3 to −13.1, corresponding to TDM modal ages between 707 and 1330Ma. These geochemical and isotope characteristics indicate that these shoshonites derived from a subduction-modified ancient subcontinental lithospheric mantle, and then underwent significantly fractional crystallization of K-feldspar, plagioclase, and accessory minerals, such as apatite and Fe–Ti oxides during magma ascent. We interpret that asthenospheric mantle upwelling (caused by eastward roll-back of a flat-slab?) triggered partial melting of the metasomatized lithospheric mantle to result in the Early Cretaceous shoshonitic magmatism in the northern Wuyi Mountains. An integration of our new results with compiled data from the interior of the South China Block reveals that the arc-like geochemical signature is confined to the Wuyi Mountains region, but becomes little or even invisible toward inland in South China. This implies that the far-field effects of the early Mesozoic subduction only reached the Wuyi Mountains, ca. 500km away from the trench, consistent with flat or shallow subduction models.

Petrology, geochemistry and zircon U–Pb geochronology of the Jurassic porphyry dykes in the Dehua gold field, Southeast China: Genesis and geodynamics

The widespread tectonic and magmatic events in Southeast China during the Mesozoic were accompanied by large‐scale metallogenesis. Here, we investigate the petrology, geochemistry, and zircon U–Pb age of the porphyry dykes in relation to the gold mineralization at Dongyang deposit in the Fujian Province. The geochemical data show silica‐rich and Mg‐ and Ca‐poor A‐type granite affinity for the dykes. The rocks have high concentrations of Light Rare Earth Elements (LREE) and Large Ion Lithophile Elements (LILE) and relatively low contents of Heavy Rare Earth Elements (HREE) and High Field Strength Element (HFSE). They display (La/Yb)N and (La/Sm)N values in the range of 5.7 to 31.4 and 6.0 to 8.3, respectively, with negative anomalies of Ba, Nb, Ta, Sr, Zr, Eu, and Ti and positive Ce anomaly. The source magma for the dykes was derived from a mixture of partially melted Al‐rich crustal material, with an input of mantle components. We present sensitive high‐resolution ion microprobe zircon U–Pb data from the dykes, which show emplacement ages in the range of 158 to 162 Ma, coinciding with the onset of the Yanshanian magmatism in Southeast China. Integrating the data obtained from studies on geology, geochronology and petro‐geochemistry, we suggest that the dykes played role in controlling the gold mineralization, and the magmatism and gold mineralization are the response of continental lithosphere thinning in Southeast China triggered by the subduction of the ancient Pacific Ocean Plate.

Modelado gravimétrico de una sección entre el margen continental del sudeste de Brasil y el área emergida adyacente

This study was conducted in a gravimetric section between the continental margin and the onshore area of southeastern Brazil, encompassing the northern portion of the Santos Basin and the Neoproterozoic Brasilia and Ribeira mobile belts. Free-Air gravimetric terrestrial and satellite data have been fixed and calculated for Free-Air anomaly and used as the basis for the 2D gravity modeling. Constraints imposed to the 2D gravity model were based on magnetotelluric model 2D inversion for the terrestrial part of the section and model of joint inversion and 2D seismic reflection migrated in time for the Santos Basin. Residual data were calculated from the Free-Air anomaly subtracted from the regional field and were used to associate the shallow part of the model to the geophysical models. In the area of the continental margin, crustal and lithospheric thinning was observed below shallow shelf and in the proximal part of the Santos Basin. Gravity data of the regional field were calculated from a polynomial fit to most Free-Air anomaly data were used to model and fix the lower crust and lithospheric mantle. The fit of the model to regional data was done only it was infered keels at the base of the lithosphere, interpreted as subducted paleoplates situated below suture zones in Brasilia and Ribeira belts. This model fits the observed gravity data with error less than 8% of the total gradient.

Jurassic rifting at the Eurasian Tethys margin: Geochemical and geochronological constraints from granitoids of North Makran, southeastern Iran

This study focuses on an east-west trending belt of granitic to intermediate intrusions and their volcanic cover in the northern Dur Kan Complex, a continental slice outcropping to the north of the exposed Makran accretionary wedge in southeastern Iran. Field observations, petrographic descriptions, trace element, and isotope analyses combined with U-Pb zircon geochronology are presented to determine the time frame of magmatism and tectonic setting during the formation of these rocks. Results document three magmatic episodes with different melt sources for (1) granites, (2) a diorite-trondhjemite-plagiogranite sequence, and (3) diabases and lavas. Granites, dated at 170–175 Ma, represent crystallized melt with a strong continental isotopic contribution. The diorite-trondhjemite-plagiogranite sequence is 165–153 Ma old and derives from a mantle magma source with minor continental contribution. East-west trending diabase dikes and bodies intruded the granitoids, which were eroded and then covered by Valanginian (140–133 Ma) alkaline lavas and sediments. Alkaline dikes and lavas have a mantle isotopic composition. Temporal correlation with plutonites of the Sanandaj-Sirjan Zone to the northwest defines a narrow, NW-SE striking and nearly 2000 km long belt of Jurassic intrusions. The increasing mantle influence in the magma sources is explained by thinning of continental lithosphere and related mantle upwelling/decompression melting. Accordingly, the formation of the studied igneous rocks is related to the extension of the Iranian continental margin, which ultimately led to the formation of the Tethys-related North Makran Ophiolites.

Hidden hotspot track beneath the eastern United States

Hotspot tracks are thought to be the surface expressions of tectonic plates moving over upwelling mantle plumes, and are characterized by volcanic activity that is age progressive. At present, most hotspot tracks are observed on oceanic or thin continental lithosphere. For old, thick continental lithosphere, such as the eastern United States, hotspot tracks are mainly inferred from sporadic diamondiferous kimberlites putatively sourced from the deep mantle. Here we use seismic waveforms initiated by the 2011 M_w 5.6 Virginia earthquake, recorded by the seismic observation network USArray, to analyse the structure of the continental lithosphere in the eastern United States. We identify an unexpected linear seismic anomaly in the lower lithosphere that has both a reduced P-wave velocity and high attenuation, and which we interpret as a hotspot track. The anomaly extends eastwards, from Missouri to Virginia, cross-cutting the New Madrid rift system, and then bends northwards. It has no clear relationship with the surface geology, but crosses a 75-million-year-old kimberlite in Kentucky. We use geodynamical modelling to show that an upwelling thermal mantle plume that interacts with the base of continental lithosphere can produce the observed seismic anomaly. We suggest that the hotspot track could be responsible for late Mesozoic reactivation of the New Madrid rift system and seismicity of the eastern United States.

Origin of basaltic magmas of Perşani volcanic field, Romania: A combined whole rock and mineral scale investigation

The Perşani volcanic field is a low-volume flux monogenetic volcanic field in the Carpathian–Pannonian region, eastern-central Europe. Volcanic activity occurred intermittently from 1200ka to 600ka, forming lava flow fields, scoria cones and maars. Selected basalts from the initial and younger active phases were investigated for major and trace element contents and mineral compositions. Bulk compositions are close to those of the primitive magmas; only 5–12% olivine and minor spinel fractionation occurred at 1300–1350°C, followed by clinopyroxenes at about 1250°C and 0.8–1.2GPa. Melt generation occurred in the depth range from 85–90km to 60km. The estimated mantle potential temperature, 1350–1420°C, is the lowest in the Pannonian Basin. It suggests that no thermal anomaly exists in the upper mantle beneath the Perşani area and that the mafic magmas were formed by decompression melting under relatively thin continental lithosphere. The mantle source of the magmas could be slightly heterogeneous, but is dominantly variously depleted MORB-source peridotite, as suggested by the olivine and spinel composition. Based on the Cr-numbers of the spinels, two coherent compositional groups (0.38–0.45 and 0.23–0.32, respectively) can be distinguished that correspond to the older and younger volcanic products. This indicates a change in the mantle source region during the volcanic activity as also inferred from the bulk rock major and trace element data. The younger basaltic magmas were generated by lower degree of melting, from a deeper and compositionally slightly different mantle source compared to the older ones. The mantle source character of the Perşani magmas is akin to that of many other alkaline basalt volcanic fields in the Mediterranean close to orogenic areas. The magma ascent rate is estimated based on compositional traverses across olivine xenocrysts using variations of Ca content. Two heating events are recognized; the first one lasted about 1.3years implying heating of the lower lithosphere by the uprising magma, whereas the second one lasted only 4–5days, which corresponds to the time of magma ascent through the continental crust. The alkaline mafic volcanism in the Perşani volcanic field could have occurred as a response to the formation of a narrow rupture in the lower lithosphere, possibly as a far-field effect of the dripping of dense continental lithospheric material beneath the Vrancea zone. Upper crustal extensional stress-field with reactivation of normal faults at the eastern margin of the Transylvanian basin could enhance the rapid ascent of the mafic magmas.

A detailed receiver function image of the lithosphere beneath the Kopeh-Dagh (Northeast Iran)

The Kopeh-Dagh fold belts are among the most seismically active areas in Northeastern Iran, which build the northern part of the Alpine–Himalayan orogen in western Asia. They act as the abrupt northeastern limit to active deformation in Iran. We perform a combined P and S receiver function analysis to detect the major discontinuities within the lithosphere beneath Northeast Iran. Our results obtained from 12 short period and broadband seismological stations significantly map the lateral variations of the Moho boundary. Based on P receiver functions, we show that the Moho depth varies from ∼43 km beneath the southern Kopeh-Dagh foreland basin to ∼49 km below the northern part of the basin. S receiver functions reliably reveal an average Moho depth of ∼50–55 km beneath the Kopeh-Dagh mountain range showing the regional shortening in response to the collision of Arabia with Eurasia. Furthermore, we observe clear conversions with negative polarity at ∼8.5–9.5 s in S receiver functions, which could be related to the conversion at the lithosphere–asthenosphere boundary. This may show a relatively thin continental lithosphere of about 85–95 km beneath the Kopeh-Dagh implying that the lithosphere was influenced by geodynamical reworking processes in the past.

The formation of a failed continental breakup basin: The Cenozoic development of the Faroe‐Shetland Basin

AbstractUltra‐large rift basins, which may represent palaeo‐propagating rift tips ahead of continental rupture, provide an opportunity to study the processes that cause continental lithosphere thinning and rupture at an intermediate stage. One such rift basin is the Faroe‐Shetland Basin (FSB) on the north‐east Atlantic margin. To determine the mode and timing of thinning of the FSB, we have quantified apparent upper crustal β‐factors (stretching factors) from fault heaves and apparent whole‐lithosphere β‐factors by flexural backstripping and decompaction. These observations are compared with models of rift basin formation to determine the mode and timing of thinning of the FSB. We find that the Late Jurassic to Late Palaeocene (pre‐Atlantic) history of the FSB can be explained by a Jurassic to Cretaceous depth‐uniform lithosphere thinning event with a β‐factor of ~1.3 followed by a Late Palaeocene transient regional uplift of 450–550 m. However, post‐Palaeocene subsidence in the FSB of more than 1.9 km indicates that a Palaeocene rift with a β‐factor of more than 1.4 occurred, but there is only minor Palaeocene or post‐Palaeocene faulting (upper crustal β‐factors of less than 1.1). The subsidence is too localized within the FSB to be caused by a regional mantle anomaly. To resolve the β‐factor discrepancy, we propose that the lithospheric mantle and lower crust experienced a greater degree of thinning than the upper crust. Syn‐breakup volcanism within the FSB suggests that depth‐dependent thinning was synchronous with continental breakup at the adjacent Faroes and Møre margins. We suggest that depth‐dependent continental lithospheric thinning can result from small‐scale convection that thins the lithosphere along multiple offset axes prior to continental rupture, leaving a failed breakup basin once seafloor spreading begins. This study provides insight into the structure and formation of a generic global class of ultra‐large rift basins formed by failed continental breakup.

Mapping crustal thickness and oceanic lithosphere distribution in the Eastern Mediterranean using gravity inversion

Oceanic and continental lithosphere distribution within the eastern Mediterranean is not well understood. A gravity inversion, incorporating a lithosphere thermal gravity anomaly correction, has been used to map Moho depth, crustal thickness and lithosphere thinning for the eastern Mediterranean, from which the distribution of oceanic and continental lithosphere, the structure of the ocean–continent transition (OCT) and the location of the continent–ocean boundary (COB) can be determined. The gravity inversion results show thin crust and high continental lithosphere thinning under the Ionian Sea and the Herodotus Basin, consistent with these basins being underlain by oceanic crust. Moho depths from gravity inversion are in agreement with seismic refraction estimates in these basins. Highly thinned continental crust is predicted under the offshore Sirte and Levant basins. The sharp decrease in crustal thickness predicted by gravity inversion off the Libyan and Egyptian coast gives an indication of COB location. Crustal thickness and continental lithosphere thinning determined from gravity inversion have also been used to explore the relationship between the Cretaceous West and Central African Rift System (WCARS: Benue Trough, Chad, Central African Shear Zone (CASZ) and Sudan basins) and the eastern Mediterranean basins; continuity between the Cretaceous WCARS and the eastern Mediterranean basins is not apparent in the gravity inversion results.