Crustal structure of the Newfoundland rifted continental margin from constrained 3-D gravity inversion

The rifting history of the Atlantic continental margin of Newfoundland is very complex and so far has been investigated at the crustal scale primarily with the use of 2-D seismic surveys. While informative, the results generated from these surveys cannot easily be interpreted in a regional sense due to their sparse sampling of the margin. A 3-D gravity inversion of the free air data over the Newfoundland margin allows us to generate a 3-D density anomaly model that can be compared with the seismic results and used to gain insight into regions lacking seismic coverage. Results of the gravity inversion show good correspondence with Moho depths from seismic results. A shallowing of the Moho to 12 km depth is resolved on the shelf at the northern edge of the Grand Banks, in a region poorly sampled by other methods. Comparisons between sediment thickness and crustal thickness show deviations from local isostatic compensation in locations which correlate with faults and rifting trends. Such insights must act as constraints for future palaeoreconstructions of North Atlantic rifting.     

Thinning of the Goban Spur continental margin and formation of early oceanic crust: constraints from forward modelling and inversion of marine magnetic anomalies

The deep seismic reflection profile Western Approaches Margin (WAM) cuts across the Goban Spur continental margin, located southwest of Ireland- This non-volcanic margin is characterized by a few tilted blocks parallel to the margin. A volcanic sill has been emplaced on the westernmost tilted block. The shape of the eastern part of this sill is known from seismic data, but neither seismic nor gravity data allow a precise determination of the extent and shape of the volcanic body at depth. Forward modelling and inversion of magnetic data constrain the shape of this volcanic sill and the location of the ocean-continent transition. The volcanic body thickens towards the ocean, and seems to be in direct contact with the oceanic crust. In the contact zone, the volcanic body and the oceanic magnetic layer display approximately the same thickness. The oceanic magnetic layer is anomalously thick immediately west of the volcanic body, and gradually thins to reach more typical values 40 km further to the west. The volcanic sill would therefore represent the very first formation of oceanic crust, just before or at the continental break-up. The ocean-continent transition is limited to a zone 15 km wide. The continental magnetic layer seems to thin gradually oceanwards, as does the continental crust, but no simple relation is observed between their respective thinnings.     

Gravity constraints on the dynamics of the crust–mantle system during Calabrian subduction

The thermomechanic evolution of the lithosphere–upper mantle system during Calabrian subduction is analysed using a 2-D finite element approach, in which the lithosphere is compositionally stratified into crust and mantle. Gravity and topography predictions are cross-checked with observed gravity and topography patterns of the Calabrian region. Modelling results indicate that the gravity pattern in the arc-trench region is shaped by the sinking of light material, belonging to both the overriding and subduction plates. The sinking of light crustal material, up to depths of the order of 100–150 km is the ultimate responsible for the peculiar gravity signature of subduction, characterized by a minimum of gravity anomaly located at the trench, bounded by two highs located on the overriding and subducting plates, with a variation in magnitude of the order of 200 mGal along a wavelength of 200 km, in agreement with the isostatically compensated component of gravity anomaly observed along a transect crossing the Calabrian Arc, from the Tyrrhenian to the Ionian Seas. The striking agreement between the geodetic retrieved profiles and the modelled ones in the trench region confirms the crucial role of compositional stratification of the lithosphere in the subduction process and the correctness of the kinematic hypotheses considered in our modelling, that the present-day configuration of crust–mantle system below the Calabrian arc results from trench's retreat at a rate of about 3 cm yr⁻¹, followed by gravitational sinking of the subducted slab in the last 5 Myr.     

New Late Jurassic palaeomagnetic data from the northern Sichuan basin: implications for the deformation of the Yangtze craton

Upper Jurassic red sandstones and red siltstones were collected from 67 layers at 12 localities in the Penglaizhen formation. This formation is in the north of Bazhong county (31.8°N, 106.7°E) in the Sichuan basin, which is located in the northern part of the Yangtze craton. Thermal demagnetization isolated a high-temperature magnetic component with a maximum unblocking temperature of about 690 °C from 45 layers. The primary nature of the magnetization acquisition is ascertained through the presence of magnetostratigraphic sequences with normal and reversed polarities, as well as positive fold and reversal tests at the 95 per cent confidence level. The tilt-corrected mean direction of 36 layers is D = 20.0°, I = 28.8° with α ₉₅ = 5.8°. A Late Jurassic palaeomagentic pole at 64.7°N, 236.0°E with A ₉₅ = 7.0° is calculated from the palaeomagnetic directions of 11 localities. This pole position agrees with the two other Late Jurassic poles from the northern part of the Yangtze craton. A characteristic Late Jurassic pole is calculated from the three poles (68.6°N, 236.0°E with A ₉₅ = 8.0°) for the northern part of the Yangtze craton. This pole position is significantly different from that for the southern part of the Yangtze craton. This suggests that the southern part of the Yangtze craton was subjected to southward extrusion by 1700 ± 1000  km with respect to the northern part. Intracraton deformation occurred within the Yangtze craton.     

The seismic structure of thinned continental crust in the northern Bay of Biscay

Summary. The stretching and thinning of the continental crust, which occurs during the formation of passive continental margins, may cause important changes in the velocity structure of such crust. Further, crust attenuated to a few kilometres' thickness, can be found underlying 'oceanic' water depths. This paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness. A single seismic refraction line shot over thinned continental crust as part of the North Biscay margin transect in 1979 was studied in detail. Tau— p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure. This was confirmed when synthetic seismograms were calculated. The thinned continental crust (β± 7.0) exhibits a two-gradient structure in the non-sedimentary crust with velocities between 5.9 and 7.4 km s⁻¹; an upper 0.8 s⁻¹ layer overlies a 0.4 s⁻¹ layer. No layer comparable to oceanic layer 3 was detected. The uppermost mantle also contains a low-velocity zone.     

Geomorphic constraints on the active tectonics of southern Taiwan

Taiwan is a region of rapid active tectonics, yet the study of the tectonic processes that shape the interior of the island is difficult due to the high rates of erosion and dense vegetation. We use digital topography to look for indications of active deformation preserved in the local geomorphology. In particular, anomalies in the regional pattern of drainage are used to infer zones of enhanced tectonic activity. The apparent anticlockwise rotation of major river systems in plan view indicates the presence of a diffuse zone of left-lateral shear running down the southeastern side of Taiwan. Asymmetries in the catchments of individual drainage basins show the influence of varying rates of uplift across southern Taiwan, with the most rapid uplift close to Taitung at the indentation point of the Luzon arc with the Chinese continental margin. Our interpretations, though based predominantly on remote-sensing observations, are consistent with the available field evidence. This study demonstrates the usefulness of drainage basins as tectonic markers in the quantification of regional strain and uplift, which may have wider applicability in other deforming parts of the world.     

Insights into the lithospheric structure and tectonic setting of the Barents Sea region from isostatic considerations

We study the tectonic setting and lithospheric structure of the greater Barents Sea region by investigating its isostatic state and its gravity field. 3-D forward density modelling utilizing available information from seismic data and boreholes shows an apparent shift between the level of observed and modelled gravity anomalies. This difference cannot be solely explained by changes in crustal density. Furthermore, isostatic calculations show that the present crustal thickness of 35–37 km in the Eastern Barents Sea is greater than required to isostatically balance the deep basins of the area (>19 km). To isostatically compensate the missing masses from the thick crust and deep basins and to adequately explain the gravity field, high-density material (3300–3350 kg m⁻³) in the lithospheric mantle below the Eastern Barents Sea is needed. The distribution of mantle densities shows a regional division between the Western and Eastern Barents and Kara Seas. In addition, a band of high-densities is observed in the lower crust along the transition zone from the Eastern to Western Barents Sea. The distribution of high-density material in the crust and mantle suggests a connection to the Neoproterozoic Timanide orogen and argues against the presence of a Caledonian suture in the Eastern Barents Sea. Furthermore, the results indicate that the basins of the Western Barents Sea are mainly affected by rifting, while the Eastern Barents Sea basins are located on a stable continental platform.