Amount Of Crustal Thinning Research Articles

The structure, evolution and symmetry of the magma-poor rifted margins of the North and Central Atlantic: A synthesis

Magma-poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting, the serpentinization of the mantle beneath crust thinned to less than 8 ± 2 km, and the unroofing of a broad zone of mantle within the continent–ocean transition, accompanied by the development of detachment and other large-offset faults. These observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes. Although the paucity of magmatism may be explained by depth-dependent extension of the lithosphere, and by pre-depleted sub-crustal lithosphere, rifting above initially cool sub-lithospheric mantle also may explain the subsidence deficit observed at some margins: synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere. As it extends, thins and cools, ductile creeping layers in the mid- and deep crust become progressively more brittle, resulting in increased coupling between the upper and lower crust, and eventually the embrittlement of the entire crust, faults cutting from the surface across the Moho, bringing water into the mantle and causing its serpentinization. Increased coupling and the development of serpentine detachments predict the development of late-stage asymmetry once the entire crust is brittle. Such detachments are imaged on some margins and inferred on others; analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric. Similar analyses show that depth-dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning. Instead this “extension discrepancy” may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting, related to the changes in the rheological character and strength of the lithosphere as it is thinned. Complex polyphase faulting continues after complete crustal separation, resulting in the exhumation of broad expanses of peridotitic basement, the top of which is everywhere marked by an exhumed slip surface, similar to the corrugated surface observed at mid-ocean ridges. The similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the COT and true oceanic crust difficult and possibly moot.

Gravity anomalies and segmentation at the East Coast, USA continental margin

SUMMARY The free-air gravity ‘edge effect’ anomaly at rifted continental margins has generally been attributed to the transition between thick continental and thin oceanic crust. While crustal thinning is a major contributor, recent studies suggest that sediment loading and magmatism may significantly modify the edge effect anomaly and cause it, at some margins, to be highly segmented along their strike. In this paper, we use a combined 3-D flexural backstripping and gravity anomaly modelling technique to determine the role that sediment loading has played in controlling the segmentation of Atlantic-type continental margins. We focus on the East Coast, USA since a substantial amount of high-quality seismic reflection and refraction, gravity, and magnetic data already exists for this margin. By calculating the gravity anomaly associated with rifting and sediment loading and iteratively comparing it to the observed free-air anomaly, we have determined the ‘best-fit’ elastic thickness, Te, structure of the margin. We show that 0 10 5 a) integrated strength of the lithosphere, these results imply that weak regions abut strong ones at the East Coast, USA margin. Te generally decreases with increase in the amounts of crustal thinning, β, and the flexed basement curvature, K, suggesting it is controlled, at least in part, by the mechanical structure of the pre-rift lithosphere and yielding due to flexural loading. However, there is considerable scatter, suggesting other factors such as along-strike changes in crustal composition. Irrespective, we show that an isostatic anomaly that takes into account the ‘best-fit’ Te distribution is significantly reduced in spectral power compared with one which is computed assuming only Airy compensation (i.e. Te = 0 km). This is not to imply that rifting and sediment loading completely accounts for the anomalies along-strike and across-strike the East Coast, USA margin. To the contrary, significant isostatic anomaly highs and lows persist, especially in inner and middle shelf regions. One of the most prominent is an arcuate, 670 km long, high with flanking lows offshore Carolina that we attribute to magmatism during the initial stages of continental break-up.

Crustal structure of the northern Main Ethiopian Rift from receiver function studies

Abstract The northern Main Ethiopian Rift captures the crustal response to the transition from continental rifting in the East African rift to the south, to incipient seafloor spreading in the Afar depression to the north. The region has also undergone plume-related uplift and flood basalt volcanism. Receiver functions from the EAGLE broadband network have been used to determine crustal thickness and average V p / V s for the northern Main Ethiopian Rift and its flanking plateaus. On the flanks of the rift, the crust on the Somalian plate to the east is 38 to 40 km thick. On the western plateau, there is thicker crust to the NW (41–43 km) than to the SW (<40 km); the thinning taking place over an off-rift upper mantle low-velocity structure previously imaged by travel-time tomography. The crust is slightly more mafic ( V p / V s ∼ 1.85) on the western plateau on the Nubian Plate than on the Somalian Plate ( V p / V s ∼ 1.80). This could either be due to magmatic activity or different pre-rift crustal compositions. The Quaternary Butajira and Bishoftu volcanic chains, on the side of the rift, are characterized by thinned crust and a V p / V s > 2.0, indicative of partial melt within the crust. Within the rift, the V p / V s ratio increases to greater than 2.0 (Poisson’s ratio, σ > 0.33) northwards towards the Afar depression. Such high values are indicative of partial melt in the crust and corroborate other geophysical evidence for increased magmatic activity as continental rifting evolves to oceanic spreading in Afar. Along the axis of the rift, crustal thickness varies from around 38 km in the south to 30 km in the north, with most of the change in Moho depth occurring just south of the Boset magmatic segment where the rift changes orientation. Segmentation of crustal structure both between the continental and transitional part of the rift and on the western plateau may be controlled by previous structural inheritances. Both the amount of crustal thinning and the mafic composition of the crust as shown by the observed V p / V s ratio suggest that the magma-assisted rifting hypothesis is an appropriate model for this transitional rift.

Ridge offsets, normal faulting, and gravity anomalies of slow spreading ridges

We develop a model relating mid‐ocean ridge normal faulting and crustal structure by examining recently available high‐resolution gravity and multibeam bathymetric data of the Mid‐Atlantic Ridge and other spreading centers. Results of the analysis reveal a consistent pattern of positive residual gravity anomalies along the crust paralleling all major Atlantic offsets studied, especially along the inside corner side of the offsets. Individual residual gravity anomaly spikes (local peaks), which have amplitudes of up to 20 mGal and typical across‐axis spacing of 10–30 km, often coincide with individual major fault scarps, suggesting crust thinned by normal faulting. Theoretical calculations indicate that the amplitude and spacing of the observed residual gravity spikes are consistent with the presence of successive, ridge‐parallel low‐angle faults that originate episodically at inside corners of ridge offset intersections. Fault scarp heights and the amounts of crustal thinning (as inferred from gravity anomalies) are consistently larger at inside corners than at outside corners, supporting a model in which tectonic extension near ridge offsets is asymmetric with low‐angle faults occurring preferentially at inside corners. These results on spatial variations in seafloor morphology and gravity anomalies further support a three‐dimensional tectonic faulting model at oceanic spreading centers with three major characteristics: (1) Low‐angle faults form preferentially at inside corners, where the mantle lithosphere is the strongest and the lithospheric plates are sufficiently decoupled across ridge axis offsets; (2) low‐angle faults decrease in throw toward midpoints of long ridge segments, where large low‐angle faults may not be sustained by a weak lithosphere; and (3) the residual gravity peaks are statistically larger at transform than near nontransform offsets, indicating that the length of a ridge offset and its tectonic style control the development of low‐angle faults.

Magmatism and the geodynamics of basin formation

The magmatic response to extension of the continental lithosphere, with the resultant formation of sedimentary basins, varies widely. Quantitative models (e.g. McKenzie and Bickle, 1988), based upon an assumption of anhydrous partial melting in both lithosphere and asthenosphere, suggest that the temperature of the sub-lithospheric upper mantle, upwelling beneath the zone of extension, exerts a fundamental control on both the chemistry and volume of the erupted magmas. High volcanicity rifts are, in general, only likely to occur when the asthenospheric potential temperature is ca. 100–300°C greater than normal, associated with the activity of mantle plumes. In addition, such models also predict a strong correlation between the volume of magma produced and the amount of lithospheric stretching (β). More complex models (e.g. Gallagher and Hawkesworth, 1992) include partial melting of volatile-rich mantle source regions with lower than normal solidus temperatures. These may reside within mantle plumes or within enriched domains within the continental lithospheric mantle. Quantitative modelling of the magma generation process in the latter case is limited by our rather poor understanding of volatile-present partial melting of the range of potential source components. Geochemical and isotopic (Sr—Nd—Pb—O) studies of the igneous rocks emplaced within a basin are of fundamental importance in evaluating the relative roles of lithospheric versus asthenospheric source components in the petrogenesis of the magmas and the depth and degree of partial melting. In addition, these data may provide important constraints on the amount of crustal contamination which has occurred, particularly by partial melting of the lower crust. Knowledge of the latter is important in the development of models to explain crustal thinning. In many basins the bulk of the mantle-derived magmas may never actually reach the surface, but are underplating and intruding the lower crust instead. Identification of such a mafic underplate is critical if the amount of crustal thinning is to be evaluated correctly. The location of eruptive sites within individual basins is frequently determined by pre-existing basement structures. Within the extensive Mesozoic rift system of West and Central Africa, Pan-African lithospheric shear zones and deep basement faults have acted as foci for rift basin development, with the faults acting as magma pathways to the surface. In western and central Europe, Neogene—Recent extension has generated an extremely varied magmatic response which can be correlated with the Hercynian structural grain of the lithosphere. The Rhine graben, trending at a high angle to the Hercynian terrane boundaries is largely amagmatic. In contrast the Ohre rift of Czechoslovakia, which trends parallel to a major (Saxothuringian—Moldanubian) terrane boundary, is a classic example of a pure shear rift with major volcanic complexes located along the axial zone.

Subsidence history, crustal structure, and thermal evolution of the Valencia Trough: A young extensional basin in the western Mediterranean

The Valencia Trough is an extensional basin that began rifting during the late Oligoceneearly Miocene, yet it is located within a region of compression between the European and African plates. Seismic reflection, seismic refraction, well, gravity, and geoid data have been used to determine the relative importance of extensional and compressional processes in controlling the structure and evolution of the trough. Although the trough appears symmetric on bathymetric maps, seismic and gravity and geoid data indicate a fundamental asymmetry in its deep crustal and upper mantle structure. Two dimensional flexural backstripping techniques have been used to estimate the total amount of crustal stretching along a “transect” of the trough between the Catalan Coastal Ranges and Mallorca. There is good agreement between the predicted depth to the Mono based on a stretching model and the seismically constrained depth beneath the Catalan margin, but poor agreement beneath Mallorca. The seismic Moho beneath Mallorca can be explained by a model in which the crust was modified by the flexural effects of thrust/fold loading. Geoid data suggest that the differences in the crustal structure between the two margins extend into the upper mantle. The Catalan margin is characterized by a thermal and mechanical boundary, which marks the transition between cold, unstretched lithosphere and hot, thin lithosphere, while Mallorca is located above a broad region of lithospheric thinning. The subsidence history and amount of crustal thinning in the Catalan margin resemble that of typical Atlantic‐type nonvolcanic margin. However, a narrow transition zone (<60 km), the lack of any evidence for a “conjugate” margin, and the absence of oceanic crust suggest fundamental differences in the way that extensional processes may occur in convergent plate settings.

Present Thermal Conditions in Baltimore Canyon and Georges Bank Basins in Relation to Extensional Models: ABSTRACT

We examine the thermal evolution of the Baltimore Canyon and Georges Bank basins to derive their present temperature structure. Our models are based on the hypothesis that these basins have formed over continental crust which was thinned by extension during the opening of the Atlantic Ocean. The cooling and subsidence history are simulated using two-dimensional finite difference techniques. The results of this work demonstrate that the extensional model is consistent with seismic, gravity, and heat-flow observations in the two basins. Since the model reliably predicts the observed features of the basins, we believe that its predictions for the temperature structure of the basins are also reliable. The thermal gradient, heat flow, and temperature at depth in the two basins are expected to vary laterally because of variations in the amount of crustal thinning, sediment distribution, and basement depth. The Baltimore Canyon basin is characterized by one large sediment depocenter, while the Georges Bank basin has several smaller depocenters. This difference is character may be attributed to the oblique, rather than normal, orientation of rifting in the Georges Bank area. The modeling technique used predicts what effects these types of features will have on the present temperatures and temperature history of these parts of the continental margin. End_of_Article - Last_Page 1363------------

Structure of the crust and upper mantle at Goban Spur, Southwest of the British Isles — some implications for margin studies

Goban Spur is a marginal plateau on a sediment-starved passive continental margin. Recent detailed geophysical investigations on and around the spur reveal four interesting aspects of passive margin structure. 1. (1) A well-exposed continent-ocean boundary, whose shallow structure may be very similar to that of the former passive margins of the Tethys ocean. The ease with which the continent—ocean boundary can be detected geophysically greatly depends on the extent of rifting-phase igneous activity however. A ridge-generated isostatic gravity anomaly, as discussed by Rabinowitz and La Brecque (1977), appears to mark the boundary in this area, but the composition of the ridge and its lateral extent are not established. 2. (2) Transverse structures (faults) that generate small offsets in the continent—ocean boundary and extented from continental into oceanic crust. These may be widespread features of margin basements. 3. (3) A zone of very thin continental crust above a mantle that is normal or of density only very slightly less than normal. Areas of very thin continental crust must be able to originate by mechanisms that do not require the anomalous, low-density upper mantle discussed by Van der Linden (1977) in his “continental spreading” hypothesis. Other thinning mechanisms (with the amount of crustal thinning as a percentage of the original crustal thickness) are crustal stretching (c. 10% per 10° rotation of fault blocks), thermal expansion and supracrustal erosion (up to ca. 15%), lateral creep of continental crust (unlimited?) and upward migration of a basalt-eclogite phase change at the Moho (c. 1% per 1 km sediment blanket). 4. (4) A possible change in upper mantle density across the margin from about 3.40 g/cm 3 beneath continental areas to about 3.36 g/cm 3 beneath oceanic areas may satisfy the regional gravity data. A global change in upper mantle density across passive margins may be expected from physical and chemical considerations. However, it is doubtful that such a change would be easily detected and a change may not always be present because of local variations in upper mantle properties.