Rezene Mahatsente

Crustal structure of the Main Ethiopian Rift from gravity data: 3-dimensional modeling

Abstract A three-dimensional interpretation of the newly compiled Bouguer anomaly map of the Main Ethiopian Rift is presented. A high-resolution 3-D model constrained with the seismic results reveals a possible crustal thickness and density distribution beneath the graben. The Bouguer anomalies along the axial portion of the rift floor, as deduced from the results of the regional and residual separation, are mainly caused by the deep-seated structures. The inferred zone of intrusion, which is the main subject of the present study, coincides with the maximum gravity anomaly of the rift floor. The intrusion is displaced at several sectors along the east–west direction, and the two major displacements coincide with the locations of the major rift offsets on the surface. Because of the asthenospheric uplift, the crust under the Main Ethiopian Rift is slightly thinned. The zone of crustal thinning (≤31 km) coincides with the location of the intrusion beneath the rift floor, and the maximum of which is attained in the northern and central sectors of the graben. The trend of the crustal thinning zone, which is from south to north, is the same as the one obtained in the Afar depression. The southeastern and western plateaus, on the other hand, show by far the largest crustal thickness in the region (38–51 km). In contrast to the Afar depression, where the crust is partly oceanized, the thickness and density of the crust suggest that the Main Ethiopian Rift is underlain by a purely continental crust. The deep and relatively large nature of the intrusion leads to the conclusion that a large-scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of the graben.

Structure and State of Stress of the Chilean Subduction Zone from Terrestrial and Satellite-Derived Gravity and Gravity Gradient Data

AbstractnIt is well known that the quality of gravity modelling of the Earth’s lithosphere is heavily dependent on the limited number of available terrestrial gravity data. More recently, however, interest has grown within the geoscientific community to utilise the homogeneously measured satellite gravity and gravity gradient data for lithospheric scale modelling. Here, we present an interdisciplinary approach to determine the state of stress and rate of deformation in the Central Andean subduction system. We employed gravity data from terrestrial, satellite-based and combined sources using multiple methods to constrain stress, strain and gravitational potential energy (GPE). Well-constrained 3D density models, which were partly optimised using the combined regional gravity model IMOSAGA01C (Hosse et al. in Surv Geophys, 2014, this issue), were used as bases for the computation of stress anomalies on the top of the subducting oceanic Nazca plate and GPE relative to the base of the lithosphere. The geometries and physical parameters of the 3D density models were used for the computation of stresses and uplift rates in the dynamic modelling. The stress distributions, as derived from the static and dynamic modelling, reveal distinct positive anomalies of up to 80xa0MPa along the coastal Jurassic batholith belt. The anomalies correlate well with major seismicity in the shallow parts of the subduction system. Moreover, the pattern of stress distributions in the Andean convergent zone varies both along the north–south and west–east directions, suggesting that the continental fore-arc is highly segmented. Estimates of GPE show that the high Central Andes might be in a state of horizontal deviatoric tension. Models of gravity gradients from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite mission were used to compute Bouguer-like gradient anomalies at 8xa0km above sea level. The analysis suggests that data from GOCE add significant value to the interpretation of lithospheric structures, given that the appropriate topographic correction is applied.

Continental subduction and exhumation: an example from the Ulten Unit, Tonale Nappe, Eastern Austroalpine

Abstract Some exhumed complexes in collisional belts consist of continental basement containing slivers of mafic and ultramafic material showing evidence of UHP metamorphism (P c. 3 GPa). Their PTt history can be interpreted in terms of subduction of continental material to depths ≥ 100 km and subsequent exhumation. This type of tectonic history is illustrated by the Late Palaeozoic evolution of the Ulten Unit, Tonale Nappe, Eastern Austroalpine. The upper crustal felsic component (c. 80% by volume) incorporated mafic material at the trench, and peridotitic material at deeper levels in the subduction zone. The peridotites show evidence of a P-increasing, T-decreasing path before incorporation in the felsic material, compatible with flow in the mantle wedge above the subducting slab. After emplacement of the peridotites, which occurred at or near peak metamorphic conditions (P ≥ 2.7 GPa, T ≥ 850 °C), the complex underwent a two-stage pre-Alpine exhumation path: a first, fast stage (c. 0.1–1 cm a−1), lasting c. 30 Ma and bringing rocks from depths ≥ 100 km to approximately 25 km; and a second, slow stage (c. 0.01–0.1 cm a−1), lasting c. 100 Ma and bringing rocks to depths <20 km. The subduction of felsic material to the required depths can be modelled by analysing the time-evolution of negative buoyancy, which confirms that relatively light continental upper crust can be subducted to depths > 200 km if attached to a mature oceanic slab that does not break-off during the early stages of continental subduction. The first exhumation stage can be accounted for by buoyancy-driven tectonic extrusion of continental slices along the subduction channel during continuing subduction. A force balance analysis shows that such a mechanism is compatible with the rheology of felsic and intermediate rocks at high temperature. The second exhumation stage is compatible with isostatic rebound and tectonic denudation following slab break-off. The conclusion that fast exhumation occurs during continuing subduction and before slab break-off is in accordance with the observed rates, which show fast movement of the rising slices with respect to the surrounding material. Slab break-off, on the other hand, generates a long-wavelength gentle upwarping of the overlying region, which is more compatible with later and slower exhumation rates.

Ridge-push force and the state of stress in the Nubia-Somalia plate system

We assessed the relative contribution of ridge-push forces to the stress state of the Nubia-Somalia plate system by comparing ridge-push forces with lithospheric strength in the oceanic part of the plate, based on estimates from plate cooling and rheological models. The ridge-push forces were derived from the thermal state of the oceanic lithosphere, seafloor depth, and crustal age data. The results of the comparison show that the magnitude of the ridge-push forces is less than the integrated strength of the oceanic part of the plate. This implies that the oceanic part of the plate is very little deformed; thus, the ridge-push forces may be compensated by significant strain rates outside the oceanic parts of the plate. We used an elastic finite element analysis of geoid gradients of the upper mantle to evaluate stresses associated with the gravitational potential energy of the surrounding ridges and show that these stresses may be transmitted through the oceanic part of the plate, with little modulation in magnitude, before reaching the continental regions. We therefore conclude that the present-day stress fields in continental Africa can be viewed as the product of the gravitational potential energy of the ridge ensemble surrounding the plate in conjunction with lateral variations in lithospheric structure within the continent regions.

Lithospheric structure and the isostatic state of Eastern Anatolia: Insight from gravity data modelling

Eastern Anatolia, Turkey, is a part of the Alpine-Himalayan collisional belt where continental crust is relatively thin for a collisional belt. The region contains part of the Zagros suture zone, which formed during collision of the Arabian and Anatolian plates in the Miocene. It is underlain by a low-velocity zone associated with asthenospheric flow in the uppermost mantle. We constructed gravity models of the crust and upper-mantle structures to assess the driving mechanism of asthenospheric flow and the isostatic state of Eastern Anatolia. Our density models are based on terrestrial and satellite-derived gravity data, and they are constrained by receiver function and seismic tomography. The gravity models show significant lithospheric thickness variations across the Anatolian and Arabian plates. The lithospheric mantle in Eastern Anatolia is thinner (~62–74 km) than the Arabian plate (~84–95 km), indicating that part of the Anatolian mantle lithosphere might have been removed by delamination. The lithospheric removal process might have occurred following the detachment of the Arabian slab in the Miocene. Widespread Holocene volcanism and high heat flow in Eastern Anatolia can be considered as evidence of lithospheric delamination and slab break-off. The upward asthenospheric flow and subsequent asthenospheric underplating beneath Eastern Anatolia might have been induced by both delamination and slab break-off. These two processes may account for the rapid uplift of the Anatolian Plateau. There is a residual topography of ~1.7 km that cannot be explained by crustal roots. Based on our gravity models, we suggest that part of the eastern Anatolian Plateau is dynamically supported by asthenospheric flow in the upper mantle. LITHOSPHERE; v. 10; no. 2; p. 279–290; GSA Data Repository Item 2018111 | Published online 22 February 2018 https://doi.org/10.1130/L685.1

Global Models of Ridge-Push Force, Geoid, and Lithospheric Strength of Oceanic plates

An understanding of the transmission of ridge-push related stresses in the interior of oceanic plates is important because ridge-push force is one of the principal forces driving plate motion. Here, I assess the transmission of ridge-push related stresses in oceanic plates by comparing the magnitude of the ridge-push force to the integrated strength of oceanic plates. The strength is determined based on plate cooling and rheological models. The strength analysis includes low-temperature plasticity (LTP) in the upper mantle and assumes a range of possible tectonic conditions and rheology in the plates. The ridge-push force has been derived from the thermal state of oceanic lithosphere, seafloor depth and crustal age data. The results of modeling show that the transmission of ridge-push related stresses in oceanic plates mainly depends on rheology and predominant tectonic conditions. If a lithosphere has dry rheology, the estimated strength is higher than the ridge-push force at all ages for compressional tectonics and at old ages (>75xa0Ma) for extension. Therefore, under such conditions, oceanic plates may not respond to ridge-push force by intraplate deformation. Instead, the plates may transmit the ridge-push related stress in their interior. For a wet rheology, however, the strength of young lithosphere (<75xa0Ma) is much less than the ridge-push force for both compressional and extensional tectonics. In this case, the ridge-push related stress may dissipate in the interior of oceanic plates and diffuses by intraplate deformation. The state of stress within a plate depends on the balance of far-field and intraplate forces.