The proposed ages for the collision of the Carnegie Ridge with the South America trench, offshore Ecuador, range from 1 to 15 Ma. In this time frame, many geological features of Ecuador are commonly linked to the subduction of the Carnegie Ridge. (1) After the ridge collided with the trench at ca. 15 Ma, the subsequent interplate coupling produced high exhumation rates of volcanic materials at ca. 9 Ma. (2) The oblique convergence of the Carnegie Ridge would have caused the northward drift of the North Andean block and the opening of the Gulf of Guayaquil. (3) During the late Miocene, the subduction of the Carnegie Ridge would have triggered a regional tectonic inversion along the forearc. (4) Along the collision front of the ridge with the trench, subduction-related erosion is occurring, and the Ecuadorian continental margin is being uplifted in the present day. (5) The chemistry of the active volcanic arc is explained as resulting from the arrival of the Carnegie Ridge into the trench. For instance, the adakitic signal, which appears at 1.5 Ma, is thought to be ridge-induced. (6) The buoyancy of the subducted Carnegie Ridge would explain the flatness of the slab beneath Ecuador. In this paper, we review the geological evolution of the Northern Andes in order to establish which of these geological events may be related to the subduction of the Carnegie Ridge. This review suggests that there is no clear deformation linked with the subduction of the Carnegie Ridge or with its landward prolongation postulated at depth.

At 12.5 Ma, after subduction below the North American plate stops, right-lateral transform motion occurs along the margin between the Pacific and North American plates. The Tosco-Abreojos fault zone, located along the western margin of southern Baja California, has been interpreted as the main transform boundary between both plates until early Pliocene, when the plate boundary was transferred to the Gulf of California, leading to the capture of Baja California Peninsula by the Pacific plate. However, the morphology and the seismic activity of the Tosco-Abreojos fault zone suggest this right-lateral strike-slip motion is still active. The Tosco-Abreojos fault zone is characterized by bathymetric scarps and asymmetric basins filled by recent sediments which are deformed. These observations are compatible with the hypothesis that the motion of the Pacific plate with respect to the North American plate is partitioned, as indicated by kinematic data (GPS versus global models) between the still active Tosco-Abreojos fault zone and the Gulf of California where most of the motion is accommodated. The Baja California Peninsula can thus be considered as an independent block limited to the west by the Tosco-Abreojos and San Benito fault zones and to the east by the Gulf of California transform boundary.

Abstract We review previous models for the Paleogene tectonic evolution of the Arabian and Eastern Somali basins and present a model based on a new compilation of magnetic and gravity data. Using plate reconstructions, we derive a self-consistent set of isochrons for Chron 27 to Chron 21 (61-46 Ma). The new isochrons account for the development of successive ridge propagation events along the Carlsberg Ridge, leading to an important spreading asymmetry between the conjugate basins. Our model predicts the growth of the outer and inner pseudo-faults associated with the ridge propagation events. The location of outer pseudo-faults appears to remain very stable despite a drastic change in the direction of ridge propagation before Chron 24 (c. 54 Ma). The motion of the Indian plate relative to the Somalian plate is stable in direction through Paleogene time; spreading velocities decrease from 6 to 3 cm a -1 . Our reconstructions also confirm that the Arabia-India plate boundary was located west of the Owen Ridge along the Oman margin during Paleogene time; some compression is predicted at about Chron 21 (47 Ma) between the Indian and Arabian plates.

Abstract We present a revised magnetic isochron map of the conjugate Arabian and Eastern Somali basins based on an up-to-date compilation of Indian, French, and other available sea-surface magnetic data. We have used the magnetic anomaly and the modulus of the analytical signal computed from the magnetic anomaly to identify and precisely locate the young and old edges of magnetic chrons in both basins. In addition to the major, well-defined anomalies, we have also used correlatable second-order features of the magnetic anomalies, the ‘tiny wiggles’, to strengthen the interpretation. The resulting isochrons and tectonic elements have been validated using the stochastic method of palaeogeographical reconstruction. The magnetic anomaly pattern in both basins depicts clear oblique offsets, characteristics of pseudofaults associated with propagating ridge segments. Our tectonic interpretation of the area revealed: (1) a complex pattern of ridge propagation between Chrons 28n (c. 63 Ma) and 25n (c. 56 Ma), with dominant eastward propagation between Chrons 26n (c. 58 Ma) and 25n; (2) numerous, systematic westward propagations between Chrons 24n (c. 53 Ma) and 20n (c. 43 Ma); (3) asymmetric crustal accretion (caused by ridge propagation and asymmetric sea-floor spreading) in the conjugate basins during the whole period; (4) a slowing of India-Somalia motion after c. 52 Ma.

Abstract We present a new tectonic fabric map of the Southern Ocean south of 45°S derived from Geosat altimeter profiles and published bathymetric charts and magnetic anomaly picks. The interpretation of the Geosat data is based on an analysis of the first derivative of the geoid profiles (i.e., vertical deflection profiles). To improve the accuracy and resolution of the vertical deflection profiles, 22 repeat cycles from the first year of the Geosat/Exact Repeat Mission (Geosat/ERM) were averaged. At wavelengths less than about 200 km, the vertical deflection is highly correlated with sea-floor topography and thus reveals major features in areas that were previously unsurveyed. The density of the Geosat data is greatest in the high latitudes where lineated bathymetric features such as fracture zones, spreading ridges, trenches, and rifted margins stand out. To construct the tectonic fabric chart, the Geosat data are analyzed in combination with available shipboard bathymetric data and magnetic anomaly identifications.