In the frame of the ANR program GAARanti, aiming to track regional emersion surfaces and potential timing of land emersion or drowning, we conducted new field studies in islands belonging to the Anguilla Bank (Saint Barthelemy, Saint Martin, Tintamarre and Anguilla), the northernmost bank located in the forearc of the Lesser Antilles subduction zone and south of the Anegada passage. Our new micropaleontological, sedimentological and paleoenvironmental investigations allow a highly accurate calibration of the Neogene successions using large benthic foraminifers, planktonic foraminifers and calcareous nannofossils. We determine three main regional unconformities cropping out on the islands of the Anguilla bank.
Interpretations of seismic lines acquired during the AntiTheSis (2016) and GARANTI (2017) cruises allow to reconstruct the tectono-sedimentary evolution of the offshore forearc basins, between South Saba Bank and Sombrero Basin. We use our well constrained onshore dataset and petroleum industry wells drilled in the Saba Bank to propose a constrained sismostratigraphy of offshore basins and onshore-offshore correlations.
At regional scale, we identify three main unconformities dated: (1) Late Eocene, (2) Mid Miocene (ca 15 Ma) and (3) Zanclean (4 Ma).
The Late Eocene unconformity is related to compressional tectonics and led to the emergence of most of our study area. Then subsidence occurred and topographic depressions were infilled by Oligocene to Early Miocene deposits, partly controlled by extensional fault activity along NW-SE and ENE-WSW faults systems bounding the Kalinago Basin and Anguilla Bank, respectively. The Mid Miocene unconformity is related to tectonics with increasing importance from south to north, thus probably related to the onset of the opening of Anegada Passage. This unconformity is related to emergence and erosion on the Anguilla and Saba Banks. The northern Antilles then subsided and most of basins reveal passive infilling during Middle Miocene to Pliocene. The Zanclean unconformity is related to localized uplifts that led to the final emergence of Anguilla, Tintamarre and St Martin carbonate platforms.
Abstract In this study, we draw on a unique combination of well‐resolved fault‐slip data and earthquake focal mechanisms to constrain spatial variations in style of faulting in the obliquely extending Main Ethiopian Rift, East Africa. These data show that both boundary and internal faults – oblique and orthogonal to the plate divergence (PD) respectively – exhibit almost pure dip‐slip motion, and indicate significant local deflection in orientation of the extension direction at rift margins. Scaled analogue models closely replicate the multidisciplinary observations from the rift and suggest that the process is controlled by the presence of a deep‐seated, pre‐existing weakness – oblique to the direction of PD – that is able to cause a local rotation in the orientation of the extension direction at rift margins. Minor counterclockwise block rotations are required to accommodate the difference in slip direction along the different fault systems, as supported by existing and new palaeomagnetic data from the rift.
Trench-parallel strike slip faults develop at lithospheric scale during oblique high-angle subduction. A “sliver” plate forms due to slip partitioning between the subduction plane (margin-normal slip) and the strike slip fault (margin-parallel slip). This process ultimately controls the location of volcanoes and earthquakes. The Great Sumatran Fault (GSF) is a showcase of this tectonic configuration located in the Sumatran section of the Sunda arc-trench system. Kinematics of the large-scale structures of the Sumatra section of the Sunda trench are well understood and tensional and compressional domains have been identified at the regional scale. However, detailed understanding of the stress distribution is still lacking yet essential for evaluating the seismic hazard potential in order to mitigate the impact of the large, hazardous earthquakes associated with this system.In this contribution, we study the present-day stress orientations of the Great Sumatran Fault at its northern section (NGSF). We deduced the state of (paleo)stress along structural features observed at two scales; (a) at meso-scale, analyzing ASTER GDEM data, and (b) at outcrop-scale, with field data measurements. We focus on the leading edge of northwestward propagating continental sliver deformation exposed on land, i.e. the northernmost tip of Sumatra (between 4,5°N and 6°N), where the NGSF bifurcates into its two major branches. These two fault branches form two structural highs bounding a graben basin in the onshore, continuing into the Pulau Weh Island in the east, and the Pulau Aceh Archipelago in the west. Given their location at the present day deformation front, these islands provide a unique opportunity to compare the sub-recent stress field with present day stresses, contributing to the understanding of the stress field evolution during northwestwards propagation of the Sumatran forearc continental sliver.
Since the acceptance of plate tectonics, the presence of calc-alkaline magmatic rocks has been recognized as evidence of subduction. But under specific geodynamic circumstances, subduction may occur without generating magmas. Here, we investigate the Cenozoic northern Lesser Antilles arc where, from sparsely exposed magmatic records, Eocene−Oligocene and Pliocene magmatic flare-ups and a Miocene lull were postulated. Nevertheless, most of the arc is submarine, so it is challenging to discern lulls and flare-ups from sampling bias. We review the magmatic evidence exposed onshore in the Lesser Antilles and investigate in detail the island of Antigua, which exposes an Eocene to Miocene volcanic sequence and platform carbonate series that coincide with the postulated lull. By combining lithostratigraphic analysis, structural mapping, 40Ar/39Ar geochronology, and biostratigraphy, we refine the magmatic history of the island and date the arrest of extensive arc magmatism at 35 Ma, with minor activity until 27 Ma. No magmatic products are interleaved with the platform sequence until the latest Oligocene, which confirms a lull in northern Lesser Antilles arc magmatism that may have lasted ca. 20 Ma. Flare-up of magmatic activity contributed to crustal thickening and land emersion, whereas magmatic lulls and related thermal cooling induced subsidence/submersion. Thus, we propose that the paleo-(bio)-geographical evolution of the eastern Caribbean region has been partly controlled by magmatic activity. Fault kinematic analysis, along with anisotropy of magnetic susceptibility, suggest that, at the island scale, magmatic arrest is not associated with a change in stress field during the Oligocene. We speculate that slab flattening triggered by progressive curvature played a role in the temporal shutdown of the northern Lesser Antilles arc.
Upper plates in subduction zones are prone to record slab dynamics as their strain pattern, uplift-subsidence records and volcanic arc activity accommodates variations of slab parameters in terms of dip, density and rheology. The ANR GAARAnti aims at tracking the timing of land emersions and submersions along the Lesser Antilles subduction zone, which is key to understand the long-term mechanical behavior of this subduction zone. In particular the strain history of the northern Lesser Antilles realm, that makes the junction with the Greater Antilles, needs to be better constrain in order to elaborate paleogeographic models. In this study we combined onshore (structural and geological mapping, PMag sampling, absolute dating and biostratigraphy) and offshore investigations (seismic profiling from the 2017 GARANTI Cruise), we evidence an unprecedently described episode of Mid-Eocene shortening, south of the Anegada Trough. Moreover, we present new paleomagnetic data from the island of St. Barthelemy, indicating a Post Oligocene ~25 counterclockwise rotation that we interpret as an accommodation of trench curvature. After a restoration of the Cayman Trough to the Mid Eocene, the regional compressive structures are interpreted to be the eastward propagation of the compressional domain that accommodated the N-S shortening triggered by the collision of the Bahamas Bank. A crustal-scale cross section drawn from the forearc to the backarc across the thrusts allows us to discuss the origin of crustal thickening, magmatism and tectonics, in the study area.
