Fracture zone subduction and reactivation across the Puysegur ridge/trench system, Southern New Zealand

A two-dimensional kinematic model for lithospheric thrusting that considers the flexural interction between the underriding and overriding plates was used to assess the mechanical implications of subducting trench-parallel fracture zones on the topography and free-air gravity anomalies of the Puysegur ridge/trench system. Fracture zones in the underriding plate are simulated by vertical discontinuities across which bending and shearing stresses cannot be supported. The implication of reactivating fracture zones on the outer trench slope is to suppresss the flexural bulge, creating topographic relief between the fracture zone and the trench, and reducing the trench gravity anomaly. In the studied region the trench subparallel l'Atalante fracture zone accounts for the lack of a flexural bulge, strong outer trench slope, and gravity gradients. The southward decrease of the trench fracture zone distance results in a decrease of the trench gravity anomaly. The effect of subducting and vertically reactivating fracture zones within the underriding plate is to segment the load of the overriding plate, creating a trough located above the underthrust fracture zone. The vertical offset and gravity modifications are functions of the fracture zone position relative to the trench. This model explains the 3500-m-deep troughs and the 1700-m subsidence observed at the Puysegure ridge. We speculate that reactiviating underthrust fracture zones can facilitate strike slip faults development withn the upper plate in case of oblique subduction and are conducive to the formation and dispersion of terranes at convergent margins. This process can account for basal tectonic erosion. (Résumé d'auteur

Preliminary constraints on paleotemperature and landscape evolution in and around Araripe Basin, northeastern Brazil, using apatite fission track analysis

The Araripe Basin, located in the Borborema Province, northeastern Brazil, developed through a sequence of distinct events related to the opening of the South Atlantic. Its geological history is recorded in the preserved stratigraphy, which ranges in age from Paleozoic to Middle Cretaceous. The most conspicuous feature associated with the Araripe Basin is the Araripe Plateau, a large mesa capped by CenomanianAlbian non-marine sandstones (Exu Formation). At the plateau base, gray to black shales ("Batateiras layers") represent deposition in a large lake system covering part of northeastern Brazil during Early Aptian. Previous researchers have used the preserved geological section in the basin to infer the landscape evolution of the Araripe Plateau, and they have suggested that its uplift and exhumation were caused by widespread epeirogenic movements associated with post-break-up events in the Borborema Province. Independent constraints on the timing and magnitude of uplift in the Araripe Plateau area can now be introduced by this preliminary study using apatite fission track analysis. A series of samples along a N-S transect of the Araripe Basin and its neighboring basement provide clear evidence of heating in the past. Results from all samples also show evidence of Late Cenozoic cooling commencing sometime in the last 40 Ma. Maximum paleotemperatures associated with this event are very similar (70° - 90°C) between samples, consistent with little differential movement across the sampled region. Fission track results from the Aptian sandstone sample (Rio da Batateira Formation) indicate an earlier event (pre-40 Ma). However, interpretation of data suggests that any paleo-heating between sediment deposition (112 Ma) and 40 Ma must be associated with paleotemperatures less than ∼100°C, pointing to minor paleoburial during the Late Cretaceous in this area. The timing of these various events are broadly consistent with the timing of events identified in and around the Borborema Plateau using the same method (Morais Neto et al. submitted), which identified two cooling episodes: a Late Cretaceous cooling event commencing sometime between 100-90 Ma and a Late Cenozoic cool ing commencing between 30-0 Ma. This coincidence in the timing of events suggests that these events may be regional in nature and therefore likely to be related to regional exhumation. The uplift of the Araripe Basin can be related to epeirogenic movements associated with (or following) the Late Cretaceous cooling event. Thermal maturity presently observed in the "Batateiras layers" suggests that any source rock in the area ceased its thermal evolution due to such process. Present relief is interpreted as the result of subsequent erosion processes. If we assume a paleogeothermal gradient of 30°C/km (and a surface temperature of 25°C), we estimate an erosion of ∼1.5 km across the Araripe area, associated with the Late Cenozoic cooling event. While important parameters on the timing and magnitude of events in this area have been resolved based on only 6 samples, further work involving additional analysis and U-Th/He thermochronology is suggested

Structural and stratigraphic evolution of the Iberia-Newfoundland hyper-extended rifted margin: a quantitative modelling approach. In : Gibson, G. M., Roure, F. & Manatschal, G. (eds). Sedimentary Basins and Crustal Processes at Continental Margins: From Modern Hyper-extended Margins to Deformed Ancient Analogues.

International audienc

Sedimentary Regimes at the Macquarie Ridge Complex: Interaction of Southern Ocean Circulation and Plate Boundary Bathymetry

Seafloor structure at the Macquarie Ridge Complex strongly influences the intensity and circulation pattern of ocean currents south of New Zealand. New marine geophysical data show heterogeneous sedimentary environments on Macquarie seafloor that reflect interaction of highly variable bathymetry with the Antarctic Circumpolar Current and north flowing Antarctic Bottom Water. Acoustic backscatter, bathymetry, and seismic reflection data collected aboard R/V Rig Seismic in 1994 show five bathymetrically constrained sedimentary provinces flanking the ridge complex: (1) northwest Macquarie hemipelagic drifts, (2) current-modified Solander submarine fan complex, (3) southwest Macquarie manganese nodule province, (4) Emerald Basin pelagic drift province, and (5) sediment-free oceanic crust related to the 53.5°S passage in the Macquarie Ridge Complex. The late Miocene-Pliocene opening of a 53.5°S passage in the ridge complex caused a major increase in the intensity of ocean current circulation, sediment reworking, and erosion in all sedimentary provinces

Long-lived mega fault-scarps and related breccias at distal rifted margins: Insights from present-day and fossil analogues

International audienceThinning of the continental crust during rifting is accommodated by a number of major faults, only a few of which produce long-lived mega fault-scarps. In this paper, we investigate mega fault-scarps and the sedimentary system located at their toe across magma-poor rifted margins. Our approach combines observations from subsurface examples along present-day margins and field analysis of fossil examples exposed in the Alpine Tethys margins. While present-day examples of rifted margins imaged by seismic techniques provide details about the architecture of the top basement and the relationship between faults and sediments, outcrops give access to the sedimentary features of rocks related to mega fault-scarps. Our study shows that: (i) mega fault-scarps are preferentially located at rift domain boundaries, implying a topographic escarpment that juxtaposes rift domains of different crustal thickness, and (ii) mega fault-scarps are long-lived local sources for syn- and post-tectonic breccia, respectively produced during and after fault activity

Direct observation of Mg²⁺ complexes in ionic liquid solutions by ³¹Mg β-NMR spectroscopy

NMR spectra of Mg2+ ions in ionic liquids were recorded using a highly sensitive variant of NMR spectroscopy known as β-NMR. The β-NMR spectra of MgCl2 in EMIM-Ac and EMIM-DCA compare favourably with conventional NMR, and exhibit linewidths of ∼3 ppm, allowing for discrimination of species with oxygen and nitrogen coordination