Structure and dynamics of the Ecuador Fracture Zone, Panama Basin

In this study, multiple geophysical data types are used to investigate the structure and dynamics of the Ecuador Fracture Zone – a complex multi-stranded strike-slip fault system located in the Panama Basin. Gravity modelling reveals a 25-30 km-wide region of ∼3 km-thick, low-density crust beneath this system, and an anomalously low-density region in the uppermost mantle. Along both edges, the transition to the ‘normal’ structure and thickness oceanic crust formed at both the Ecuador and Costa Rica Rifts is abrupt. Within the Ecuador Fracture Zone itself, normal faults bound the median ridges. These faults traverse the entire thickness of accumulated sediment and offset the seabed, while sediment layer geometries document multiple phases of relative uplift, with the most recent phase still ongoing. Active extensional faulting, with an approximately spreading ridge-parallel strike, is also observed in 6-7 Ma Costa Rica Rift crust. The median ridges and the transverse ridge at the eastern edge of the Ecuador Fracture Zone also have contrasting crustal density structures. Both median ridges have a lower density crust than between the intervening valleys, while the transverse ridge crust has an equivalent thickness and density structure to that formed at the Costa Rica Rift. The active median valley basement-cutting normal faults allow seawater ingress and alternation of the crustal footwall, and also flow to mantle depth where, based on gravity modelling, 30-50% serpentinization of mantle peridotite occurs. The resulting serpentinite-driven buoyancy acts as the primary control on the observed median ridge relative vertical tectonism. In contrast, the relative uplift of the transverse ridge results from lithospheric flexure in response to a change in spreading direction between the Ecuador and Costa Rica Rifts. Contrary to the widely accepted assumption that fracture zones are tectonically inactive systems, the Ecuador Fracture Zone provides evidence of extension, serpentinization due to ongoing hydrothermal circulation, and relative uplift

Research and Outreach at the Mid-Atlantic Ridge

Roger Searle, Christine Peirce and Angela Bentley look at research on oceanic core complexes and detachments at the Mid-Atlantic Ridge, combined with innovative geophysics outreach

Crossover Behavior in Burst Avalanches of Fiber Bundles: Signature of Imminent Failure

Bundles of many fibers, with statistically distributed thresholds for breakdown of individual fibers and where the load carried by a bursting fiber is equally distributed among the surviving members, are considered. During the breakdown process, avalanches consisting of simultaneous rupture of several fibers occur, with a distribution D(Delta) of the magnitude Delta of such avalanches. We show that there is, for certain threshold distributions, a crossover behavior of D(Delta) between two power laws D(Delta) proportional to Delta^(-xi), with xi=3/2 or xi=5/2. The latter is known to be the generic behavior, and we give the condition for which the D(Delta) proportional to Delta^(-3/2) behavior is seen. This crossover is a signal of imminent catastrophic failure in the fiber bundle. We find the same crossover behavior in the fuse model.Comment: 4 pages, 4 figure

Energy bursts in fiber bundle models of composite materials

As a model of composite materials, a bundle of many fibers with stochastically distributed breaking thresholds for the individual fibers is considered. The bundle is loaded until complete failure to capture the failure scenario of composite materials under external load. The fibers are assumed to share the load equally, and to obey Hookean elasticity right up to the breaking point. We determine the distribution of bursts in which an amount of energy $E$ is released. The energy distribution follows asymptotically a universal power law $E^{-5/2}$, for any statistical distribution of fiber strengths. A similar power law dependence is found in some experimental acoustic emission studies of loaded composite materials.Comment: 5 pages, 4 fig

Crustal structure of the French Guiana margin, West Equatorial Atlantic

Geophysical data from the Amazon Cone Experiment are used to determine the structure and evolution of the French Guiana and Northeast Brazil continental margin, and to better understand the origin and development of along-margin segmentation. A 427-km-long combined multichannel reflection and wide-angle refraction seismic profile acquired across the southern French Guiana margin is interpreted, where plate reconstructions suggest a rift-type setting. The resulting model shows a crustal structure in which 35–37-km-thick pre-rift continental crust is thinned by a factor of 6.4 over a distance of ∼70 km associated with continental break-up and the initiation and establishment of seafloor spreading. The ocean–continent boundary is a transition zone up to 45 km in width, in which the two-layered oceanic-type crustal structure develops. Although relatively thin at 3.5–5.0 km, such thin oceanic crust appears characteristic of the margin as a whole. There is no evidence of rift-related magmatism, either as seaward-dipping sequences in the reflection data or as a high velocity region in the lower crust in the P-wave velocity model, and as a such the margin is identified as non-volcanic in type. However, there is also no evidence of the rotated fault block and graben structures characteristic of rifted margins. Consequently, the thin oceanic crust, the rapidity of continental crustal thinning and the absence of characteristic rift-related structures leads to the conclusion that the southern French Guiana margin has instead developed in an oblique rift setting, in which transform motion also played a significant role in the evolution of the resulting crustal structure and along-margin segmentation in structural style

Three-dimensional S-wave velocity structure of oceanic core complexes at 13N on the Mid-Atlantic Ridge

13°N on the Mid-Atlantic Ridge is regarded as a type site for oceanic core complexes (OCCs). Within ~70 km along the spreading centre, it hosts four OCCs in different stages of their life cycle making this an ideal location to determine how OCCs are formed, and what drives the hydrothermal circulation that sustains the vent fields associated with them. Here we describe the results of S-wave seismic tomographic modelling within a 60 x 60 km footprint containing several OCCs, the spreading centre and both flanks. A grid of 17 wide-angle seismic refraction profiles was shot within this footprint and recorded by a network of 46 ocean-bottom seismographs (OBS). Approximately 6200 S-wave arrival travel times have been modelled, constraining primarily the velocity-depth structure of the upper-to-mid-crust. Depth slices through the resulting 3-D S-wave velocity (Vs) model reveal the OCCs located at 13°20’N and 13°30’N to each have a region of relatively low Vs (3 km s-1 ) in the inter-OCC basin and regions surrounding the detachments. Using the equivalent 3-D P-wave velocity (Vp) model of Simão et al. (2020), the corresponding Vp/Vs model is calculated to investigate lithology, permeability and the existence of any off-axis magmatic intrusions that may drive fluid flow. The Vp/Vs model clearly shows that the crust beneath the deep lava-floored inter-OCC basin is characteristically oceanic (Vp/Vs ratio of 1.85, suggesting that they formed under magma poor (tectonic) conditions. The Vp/Vs model also shows that the OCCs are not connected, at least to mid-crustal level. Alternatively, if the OCCs lie on the same detachment surface, that surface would have to undulate >3 km in amplitude over a distance of <20 km for these OCCs to appear to be unconnected. Our 3-D Swave and Vp/Vs models thus support MacLeod et al.’s (2009) model of localized OCC evolution. Our S-wave velocity model also suggests that the Irinovskoe (13°20’N) and Semyenov (13°30’N) vent fields have different hydrothermal circulation drivers, with the Semyenov field being driven by magma intrusion(s) and the Irinovskoe field being driven by the spreading centre thermal gradient and pervasive flow along open permeability within the detachment footwall, perhaps further opened by roll-over to lower dip angle as it exhumes at the seabed