The oceanic crust in 3D: Paleomagnetic reconstruction in the Troodos ophiolite gabbro

The Troodos complex, Cyprus, provides an opportunity to study the structural configuration along a fossil intersection of a spreading axis and a transform fault. We complement studies at Troodos that have reconstructed the brittle deformation of the upper crust by new paleomagnetic data from the gabbro suite. The gabbro suite is exposed at the extinct spreading axis continuing the Solea graben toward the intersection with the fossil Arakapas oceanic transform. This is a unique exposure of deep crustal rocks formed at both an inside-corner and an outside-corner of a ridge-transform intersection. Remanence directions from gabbros (23 sites) were used as indicators for rigid body rotation. The spatial distribution of rotation axes allow recognition of three regions to which deformation is partitioned: 1) a western region (outside corner) that experienced primarily tilt about horizontal axis 2) a central region with minor rotation and, 3) an eastern area (inside corner) where vertical axis rotations are dominant. The absence of significant rotation in the 6 km-wide central domain together with its location between the inside- and the outside corner uncover the root of a fossil axial volcanic zone, a zone sufficiently hot so the upper crust can decouple from the substrate. Clockwise rotation in the gabbro increases from the axial zone eastward, similar to that in the overlying dikes, indicating coupling of the lower crust with the brittle upper oceanic crust. The transition from the decoupled layers of sheeted dikes and gabbro in the axial zone to the dikes-gabbro coupling in the inside corner is in keeping with deepening of the brittle-ductile transition from the dike-gabbro boundary into the lower crust away from the axial zone. Our conclusions are consistent with one of the previous reconstructions in which the Solea spreading axis was orthogonal to the Arakapas trunsform fault, and with recent studies of the present-day lower oceanic crust. However, the newly inferred surface trace of the Solea spreading axis is further to the east, probably reflecting the tilt of axial upper crust rotated blocks. (c) 2006 Elsevier B.V. All rights reserved

Direct evidence for dynamic magma supply fossilized in the lower oceanic crust of the Troodos ophiolite

International audienceTemporal and spatial variabilities of mantle upwelling and melt supply in mid-ocean ridges (MORs) have long been documented. Such variabilities span a range of scales and have a profound effect on the structure as well as the composition of the oceanic crust. Previous seismic and gravity studies have suggested that the lower oceanic crust plays a major role in accommodating these changes in melt supply. Here we report the first direct evidence for a sharp transition from coherent sub-horizontal to near vertical magma flows frozen in the lower oceanic crust of the Troodos ophiolite at the segment edge near a fossil ridge-transform intersection. We constrain the preferred petrofabric lineation directions at 13 gabbroic sites using anisotropy of magnetic susceptibility (AMS) verified by electron backscatter diffraction. Pre-emplacement accretion-related rotations were corrected using magnetic remanence directions. We identify two provinces of nearly uniform susceptibility directions (principal axes) and attribute them to two magmatic episodes. A more focused mantle upwelling and melting episode near the segment midpoint may have resulted in lower crustal lateral magma flows along the fossil segment-edge, whereas uniform mantle upwelling and melt supply along the entire axis may have resulted in vertical magma flows at the segment-edge. Overall, our data verify the vital role of the lower oceanic crust in accommodating changes in mantle upwelling and melt supply beneath MORs

SAR Interferometry for Sinkhole Early Warning and Susceptibility Assessment along the Dead Sea, Israel

During the past three decades, the Dead Sea (DS) water level has dropped at an average rate of ~1 m/year, resulting in the formation of thousands of sinkholes along its coastline that severely affect the economy and infrastructure of the region. The sinkholes are associated with gradual land subsidence, preceding their collapse by periods ranging from a few days to about five years. We present the results of over six years of systematic high temporal and spatial resolution interferometric synthetic aperture radar (InSAR) observations, incorporated with and refined by detailed Light Detection and Ranging (LiDAR) measurements. The combined data enable the utilization of interferometric pairs with a wide range of spatial baselines to detect minute precursory subsidence before the catastrophic collapse of the sinkholes and to map zones susceptible to future sinkhole formation. We present here four case studies that illustrate the timelines and effectiveness of our methodology as well as its limitations and complementary methodologies used for sinkhole monitoring and hazard assessment. Today, InSAR-derived subsidence maps have become fundamental for sinkhole early warning and mitigation along the DS coast in Israel and are incorporated in all sinkhole potential maps which are mandatory for the planning and licensing of new infrastructure