Seismic imaging of deep low-velocity zone beneath the Dead Sea basin and transform fault : implications for strain localization and crustal rigidity

Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 33 (2006): L24314, doi:10.1029/2006GL027890.New seismic observations from the Dead Sea basin (DSB), a large pull-apart basin along the Dead Sea transform (DST) plate boundary, show a low velocity zone extending to a depth of 18 km under the basin. The lower crust and Moho are not perturbed. These observations are incompatible with the current view of mid-crustal strength at low temperatures and with support of the basin's negative load by a rigid elastic plate. Strain softening in the middle crust is invoked to explain the isostatic compensation and the rapid subsidence of the basin during the Pleistocene. Whether the deformation is influenced by the presence of fluids and by a long history of seismic activity on the DST, and what the exact softening mechanism is, remain open questions. The uplift surrounding the DST also appears to be an upper crustal phenomenon but its relationship to a mid-crustal strength minimum is less clear. The shear deformation associated with the transform plate boundary motion appears, on the other hand, to cut throughout the entire crust.Funded by USAID Middle Eastern Regional Cooperation Program grant M21-012, with matching funds by the participating institutions

Magnetic character of a large continental transform : an aeromagnetic survey of the Dead Sea Fault

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 8 (2007): Q07005, doi:10.1029/2007GC001582.New high-resolution airborne magnetic (HRAM) data along a 120-km-long section of the Dead Sea Transform in southern Jordan and Israel shed light on the shallow structure of the fault zone and on the kinematics of the plate boundary. Despite infrequent seismic activity and only intermittent surface exposure, the fault is delineated clearly on a map of the first vertical derivative of the magnetic intensity, indicating that the source of the magnetic anomaly is shallow. The fault is manifested by a 10–20 nT negative anomaly in areas where the fault cuts through magnetic basement and by a <5 nT positive anomaly in other areas. Modeling suggests that the shallow fault is several hundred meters wide, in agreement with other geophysical and geological observations. A magnetic expression is observed only along the active trace of the fault and may reflect alteration of magnetic minerals due to fault zone processes or groundwater flow. The general lack of surface expression of the fault may reflect the absence of surface rupture during earthquakes. The magnetic data also indicate that unlike the San Andreas Fault, the location of this part of the plate boundary was stable throughout its history. Magnetic anomalies also support a total left-lateral offset of 105–110 km along the plate boundary, as suggested by others. Finally, despite previous suggestions of transtensional motion along the Dead Sea Transform, we did not identify any igneous intrusions related to the activity of this fault segment.The project was funded by U.S.-AID Middle Eastern Regional Cooperation grant TA-MOU-01-M21-012

The Upper Rhine Graben (URG) revisited: Miocene transtension and transpression account for the observed first-order structures

International audienceAlthough the Upper Rhine Graben (URG) has been studied extensively for years, the origin of some of its first-order structures is still under debate, particularly the relatively young uplift of the Vosges Mountains (VM) and Black Forest Mountains (BFM). Their uplift appears to be temporally related to the change of the URG into a continental transform, the rapid subsidence of its deep northern basin, and the onset of erosional and nondepositional phase south of the Northern Basin. Recent observations from newly released seismic reflection data, coupled with older geologic and seismic observations, are used to explain this correlation. We suggest that when the URG turned into a continental transform during the early Miocene, not only was its northern basin transtensionally subsiding as previously suggested, but the VM and BFM were transpressionally uplifted. Transpression became weaker with growing distance from the Alpine front, and north of Baden-Baden the transpression is expressed only by down-buckling of the sediments, forming a deep, elongated syncline. The largest uplifts and erosion associated with this event occurred along both boundaries of the southern URG. However, the center of the graben was also affected to some extent, causing widespread erosion of pre-early Miocene sediments and subsequent nondeposition. The arcuate Vosges and Black Forest fault systems, which formed the boundary faults of the URG during the Oligocene, became mechanically unfavorable during the Miocene transpressional regime. Instead, more linear normal faults took over as the dominant boundary faults, forming the western and eastern Rhine Fault systems and assuming a strike-slip component of motion

Refined spreading history at the Southwest Indian Ridge for the last 92 Ma, with the aid of satellite gravity data

The spreading history of the oceans is modelled mostly by using magnetic anomalies and the fracture zone geometry. The high-quality, satellite-derived gravity data, that became available in recent years, reveal the details of fracture zones, which can be used as flow lines to control spreading models. We have applied this approach to the Southwest Indian Ridge (SWIR) in order to refine its spreading history. This is particularly useful for the period of complex spreading between magnetic anomalies 33 and 23, where the magnetic anomalies alone cannot resolve the detailed spreading history. We find four main stages in the spreading history of the SWIR since 96 Ma, including two that were not noted previously, between 96 Ma and anomaly 33 (76.3 Ma) and between anomalies 23o (51.7 Ma) and 18o (40.1 Ma; o denotes old boundaries of normal magnetization period). We also find that the start of the period of complex spreading was at anomaly 33, somewhat earlier than previously proposed. We discuss the characteristics of the extension that the old transform faults underwent during the complex spreading phase, in response to the counterclockwise rotation of spreading. New transform faults appeared at that time, considerably widening the transform zones

Structure and tectonic setting of the 77°E and 75°E Grabens, Kerguelen Plateau (South Indian Ocean)

The Central Kerguelen Plateau (South Indian Ocean) is characterized by abundant north-south striking normal faults, which comprise two prominent north-south rifts known as the 77°E and 75°E grabens. The 77°E Graben is a well-defined structure which extends over some 800 km from the eastern margin of the Kerguelen Plateau to about 58.5°S. Over most of its length it is associated with a 10–30 km wide axial rift and with a 100–150 km wide uplift. The 75 °E Graben is less well documented, but the available data suggest that its dimensions and internal structure resemble that of the 77°E Graben. In the better documented 77°E Graben, six rift segments, 50–100 km long, are identified. Faulting is more developed at the northern and southern ends of the 77°E Graben, possibly resulting from the interaction with other rifts. To the north, the 77°E Graben abuts the highly faulted eastern margin of the Kerguelen Plateau and the northern part of an even larger rift zone, the Plate Boundary Rift Zone, which extends along the boundary with the Australian-Antarctic Basin. To the south, the 77°E Graben adjoins the northwestern end of the Southern Kerguelen Plateau Rift Zone. The 77°E and 75°E grabens, and the other rift zones on the Kerguelen Plateau, appear to have been formed at approximately the same time, between 72 and 60 Ma. They are all part of an important extensional phase which occurred in the region and mark the beginning of the process which led to the development of the Plate Boundary Rift Zone into the Southeast Indian Ridge, between 46 and 43 Ma. The north-south trend of the 77°E and 75°E grabens is different from that of the other rift zones, which are oriented northwest-southeast. This geometry suggests that some strike-slip motion may have occurred along the north-south trending grabens as a result of extension on the northwest-southeast trending rifts, particularly the Southern Kerguelen Plateau Rift Zone. However, since near-surface extension estimates for the Southern Kerguelen Plateau Rift Zone are small, the strike-slip motion along the 77°E Graben must be equivalently small. Also, the available seismic data from this graben do not show typical seismic characteristics of a strike-slip environment, such as zones of compression or flower structures. Thus the results of this work are inconsistent with any model for the development of the South Indian Ocean which requires significant amount of transform motion on the 77°E or 75°E grabens. Finally, the seismic data from this area provide a unique opportunity to compare rifting on an oceanic plateau environment with continental rifting. We find great similarities between the two processes, in the segmentation of the rifts, the asymmetric cross section, and the associated shoulder uplifts. <br/