Anatomy of a Large Fault Zone- from Lithospheric to Microscopic Scale (The Dead Sea Transform)
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  • Radwan J. El-Kelani

Fault/shear zones (FZ) are the locations where movement within the Earth occurs and where motion of tectonic plates, often associated with earthquakes, is accommodated. Despite a rapid increase in the understanding of faults in the last decades our knowledge of their form and their controlling processes remains incomplete. The central questions addressed here, studying the Dead Sea Transform (DST) in the Middle East in detail, are: (1) What is the structure and dynamics of a large fault zone? (2) What controls its structure and dynamics? and (3) How does the FZ studied compare to other large fault zones?
The DST has accommodated left-lateral transform motion of 105 km between the African and Arabian plates since early Miocene (≈20 My). The DST segment between the Dead Sea and the Red Sea, called Arava/Araba Fault (AF), is studied using a multi-disciplinary and multi-scale approach from µm to plate tectonic scale.
We show first, that under the DST a narrow, sub-vertical zone cuts through crust and lithosphere to more than 50 km depth. The Moho increases smoothly from 26 km to 39 km under the AF from W to E and a sub-horizontal lower crustal reflector is detected east of the AF. Second, several faults exist in the upper crust in a 40 km wide zone centered on the AF, but none has kilometer-size zones of decreased seismic velocities nor zones of high electrical conductivities typical for large damage zones. The AF acts as a barrier to fluids and shows abrupt changes in lithology to a depth of 4 kilometers. The AF is the main active fault of the DST system, but it has only accommodated a limited part (up to 60 km) of the overall 105 km of sinistral plate motion. Until about 5 Ma ago fault strands in the vicinity of the present day AF took up lateral motion, then the main, active fault trace shifted ca. 1 km westward to its present position. Third, in the top few hundred meters of the AF a locally transpressional regime occurs in a 100 to 300 m wide zone of deformed and displaced material, bordered by sub-parallel faults forming positive flower structures. The damage zones of the individual faults are only 5 to 20 m wide, i.e. significantly smaller than at other major faults. Fourth, two areas on the AF show meso- to micro-scale faulting and veining in limestone sequences with faulting depths between 2 and 5 km. Fluids in the AF are of marine origin (of Pliocene age), some originated from meteoric fluids carried downward into the fault zone and to a lower extent from ascending hydrothermal fluids; but on kilometer-scale the AF does not act as an important fluid conduit. Furthermore, hydro-thermal reactions do not change the strength and behavior of the narrow and strong AF. Fifth, Most of these findings are corroborated using thermo-mechanical modeling showing that shear deformation in the lithosphere under the DST/AF trace first localizes in a 20 to 40 km wide zone with a mechanically weak decoupling zone extending sub-vertically through the entire lithosphere. As time progressed upper crustal deformation became quickly focused in a few faults. On plate tectonic scale the AF is a system of predominantly strike-slip faulting with less than 3 km transform-perpendicular extension. Prominent similarities between the DST and the SAF are the asymmetry in sub-horizontal lower-crustal reflectors and deep reaching deformation zones. Comparing the AF and the SAF at Parkfield also shows that both faults do not act as important fluid conduits at crustal scale and that both have flower structures in transpressional regimes at local scale. Such features are most likely fundamental characteristics of large transform plate boundaries.

American Geophysical Union (AGU)
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