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.