Deformation processes along continental transform faults: the southern Dead Sea Fault System, Israel

Abstract

The mechanical weakening processes involved in the development of major crustal fault systems have been widely documented, and it is recognised that clay-bearing fault rocks frequently have a significant influence on fault strength and slip behaviour in the upper crust. It is less well-understood how mechanical processes, such as cataclasis and the entrainment of shales along fault zones, interact with chemical processes, such as clay mineral transformations, during fault rock development. These processes can combine to form fault zones that may be both lithologically and mechanically heterogeneous, and which may also evolve over time, changing the nature of observed heterogeneities.

Data are presented here from a suite of exhumed fault sections of the southern Dead Sea Fault System (DSFS), Israel. The DSFS is an active continental transform fault that has accumulated approximately 105 km of sinistral displacement since the mid-Miocene; 60 km in an initial phase (20-18 Ma) and a further 45 km within the last 5 Ma. The studied faults lie immediately to the west of the active fault trace, west of the town of Elat, southern Israel, and are estimated to have been exhumed from shallow depths (<5 km, but potentially significantly less). Fieldwork has been carried out to document the architecture of the fault outcrops, recording comprehensive structural data, and to collect samples of a range of fault rocks. Samples have been analysed by optical and scanning electron microscopy to record microstructures and mineralogy of framework minerals, by X-ray diffraction (XRD) to record mineralogy of clay minerals, and by fusion inductively coupled plasma mass spectrometry (FUS-ICP/MS) to quantify elemental composition.

Results show the fault sections to be highly heterogeneous and comprise a range of fault rocks: variably fractured damage zones hosted in crystalline basement and sedimentary cover rocks; crushed crystalline basement rocks; mechanically entrained shale gouges; and fault gouges formed by a combination of cataclasis and neomineralisation of Mg-bearing smectite. Through operation of grain-size reduction and limited fluid-rock interactions, there is a bulk change from fault rocks dominated by frictionally strong phases, such as quartz, feldspars and calcite, displaying no obvious fabric, through to foliated phyllosilicate-rich fault gouges that likely have much lower frictional strengths. Elemental compositions across the fault zones suggest limited ingress of chemically reactive exotic fluids during neomineralisation. Mechanically entrained shale that has not undergone significant brittle deformation is also present in relatively large volumes in some instances and it is likely that the incorporation of this material inhibits further cataclastic deformation within the fault zones. Phyllosilicate-rich gouges contain microfolds on the centimetre to micron-scales, and preserve evidence of distributed deformation at shallow depth and low temperature conditions.

The heterogeneous nature of mechanically complex fault zones is influenced largely by the initial mineralogy of protolith rocks, but also by syn-tectonic processes, leading to the evolution of fault rock mineralogy with time. The development of layers of aligned phyllosilicate minerals have the potential to significantly alter the physical properties and mechanical strength of a fault zone, even if they are not present in large volumes (perhaps as little as 10-20%). The precipitation and/or entrainment of weak mineral phases may account for the evidence of both aseismic creep (microfolding) and coseismic slip (rock pulverisation) within these fault zones, recording different stages in their evolution.