Velocities and driving pressures of clay-rich sediments injected into clastic dykes during earthquakes

Abstract

S U M M A R YWe studied the velocities and driving pressures associated with clastic-dyke formation in the Ami'az plain, where hundreds of clastic dykes cross-cut the soft rock of the late Pleistocene lacustrine Lisan Formation, within the seismically active Dead Sea basin. Flow of clastic material into fractures and opening of the fractures are two mechanisms that occur during earthquake-induced clastic dyke emplacement. Two analytic models were established, based on field observations and experimental viscosity tests, to estimate the velocities and driving pressures that were associated with dyke emplacement: (a) a channel flow for upward injection of a clay-water mixture and (b) a profile of fracture dilation based on the elastic theory analysis. The two models predict that pressures between 1 and 10 MPa are generated in the source layer and dykes in the last stage of the injection process. In addition, the channel flow model predicts that the injection velocity reaches metres to tens of metres per second and the emplacement time of the clastic dykes is on a scale of seconds. It is suggested that the high pressure values represent the static stress drop during earthquake events or represent dynamic stresses resulting from the seismic waves which passed through the soft lacustrine rocks. In both cases, the predicted high pressure values indicate that the clastic dyke was emplaced in close proximity of an active segment of the Dead Sea Fault during the late Pleistocene-Holocene.

Magnetic fabric (anisotropy of magnetic susceptibility) constraints on emplacement mechanism of clastic dikes

“…Some dikes are as thick as 10 cm and occasionally as high as 15 cm. The dikes have generally tabular or rarely sinuous geometry and [Delaney and Pollard, 1981;Rickwood, 1990;Poland et al, 2004;Levi et al, 2006bLevi et al, , 2008Morris et al, 2008]. Obliquely intersected dikes, indicative of their relative chronology, are also observed ( Figure 3c).…”

“…The infilling materials, which were apparently derived from the overlying Quaternary conglomerate, are poorly lithified and composed of coarse sands, rounded pebbles (partly cobbles), and interstitial clays and silts, which decrease in grain size downward. Most of the dikes become narrower downward and pinch out at a depth of several meters, having the typical geometry of neptunian dikes [ Eyal , ; Levi et al ., , , ; Moretti and Sabato , ]. The overlying conglomerate is composed of disorganized, clast‐supported, poorly sorted, rounded to subrounded, pebble‐ to boulder‐sized clasts and finer interstitial detritus.…”

“…The source layers are light yellow to greenish sandstones. Some dikes are disconnected, and stepped linkage of dike segments is observed in vertical (Figures c and f) and horizontal sections (Figure g), indicating lateral and vertical injections of the infilling materials, respectively [ Delaney and Pollard , ; Rickwood , ; Poland et al , ; Levi et al , , ; Morris et al ., ]. Obliquely intersected dikes, indicative of their relative chronology, are also observed (Figure c).…”

Identifying soft-sediment deformation in rocks

“…Further evidence that the Lisan Formation remained poorly cemented at the time of deformation is provided by analysis of thin sections that reveal a lack of brecciation (Alsop and Marco, 2011). Re-mobilisation of sediments following thrusting (Alsop and Marco, 2011), and injection of numerous clastic dykes that cut the MTD horizons, and are sourced from within the lower portions of Lisan Formation (Levi et al, 2008), also demonstrate that structures within the Lisan Formation were unequivocally created during SSD.…”

Characterizing seismites with anisotropy of magnetic susceptibility

“…During an earthquake, the evolved deformation in a rock can differ from place to place and from time to time, depending on the local geological conditions and the material properties (e.g., McCalpin, 1996;Marco and Agnon, 2005;Alsop et al, 2012Alsop et al, , 2017Levi et al, 2006Levi et al, , 2008Weinberger et al, 2016Weinberger et al, , 2017. In order to relate magnetic fabrics of sedimentary rocks with different deformation processes, several plots of AMS parameters have been used so far, including the bootstrapped eigenvalue histogram (Tauxe, 1998;Schwehr and Tauxe, 2003); L-F Pluijm, 2002, 2003;) P-k m (e.g., Ferré el al., 2014) T-P (e.g., Cifelli et al, 2009;Aubourg, et al, 2014).…”