Lower crustal flow and the role of shear in basin subsidence: an example from the Dead Sea basin (original) (raw)
Related papers
Tectonics, 2008
1] Contrary to other examples, like Death Valley, California, and the Sea of Marmara, Turkey, the Dead Sea-type pull-apart basins form within a narrow transform corridor between strike-slip faults that are less than 10 km apart, much smaller than the crustal thickness of 35 km. In this paper we investigate the role of fault zone width versus thickness and rheology on the mechanics of pull-apart basins through a series of laboratory experiments. Results show that pull-apart basins that develop above a small step over (i.e., smaller than the thickness of the brittle layer") are narrow and elongated parallel to the overall motion. This is enhanced by increased decoupling along a basal ductile layer. The experiment with the highest degree of mechanical decoupling shows a striking resemblance to the Dead Sea Basin (DSB). Comparison with modeling results suggests that the DSB's flat basin floor is bordered over its full length by strikeslip faults that control the basin geometry and temporal and spatial basin migration. This is in strong contrast to Death Valley-type pull-apart basins that are highly oblique to the transform direction with transverse normal faults dominating over longitudinal strike-slip faults. Results imply that lithosphere rheology and the ratio of basin width to crustal thickness are controlling factors in the mechanics of pull-apart basin formation within transform corridors like the Dead Sea Fault. Citation: Smit, J., J.-P. Brun, S. Cloetingh, and Z. Ben-Avraham (2008), Pull-apart basin formation and development in narrow transform zones with application to the
The structure of the Dead Sea basin
Tectonophysics, 1996
The Dead Sea basin is located along the left-lateral transform boundary between the Arabian and Sinai plates. Its structure and history are known from surface geology, drilling, seismic reflection and other geophysical data. The basin comprises a large pull-apart, almost 150 km long and mostly 8-10 km wide, which is flanked by a few kilometres wide zones of normal faulting. The basin formed at about 15 Ma or earlier, close to the beginning of the transform motion, and it reached about half its present length before the end of the Miocene. A strong negative gravity anomaly records a thick sediment basin fill: >5 km under half its length, reaching a maximum of _> 10 km. The fill includes a few km of salt (ca. 6-4 Ma) which forms several diapirs. At any one time large parts of the basin subsided simultaneously, but the site of fastest subsidence seems to have shifted northward. Sedimentation rates reached at least hundreds of metres per million years or more in the Miocene, and > 1 km/Myr in later periods.
Structure of the Dead Sea pull-apart basin from gravity analyses
Journal of Geophysical Research: Solid Earth, 1993
Analyses and modeling of gravity data in the Dead Sea pull-apart basin reveal the geometry of the basin and constrain models for its evolution. The basin is located within a valley which defines the Dead Sea transform plate boundary between Africa and Arabia. Three hundred kilometers of continuous marine gravity data, collected in a lake occupying the northern part of the basin, were integrated with land gravity data from Israel and Jordan to provide coverage to 30 km either side of the basin. Free-air and variable-density Bouguer anomaly maps, a horizontal first derivative map of the Bouguer anomaly, and gravity models of profiles across and along the basin were used with existing geological and geophysical information to infer the structure of the basin. The basin is a long (132 km), narrow (7-10 km), and deep (-<10 km) full graben which is bounded by subvertical faults along its long sides. The Bouguer anomaly along the axis of the basin decreases gradually from both the northern and southern ends, suggesting that the basin sags toward the center and is not bounded by faults at its narrow ends. The surface expression of the basin is wider at its center (<16 km) and covers the entire width of the transform valley due to the presence of shallower blocks that dip toward the basin. These blocks are interpreted to represent the widening of the basin by a passive collapse of the valley floor as the full graben deepened. The collapse was probably facilitated by movement along the normal faults that bound the transform valley. We present a model in which the geometry of the Dead Sea basin (i.e., full graben with relative along-axis symmetry) may be controlled by stretching of the entire (brittle and ductile) crust along its long axis. There is no evidence for the participation of the upper mantle in the deformation of the basin, and the Moho is not significantly elevated. The basin is probably close to being isostatically uncompensated, and thermal effects related to stretching are expected to be minimal. The amount of crustal stretching calculated from this model is 21 km and the stretching factor is 1.19. If the rate of crustal stretching is similar to the rate of relative plate motion (6 mm/yr), the basin should be-•3.5 m.y. old, in accord with geological evidence. ment discontinuities across en echelon faults in a brittleelastic medium [Rodgers, 1980; $egall and Pollard, 1980; Bilham and King, 1989]. The evolution of deep basins (deeper than 2-3 km) is expected to be more complicated as they result from either larger displacements along the fault system or from rotation of the axis of extension relative to the fault system. Furthermore, the deformation of deep 1U.S. Geological Survey,
Geophysical Research Letters, 2006
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.
Lower-crustal strength under the Dead Sea basin from local earthquake data and rheological modeling
We studied the local seismicity of the Dead Sea basin for the period 1984-1997. Sixty percent of well-constrained microearthquakes (ML <= 3.2) nucleated at depths of 20-32 km and more than 40% occurred below the depth of peak seismicity situated at 20 km. With the Moho at 32 km, the upper mantle appeared to be aseismic during the 14-year data period. A relocation procedure involving the simultaneous use of three regional velocity models reveals that the distribution of focal depths in the Dead Sea basin is stable. Lower-crustal seismicity is not an artifact created by strong lateral velocity variations or data-related problems. An upper bound depth uncertainty of +- 5 km is estimated below 20 km, but for most earthquakes depth mislocations should not exceed +- 2 km. A lithospheric strength profile has been calculated. Based on a surface heat flow of 40 mW /(m*m) and a quartz-depleted lower crust, a narrow brittle to ductile transition might occur in the crust around 380 C at a depth of 31 km. For the upper mantle, the brittle to ductile transition occurs in the model at 490 C and at 44 km depth. The absence of micro-seismicity in the upper mantle remains difficult to explain.
Salt tectonics in pull-apart basins with application to the Dead Sea Basin
Tectonophysics, 2008
The Dead Sea Basin displays a broad range of salt-related structures that developed in a sinistral strike-slip tectonic environment: en échelon salt ridges, large salt diapirs, transverse oblique normal faults, salt walls and rollovers. Laboratory experiments are used to investigate the mechanics of salt tectonics in pull-apart systems. The results show that in an elongated pull-apart basin the basin fill, although decoupled from the underlying basement by a salt layer, remains frictionally coupled to the boundary. The basin fill, therefore, undergoes a strike-slip shear couple that simultaneously generates en échelon fold trains and oblique normal faults, trending mutually perpendicular. According to the orientation of basin boundaries, sedimentary cover deformation can be dominantly contractional or extensional, at the extremities of pull-apart basins forming either folds and thrusts or normal faults, respectively. These guidelines, applied to the analysis of the Dead Sea Basin, show that the various salt-related structures form a coherent set in the frame of a sinistral strike-slip shearing deformation of the sedimentary basin fill.
Tectonophysics, 2002
The Dead Sea Basin is a morphotectonic depression along the Dead Sea Transform. Its structure can be described as a deep rhomb-graben (pull-apart) flanked by two block-faulted marginal zones. We have studied the recent tectonic structure of the northwestern margin of the Dead Sea Basin in the area where the northern strike-slip master fault enters the basin and approaches the western marginal zone (Western Boundary Fault). For this purpose, we have analyzed 3.5-kHz seismic reflection profiles obtained from the northwestern corner of the Dead Sea. The seismic profiles give insight into the recent tectonic deformation of the northwestern margin of the Dead Sea Basin. A series of 11 seismic profiles are presented and described.