Northward migration of the southern diagonal fault of the Hula pull-apart basin, Dead Sea Transform, northern Israel

Abstract

The Hula Valley, located in northeastern Israel, is a 4 million-year old pull-apart basin bounded by two left stepped segments of the Dead Sea Transform. Basic models of pull-apart basins suggest four faults at all sides of the basin, a situation which is not always the case. In the Hula pull-apart basin the longitudinal (N–S) faults are clearly evident while the transversal (E–W) faults are less explicit. An NW oriented lineament, expressed as a north-facing continuous scarp 1–3 m high, was recognized in the southern part of the valley. Morphological, seismic and sedimentary considerations suggest that this scarp is the surface expression of a deep-seated fault. This fault, termed the Yesod HaMa'ala fault, forms the present southern boundary of the Hula pull-apart basin, and appears to be the southern diagonal fault, which links the Jordan fault in the east with the Yammunneh fault in the west. It is suggested that this fault scarp formed a shallow morphological barrier that partly blocked the southern outlet of the valley, enabling a permanent lake in the southern part of the Hula Valley. Thus, the uplifted southern block determined the SW shoreline of the lake. It is proposed that the Hula basin is a version of the classic pull-apart model. It is also suggested that in the absence of upwelling mantle material, and extension along the length of the basin, a crustal thinning and subsidence might have occurred through normal faulting of the upper brittle crust. The thick depositional sequence within the southern part of the basin shows that the present margins have been a major depositional site for a long time, and that the Hula Valley, has thus developed as an asymmetric, pull-apart (rhomb-shaped) basin, bounded by steep north-striking boundary normal (as well as strike-slip) faults, W–NW striking and northward-dipping normal faults, and a south-facing bending flexure. As activity within the basin continued, and northward migration of the faulting occurred, a new fault developed. According to this perception, the present active fault is the Yesod HaMa'ala fault, while the future fault will be located northward, as can be presumed from the present micro-seismic activity in the valley.