The SISZ

$\\$The work has focussed on aerial inspection of the main fault systems. Some of the faults have been selected for a detailed areal study for comparison with on-land field studies. The principal questions addressed are: Why are the fault systems in the Holocene lava flows so complex? How do these systems compare with the nearby faults in the Pleistocene rocks.

With the view of answering these questions partly, a detailed aerial inspection and mapping of the Leirubakki-Svínhagi seismogenic fault has been made. This fault is just over 7 km long strikes N12$^{\circ }$. The fault consists of many segments within a zone that is 100-250 m wide. In the northern part of the zone, individual fractures strike around N12$^{\circ }$, around N15$^{\circ }$in the middle part, and around N25$^{\circ }$ in the southernmost part. The fractures thus become more easterly striking towards the south. The fractures consist of en echelon segments, with small push-ups or hillocks as well as pull-apart structures between the nearby ends of the segments. These structures are associated with areas of transpression and transtension, respectively, along the main strike-slip fault.

The Leirubakki-Svínhagi fracture is location in the 9000 years old Thjórsárhraun lava flow. The age of the fracture itself is, however, not known. It is obviously less than 9000 years old, but may be only several hundred years old. At the southern end of the fracture, near the farm Svínhagi, hot water (15-20$^{\circ }$C) emits from the fracture. The presence of warm water in the fracture indicates that it is not very old as otherwise it would have been sealed up with silica.

There are at least three main trends of fractures associated with the Leirubakki-Svínhagi fault: NNE, ENE and WNW. Most of these cracks are presumably strike-slip faults. In addition, there are NE trending tension fractures. All these trends are also found in the fault populations in the nearby Pleistocene rocks. Nevertheless, the WNW trend of segments associated with the Leirubakki-Svínhagi fault is unusually clear and conspicuous and must be explained by any model that attempts to account for the fracture pattern associated with the SISZ.

Many other faults in the Holocene lava flows have been studied, on land, by aerial inspection, and from aerial photographs. These results have been compared with those obtained by field studies of faults in the nearby Pleistocene areas of the SISZ. As was to be expected, the same populations of faults occur in the Holocene lava flows as in the Pleistocene rocks; these populations are discussed in more detail in the section on fault populations below.

Work on the SISZ as regards the field studies and paleostresses is in close collaboration with Jacques Angelier, Francoise Bergerat, Sigurdur Th. Rögnvaldsson and Helgi Torfason. Concerning analytical studies on fault development, a close collaboration is with Maurizio Bonafede and Maria Elina Belardinelli. A manuscript on the stress fields and fracture pattern in the central part of the SISZ has already been submitted, and another manucript, combining field and theoretical studies on faults in the eastern part of this zone, is in preparation.