Rob Butler1, Helena Griffiths1 and Sara Spencer2
1Department of Earth Sciences, The University of Leeds, Leeds LS2
9JT.
2Department of Geology, American University of Beirut, Beirut, Lebanon
Recent mechanical models of continental transform tectonics relate vertical and horizontal displacements not only to the partioning of relative plate motion relative to transform obliquity but also to rheological coupling within the crust, the width of the transform zone and the strength of upper crustal fault segments. For deformations in the upper continental crust, high quality data may be extracted from geomorphological studies of active tectonics. This contribution is concerned with the acquisition strategy and utility of these data in understanding the evolution of a particularly well-constrained tectonic setting: the Dead Sea Transform.
One of the most striking features of the Dead Sea Transform is the contrast in deformation between its southern and northern segments. In the south the fault zone is about 15km across with sub-parallel fault strands that accommodated the total displacement of about 110 km sinistral strike-slip. However, moving north away from the Arabian continent and onto its Mesozoic passive margin, deformation associated with the transform is distributed across a broad belt almost 100km across. Within this belt, largely confined to Lebanon, the transform forms a finite restraining bend with extensive, long-wavelength folds, over 3000m of topography and an array of strike-slip fault strands. The age of terrestrial geomorphic surfaces is established by eruption ages of lavas which form regional sheets and palaeo-valley fills. These features, linked to the patterns of drainage basins, not only show lateral and vertical offsets but also seal fault strands. Additional timing information comes from various uplifted marine terraces which both truncate and are offset by fault segments. Much of the pre-Pliocene sinistral displacement was accommodated on the regionally extensive Yammouneh fault. This was associated with transpressional uplift of the Mount Lebanon anticline and weak transtension in the west. However, more recently the Yammouneh fault strand has been abandoned and is sealed by the lower Pliocene Homs basalt. The Lebanese sector is evolving into a releasing bend with active fault strands linking westwards towards the Levant continental margin. This behaviour began in the Miocene with weak NW-SE extension. Active faulting is currently stepping offshore along the weakly-transtensional Roum fault system. The main Dead Sea fault segment to the south links with the Roum along an array of transtensional relay ramps.
These results provide the qualitative conclusion that continental transform zones can show complex interactions between strike-slip fault activities and linkages which can show overprinting transpression and transtension. The variations in transform zone complexity correlate with the change in pre-existing structure of the continental crust. However, vertical and horizontal displacements may be quantified and timed. Topographic variations across the Roum system show approximately equal amounts of uplift and subsidence between areas of fault segment overlap and segment tips (about 200m). Approximately 9.5 km lateral movement is indicated by offset drainage basins, allowing for stream capture. These data are used to examine numerical models for strike-slip deformations. Model results suggest that coupling between the Roum and the main Dead Sea fault strands is strong. Enhanced development of rhomboid basins and uplift along the relays of the Roum system suggests that fault strength decreases northwards. The long-term complexity of deformation in the Lebanese transect of the Dead Sea Transform, together with the insights from mechanical models suggests an interaction between structural inheritance and geometric hardening. The study illustrates the use of tectonic geomorphology on charting deformation in continental transforms to understand fault array evolution and displacement partitioning.
Paper presented at: Continental transpressional and transtensional tectonics. 5-6 March 1997. Tectonic Studies Group, Burlington House.