The magnetism in tectonically controlled travertine as a palaeoseismological tool: examples from the Sicak Cermik geothermal field, central Turkey

Abstract

In regions of neotectonic activity geothermal waters flow into extensional fissures and deposit successive layers of carbonate as fissure travertine incorporating small amounts of ferromagnetic grains. The same waters spill out onto the surface to deposit bedded travertine, which may also incorporate wind-blown dust with a ferromagnetic component. Travertine deposits are linked to earthquake activity because geothermal reservoirs are reset and activated by earthquake fracturing but tend to become scaled by deposition of carbonate between events. A weak ferromagnetism records the ambient field at the time of deposition and Sequential deposition can identify cycles of secular variation of the geomagnetic field to provide a means of estimating the rate of travertine growth. The palaeomagetic record in three travertine fissures from the Sicak Cermik geothermal field in central Anatolia dated to between 100 and 360 ka by U-Th determinations has been examined to relate the geomagnetic signature to earthquake-induced layering. Sequential sampling from the margins (earliest deposition) to the centres (last deposition) identifies directional migrations reminiscent of geomagnetic secular variation. On the assumption that these cycles record time periods of 1-2 ka, the number of travertine layers identifies resetting of the geothermal system by earthquakes every 50-100 years. Travertine precipitation occurs at rates of 0.1-0.3 mm a(-1) on each side of the extensional fissures and at a rate an order higher than for bedded travertine on the surface. Earthquakes of magnitude M <= 4 occur too frequently in the Sivas Basin to have any apparent influence on travertine deposition but earthquakes with M in the range 4.5-5.5 occur with a frequency compatible with the travertine layering, and it appears to be events of this order that are recorded by sequential travertine deposition. Two si-natures of much larger earthquakes on a 1-10 ka time scale are also present in the travertine deposition: (1) the incidental emplacement of massive travertine or fracturing of earlier travertine without destruction of the fissure as a site of travertine emplacement; (2) termination of the fissure as a site of deposition with transfer of the geothermal activity to a new fracture. The presence of some 25 fractures in the c. 300 ka Steak Cermik field growing at rates of 0.1-0.6 mm a(-1) suggests that the type (2) signature may be achieved by an M c. 7.5 event approximately every 10 ka.