Palaeomagnetic and rock magnetic properties of travertine: Its potential as a recorder of geomagnetic palaeosecular variation, environmental change and earthquake activity in the Sicak Cermik geothermal field, Turkey

摘要

Travertine, the product of incremental growth of inorganic carbonate, is potentially a high-resolution recorder of geomagnetic palaeosecular variation (PSV) when it incorporates small amounts of ferromagnetic material. It grows most regularly in regions of neotectonic activity where geothermal waters feed into extensional fissures and deposit successive layers of carbonate as fissure travertine. The same waters spill out onto the surface to deposit bedded travertine which may incorporate wind blown dust including ferromagnetic particles. Tectonic travertine deposits are linked to earthquake activity because the geothermal reservoirs are reset and activated by earthquake fracturing but tend to become sealed up by carbonate deposition between events. This study investigates whether sequential deposition can identify cycles of PSV and provide a means of estimating rates of travertine growth and earthquake frequency. The palaeomagnetic record in three travertine fissures from the Sicak Cermik geothermal field in Central Anatolia and nearby bedded travertines dated up to 360,000 years in age (U–Th) are investigated to evaluate magnetic properties and relate the geomagnetic signature to earthquake-induced layering. Sequential sampling of bedded travertine from the margins (earliest deposition) to centres of fissures (last deposition) identifies directional migrations reminiscent of PSV. Thermal demagnetisation shows that goethite pigment is not a significant remanence carrier; instead hematite, and more rarely magnetite, is the carrier. Magnetic susceptibility of fissure travertine is proportional to the calcite:aragonite ratio. Two-frequency susceptibility analysis identifies a ferromagnetic content in bedded travertine dominated by fine superparamagnetic grain sizes whereas the fissure travertine has mostly single and multidomain grain sizes, a difference interpreted to reflect contrasting energies of the two environments plus atmospheric input in the bedded travertine. Fissure travertine possesses strong lineated anisotropy of magnetic susceptibility (AMS), with horizontal kmax axes oriented along the fissure axes and kint and kmin distributed within the orthogonal plane; this is explained by rolling of ferromagnetic grains up the side of the fissure during repeated water ejection until fixed by the host carbonate precipitation. In contrast bedded travertine has low magnitude AMS with near neutral ellipsoid shapes controlled by settling of grains during weak outflow from the axis of the fissure ridge. The source of the magnetic minerals in the fissure travertine is probably in material washed down by meteoric waters from the local terra rossa soil and concentrations of these minerals (and hence magnetic susceptibility) could be a signature of pluvial environments. Fissure travertine is a reliable recorder of the ambient field when layered although bedded travertine is found to exhibit inclination shallowing. On the assumption that PSV cycles record periods of 1–2 ky, layering in the travertine identifies resetting of the geothermal system by earthquakes every 50–100 years in this region. Travertine precipitation occurs at rates of 0.1–0.3 mm/year on each side of the extensional fissures and possibly at a rate an order higher as bedded travertine on the surface. Earthquakes of magnitude M ≤ 4 occur much too frequently to have any apparent influence on travertine deposition but earthquakes with M = 4.5–5.5 occur with a frequency compatible with the travertine layering and appear to be the events recorded by the layering. Two signatures of much larger earthquakes on a 1–10 ky timescale are also recorded by travertine deposition. These are (i) incidental emplacement of massive travertine or fracturing of earlier travertine without destruction of the fissure as a venue of travertine emplacement and (ii) termination of the fissure as a site of deposition with transfer of geothermal activity to a new fracture. Palaeomagnetic estimates o