Thermal history of the central part of the Karst Dinarides, Croatia: Combined application of clay mineralogy and low-T thermochronology

This study was undertaken to obtain the first thermochronological and clay mineralogy data to unravel the thermal history (maximum temperatures, timing, origin of the temperature increase, rate of exhumation) of the central part of the Karst Dinarides, exposed along the NE Adriatic coast in the Velebit Mt. and neighbouring areas. An additional research objective was tracing the behaviour of haematite crystals during diagenesis. The lower, partly elastic part of the sedimentary section (Upper Carboniferous to Triassic), covered by a thick succession of Mesozoic carbonates has been studied by a combination of mineralogical techniques (XRD + SEM), K-Ar dating of illite, apatite fission track (AFT), as well as apatite and zircon (U-Th-Sm)/He thermochronology. A consistent model of the thermal history of the study area was obtained. The Carboniferous to Triassic sequences SW of the Split-Karlovac Fault experienced maximum burial temperatures between 200 and 270 degrees C, while lower palaeotemperatures (ca. 150 degrees C) were detected in the Middle Triassic rocks to the NE of the fault. The maximum palaeotemperatures were recorded earlier (during the Late Cretaceous-Palaeocene) than expected during the period of maximum sedimentary and/or tectonic burial in Middle Eocene and Early Oligocene, corresponding to the major thrusting phase in the studied part of the Dinarides. Rapid exhumation started in the studied structural domains between 80 and 35 Ma, i.e. between the Campanian and the end of Eocene, followed in some domains by a younger exhumation and cooling pulse. Haematite, the main carrier of the palaeomagnetic signal in the studied area, was shown to recrystallize at temperatures above 120 degrees C, which explains the appearance of secondary magnetization well below the Curie point of haematite, such as reported recently in the Central Velebit Mt. area.