FRASNIAN-FAMENNIAN BIOTIC CRISIS - UNDERVALUED TECTONIC CONTROL

The prime cause of the Late Devonian Kellwasser crisis, culminating in a mass extinction event near the Frasnian-Famennian (F-F) boundary, r emains conjectural. Nevertheless, rapid sea-level fluctuations of unce rtain origin during tectono-eustatic highstand, paired with repeated o ceanic anoxia and climatic changes, are usually thought at present to be one of the main immediate triggers. The Cathles-Hallam model of str ess-induced changes in plate density, accompanying rapid rift formatio n, furnishes an alternative for understanding the enigmatic sudden eus tatic variations in the non-glacial time. Late Devonian tectonic exten sion, causing rifting and volcanicity, appears to be strongly marked i n several regions of Eurasia, particularly in Kazakhstan and eastern L aurussia. All larger Devonian continents were more or less tectonicall y affected. A subtle record of this tectonic rearrangement is implied even for distant and apparently quiet carbonate platforms in local ext ensional block faulting and tilting, hydrothermal mineralisation, geoc hemical anomalies, and localized blooms of siliceous biota. Interpreti ng the late Frasnian regressive-transgressive pattern in terms of the Cathles-Hallam tectono-eustatic model, two major rifting events are hy pothesized: one at about the beginning of the rhennna Zone, and a seco nd principal pulse in the late linguiformis Zone, that encompassed the F-F transition. Developing the Veimarn-Milanovsky scenario of the glo bal extensional pulse, it is assumed that the key endogenous factors w ere related to episodic (super)plume activity. The tectonically trigge red changes climaxed in thermal and nutrient pulses, and induced the s tepdown ecosystem destabilization observed in the F-F bio-crisis. Mino r cometary strike(s) might have eventually participated in this prolon ged multicausal environmental stress, mainly due to additional thermal shocks, but perhaps effective on a regional scale only. (C) 1998 Else vier Science B.V. All rights reserved.