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, remains conjectural. Nevertheless, rapid sea-level fluctuations of uncertain origin during tectono-eustatic highstand, paired with repeated oceanic anoxia and climatic changes, are usually thought at present to be one of the main immediate triggers. The Cathles-Hallam model of stress-induced changes in plate density, accompanying rapid rift formation, furnishes an alternative for understanding the enigmatic sudden eustatic variations in the non-glacial time. Late Devonian tectonic extension, causing rifting and volcanicity, appears to be strongly marked in several regions of Eurasia, particularly in Kazakhstan and eastern Laurussia. All larger Devonian continents were more or less tectonically affected. A subtle record of this tectonic rearrangement is implied even for distant and apparently quiet carbonate platforms in local extensional block faulting and tilting, hydrothermal mineralisation, geochemical anomalies, and localized blooms of siliceous biota. Interpreting the late Frasnian regressive-transgressive pattern in terms of the Cathles-Hallam tectono-eustatic model, two major rifting events are hypothesized: one at about the beginning of the rhennna Zone, and a second principal pulse in the late linguiformis Zone, that encompassed the F-F transition. Developing the Veimarn-Milanovsky scenario of the global extensional pulse, it is assumed that the key endogenous factors were related to episodic (super)plume activity. The tectonically triggered changes climaxed in thermal and nutrient pulses, and induced the stepdown ecosystem destabilization observed in the F-F bio-crisis. Minor cometary strike(s) might have eventually participated in this prolonged multicausal environmental stress, mainly due to additional thermal shocks, but perhaps effective on a regional scale only. (C) 1998 Elsevier Science B.V. All rights reserved.