Olistostromes and olistoliths: a historical review and modern perspectives

The terms olistostrome and olistolith have been introduced by G. Flores (4th World Petroleum Congress, Rome, 1955) to indicate either mass-transported bodies with a chaotic block-in-matrix fabric, or single slide blocks, intercalated between layered sequences in the Tertiary succession of Sicily. Both terms soon became extensively used by the international geological community world-wide. With the extended usage, they evolved to generally indicate stratally disrupted to chaotic complexes and “exotic” bed packages, which originated by mass-transport events, mainly recycling extrabasinal rocks. In this extended meaning, the concept of olistostrome played a significant role in other, important scientific debates, such as the origin of mélanges, being paradigmatically assumed as indicative of sedimentary processes (olistostromal mélanges). We will briefly discuss what happened to the terms during the 50 years of their history and how this lead to the following points: 1) Are the terms olistostrome and olistolith still to be used, and, if yes, with what meaning. 2) Olistostromes have been considered markers of either phases of basin instability and regional-scale tectonic events, or of peculiar geodynamical or tectonic stages. Are these assumptions true, or are olistostromes merely related to a specific condition of slope instability. 3) Olistostromes have been only seldom studied from sedimentological point of view. Therefore, their translational and depositional mechanics, as well as the internal processes of stratal disruption and dispersion leading to their classic, breccia-like, block-in-matrix fabric, are still poorly known. 4) The two previous points concern the genetically and regional relationships between the bodies defined to as olistostromes and the more general category of mass-transport complexes (MTC), with a particular emphasis on the basin-wide ones. In the sedimentary record of collisional chains, the majority of fossil MTC, including olistostromes, originated during the stages of intracontinental deformation, having been deposited in foreland and wedge-top basins. In some cases, collisional orogeny has allowed MTC related to extensional tectonics and passive margin to become exposed. This contrasts with the observed abundance of present-day MTC, which prevail in passive and divergent margins and along the flanks of volcanic islands atop the oceanic crust. The present-day submerged contractional margins, however, do not show a significantly high concentration of MTC, apart from the erosional margins off the coasts of Peru. Moreover, basin-wide MTC are only present when catastrophic events occur, as in the case of the subduction of seamounts and volcanoes. Some, possibly concomitant, solutions to these discrepancies will be discussed in this communication, with a special emphasis on the origin of mélanges in the accretionary wedges and relations between mass-transport processes, slope tectonics, contractional tectonics and mud diapirism.