FORMATION OF MELANGES, BROKEN FORMATIONS, AND MASS-TRANSPORT CHAOTIC DEPOSITS DURING THE ORDOVICIAN TACONIC OROGENY: EXAMPLES FROM CENTRAL AND NORTHERN APPALACHIAN OROGENIC BELT, EASTERN USA

Different types of unmetamorphosed mélanges related to the evolution of the accretionary wedge-front of the Taconic Allochthon have been studied in the Central and Northern Appalachian orogenic belt (in eastern Pennsylvania, New York and Vermont).

Mélanges, broken formations, and mass-transport chaotic deposits (mélanges s.l.) display structural evidences for progressive stratal disruption of a Lower-to Upper Ordovician succession during the subduction-accretion-to collision tectonic episodes of the Ordovician Taconic Orogeny. They represent the products of different processes (tectonic, sedimentary and diapiric) operating during the evolution of the accretionary wedge.

Geologic mapping and stratigraphic-structural observations indicate that different types of mélanges s.l. were formed at different structural positions with respect to the wedge front, and that their own type of chaotic arrangements and deformation intensities depend on their origin, evolution, and tectonic position.

Downslope mass-movements, which are consistent with different gravity-driven processes (such as slides, slumps and debris flows), produced mass-transport deposits, including debris flow deposits and/or olistostromes, that were emplaced at the wedge-front as precursors of the advancing Taconic Allochthon providing exotic material into the flysch successions. These sedimentary mélanges were locally overridden by the advancing thrust sheets and incorporated into the shear zones forming an olistostromal carpet. Shearing led to the juxtaposition and mixing of rocks (in some cases including exotic blocks) of various ages, and subsequently to the formation of boudinage, enucleation of isoclinals folds, and phacoidal microshear cleavages.

Broken formations were mainly formed at the base of the wedge-front and Taconic thrust fault systems. During early stages of accretion the layered and still water-rich flysch sediments were locally dismembered by in-situ layer-parallel extension caused by tectonic loading and increase of fluid pressure. In other cases stratal disruption appears to be produced by compressional deformation of consolidated sediments through thrusting and shearing at the base of the accretionary wedge-front and/or Taconic Allochthon thrust faults. Both types of broken formations show a gradual transition from originally coherent stratigraphic successions to variously disrupted strata, and, finally, to block-in-matrix arranged units that lack a stratigraphic continuity and do not contain exotic blocks.

Diapiric mélanges containing exotic and/or non-exotic blocks formed by the increasing of sedimentary and/or tectonic load at the base of or within the accretionary wedge that, in turn, induced local over-pressure conditions promoting the upward rise of under-consolidated sediments.

In conclusion, the final mélange products studied in the Central and Northern Appalachian orogenic belt generally preserves the artefacts of the only last process of formation, but may represent the product of a complex interaction of superimposed processes that in many cases are reminiscent of a tectono-sedimentary evolution of a mélange developed in a single geodynamic setting.