BLOCK-IN-MATRIX ROCKS AT THE LEADING EDGE OF THE HUMBER ARM ALLOCHTHON, PORT AU PORT PENINSULA, NEWFOUNDLAND APPALACHIANS

Port au Port Peninsula, in western Newfoundland, sits at the western edge of the Appalachian orogen. Middle Ordovician foreland basin strata deposited on the Laurentian margin are primarily derived from, and overridden by, Cambrian to Ordovician deep-water rocks previously mapped as mélange and assigned to the Humber Arm Allochthon.

The Humber Arm Allochthon comprises the Cooks Brook, Middle Arm Point and Eagle Island formations, and is structurally highly disrupted. More coherent, folded outcrops of allochthonous rocks show three fold generations; early tight to isoclinal folds are overprinted by two later deformation events which we link to Acadian (Devonian) orogenesis and inversion, and subsequent Carboniferous strike-slip along high-angle faults. More disrupted sections of the allochthon ("broken formation") contain densely spaced fractures, some of which contained liquid hydrocarbons at the time of deformation. Autobrecciation, dewatering structures and sandstone dykes imply high fluid pressures during deformation. The allochthon underwent coaxial extension within an overall compressional setting as a result of high fluid pressure due, in part, to the expulsion of hydrocarbons within an advancing tectonic wedge. Mélange, representing the most heterogeneous parts of the allochthon, commonly contains igneous blocks from a variety of sources. Mélange at outcrop scale shows, on average, 24% blocks to 76% scaly shale. Measured blocks within mélange range from thin-section scale (0.5 mm) to 150 cm. No metamorphic rocks are present. We interpret the igneous blocks as originating by debris-flow processes at the Ordovician deformation front.

Thus, structures in broken formation can be explained largely by tectonic processes assisted by early expulsion of fluids, whereas Earth-surface processes are necessary to explain the entrainment of exotic blocks in mélange.