Melange fabrics of the northern Appalachians

Abstract

Melange and phacoidal or scaly cleavage have been observed in both ancient and modern day accretionary tectonic environments throughout the world. This unique structural fabric reflects common structural elements from microscopic to macroscopic scales of observation and from one region to another. Structures include: distinct individual polyhedral phacoids, phacoidal shear-aparts, rootless dismembered bedlets of silt, isolated rootless near isoclinal fold noses, seams of preferentially oriented phyllosilicates that generally parallel the foliation, abundant pyrite that ranges in form from large globular masses to small euhedral framboids, and calcite present as foliation-parallel veins and as intergranular precipitates. The characteristic phacoid shape is defined by an anastomosing network of regularly intersecting curviplanar slip surfaces whose average orientation defines a macroscopic and microscopic foliation. In three dimensions these intersections form individual phacoidal forms. The relationship of these lensoid shapes to their internal geometry and the overall stress environment in which they form is the least understood aspect of this fabric. By comparing structural data from Taconic melange in western Newfoundland, Canada and eastern New York State insight into the nature of the fabric has been gained. Three primary analytical techniques were used to obtain data for this study: microstructural analysis of thin-sections, analysis of individual phacoid specimens, and manual dissection of large hand samples. Because of the friable nature of phacoidally cleaved material, a method of dissection was developed to measure structural data such as phacoidal surface orientations and associated slickenline orientations. Data collected from these techniques was analyzed using standard stereographic methods using the computer program Orient. In addition, a stress analysis of the foliation and lineation data was done. The results of these analyses suggest that the lensoid shape of individual phacoids is significant at all scales, similar structures are observed from one location to another, and from ancient to modern day tectonic environments. In addition, the presence of phacoidal cleavage in shales and shaly sediments can be used, along with other geologic parameters, as a genetic indicator of the accretionary environment. The distinct phacoidal shape ranges from highly euhedral polyhedrons with triclinic symmetry to subhedral faceted forms that are best described as elongated oblate ellipsoids. These shapes are interpreted to reflect the internal arrangement of seams of preferentially oriented phyllosilicate grains that have apparently undergone rotation, intergranular particulate flow, and possibly recrystallization in an environment of high shear stress, flattening, and progressive deformation. In addition, conjugate microfaulting along phacoidal surfaces that generally parallel the seams acts in concert with the above mechanisms to accommodate the deformation in the accretionary prism environment. The presence of abundant precipitates of pyrite and calcite are interpreted to reflect dewatering processes that are syntectonic to the development of the fabric in the accretionary environment. The results of the stress analysis has demonstrated that the fabric axes, as defined by individual phacoid axes which are generally symmetrical to the axes of the fabric as a whole, are symmetrical to the principal stress axes. Comparison of the geometric, and petrographic data with the stress analysis data confirms this relationship and places the maximum principal stress at a high angle to the average orientation of the dominant foliation. This further indicates that the fabric is the result of shortening symmetrical to the fabric. The state of strain in phacoidally cleaved shales remains ambiguous because of a lack of strain markers and because there is no prefabric frame of reference with which to evaluate it.