SLATE AND SLATE BELTS
Slates are a common rock lype in many of the worlds orogenic belts. Typically they outcrop in narrow zones that are elongate parallel to the orogenic belt, but there are exceptions. The area of North Wales occupied by slates, for example, is almost equidimensional. However, if this area is subdivided on the basis of age, the Cambrian slates are found to occur in a narrow belt along its northern edge. In view of its general applicability, the term slate belt is used to describe this association despite the fact that not all slate belts have a linear outcrop pattern.
The definition of the slate belt association is, of course, as dependent on lithology as on structure. Slates are low-grade metamorphic rocks of restricted parentage. Metamorphically they belong in the greenschist facies and are commonly found below the biotite isograd. Typically they are metasediments of silt or finer grain size but they are sometimes derived from igneous rocks; for example, some of the slates of the English Lake District are derived from lapilli tuffs and some of the Mariposa slate of California is believed to be derived from igneous rock of gabbroic composition. Generally slates are interbedded with other rocks of sedimentary origin, such as sand and gritstones, conglomerates, novaculites, and limestones. They are also very commonly pyritic and/or carbonaceous. In some belts, such as the commercially producing slate areas, the slate may be
the most abundant rock type, but elsewhere, it may be a minor constituent of the stratigraphic column.
The most obvious structural feature of slate is the cleavage along which it is readily split. Less obvious and less perfect is another surface parallel to which the rock may be split: the grain. This surface is exploited by quarrymen in the dressing of slate into workable pieces or into the final product. It is perpendicular to the cleavage and parallel to a lineation that is generally visible on the cleavage plane. This particular lineation is usually inclined to the trace of bedding on the cleavage surface, at an angle close to it. It is referred to as the down dip lineation because of its common orientation parallel to the direction of dip of the cleavage.
Folding in slate belts is generally simple, although the folds are commonly tight to isoclinal. In most areas the folds are upward facing, hinge lines are doubly plunging with shallow plunges predominating, and axial planes are steep to vertical with the axial planes of parasitic folds commonly fanning across the major anticlinoria and synclinoria. In commercially producing slate areas the slate beds are generally thick (several meters or more) and folds tend to be large with amplitudes measurable in meters or tens of meters. In areas where thin (a few centimeters) beds of different lithology are intercalated, small parasitic folds, with amplitudes measurable in centimeters, generally abound. If the slates are interbedded with other lithologies, the cleavage generally changes orientation as it passes from one rock type to another. In the slate the tans are almost invariably divergent though the amount of divergence may be very small and in the other rock types the cleavage is convergent.
Various joint patterns are found in slate belts but two systems occur recurrently: a system of joints perpendicular to the down dip lineation and a system of ac joints, which are often parallel to the grain. Both systems, but particularly the first, are commonly occupied by quartz and/or carbonate veins. Various patterns of faulting may also be found in such regions, but again there is one type that is common in this association, and that is faulting parallel to the cleavage. Such faults are very well developed in some areas and vary in magnitude of displacement from a few millimeters to tens of meters. They mayor may not be genetically related to the development of the cleavage.
It is common to find kinks and/or crenulation cleavage overprinting slaty cleavage in slate belts. The kinks tend to occur in "swarms" that, in some areas, are spatially related to large faults. They quite commonly occur as conjugate pairs but even where they are abundant, their contribution to the large-scale structure is generally small. The crenulation cleavage may be related to a second generation of folds and, if such folds are large, the structure becomes more complex and there is gradation into the multiply deformed belt association. In other areas however the crenulation cleavage is not accompanied by much folding and the structure remains comparatively simple.
Various attempts have been made to analyze the strain in slate belts using such objects as deformed fossils, lapilli, or reduction spots. They indicate that the cleavage is now perpendicular, or approximately perpendicular, to the direction of maximum shortening. The most reliable of these analyses appear to be ones based on reduction spots and they indicate shortening perpendicular to cleavage generally in excess of 55 percent. Another interesting point has emerged from Wood's work (1974) on reduction spots in the slates of North Wales. He has shown that the amount of shortening perpendicular to the vertically oriented cleavage varies from point to point within the belt and that, where the amount of shortening is large, the amount of vertical extension is also large, and conversely, where shortening is small, vertical extension is small. Furthermore, there is a spatial correlation between
shortening and volume loss.