Topographic lineament swarms: Clues to their origin from domain analysis of Italy

Abstract

Regional-scale, subparallel linear topographic features characterize almost all planetary surfaces. This experiment concerns their tectonic meaning by focusing on map limits or domains through which individual swarms of these linear features are developed. These domain limits are contrasted with map boundaries of better-known structural features in a tectonically active region (Italy) to seek clues to stress environments and times of origin of the lineaments. The study uses regional raised relief maps to filter the lineament data to a very simple subset: those strictly topographic features of a length and prominence capable of retaining detectability through generalizations required by relief-map production. A total of 5,372 lineaments were drawn using 4 differently lighted images of 1:1,000,000 scale relief maps of Italy. Seven different tests were used for reliability and reproducibility of the data. Rose diagrams were prepared for 86 subareas by computer fitting of gaussians to azimuth-frequency histograms. Individual azimuthal “petals” of these roses were then correlated to delimit the general area over which a given azimuthal swarm is developed. The precise swarm boundaries were then located by computer contouring the population density of lines of each swarm on the original data set. In this way, 48 local swarms were mapped. Boundaries of these swarms correlate poorly with traditional litho-tectonic provinces. Instead, they seem to be associated with basin axes, broad arches, coastal flexures, areas of normal fault swarms, and projections of structural grain from adjacent sea floors. The 48 domains may be grouped into 8 noncontiguous but azimuthally compatible super-swarms covering much of Italy. The most prominent of the super-swarms are greatly expanded versions of regional structural grains: trend of the Po Basin, axis of the upper Adriatic Basin, zone of south Alpine underthrusting, and landward extensions of structural grain of the Tyrrhenian Sea. From these relationships, a model for lineament-swarm origins is proposed, involving very minor regional stretching of the thin, brittle carapace of extremely large, sometimes subtle, structures deforming by ductile mechanisms at depth.