Lineaments: Their Role in Tectonics of Central Rocky Mountains

Abstract

Lineaments are important structural elements in the Central Rocky Mountains. Evaluation of the role of lineaments has been hampered by confusion of terminology, inadequate analysis of available data, and oversimplified speculation about very complex features. Lineaments and linears should not be used as equivalent terms. Linears are single rectilinear elements commonly, but not necessarily, of structural origin. Lineaments are generally rectilinear lines or zones of structural discordance of regional (100 km or longer) extent. Lineaments are expressed at the surface by alignments of combinations of linears of several types. In many cases they have been demonstrated to be very old deep faults of composite character that have a long history of repeated activity involving different modes of deformation and senses of net slip. Attempts to fit lineaments of the Central Rocky Mountains into a simple wrench-fault system run into problems by not accounting for the abundant documentation of Cenozoic vertical movements, by not accounting for all lineament directions, by confusing separation and slip, by assuming that shear zones necessarily indicate horizontal movement, and by insisting that all lineaments of a given trend must have the same sense of horizontal movement in spite of evidence to the contrary. Available evidence suggests that the amount of strike-slip displacement is small. There are four main trends, N-S, E-W, NE-SW, and NW-SE. Extensions of present known lineaments and connection of lineaments from one region to another across gaps can be considered only speculative. There is serious question that the lineaments, together with all the other Cenozoic structures, can be accounted for by a single stress configuration arising from an E-W-directed horizontal compressive stress. Much more detailed description and kinematic analysis of lineaments are required to answer the problem of the relative importance and timing of vertical versus horizontal movements in the development of the Central Rocky Mountains during the Cenozoic.