USE OF FORELAND MONITORS IN THE CORRELATION AND ANALYSIS OF OROGENIC PULSES

Abstract

Theories on crustal evolution have been developed from geologic investigations in orogenic belts and in the less deformed foreland or epeirogenic areas. In this paper, it is suggested that, because one global mechanism must govern the evolution of both orogenic and epeirogenic areas, the two must be closely related. Because the interpretation of orogenic belts is complicated so greatly by extreme deformation, metamorphism, and loss of data through erosion, it is logical to seek interpretative leads on their orogenic style and timing by precise observations and analyses of structural “monitors” in the less complex borderlands.The three foreland monitors considered are: (1) block faulting, (2) salt diapirism, and (3) volcanicity. All three monitors support the concept that tectonic pulses during measurable geologic time have been short lived and synchronous around the globe. They also indicate that the dominant structural stress which triggered these tectonic pulses has been extensional.If extensional stress also has governed the evolution of orogenic belts, then the interpretations which advocate fundamental vertical tectonics, secondary gravity sliding, and intrusion by mantle diapirism are more acceptable than those which advocate crustal shortening and compression. Moreover, if tensional tectonics has governed both orogenic and epeirogenic areas as well as the ocean‐spreading centers, then the hypothesis of global expansion is more logical and harmonious than the popular and heuristic assumptions on crustal subduction.For the successful resolution of regional tectonic problems, geologists must free themselves of dogmatic constrictions and broad generalisations. As in the exploration for hydrocarbons, what is needed is a scrupulous and patient attention to geologic detail, set within the most precise time framework attainable.