Convergent Plate Margins for Field Geologists

Abstract

ABSTRACT The variability and complexity of active plate margins require that the identification of units of previously active margins in orogenic belts be done with care and sophistication. Tectonostratigraphic units from active margins vary not only as sedimentation and plate kinematic parameters change spatially, but also temporally, as these parameters evolve. Deposits from the trench, lower slope basins and upper slope or forearc basins may be distinguished using the distinctive structural styles, sedimentation pat terms, and lithologic content observed within each unit. The relative positions and characteristics among the units are equally important criteria. INTRODUCTION Recent geological and geophysical investigations, interpreted in the framework of plate tectonics, have produced a general geological model of convergent plate margins. Nevertheless, one of the most important objectives, that of identifying various elements of a previously active margin within an orogenic belt, remains elusive. The field geologist, if he is to interpret the evolution of the complexly deformed mountain belts formed by plate convergence, must be able to deduce the original tectonic settings of the various tectonostratigraphic units which are now deranged by younger overprinting and by large-displacement faulting. The purpose of this paper is to review diagonistic characteristics of the various packages of rocks produced within a convergent plate margin with which these units might be identified after their morphology is destroyed. An admonition is immediately required. There is no typical arc system, but rather a broad spectrum of responses arising from a single basic process. As a result, in this setting, perhaps more than in any other, identification using simple criteria can lead to serious error. Only a comprehensive integration of data and an adequate knowledge of arc behavior is likely to produce the correct interpretation. Convergent plate margins range from oceanic island arcs involving very little sediment to continental margin arcs which often have several kilometers of sediment on the descending plate. These differences are reflected in wide variations in size, shape, and lithologic content of units [Fig. 1]. Not only do convergent margins vary spatially, but also temporally as sedimentation and kinematic parameters change. To compound the difficulty, morpho tectonic units are the basis for subdivision in active arc systems whereas in orogenic belts, only lithologic structural entities (tectonostratigraphic units) remain. These may differ because a given tectonostratigraphic unit may be involved in several different morphotectonic units the arc evolves. For example, a block of trench turbidite rapidly becomes part of the lower trench slope. Later this same block of rock may become part of the basement beneath a forearc basin or even later may become part of the frontal arc, but when mapped in the field as a melange, it cannot be used to locate that frontal arc until temporally correlative information on other arc units is available.