THE GEOLOGY OF CRYPTIC SUTURES: AN EXAMPLE FROM THE WESTERN GREATER CAUCASUS

Eldridge Moores recognized ophiolites as fragments of relict ocean basins that closed due to subduction and collision, and ophiolites are now widely viewed as a diagnostic feature of suture zones. Identifying sutures is critical for locating ancient plate boundaries and building a record of continental assembly. However, Moores (1981) noted that not all sutures expose ophiolites and that other criteria are needed to recognize such cryptic sutures. Key geologic indices of Moores (1981) include the thrusting of deep-water rocks onto continental margin rocks, discrepancies in paleomagnetism, mylonitic shear zones, and discontinuities in structural orientation.

New analytical techniques have improved the ability to differentiate units joined by a cryptic suture. Here, we propose revised geologic criteria for identifying former ocean basins closed by cryptic sutures. Major differences in radiometric age of igneous or metamorphic rocks provide a first-order indication of a suture, though like Moores (1981), we emphasize the importance of multiple lines of evidence. Provenance data, including U-Pb ages and Hf isotopes in detrital zircon, can identify differences in crustal evolution between domains juxtaposed by a suture. Finally, differences in thermal histories across sutures may be identified with thermochronology and geothermometry.

The Greater Caucasus (GC) defines the northern margin of Arabia-Eurasia collision and localizes most orogen-perpendicular shortening in the region. The GC formed by closure of a Mesozoic marine basin between GC basement and the Lesser Caucasus (LC) arc. In one model, subduction of a back-arc basin culminated in Miocene-Pliocene collision and suturing. An alternate view is Eocene-Oligocene rift inversion without suturing. Despite the absence of an ophiolite, detrital zircon U-Pb analyses indicate a spatial shift in provenance from GC to LC affinity within the closed basin, suggesting stratigraphic variation. (U-Th)/He analyses suggest that this shift correlates with offsets in magnitude of exhumation, implying structural juxtapositions. Additional radiometric, geochemical, and Raman spectroscopic analyses will help evaluate whether there is a cryptic suture in the GC and unravel the complex history of continental assembly in the Arabia-Eurasia collision zone.