H. G. Avé Lallemant

TWO CONTRASTING PRESSURE-TEMPERATURE-TIME PATHS IN THE VILLA DE CURA BLUESCHIST BELT, VENEZUELA : POSSIBLE EVIDENCE FOR LATE CRETACEOUS INITIATION OF SUBDUCTION IN THE CARIBBEAN

The Villa de Cura blueschist belt is one of several east-west–trending allochthonous belts comprising the Caribbean Mountain system of northern Venezuela. This blueschist belt consists of four structurally coherent subbelts that also trend east-west; from north to south these are characterized by: (1) pumpellyite-actinolite, (2) glaucophane-lawsonite, (3) glaucophane-epidote, and (4) barroisite. The retrograde pressure-temperature ( P-T ) path of the northern three subbelts generally parallels their prograde path. Such P-T paths are typical for Franciscan-style subduction settings and are characterized by relatively low geothermal gradients indicative of refrigeration during subduction-zone-parallel ascent and exhumation of these rocks. The barroisite subbelt formed at high pressures similar to those of the glaucophane-epidote subbelt, but at substantially higher temperatures, and followed a counterclockwise P-T path. New 40 Ar/ 39 Ar ages record peak metamorphism at 96.3 ± 0.4 Ma for the barroisite subbelt and 79.8 ± 0.4 Ma for the northern three subbelts. The Caribbean plate is thought to have been a fragment of the Farallon plate, which together with the “Great Arc of the Caribbean” (Greater Antilles–Aves Ridge–Lesser Antilles–Leeward Antilles) migrated northeastward after a subduction polarity reversal and overrode the young Proto-Caribbean lithosphere that had formed by spreading between North America and South America. The more silicic barroisite subbelt may have been part of the arc that was subducted immediately after polarity reversal, whereas the other three belts formed much later when the geothermal gradient had decreased substantially. The Villa de Cura belt was exhumed in two stages, first by Late Cretaceous arc-parallel extension, and second by Miocene southward thrusting onto the South American continent.

Mechanisms of preferred orientations of olivine in tectonite peridotite

Preferred orientations of olivine have been produced by syntectonic recrystallization in dunite samples that have been deformed moderately in a general stress field at high temperatures and pressures: X -olivine maxima ( X = [010]) developed parallel to σ 1 and Z -olivine maxima ( Z = [100]) developed parallel to σ 3 . Similar fabrics are expected to form by mechanical rotation due to translation gliding on (010)[100], but much larger strains are required. The experimental results indicate that the orientation patterns are caused by preferred growth of newly formed nuclei that have low coefficients of resolved shear stress for slip on (010)[100].

Ellesmerian(?) and Brookian deformation in the Franklin Mountains, northeastern Brooks Range, Alaska, and its bearing on the origin of the Canada Basin

Structural analysis of deformed rocks in the Franklin Mountains, northeastern Alaska, indicates that (1) pre-Carboniferous rocks were transported southeastward during mid-Devonian (Ellesmerian.) thrusting, (2) Cretaceous and older rocks were transported northward during Mesozoic-Cenozoic Brookian thrusting, and (3) the pre-Carboniferous rocks were strongly involved in the Brookian deformation. The strong involvement of these rocks in Brookian structures suggests that the magnitude of northward thrusting during Brookian tectonism is virtually uniform from west to east along the axis of the Brooks Range fold and thrust belt. In addition, the newly recognized southern vergence of pre-Carboniferous structures is comparable with that of coeval structures exposed in Arctic Canada to the east. These data are not easily reconciled with the orocline model for the origin of the Canada Basin but are consistent with left-lateral transport on a north-south-striking transform fault along the Canadian Arctic islands. 19 references.