EVIDENCE FOR THRUST IMBRICATION OF BALTICA BASEMENT, AND SYN-COLLISIONAL EXHUMATION OF HIGH-PRESSURE ROCKS, WESTERN GNEISS REGION, NORWAY

Integration of tectono-stratigraphic, geochronologic, structural, and petrologic data from the central and southern parts of the Western Gneiss Region (WGR) leads to better understanding of exhumation processes for high-pressure rocks. In the south, the Lindås Nappe lies structurally above an extensional fault and was subject to eclogite-facies metamorphism. New ion probe results indicate that the lower intercept age of 419 ± 4 Ma was the time of eclogite-facies metamorphism that overprinted an earlier Sveconorwegian granulite-facies metamorphism (929 ± 1 Ma). These results indicate that Sveconorwegian Baltica basement originally lying outboard of the present WGR was subducted to a depth of 65 km, imbricated, and later thrust toward the foreland and exhumed through amphibolite-facies conditions, possibly at 409 Ma. A repetition of similar events occurred in UHP rocks in the central part of the WGR, including microdiamond-bearing kyanite-garnet gneiss and associated kyanite eclogites. UHP rocks show thrust kinematics under eclogite-facies conditions that occurred from 407 ± 2 to 402 ± 2 Ma. These UHP rocks are interpreted to have been imbricated and brought in contact with HP basement by thrusting between 402 and 395 Ma. The age range for UHP metamorphism overlaps with the deposition of sediments in the high-level Hitra extensional basin over a minimum range of 403 to 394 Ma. Monazite grains from the microdiamond-bearing kyanite-garnet gneiss also contain interior domains, possibly detrital, that yield ages near 1060 Ma, interpreted to reflect an earlier Sveconorwegian metamorphic event that took place outboard of the WGR. Implications: 1) Syn-collisional exhumation was the primary mechanism for exhumation of HP and UHP rocks from 419 to 395 Ma. It was accomplished by synchronous deep thrust imbrication and upper crustal sinistral trans-extension along a sinstral transpressive plate boundary. 2) Sveconorwegian basement was present west of older 1500 to 1680 Ma basement gneiss. 3) Imbrication of HP Baltica basement and cover into relatively thin tectonic units indicates that models invoking imbrication of entire crustal thicknesses are not applicable to the Scandinavian Caledonides.