40AR/39AR GEOCHRONOLOGICAL AND STRUCTURAL CONSTRAINTS FOR THE TACONIAN OROGENY IN THE HUMBER ZONE OF THE GASPÉ PENINSULA, QUÉBEC

In the Gaspé Peninsula, Cambrian-Ordovician rocks of the Appalachian orogen are divided into five geological domains which belong to the Humber and Dunnage zones, from south to north: ophiolites of the Mont Albert Complex, the metamorphic sole (Amphibolite du Diable), internal and external Humber zones, and the foreland thrust belt. The tectonic evolution of the Humber zone is the result of several deformation events. The Taconian (Late Ordovician) and Acadian (Middle to Late Devonian) orogenic events are separated by a period of Silurian tectonic activity. Major structures within the Humber zone are Taconian and associated with an important tectonometamorphic event within the internal zone. Minor Acadian folds are present whereas Silurian structures are principally localized at the southern border of the Humber zone, along the Shickshock Sud fault. Based on new 40Ar/39Ar and structural data, a new Taconian tectonic model is proposed for the Humber zone. This model gives importance to transpressive deformation in the internal Humber zone. The Early Taconian phase (pre-D1) is responsible for the intraoceanic ophiolite obduction and the formation of a metamorphic sole at amphibolite-to-granulite facies. This high temperature facies is, preserved as a relic paragenesis and its age is estimated to be pre- 465 Ma. The Middle Taconian (D1) is characterized by northwest-verging nappe thrusting in the external Humber zone and by syn-metamorphic faults in the internal Humber zone. The regional metamorphism is at greenschist to amphibolite facies and associated with a mineral and stretching orogen-parallel lineation (L1) perpendicular to the nappe transport direction, suggesting an oblique collision and a transpressive regime of deformation for D1. 40Ar/39Ar data on muscovites and amphiboles yield metamorphism age of 459 to 456 Ma. The Late Taconian (D2) event corresponds to the continuation of foreland-directed thrusting in the external zone. In the internal zone, D2 is expressed by northeast-trending folds with a penetrative crenulation cleavage, and development of dextral strike-slip and reverse faults. D2 in the internal zone is the result of a continued transpression initiated during D1. 40Ar/39Ar analyses on muscovites and amphiboles give an age at ca. 447 +/- 2 Ma for this event.