Geology of the Lewisporte/Loon Bay Area, Newfoundland, Canada

The Lewisporte/Loon Bay area is located within the Central Volcanic belt of Newfoundland. It contains rocks of (?)upper Cambrian to Ordovician in age. The most important group of rocks within this area is the three part Campbellton sequence. The oldest unit of the Campbellton sequence is the Loon Harbour Formation (500-1,000 m thick) composed of mafic volcaniclastics that is conformably overlain by manganiferous cherts of the Luscombe Formation. The Luscombe Formation (370 m thick) is composed of manganiferous cherts that grade upward into highly argillaceous chert. Conformably overlying the Luscombe Formation is the Riding Island greywacke that represents the uppermost unit of the Campbellton sequence. Other units found within the map area include the New Bay Formation, Dunnage melange, Burnt Bay chert - (new), Caradocian age black slate, Goldson Formation and the Botwood Group. The Campbellton sequence is interpreted to underlie the Dunnage melange with the Riding Island greywacke representing a member of the New Bay Formation. The Burnt Bay chert (new) is inferred to directly underlie the Caradocian age black slate and overlie both the New Bay Formation and Dunnage melange. Greywackes correlative with the Sansom/Point Leamington greywackes are not found within the map area. The dominant structure of the Lewisporte/Loon Bay area consists of a series; of moderate to steeply southeast -plunging inclined to reclined, close to tight, overturned macroscopic folds. Axial surface cleavage of these folds forms a regional penetrative cleavage that affects all rocks within the map area. These folds are interpreted to represent the first major phase of deformation that affected this area (Dl). Various local complications exist such as minor differences in style and orientations of the folds and associated lineations and cleavage. Some of the minor changes in fold orientations may be a result of a series of north-northeast trending sinistral faults that kink and offset cleavage. Important non-penetrative soft sediment structures pre-dating the major regional folding episode are found in several units. The Carmanville melange may represent the accretionary prism of a west dipping subduction zone of the central volcanic belt of Newfoundland during the late Cambrian to medial Ordovician. Deposition of the Luscombe Formation most likely occurred in the forearc basin of this arc system during its incipient development as nearby arc-related subsea volcanism pumped large quantities of Mn, Fe and Si into the sea water to be precipitated as manganiferous chert. Development of this arc system through time lead to the deposition of the New Bay Formation and Lawrence Head volcanics adjacent to the forearc trough resulting in gravitational slope instabilities and the olistotromic deposition of the Dunnage melange in this forearc trough

From ductile to brittle: evolution and localization of deformation below a crustal detachment (Tinos, Cyclades, Greece)

International audienceThe Cycladic Oligo-Miocene detachment of Tinos island is an example of a flat-lying extensional shear zone evolving into a low-angle brittle detachment. A clear continuum of extensional strain from ductile to brittle regime is observed in the footwall. The main brittle structures marking extension are shallow- and steeply dipping normal faults associated with subvertical extensional joints and veins. The earliest brittle structures are lowangle normal faults which commonly superimpose on, and reactivate, earlier (precursory) ductile shear bands, but newly formed low-angle normal faults could also be observed. Low-angle normal faults are cut by late steeply dipping normal faults. The inversion of fault slip data collected within, and away from, the main detachment zone shows that the direction of the minimum stress axis is strictly parallel to the NE-SW stretching lineation and that the maximum principal stress axis remained subvertical during the whole brittle evolution, in agreement with the subvertical attitude of veins throughout the island. The high angle of s1 to the main detachment suggests that the detachment was weak. This observation, together with the presence of a thick layer of cataclasites below the main detachment and the kinematic continuum from ductile to brittle, leads us to propose a kinematic model for the formation of the detachment. Boudinage at the crustal scale induces formation, near the brittle-ductile transition, of ductile shear zones near the edges of boudins. Shear zones are progressively exhumed and replaced by shallowdipping cataclastic shear zones when they reached the brittle field. Most of the displacement is achieved through cataclastic flow in the upper crust and only the last increment of strain gives rise to the formation of brittle faults. The formation of the low-angle brittle detachment is thus ''prepared'' by the ductile shear zone and the cataclasites and favored by the circulation of surface-derived fluids in the shear zone