Younging Of Ages Research Articles

Progradation of the Alpidic Central Western Carpathians orogenic wedge related to two subductions: constrained by 40Ar/39Ar ages of white micas

To reveal a model of the polystage evolution of an orogenic wedge subsequent to two subductions, 40Ar/39Ar dating of white micas (Wmca) across the shear zones in the Central Western Carpathians (CWC) has been carried out. This wedge is limited by the two suture zones, as a remnant after subducted oceanic crust of the Neotethyan Meliatic (in the S) and South-Penninic (in the N) oceans. It is composed of a system of basement and cover nappes, separated by major shear zones. Wmca aggregates from different microstructural and grain-size domains of lower- to medium-grade blastomylonites were dated by the 40Ar/39Ar method. The minimum age of NW-ward thrusting of the Gemeric and Meliatic nappes over the South-Veporic nappes in the southern part of this wedge was detected around 100 Ma. The NE-SW trending sinistral transpression, combined with a top-to-the SE extensional exhumation was dated at 90 – 85 Ma in the South-Veporic unit. Collision continued at ca. 95-90 Ma in the central part of the North-Veporic unit, while in the frontal part and in the Tatric and Infratatric units it was identified at 80 – 70 Ma. Lateral extrusion of the Tatric unit towards the NE occurred in Early Eocene, at ca. 50 Ma, indicating a final evolutionary stage of this wedge. The younging of ages from S to N revealed that footwall-propagating thrust-faults were responsible for the growth of the orogenic wedge.

Constraining the timing and migration of collisional tectonics in the Damara Belt, Namibia: U-Pb zircon ages for the syntectonic Salem-type Stinkbank granite

A U-Pb zircon LAM-ICP-MS age of 549 ± 11 Ma from the Stinkbank granite in the south Central Zone of the Damara belt represents the first robust age for the crystallization of a member of the regionally widespread suite of syntectonic Salem-type granites. This age confirms previous assertions that suggest that the main phase of crustal shortening related to the collision between the Congo and Kalahari Cratrons occurred between 550 and ~540 Ma in the south Central Zone, the leading edge of the Congo Craton. Ages from syntectonic granites reported in the literature from the north Central and the Northern Zone record progressively younger ages of ~530 to ~515 Ma and late-stage folding in the Northern Zone is suggested to have occurred between ~510 to ~500 Ma. The younging of ages that date the main collisional phase towards the north is interpreted to track the northward propagation of the deformation and metamorphic front from the main suture zone to the northern foreland of the Damara belt. Geochronological data are sparse, and the lack of detailed structural data precludes the retrodeformation of the fold-and-thrust belt along this 250 km long section. However, the age brackets suggest a migration of the orogenic front at a rate of at least 5 to 7 mm/a, but more likely in the order of several centimetre per year, if the internal strain is removed.