Multi-stage mountain building vs. relative plate motions in the Betic Cordillera deduced from integrated microstructural and petrological analysis of porphyroblast inclusion trails

摘要

The tectonic evolution of the Betic Cordillera has been investigated through integrated microstructural and petrological analysis of 93 samples of garnetiferous Grt. ±. Cld. ±. Plg. ±. Ky. ±. St phengite schists from its lowermost allochthon, the Nevado-Filabride Domain. Porphyroblast inclusion trails in these samples exhibit well-developed preferred vertical and horizontal orientations and truncational relationships. These features indicate a complex history of multiple crenulation cleavage development during alternating contractional and gravity-induced deformations. Associated Foliation-Intersection Axes preserved in porphyroblasts (FIA) were determined separately for 2 groups of samples using the "asymmetry technique" (Bell et al., 1995) and the "FitPitch" method (Aerden, 2003), respectively. Broadly consistent results establish a succession of 4 FIA sets with specific trends, which from the oldest to the youngest FIA set are: NE-SW (FIA set 1), NW-SE (FIA set 2), W-E (FIA set 3) and NNW-SSE (FIA set 4). This trend sequence is also reflected in the orientations of successive fold generations and associated lineations. More remarkably, the FIA-trend sequence mimics known changes in the plate motions of Africa and Adria relative to Iberia in the 50-10. Ma interval. This is also the time span indicated by published radiometric ages. Garnet-isopleth thermobarometry applied to selected samples and compositional zoning of garnets indicate that the earliest 3 FIA sets formed along a prograde metamorphic path from ca. 5. kb/500. °C to 10. kb/550. °C, whereas the latest stages of FIA set 3 and FIA set 4 developed during exhumation. Exhumation involved a period of heterogeneous reheating that was spatially and temporally linked to emplacement of the overlying Alpujarride Complex.