BIARJMAND METAMORPHIC CORE COMPLEX: NEW EVIDENCE FOR LATE CRETACEOUS-PALEOCENE EXTENSIONAL TECTONICS ALONG THE NORTHERN MARGIN OF CENTRAL IRANIAN PLATEAU

The Biarjmand metamorphic core complex is a ~60 km long, NE-trending structure which lies in NE central Iran near the boundary with the eastern Alborz mountains ~140 km WSW of the city of Sabzevar. The complex consists of three structural units separated by two major low-angle normal faults. (1) A core consisting of amphibolite-grade gneisses, amphibolites, mica schists, dolomitic marbles, and quartzites derived from the Upper Proterozoic-Lower Cambrian Kahar and Soltanieh Formations, and Pan African granites with U-Pb zircon ages of 580-530 Ma. Penetrative mylonitic foliation, S-C and S-C' shear bands, and stretching lineations are well developed. Foliation planes specify a doubly plunging dome. Stretching lineations trend NE and are parallel to the long axis of the dome. Sheath folds with axes parallel to stretching lineations are also abundant. The intensity of mylonitization increases with depth. (2) A low-grade metamorphic carapace of Jurassic mylonitic volcaniclastic rocks and calcareous slates is separated from the core by a detachment fault dipping 15° to 20° N. 3) An unmetamorphosed cliff-forming cover section consisting of Jurassic-Cretaceous carbonates exhibits only brittle deformation. A second detachment fault separates units 2 and 3. All around the complex, clasts of the core rocks dominate Paleocene-Eocene conglomerates, which are in turn concordantly capped by 46 Ma (middle Eocene) trachybasalts.

40Ar/39Ar total-gas ages of 210 Ma (hornblende), 153 Ma (muscovite) and 135 Ma (biotite), along with a (U-Th)/He zircon age of 80 Ma suggest that the core cooled slowly from ~550 to 180°C. An 40Ar/39Ar age spectrum for K-feldspar is consistent with the above and shows that slow Mesozoic cooling continued until ~70 Ma, followed by abrupt rapid cooling.

Rapid extensional exhumation of the Biarjmand mylonites during Late Cretaceous-Paleocene time signifies the early stage of a propagating intraarc extensional system in the Cretaceous-Paleogene continental arc that once covered the central Iran-Alborz region as a single belt. This extension was likely related to closure of the Neotethyan ophiolitic backarc basin in Sabzevar region. Intraarc spreading slowly propagated westward and reached regions south of the central to western Alborz and Takab-Zanjan in the early Oligocene and early Miocene, respectively.