HR: 1340h
AN: T23C-1533    [Abstracts]
TI: Structural and drainage pattern evolution of the Alborz Mountains (N Iran) inferred from provenance data and magnetostratigraphy
AU: * Ballato, P
EM: ballato@geo.uni-potsdam.de
AF: University of Potsdam Institute of Geosciences, K. Liebknecht Strasse 24, Potsdam, 14476, Germany
AU: Landgraf, A
EM: landgraf@geo.uni-potsdam.de
AF: University of Potsdam Institute of Geosciences, K. Liebknecht Strasse 24, Potsdam, 14476, Germany
AU: Strecker, M R
EM: strecker@geo.uni-potsdam.de
AF: University of Potsdam Institute of Geosciences, K. Liebknecht Strasse 24, Potsdam, 14476, Germany
AU: Friedrich, A
EM: friedrich@iaag.geo.uni-muenchen.de
AF: LMU Munich, Luisen Strasse 37, Munich, 80333, Germany
AU: Tabatabaei, S
EM: htabatabaei@bhrc.ac.ir
AF: Building and Housing Research Center, Sheikh Fazlollah Exp. Way, Tehran, 13145-1696, Iran (Islamic Republic of)
AB: The Alborz Mts, N Iran, constitute an active, E-W-oriented, double verging orogen, within the continental collision zone between Arabia and Eurasia. GPS data show that currently convergence is partitioned in the Alborz by 6 mmyr-1 of NNE directed shortening and 4 mmyr-1 of left-lateral motion. The growth of the Alborz was associated with the development of adjacent foreland basins. Here, we discuss the evolution of the southern Alborz and its foreland basin. We present a sandstone and conglomerate provenance study, combined with sedimentary facies analysis, and magnetostratigraphy of a 7200-m-thick sedimentary section in the foreland basin, previously thought to be Mio- Pleistocene in age. This section includes three first-order coarsening and thickening upward units and documents a gradual transition from restricted marine to lacustrine, braided river (U1 and U2), and alluvial fan depositional environments (U3). These units were deposited between 17.5 and 7.8 Ma. Assuming a constant accumulation rate for the alluvial-fan conglomerates of U3, the top of the section is approximately 6 m.y. old. The derived accumulation rates correlate with the cyclicity of the recognized units: at the bottom of each cycle in association with the fine-grained facies, the accumulation rates are faster, while with the appearance of coarse- grained facies the accumulation rates decrease. Fifty sandstone samples were analyzed using the Gazzi-Dickinson sandstone provenance method. The detrital modes of lithic grains indicate at least two first-order variations in the source area. The first major change occurred at the bottom of U1 and is characterised by an increase of low-grade metamorphic clasts and a concomitant decrease in volcanoclastic grains. The second major change is manifested at the base of U3 with a progressive decrease in low-grade metamorphic clasts and the increase of carbonates and volcanic lithics. The analysis of conglomerate clasts shows a relatively uniform composition with dominant volcanoclastic pebbles up to U3, where an important change occurs with the appearance of low-grade metamorphic clasts, carbonates, and intrabasinal sandstones. Faster accumulation rates combined with fine-grained facies and variations in sandstone petrography suggest tectonically controlled flexural loading associated with erosional unroofing. The composition of conglomerate clasts indicates the southward propagation of the Alborz deformation front coupled with the erosion of an adjacent source and the onset of basin inversion and clast recycling. In addition, our data suggest that the locus of tectonic activity and unroofing shifted forward and backward and/or that the shortening rate varied through time. Interestingly, the accumulation rates available for northern foreland basin in the realm of the South Caspian Basin, do not record any important changes earlier than Pliocene time. This suggests that either the flexural load was focused on the southern foreland rather than the Caspian side during the Miocene or that most of the sediments eroded from the Alborz were delivered to the southern sectors before the Pliocene. In any case, important drainage pattern reorganization must have taken place before the Pliocene. This reorganization appears to be coeval with of the onset of pronounced exhumation at 7-6 Ma.
DE: 1520 Magnetostratigraphy
DE: 8108 Continental tectonics: compressional
DE: 8169 Sedimentary basin processes
DE: 9604 Cenozoic
SC: Tectonophysics [T]
MN: 2007 Fall Meeting