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Late Cenozoic shortening in the west-central Alborz Mountains, northern Iran, by combined conjugate strike-slip and thin-skinned deformation

¹ Department of Earth and Space Sciences, University of California, Los Angeles, California 90095-1567, USA
² Department of Geology, University of Tehran, Tehran, Iran

View larger version (115K): [in this window] [in a new window]   Figure 1. Shaded relief map showing the Arabia-Eurasia collision zone. Iran's border is shown in black. Labels refer to major physiographic and tectonic units. ZMFF—Zagros Mountain front flexure. The box over the western Alborz Mountains shows the location and coverage of Figure 2.

View larger version (89K): [in this window] [in a new window]   Figure 2. Map showing regional geology of the western Alborz. Insets show Alborz structural zones. Dashed rectangles show locations of maps in later figures. BF—Banan fault; BFZ—Barir fault zone; BIF—Binaksar fault; CR—Chalk Rud fault; DHF—Doh Hezar fault (informal name); KF—Khashachal fault; KT—Kandavan thrust; LF—Lahijan fault; LPT—lower Parachan thrust; MFT—Mosha Fasham thrust; NAFS—north Alborz fault system; NQ—north Qazvin fault; NTT—North Tehran thrust; NF—Nusha fault; TGFZ—Tang-e-Galu fault zone; TT—Takieh thrust; TFZ—Taleghan fault zone; UPT—upper Parachan thrust; TB—Taleghan Basin; AB—Alamut basin.

View larger version (49K): [in this window] [in a new window]   Figure 3. Schematic tectonostratigraphy of the Alborz, modified from Allen et al. (2003) by including Cretaceous deformational event.

View larger version (62K): [in this window] [in a new window]   Figure 4. Map showing the portion of the northern zone that was examined in detail. Stereonet plots of structural data are tied to the locations where the data were collected. Letter and number labels indicate structures referred to in the text and in later figures.

View larger version (60K): [in this window] [in a new window]   Figure 5. A: Two stereonet plots of faults measured at map locations 1 and 2 on Figure 4. B: Stereonet plot of faults and tension fractures measured in SW Dalir Valley, shown in photograph at right. Photograph was taken near map locality 3 (Fig. 4) looking downdip from outcrop containing two fault surfaces (A, B) bounding numerous tension fractures (gray dashed line). Compass is for scale, with compass needle indicating north. Fault planes dip NE (n = 8). Solid black lines are faults which bound tension fractures (A, B). Solid gray lines are faults in the same outcrop. Dashed gray lines are tension fracture planes (n = 14). Thick black line (C) represents cylindrical best fit of tension fracture planes. The faults juxtapose Cambrian Lalun Formation with Ordovician Lashkerak Formation; however, the arrows indicate a sinistral and normal sense of motion in the hanging wall.

View larger version (59K): [in this window] [in a new window]   Figure 6. Map showing the thin-skinned zone and stereonet plots of structural data from this region. Blue arrow shows the location and viewing direction for photograph shown in Figure 7.

View larger version (62K): [in this window] [in a new window]   Figure 7. Photo with line drawing of Khowchireh anticline (blue arrow on Figure 6 shows location and viewing direction). Eocene tuffaceous clastic rocks (gray fill) are draped unconformably in Eocene normal fault–generated grabens. Faults cut Paleocene isoclinal folds in Cretaceous Tiz Kuh (Kls) limestone overlapped, locally, by Fajan conglomerate (Fc). To the south (left) Kahar Formation quartzites, Soltanieh dolomites (yellowish rocks), and carbonaceous Jurassic Shemshak Formation (Jsh, dark slope former) are juxtaposed with the younger units to the north across two strands of the sinistral-reverse Taleghan fault zone (TFZ; thrust and high-angle fault to north).

View larger version (66K): [in this window] [in a new window]   Figure 8. Map showing the Taleghan range and the southern folded zone and stereonet plots of structural data collected throughout this region. Blue arrow indicates the viewing direction and location of the region shown in Figure 9. Red and yellow arrows along the Purkan thrust show the locations of fault bends in this thrust that correspond to northeast-southwest–trending anticlines and synclines (fault-bend folds) in the upper plate. See Figures 4 and 6 for definitions.

View larger version (72K): [in this window] [in a new window]   Figure 9. Photo with lines for clarity of the Mosha fault linking via a right bend to the Taleghan fault zone. The ridge line (hanging wall) exposes subhorizontal Eocene Karaj Formation (Ek) tuffaceous clastics deposited in an Eocene half graben over Paleocene-Eocene topography developed on folded Cretaceous limestones (Kls) (entire package in hanging wall of the Mosha-Taleghan fault zone). Footwall is Late Proterozoic Kahar Formation. North-tilted Karaj Formation tuffaceous clastics overlain by small amounts of Neogene conglomerate lie beneath the Mosha fault to the south. Jsh—Jurassic Shemshak Formation; Pz—Paleozoic rocks.

View larger version (43K): [in this window] [in a new window]   Figure 10. Schematic fault map of the northern zone and thin-skinned belt showing the kinematic regimes that control the orientation and kinematics of the small and large structures in the region (see Figure 4 for an explanation of abbreviations). In the northern zone (dark-gray area), small sinistral conjugate faults intersect the major dextral faults at the outcrop scale. Offset markers (yellow faults and light-blue folds) for the Tang-e-Galu fault zone (TGFZ) indicate 25 km of dextral offset. In the thin-skinned belt, a microdiorite dike swarm intruded at ca. 6–9 Ma. The dikes are roughly orthogonal to thrusts and folds and locally intruded across, and were subsequently offset by, thrust faults (e.g., upper Parachan thrust [UPT]). This suggests that the dikes intruded parallel to the shortening direction during active contractional deformation.

View larger version (20K): [in this window] [in a new window]   Figure 11. Schematic fault map of the Taleghan range and southern folded zone showing the right-hand bend in the Mosha-Taleghan fault system and adjacent transpressional deformation.

View larger version (26K): [in this window] [in a new window]   Figure 12. A: Schematic fault map of the west-central Alborz showing the presence of a tectonic wedge (gray shaded region) bounded by the dextral Nusha, Banan, Tang-e-Galu, and Kandavan faults to the north and the Mosha-Taleghan fault system to the south. This is probably a conjugate fault system that developed in a sinistral-transpression kinematic regime. B: Geometric representations of this conjugate system. Top shows the angle between the conjugate faults before 12.5 km of internal shortening within the wedge accumulates (52°) and the present angle (41°). The 30 km estimated slip in the Mosha thrust is represented by offset, black half circles. Bottom shows a simplified geometric wedge extrusion model that predicts 17 km of range-perpendicular strike-slip shortening when the wedge is extruded to the west, such that 20 km of sinistral slip accumulates on the Mosha-Taleghan fault system and 27 km of slip accumulates on the dextral system to the north.

View larger version (73K): [in this window] [in a new window]   Plate 1.

View larger version (42K): [in this window] [in a new window]   Plate 2.

View larger version (113K): [in this window] [in a new window]   Plate 3.