Chapter 2: Plate T-42

THE ZAGROS MOUNTAINS: ARABIAN/ASIAN COLLISION

Plate T-42	Map

The morphology of Iran and Pakistan north of the Straits of Hormuz spectacularly expresses the surficial traces of several plate boundaries. These boundaries are the tectonic effects of the collision of the Arabian platform with the Asian landmass to the north, or more accurately, with a microcontinent fragment of the Asian landmass known as the Central Iranian Microcontinent (CIM). The CIM block split away from the Asian continent and drifted slowly southward during the Triassic opening of the Neo-Tethys ocean. By the Jurassic, the Neo-Tethys began to close so that the CIM block was caught in a tectonic vise during the Cretaceous. At the same time the Arabian platform was suturing to the southern margin of the CIM block, the Asian continent was being sutured to the northern margin of the CIM block. The series of units making up the "colored mélange," delineated on the index map by dark patterns, are characteristic of interstructured groups of flysch and ophiolitic rocks associated with the northern margin of the suture zone between the CIM block and the Arabian platform. The mélange separates the "worm-like" anticlinal folds to the south from obviously complex geology of the CIM to the north (Stocklin, 1968, 1974; Roohi, 1976).

Figure T-42.1	Figure T-42.2

The Central Iranian Microcontinent is divided by the northerly striking Nayband fault (clearly visible in the mosaic) into two areas of distinctly different structure. The eastern, more stable platform area is called the Lut Massif; the western, highly faulted area is known as the Tabas Block. The Lut Massif has a relatively low degree of Cretaceous Alpine deformation. Most of the area is covered with Tertiary continental sediments and volcanic rocks with scattered outcrops of Mesozoic and Paleozoic rocks. In marked contrast, the Tabas block includes a succession of arcuate fault-bounded grabens with Precambrian basement exposed within the intervening horsts. The arcuate normal faulting that controls the horst and graben structures is convex to the west, with increasing curvature from east to west. The extreme western fault marks the curved western boundary of the CIM block (Dykstra and Birnie, 1979; and Stocklin, 1974).

Figure T-42.3	Figure T-42.4

The Nayband fault continues south and becomes an integral part of a broad structural discontinuity known as the "Oman Line" that extends northward from Oman across the Straits of Hormuz. This line divides the flysch-rich eugeosynclinal sediments of the Makran Ranges on the east from the miogeosynclinal shelf sediments of the Zagros Mountains to the west. To the west of the Oman Line, the Zagros Crush Zone marks the location of a continent/continent-style active margin where the Arabian platform collided, and continues to collide, with the CIM block. To the east, the active margin is a continent/ocean-style boundary with the oceanic lithosphere of the Indian Ocean being subducted beneath the CIM and other more easterly microcontinental blocks. The Oman Line can therefore be interpreted as marking the eastern boundary of the Arabian platform. The close association of the Nayband fault and the Oman Line reflects the marked structural contrast across the Nayband fault. This probably indicates that, west of the fault, the CIM block is caught in a highly compressive continent/continent-style collision, whereas to the east, the Lut Massif section of the CIM block is affected by the smaller compressive forces associated with the subduction along a continent/ocean margin. The continued subduction of oceanic lithosphere east of the Oman Line has led to the flysch-rich accretionary prism comprising the Makran Range of southern Baluchistan (Glennie, 1974; Glennie et al., 1973; Stocklin, 1974).

Figure T-42.5	Figure T-42.6

Part of the southern Zagros Mountains is shown in detail in a Landsat scene (Figure T-42.1) and again from an oblique perspective in a Gemini photograph (Figure T-42.2). These mountains consist predominantly of miogeosynclinal shelf sediments of Mesozoic and Lower Tertiary age, The sediments were deposited largely on the northern edge of the Arabian platform. During the Maastrichtian, after the beginning of the continent/continent collision, the Zagros and regions immediately to the north were uplifted. This, in association with the lateral forces generated by the continental collision, created southward thrusting and folding of the Zagros sediments (Dykstra and Birnie, 1979). The shortening has been estimated as approximately 50 km with a probable décollement surface along the Hormuz salt beds of Infracambrian age. Several salt diapirs have pierced the folded sediments and are clearly visible on the Landsat imagery as salt glaciers (circular masses of darker colored material). Figure T-42.3 zeros in on one of these diapirs.

Figure T-42.7	Figure T-42.8

Plunging anticlines in all stages of dissection are visible, some appearing as if layers were peeled off (Oberlander, 1965). Tilted resistant beds in the anticlines are eroded to form hogbacks, triangular flatirons, and infacing scarps. Synclinal basins, where eroded, have outfacing scarps. These folds have not reached the degree of dissection of the folded Appalachians. Figures T-42.4, Figure T-42.5, Figure T-42.6 and Figure T-42.7 offer striking views of anticlinal landforms in both the central and southern Zagros Mountains, as these are experiencing various stages of breaching. The structural style so evident in Figure T-42.1 is clearly reflected in the topography and drainage patterns. The V's where streams cross tilted beds form scalloped cuestas. Fans and braided rivers are abundant.

Figure T-42.9

In the central Zagros, the folds are closed, as evident in a Landsat RBV view (Figure T-42.8) of structures in the main petroleum fields between the head of the Persian Gulf and Esfiahan. The folds of the main belt in the northern Zagros Mountains (Figure T-42.9) are more tightly spaced and elongated, without obvious plunging, where this orogen approaches the Anatolian plate along the Bitlis Suture (Plate T-39). South of this belt are well-exposed but seemingly isolated plunging folds separated by valley fill that now covers other structures. (GCW: J. D. Dykstra) Landsat Mosaic.