See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/236233858 Progressive unconformities within an evolving foreland fold–thrust belt, Zagros Mountains Article · November 2001 CITATIONS READS 17 2,483 5 authors, including: Hemin Koyi Christopher J Talbot Uppsala University Uppsala University 330 PUBLICATIONS 3,895 CITATIONS 172 PUBLICATIONS 4,039 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: South China Sea extension tectonics View project Kinematic evolution of deepwater fold-and-thrust belts: "gravity-driven" vs. "tectonic-driven". View project All content following this page was uploaded by Christopher J Talbot on 21 November 2014. The user has requested enhancement of the downloaded file. Journal of the Geological Society, London, Vol. 158, 2001, pp. 969–981. Printed in Great Britain. Progressive unconformities within an evolving foreland fold–thrust belt, Zagros Mountains KHALED HESSAMI 1 , HEMIN A. KOYI 1 , CHRISTOPHER J. TALBOT 1 , HADI TABASI 2 & ESMAEL SHABANIAN 2 1 Hans Ramberg Tectonic Laboratory, Department of Earth Sciences, Uppsala University, SE-752 36 Uppsala, Sweden (e-mail:
[email protected]) 2 International Institute of Earthquake Engineering and Seismology, PO Box 19395/3913, Tehran, Iran Abstract: A major angular unconformity between the Bakhtyari conglomerates and the underlying Agha Jari Formation has long been interpreted as indicating that orogeny in the Zagros Simply Folded Zone took place in Plio-Pleistocene times. This study uses field evidence of unconformities between older units in conjunction with geological maps and cross sections to argue that the front of the Zagros Simply Folded Zone has propagated in time and space. These unconformities indicate that deformation started as early as end Eocene in the northeast of the Simply Folded Zone and propagated progressively to the southwest, where unconformable contacts are only seen between younger units. As shortening continued, the southwest migration of the deformation front drove the foreland basin in front of it to its present position along the Persian Gulf and Mesopotamia. The climax of orogeny took place at end Pliocene time when the most extensive unconformity in the Zagros Simply Folded Zone developed between the (upper) Bakhtyari Formation and older units. Active seismicity and documented present uplift imply that the Simply Folded Zone is still propagating southwestward. Keywords: Zagros, Iran, orogeny, unconformities, folds. Different hypotheses for the mechanism and timing of the Assemblage, the Sanandaj–Sirjan Zone, and the Zagros Simply final closure of Neotethys and the beginning of orogeny in Folded Zone. the Zagros Mountains have long been suggested by many All these workers agree that the same Simply Folded Zone workers (e.g. Stöcklin 1968, 1974; Falcon 1969, 1974; Takin forms the most southwesterly deformation zone of the Zagros 1972; Haynes & McQuillan 1974; Kashfi 1976; Stoneley 1975, orogen (Fig. 1). The northeastern limit of the Simply Folded 1981; Berberian & King 1981; Alavi 1994). The following Zone is marked by the southwestern boundary of a narrow basic facts account for the variations and complications (70–90 km wide) imbricate zone of SW-verging overthrust among these hypotheses. The emplacement of ophiolites anticlines and thrusts (Fig. 1). There is no clear surface upon the northeastern margin of Arabia in Late Cretaceous boundary to the frontal edge of the Simply Folded Zone where (Campanian–Maastrichtian) prompted early workers (Takin folding is gentle both on land and beneath the Persian Gulf. 1972; Stöcklin 1974; Berberian & King 1981) to infer that However, the Zagros deformation front can be defined at Neotethys closed in Late Cretaceous. However, continued different levels by the shape in map view of the oil and gas deposition of marine sediments near the suture until the fields (Talbot & Alavi 1996; Fig. 1) and the seismicity and Early Miocene discredits this hypothesis (Falcon 1974; Koop topography (Jackson & McKenzie 1984). Despite its name, the & Stoneley 1982). In addition, by demonstrating an oceanic Simply Folded Zone involves thrusts in both the sedimentary melange of Maastrichtian to early Tertiary age along part cover and its underlying basement (Ameen 1991; Talbot & of the Zagros suture zone, Stoneley (1981) proved that at Alavi 1996). The Zagros Simply Folded Zone can be distin- least part of Neotethys remained open until Early Miocene guished from the rest of the orogen by NW–SE-trending time. parallel folds and thrust-cored anticlines which verge to the Several major zones across the Zagros orogenic belt (Fig. 1) SW in a 6–12 km thick cover sequence (Falcon 1969, 1974; record several discrete orogenic episodes since Cretaceous time Colman-Sadd 1978; Talbot & Alavi 1996). Dividing the Zagros (Falcon 1969, 1974; Haynes & McQuillan 1974; Koop & Simply Folded Zone into two distinct along-strike segments, Stoneley 1982; Alavi 1994) and previous workers have sug- Talbot & Alavi (1996) pointed out detailed differences in gested several different sub-divisions. Stöcklin (1968, 1974) deformation style along the Simply Folded Zone. This paper is defined the Main Zagros Thrust (Fig. 1) as the suture between not concerned with the timing of the Zagros orogeny NE of the Arabia and Central Iran and the northeast limit of the Zagros Simply Folded Zone, where Cretaceous movements are con- belt. Falcon (1969, 1974) divided the Zagros orogen into three sidered to be the most significant (Falcon 1969, 1974; Koop & zones: the thrust zone, the imbricate zone and the Simply Stoneley 1982; Alavi 1994). Instead, we focus on the timing of Folded Zone. However, Haynes & McQuillan (1974) consid- deformation in the ‘Simply Folded Zone’. ered the Sanandaj–Sirjan zone (Fig. 1) along the southeastern A major, regional angular unconformity between the Agha margin of Central Iran (Stöcklin 1968) as a segment of the Jari and Bakhtyari formations is generally considered to have Zagros orogen, and sub-divided Falcon’s imbricate zone into marked the Late Pliocene climax of orogeny in the Simply two zones. Most recently Alavi (1994) divided the Zagros Folded Zone (James & Wynd 1965; Haynes & McQuillan orogeny into three zones: the Urumieh–Dokhtar Magmatic 1974; Kashfi 1976). Nevertheless, syn-orogenic clastic rocks of 969 970 K. HESSAMI ET AL. o o o o o o o 48 50 52 32 40 48 56 M C entral 38 o S 34 o Iran a n a a Z n 34 o a M C entral d i a g Iran n j- r Z S o ir 30 o T F ig. 6 Z ja s D a n 26 o Arabian 32 o g Z P latform r Z o M a B o n 22 o e s F or a s g e s Africa ro op e l in Im ot a n s T 18 o am d B S b im B ri h ia c h n a o p F ig. 5 te 14 rr uu o 30 ly K aze run F . S Z on ss F ig. 3 e tt a F F ig. 4 b o ld P o ed c Fig. 1. Location map, SW Iran. Legend: 28 e a, anticline axes; b, strike-slip faults with e A d Zone r L in Za sense of motion; c, southwestern edge of s Arabian gr e the Zagros deformation front; d, reverse i os P latform F ro a nt faults; e, Zagros major zone boundaries. n of it Initials indicate major cities as follows: H ormuz ra 26 o G St B, Behbahan; D, Dezful; S, Shiraz. u l Localities illustrated in later figures are an f 0 100 200 indicated. The shaded area indicates the m km O O ma n S ea Zagros Simply Folded Zone. A and B o o o o o o locate the geological cross sections in 48 50 52 54 56 58 Figure 8. the Agha Jari Formation overlying the Early–Mid-Miocene The Cenozoic stratigraphic column of the Zagros Simply shallow marine sediments of the Gachsaran and Mishan Folded Zone starts with a marine transgression at the begin- formations have been related to earlier movements before this ning of Palaeocene. This transgression led to the accumu- major unconformity (Mina et al. 1967; Falcon 1969, 1974; lation of pelagic marl, shales and limestones of the Pabdeh Colman-Sadd 1978; Stoneley 1981; Koop & Stoneley 1982; Formation along the main troughs (Fig. 2). Contemporaneous Motiei 1993). Several other interruptions within the Cenozoic massive dolomite of the Jahrom Formation accumulated on stratigraphy of the Zagros region have long been attributed to shallow ridges as well as the northeastern part of the Simply epirogeny (Henson 1951; James & Wynd 1965; Stöcklin 1968; Folded Zone until the end of Eocene. A regional regression Falcon 1974; Koop & Stoneley 1982). Here, they are shown to then exposed the whole area to subaerial weathering (James & involve onlaps, offlaps, and angular unconformities which Wynd 1965; Mina et al. 1967; Koop & Stoneley 1982; Motiei record the propagation to the SW of a series of anticlines 1993). In the SW of the Zagros basin (NE Arabia), clastic marking the Zagros deformation front since the end Eocene. sediments accumulated after the end Eocene (Mina Previous field and model studies of the architectures of et al. 1967; Motiei 1993). A second marine transgressive cycle syntectonic sediments beside growth folds in exposure, seismic developed early in Oligocene time. This began with deposition and well data in the Appenines (e.g. Torrente & Kligfield of the shallow-marine limestone of the Asmari Formation 1995), Pyrenees (e.g. Poblet et al. 1997) and elsewhere (e.g., until the Early Miocene. In the northeastern part of the basin, Novoa et al. 2000) have focused on the mechanisms of the uppermost part of the Asmari Formation consists of a individual folds and related tectonosedimentary structures. sequence of conglomerates with clasts derived from the Asmari Here we begin documentation of a suite of progressive uncon- limestone itself (Motiei 1993). formities throughout a fold–thrust belt that allow constraint of The litho-facies of the Gachsaran (and its time-equivalent, the rate that its front propagated in time and space. the Razak Formation), Mishan and Agha Jari formations (James & Wynd 1965), represent a general regressive cycle that began in a NW–SE-trending basin in the Early Miocene Cenozoic stratigraphy (Fig. 2). The Early Miocene Gachsaran Formation is com- posed of evaporites (salt and anhydrite), marl and limestone. At the beginning of the Cenozoic, the area of deposition in Towards the NNE of the Gachsaran basin, the Razak northeast Arabia was complicated by NW–SE-trending marine Formation consists of littoral clastics (mainly red, green and troughs which were partially separated by linear NW–SE grey marls and sandstones) indicating a change in depositional ridges (Henson 1951; James & Wynd 1965; Koop & Stoneley environment during the Early Miocene. Conglomerates are 1982; Motiei 1993). These troughs were in turn separated reported at both the base and near the top of the Razak from what remained of Neotethys by a NW–SE-trending high Formation (e.g., Motiei 1993). The Mishan Formation is a which later formed the Zagros imbricate zone (see Stoneley shallow-marine grey marl with intercalations of clayey lime- 1981). stone indicating a relatively weak marine transgression across UNCONFORMITIES IN FOLD–THRUST BELT 971 NE SW AGE OROGENIC (Ma) TIME EPOCH LITHOLOGY PHASES Holocene 0.01 Late Pleistocene 0.76 Pasadenan Early Pleistocene U.Ba Bakhtyari Fm., conglomerate/ 1.75 Wallachian Lahbari Fm., siltstone Late L.Ba La and sandstone Pliocene 3.4 Early Pliocene 5.3 Agha Jari Fm., Late marl and sandstone Miocene 11 Late Styrian Middle Mishan Fm., Miocene marl and limestone 15.8 Razak Fm., marl and sandstone/ Ga Gachsaran Fm., evaporite, Early Ra marl and limestone Miocene 20.3 Main Himalayan Asmari Fm., limestone Fig. 2. Generalized stratigraphic divisions Oligocene and times of orogenic movements in the 33.7 Pyrenean? Eocene Jahrom Fm., dolomite/ Zagros Simply Folded Zone. Based on: 53 Ja Pa Pabdeh Fm., shale, marl James & Wynd (1965), Pedrami (1987) 65 Paleocene and limestone and Motiei (1993). the Zagros basin during Mid-Miocene. The Agha Jari similar to Busk & Mayo’s upper Bakhtyari and synonymous Formation consists of alternations of red marl and sandstone with the conventional Bakhtyari Formation defined by James of Late Miocene to Pliocene age which were deposited in both & Wynd (1965). Pedrami (1987) named the equivalent shallow-marine and estuarine environments, with the latter fine-grained estuarine sandstones and siltstones the Lahbari being dominant (James & Wynd 1965; Mina et al. 1967; Formation and divided it into two parts; the Upper Pliocene Pedrami 1987; Motiei 1993). The upper fourth of the Agha Jari and the Lower Pleistocene. Towards the NE, both the (lower) Formation consists of siltstone, marl and sandstone of the Lahbari and (lower) Bakhtyari formations tend to correlate Lahbari member and has a disputable age, but is generally with the upper part of the Agha Jari Formation (James & considered to be Pliocene and younger (James & Wynd 1965; Wynd 1965; Pedrami 1987; Motiei 1993; Fig. 2). Pedrami 1987). The age of the coarse conglomerates of the overlying Bakhtyari Formation is the clue to the timing of the most Observations vigorous phase of orogeny in the Zagros Simply Folded Zone. Pilgrim (1908 in James & Wynd 1965) was the first to refer to Field evidence the sequence rich in conglomerates overlying the older for- This section documents for the first time some progressive mations both conformably and unconformably, as the unconformities and the architectures of their growth sediments Bakhtyari Formation. Later, Busk & Mayo (1918) divided the that we have identified in the Cenozoic stratigraphy within the Bakhtyari Formation into two parts: the lower Bakhtyari, in Zagros Simply Folded Zone. We then discuss the significance which sandstone and shales are dominant, and the upper of these relationships with regard to the timing and mechanism Bakhtyari, which consists mainly of massive conglomerates. of deformation. However, James & Wynd (1965) considered only the upper Bakhtyari as representative of the entire Bakhtyari Formation The Pabdeh–Asmari contact. A relatively slight (c. 15) angular and classified the lower Bakhtyari as the Lahbari member of unconformity of end Eocene–early Oligocene age occurs the Agha Jari Formation. No fossils have been found in the between the Pabdeh and Asmari formations about 75 km west Bakhtyari Formation. but it is generally considered to be Late of Shiraz along the northeastern part of the Zagros Simply Pliocene or younger because it overlies the fossiliferous Agha Folded Zone (Fig. 3a & b). Tectonic movement locally folded Jari Formation of Late Miocene–Pliocene age (James & Wynd and uplifted the Pabdeh Formation exposing it to erosion 1965; Mina et al. 1967; Falcon 1974; Motiei 1993). Pedrami along the crests of anticlines before this unconformity was (1987) used lithologies, geomorphic levels, and unconformities buried beneath the Asmari Limestone. However, to the NE of to divide a sequence of fan deposits about 1000 m thick into this unconformity, the shelf deposits of the Jahrom Formation two halves (similar to Busk & Mayo’s 1918) in the north of (time-equivalent of the Pabdeh Formation) do not show this Dezful (Figs 1 & 2). The lower half formed during the Late relationship. Instead, they have been exposed to subaerial Pliocene and is mainly composed of medium to fine grain weathering (James & Wynd 1965; Mina et al. 1967; Motiei clastic sediments above a 7 m thick basal conglomerate. The 1993; see also, National Iranian Oil Company 1977) ever since upper half is mainly conglomerate deposited during the Early the onset of this tectonic movement. We interpret the uncon- Pleistocene. The upper half of the Bakhtyari Formation is formity between the Pabdeh and Asmari formations and its 972 K. HESSAMI ET AL. The Mishan–Agha Jari contact. A rather pronounced (about 20–25) angular unconformity between the Mishan and Agha Jari formations can be seen some 55 km to the SE of Behbahan (Fig. 5a & b). This unconformity can be identified in the field much more easily than the two previous unconformities. At this locality, the upper layers of the Mishan Formation are truncated below the unconformable contact at the base of the overlying Agha Jari Formation (Fig. 5c). Similarly, divergence of the Agha Jari bedding planes also demonstrates a slight offlap indicating syndepositional growth of this anticline (Fig. 5c). Towards the WSW of this section, the Agha Jari is unconformably overlain by (upper) Bakhtyari conglomerates. A major angular unconformity of 20 cutting out the Mishan Formation some 100 km west of Shiraz has been related to movement of the Kamarij salt plug on the Kazerun Fault line (Kent 1979). However, since folding in the locality mentioned above (Fig. 5a–c) is not related to salt movement we interpret this unconformity as indicating that folding had reached this area during the Mid–Late Miocene and that its arrival triggered diapirism of a former salt pillow (Talbot & Alavi 1996). As mari Limes tone The Agha Jari–Bakhtyari contact. The most obvious, vigorous and widespread phase of folding is recorded by the only angular unconformity already documented well in the Simply Folded Zone. This unconformity, which developed during the end of deposition of the Agha Jari Formation, records the main phase of the Zagros orogeny at the end Pliocene. The unconformity between the Agha Jari and (upper) Bakhtyari formations can be seen clearly some 40 km north of Dezful io n (Fig. 6a & b). As the angle of unconformity increases (from 0 ma t F or a bd eh in the south to nearly 80 at the northernmost end of this P section) the thickness of the Bakhtyari Formation decreases suggesting that lower layers of the Bakhtyari were not deposited further north (Fig. 6c). Towards the north of Fig. 3. (a) Photograph and (b) line drawing showing the angular Figure 6b, the upper layers of the Bakhtyari overlap the Agha unconformity between the Pabdeh (Paleocene–Eocene) shales and Jari, while in the south the lower Bakhtyari offlaps the Agha Asmari (Oligocene–Miocene) limestone, some 75 km west of Shiraz. Jari Formation (Fig. 6c). Such relationships imply that the The height of the cliff is about 100 m (looking SW, see Fig. 1 for location). northern part of the section rose above depositional level while deposition continued in the south. weathering as indicating that this phase of orogeny began with a few individual folds and uplifted the area to the NE at the end of the Eocene. The migration of frontal folds and sedimentation The geometries of progressive unconformities and growth The Asmari–Razak contact. In many places within the Simply deposits associated with growing detachment and fault- Folded Zone of the Zagros (e.g. 120 km west and 5 km east of associated folds have been shown to depend on various Shiraz), the Razak Formation onlaps the Asmari Formation. factors, including fold kinematics (e.g. hinge migration, limb One of the most obvious exposures of this onlap crops out rotation and/or lengthening) and how the syntectonic sedi- some 100 km south of Shiraz (Fig. 4a & b). In this locality, the ments deform, but mainly the relative rates of synsedimentary top 20 m of the Asmari Limestone exhibits palaeo-weathering uplift and syntectonic sedimentation (Poblet et al. 1997; Ford along joints suggesting subaerial exposure of the Asmari near et al. 1997). Syn-growth facies architectures over the crests of the crest of an anticline. A monomict conglomerate with clasts anticlines are key localities, but are rarely preserved in the composed only of the Asmari Limestone occurs at the top of Zagros. In general, offlap indicates tectonic uplift faster than the Asmari Limestone 5 km east of Shiraz. Similar conglom- syntectonic sedimentation, onlap indicates sedimentation erates are reported at the same stratigraphic level in the NW faster than uplift and overlap indicates sedimentation much Zagros of Iraq (Motiei 1993). Such onlaps and conglomerates faster than tectonic uplift. Growth strata parallel to pre- are due to gentle movements at the end of Asmari times growth strata indicate tectonic uplift and sedimentation at (Burdigalian). They indicate that particular anticlines capped similar rates. by the Asmari Formation reached wave-base or even subaerial Each of the unconformities documented in the Zagros conditions in these areas. Hence, present outcrops of the Fimply Folded Zone represents an area in which specific Asmari Formation in these areas of the Simply Folded Zone stratigraphic units were uplifted by deformation at particular record local erosion of highs uplifted by folding so that time intervals. The uplifted fold belt episodically widened by younger sediments were deposited only along syncline axes. the addition of a few active folds at a time. The front of the UNCONFORMITIES IN FOLD–THRUST BELT 973 Razak Shale As s m ari Lim est Razak Shales on e Asm ari Raza Lim k Sh ales est on e Fig. 4. (a) Photograph and (b) line drawing showing the Razak Formation (marls and sandstones) onlapping the Asmari Limestone some 100 km south of Shiraz. The height of the cliff is about 200 m (looking WSW, see Fig. 1 for location). Zagros Simply Folded Zone therefore underwent progressive migrating deformation front reached this area in the Mid–Late advance, as described below. Miocene after deposition of the Gachsaran and Mishan Outcrops of the Palaeocene–Eocene Jahrom Formation are formations (Figs 7III, 8a & b). The Mid–Late Miocene phase limited to the northeast rim of the Simply Folded Zone where of folding documented here was coeval with the change from they were uplifted by folding in end Eocene (Figs 7I & 8a, also shallow-marine sediments of the Mishan Formation to the see, geological maps of the Zagros region, e.g. National estuarine sediments of the Agha Jari Formation along a Iranian Oil Company 1975a, 1977). This uplifted area has been narrow foredeep that had migrated SW as well as along the subjected to subaerial weathering since the end of the Eocene synclinal troughs across the rest of the Simply Folded Zone in (James & Wynd 1965; Mina et al. 1967; Motiei 1993). Con- the NE. The migration of this deformation front forced local temporaneous with the end Eocene phase of folding, the southwestward regression (see Fig. 5c and Gawthorpe et al. shallow-water shelf carbonates of the Oligo-Miocene Asmari 2000) and resulted in offlap seen in the Agha Jari Formation Formation accumulated in the trough, which had been driven across the synclinals developing in the NE part of the Late to the SW by the serial migration of frontal folds (Fig. 7I). Miocene trough. Outcrops of the Asmari Formation and older units along a The most dramatic phase of the Zagros orogeny in the narrow strip in the Simply Folded Zone record local erosion of Simply Folded Zone was when estuarine sediments of the highs uplifted by folding that reached this area after deposition Agha Jari (and lower Bakhtyari) gave way to the massive of the Asmari Limestone in the Early Miocene (Figs 7II, 8a conglomerates of the (upper) Bakhtyari which were then & b). The growth of structures along this strip was coeval with folded across whole of the Simply Folded Zone at the end deposition of the shallow-marine sediments of the Early–Mid- Pliocene (Figs 7IV, 8a & b). Figures 7IV and 8 highlight the Miocene Gachsaran and Mishan formations in the marine area which was added to the Simply Folded Zone by the end trough that had migrated to the SW. Loading by the Razak Pliocene phase of deformation which folded the Agha Jari as Formation aided the rise (Nalpas et al. 1999) of anticlines of well as the (lower) Bakhtyari formations. As this orogenic Asmari limestone which forced local regression (Gawthorpe phase developed, the main trough of deposition moved et al. 2000) of the Gachsaran basin southwestward and towards the SW, to the NE part of the present-day Persian resulted in the Razak Formation onlapping the Asmari Gulf (Fig. 7IV). Post-end Pliocene conglomerates of the Limestone along the deformation front. The Early–Mid- (upper) Bakhtyari Formation overlie the older for- Miocene marine trough, in turn, was uplifted when the mations, conformably in front of the deformation front and 974 K. HESSAMI ET AL. Agha J ari F ormation G achs aran F ormation Mis han F ormation ~ 200 m W- S W E- N E Bakhtyari Fm. Gachsaran Fm. Agha Jari Fm. (overturned) Recent Alluvium Mishan Fm. F ~ 500 m Fig. 5a Fig. 5. (a) Photograph, (b) line drawing and (c) field-sketch showing the angular unconformity between the Mishan (marl and limestone) and Agha Jari (red marl and sandstone) formations observed some 55 km SE of Behbahan (looking west, see Fig. 1 for location). unconformably over the rest of the belt to the NE. The (upper) (Fig. 7V). The present foreland basin is located along the Bakhtyari Formation was folded right across the zone (Busk & Mesopotamian foredeep and the Persian Gulf. Figure 9a Mayo 1918; Falcon 1974; Stoneley 1981) during the mid- summarizes the increments of growth by progressive folding of Pleistocene orogenic phase (Pedrami 1987) and the grey area the Simply Folded Zone as it widened from the end of the in the SW was added to the Zagros Simply Folded Zone Eocene to the present. Upper B akhtyari C onglomerates U. B akhtyari C onglomerates UNCONFORMITIES IN FOLD–THRUST BELT Agha J ari F ormation Fig. 6a S N Upper Bakhtyari Fm. Lower Bakhtyari Fm. Fig. 6. (a) Photograph, (b) line drawing and (c) field-sketch showing the offlap structure in the Agha Jari (marl and sandstone) and the lower Bakhtyari conglomerates some 40 km in the north Agha Jari Fm. of Dezful (looking WSW, see Fig. 1 for location). The upper Bakhtyari overlaps both the lower Bakhtyari and the Agha ~ 2 Km Jari formations. The lower Bakhtyari of about 500 m thick and also major part of the upper Bakhtyari are missing in the north (partly based on Busk & Mayo 975 1918; Pedrami 1987). 976 K. HESSAMI ET AL. I E nd E ocene II E arly Miocene 48 o 50 o 52 o 32 o 40 o 48 o 56 o 48 o 50 o 52 o 32 o 40 o 48 o 56 o M C entral 38 o M C entral 38 o S S 34 o Iran 34 o Iran a a n n a G a a a Z Z n 34 o a M a n 34 o a M C entral d C entral d i i a g c a g Z Iran T h Z Iran n n j- r ro s j- r S 30 o o a S 30 o o ir T u ra ir T O Z ja s g h n Z ja s li a n 26 o Arabian a n 26 o Arabian 32 o g g Z P latform 32 o g Z P latform o r o r o -M o n e 22 o G o n e 22 o s Africa s Africa io a Im Im c o o c b T 18 b T 18 h S K azerun F . ri ri e s c im c a h h n a te o p a te o ra o 14 o 14 30 r u 30 r u ly K azerun F . e Z Z n on on e s e s Za t a t a gro A s sF b b m a ro n a n P P c Za c d 28 o ri 28 o t gr o e e e e d sF d M T r is r o nt r r L in L in o u g ha s s Arabian h e Arabian e n T i i P latform P latform rough a a it of n n of tra it H ormuz ra H ormuz S 26 o G 26 o G St u l u l an an f f 0 100 200 0 100 200 m m km km O O O ma n S ea O ma n S ea 48 o 50 o 52 o 54 o 56 o 58 o 48 o 50 o 52 o 54 o 56 o 58 o III Middle-Late Miocene IV E nd P liocene o o 48 o 50 o 52 o 32 o 40 o 48 56 o 48 o 50 o 52 o 32 o 40 o 48 56 o M C entral 38 o M C entral 38 o S 34 o 34 o S Iran Iran a n a a n a a a Z Z n 34 o a M n 34 o a M d C entral d C entral i i a g a g Z Iran n n j- r Z Iran j- r S 30 o o S 30 o o ir T ir T Z ja s Z ja s a n Arabian a n Arabian 26 o 26 o 32 o g Z P latform 32 o g Z P latform r o r o o n e 22 o o n e 22 o s Africa s Africa Im Im o b T 18 b T 18 o ri ri c c h h a o a te 14 te 14 o K azerun F . 30 o 30 o K azerun F . r u Z Z r u on on e s e s t a t a b b P P c c 28 o 28 o e e e e Za d d r r L in L in gr os s s e Arabian Za e F ro gr i i Arabian nt P latform os a a F ro P latform of nt of n n it it H ormuz H ormuz ra ra 26 o G St 26 o G St u l u l an an f f 0 100 200 0 100 200 m m km km O O O ma n S ea O ma n S ea 48 o 50 o 52 o 54 o 56 o 58 o 48 o 50 o 52 o 54 o 56 o 58 o V Mid-P leis tocene 48 o 50 o 52 o 32 o 40 o 48 o 56 o M C entral 38 o S o Iran a 34 n a a Z n 34 o a M d C entral i a g Z Iran n j- r S 30 o o ir T Z ja s a n Arabian 26 o 32 o g Z P latform r o o n e 22 o s Africa Im b T 18 o ri c Fig. 7. Diagram showing the southwest migration of a few h a te 14 o K azerun F . o 30 r u Z on individual folds along the Zagros front during Cenozoic s e t a times. The deformation front of the Zagros Simply Folded b Zone has been driving the foreland basin in front of it as it c has propagated episodically to the SW since the end of the P o 28 e e d Eocene. The strips shaded grey in I-V are the areas uplifted L in r Arabian e s by folding in the different labelled episodes. (I) end Eocene, i P latform a of (II) Early Miocene, (III) Mid–Late Miocene, (IV) end n Zag t H ormuz ros a n tra i 26 o F ron G t Pliocene, (V) mid-Pleistocene. Legend: a, anticline axes; b, S u l f 0 100 200 strike-slip faults with sense of motion; c, southwestern edge of m km O O ma n S ea the Zagros deformation front; d, reverse faults; e, major zone o o o o o o 48 50 52 54 56 58 boundaries of the Zagros orogen. UNCONFORMITIES IN FOLD–THRUST BELT Fig. 8. Geological cross sections across the Zagros. (a) SE of the Kazerun Fault (section A on Fig. 1), (b) NW of the Kazerun Fault (section B on Fig. 1). Shading indicates age of folds. The youngest marine sediments in strip ‘a’ are Eocene and were uplifted by folding in end Eocene. In strip ‘b’ the Asmari limestone is the youngest marine deposit was uplifted in Early Miocene. The Mishan Formation is the youngest marine sediment folded in strip ‘c’ which was uplifted by folding in Mid–Late Miocene. While, littoral clastics of the Razak in strip ‘c’ were restricted to the synclines. The strip ‘d’ was folded by the end Pliocene folding. Formations names are abbreviated as follows: Ep, Pabdeh Fm; Ej, Jahrom Fm; ICh, Hormuz salt; Kx, Cretaceous sediments in general; Mr, Razak Fm; Mgs, Gachsaran Fm; Mm, Mishan Fm; Ma, Agha Jari Fm; OMa, Asmari Fm; Plb, Bakhtyari Fm; Qal, Quaternary alluvium (modified after National Iranian oil company 1975b, 1976). 977 978 K. HESSAMI ET AL. Fig. 9. (a) Summary of the increments of growth by progressive folds of the Simply Folded Zone as it widened from the end Eocene to the present. Shading lightens successively through strips of frontal folds that become younger southwestward. Key as in Figure 8 with addition of ‘e’. Area ‘e’ is the southwestern most part of the Zagros deformation zone which is speculated to have folded in the mid-Pleistocene. The entire Simply Folded Zone folded simultaneously as it widened progressively towards the SW. Based on Geological maps and cross sections of the Zagros (National Iranian Oil Company 1975a, b, 1976, 1977). (b) Cumulative average propagation rate of the deformation front of the Simply Folded Zone. (c) Average propagation rate of individual phases. Solid dots and small circles represent the propagation rates for the SE and the NW of the Kazerun Fault, respectively. Why are the unconformities not due to local older (e.g. the end Eocene) angular unconformities are deformations? restricted to the northeastern part of the Simply Folded Zone, the younger unconformities cross the entire zone. Progressive unconformities associated with syntectonic sedi- ments offlapping the distal flanks of rapidly rising frontal folds are localized to the Zagros deformation front, but extend for long distances along it. Whether these folds overlie piggy-back Propagation rate of the deformation front thrusts is not yet clear. However, they are not the local A zone of a few active folds has formed along a deformation unconformities associated with the extrusion of the 120 or so front migrating progressively to the southwest since the end of diapirs of Hormuz salt east of the Kazerun line. Halokinetic the Eocene. Dividing the width of the Zagros Simply Folded unconformities associated with salt extrusion (Giles et al. 1999) Zone by the time since deformation began; i.e. end Eocene are easy to recognize in the Zagros for they are characterized (33.7 Ma ago), constrains the average rate of propagation of by spreads of distinctive red to black insoluble Hormuz the deformation front to about 6 mm a−1 NW of the Kazerun material interbeded with the surrounding honey coloured Fault and 9 mm a−1 SE the Kazerun Fault. These long-term carbonates within a few km of individual diapirs (e.g. Kent average rates of migration of the deformation front are three 1958, 1979). Extrusions of salt recognized in the easily distin- to five times slower than the rate of lateral shortening of guished local halokinetic unconformities are much more 29 mm a−1 within the restricted zone of Holocene folding localized than the regional orogenic phases recorded in the calculated by Vita-Finzi (1987). Moreover, Mann & Vita-Finzi different tectonostratigraphic units. This is because the pro- (1988) inferred that the folding in the SE Zagros during the gressive unconformities discussed here also extend across areas Quaternary had advanced the deformation front at 17 mm a−1. (such as the Fars platform and northwestward of the Kazerun However, since the entire Simply Folded Zone shortened line) where no diapirs of Hormuz salt reach the surface. during each orogenic phase, the migration rate of deformation Contemporaneous disconformities, previously attributed front does not represent the rate of shortening. Put another to epirogeny between time equivalents of the formations way, if the deformation has not always been confined to the mentioned earlier have also been reported both in other parts frontal folds, then the rate of shortening is expected to exceed of the Zagros (James & Wynd 1965; Motiei 1993) as well as at the rate of advance of the deformation front. The rates of many surface and underground locations in neighbouring migration of the deformation front calculated for each of the areas (Henson 1951; Mina et al. 1967; Motiei 1993). This different orogenic phases described in this study are plotted observation alone supports the idea that the unconformities in Figure 9b. However, since all ages are referred to end documented here, are not due to local holokinesis or tilting of Eocene time, these results represent cumulative average rates fault blocks but are due to regional folding. Although, the of propagation. Measurements were made across two areas UNCONFORMITIES IN FOLD–THRUST BELT 979 SW NE Arabia Foreland Zagros trough and ridge Zagros imbricate Remnants of Tethys ocean Sanandaj-Sirjan shelf zone zone Zone Sea Level U p Obducted ophiolite I Paleocene-Eocene, p e Deposition of Jahrom/Pabdeh Fms. r C r e t a c e o u s a n d o l d e r u n i t s Asmari overlies Pabdeh Fm. Hormuz salt with angular unconformity, see Fig.3 II End Eocene. Folding of Jahrom/Pahdeh Fms. Deposition of Asmari Fm. Razak Fm. onlaps Asmari Fm., see Fig.4 III Early Miocene. Folding of Asmari Fm. Deposition of Razak/Gachsaran and Mishan Fms. Agha Jari Fm. overlies Mishan Fm. with angular unconformity, see Fig.7 IV Middle-Late Miocene. Folding of Gachsaran and Mishan Fms. Deposition of the estuarine sediments of Agha Jari and (lower) Bakhtyari/Lahbari Fms. (Upper) Bakhtyari Fm. Pabdeh Fm. overlies Agha Jari Fm. Agha Jari and lower Bakhtyari Fms. Jahrom Fm. with angular unconformity, Gachsaran and Mishan Fms. see Fig.8 Asmari Fm. V End Pliocene Deposition of the /(upper) Bakhtyari/Lahbari Fm. Arabia Foreland Simply folded zone Imbricate Sanandaj-Sirjan shelf zone zone Zone Fig. 10. Schematic diagrams showing the sequence of orogenic phases leading to the folding of the entire Zagros Simply Folded Zone. Note that the deformation front migrates southwestward causing formation of local unconformities at the end of the Eocene, Early Miocene (Burdigalian), Mid–Late Miocene, and end Pliocene. The area affected by folding during each orogenic phase must have been wider than the area uplifted by folding and subjected to subaerial weathering. The imbricate zone is assumed to be the stationary backstop to the Zagros Simply Folded Zone. The thickness of the formations is not to scale. northwest and southeast of the Kazerun Fault. Each strip on 37W and 25N (Bird et al. 1975) and the rate of horizontal Figures 7 and 9a represents a minimum area uplifted by shortening of 29 mm a−1 calculated from an uplift rate of folding of a particular age. The propagation rate for the 1.8–6.6 mm a−1 by folding (Vita-Finzi 1987). More data, deformation front began slowly but settled to a steady rate of especially GPS measurements are required to take this study about 7.3 0.6 mm a−1 over a high friction decollement further. (where salt is presumably absent or very thin) and 9.5 1 mm a−1 over a low friction decollement (represented by the Hormuz Salt Formation decoupling the cover sediments from the underlying basement). Discussion The rate of southwestward migration of the Zagros front A general southwestward migration of marine troughs across within each orogenic phase is plotted in Figure 9c. There is one the complete width of the current Zagros orogen from the surprising aspect of migration rate which deserves discussion. early Cretaceous to Plio-Pleistocene times led early workers to The migration rate appears to have undergone a dramatic suggest several separate phases of tectonic movements across increase of about an order of magnitude, from ‘d’ to ‘e’ the Zagros orogen (Henson 1951; Falcon 1974; Koop & (Fig. 9a & c). Any error introduced while measuring the width Stoneley 1982). Here, we apply the same principle to the of strips a–d is considered negligible. Any miscalculation is Simply Folded Zone. likely to result from the absolute ages considered for the rock The lessons from the tectonostratigraphic relationships units. Although both the NE and SW boundaries of the 40 km described above are summarized in Figure 10. This figure wide strip ‘e’ shown on Figure 9a are uncertain (marked by shows that deformation of the Simply Folded Zone began dashed and dotted lines), the rate measured across it for the propagating from the imbricated zone at the end Eocene last 1 Ma is likely to be closest to the rate of shortening. It is and has propagated southwestwards by both the addition of therefore reasonable that this figure of 40 mm a−1 is between frontal folds and shortening of existing folds ever since. The the rate of 47 mm a−1 at which the Arabian and Asian plates southwestward migration of the deformation front drove the on either side of the Zagros orogen converge about a pole near foreland basin in front of it (Figs 7I–V & 10I–IV). The area 980 K. HESSAMI ET AL. affected by folding during each orogenic phase must have been The rate of propagation of the deformation front across the wider than the area that was uplifted and subjected to sub- Simply Folded Zone, using the stratigraphy and structures in aerial weathering. The orogenic phase that occurred at the end the cover, appears to have been close to constant but was not of the Eocene was the least widespread and restricted to the necessarily either continuous or steady (Fig. 9b & c). Local most northeasterly rim of the present Simply Folded Zone unconformities and deposition in the foreland basin and (Figs 7I & 10II). As the Simply Folded Zone widened progres- intra-mountain synclinal basins occurred in discrete phases. sively (by the addition of new frontal folds) towards the SW, These are the end Eocene, Early Miocene, Mid–Late Miocene, old folds continued to shorten and out-of-sequence structures the end Pliocene, mid-Pleistocene, and present-day when may have formed so that the entire belt folded simultaneously. reactivation of old faults in the basement led to extrusion of The evidence for this claim is the simultaneous Early–Mid- salt (Hessami et al. 2001). Such episodic movements are also Miocene folding of both the littoral sediments of the Razak known elsewhere in the neighbouring region: e.g., in the Formation (deposited in the synclinal troughs, formed between Sananaj–Sirjan Zone (Alavi 1994), and in the southern Tethys anticlinal axes rising in the NE) and its time-equivalent, region (Henson 1951; Stoneley 1975). the Gachsaran Formation (deposited in the main marine depocentres). In addition, the folding of the continental deposits of the Agha Jari and Bakhtyari formations right Conclusion across the Simply Folded Zone indicates that the end Pliocene Field evidence for offlap and onlap associated with angular and mid-Pleistocene phases of orogeny folded the whole width unconformities and conglomerates within the Cenozoic of the Zagros Simply Folded Zone. Furthermore, seismic stratigraphy of the Zagros demonstrates that stratigraphic activity throughout the Simply Folded Zone indicates that the breaks, previously attributed to epirogeny, are the result of zone is still deforming across its entire width. As a result, the separable pulses of deformation across the Simply Folded sedimentary cover at the present-day deformation front has Zone. The oldest (end Eocene) angular unconformity recorded folded only during the last phase of orogeny whereas it has between the Pabdeh (Palaeocene–Eocene) and Asmari suffered increasing number of orogenic phases with increasing (Oligo-Miocene) formations is confined to the northeastern distances behind. We attribute concomitant folding southeast rim of the Simply Folded Zone. This first phase of deformation of the Kazerun Fault to slippage of sedimentary cover over the was followed by a series of more intense phases culminating Hormuz Salt at the base of the cover sequence. However, in the climax of deformation which shortened the whole width NW of the Kazerun Fault, the Lower Miocene Gachsaran of the existing Simply Folded Zone at end Pliocene when Formation plays the same role of a low-friction decollement the Agha Jari Formation was unconformably buried by the much higher in the cover sequence. Modeling results by (upper) Bakhtyari conglomerates. Later events are indicated Letuzey et al. 1995; Cotton & Koyi (2000) and Koyi et al. by folding of younger sediments. Local unconformities at the (2000) and field data by Lillie et al. (1987) and Butler et al. end Eocene, Early Miocene [Burdigalian], Mid–Late Miocene, (1987) show that shortening of sedimentary units above a end Pliocene, mid-Pleistocene indicate a southwestward mi- low-friction decollement provided by a layer of salt, spreads gration of the Zagros deformation front that was pulsed in the zone of active deformation to a wide area in which several time. Changes in lithology and salt extrusion contempor- fold/thrusts are active simultaneously. Thus, we conclude that aneous with each unconformity also demonstrate that the the folds in the Zagros Simply Folded Zone have developed tectonic movements were episodic. There is plenty of room in both serially and concomitantly in time and space. the Zagros Simply Folded Zone for applying modern advances The different orogenic phases distinguished in the tectono- in interpreting the interrelated architectures of syntectonic stratigraphic units of the Simply Folded Zone (Fig. 9) are sediments and synsedimentary structures. also evident in the timing of extrusions of the salt diapirs, which have ages spanning the entire time window of the We dedicate this work to the late Manoochehr Pedrami who directed orogeny. Diapirs of Hormuz Salt were extruded at different us to most of the sections described in this study. Our presentation was times in front, at and behind the deformation front in the considerably improved by comments by Rob Strachan and Ian Alsop. Zagros Simply Folded Zone. 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