Holocene Sedimentary Record and Coastal Evolution in the Makran Subduction Zone (Iran)

The western Makran coast displays evidence of surface uplift since at least the Late Pleistocene, but it remains uncertain whether this displacement is accommodated by creep on the subduction interface, or in a series of large earthquakes. Here, we address this problem by looking at the short-term (Holocene) history of continental vertical displacements recorded in the geomorphology and sedimentary succession of the Makran beaches. In the region of Chabahar (Southern Iran), we study two bay-beaches through the description, measurement and dating of 13 sedimentary sections with a combination of radiocarbon and Optically Stimulated Luminescence (OSL) dating. Our results show that lagoonal settings dominate the early Holocene of both studied beach sections. A flooding surface associated with the Holocene maximum transgression is followed by a prograding sequence of tidal and beach deposits. Coastal progradation is evidenced in Pozm Bay, where we observe a rapid buildup of the beach ridge succession (3.5 m/years lateral propagation over the last 1950 years). Dating of Beris Beach revealed high rates of uplift, comparable to the rates obtained from the nearby Late Pleistocene marine terraces. A 3150-year-old flooding surface within the sedimentary succession of Chabahar Bay was possibly caused by rapid subsidence during an earthquake. If true, this might indicate that the Western Makran does produce large earthquakes, similar to those that have occurred further east in the Pakistani Makran.


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The Makran coast, in southeastern Iran, sits above oceanic lithosphere of the Arabian plate that The bedrock geology at the coastal plain [37,38,5,[39][40][41]2,1,42,43] is dominated by erodible Continental uplift has an impact on the relative sea-level curve and should be taken into 123 consideration. Our limited knowledge of the uplift rate variations during this period creates 124 uncertainty regarding the relative sea-level curve of the Makran. However, the Makran coast has 125 been continuously uplifting during the Late Pleistocene, as shown by the presence of marine terraces 126 [19]. Therefore, the resulting relative sea-level curve should peak at around 6000 ka, hereafter 127 referred to as the mid-Holocene relative highstand, and then slowly fall until present time. The 128 magnitude of this peak is not well known due to the uncertainties mentioned above (Fig. 2b shows

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Here we present the main results of our research on the Holocene beach evolution based on their 137 geomorphological and sedimentological characteristics. Our focus is on three main sites, Beris 138 Beach, Chabahar Bay and Pozm Bay (Fig. 3), which we describe separately in the following sections.

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Details of the methods used are reported in supplementary S2. Dating results are summarized in Table 1 and 2, and more analytical details are provided in the data repository [56]. The facies 141 description and interpretation of the depositional setting is summarized in Table 3  The morphology of the Makran coast is strongly influenced by the spatial distribution of 147 sandstones and marls, which have a marked contrast in resistance (and erodibility) (e.g., [57][58][59]).

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Wave action erodes faster through soft marl bedrock than through indurated sandstones, which 149 causes the coastline to develop into deep bays and protruding headlands. Material eroded from 150 headlands, exposed to wave attack, is transported by alongshore currents and preferentially 151 redeposited in embayments, together with continental fluvial input, to form prograding beaches, 152 progressively protecting the bays from coastal retreat [59,57]. The ability of a coastline to either 153 develop a large amplitude, or evolve towards a smooth profile depends on many factors, such as 154 bedrock lithology or wave regime, but it is primarily a function of the sedimentary budget [57,59].

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We infer that the sedimentary input supplying the Makran beaches mainly originates from 4 156 sources (e.g., [60,45,61,57,62,59]); (1) alongshore transport of littoral sediments, (2) erosion of nearby linked to climatic conditions, which remained relatively constant in the Makran since the start of the 159 Holocene. However, we observed the presence of abandoned river channels within the low coastal 160 plain (Fig. 4a, 4b) indicating that the Makran Rivers have switched from one bay to another, 161 drastically modifying the sandy fluvial input towards each local bay beach, throughout the 162 Holocene. We gathered information on river watersheds in order to understand where fluvial 163 sediments input the Oman Sea and how fluvial sedimentary input can influence beach progradation.

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Although the presence of this wide strandplain hints towards a high input of sediment,

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Chabahar Bay currently receives sediments from only two small watersheds (max 500 km 2 ) draining 271 mainly the fined grained rocks of the coastal plain (Fig. 4a). Part of the sand input comes from 272 erosion of the nearby headlands (mainly Chabahar headland, due to its size, upstream position and 273 sandstone dominated bedrock). However, the abandoned river channels observable around

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Konarak airport (Fig. 4b) suggest that the Sergan River used to flow into the Chabahar Bay, nearly 275 tripling its coarse-grained fluvial input (Fig. 4a,  towards ancient river routes towards the Chabahar Bay (Fig. 4a). However, these routes were 281 probably diverted due to rock uplift, on timescales greater than the Holocene.   We measured eleven logs (Fig. 7) along a 4.5 km long man-made trench through the coastal 289 plain near Konarak airport, within Chabahar Bay (Fig. 1, 3c). At this locality, the contact between the

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Therefore, a relative sea-level rise, or flooding surface, seems to occur within the sedimentary 309 successions, whereas the relative sea-level curve of the Holocene on an uplifting coast would rather 310 be expected to be globally falling (Fig. 2b).

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We attempted to date the episode of relative sea-level rise observed within the sedimentary logs 326 K3-K9 (outlined blue arrows in Fig. 7) in order to understand if this was a slow or fast event, or if it Therefore, we propose two different interpretations based on either method, since combining both 330 leads to ambiguous conclusions.
2) which seems at odds with the prograding sequence of sediments below the flooding surface (see 334 above). Eventually, the sequence becomes immerged and the coastal regression is expressed in the 335 logs by the flooding surface. After the maximum transgression ~6000 years ago, the relative sea level 336 falls, and the system progrades. Hence, the flooding surface is associated with Early Holocene 337 sea-level rise.

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Based on OSL dating, the system postdates the mid-Holocene relative highstand. Samples 339 below and above the flooding surface date at the same age within errors of ~3150 years ago.

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Therefore, the prograding lagoonal system has been flooded very quickly around 3150 years ago.

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This rapid relative sea-level rise is at odds with the seemingly steady and undisturbed nature of the 342 sea-level curve at that time (Fig. 2). 349 km 2 , respectively, currently discharge into Pozm Bay, though sporadic river avulsion towards the 350 nearby bays has happened throughout the Holocene (Fig. 4a, 4b). The oldest ridges, situated further

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The profile is projected on the black line (see supplementary table S1.3).

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We have dated four shell samples from the beach ridges (from the sea, beach ridge N°3, 7, 15 373 and 18) (Fig. 3b) to better understand the prograding history of the strandplain. Unfortunately, the 374 two first samples yielded very young conventional ages that could not be accurately calibrated.

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Nonetheless, we know they are recent, (a maximum of several hundred years). The 15 th and 18 th 376 beach ridges yielded calibrated ages of 184 ± 116 BP and 931 ± 112 BP respectively (Fig. 9). We also 377 sampled one of the oldest beach ridges, close to the observed paleocliff, which yielded an

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In Chabahar Bay, we observed a layer of supratidal facies (i.e., deposited above the mean 395 sea-level) overlain by sediments deposited in the lower intertidal zone (i.e., below mean sea-level).

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This transition is observable in several logs along a distance of more than 500 m, hence, it is not