Tectono‐Sedimentary Evolution of Shale‐Related Minibasins in the Karvandar Basin (South Sistan, SE Iran): Insights From Magnetostratigraphy, Isotopic Dating, and Sandstone Petrology

Sediments deposited into foreland basins can provide valuable insights related to the geological evolution of their hinterlands. Located in the peripheral foreland of the South Sistan Suture Zone (SE Iran), the Karvandar Basin exhibits a several‐kilometer‐thick shallow‐marine to continental clastic sedimentary sequence forming elongated sub‐circular synclines. These synclines overlie a mud‐dominated formation with exotic volcanic blocks that hosts one of Iran's largest mud volcano, known as Pirgel. In this study, we present a ∼3.5‐km‐thick magnetostratigraphic section and U‐Pb zircon ages of interlayered tuffs that constrain a depositional age of the Karvandar Basin of ∼24–17 Ma. Sandstone and microconglomerate framework analyses and paleocurrent directions suggest a first‐cycle active volcanic arc source to the northeast of the basin. We interpret the mud‐dominated lithology with volcanic blocks as an olistostrome originating from a similar source as the overlying clastic sequence. The deposition of the olistostrome is dated at ∼24.5 Ma by a U‐Pb calcite age from a coral block. The absence of large‐scale anticlines and the occurrence of angular unconformities suggest that the sub‐circular synclines in the Karvandar Basin formed by gravity‐driven downbuilding into the unconsolidated fluid‐saturated olistostrome, resembling salt‐related minibasins. Integrated results indicate that a late Oligocene to early Miocene Makran volcanic arc represents the source of the clastic sequence. Hence, our results provide new constraints on the initiation of arc volcanism related to the Makran subduction zone, predating earliest reported ages from the Mirabad pluton (19 Ma) to the northeast of the Karvandar Basin by ∼5 Myr.


Geotectonic Setting
The Cenozoic Karvandar Basin (also known as Kaskin Basin in some publications; e.g., Bagheri & Damani Gol, 2020;Davoudzadeh, 1997;Huber, 1978;Mohammadi, Burg, & Winkler, 2016;Sahandi, 1996) is situated at the intersection of the SSZ and the Makran accretionary wedge (Figure 1a).This intersection represents the junction of the continental Lut and Afghan/Helmand blocks in the west and the east, respectively, and the northern extent of the Makran accretionary wedge in the south (hereafter called Makran-Sistan triple junction).The tectonic history of this region began after the separation of Central Iran (Yazd, Tabas, and Lut blocks), the Afghan/Helmand block, and the Sanandaj-Sirjan and Bajgan-Durkan blocks from the edge of Gondwana during the Permian and no later than Early Triassic (Burg, 2018;Senghor et al., 1988;Sudi Ajirlu et al., 2016), resulting in the formation of the Neo-Tethys Ocean (Muttoni et al., 2009).During the Late Jurassic and Early Cretaceous, several smaller oceans connected to the Neo-Tethys formed, partially as backarc basins resulting from the northward subduction of the Neo-Tethys (Hunziker et al., 2015;Lechmann et al., 2018;Moghadam et al., 2009;Mohammadi, Burg, & Guillong, 2022;Mohammadi, Ruh, et al., 2022;Rossetti et al., 2010).In the Late Cretaceous, the plate tectonic configuration resembled the present-day Mediterranean region, with multiple microcontinents separated by smaller branches of the Neo-Tethys (Nain-Baft, Fannuj, Sistan, Sabsevar, and Khoy oceans) RUH ET AL.
10.1029/2023TC007971 3 of 28 surrounding the Central Iranian Blocks and the main Neo-Tethys Ocean to the south (McCall, 1997).The SSZ and the Makran accretionary wedge formed as a result of the (in the case of the Makran still ongoing) closure of different branches of the Neo-Tethys Ocean that initiated during the Cretaceous (e.g., Alavi, 2007; F. Berberian et al., 1982;Ricou, 1994).
The SSZ is structurally overprinted by an active large-scale dextral strike-slip system that affects entire eastern Iran (e.g., M. Berberian et al., 1999Berberian et al., , 2000;;Freund, 1970;Walker & Jackson, 2004), induced by the indentation of the Arabian continent during the Zagros collision (Mattei et al., 2012), and accompanied by regional intra-plate alkali basaltic magmatism between 15 and 1.5 Ma (Pang et al., 2012;Walker et al., 2009).Furthermore, Jentzer et al. (2017) demonstrated how the principle stress orientation in the north Sistan rotated from purely compressional during the Miocene to dextral transpressional in the late Pliocene to present.Their temporal constraint contradicts with the Karvandar Basin being a strike-slip-related pull-apart basin already active in the Paleocene (Aghanabati et al., 1994;Bagheri & Damani Gol, 2020).Besides the tectonic overprint related to the SSZ and the East Iranian strike-slip system, the North Makran terranes south of the Karvandar Basin (Figure 1a) were imbricated and uplifted in the Miocene (∼23-5 Ma) based on apatite fission track ages (Burg, 2018;Dolati, 2010), probably related to sediment underplating occurring across the Makran accretionary wedge (Platt et al., 1985).The uplift of the North Makran helped establishing the Jaz Murian as a forearc basin that, at least locally, was connected to the Karvandar Basin (Figure 1).

Karvandar Basin
The Karvandar Basin is an elongated structural basin west of the Karvandar Fault that has been proposed to represent the frontal fault, that is, western tectonic boundary, of the SSZ (Bagheri & Damani Gol, 2020).The local basin measures ∼150 km in N-S direction and maximally 35 km along an E-W transect (Figure 1a) and was deposited onto the south-eastern margin of the microcontinental Lut Block (the easternmost microplate forming Central Iran).The Karvandar Basin consists of several doubly plunging synclines trending roughly N-S, parallel to the Karvandar Fault.The southern part of the basin consists of a single large syncline, while the basin becomes narrower toward the north, where several smaller dish-shaped synclines are closely assembled (Figure 1b).Anticlines are generally absent throughout the entire basin and synclines are separated by mud-dominated strata, which are in stratigraphic contact, or they are welded against each other (Figure 2).A prominent tuff horizon can be identified on satellite imagery in the northeastern syncline, which along its southern edge forms an angular unconformity, that is, a sedimentary onlap, potentially indicating syn-tectonic deposition (Figures 2c and 2d).
Detailed sedimentological mapping was not attempted previously (see area of geological mapping covered by Intercon-Texas Instruments in McCall (1997)).However, the detrital zircon U-Pb age population of a turbiditic sandstone sample in the southeast of the Karvandar Basin suggests an upper Eocene depositional age (Figure 1b; sample 134 from Mohammadi, Burg, and Winkler (2016)).These turbidites were potentially deposited into a shallow basin, demonstrated by the occurrence of patch reefs (Figure 1b).Strata overlaying Eocene deposits are composed of Oligocene to Pliocene shallow-marine and fluvial sediments, based on lower Miocene foraminiferal assemblages of one sample indicated on the Karvandar geological map of the Geological Survey of Iran (Figure 1b; Ghorashi & Vaezipour, 1987;McCall, 1997).However, data points are very scarce and the ages of the younger fluvial strata is interpretative.The exposure of Oligocene reefal limestones documented south of the Karvandar Basin (Huber, 1978) led McCall (1997) to argue that there has probably been a shallow sea extending westward toward Sirjan, where late Oligocene to Miocene reefal limestones are exposed (McCall et al., 1994).In this scenario, the Karvandar Basin is part of this shallow sea inland of the Mesozoic ophiolitic belt (Hunziker et al., 2015;Omrani et al., 2017;Saccani et al., 2018), which is currently mostly covered by Quaternary deposits in the Jaz Murian depression (McCall, 1997;Shahabpour, 2010).

Sedimentological Field Observations
According to Mohammadi et al. (2016b), the oldest sediments exposed in the Karvandar Basin (southeast) are middle to upper Eocene (youngest detrital zircon ages) marine slope deposits consisting of steeply WSW-dipping  occurrence of layered deposits covering the mud-dominated lithology (Figure 3).The base of this section correspond to a ∼40-m-thick turbiditic interval dominated by mud, where fine-grained sandstones of centimetric and occasionally decimetric thickness with parallel lamination define the bedding.Above the turbidites, the occurrence of fine-grained sandstones with well-developed tractive depositional structures, such as ripple marks, and the scarcity of mud and silty beds indicate an increase in depositional energy (Figure 4a).The presence of carbonate beds rich in corals and large benthic foraminifera further supports this upward shallowing trend.This trend is also confirmed by the appearance of the first continental conglomeratic layer after 73 m into the section (Figure 5a).Above this stratigraphic level, an alternating sequence of conglomerate, sandstone, silt and clay dominates up to the top of the section.This depositional style is only interrupted by three marine incursions in the lowermost kilometer of the section, potentially linked to occurrences of sea level fluctuation or local short-lived basin subsidence (Figure 5a).
The conglomeratic unit within the Karvandar Basin exhibits distinct characteristics, consisting of amalgamated channels with sheet-like geometries (Figures 4b and 4c), often wider than several kilometers.Channels occur as multi-story fills with occasional trough and planar cross-stratification and tabular strata.Individual channel fills are generally 1-5-m-thick (Figures 6b and 6c), while vertically stacked amalgamated bodies reach up to 30 m in thickness, particularly above meter 3,300 in the section (Figure 5a).Fine-grained material, such as clays and silts, dominates some intervals (e.g., between meters 2,000-2,100 in the section).Sporadic carbonate-rich paleosoils develop among the fine-grained intervals, for instance at meters 1,560 and 2,400 (Figure 5a).The general architectural arrangement and facies association of the conglomerate unit suggest the depositional environment to represent the middle to distal parts of a distributive fluvial system with a distributed source area (Davidson et al., 2013;Huerta et al., 2011;Weissmann et al., 2010).The sequence was probably deposited with a high relative water discharge and high sediment supply (Hartley et al., 2010), likely under semiarid conditions (no organic beds, no plant remains, no lacustrine deposition).The general upward increase of conglomerate thickness and more frequent occurrences of paleosoils suggest retrogradation, basin fill-up, and a farther shoreline upwards over time.The high sediment supply compensates for the high subsidence rate, resulting in a long-term trend of sediment accumulation and filling of the basin.
Sedimentary bedding is steep and partly overturned in the lowermost 100 m of the sampled section (Figure 5b).Across the magnetostratigraphic section, the bedding of the clastic strata flattens continuously within each subsection to reach dip values of ∼30° at the top.
The Pirgel mud volcano is located at coordinates 27°59′20″N and 60°29′03″E and is squeezed in between the two nearly welded northern synclines (Figure 3).The mud volcano and its extrusion product cover an area of ∼1 km 2 with a vertical relief of ∼100 m relative to the surrounding areas (Figure 6a).The mud volcano consists of a central plateau (Figure 6a) with several active and inactive mud gryphons and pools (Figure 6b).
The poorly lithified mudstones forming the frame of the mud volcano contain centimeter-to meter-scaled randomly oriented angular to sub-rounded blocks of mostly volcanic (andesitic) material (Figure 6c).In addition to the volcanic blocks, there are also fewer occurrences of corals within the mudstones (Figure 6d).Pebbles of the same composition as the larger blocks furthermore cover the mud edifice surface (Figures 6c and 6d), except in places where fresh mud flows appear (Figure 6b).It is noteworthy that these pebbles and blocks are not found anywhere else throughout the investigated area, suggesting a localized source or transport mechanism specific to the mud-dominated lithology.

Magnetostratigraphic Sampling and Measurements
Due to the limited transport infrastructure, magnetostratigraphic sampling was conducted on the north-eastern syncline of the Karvandar Basin.This section was accessible via a dirt road that connects the Pirgel mud volcano with the Zahedan-Iranshahr main road to the east of the basin (Figure 3).To cover the entire section comprehensively, we divided it into five subsections, following dry river creeks crosscutting the sedimentary sequence.A total of 170 sites were sampled, starting from the south of the Pirgel mud volcano (Figure 3).At each site, cores of mud-, silt-or fine-grained sandstone were sampled with a battery-driven drill.The spacing between sampling sites range between 10 and 25 m depending on the suitability of lithologies for magnetostratigraphy, considering a high sedimentation rate setting, similar to other conglomerate-dominated foreland basins, where accumulation rates of >100 cm/kyr were observed (e.g., Schlunegger et al., 1997).
Natural remanent magnetization (NRM) of the samples were measured during stepwise thermal demagnetization using a tri-axial DC superconducting quantum interference device rock magnetometer (2G Enterprise 755R) with a sensitivity of ∼3•10 −8 A/m at the Laboratory for Natural Magnetism at ETH Zurich.The NRM and thermal demagnetization process included 20 steps  10.1029/2023TC007971 9 of 28 from 0°C to 680°C, with intervals of 50°C up to 400°C, 20-30°C up to 660°C, and 10°C up to 680°C.Sample heating was conducted with an ASC TD48 oven (internal field <5 nT).The resulting demagnetization paths were plotted on vector diagrams for the identification of different magnetic components, unblocking at different temperatures (Zijderveld, 1967).The program Remasoft was applied to identify distinct magnetic components (Chadima, 2006), where line fitting of principal components was achieved by the method of Kirschvink (1980) and statistical parameters of magnetic directions were calculated by the method of Fisher (1953).Magnetic susceptibility of all samples was measured before each heating step with an AGICO KLY 2 Kappabridge in order to monitor mineralogy changes related to the heating.Remasoft-type files (.rs3) of each measured sample are found in Data Set S1.

Framework Composition and Paleocurrents
Along the same section, a sandstone and microconglomerate framework study was performed aiming to identify detrital grain types and interpret their source areas.Modal framework analysis was carried out on 55 mediumto coarse-grained sandstones.For each sample thin-section, at least 300 points were counted following the Gazzi-Dickinson point-counting method (Ingersoll et al., 1984).Additionally, 14 microconglomerate samples were analyzed, where at least 100 granule grain points were counted per sample thin-section.Attention focused on identifying rock fragments to define the fabric, texture, and lithology of the parent rocks.Results are presented in standard complementary ternary diagrams (Dickinson, 1985;Dickinson & Suczek, 1979;Folk, 1980).
A total of 32 paleocurrent indicators were measured across the sampled section, including grooves, flute casts, tool casts/marks, erosion channels and cross-bedding.Paleocurrents were separated into 11 directional and 21 non-directional orientations depending on whether a clear transport direction could be interpreted.Measurements were rotated to a horizontal plane around the strike direction of bedding.

Zircon U-Pb Geochronology
Two prominent tuff layers were detected in the Karvandar Basin and sampled for zircon dating to provide independent age constraints for the magnetostratigraphy.A ∼50-cm-thick tuff layer was sampled at the base of the northwestern syncline, ∼500 m west of the Pirgel mud volcano (Figure 3).Considering the dip of the marine strata of 45° at the base of the western syncline results in stratigraphic position of ∼350 m in section.As both the bases of the western and eastern syncline are in direct stratigraphic contact with the underlaying mud-dominated lithology, the tuff layer is expected to be positioned at the same stratigraphic level in the eastern syncline, where the magnetostratigraphic section was sampled.There however, first arrivals of conglomerate probably eliminated the thin tuff horizon.In addition, two samples were collected from a ∼25-m-thick tuff horizon situated several kilometers to the north of the magnetostratigraphic section, as indicated on the Ziruki-ye-Goharkuh geological map of the Geological Survey of Iran (Figure 2c; Z. Hosseini & Afsharianzadeh, 1993).
The U-Pb ages of individual zircon grains of the tuff layers were analyzed using laser ablation inductively coupled mass spectrometry (LA-ICP-MS) at the Institute of Geochemistry and Petrology at ETH Zurich with a similar analytical protocol as applied in Guillong et al. (2014).The measurements were conducted with a laser repetition rate of 4 Hz, an energy density of ca. 2 J/cm 2 and a spot diameter of 19 μm.The raw data was corrected by primary reference material "GJ-1" (601.86 ± 0.37 Ma, Horstwood et al., 2016), and the secondary RMs "Plešovice" (337.13 ± 0.37 Ma, Sláma et al., 2008), "91500" (1,063.51± 0.39 Ma, Wiedenbeck et al., 1995), "AusZ7-1" (38.896 ± 0.01 Ma, Kennedy et al., 2014), and "AusZ7-5" (2.41 ± 0.01 Ma, von Quadt et al., 2016) were analyzed for quality control.The measured data was plotted on Concordia diagrams using the IsoplotR software (Ludwig, 2003;Vermeesch, 2018).Uncertainties are given as 2SE and "SE including 1.5% systematic external uncertainty (Horstwood et al., 2016).Only concordant measurements of 206 Pb/ 238 U and 207 Pb/ 235 U ages were considered for all three samples to ensure the exclusion of data affected by lead loss and initial Pb.Both cores and rims of zircon single grains were targeted to detect and exclude inherited crystallization ages (Figure S1 in Supporting Information S1).

Calcite U-Pb Geochronology
Calcite U-Pb dating was performed on a coral sample collected from the Pirgel mud volcano (Figure 3).The polished sample was examined using LA-ICP-MS by the same instrument as for zircon dating (see Section 4.3.),but with a laser repetition rate of 5 Hz and a spot diameter of 110 μm.Variable single-spot ablation signals were integrated independently following the method described by Guillong et al. (2020).The raw data was corrected by primary RMs "NIST 614" and "WC-1" (254.4 ± 4 Ma, Roberts et al., 2017) and secondary RMs "JT" (13.797 ± 0.031 Ma, Guillong et al., 2020) and "ASH-15D" (2.965 ± 0.011, Nuriel et al., 2021) were analyzed for quality control.U-Pb ages were interpreted from the lower intercepts of Tera-Wasserburg Concordia diagrams using the IsoplotR software (Ludwig, 2003;Vermeesch, 2018).Uncertainty is given as 2SE and "SE including 2.5% systematic external uncertainty (Guillong et al., 2020).

Nannofossils and Foraminifera
Three liquid samples collected from active mud pools and gryphons of the Pirgel mud volcano were analyzed to identify their micropaleontological content (calcareous nannofossils and foraminifera) to obtain a first-order age control of the source lithology.For the nannofossil content, the samples were prepared according to the smearing technique in order to prevent assemblage selection and to retain the original composition (Bown & Young, 1998).The analyses were performed using a polarized light microscope with a magnification of 1250X.Nannofossil assemblages were semi-quantitatively estimated by counting all the nannoliths recorded in 300 fields of view.Taxonomic attribution follows Perch-Nielsen (1985), Bown and Young (1998), and the website https://mikrotax.org/Nannotax3 (Young et al., 2017).Calcareous nannofossil content was described with reference to Agnini et al. (2014) and Backman et al. (2012).
For the analysis of foraminiferal content, the same samples were prepared as washed residues following standard procedures.The muddy mixture was washed and sieved (with mesh sizes of 425, 150, and 63 μm).The obtained washed residues were then oven-dried for one day at 40°C and examined under a stereomicroscope at magnifications varying from 6.3 to 50X.Taxonomic attribution of planktonic marker species are based on the online catalog "pforam@mikrotax" (http://www.mikrotax.org),integrated by the online World Foraminifera Database (http://www.marinespecies.org/foraminifera;Hayward et al., 2022) for benthic taxa.

Magnetostratigraphic Analysis
The NRM of all measured samples display values larger than 3•10 −6 A/m with a mean value of 1.1•10 −4 A/m, slightly increasing toward the top of the section (Figure S2 in Supporting Information S1).A similar tendency can be observed for the dimensionless magnetic susceptibility χ (SI), which displays values of 10 −4 -10 −3 in the lower part and 10 −3 -10 −2 in the upper part of the section (Figure S2 in Supporting Information S1).This increase indicates an increasing magnetite concentration toward the top of the section.The resulting normalized intensity (NRM/χ) shows no significant variation across the entire section (∼10 −1 A/m; Figure S2 in Support ing Information S1).Representative thermal demagnetization curves reveal that most samples (89%) contain a low-temperature magnetic component that is removed at temperatures of 250-300°C (Figure 7).An additional stable magnetic component is found in 96% of the measured samples, typically fully demagnetized above 620°C, indicating hematite as the primary magnetic carrier (Figure 7).Characteristic remanent magnetic components (ChRM) are classified depending on the median absolute deviation of the demagnetization curve, where type 1 indicates a MAD ≤ 10 (44%), type 2 a MAD ≤ 20 (45%), and type 3 a MAD > 20 (11%).Only samples with quality type 1 and 2 are used to constrain the magnetopolarity stratigraphy (MPS).All paleomagnetic data can be found in Table S1.
Statistical grouping of the low-temperature magnetic components (without correcting for bedding tilt) yields a mean direction with a declination of 3° and an inclination of 42.3°, with a strong statistical significance (Figure S3 in Supporting Information S1), and is similar to the present Earth's magnetic field direction at the corresponding longitude and latitude (dec: 1.5° and inc: 43.1°).Of the 170 measured samples, 145 manifest ChRM directions of quality type 1 and 2 (MAD ≤ 20).These magnetic directions are projected onto lower-hemisphere equal-area Schmidt nets before (Figure 8a) and after correcting for bedding tilt (Figure 8b).Tilt-corrected magnetic directions indicate both normal and reversed polarities, showing an antiparallel distribution (Figures 8a and 8b).To obtain the mean orientations for all 145 samples before and after tilt correction, ChRM directions are transposed onto the same hemisphere (Figures 8c  and 8d).The statistical distribution of tilt-corrected directions is slightly more significant (k = 4.25, α 95 = 6.5°) than uncorrected direction (k = 3.76, α 95 = 7.0°), although only to an irrelevant degree.The resulting mean of the transposed tilt-corrected magnetic directions shows a declination of 327.3° and an inclination of 29.4° (Figure 8d).
The tilt-corrected ChRM directions were used to calculate the virtual geomagnetic pole (VGP) for each individual sample, considering the sampled site location using a longitude of 60.5°E and a latitude of 28°N (Figure 9).For samples PG130-170, a fold axis with an orientation of 325-15 was additionally applied for tilt correction, related to the syncline geometry to the north (Figure 3).Corrected VGP directions are calculated for tilt-corrected ChRM directions rotated 30° in clockwise orientation, corrected for the apparent rotation shown by the tilt-corrected ChRM directions (Figure 9e).The resulting VGP corr is used to define the magnetic polarity of each ChRM, where positive VGP corr latitudes represent normal polarities and negative VGP corr latitudes reverse polarities.Sixty samples exhibit VGP corr latitudes of ≥45° and are considered clearly positive, while 61 samples exhibit latitudes of <−45° that are considered clearly reverse.Forty-two samples exhibit VGP corr latitudes between −45° and 45° and are considered transitional, either indicating polarity switches, polarity excursions, or a ChRM affected by a secondary magnetization.To construct the local MPS, at least two consecutive samples of quality type 1or 2 with a clear polarity (VGP corr ≥ |45|) are required.This results in a robust reversal patter for sections B to E, and a less robust pattern for section A, with several occasions of single reverse ChRM's (Figures 9e and 9f).
A reversal test was conducted to verify the antiparallel configuration of the normal and reverse magnetic directions (McFadden & McElhinny, 1990).A reversal test is considered successful if the transposed mean direction of one polarity group lays within the 95% confidence level (α 95 ) of the other polarity group.The paleomagnetic data set of this study does not pass the reversal test.This can be due to the relatively high percentage of transitional ChRM directions that mostly plot in the northeastern quartile of the lower hemisphere Schmidt net (Figure 8b) and therefore close the angle between the two mean directions.Another reason may be a secondary NRM that could not have been fully removed from individual ChRM's.However, the importance of the reversal test diminishes when it comes to magnetostratigraphy, as magnetic polarity can frequently be determined from partially overlapping paleomagnetic orientations (Garcés, 2015).Furthermore, fold tests were conducted independently for both normal and reverse polarity groups (Tauxe & Watson, 1994), regarding the variable bedding dip across the magnetostratigraphic section (Figure 5b).Fold test of both polarity clusters show a more significant statistical grouping for unfolded strata (Figure S4 in Supporting Information S1).

Framework Composition and Paleocurrents
Across the investigated section, sandstones are predominantly medium-to very-coarse-grained, moderately sorted, with sub-rounded to angular grains (Figure S5a in Supporting Information S1).These sandstones are rich in first-cycle rock fragments, corresponding to litharenite/volcanic arenites (Figure 10a).Among the rock fragments, volcanic lithic fragments are the most abundant accounting for an average of 77% of the total lithic grains (Table S2).Microlithic andesites, basalts and diabase are the main components of the volcanic lithic fragments (Figures S5b-S5d in Supporting Information S1).Metamorphic lithic fragments make up about 15% of total lithic grains and are mainly composed of slate, quartz-sericite, chloritoschist and prasinite lithic fragments (Figures S5e-S5h in Supporting Information S1).Other components present in smaller proportions include radiolarian cherts (4%), siltstones (2%), ultramafic grains (2%) and extra-basinal carbonates (Figures S6a-S6d in Supporting Information S1).On average, 22% of all grains are quartz, of which ∼75% are monocrystalline (Table S2).Monocrystalline quartz grains exhibit idiomorphic volcanic shapes with straight sides and rounded corners (Figure S6e in Supporting Information S1).Polycrystalline quartz grains, on the other hand, are predominantly recrystallized, schistose and stretched metamorphic types with multiple sub-grains (Figures S6f-S6g in Supporting Information S1).Feldspars make up about 11% of the modal composition (Figure S6h in Supporting Information S1), with ∼67% of them being plagioclase.Standard provenance ternary diagrams (Dickinson, 1985) display average grain compositions of Qt 25 F 11 L 64 and Qm 16 F 11 Lt 73 and Karvandar sandstones mainly plot in the fields for transitional to undissected arcs and/or recycled orogens (Figures 10b and 10c).
The stratigraphic trend along the sampled section in terms of sandstone classification remains relatively consistent, with no major deviation from the overall average composition with 22% quartz, 11% feldspar, and 67% rock fragments (Figure 11c).However, a notable increase in metamorphic lithic grains can be observed toward the top of the section, potentially causing the increase in magnetite content manifested by magnetic susceptibility (Figure S2 in Supporting Information S1).Furthermore, rock fragments of sandstones show a slightly higher  (Folk, 1980), (b) QtFL, and (c) QmFLt (Dickinson, 1985).Detailed data and definitions are given in Tables S1 and S2.Black dots: Karvandar sandstones.Gray dots: Sandstones from the South Sistan Basin (Mohammadi, Burg, & Winkler, 2016).abundance of ultramafic, chert, siliciclastic, and carbonate grains at the base of the section (Figure 11d).Microconglomerates exhibit a similar pattern, with slightly higher abundances of chert, pelitic, and ultramafic grains at the base (Figure 11e).
The paleocurrents measured on flute casts with a clear flow direction (Figure 4d) indicate a general northeast to southwest transport direction (Figure 12a).Paleocurrent measurements based on bi-directional symmetric ripple marks and tool marks show similar orientations with a dominant northeast-southwest component (Figure 12b).The obtained orientations are similar to paleocurrent directions from the neighboring Eocene Neh Complex (Figure 12c, Mohammadi, Burg, & Winkler, 2016).

Zircon U-Pb Ages
Samples PG-T01 and T04 represent the base and the top of a ∼25-m-thick andesitic tuff horizon in a concentric syncline located several kilometers north of the magnetostratigraphic section deposited after the last collected sample site PG170 (Figures 2c and 2d).Zircon grains show oscillatory zoning indicative of magmatic growth and sometimes contain cores that are 1-5 Myr older (Figures S1a and S1b in Supporting Information S1).Some zircons exhibit Ediacaran cores (Figure S1b in Supporting Information S1).Concordant crystallization ages of 11 single grain zircons of the andesitic tuff sample PG-T01 result in a Concordia age of 17.65 ± 0.14 | 0.30 Ma (Figure 13a).The Concordia age of sample PG-T04 is 17.81 ± 0.13 | 0.30 Ma based on 11 single grain zircon ages (Figure 13b).Zircons of tuff sample PG-T05 from the base of the shallow marine succession west of the Pirgel mud volcano (Figure 3) also show two phases of growth with round to angular cores and oscillatory overgrowth (Figure S1c in Supporting Information S1).The youngest concordant cluster of 24 U-Pb zircon ages indicates a crystallization age of 22.84 ± 0.16 | 0.38 Ma (Figure 13c).Isotopic ratios and ages are given in Table S4.

Calcite U-Pb Ages
Figure 14a shows an image of the thin section of coral sample PG-Pir05 under cross-polarized light.The sample was collected from a larger block found within the mud matrix on the Pirgel mud volcano (Figure 3).A total of 58 calcite U-Pb age point measurements yield a discordant lower intercept age of 24.49 ± 1.15 | 1.30 Ma (Figure 14b).A mean square weighted deviation of 0.44 indicates a closed system behavior (Guillong et al., 2020).Isotopic ratios are given in Table S5.

Nanno-and Foraminiferal Age Constraints
Nannofossil assemblages from extruded mud include Cyclicargolithus abisetctus, Cyclicargolithus floridanus, Discoaster deflandrei, Sphenolithus dissimilis, and Sphenolithus moriformis (Table S6).Although zonal markers were not recorded, maybe because in very poor materials only heavily calcified species of assemblages are preserved, samples can be referred tentatively to the Oligocene-Miocene transition interval.

Age Constraints on the Deposition of the Karvandar Basin
To establish the temporal framework for the deposition of the Karvandar Basin, the results of different dating techniques were combined.The U-Pb zircon ages of three tuff layers (Figure 13) and the U-Pb calcite age of the coral block within mud deposits of the Pirgel mud volcano (Figure 14) help correlating the MPS with the geomagnetic polarity time scale (GPTS, Gradstein & Ogg, 2020).The U-Pb calcite dating of the coral sample taken from a block within the mud-dominated lithology provides an age of 24.49 ± 1.15 | 1.30 Ma (Figure 14b), indicating a maximum age for the base of the neritic-continental sequence that overlies the mud-dominated lithology.Furthermore, nannofossil and foraminiferal assemblages from extruding mud samples of the Pirgel mud volcano suggest a Chattian-Aquitanian depositional age (Tables S6 and S7).The age constraint provided by tuff sample PG-T05 (Figure 13c, 22.84 ± 0.16 | 0.38 Ma) at ∼350 m from the base of the western syncline (Figure 3) offers a minimum age estimate for the base of the shallow marine to continental sequence.Furthermore, it position in the magnetostratigraphic section of the eastern syncline is estimated to ∼300-450 m from the base, based on similar to slightly increased sediment accumulation rates of the first conglomerate layers.The   2c and 2d) at the top of the section.(c) Sample PG-T05 is from western syncline (location in Figure 3) at the bottom of the magnetostratigraphic section.Only concordant ages were used to constrain final age.Uncertainty ellipses are 2SE.See Table S4 for isotopic ratios and ages.
The combination of independent age constraints from U-Pb ages and the high resolution of the magnetostratigraphic section (except for Section A, Figure 9f) allow for a robust correlation of the measured MPS with the GPTS (Figure 15).Section A displays several potential reverse magnetozones characterized by only one ChRM direction, which have to be interpreted with caution.The lowermost normal magnetozone of the MPS is correlated to chron C7n, post-dating the U-Pb calcite age of the coral block.The two normal magnetozones between ∼250 and 500 m of the MPS are interpreted as belonging to chron C6Cn, in accordance with the U-Pb zircon age of PG-T05 from the shallow-marine deposits of the neighboring syncline (Figure 3).Continuing up the section, the normal magnetozone with potential, but not fully resolved, reversals between ∼630 and 850 m of the MPS are linked to chron C6Bn.Above, a long reversed interval is punctuated by two mixed polarity intervals at 915 and 1,060 m, interpreted as C6AAr.2nand C6AAr.1n,respectively, and by a short normal magnetozone starting at 1,380 that correlates to chron C6AAn (Figure 15).Between ∼1,750 and the top of the section, three long normal magnetozones correspond to C6A and C6n normal magnetochrons.The top of the section yields an age of <19.72 Ma (base C6n), which is corroborated by the upper tuff horizon 430-530 m, located above the top of the section (Figure 15).An alternative interpretation suggesting that the topmost normal interval represents chron C5En does not align well with the underlaying MPS.
The resulting age constraint for the shallow marine-to-continental sequence of the Karvandar Basin indicates that the first shallow marine layered sediments deposited onto the chaotic shale-dominated formation in late Chattian, just before the Oligocene to Miocene boundary (Figure 15).The transition of shallow marine to continental facies occurred during the Chattian in the eastern syncline, perhaps favored by the global sea-level drop at the Ologicene-Miocene boundary (OMB, Liebrand et al., 2011).The western and southern synclines exhibit more prevalent marine sedimentation at their base, and the marine-to-continental transition, at least in the western syncline, occurred in early Aquitanian, based on U-Pb zircon ages of tuff sample PG-T05 (Figures 3  and 14).The continentalization of the eastern syncline close to the OMB is accompanied by an increase of average compacted sediment accumulation rate from ∼15 to ∼50 cm/kyr.Upsection, sediment accumulation rate increases constantly to reach >120 cm/kyr between ∼2,200 and 2,700 m, corresponding to 20-20.5 Ma (Figure 15).After ∼20 Ma, the sediment accumulation rate gradually decreases to ∼50 cm/kyr near the top of the  (Guillong et al., 2020).See Table S5 for isotopic ratios.
section.The magnetostratigraphy presents the irst age constraints from the Karvandar Basin and contributes to our broader understanding of the basin's geological history, providing a framework to discuss in the following the driving mechanisms behind the observed changes in sedimentation patterns.

Tectono-Stratigraphic Evolution of the Karvandar Basin
The stratigraphically lowermost (i.e., oldest) sediments in the Karvandar Basin consist of Eocene turbiditic sequences interbedded with occasional patch reefs (Figures 1 and 2).Published data on provenance analysis suggest that these turbidites are part of the Neh accretionary Complex (NAC, Figure 16a) with a dissected arc or recycled orogenic sediment source to the northeast (sample 134 in Mohammadi, Burg, and Winkler ( 2016)).The  (Gradstein & Ogg, 2020).The correlation is anchored by the U-Pb calcite age of the coral that pre-dates the base of the section (blue dotted line) and the tuff layers at the base and above the section (red dotted lines).Shaded red indicate estimations of the position of the tuff layers with regard to the magnetostratigraphic section.Resulting sediment accumulation rates fit the observed transition from marine to continental facies.The rates of sedimentation are illustrated beside the curve in cm/kyr.presence of patch reefs indicates, at least occasionally, a shallow-marine sedimentary environment (Figure 1b).During the Eocene, the region experienced potential local uplift and subsidence, which can be attributed to the onset of continental collision and subsequent lithospheric delamination along the SSZ at the corresponding latitude (Mohammadi, Burg, Bouilhol, & Ruh, 2016).Occurrences of thinned Eocene turbidites along the western edge of the Karvandar Basin suggests that the extent of the basin did not considerably change since the Eocene (Figure 16a).
Above the Eocene deposits, the lithology of the Karvandar Basin is characterized by mud-dominated sediments that separate the large-scale synclines and forms the framework of the Pirgel mud volcano (Figures 1-3).Chaotically distributed meter-size angular to sub-angular blocks of mostly volcanic origin (Figure 6c) suggest that this lithology represents an olistostrome, that is, a submarine debris flow (Figure 16b).Similar gravitationally emplaced olistostromes that traveled up to hundreds of kilometers along gently dipping slopes were reported in other regions such as the Canary Islands (Masson et al., 1998), the Gulf of Cádiz (Medialdea et al., 2004), and the Makran accretionary wedge (Burg et al., 2008).The abundance of volcanic blocks indicates that the source area was located on the slope of a volcanic edifice.The olistostrome was deposited into a shallow basin, overlaying the Eocene clastic slope sediments of the NAC (Figures 2b and 16b).Nannofossil and foraminiferal assemblages derived from the extruded mud from the Pirgel mud volcano and the U-Pb calcite age (24.49Ma, Figure 14) of a coral block in the mud matrix (Figure 6d) suggest olistostrome emplacement in the latest Oligocene.This is corroborated by the U-Pb zircon age of 22.84 Ma obtained from the lowermost part of the overlying sequence (Figures 3 and 13c).Besides, the occurrence of Cretaceous foraminifera can be related to the reworking of older strata, which is commonly observed in debris flow deposits.
The first layered deposits overlaying the mud-dominated lithology consist of shallow-marine shale and sandstones with asymmetric ripple marks (Figure 4a).The thickness of the marine sequence, distinguishable by its greenish color in satellite images, compared to reddish colors of the continental deposits, varies significantly from northeast to southwest within the Karvandar Basin (Figures 1-3).In the sampled section, marine deposits account for less than 100 m thickness (Figure 5a), whereas on the western side of the Pirgel mud volcano, the marine strata at the base extend to a thickness of ∼1 km (Figures 2c,2d,and 3).A similar increase in thickness of the marine strata is observed in the eastern syncline from north to south (Figures 1b, 2a, and 2b).These variations in marine strata thickness indicate that a shallow sea opened toward the southwest during earliest Miocene (Figure 16c).The measured paleocurrents within the Karvandar Basin further support this interpretation, indicating sediment transport from northeast to southwest, or east-northeast to west-southwest for a counter clock-wise rotated Karvandar Basin (Figures 12a and 12b).Furthermore, the distinct difference in marine strata thickness at the base of the western and eastern synclines along the mud-dominated lithology hosting the Pirgel mud volcano suggests that this ridge already existed in early Miocene, forming the coastline (Figure 16c).
During the latest Aquitanian (∼20.5 Ma), the Karvandar Basin underwent a phase of aggradational continental sedimentation (Figure 16d), indicated by the presence of interlayered redbeds and amalgamated conglomerates (Figures 4b and 4c).Sandstone framework data along the investigated section do not indicate any significant changes in sediment sources supplying the Karvandar Basin (Figures 5c-5e).The basin experienced consistently high sediment accumulation rates of ∼100 cm/kyr, extending at least until the end of the Burdigalian stage (Figure 15).Based on these observations, we propose that the consistent depositional environment in the Karvandar Basin reflects a balance between local subsidence and sediment accumulation, related to downbuilding of the synclines into the underlaying olistostrome (Figure 16d).This interpretation is consistent with an early existence of a mud ridge between the synclines, the general absence of major anticlines throughout the Karvandar Basin and the detection of syn-tectonic sedimentary onlaps and growth strata (Figure 2).In this sense, the mud ridge represents an elongated diapiric structure induced by the density contrast of the fluid-saturated olistostrome mass and the rapidly accumulating overlying continental sequence (Figure 16d).The Pirgel mud volcano pinches through this diapiric structure and fluids from the overpressures mud lithology are released.
After deposition of the clastic sequence related to the erosion of the volcanic arc, the minibasin synclines of the Karvandar Basin most probably underwent a phase of fold tightening, as E-W-oriented compression has been reported to affect the SSZ until late Miocene (Jentzer et al., 2017).Furthermore, mean magnetic directions with a declination of 328° indicate a counter-clockwise (CCW) rotation of the Karvandar Basin of ∼30° since its deposition, because the apparent polar wander path of Eurasia only provides a deviation between 1.31° and 2.64° since the last 20 Myr (Besse & Courtillot, 2002;Torsvik et al., 2012).Walker and Jackson (2004) interpreted several strike-slip fault systems in East Iran to have been responsible for CCW block rotation of 10-25°.They related this block rotation to 75-105 km of right-lateral shear in East Iran since ∼5-7 Ma.Furthermore, Mattei et al. (2012) reported CCW rotations of up to 35° in Oligocene-Miocene sedimentary rocks from Central Iran, related to large-scale right-lateral strike-slip systems.This supports that vertical-axis rotation of the Karvandar Basin is related to regional tectonics along the East Iranian strike-slip system and that rotation occurred after the deposition of the clastic sequence.

Regional Tectonic Implications
The late Oligocene to early Miocene marine strata of the Karvandar Basin were deposited in a shallow sea extending toward the southwest of the investigated area.Previous studies suggested that the westward extension of this shallow sea (today Jaz Murian Depression, Figure 1a) resulted from local uplift related to the imbrication of the North Makran toward the south during the Late Cretaceous to Eocene (McCall, 1997;Shahabpour, 2010).The Jaz Murian Depression remained a shallow sea until at least the early Miocene, as evidenced by the shallow-marine reefal limestones and clastic deposits of the Karvandar Basin (Ghorashi & Vaezipour, 1987;McCall, 1997;McCall & Eftekhar Nezhad, 1994;McCall et al., 1994;this study).Furthermore, extensive provenance analysis points toward the abandonment of the Cretaceous arc south of the Jaz Murian and the formation of a new arc, probably the eastward continuation of the Urumieh-Dokhtar arc in the Zargos (e.g., Alavi, 1994;Farhoudi & Karig, 1977; M. R. Hosseini et al., 2017), north of the Jaz Murian in the Eocene (Mohammadi, Burg, Winkler, et al., 2016;Mohammadi et al., 2017).This implies a transition of the Jaz Murian Depression from a backarc to a forearc basin during the early Paleogene.
As outlined above, the Karvandar Basin was probably connected to the Jaz Murian shallow sea in early Miocene.However, several key features suggest that the Karvandar Basin also functions as a Molasse-type foreland basin to the SSZ.These features include the large-scale structural disposition sub-parallel to the Karvandar Fault, the tectono-stratigraphic composition of the South Sistan (Figure 1a), and paleocurrents indicating sediment transport from the northeast to the southwest (Figure 12).Taking into consideration a CCW rotation of the basin since the late Miocene, paleocurrents indicate an east-northeast-west-southwest sediment transport, perpendicular to the Karvandar Fault (Figures 12a and 12b).The creation of new accommodation space in the foreland of the SSZ resulting in the formation of the Karvandar Basin is not trivial, as the continental sequence post-dates SSZ collision for ∼10-15 Myr (Jentzer et al., 2022;Mohammadi, Burg, Bouilhol, & Ruh, 2016;Tirrul et al., 1983).One possible explanation for this delayed response is the reported post-collisional westward-propagating lithospheric delamination (Mohammadi, Burg, Bouilhol, & Ruh, 2016).Similar effects of mantle deformation on the evolution of foreland basins were documented in the Swiss Molasse Basin, where processes such as slab rollback and plate bending provided additional accommodation space without significant changes in surface topography (Schlunegger & Kissling, 2015).Additionally, slab breakoff may have resulted in delayed isostatic rebound and increased sediment fluxes (Schlunegger & Castelltort, 2016).
Sandstone and microconglomerate framework of the Karvandar sequence consisting mainly of andesitic-basaltic volcanic rock fragments (Figures 5c-5e and 10a, Figures S4-S6 in Supporting Information S1) indicate a homogeneous and continuous sediment source of volcanic rocks, with subordinate contributions from ophiolitic and metamorphic source rocks.These sediments were likely eroded from an active volcanic arc and an uplifted subduction complex (Figures 10b and 10c, Dickinson, 1985;Dickinson & Suczek, 1979).Comparison to the more mature clastic deposits of the South Sistan Basin (Mohammadi, Burg, & Winkler, 2016) indicate a primary volcanic source without reworking of the Sistan turbidites (Figure 10).Furthermore, up to ∼25-m-thick tuff layers (Figures 2c and 2d) suggest that the Karvandar Basin was filled in the proximity of active volcanism.We interpret that a volcanic system, possibly located to the northeast, served as the source of the volcanic blocks in the late Oligocene olistostrome, as well as the tuff layers at the base and the top of the sequence, and the clastic sequence itself.Magmatism of similar age has been reported from the Mirabad pluton ∼70 km to the northeast (∼19 Ma, Pang et al., 2014) and the Bazman volcanic complex (∼21.7 Ma, Sholeh et al., 2016) to the west of the Karvandar Basin.Trace element ratios from the Mirabad pluton indicate a subduction-related mantle source, similar to the nearby Taftan volcano, which forms the recent Makran magmatic arc together with the Bazman volcano to the west and the Koh-i-Soltan volcano to the east (Pang et al., 2014).
Based on our age constraints and framework analysis of the shallow-marine to continental upper Oligocene to lower Miocene sequence within the Karvandar Basin, we propose that active volcanism related to arc magmatism of the northward-directed Makran subduction zone initiated in the region of Mirabad and present-day Taftan volcano no later than in the late Aquitanian, at 24.5 Ma, when a gravitational collapse of a volcano flank occurred, resulting in the emplacement of the Karvandar olistostrome (Figure 16b).The volcanic activity persisted until at least until 17.7 Ma (Figure 13a), indicated by the presence of thick tuff layers within the Karvandar Basin, suggesting a nearby source.It is likely that volcanic rocks covered extensive areas surrounding the Taftan volcano (Aghanabati et al., 1994), considering the first-cycle nature of the investigated sandstone framework.This finding excludes potential recycling of clastic rocks from the NAC.

Conclusions
We conducted magnetostratigraphy and sandstone framework of the clastic shallow-marine to continental sequence of the Karvandar Basin, a peripheral foreland basin of the South SSZ.We draw the following conclusions from the interpretation of the multidisciplinary results presented in this study: • The magnetostratigraphic section, anchored to the GPTS by U-Pb zircon ages of interlayered tuffs, reveals a late Oligocene to early Miocene depositional age of the Karvandar sequence (∼24-18 Ma).

Figure 1 .
Figure 1.Geology of southeast Iran.(a) Tectono-stratigraphic setting of the ∼N-S-trending South Sistan Basin (white patterns; modified from Mohammadi, Burg, and Winkler (2016)) and the W-E-trending Makran Accretionary Wedge (gray patterns; modified from Burg (2018)).The Bazman, Taftan and Kuh-e-Sultan volcanics represent the recent magmatic arc of the Makran subduction zone.Yellow colors indicate the Tertiary Karvandar Basin.Pink colors denote volcanic and ultrabasic slivers of the Karvandar Exotic Complex.Green colors illustrate exposures of the Tortonian olistrostrome in the Makran (Burg et al., 2008).Red dashed lines: Country borders.(b) Satellite image shows the Karvandar Basin with NNW-SSE-trending elongated, sub-circular synclines, bounded by the Karvandar Fault in the east.Study area outlined in panel (a).Star: Detrital zircon sample location ofMohammadi et al. (2016b).Triangle: Fossil sample location ofGhorashi and Vaezipour (1987).

Figure 2 .
Figure 2. Satellite imageries (left) and corresponding interpretations of synclinal structures (right).Whitish colors in panels (a, c) indicate mud-dominated lithology underlying the shallow-marine to continental sequence.Imagery of the northern syncline (c) shows the ∼25-m-thick tuff layer (samples PG-T01 and T04).Major angular unconformities, that is, onlaps, are visible.Locations outlined in Figure 1b.

Figure 3 .
Figure 3. Sampling sites along the magnetostratigraphic section.Black line outlines the Pirgel mud volcano.Red-white dashed layer indicates the rural road leading to the mud volcano.Square shows the location of tuff sample PG-T05.PG170 indicates the position of the uppermost magnetostratigraphic section sample.White arrow indicates the orientation of the fold axis.Location outlined in Figure 1b.

Figure 4 .
Figure 4. Photographs of lithologies in the studied section.(a) Thin-bedded sandstones with ripple marks of the shallow-marine lowermost part.(b) Vertically stacked conglomerate-sandstone intervals above finer-grained intervals.(c) Channelized coarse-grained sandstones and conglomerates.(d) Flute cast indicating flow direction.

Figure 5 .
Figure 5. Stratigraphic section.(a) Detailed stratigraphic description with locations of selected paleocurrents, marine incursions, and first occurrences of conglomerate reworking.(b) Measured bedding dips.

Figure 6 .
Figure 6.Photographs of the Pirgel mud volcano and the mud-dominated lithology.(a) Eastward view of the mud volcano.(b) Active mud gryphon in front, inactive mud cone in back.(c) Sub-angular volcanic block in mud matrix.(d) Volcanic and coral pebbles covering the mud volcano.

Figure 7 .
Figure 7. Selected diagrams of tilt-corrected Natural remanent magnetization thermal demagnetization curves.White and black dots represent vertical and horizontal projections (after Zijderveld (1967)) of (d, f) normal and (a-c, e) reversed polarities.

Figure 8 .
Figure 8. Paleomagnetic characteristic remanent magnetization (ChRM) directions, without and with tectonic tilt-correction.(a, b) All samples with clear ChRM.(c, d) All ChRM transposed onto same hemisphere for statistical significance.Mean direction of tilt-corrected transposed data indicate a counter-clockwise rotation of ∼30° (d).

Figure 11 .
Figure 11.Sediment framework along the sampled section.(a) Variation in sandstone framework.(b) Variations in sandstone rock fragment composition.(c) Variations in microconglomerate fragment composition.

Figure 12 .
Figure 12. Rose diagram indicating (a) directional and (b) non-directional paleocurrent orientations from the Karvandar Basin with dominant southwest directions.(c) Paleocurrents from the South Sistan Basin for comparison (Mohammadi, Burg, & Winkler, 2016).Light gray in panels (a, b) indicate paleocurrents from the Karvandar Basin corrected for an observed counter clock-wise rotation of 30°.

Figure 13 .
Figure 13.U-Pb zircon ages of three tuff samples.(a, b) Sample PG-T01 and T04 are from northern syncline (location in Figures2c and 2d) at the top of the section.(c) Sample PG-T05 is from western syncline (location in Figure3) at the bottom of the magnetostratigraphic section.Only concordant ages were used to constrain final age.Uncertainty ellipses are 2SE.See TableS4for isotopic ratios and ages.

Figure 15 .
Figure 15.Correlation of the local magnetopolarity stratigraphy from the Karvandar Basin with the geomagnetic polarity timescale(Gradstein & Ogg, 2020).The correlation is anchored by the U-Pb calcite age of the coral that pre-dates the base of the section (blue dotted line) and the tuff layers at the base and above the section (red dotted lines).Shaded red indicate estimations of the position of the tuff layers with regard to the magnetostratigraphic section.Resulting sediment accumulation rates fit the observed transition from marine to continental facies.The rates of sedimentation are illustrated beside the curve in cm/kyr.

Figure 16 .
Figure 16.Schematic evolution of the Karvandar Basin from Eocene to early Miocene.(a) Pre-to syn-collisional trench deposition of typical Neh Accretionary Wedge deep-marine turbidites.(b) Emplacement of olistostrome with volcanic blocks sourced from gravitational collapse of a proximal volcano.(c) First deposition of continental sandstones and conglomerates in the east and shallow-marine facies in the west.(d) Sediment load leads to downbuilding into water-saturated, non-lithified olistostrome mass.
• First-cycle volcanic nature of the sandstone framework and the occurrence of up to 25-m-thick tuff layers indicate syn-sedimentary active volcanism during the deposition of the Karvandar Basin.• The mud-dominated lithology underlying the clastic sequence is interpreted to represent an olistostrome sourced from a volcanic flank, based on the occurrence of randomly oriented centimeter-to meter-sized angular to sub-angular blocks of volcanic origin.• A calcite U-Pb age of a coral block in the olistostrome indicates an emplacement age of 24.5 ± 1.3 Ma. • Angular unconformities, the absence of anticlines, and significant thickness variations of the marine strata across mud ridges suggest early mud diapirism and gravity-driven minibasin downbuilding into the olistostrome.• Subduction-related magmatic intrusions of early Miocene age toward the northeast of the Karvandar Basin with geochemical signatures similar to the quaternary Taftan volcano indicate that magmatic arc volcanism related to the Makran subduction zone started in latest Oligocene and represents the source of the volcanic blocks in the olistostrome and Karvandar clastic sequence and tuffs.