Makran Continental Margin Research Articles

Evolution Process of Chemical Weathering and Sediment sources in the Makran Continental margin since the Younger Dryas

The chemical weathering processes and sedimentary source evolution since the Younger Dryas (YD) in the low-latitude arid continental margin have been investigated. Two sediment cores, MK07G and MK09G, were retrieved from the Makran continental margin in the northern Arabian Sea and subjected to analyses of major and trace elements, along with AMS14C dating. The results show that since the YD, the weathered parent rocks of Makran sediments have remained relatively stable, predominantly consisting of felsic rocks, with some contributions from mafic rocks. The Makran sediments exhibit initial to moderate weathering, with no discernible effects from grain size sorting or disturbances from sediment recycling, indicating primary deposition. Significant contributions of terrigenous eolian dust from surrounding continents (e.g., the Indian subcontinent, Arabian Peninsula, and northeastern Africa) were identified, along with riverine inputs from the Dasht River and fine-grained components from the Late Pleistocene Indus delta sediment, as well as proximal basin sedimentation. The evolution of sediment sources in the study area is significantly influenced by the Indian Monsoon and westerly wind systems, with intensified monsoon phases and westerly conditions correlating with increased fluvial input. Furthermore, chemical weathering processes since the YD are closely linked to local precipitation patterns, where intensified rainfall enhances weathering intensity. Records from the Makran continental margin indicate a teleconnection between chemical weathering and sedimentary processes in the Arabian Sea and Bond events in the North Atlantic, highlighting the extensive influence of Northern Hemisphere climate fluctuations.

Seafloor methane emission on the Makran continental margin

Seafloor methane emission is widespread on both active and passive continental margins, which may exerts significant impact on global climate change, ocean acidification, cold seep ecosystem, and global carbon cycle. However, due to the limitation of the thick water body, systematic knowledge of detection, quantification and activity of the submarine methane seepage is still unreachable, which greatly limits the assessment of the environmental impact. In 2018, a comprehensive geological survey, including multibeam mapping, seafloor observation, and seismic reflection profiling, was conducted using R/V “Haiyangdizhi 10” on the Makran continental margin. Sixty-five gas flares, which indicated seafloor methane seepage, were detected in a total survey area of 32,000 km2. The total methane flux of the surveyed area is estimated to be 4.7-5.9 × 103 Mg/yr, accounting for 0.013-0.016% of the global seafloor methane emission. In addition, three gas seeps, which were active in 2007, were inactive during our survey in 2018. It is inferred that the intermittent activity might be related to the periodic pressure release and accumulation in the system. All the flares vanish in the water column, which indicates that all the methane gas was oxidized and/or dissolved by seawater. No methane was observed entering the atmosphere in gas phase. In this study, we present new data sets of methane seeps on the Makran continental margin, which are useful to better understand the behavior of the submarine methane seepage.

Distribution and development of submarine mud volcanoes on the Makran Continental Margin, offshore Pakistan

Abstract In 2018, China Geological Survey and National Institute of Oceanography of Pakistan conducted a comprehensive geological survey on the Makran Continental Margin offshore Pakistan using the R/V Haiyangdizhi10. Seismic profiles, multibeam bathymetric and water column data, and seafloor observations were obtained during the expedition. Twelve mud volcanoes and three pockmarks were discovered in the surveyed area on the Makran Continental Margin. The heights of the mud volcanoes and the depths of the pockmarks are 12–121 m and −108 to −26 m, respectively, with diameter of 500–1400 m. The mud volcanoes are distributed on top of the W-E trending ridges at 1200–2800 m water depth. Six mud volcanoes and one pockmark were active during the investigation as indicated by the gas flares mapped in the multibeam water column data. Acoustic blanking zones, faults and gas chimneys are distributed ubiquitously beneath the mud volcanoes on the seismic profiles. Their spatial correlation with mud volcanoes indicates that the faults and gas chimneys provide pathways for fluid migration. In addition, the Bottom Simulating Reflectors (BSR), interface of gas hydrate and free gas, were also discovered in multiple seismic profiles. Based on the BSR discontinuity and piercing of the gas chimneys, it is inferred that mud volcano activities might cause gas hydrate dissociation, leading to the upward movement of the BSR. Tectonic uplifting, seismic-induced shock and fluid overpressure induced by rapid sedimentation are the major driving forces for the mud volcano formation and development. Gases from deep strata and gas hydrate decomposition are suspected to provide additional forces for fluid migration. Pressure decrease in the deep subsurface and gas hydrate sealing of the faults and fracture are speculated to be important in controlling the intermittent mud volcano activities and potential pockmark formation. The new model of mud volcano activities depicted in this study might be applicable to other mud volcanoes elsewhere.

Sea Level Change Controlled the Sedimentary Processes at the Makran Continental Margin Over the Past 13,000 yr

AbstractClimate change, sea level fluctuations, and tectonic uplift play key roles in the evolution of sedimentary systems on continental margins. In the Arabian Sea, the Makran continental margin is suitable for studying these processes during the Holocene due to its high sedimentation rate and continuous sequence of Holocene sediments. Here, high‐resolution clay mineral and grain size analyses have been conducted to better understand the sedimentary process and controlling factors over the past 13,000 yr. The results show that the influence of Indus‐derived sediments on the Makran continent margin is negligible. Holocene relative sea level fluctuations can strongly control the shelf accommodation space due to the narrow shelf, which in turn affects the deposition process at the Makran continental margin. The sediment records at the central Makran continental margin are more suitable for studying ancient earthquakes or tsunamis.

Geochemistry of Bazman thermal springs, southeast Iran

Thermal springs of the Bazman geothermal field, 27km south of the Holocene Bazman volcano caldera in Makran continental margin, SE Iran, were studied for the first time. New data on major components, selected trace elements (Li, Rb, Cs, Ba, Sr, Fe, Al), water isotopic composition (H, O and S), chemical and isotopic (δ13C of CO2 and CH4, 3He/4He and 40Ar/36Ar) composition of dissolved and bubbling gases from the hottest vents are presented. Four groups of springs with temperature range of 27–44°C discharge Na-Cl waters with total dissolved solids (TDS) of ∼800 to ∼7000mg/L from different aquifers composed of diverse type of rocks including granites and limestone. Only the hottest and most saline springs of the Bazman field with low bicarbonate are close to equilibrium with surrounding rocks whereas the others discharge immature waters. Geothermometry based on SiO2 concentrations, Na-K and Na-K-Ca-Mg systems shows low equilibrium temperatures of up to 130°C, consist with the temperatures estimated by alumino-silicate minerals saturation indices. The water isotopic composition (δ18O and δD) indicates meteoric origin with a small oxygen isotopic shift measured in the saline waters. The δ34S values of SO4 indicate influence of gypsum and anhydrite dissolution from the host rock. Dissolved and free gases are N2-rich (>95vol.%) with a high He content (0.5vol.%). A biogenic origin may be suggested for CH4 and CO2 based on their carbon isotopic characteristics (−62‰ and −13‰ vs. V-PDB, respectively). The R/Ra value of 0.5 indicates a contribution of about 6 % of He from the mantle. It is suggested that the Bazman thermal waters are heated at considerable depth (4−5km) in a deep fault system most probably by the regional heat flow according to the local geothermal gradient.

Makran continental margin sedimentation during the Late Holocene

Sedimentation in the Makran active margin is governed by a complex interaction of atmospheric, tectonics, and hydrodynamic setting of the northern flank of the Gulf of Oman. The mixed clastic carbonate sediments in the tectonically and hydrodynamically active environment have complicated the distribution pattern. The region is suffering from basic sedimentological data, and specifically, the sedimentation history of the Holocene deposits has been rarely studied in the Iranian coast. To deal with this deficiency, surface and core sediment samples from the Iranian continental shelf and upper slope of the Gulf of Oman have been studied using standard sedimentological techniques. The overall sediment distribution pattern demonstrates that the grain size gradually decreases from the shoreline to the deeper zones. However, some medium- to coarse-grained sand patches can be found in the deeper parts, especially in the middle part of the studied area that can be related to sediment supply of ephemeral rivers discharging into the sea in rainy seasons and (or) high-energy events (i.e., turbidites and tsunamis during the Holocene). Several horizons of the coarse-grained detrital sediments are detectable in the upper slope sediment cores. The coarse-grained materials are received from the hinterland during flash floods and could be accumulated due to mass wasting events. The elevated amount of organic materials in the upper slope indicates a deficit of dissolved oxygen that leads to preservation of organic materials in the bottom sediments.

Formation of seep carbonates along the Makran convergent margin, northern Arabian Sea and a molecular and isotopic approach to constrain the carbon isotopic composition of parent methane

Authigenic carbonate deposits have been sampled with the remotely operated vehicle ‘MARUM-QUEST 4000m’ from five methane seeps between 731 and 1823m water depth along the convergent Makran continental margin, offshore Pakistan (northern Arabian Sea). Two seeps on the upper slope are located within the oxygen minimum zone (OMZ; ca. 100 to 1100m water depth), the other sites are situated in oxygenated water below the OMZ (below 1100m water depth). The carbonate deposits vary with regard to their spatial extent, sedimentary fabrics, and associated seep fauna: Within the OMZ, carbonates are spatially restricted and associated with microbial mats, whereas in the oxygenated zone below the OMZ extensive carbonate crusts are exposed on the seafloor with abundant metazoans (bathymodiolin mussels, tube worms, galatheid crabs). Aragonite and Mg-calcite are the dominant carbonate minerals, forming common early diagenetic microcrystalline cement and clotted to radial-fibrous cement. The δ18Ocarbonate values range from 1.3 to 4.2‰ V-PDB, indicating carbonate precipitation at ambient bottom-water temperature in shallow sediment depth. Extremely low δ13Ccarbonate values (as low −54.6‰ V-PDB) point to anaerobic oxidation of methane (AOM) as trigger for carbonate precipitation, with biogenic methane as dominant carbon source. Prevalence of biogenic methane in the seepage gas is corroborated by δ13Cmethane values ranging from −70.3 to −66.7‰ V-PDB, and also by back-calculations considering δ13Cmethane values of carbonate and incorporated lipid biomarkers. These calculations (Δδ13Cmethane–carbonate, Δδ13CANME–methane, Δδ13CMOX–methane) prove to be useful to assess the carbon stable isotope composition of seeping methane if this has not been determined in the first place; such an approach represents a useful tool to reconstruct fluid composition of ancient seeps. AOM is also revealed by lipid biomarkers of anaerobic methane oxidizing archaea such as crocetane, pentamethylicosane (PMI), and sn2-hydroxyarchaeol strongly depleted in 13C (δ13C values as low as −127‰ V-PDB). Biomarkers of sulphate-reducing bacteria are also abundant, showing slightly less negative δ13C values, but still significantly 13C-depleted (average values as low as −101‰). Other bacterial biomarkers, such as bacteriohopanepolyols (BHPs), hopanols, and hopanoic acids are detected in most carbonates, but are particularly common in seep carbonates from the non-OMZ sites. The BHP patterns of these carbonates and their low δ13C values resemble patterns of aerobic methanotrophic bacteria. In the shallower OMZ sites, BHPs revealed much lower contents and varying compositions, most likely reflecting other sources than aerobic methanotrophic bacteria. 230Th/U carbonate ages indicate that AOM-induced carbonate precipitation at the deeper non-OMZ seeps occurred mainly during the late Pleistocene–Holocene transition, i.e. between 19 and 15ka before present, when the global sea level was lower than today.

Tectonics of the Musandam Peninsula and northern Oman Mountains: From ophiolite obduction to continental collision

ABSTRACT The tectonics of the Musandam Peninsula in northern Oman shows a transition between the Late Cretaceous ophiolite emplacement related tectonics recorded along the Oman Mountains and Dibba Zone to the SE and the Late Cenozoic continent-continent collision tectonics along the Zagros Mountains in Iran to the northwest. Three stages in the continental collision process have been recognized. Stage one involves the emplacement of the Semail Ophiolite from NE to SW onto the Mid-Permian–Mesozoic passive continental margin of Arabia. The Semail Ophiolite shows a lower ocean ridge axis suite of gabbros, tonalites, trondhjemites and lavas (Geotimes V1 unit) dated by U-Pb zircon between 96.4–95.4 Ma overlain by a post-ridge suite including island-arc related volcanics including boninites formed between 95.4–94.7 Ma (Lasail, V2 unit). The ophiolite obduction process began at 96 Ma with subduction of Triassic–Jurassic oceanic crust to depths of > 40 km to form the amphibolite/granulite facies metamorphic sole along an ENE-dipping subduction zone. U-Pb ages of partial melts in the sole amphibolites (95.6– 94.5 Ma) overlap precisely in age with the ophiolite crustal sequence, implying that subduction was occurring at the same time as the ophiolite was forming. The ophiolite, together with the underlying Haybi and Hawasina thrust sheets, were thrust southwest on top of the Permian–Mesozoic shelf carbonate sequence during the Late Cenomanian–Campanian. Subduction ended as unsubductable cherts and limestones (Oman Exotics) jammed at depths of 25–30 km. The Bani Hamid quartzites and calc-silicates associated with amphibolites derived from alkali basalt show high-temperature granulite facies mineral assemblages and represent lower crust material exhumed by late-stage out-of-sequence thrusting. Ophiolite obduction ended at ca. 70 Ma (Maastrichtian) with deposition of shallow-marine limestones transgressing all underlying thrust sheets. Stable shallow-marine conditions followed for at least 30 million years (from 65–35 Ma) along the WSW and ENE flanks of the mountain belt. Stage two occurred during the Late Oligocene–Early Miocene when a second phase of compression occurred in Musandam as the Arabian Plate began to collide with the Iran-western Makran continental margin. The Middle Permian to Cenomanian shelf carbonates, up to 4 km thick, together with pre-Permian basement rocks were thrust westwards along the Hagab Thrust for a minimum of 15 km. Early Miocene out-of-sequence thrusts cut through the shelf carbonates and overlying Pabdeh foreland basin in the subsurface offshore Ras al Khaimah and Musandam. This phase of crustal compression followed deposition of the Eocene Dammam and Oligocene Asmari formations in the United Arab Emirates (UAE), but ended by the mid-Miocene as thrust tip lines are all truncated along a regional unconformity at the base of the Upper Miocene Mishan Formation. The Oligocene–Early Miocene culmination of Musandam and late Cenozoic folding along the UAE foreland marks the initiation of the collision of Arabia with Central Iran in the Strait of Hormuz region. Stage three involved collision of Arabia and the Central Iran Plate during the Pliocene, with ca. 50 km of NE-SW shortening across the Zagros Fold Belt. Related deformation in the Musandam Peninsula is largely limited to north and eastward tilting of the peninsula to create a deeply indented coastline of drowned valleys (rias).

Quantification of gas bubble emissions from submarine hydrocarbon seeps at the Makran continental margin (offshore Pakistan)

Evidence for twelve sites with gas bubble emissions causing hydroacoustic anomalies in 18 kHz echosounder records (‘flares’) was obtained at the convergent Makran continental margin. The hydroacoustic anomalies originating from hydrocarbon seeps at water depths between 575 and 2870 m disappeared after rising up to 2000 m in the water column. Dives with the remotely operated vehicle ‘Quest 4000 m’ revealed that several individual bubble vents contributed to one hydroacoustic anomaly. Analyzed gas samples suggest that bubbles were mainly composed of methane of microbial origin. Bubble size distributions and rise velocities were determined and the volume flux was estimated by counting the emitted bubbles and using their average volume. We found that a low volume flux (Flare 1 at 575 mbsl: 90 ml/min) caused a weak hydroacoustic signal in echograms whereas high volume fluxes (Flare 2 at 1027 mbsl: 1590 ml/min; Flare 5 C at 2870 mbsl: 760 ml/min) caused strong anomalies. The total methane bubble flux in the study area was estimated by multiplying the average methane flux causing a strong hydroacoustic anomaly in the echosounder record with the total number of equivalent anomalies. An order‐of‐magnitude estimate further considers the temporal variability of some of the flares, assuming a constant flux over time, and allows a large range of uncertainty inherent to the method. Our results on the fate of bubbles and the order‐of‐magnitude estimate suggest that all of the ∼40 ± 32 × 106 mol methane emitted per year within the gas hydrate stability zone remain in the deep ocean.

Mud volcanoes at the front of the Makran accretionary complex, Pakistan

Abstract Conical mounds, 1–1.5 km in diameter, and up to 65 m high were mapped at the foot of the active Makran continental margin. The mounds developed seaward of the accretionary front in a relatively planar zone where the beginning of build-up of tectonic pressure initiates deformation. Based on shallow high-resolution 4 kHz sediment echosounding, the sedimentary sequence in this area is generally well stratified, as indicated by closely spaced horizontal reflections. However, in the vicinity of the mounds the sediment is characterised by many acoustically transparent zones, which are 100–300 m in diameter and cut near-vertically through the horizontal reflectors. Two sediment cores from the top of the largest cone and a neighbouring acoustically transparent zone reveal small-scale post-depositional deformation in a stratified sequence and methane concentrations up to 40,000 ng/g. This deformation and disruption of potential reflectors provides a clue to explain the acoustic transparency: we interpret it as caused by the rise of charged fluids and mud, leading initially to the (slight) disturbance of the generally good acoustic reflectors and eventually to the formation of conical mud mounds (mud volcanoes). MCS data, showing a buried mound in an analogous structural position, support the idea of tectonically induced mud/fluid expulsion seaward of the accretionary front.

Controls on terrigenous sediment supply to the Arabian Sea during the late Quaternary: the Makran continental slope

The input of terrigenous sediment along the tectonically active Makran continental margin off south-western Pakistan (Gulf of Oman, northern Arabian Sea) is studied on the basis of sediment cores distributed along a transect from the upper slope to the abyssal plain. Spatial and temporal variations in sediment composition, sedimentation rate and turbidite frequency in late Pleistocene–Holocene time (last ∼20 14C ka) will be discussed and related to changes in sea level and climate, and tectonic activity. End-member modelling of the grain-size distributions of the hemipelagic and turbiditic sediments indicate that the sediments are adequately described as mixtures of three end-members, which represent turbidite sand, turbidite silt or eolian dust, and fluvial mud. The geochemical and mineralogical compositions of the hemipelagic sediments indicate that the eolian dust was dominantly supplied from the northern Arabian Peninsula and the Persian Gulf region, and that the fluvial input is from the Makran margin. The ratio of contributions of eolian and fluvial sediment in the hemipelagic intervals is used as an indicator of continental aridity, i.e. summer monsoon intensity. Highest Holocene turbidite frequencies and sedimentation rates are recorded at the deepest coring sites. They are related to the proximity of the deformation front of the accretionary prism. Turbidite sedimentation on the upper continental slope was most frequent during the last glacial period of sea-level lowstand, and continued during the entire deglaciation and sea-level rise. Infrequent turbidite sedimentation occurred during the Holocene highstand of sea level. Turbidite sedimentation during the period of late sea-level rise and the Holocene sea-level highstand is inferred to be due to the strong impact of episodic events (i.e. flash floods and earthquakes) and because of the narrow shelf of the active Makran continental margin.

Evidence for a thick free gas layer beneath the bottom simulating reflector in the Makran accretionary prism

Seismic reflection data from the Makran continental margin indicate the presence of a strong and widespread bottom simulating reflector (BSR). We apply a nonlinear full waveform inversion technique to multichannel reflection data from this area, to investigate the detailed velocity structure and hence the origin of the BSR. Our result shows an abrupt decrease in the compressional wave velocity from 2.2 to 1.3 km/s at a depth of 500 m below the sea-bed. The low velocity zone is unusually thick (∼200–350 m), and may contain large quantities of free gas, similar to some of the recently drilled Blake Ridge sites of ODP Leg 164. The voluminous free gas may have been generated by the dissociation of gas hydrates as a consequence of the upward movement of the base of gas-hydrate stability field, relative to the sediment column, due to uplift and sedimentation in the accretionary wedge.

Sediment compaction and fluid migration in the Makran Accretionary Prism

The Makran continental margin in the Gulf of Oman forms the seaward extremity of an accretionary sediment prism which extends several hundred kilometers inland. A recently acquired multichannel seismic reflection profile shot across the margin imaged the structure of the prism in greater detail than was previously possible and allowed us to investigate the relationship between deformation and pore fluid motion in the region. Velocity analyses of the common midpoint gathers reveal a marked change in velocity structure at the toe of the accretionary wedge, as seen in previous sonobuoy wide‐angle data. Accreted sediments show significantly higher vertical velocity gradients than those of sediments entering the prism; this change is interpreted as due to porosity reduction as pore fluids are squeezed out of the compacting sediment. A prominent “bottom simulating reflector” appears 500–800 m beneath the sea bed. Several lines of evidence suggest that this reflector represents the base of a gas hydrate zone underlain by widespread free gas, which may be exsolved from pore water migrating from deep within the sediment pile up permeable fault planes imaged in the profile. The hydrate reflector appears to shallow in the region of some faults, suggesting a temperature anomaly due to the presence of warm pore fluids. A heat flow profile derived from the depth of the hydrate reflector does not show the expected landward decrease as the sediment pile thickens. Simple thermal modeling suggests that advective heat flow within the prism may explain this anomaly. The inferred presence of overpressured pore fluids in the Makran suggests that accreted sediments have a low permeability. The seismic evidence suggests a two‐stage compaction process, with rapid initial dewatering through intergranular permeability as sediment enters the prism followed by a buildup of pore pressure as the permeability decreases and fluid migration is restricted to fault zones.

Deformation of the Makran accretionary sediment prism in the Gulf of Oman (north-west Indian Ocean)

Summary The offshore Makran of Pakistan and Iran in the Gulf of Oman forms the seaward margin of a folded and faulted accretionary sediment wedge which extends several hundred kilometres inland across the onshore Makran. Sediment is being scraped off the oceanic Arabian plate as it subducts shallowly northward beneath the Eurasian plate. The submerged front of the accretionary prism has been traced along the entire width of the Makran continental margin, from the Strait of Hormuz in the west to the boundary with the Indian plate in the east. The structure of a 70 by 90 km portion of the deformed sediment belt was mapped in detail by a grid of continuous seismic reflection profiles. This survey demonstrates that the folds are laterally continuous and are well lineated, that the deformed belt appears to be migrating southward at a rate of about 10 km every million years and that folding of the initially undisturbed abyssal plain sediments occurs in the southernmost, or ‘frontal’ fold. Mapping of the three-dimensional structure of the frontal fold shows that it systematically increases then decreases in height along strike over a distance of about 80 km. Folding is initially restricted to the uppermost 2½ km of the sediment pile, and there is local evidence of small-scale shale diapirism. The frontal fold is then incorporated into the accretionary prism by uplift along a thrust fault which flattens out within the sedimentary section, possibly in overpressured shales. There is little subsequent deformation of the uplifted material or of the sediment which infills the basins between the fold ridges until some 70 km north of the frontal fold when further rapid uplift occurs, eventually elevating the sediment above sea-level. I attribute this second phase of uplift to major imbricate thrust faulting extending down to the oceanic basement; most of the shortening and thickening of the sediment wedge occurs in this region. The relatively simple pattern of uplifted folds and intervening basins in the frontal 70 km of the accretionary wedge results from a balance of compressive and normal stresses on the thick sediment section. This simple deformational style of the seaward part of the accretionary prism can be traced eastwards until it becomes chaotically disrupted by the consumption of a basement ridge which protrudes to just beneath the seafloor.

Tectonics of the western Gulf of Oman

The Oman line, running northward from the Strait of Hormuz separates a continent‐continent plate boundary to the northwest (Persian Gulf region) from an ocean‐continent plate boundary to the southeast (Gulf of Oman region). A large basement ridge detected on multichannel seismic reflection and gravity profiles to the west of the Oman line is probably a subsurface continuation of the Musandam peninsula beneath the Strait of Hormuz. Collision and underthrusting beneath Iran of the Arabian plate on which this ridge lies has caused many of the large earthquakes that have occurred in this region. Convergence between the oceanic crust of the Arabian plate beneath the Gulf of Oman and the continental Eurasian plate beneath Iran to the north is accommodated by northward dipping subduction. A deformed sediment prism which forms the offshore Makran continental margin and which extends onto land in the Iranian Makran has accumulated above the descending plate. In the western part of the Gulf of Oman, continued convergence has brought the opposing continental margin of Oman into contact with the Makran continental margin. This is an example of the initial stages of a continent‐continent type collision. A model of imbricate thrusting is proposed to explain the development of the fold ridges and basins on the Makran continental margin. Sediments from the subducting plate are buckled and incorporated into the edge of the Makran continental margin in deformed wedges and subsequently uplifted along major faults that penetrate the accretionary prism further to the north.

Recent fold development in the Gulf of Oman

Detailed examination of the structure of folds at the edge of the Makran continental margin in the Gulf of Oman suggests that only the uppermost 2.5 km of sediment is initially folded. Deeper layers show little evidence of buckling. Compression rather than diapirism is probably the major driving force, but overpressured clays may play a significant role in determining the zone of décollement between folded and unfolded strata.