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Supplementary figure c: Bamboo Creek Sector Sout

Fault interpretation in seismic reflection data: an experiment analysing the impact of conceptual model anchoring and vertical exaggeration

The use of conceptual models is essential in the in- terpretation of reflection seismic data. It allows interpreters to make geological sense of seismic data, which carries inherent uncertainty. However, conceptual models can create powerful anchors that prevent interpreters from reassessing and adapting their interpretations as part of the interpretation process, which can subsequently lead to flawed or erroneous outcomes. It is therefore critical to understand how conceptual models are generated and applied to reduce unwanted effects in interpretation results. Here we have tested how interpretation of vertically exaggerated seismic data influenced the creation and adoption of the conceptual models of 161 participants in a paper-based interpretation experiment. Participants were asked to interpret a series of faults and a horizon, offset by those faults, in a seismic section. The seismic section was randomly presented to the participants with different horizontal-vertical exaggeration (1 : 4 or 1 : 2). Statistical analysis of the results indicates that early anchoring to specific conceptual models had the most impact on interpretation outcome, with the degree of vertical exaggeration having a subdued influence. Three different conceptual models were adopted by participants, constrained by initial observations of the seismic data. Interpreted fault dip angles show no evidence of other constraints (e.g. from the application of accepted fault dip models). Our results provide evidence of biases in interpretation of uncertain geological and geophysical data, including the use of heuristics to form initial conceptual models and anchoring to these models, confirming the need for increased understanding and mitigation of these biases to improve interpretation outcomes

Linkage of crustal deformation between the Archaean basement and the Proterozoic cover in the Peräpohja Area, northern Fennoscandia

Constraints from restoration of minor rift-related faults preserved from subsequent Palaeoproterozoic overprint along the southern margin of the Peräpohja Belt, northern Fennoscandia indicate local NE-SW extension directions prevailed at the time of 2.44 Ga rifting of the Archaean continent. This palaeostress field is attributed to pull-apart setting of the Peräpohja Belt at a left overstepping zone of major sinistral N-S trending deformation zones. Consequently, the variably overprinted NW-SE trending structures of the Peräpohja Belt, including the NW-SE central graben were originally generated as normal faults. The proposed setting is compatible with a conducted reconnaissance sandbox analogue model where the orientation of the major normal faults was controlled by the ambient stress field within the step-over zone between strike-slip faults. By contrast, the faults bounding E-W to ENE-WSW trending basement horsts are the result of reactivation of older basement structures underlying the pull-apart basin. Based on the results, we infer that the final break-up of the Archaean continent in northern Fennoscandia utilized the 2.44 Ga rift-related NW-SE trending crustal anisotropy (parallel with dyke swarms), with pre-existing Archaean N-S trending structures reactivated as second-order shear zones in-between. The voluminous syn-rift 2.5-2.4 Ga and later pulses of mafic magmatism (2.32-1.98 Ga) are considered indicative of active-type rifting which onset was controlled by a mantle plume centred within a supercraton formed by the Superior, Hearne and Karelian (-Kola) cratons. The presented model supports the continuity of the Archaean craton beyond the Caledonian Orogen towards north-west and explains its segmentation, suggesting a model for the localization of the Palaeoproterozoic supracrustal belts of Northern Fennoscandia. Moreover, this paper provides an approach to understand the basement-cover relationships and the structural signatures within these relatively shallow supracrustal belts which are highly prospective for mineral deposits.</p

Archaean basin margin geology and crustal evolution : an East Pilbara traverse

A palinspastic reconstruction of a 100 km long traverse through Archaean rocks of the East Pilbara, Western Australia, includes new observations of the deformation preceding the now visible greenstone belt pattern. The restoration is time-calibrated with all available U–Pb datings. Between incompletely preserved basin sequences, two superposed Palaeoarchaean volcano-sedimentary basins (the Coongan and Salgash Basins) are separated by an eastwards time-transgressive interface tentatively interpreted as an onlap surface. For over 140 Ma, the basin margin architecture was structurally controlled by superposed extensional growth fault arrays (D1) with associated dyke swarms in a curved pattern spatially not related to that of the actual distribution of granite domes and greenstone belts. The basins are interpreted to have formed by collapse after arching above hotspots due to phase transitions by mini-subduction of slabs of cooled water-saturated basalt towards the base of an originally c. 45 km mafic crust. At c. 3.31 Ga, the extension was replaced by plate-driven regional NW–SE compression (D2) inferred from NW-over-SE shear and ramp-and-flat thrusts, partly reversing offsets of the D1 extension. The recognition of widespread D2 pre-doming compression is important because it triggered the c. 3.18 Ga start of formation of the dome-and-keel pattern (D3) visible today, which culminated at c. 2.9 Ga

2.45 Ga break-up of the Archaean continent in Northern Fennoscandia: Rifting dynamics and the role of inherited structures within the Archaean basement

Constraints from restoration of minor rift-related faults preserved from subsequent Palaeoproterozoic overprint along the southern margin of the Peräpohja Belt, northern Fennoscandia indicate local NE-SW extension directions prevailed at the time of 2.44 Ga rifting of the Archaean continent. This palaeostress field is attributed to pull-apart setting of the Peräpohja Belt at a left overstepping zone of major sinistral N-S trending deformation zones. Consequently, the variably overprinted NW-SE trending structures of the Peräpohja Belt, including the NW-SE central graben were originally generated as normal faults. The proposed setting is compatible with a conducted reconnaissance sandbox analogue model where the orientation of the major normal faults was controlled by the ambient stress field within the step-over zone between strike-slip faults. By contrast, the faults bounding E-W to ENE-WSW trending basement horsts are the result of reactivation of older basement structures underlying the pull-apart basin. Based on the results, we infer that the final break-up of the Archaean continent in northern Fennoscandia utilized the 2.44 Ga rift-related NW-SE trending crustal anisotropy (parallel with dyke swarms), with pre-existing Archaean N-S trending structures reactivated as second-order shear zones in-between. The voluminous syn-rift 2.5-2.4 Ga and later pulses of mafic magmatism (2.32-1.98 Ga) are considered indicative of active-type rifting which onset was controlled by a mantle plume centred within a supercraton formed by the Superior, Hearne and Karelian (-Kola) cratons. The presented model supports the continuity of the Archaean craton beyond the Caledonian Orogen towards north-west and explains its segmentation, suggesting a model for the localization of the Palaeoproterozoic supracrustal belts of Northern Fennoscandia. Moreover, this paper provides an approach to understand the basement-cover relationships and the structural signatures within these relatively shallow supracrustal belts which are highly prospective for mineral deposits.</p

Archaean basin margin geology and crustal evolution : an East Pilbara traverse

A palinspastic reconstruction of a 100 km long traverse through Archaean rocks of the East Pilbara, Western Australia, includes new observations of the deformation preceding the now visible greenstone belt pattern. The restoration is time-calibrated with all available U–Pb datings. Between incompletely preserved basin sequences, two superposed Palaeoarchaean volcano-sedimentary basins (the Coongan and Salgash Basins) are separated by an eastwards time-transgressive interface tentatively interpreted as an onlap surface. For over 140 Ma, the basin margin architecture was structurally controlled by superposed extensional growth fault arrays (D1) with associated dyke swarms in a curved pattern spatially not related to that of the actual distribution of granite domes and greenstone belts. The basins are interpreted to have formed by collapse after arching above hotspots due to phase transitions by mini-subduction of slabs of cooled water-saturated basalt towards the base of an originally c. 45 km mafic crust. At c. 3.31 Ga, the extension was replaced by plate-driven regional NW–SE compression (D2) inferred from NW-over-SE shear and ramp-and-flat thrusts, partly reversing offsets of the D1 extension. The recognition of widespread D2 pre-doming compression is important because it triggered the c. 3.18 Ga start of formation of the dome-and-keel pattern (D3) visible today, which culminated at c. 2.9 Ga

Growth faults and avalanches: Reconstructing Paleoarchean basins in the Pilbara and Kaapvaal cratons

Four new volcano-sedimentary complexes (VSC's), respectively c. 3510, 3460, 3430, and 3320 Ma old, are identified in the East Strelley, Coongan, and Kelly belts of the East Pilbara craton and compared with the extraordinarily complete c. 3450 Ma Buck Reef-VSC in the southern African Kaapvaal craton. The VSC's reveal an intricate relationship between volcanic deposition, sedimentation, development of syndepositional fault arrays, both extensional and contractional, and magmatism. The geometry and kinematics of these fault systems were analyzed after restoring the tilt of the stratigraphic sequences back to the depositional horizontal. The growth fault arrays are interpreted to have been generated by lack of lateral support of depositional basin margin prisms as known from present-day deltas and passive margins. This ‘basin margin collapse’ scenario for deformation and kinematics of the topmost section of the crust relies on topographic relief combined with vertical crustal oscillation. The relationship between supracrustal collapse and coeval deformation on deeper-seated detachments within the basement of the Pilbara remains as yet unsolved. The restoration corroborates the (semi-)circular basin architecture we proposed in 2017, which preceded, and is unrelated to, the present-day configuration of granitoid complexes and greenstone belts. The new findings also assess an early, syndepositional presence of the east–west Warrawoona Lineament as a major dividing line of as yet unknown structural character between two areas of basin superposition. For the Coonterunah Subgroup a multistorey architecture is established: the Table Top/Coucal-VSC with its near-water level chert top is overlain by regularly bedded Double Bar Basalt. The ensemble is truncated by the newly introduced tectono-stratigraphic Bergamina Unit, interpreted as a mega-avalanche emplaced somewhere between 3496 and 3466 Ma, possibly time-equivalent to the Duffer Fm. The crustal depths of the detachments below the (water-level) tops of all VSC's identified, in other words the thicknesses of the VSC's, are used as proxy for determining minimum basin centre depths ranging from 1000 to 3800 m. The greatest depth was reached in the North and South Coongan Basins, where voluminous bimodal volcanism of the Duffer Fm, maximum subsidence and deposition rates resulted in maximum basin margin instability

C: Bamboo Creek Sector South. Archaean basin margin geology and crustal evolution: an East Pilbara traverse

Supplementary figure c: Bamboo Creek Sector Sout

GPS list of observation sites. Archaean basin margin geology and crustal evolution: an East Pilbara traverse

GPS list of observation sites 1-22

D: Site 1. Archaean basin margin geology and crustal evolution: an East Pilbara traverse

Supplementary figure d: Site