Scaled Analogue Models Research Articles

Analogue models of caldera collapse in strike-slip tectonic regimes

Regional-scale faulting, particularly in strike-slip tectonic regimes, is a relatively poorly constrained factor in the formation of caldera volcanoes. To examine interactions between structures associated with regional-tectonic strike-slip deformation and volcano-tectonic caldera subsidence, we made scaled analogue models. Tabular (sill-like) inclusions of creamed honey in a sand/gypsum mix replicated shallow-level granitic magma chambers in the brittle upper crust. Lateral motion of a base plate sited below half the sand/gypsum pack allowed simulation of regional strike-slip deformation. Our experiments modelled: (1) strike-slip deformation of a homogeneous brittle medium; (2) strike-slip deformation of a brittle medium containing a passive magma reservoir; (3) caldera collapse into sill-like magma reservoirs without regional strike-slip deformation; and (4) caldera collapse into sill-like magma reservoirs after regional strike-slip deformation. Our results show that whilst the magma chamber shape principally influences the development and geometry of volcano-tectonic collapse structures, regional-tectonic strike-slip faults (Riedel shears and Y-shears) may affect a caldera’s structural evolution in two main ways. Firstly, regional strike-slip faults above the magma chamber may form a pre-collapse structural grain that is exploited and reactivated during subsidence. Our experiments show that such faults may preferentially reactivate where tangential to the collapse area and coincident with the chamber margins. In this case, volcano-tectonic extension in the caldera periphery tends to localise on regional-tectonic faults that lie just outside the chamber margins. In addition, volcano-tectonic reverse faults may link with and reactivate pre-collapse regional-tectonic faults that lie just inside the chamber margins. Secondly, where regional-tectonic strike-slip faults define corners in the magma chamber margin, they may halt the propagation of volcano-tectonic reverse faults. The experiments also highlight the potential difficulties in assessing the relative contributions of volcano-tectonic and regional-tectonic subsidence processes to the final caldera structure seen in the field. Disruption of the pre-collapse surface by regional-tectonic faulting was preserved during coherent volcano-tectonic subsidence to produce a caldera floor of differentially-subsided fault blocks. Without definitive evidence for syn-eruptive growth faulting, thickness changes in caldera fill across such regional-tectonic fault blocks in nature could be mistaken as evidence for piecemeal volcano-tectonic collapse.

地質構造形成過程のアナログモデル実験における材料物性の影響

Scaled analogue model experiments are an excellent technique to examine underground geometry of geologic structures and its development processes, thus have been widely applied to the petroleum exploration. The experimental results are strongly affected by the physical properties of the materials, thus this paper measured the grain size and the internal friction angle of popular granular materials and compared them with the model results by a series of shortening experiments. The shear-box-test results showed that micro glass beads have a smaller internal friction angle than that of sand grains, and the grain size has little effect on the internal friction angle of micro beads. The results of the analogue experiments suggested that the internal friction angle defined by the grain shape affected the thrust fault angle as well as the surface inclination of the uplifted wedge-shaped structure. The material of a higher friction angle extended the fault spacing and increased the displacement of each fault. The grain size of the material has an effect of strain localization, and the number of faults was increased in the experiments of smaller grains.

The Managua Graben and Las Sierras-Masaya volcanic complex (Nicaragua); pull-apart localization by an intrusive complex: results from analogue modeling

There is a well-documented association between pull-apart basins in strike-slip zones and large volcanic caldera complexes. Las Sierras-Masaya volcanic complex, Nicaragua, is a large basaltic lava and ignimbrite shield with nested calderas. The widest caldera is partly underlain by a large and dense intrusive complex, evidenced by a positive gravity anomaly. The Las Sierras Caldera, relating to this cumulate complex, is less than 30,000 years old and is probably still hot and ductile. The inner Masaya Caldera hosts an active magmatic system. The volcanic complex is in a dextral transtensional tectonic context of the Nicaraguan Depression. The highly active Managua Graben is on the northern part of the volcano. We speculate that the graben and volcano are linked tectonically, with the graben initiating in response of a regional stress field modified around the dense, ductile intrusive complex. Previous field work and reappraisal of structures with digital elevation model morphological analysis show that the volcano is surrounded by a rhombic fault pattern that may form a nascent pull-apart basin. We have done scaled analogue models to test the effect of intrusion density and rheology anomalies on strike-slip fault geometries. The models show that intrusion density variations alone do not significantly change fault patterns. In contrast, ductile rocks (silicone in the models as analogue for hot mafic intrusive rocks) markedly alter strike-slip fault patterns. In transtension the presence of a ductile intrusion causes the formation of a pull-apart, while in pure strike-slip or transpression, uplift and thrusting is generated. Pull-apart and uplift structures are rhomb-shaped even when the ductile inclusion is circular. We conclude that a pull-apart is developing at Las Sierras-Masaya volcanic complex in response to the transtensive regional deformation regime and the dense, ductile intrusive complex. We suggest that the volcano and graben are one dynamic system and should be monitored as one entity.

Time-lapse analogue modelling of turbidite channel sands

This paper describes a research program at the Australian Resources Research Centre to establish and use an analog model of a turbidite channel reservoir to investigate issues with reservoir simulations and time-lapse seismic monitoring of these complex fields. The project goal is to gain a better understanding of issues relating to uncertainty in reservoir simulations of channelised fields and their seismic expression. The results demonstrate the potential of using carefully scaled analog models to reproduce realistic reservoir phenomena in a controlled laboratory environment for integrated reservoir engineering and time-lapse seismic research.

Fault reactivation in brittle–viscous wrench systems–dynamically scaled analogue models and application to the Rhine–Bresse transfer zone

Analogue experiments on oblique rifting and subsequent transpressional reactivation were performed with two-layer slabs of sand and silicone. In this brittle–viscous system, transtensional and transpressional wrench faulting was induced by movements of a basal rigid plate. The dynamically scaled analogue models are confronted with the structural evolution of the Rhine–Bresse transfer zone (RBTZ) that linked Palaeogene rifting in the Upper Rhine and Bresse Grabens and that appears to have been transpressively reactivated in Neogene to recent times. Fault patterns produced in the sand layer above the basal silicone layer are compared with structural elements in the sedimentary cover, separated from the basement by an evaporitic décollement layer. In order to investigate strain-rate dependence of fault reactivation and graben inversion, transpressional shortening was performed under different displacement rates. Experimental results suggest that the reactivation of pre-existing structures in a brittle cover above a viscous décollement is strongly dependent on the strain rate within the viscous layer. Under low to intermediate displacement rates (2.6 cm h −1), deformation concentrates within the basal viscous layer and former normal faults within the cover are not reactivated. The reactivation at higher displacement rates (5 cm h −1) results in a complete inversion of graben structures within the cover. Ongoing shortening produces lobed thrust fronts, which crosscut pre-existing normal faults. Late Pliocene to recent en-échelon aligned folds and isolated thrust faults in the cover of the RBTZ are attributed to thick-skinned reactivation of basement faults. A comparison of natural and experimentally obtained structures suggests that fault reactivation occurred under low displacement rates (<1 mm/a). This results in a low mechanical coupling between basement and cover in areas with significantly thick décollement layers, providing an explanation for decoupled stresses between basement and cover, such as observed in the northern Jura Mountains.

3D evolution of fold and thrust belts formed by oblique convergence

This paper presents the results of a series of scaled analogue models designed to simulate thrust belt development formed by orthogonal and oblique convergence. The models were constructed to produce doubly-vergent orogenic wedges with a pro-wedge and a retro-wedge separated by an uplifted orogenic core. Models were constructed from homogeneous layered sandpacks in a 3 m×1.3 m deformation rig. The progressive evolution of the models was recorded using digital photography, and the photographs were animated for analysis. The experiments investigated convergence obliquities from orthogonal (90°) to as low as 15° obliquity. Orthogonal models produced long, linear, critically tapered pro-wedge fold and thrust belts parallel to the convergence margin together with a narrow, uplifted core and a steep retro-wedge thrust system. The taper angle of the pro-wedge was typically 11–12° whereas the retro-wedge maintained a steeper taper of 38–42°. Models where the convergence vector was oblique at angles of 60 and 45° to the margin produced doubly-vergent thrust wedges with thrust faults trending parallel to the margin. There was little evidence of linked, penetrative strike-slip faulting but en-echelon Riedel like shear systems developed in the 45° oblique model. Subordinate oblique-slip motion was observed on some thrust faults. In contrast, for 30 and 15° oblique convergence models, strong deformation partitioning developed with steeply sloping pro-wedges that did not have critical tapers. Through-going linked strike-slip faulting was developed that cut the uplifted axial zones of these models. Analysis of the models by animation of the digital photographs shows how the thrust systems initiated and propagated. In particular it is clear that at any one time several thrusts were moving simultaneously. Serial cross sections through the moderately oblique convergence models (60–45°) are almost indistinguishable from the orthogonal model cross-sections. The results of orthogonal convergence experiments compare well to the map patterns of faults and folds in the Salt Range, Pakistan. 60 and 45° oblique convergence models compare well to the Zagros where some strain partitioning on strike-slip faults occurs. The highly oblique models show very strong strain partitioning with penetrative margin-parallel strike-slip faulting similar to that found in NE Venezuela.

INFLUENCE OF HETEROGENEOUS LITHOSPHERIC STRUCTURE ON CONTINENTAL RIFTING: POSSIBLE IMPLICATIONS FOR THE LIGURE-PIEMONTESE OCEAN

Continental rifting represents the thermo-mechanical process by which continents break. This process is characterised by an high variability in terms of duration, volcanicity, width and symmetry. Models formulated to explain such a structural variability consider differences in (e.g., Davison, 1997; Ziegler and Cloetingh, 2004): initial rheological or thermal structure of the lithosphere, rift kinematics (e.g., oblique vs. orthogonal) and mechanics (e.g., pure vs. simple shear), strain rate, melt volumes, presence of pre-existing weakness zones. Among these processes, the lithospheric structure inherited from previous deformation phases and the presence of weakness zones have been shown to play a major role in controlling continental extension (e.g., Dunbar and Sawyer, 1989; Ziegler and Cloetingh, 2004). As a consequence, rifting processes often affect continental lithospheres characterized by important lateral variations in rheology and these variations are able to influence parameters as the width and symmetry of the rift, the patterns of uplift and subsidence, the location and amount of volcanic products. Extension in the Ligure-Piemontese ocean has been shown to be superimposed onto a previous ercinic chain, thus configuring a lithospheric weakness zone. In this frame, lateral rheological heterogeneities is expected to have played a major role in controlling the structure of the passive margins of the Adria/Europe system. In this work, this influence is investigated through scaled analogue modelling. Small-scale experiments were performed in an artificial gravity field of 31g, by using the Large Capacity Centrifuge of the Tectonic Modelling Lab of the CNR-IGG, settled at the Earth-Science Department of Florence University. The models, with dimensions of 10cm x 16cm x 4cm, were built with suitable analogue materials (sand, silicone, oleic acid) able to reproduce the brittle/ductile stratification of the continental lithosphere and were properly scaled to be directly comparable to natural prototypes. The experimental series investigated the occurrence of three different continental lithospheres: 1) a weak orogenic lithosphere (WL; Thickness: 80-100km; Thermo-mechanic Age: 50 Ma; Effective Elastic Thickness: 20km); 2) a normal 4-layer lithosphere (NL; Th: 120km; TA: 250 Ma; EET: 40km); 3) a strong cratonic lithosphere (CL; Th: 140km; TA: 400 Ma; EET: 70km). Results show that in the simple case of WL, continental extension is taken up by regularly-spaced faulting in the upper brittle layer coupled to flow of the ductile lower crust and uppermost lithospheric mantle. No strain localisation configures with this initial rheological stratification; instead, the deformation style can be ascribed to a wide-rifting mode (e.g., Buck, 1991; Brun, 1999). The base of the lithosphere in these models was rather flat, suggesting that no major asthenospheric upraising took place during deformation. In case of lateral variation between WL and CL, deformation was mostly localised at the craton border and within the weak domain; no deformation developed within the strong lithosphere. The largest normal fault developed at the contact between WL and CL, whereas regularly-spaced faults and lower crustal flow (with development of core complex structures) characterised the weakest portion of the models. Extension was accommodated by downward flexure of the WL and elastic rebound with strong shoulder uplift of CL; limited asthenospheric upraising was mainly localised within the WL. In models with NL and CL separated by a central WL, extensional deformation was strongly localised by the weakness zone. This rheological configuration leaded indeed to a rapid necking of the lithosphere and asthenospheric upraising leading to continental break-up in the central part of model. The modelling results support an important role played by the initial lithospheric structure on rift structure and duration and suggest that lateral rheological variations may have an important influence in controlling the local pattern of uplift/subsidence and faulting in continental rift settings. These results may have also relevance for the opening of the Ligure-Pemontese ocean. Preliminarily, the structural conditions of continental extension between the European and Adriatic margin may be correlated to the experiment reproducing the strength contrast between weak and cratonic lithospheres. In particular, the downward flexure associated with low subsidence rates of the WL may well account for the development of the Calcare Massiccio fm. on the Adriatic margin, whereas the areas characterised by high subsidence at the WL-CL contact may be correlated with the coeval development of the Brianconnaise domain in the European margin.

Detachment folding, fold amplification, and diapirism in thrust wedge experiments

The relations between detachment folding, fold amplification, and salt diapirism in contractional settings have been investigated by means of scaled analogue models. The viscosity of the silicone layer simulating salt in nature and the shortening rates were combined in order to reproduce weak (type 1 models) and strong (type 2 models) décollements. Deformation patterns in the roof sequence exhibited two contrasting styles, (1) outward propagation of detachment folding along the décollement (OFP mode) and (2) passive roof duplex (PRD mode). In type 2 models, detachment folding propagated away from the most external thrust in the floor sequence, while in type 1 models, long‐lived detachment folds almost invariably localized amplified above a floor thrust tip as a result of strain localization. A silicone wall intruded occasionally into the crestal graben of detachment folds in type 1 and OFP models. Best fitting of transition models data points indicates nonlinear relations with regression curves close to the equilateral hyperbola equation for both OFP‐PRD and amplified detachment folds‐box folds transitions. A quantitative comparison of model results with nature has been attempted by plotting salt‐based fold‐and‐thrust belts data points on the scaled transition curves obtained from the modeling. Such a comparison relates shear stress products and ratios to the conditions favoring the amplification of detachment folds and the potential emplacement of ductile diapirs in their core. By reducing the roof sequence strength, pore fluid pressure λb is inferred to shift the equilibrium of fold‐and‐thrust belts toward the field of OFP and diapirism.

Insights from scaled analogue modelling into the seismotectonics of the Iranian region

We show how analogue modelling provides insights into complex crustal block kinematics and the seismotectonics of active areas undergoing contraction, such as the Iranian region. The seismotectonic model deriving from this analysis is consistent with partitioning of the N-directed Arabia indentation into a composite system of collision-oblique and collision-parallel seismogenetic belts. These features include (1) two main conjugate transpressive belts (the dextral NW–SE-trending Zagros belt and the NE–SW-trending sinistral Elburz-Aran-Torud (EAT) belt), and (2) a modest lateral escape of Central Iran towards the Lut block along the Nayband belt. This cinematic model fits the seismic activity and the current fault pattern of the region. Also, the velocity field scaled from the model to nature shows a good similarity with previous numerical modelling as well as with deformation rates along the first-order seismogenetic belts determined by plate tectonic models and GPS data.

Effect of spatial distribution of Hormuz salt on deformation style in the Zagros fold and thrust belt: an analogue modelling approach

Scaled analogue models of thin-skinned simultaneous shortening above adjacent viscous and frictional décollements simulate the effect of Hormuz salt on the shortening in the Zagros fold and thrust belt. The models consisted of sand layers that partly overlay a viscous layer of silicone and were shortened from one end. Spatial distribution of the viscous décollement varied along strike and dip, as occurs in part of the Zagros fold and thrust belt. In this belt, Phanerozoic sedimentary cover was shortened partly above the Hormuz salt lying on the Precambrian crystalline basement, behaving as a basal viscous décollement. Model results display how the nature of the décollement affects the evolution of an orogenic belt. Using model results, we explain the development of deflection zones, and discuss strain partitioning, formation of different topographic wedges and differential sedimentation along the Zagros fold and thrust belt. Model results suggest the formation of a gentle taper, consisting of both foreward and backward thrusts above a viscous décollement and a relatively steeper taper consisting only of forward-vergent imbricates above a frictional décollement. However, in our models, the steepest wedge with the highest topography formed where the viscous substrate had a limited extent with a transitional boundary (pinch-out) perpendicular to the shortening direction. Shortening of this boundary led to development of frontal ramps associated with significant uplift of the area behind the deformation front.

Effect of ductile and frictional décollements on style of extension

Scaled analogue models were used to study the effect of frictional and ductile detachments on thin-skinned extension. Models consisted of two halves; one half is the ductile and the other has a frictional detachment. Extension occurred above two different basal configurations: a stretchable rubber sheet and a folded, banded sheet intended to produce homogeneous and heterogeneous extension, respectively. Model parameters varied systematically and included the brittle/ductile thickness ratio, rheologies, and bulk strain. Structures in the two halves are compared in profiles and plan views. A series of graben developed above both halves of models extended above a banded sheet, although there were differences in style, propagation rate and width of the deformation zone between the two halves. Different rates of propagation of structures in the two halves led to the formation of an accommodation or transfer zone parallel to the extension direction. Most relay ramps and inflection of normal faults in this zone indicate differential extension between the two halves. In contrast, in models extended above a stretchable rubber sheet, extensional structures such as horst and graben developed only above the ductile detachment. Model results indicate that heterogeneous mechanical stratigraphy and displacement rate have no effect on extensional structure above a rubber sheet. However, above 20% bulk extension, deformation becomes heterogeneous along multiple sets of conjugate faults oblique to the extension direction.

Three‐dimensional analogue modelling of an alluvial basin margin affected by hydrological cycles: processes and resulting depositional sequences

ABSTRACTThe dynamics between sediment erosion and accumulation at an alluvial basin margin affected by changes in the surface hydrology are explored using scaled analogue models produced in a flume. The presented results differ from previous counterparts in that accumulation or erosion has not been forced at a spreading outlet, but occurred at a slope change produced by previously accumulated sediment. Cyclical upstream incision produced by increased stream discharge generated incised valleys, and these were subsequently filled by sediment carried by less efficient streams generated during the low discharge period. High resolution mapping using 2.5 mm contour maps allowed the study of sediment accumulation and terrain modelling. The results of three selected experiments are analysed. The only variable explored was discharge. The basin margin was simulated by a ramp inserted in a low sloping flume, consisting of two segments of different slopes selected to emulate high and low efficiency flume fans produced elsewhere. Water and fine‐medium sand entered the ramp along a narrow (0.1 m) channel and flow expanded but without occupying the complete 1.2 m flume width. Flows were highly concentrated and noncohesive.Fan‐like accumulation (slope: 0.11) began during low discharge (LD) periods at the ramp slope break, and proceeded upstream, onlapping quickly at first, but shifting to mostly progradation at the end of the period. High discharges (HD) usually generated two or three incised channels at the beginning of the period, but one of them prevailed and rapidly eroded parts of the LD fan and moved the sediment to a more distal low‐sloping fan (slope: 0.045). Both LD and HD fans passed downstream into a system of small parallel channels resembling a braided alluvial plain ending in sediment lobes. The mapping of the accumulated sediment during the various periods allowed calculation of sediment budgets for the entire flume. The stratal architecture of the deposits was investigated along five parallel trenches cut after experiment termination. The regression analysis of depositional profiles at fan‐like features (expanding flow) and at braided plains (parallel flow) indicated that these fan‐like systems are linear and dependent on applied discharge, while the latter showed an exponential decrease of slope downstream, with a starting value set up by the fan slope.Two main types of stratigraphic units were generated, the LDST and HDST (system tracts). The LDST has a nonerosive base over ‘bedrock’ and the previous HDST, filling proximal erosional topography and prograding as well, generating an onlap–downlap array. Its geometry is highly variable and dependent on pre‐existing topography. The HDST base is an important erosive surface comparable to sequence boundaries. However, there are places without erosion due to a marginal position with respect to the main stream. Indeed, the results suggest that the three‐dimensional variability of erosion and depositional processes might produce very different architectures along the same basin margin.

Analogue modeling of arc and backarc deformation in the New Hebrides arc and North Fiji Basin

Research Article| April 01, 2002 Analogue modeling of arc and backarc deformation in the New Hebrides arc and North Fiji Basin W.P. Schellart; W.P. Schellart 1Epsilon Laboratory, Australian Crustal Research Centre, School of Geosciences, P.O. Box 28E, Monash University, Melbourne, Victoria 3800, Australia Search for other works by this author on: GSW Google Scholar G.S. Lister; G.S. Lister 1Epsilon Laboratory, Australian Crustal Research Centre, School of Geosciences, P.O. Box 28E, Monash University, Melbourne, Victoria 3800, Australia Search for other works by this author on: GSW Google Scholar M.W. Jessell M.W. Jessell 1Epsilon Laboratory, Australian Crustal Research Centre, School of Geosciences, P.O. Box 28E, Monash University, Melbourne, Victoria 3800, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information W.P. Schellart 1Epsilon Laboratory, Australian Crustal Research Centre, School of Geosciences, P.O. Box 28E, Monash University, Melbourne, Victoria 3800, Australia G.S. Lister 1Epsilon Laboratory, Australian Crustal Research Centre, School of Geosciences, P.O. Box 28E, Monash University, Melbourne, Victoria 3800, Australia M.W. Jessell 1Epsilon Laboratory, Australian Crustal Research Centre, School of Geosciences, P.O. Box 28E, Monash University, Melbourne, Victoria 3800, Australia Publisher: Geological Society of America Received: 13 Sep 2001 Revision Received: 28 Nov 2001 Accepted: 28 Nov 2001 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2002) 30 (4): 311–314. https://doi.org/10.1130/0091-7613(2002)030<0311:AMOAAB>2.0.CO;2 Article history Received: 13 Sep 2001 Revision Received: 28 Nov 2001 Accepted: 28 Nov 2001 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation W.P. Schellart, G.S. Lister, M.W. Jessell; Analogue modeling of arc and backarc deformation in the New Hebrides arc and North Fiji Basin. Geology 2002;; 30 (4): 311–314. doi: https://doi.org/10.1130/0091-7613(2002)030<0311:AMOAAB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In most backarc basins, extension is perpendicular to the arc. Thus individual spreading ridges extend approximately parallel to the arc. In the North Fiji Basin, however, several ancient and active spreading ridges strike 70°–90° to the New Hebrides arc. These high- angle spreading ridges relocated southward during the asymmetric opening of the North Fiji Basin. We have simulated the structural development of the North Fiji Basin and the New Hebrides arc with scaled analogue models, and the results have inspired us to come to several tentative conclusions. We interpret the orientation of the high-angle spreading ridges to be related to the asymmetric opening of the backarc basin around a hinge, where they form close to the hinge. Relocation of these spreading ridges is most likely related to subduction of the West Torres Plateau along the New Hebrides Trench. This resulted in localized collision, retarded rollback of the subducting slab along the northwest corner of the trench, and reduced extension and shearing in the northwest corner of the North Fiji Basin. Backarc extension continued in the rest of the North Fiji Basin owing to continued rollback of the southern part of the subducting slab. Here, active extension was separated from the slightly or nonextending northwest corner by a zone striking at high angle to the New Hebrides arc, i.e., the Hazel Holme extensional zone. Moreover, impingement of the d'Entrecasteaux Ridge into the overriding plate led to local deformation and fragmentation of the arc. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Passive roof thrusting and forelandward fold propagation in scaled brittle‐ductile physical models of thrust wedges

A set of scaled analogue models was performed to investigate the role of a décollement layer in the structural style of fold‐and‐thrust belts. Silicone putty (SGM 36) has been employed to represent the ductile décollement, while frictional material (quartz sand) has been used to simulate the brittle behavior of roof and floor sequences, situated above and below the décollement, respectively. In order to explore a wide combination of strength profiles, models have been deformed under five distinct rates of compression (0.15, 0.3, 0.45, 0.6, and 1.5 cm h−1) employing four different thicknesses for the silicone layer (0.1, 0.2, 0.4, and 0.8 cm). Experimental results indicate that roof sequences exhibit two distinct styles of deformation: (1) passive roof duplex (PRD) geometry [e.g., Banks and Warburton, 1986] and (2) outward propagation of folding (OFP) along the décollement [e.g., Davis and Engelder, 1985]. In PRD geometry the roof sequence remains relatively “stationary” being underthrust by a wedge‐shaped floor duplex, while in OFP, displacement along the floor thrusts is transferred horizontally along the décollement layer. Experiments suggest the occurrence of a genetic relationship between shear stresses (at the base of the roof sequence τb and within the ductile layer τd) and the development of PRD or OFP geometry. For a given strain rate the deformation pattern of roof sequences depends upon the τb/τd ratio as well as the absolute τb and τd values. The transition field between OFP and PRD styles has been approximated by curvilinear regression of transition‐style experimental data points, with a minimum value of τd, varying between about 112 and 160 Pa, to enter the PRD field. Syntectonic sedimentation in the molasse basin is found to inhibit development of folding into the foreland but to promote PRD style. Experiments simulate many of the principal characteristics of fold‐and‐thrust belts developed above a basal evaporite décollement, as well as the development of passive roof duplexes at mountain fronts. Shear stress values inferred for many fold‐and‐thrust belts correlate well the deformation fields predicted by the present experimental study, allowing to propose a simple model describing the mechanical behavior of natural roof sequences underlain by a décollement layer.

Epicenter distribution and magnitude of earthquakes in fold‐thrust belts: Insights from Sandbox Models

Scaled analogue models are used to illustrate the effect of basal friction and erosion on fault activity and hence on epicentre distribution and magnitude of earthquakes in the sedimentary cover of active fold‐thrust belts. Model results suggest that in fold‐thrust belts shortened above low‐friction ductile decollements (rock salt or over‐pressured mudstone), low‐ to moderate‐magnitude earthquakes (Mw = 5.3–5.6), distributed over a wide area, occur along several long‐lived thrust faults. Conversely, in areas shortened above high‐friction decollements large‐magnitude earthquakes (Mw = 6.6–6.8), distributed over a narrow zone are likely to occur along few short‐lived thrust ramps. Calculated magnitude of earthquakes from models and their distribution are in agreement with recorded earthquake pattern from the Zagros mountain belt, which is partially shortened above a ductile decollement of Hormuz salt formation. Model results also showed that erosion reactivates older inactive thrusts and promotes formation of out‐of‐sequence thrusts.

Tectonic Evolution of the Sanga Sanga Block, Mahakam Delta, Kalimantan, Indonesia

The Sanga Sanga Block contains four large to giant hydrocarbon fields in mid-to upper Miocene deltaic sandstones of the Mahakam Delta, eastern Kalimantan (Indonesia). These fields occur in the northeast-trending Mahakam fold belt, which is characterized by long, tight, fault-bounded anticlines and broad synclines and cored by overpressured shales. Onshore sections of the fold belt are strongly deformed, uplifted, and eroded, whereas the eastern offshore sections are little deformed and buried by the progradational delta wedge. Section balancing of depth-converted seismic lines, together with scaled analog modeling, was used to develop a new tectonic model of inverted delta growth faults for the evolution of the Mahakam fold belt. Section balancing shows that the fault-bounded anticlines of the Sanga Sanga Block are formed by contractional reactivation of early delta-top extensional growth faults. The change from gravity-driven extension to regional contraction occurred at around 14 Ma. Anticlinal folds controlled local sedimentation patterns and influenced the distribution of the reservoir channel sands in the main hydrocarbon fields. Scaled analog models of progradational loading above a ductile substrate produced delta-top extensional growth faults and together with delta-toe fold-thrust. Contraction inverted the extensional growth faults and depobelts, producing tight, fault-bounded anticlines. The results support the model of delta inversion and, thus, the most viable explanation for the geometric, kinematic, and mechanical evolution of the structures in the Sanga Sanga Block. The inverted delta model has applications to other hydrocarbon-bearing deltas around Borneo and in other contracted delta systems.

Geometry of granite emplacement in the upper crust: contributions of analogue modelling

Abstract Granite emplacement in the brittle crust can be modelled by means of the injection of a Newtonian fluid (low-viscosity silicone putty) into sandpacks. This paper describes dynamically scaled analogue models of granite intrusions in the upper crust under different tectonic regimes. Experiments analyse three boundary conditions: (1) static conditions, with different rheological profiles (single sand-layer system, two-layer silicone-sand system, three-layer sand-silicone-sand system and five-layer sand-silicone-sand-silicone-sand system), (2) extensional regime, including gravitational sliding and divergent basal plate, with both mobile and fixed velocity discontinuities and (3) strike-slip regime, induced by two mobile basal plates. The results obtained indicate that: (1) a soft level between two competent units is necessary for laccolith formation—the critical thickness of the soft layer necessary for laccolith formation decreases with increasing depth, (2) when there are two soft layers in the brittle crust, laccolith emplacement occurs in the deeper soft level, even when this is thin and the overburden does reach its critical thickness, (3) in extensional regimes the geometry of intrusions is mainly controlled by normal faults and at the same time intrusions determine the location of faults within the cover and (4) in strike-slip zones intrusions are elongate and their long axis tends to track the principal stretching direction associated with the strike-slip regime. Some natural examples of granitic bodies were considered to test the applicability of experimental results.

Recent advances in analogue modelling: uses in section interpretation and validation

Abstract Scaled analogue modelling is a powerful tool for the understanding of the geometric and kinematic development of extensional, strike-slip and inverted fault systems, as well as for some thrust systems. In this paper the results of 2D and 3D scaled analogue model experiments designed to simulate listric fault systems, rift fault systems, and inverted listric faults are reviewed. Simple 2D rift models are characterized by asymmetric rift graben structures with planar rift border faults and domino-style intra-rift fault systems. 2D listric fault models produce roll-over anticlines with associated crestal collapse grabens whereas the ramp-flat listric fault models produce roll-over anticlines associated with each concave-up segment of the main fault system and also a syncline above the convex-up ramp section of the main detachment. The 2D analogue models are directly comparable to natural examples of extensional fault systems. The results of simple 3D analogue models of an orthogonal and a 60° oblique rift are presented. Orthogonal rift models are characterized by both rift-border and intra-rift faults that are nearly orthogonal to the extension direction. Like-dipping intra-rift faults overlap to form relay ramps that transfer displacement between the faults. In contrast oblique rift models are characterized by en-echelon arrays of segmented extensional faults. Rift border fault systems are parallel to the underlying zone of basement stretching whereas the intra-rift faults are segmented and at a high angle to the extension direction. Oblique accommodation zones formed by interlocking conjugate fault arrays separate individual depocentres within the models. No strike-slip or oblique-slip transfer faults are found in these models. The 3D rift models produce fault patterns that are comparable with natural examples of rift systems. Analogue modelling of inverted listric faults is described and the results compared with natural examples of inverted fault systems. The results of the scaled analogue models described in this paper provide important constraints on section interpretation and restoration in that they permit an understanding of the geometric and kinematic evolution of fault (and fold) structures as well as indicating the sequences of faulting and activity on faults. They provide templates for the interpretation of seismic sections in regions of poor data quality and for projecting surface data into the sub-surface.

Analogue model studies of induction effects at auroral latitudes

Abstract. In addition to field observations and numerical models, geomagnetic induction effects can be studied by scaled analogue model experiments. We present here results of analogue model studies of the auroral electrojet with an Earth model simulating the Arctic Ocean and inland conductivity structures in northern Fennoscandia. The main elements of the analogue model used were salt water simulating the host rock, an aluminium plate corresponding to the ocean and graphite pieces producing the inland highly conducting anomalies. The electrojet was a time-harmonic line current flowing at a (simulated) height of 100 km above northern Fennoscandia. The period simulated was 9 min. The analogue model results confirmed the well-known rapid increase of the vertical field when the coast is approached from the continent. The increase of the horizontal field due to induced ocean currents was demonstrated above the ocean, as well as the essentially negligible effect of these currents on the horizontal field on the continent. The behaviour of the magnetic field is explained with a simple two-dimensional thin-sheet model. The range, or the adjustment distance, of the ocean effect inland was found to be some hundreds of kilometers, which also agrees with earlier results of the Siebert-Kertz separation of IMAGE magnetometer data. The modelled inland anomalies evidently had too large conductivities, but on the other hand, their influence decayed on scales of only some tens of kilometers. Analogue model results, thin-sheet calculations, and field observations show that the induction effect on the horizontal magnetic field Bx near the electrojet is negligible. On the other hand, the vertical component Bz is clearly affected by induced currents in the ocean. Evidence of this is the shift of the zero point of Bz 0-1° southwards from the maximum of Bx. The importance of these results are discussed, emphasizing the determination of ionospheric currents.

Analogue models of laccolith formation

This paper describes dynamically scaled analogue models of laccolithic intrusions. Experiments consisted of the injection of a Newtonian fluid (low-viscosity silicone putty) into a sandpack, with or without an interbedded ductile layer of silicone putty acting as a potential décollement level. Boundary conditions were chosen to analyze the influence of the thickness of the brittle cover and of the décollement layer on the pattern of intrusion. Further experiments were made to examine the effects of an extensional regime during intrusion. Experiments showed that: (1) laccolith formation requires the occurrence of a décollement layer between two competent units, (2) the critical thickness of the décollement layer necessary for laccolith formation decreases with increasing depth, (3) laccoliths change from lenses to bell-shaped with decreasing overburden, (4) for a constant thickness of the décollement layer, the largest diameter of laccoliths is proportional to the thickness of the overburden, and (5) a syn-injection gravitational sliding regime results in an asymmetric laccolith, with amplification of the bell shape.