Structural Fieldwork Research Articles

On the Billefjorden fault zone in Garmdalen, central Spitsbergen: implications for the mapping of major fault zones during geological fieldwork and for the tectonic history of Svalbard.

The present contribution reexamines the geometry of a segment of a presumably long-lived fault in Svalbard, the Balliolbreen Fault segment of the Billefjorden Fault Zone, along which presumably two basement terranes of Svalbard accreted in the early-mid Paleozoic after thousands of kilometers strike-slip displacement. We performed structural fieldwork to Billefjorden in central Spitsbergen and interpreted satellite images. Field observations demonstrate that the Balliolbreen Fault formed as a top-west thrust fault in the early Cenozoic and that weak sedimentary units such as shales of the Lower Devonian Wood Bay Formation and coals of the uppermost Devonian-Mississippian Billefjorden Group partitioned deformation, resulting in significant contrast in deformation intensity between stratigraphic units. For example, tight early Cenozoic folds are localized in shales of the Wood Bay Formation and contemporaneous top-west brittle-ductile thrusts within coals of the Billefjorden Group, whereas Pennsylvanian deposits of the Hultberget (and/or Ebbadalen?) Formation are simply folded into gentle open folds. Rheological contrasts also resulted in the development of décollements locally, e.g., between tightly folded strata of the Wood Bay Formation and Billefjorden Group and flat-lying, brecciated limestone-dominated strata of the Wordiekammen Formation. Despite the limited quality and continuity of outcrops in the area, the eastward-thickening character (i.e., away from the fault) of Pennsylvanian deposits of the Hultberget, Ebbadalen, and Minkinfjellet formations suggests that the fault did not act as a normal fault in Pennsylvanian times. The study suggests that strain partitioning of early Cenozoic Eurekan contraction alone may explain the deformation patterns in Paleozoic rock units in central Spitsbergen, i.e., that Late Devonian Svalbardian contraction is not required, and that a major segment of the Billefjorden Fault Zone formed in the early Cenozoic. The present work illustrates the crucial need for interdisciplinary approaches and composite educational backgrounds in science.

Paleostress analysis and rift kinematics of the petroliferous Barmer rift basin, western Rajasthan, India

The NW-trending Barmer rift in western Rajasthan is one of the major petroliferous Indian basins. The basin rifted twice: (i) NW – SE extension in Early Cretaceous, and (ii) NE – SW extension during Late Maastrichtian to Early Paleogene. We performed structural fieldwork mainly along the rift margins to decipher the rift kinematics through geologic time. We collected ∼500 fault-slip data from limited outcrops along the basin's western and eastern margins. Paleostress analysis was performed for the first time in this basin using Win_Tensor, T-Tecto and FaultKin software. The outcome of this analyses shows that a strike-slip stress regime was prevalent in the western rift margin whereas the eastern margin experienced extension to transtension. Four paleostress regimes, DΔ, Dα, DΦ, and Dβ, were identified by inverting the fault-slip data. The two separate extensional regimes, during the Early Cretaceous and in the Late Maastrichtian to Early Paleogene, are confirmed. These are Dα (NW – SE extension) and Dβ (NE – SW extension). The analysis also presents two strike-slip regimes with varying maximum horizontal stress (SHMax) orientation, viz. DΔ and DΦ. Exact age correlation for these strike-slip regimes remains indeterminate. However, we deduced DΔ and DΦ were associated with the first extensional phase (Dα) during Early Cretaceous linked to India – Madagascar oblique separation. The Barmer rift extended obliquely guided by pre-existing structures. Remote sensing image analysis indicates that part of the western margin is neotectonically active as inferred from the drainage morphometry. The stress inversion analysis of earthquake focal mechanism data depicts a strike-slip transpressive regime prevailing at present in the NW India along with ∼ N-trending SHMax. This is mainly responsible for the neotectonic activities in the basin. We documented several fault gouge zones along moderately-dipping faults in the Cretaceous outcrops at the eastern margin. The lower the fault dips, the thicker the gouge zones associated with the faults. Such thick fault gouge of around 100 cm or more may act as a structural fault bound trap for Cretaceous reservoirs at depth. (Words: 324).

The effects of positive tectonic inversion structures on the formation of thrust systems on the western Domeyko Cordillera, northern Chile: Implications for the orogenic development of the outer Andean forearc

This work aims to propose a novel structural setting for the outer Andean forearc. Specifically, we propose a new deformation mechanism related to thrusting belt systems induced by positive tectonic inversion processes in the western flank of the Domeyko Cordillera in northern Chile. Thrust belt systems have been rarely understood for the outer Andean forearc; in fact, the mention of thrusting is scarce and structural interpretations are restricted only to strike-slip faulting and positive tectonic inversion processes. In this study, we have constructed novel balanced cross-sections from new structural fieldwork and combined them with 2-D reflection seismic line re-interpretations, thus offering new ideas about the evolution and development of the forearc of the Central Andes. Based on this, we propose integrated kinematic models that describe the relationships between inherited extensional structures, positive tectonic inversion structures, and the formation of thrust systems. The proposed balanced cross-sections have been restored to a pre-shortening condition. In this way, two structural styles are proposed for the western flank of the Domeyko Cordillera. The first corresponds to west-directed positive tectonic inversion structures, which expose and extrude the Middle Jurassic syn-rift deposit of the ancient Mesozoic rift basin. The second consists of east-directed thin-skinned thrust systems, mainly affecting the Upper Cretaceous to Eocene volcano-sedimentary synorogenic sequences. The interaction between inversion structures and thrust system structural styles would generate a west-directed intracutaneous wedge. Finally, the minimum shortening obtained from our balanced cross-section restorations is ca. 11% for the Río Loa and the San Salvador Canyons segment and ca. 56% for the Quebrada Mala segment. The formation of thrusting structures is controlled by a major detachment fault activated from a regional-scale angular unconformity, which we have named the Quebrada Mala detachment fault. An important part of the shortening was taken up and accommodated by buttressing structures and their associated intraformational detachment-thrust levels.

The Role of Faults in Groundwater Circulation before and after Seismic Events: Insights from Tracers, Water Isotopes and Geochemistry

The interaction between fluids and tectonic structures such as fault systems is a much-discussed issue. Many scientific works are aimed at understanding what the role of fault systems in the displacement of deep fluids is, by investigating the interaction between the upper mantle, the lower crustal portion and the upraising of gasses carried by liquids. Many other scientific works try to explore the interaction between the recharge processes, i.e., precipitation, and the fault zones, aiming to recognize the function of the abovementioned structures and their capability to direct groundwater flow towards preferential drainage areas. Understanding the role of faults in the recharge processes of punctual and linear springs, meant as gaining streams, is a key point in hydrogeology, as it is known that faults can act either as flow barriers or as preferential flow paths. In this work an investigation of a fault system located in the Nera River catchment (Italy), based on geo-structural investigations, tracer tests, geochemical and isotopic recharge modelling, allows to identify the role of the normal fault system before and after the 2016–2017 central Italy seismic sequence (Mmax = 6.5). The outcome was achieved by an integrated approach consisting of a structural geology field work, combined with GIS-based analysis, and of a hydrogeological investigation based on artificial tracer tests and geochemical and isotopic analyses.

Reply to Norini and Groppelli's comment on “Estimating the depth and evolution of intrusions at resurgent calderas: Los Humeros (Mexico)” by Urbani et al. (2020)

Abstract. Structural studies in active caldera systems are widely used in geothermal exploration to reconstruct volcanological conceptual models. Active calderas are difficult settings to perform such studies mostly because of the highly dynamic environment, dominated by fast accumulation of primary and secondary volcanic deposits, the variable and transient rheology of the shallow volcanic pile, and the continuous feedbacks between faulting, secondary porosity creation, and geothermal fluid circulation, alteration and cementation that tend to obliterate the tectonic deformation structures. In addition, deformation structures can be also caused by near- and far-field stress regimes, which include magmatic intrusions at various depths, the evolving topography and regional tectonics. A lack of consideration of all these factors may severely underpin the reliability of structural studies. By rebutting and providing a detailed discussion of all the points raised by the comment of Norini and Groppelli (2020) to the Urbani et al. (2020) paper, we take the opportunity to specify the scientific rationale of our structural fieldwork and strengthen its relevance for geothermal exploration and exploitation in active caldera geothermal systems in general and, particularly, for the Holocene history of deformation and geothermal circulation in the Los Humeros caldera. At the same time, we identify several major flaws in the approach and results presented in Norini and Groppelli (2020), such as (1) the lack of an appropriate ranking of the deformation structures considering an inventory method for structural analysis; (2) the misinterpretation and misquoting of Urbani et al. (2020) and other relevant scientific literature; and (3) irrelevant and contradictory statements within their comment.

Structural and thermal evolution of the eastern Aar Massif: insights from structural field work and Raman thermometry

The thermo-kinematic evolution of the eastern Aar Massif, Swiss Alps, was investigated using peak temperature data estimated from Raman spectroscopy of carbonaceous material and detailed field analyses. New and compiled temperature-time constraints along the deformed and exhumed basement-cover contact allow us to (i) establish the timing of metamorphism and deformation, (ii) track long-term horizontal and vertical orogenic movements and (iii) assess the influence of temperature and structural inheritance on the kinematic evolution. We present a new shear zone map, structural cross sections and a step-wise retrodeformation. From text{ca.;26,Ma} onwards, basement-involved deformation started with the formation of relatively discrete NNW-directed thrusts. Peak metamorphic isograds are weakly deformed by these thrusts, suggesting that they initiated before or during the metamorphic peak under ongoing burial in the footwall to the basal Helvetic roof thrust. Subsequent peak- to post-metamorphic deformation was dominated by steep, mostly NNW-vergent reverse faults (text{ca.} 22–14 Ma). Field investigations demonstrate that these shear zones were steeper than 50^{circ} already at inception. This produced the massif-internal structural relief and was associated with large vertical displacements (7 km shortening vs. up to 11 km exhumation). From 14 Ma onwards, the eastern Aar massif exhumed “en bloc” (i.e., without significant differential massif-internal exhumation) in the hanging wall of frontal thrusts, which is consistent with the transition to strike-slip dominated deformation observed within the massif. Our results indicate 13 km shortening and 9 km exhumation between 14 Ma and present. Inherited normal faults were not significantly reactivated. Instead, new thrusts/reverse faults developed in the basement below syn-rift basins, and can be traced into overturned fold limbs in the overlying sediment, producing tight synclines and broad anticlines along the basement-cover contact. The sediments were not detached from their crystalline substratum and formed disharmonic folds. Our results highlight decreasing rheological contrasts between (i) relatively strong basement and (ii) relatively weak cover units and inherited faults at higher temperature conditions. Both the timing of basement-involved deformation and the structural style (shear zone dip) appear to be controlled by evolving temperature conditions.

Interpreting Inverse Magnetic Fabric in Miocene Dikes From Eastern Iceland

AbstractAnisotropy of Magnetic Susceptibility (AMS) is a valid tool to investigate magma flow direction within dikes. However, geometrically inverse magnetic fabric characterized by maximum magnetic susceptibility axis (kmax) perpendicular to the dike wall may complicate the interpretation of flow trajectories. To better understand the nature of this fabric, we present a multiscale study on 19 dikes (383 samples) in the Miocene Alftafjordur volcanic system (Iceland), where 80% of the samples show a geometrically inverse magnetic fabric. We carried out (1) AMS measurements at different magnetic fields and temperatures, along with Anisotropy of Anhysteretic Remanent Magnetization (AARM) analysis; (2) hysteresis loops and FORC diagrams; (3) thin section analysis; (4) structural fieldwork. A variable Ti‐content (0.1 < x < 0.6, Fe3‐xTixO4) titanomagnetite is the main magnetic carrier, and the contribution of the paramagnetic elongated crystals to the magnetic fabric is negligible. Single domain is not the prevailing domain state of the magnetic particles, suggesting that its occurrence cannot be the main cause for the inverse fabric. AMS analysis at different fields and temperatures along with AARM allow us to exclude any mineral phase change of the titanomagnetite across the dike. Nevertheless, kmax is parallel to a diffuse horizontal column‐like fracture pattern perpendicularly oriented with respect to the dike strike. This suggests that the Ti‐magnetite mineral orientation during dike cooling was affected by the fracture network progressively developing columnar basalts. This study demonstrates that the interpretation of AMS data on old and deep volcanic bodies is not straightforward and observations at different scales are required.

Effect of Mineral Processes and Deformation on the Petrophysical Properties of Soft Rocks during Active Faulting

We have studied damage zones of two active faults, Baza and Padul faults in Guadix-Baza and Granada basins, respectively, in South Spain. Mineral and microstructural characterization by X-ray diffraction and field emission electron microscopy studies have been combined with structural fieldwork and in situ measurements of rock properties (permeability and Young’s modulus) to find out the relation between deformation behavior, mineral processes, and changes in the soft rock and sediment properties produced by fluid flow during seismic cycles. Our results show that microsealing produced by precipitation of dolomite and aragonite along fractures in the damage zone of Baza Fault reduces the permeability and increases the Young’s modulus. In addition, deformation bands formed in sediments richer in detrital silicates involved cataclasis as deformation mechanism, which hamper permeability of the sediments. In the Granada Basin, the calcarenitic rocks rich in calcite and clays in the damage zone of faults associated to the Padul Fault are characterized by the presence of stylolites without any carbonate cement. On the other hand, marly lithofacies affected by faults are characterized by the presence of disaggregation bands that involve cracking and granular flow, as well as clay smear. The presence of stylolites and deformation bands in these rocks reduces permeability.

Opposing Observations and the Political-Economy of Malaria Vulnerability: A Community-Based Study in Bududa, Uganda

Malaria is a parasitic infection that remains a leading threat to health and development in many communities, especially in Sub-Saharan Africa. In Bududa, Uganda, malaria represents a key threat to health and well-being. However, whether or not malaria rates are improving in the district over time represents a conundrum. By using principles of structural fieldwork and drawing on multiple data sources that include the more- and less- powerful, opposing observations emerge, where community members perceive marked increases in malaria rates over time, while official district-level data depict the opposite. World-systems analysis illuminates the reasons behind this discrepancy, along with the factors that community members use to explain the rise in malaria suffering, including environmental changes and decreased healthcare access. Overall, this research demonstrates how global economic policies and structures create unequal health impacts, placing those in Bududa at disproportionate and elevated vulnerability to malaria.

The political economy of landslides and international aid relief: a qualitative investigation in rural Uganda

<p>This is a qualitative study of perspectives from community members on landslides in Bududa, Uganda. Interviews with community members reveal their perceptions of the causes, effects, and aid response to landslides. We employ a 'structural fieldwork' approach to explain community member's thoughts and experiences using critical macro-comparative perspectives relating to political ecology. This research brings attention to how large-scale unequal relationships in trade and international aid increase landslide vulnerability and there are ineffective relief efforts in a particular locale. Linking environmental degradation in Bududa to political, economic, and social factors provides a broader context in which to view risk from landslides in this community, as a critical case in demonstrating how economic globalization benefits some at the expense of others.</p><p><strong>Keywords: </strong>Landslides, unequal exchange, disaster, NGOs, political economy<strong></strong></p>

Deformation around a detached half-graben shoulder during nappe stacking (Northern Calcareous Alps, Austria)

This study describes the inversion of a rift-related graben shoulder during emplacement and transport of a major thrust sheet in the external part of the western Northern Calcareous Alps (NCA). Structural fieldwork was carried out along the Lechtal thrust, separating the tectonically deeper Allgäu thrust sheet from the Lechtal thrust sheet. An irregularly shaped Early Jurassic normal fault and an adjacent basin are present in the immediate footwall of the Lechtal thrust. The Early Jurassic age of the basin formation is indirectly established by thickness differences in the syntectonic deposits. Scaly fabric and small-scale drag folds document top S to top SW kinematics at the normal fault. During Alpine orogeny the Lechtal thrust is forced to recess around the graben shoulder in the footwall. A transpressive, dextral tear-fault develops in the hanging wall to compensate for lateral differences of shortening. The normal fault is too steep for inversion, and a shortcut thrust across the half-graben shoulder developed at a shallower angle. It transports the detached half-graben shoulder into the neighbouring basin. Albian to Paleogene transport of the Lechtal thrust sheet caused deformation below the Lechtal thrust, creating a shear zone with isoclinal folds, break thrusts, boudinaged beds and development of S–C fabric. Kinematic analyses of S–C fabrics and small-scale fold axes along the Lechtal thrust show consistent top NW to top N shortening directions in accordance with Cretaceous to Paleogene thrust directions in the NCA.

Interactions between plutonism and detachments during metamorphic core complex formation, Serifos Island (Cyclades, Greece)

In order to better understand the interactions between plutonic activity and strain localization during metamorphic core complexes formation the Miocene granodioritic pluton of Serifos (Cyclades, Greece) is studied. This pluton (11.6-9.5 Ma) intruded the Cycladic Blueschists during thinning of the Aegean domain along a system of low-angle normal faults belonging to the south-dipping West Cycladic Detachment System (WCDS). Based on structural fieldwork, together with microstructural observations and anisotropy of magnetic susceptibility (AMS), we recognize a continuum of deformation from magmatic to brittle conditions within the magmatic body. This succession of deformation events is kinematically compatible with the development of the WCDS. The architecture of the pluton shows a marked asymmetry resulting from its interaction with the detachments. We propose a tectonic scenario for the emplacement of Serifos pluton and its subsequent cooling during the Aegean extension. (1) A first stage corresponds to the metamorphic core complex initiation and associated southwestward shearing along the Meghalo Livadhi detachment. (2) In a second stage the Serifos pluton has intruded the dome at shallow crustal level, piercing through the ductile/brittle Meghalo Livadhi detachment. Southwest-directed extensional deformation was contemporaneously transferred upward in the crust along the more localized Kavos Kiklopas detachment. (3) A third stage was marked by syn-magmatic extensional deformation and strain localization at the contact between the pluton and the host rocks resulting in narrow shear zones nucleation which (4) continued to develop after the pluton solidification.

Interrelations between extensional shear zones and synkinematic intrusions: The example of Ikaria Island (NE Cyclades, Greece)

The Aegean extensional backarc domain is an ideal place to investigate the interrelations between syntectonic intrusions and large-scale detachments, understanding whether the intrusions have a triggering effect on the inception of metamorphic core complex formation or not. A new field study of Ikaria Island (NE Cyclades, Greece) in two Miocene granitoid plutons intruding a high-temperature metamorphic dome, namely the Raches and Karkinagrion granites, leads to the identification of a major structure that has been omitted in previous studies: the Gialiskari Detachment. This regional-scale detachment is correlated with the North Cycladic Detachment System. Structural fieldwork at the scale of the entire Raches and Karkinagrion intrusions allows us to propose a map of strain intensity in these granites. This map, based on macrostructural criteria, highlights a strong strain gradient in both intrusions from base to top, when approaching the Gialiskari Detachment. Along the strain gradient, a continuum of top-to-the north shearing deformation is recognized, including i) syn-magmatic deformation, ii) high-temperature ductile deformation at submagmatic state, iii) mylonitic to ultramylonitic deformation, and iv) low-temperature brittle deformation. A scenario of the interactions between the detachment and the Raches and Karkinagrion intrusions during their emplacement is proposed. We show that these granites were emplaced while the exhumation of the Ikaria metamorphic dome was already underway. We conclude that these plutons do not initially localize detachments, and instead that detachments localize and control the ascent of plutons. Intrusions then interact with detachments, accommodating their upward migration in the crust. This scenario is described on other Cycladic islands such as on Tinos or Mykonos. At a larger scale, the final emplacement of plutons may result from the flow of molten crust toward metamorphic core complex during extension.

Fission-track constraints on the thermal and tectonic evolution of the Apuseni Mountains (Romania)

New zircon and apatite fission-track (FT) data, including apatite thermal modelling, are combined with an extensive literature survey and reconnaissance-type structural fieldwork in the Eastern Apuseni Mountains. This leads to a better understanding of the complex structural and thermal history of a key area at the boundary between two megatectonic units in the Balkan peninsula, namely the Tisza and Dacia Mega-Units. Following Late Jurassic obduction of the Transylvanian ophiolites onto a part of the Dacia Mega-Unit, that is, the Biharia nappe system, both units were buried to a minimum of 8 km during late Early Cretaceous times when these units were underthrust below the Tisza Mega-Unit consisting of the present-day Codru and Bihor nappe systems. Tisza formed the upper plate during Early Cretaceous (‘Austrian’) east-facing orogeny. Turonian to Campanian zircon FT cooling ages (95–71 Ma) from the Bihor and Codru nappe systems and the Biharia and Baia de Arieş nappes (at present the structurally lowest part of the Dacia Mega-Unit) record exhumation that immediately followed a second Cretaceous-age (i.e. Turonian) orogenic event. Thrusting during this overprinting event was NW-facing and led to the overall geometry of the present-day nappe stack in the Apuseni Mountains. Zircon FT ages, combined with thermal modelling of the apatite FT data, show relatively rapid post-tectonic cooling induced by a third shortening pulse during the latest Cretaceous (‘Laramian’ phase), followed by slower cooling across the 120°–60 °C temperature interval during latest Cretaceous to earliest Paleogene times (75–60 Ma). Cenozoic-age slow cooling (60–40 Ma) was probably related to erosional denudation postdating ‘Laramian’ large-scale updoming.

Miocene thrusting in the eastern Sila Massif: Implication for the evolution of the Calabria-Peloritani orogenic wedge (southern Italy)

Alpine orogens in the central Mediterranean region have revealed the concomitance of crustal extension in back-arc domain and crustal shortening in frontal domain. Quantitative data of deformation in the frontal orogenic wedges are necessary to understand how the shortening-extension pair evolves in terms of structures, orogenic transport, timing, and exhumation rate. This paper deals with kinematics and ages of the frontal thrust systems of the Calabria-Peloritani Arc (Italy) exposed in the eastern Sila Massif. We first present structural fieldwork, onshore and offshore well log data, and new apatite fission-track (AFT) thermochronology. Then, we describe the structural architecture of the studied area as an ENE-verging stacking of thrust sheets involving basement units and syn-orogenic sediments. The AFT study documents that thrust sheets entered the partial annealing zone from 18Ma to 13Ma. This Early-Middle Miocene thrusting phase was coeval with exhumation of high-pressure/low temperature metamorphic rocks in the hinterland of the orogen (Coastal Chain area), mainly driven by top-to-the-W extensional tectonics. Opposite kinematic shear senses (contractional top-to-the-E and extensional top-to-the-W) and different exhumation rates (slow in the frontal, more rapid in the hinterland) are framed in a tectonic scenario of a critically tapered orogenic wedge during the eastward retreating of the Apennine slab.

Revisional notes on the Permian tetrapod ichnofauna from the Tiddas Basin, central Morocco

During the mid-1980s, structural geological field work in the up to 300 m thick red-bed sequence of the Permian Tiddas Basin, central Morocco, revealed the first occurrence of Palaeozoic tetrapod footprints in NW Africa. Preservational ambiguity of part of the few published trace fossils and resulting taxonomic inconsistency necessitate a long-needed reanalysis of the type locality. Based on 110 recently collected specimens the revised tetrapod ichnofauna of the study area comprises tracks of Batrachichnus Woodworth, 1900 — Limnopus Marsh, 1894, Amphisauropus Haubold, 1970, Dimetropus Romer and Price, 1940, Varanopus Moodie, 1929 — Hyloidichnus Gilmore, 1927, and Dromopus Marsh, 1894 which can be referred to small and medium-sized temnospondyl, seymouriamorph, basal synapsid (‘pelycosaur’), captorhinomorph, and araeoscelid trackmakers. This assemblage is interpreted to represent a late Early Permian (Artinskian–Kungurian) tetrapod ichnofauna. Due to the relative abundance of footprints assigned to semiaquatic tetrapods and sedimentological evidence for pronounced humid palaeoclimatic conditions we propose correlation of the track-bearing strata with the late Artinskian/early Kungurian wet phase. Regarding the quality, quantity, and peculiar diversity of its fossils, including the earliest record of seymouriamorph and advanced captorhinomorph tracks from the African continent, the Tiddas Basin could be of great importance for the reconstruction of late Early Permian terrestrial ecosystems in proximity to the interface of Euramerican and Gondwanan biotic provinces.

People, Place, and Time: How Structural Fieldwork Helps World-Systems Analysis

Some of the most insightful work in the political economy of the world-system area has been produced by researchers whose extensive fieldwork offers them deep familiarity with people and locales. Few other methods are as useful to understand the impacts of structural change on daily life and the ways agents resist, alter, and shape emerging structures. Yet such structural fieldwork is marginalized by the over-reliance of pedagogical materials on social constructionist, social psychological, or interactionist perspectives and also in world-systems research and writing by the privileging of long durée historical or quantitative cross-national methods. This paper introduces the concept of structural fieldwork to describe a qualitative field methodology in which the researcher is self-consciously guided by considerations emerging out of macro-sociological theories. We identify four advantages of structural fieldwork: the illumination of power’s multiple dimensions; examination of agency and its boundaries or limitations within broad political and economic structures; attention to nuances of change and durability, spatial and temporal specificities, and processes of change and durability; and challenging and extending social theory. These advantages are illustrated in select examples from existing literature and by discussion of the two author’s fieldwork-based research. The paper concludes that explicit attention to fieldwork may strengthen political economy and world-systems research and also de-marginalize political economy informed by structural fieldwork.

A case study of lateral spreading: the Precambrian Svecofennian Orogen

Abstract We have studied the crustal structures of the Palaeoproterozoic Svecofennian ( c. 1.9 Ga) Orogeny with the help of large scale seismic reflection surveys (FIRE 1–3), preliminary structural field work and geological and geophysical databases. The central part of the orogen is occupied by the Central Finland Granitoid Complex, which comprises two suites of granitoid rocks and associated mafic and volcanic rocks. The complex and the surrounding supracrustal belts are cut and deformed by numerous shear zones and faults; here divided into six groups. The most prominent reflections are usually shear zones or faults on outcrops. The granitoid complex is interpreted as a deep, lower-level section of an old core complex, where the younger granitoid intrusions form the basins and older granitoid intrusions and associated volcanic rocks form the horsts. The upper–middle crust detachment zone is exposed at the northeastern edge of the complex and middle crust is exposed in the migmatitic domes at northern and western margins. The seismic reflection sections display a frozen image of orogenic thickening and lateral spreading. The decoupling of the upper, middle and lower crust during spreading resulted in the formation of layered superstructure–infrastructure of the crust.

A workflow to facilitate three-dimensional geometrical modelling of complex poly-deformed geological units

The three-dimensional (3D) geometry of complexly deformed regions is often beyond the scope of simple 2D and 2.5D representation in cross-sections and block diagrams. Methods must be developed for fully three-dimensional representation. A workflow for such three-dimensional modelling of structurally complex areas is presented. Data requirements are tailored to typical results of structural field work in strongly deformed rocks from mid-crustal levels. The workflow is based on data evaluation and data export using standard geographical information system and database management systems. Three-dimensional modelling in polyphase deformed areas is highly dependent on the correct interpretation of variations in the orientation of the dominant planar fabrics. The computer-aided earth-modelling software 3D GeoModeller is well adapted for managing this problem. It calculates the geometry of geological interfaces taking into account simultaneously the foliation data and location of lithological contacts. The workflow is based on iterative model refinement based on interactive editing of the geometry in section and map views with data assessment and pre-processing using GIS software. This approach assures internal consistency of the resulting three-dimensional models. Modelling of the Lower Lepontine Nappes in the Central Alps is used as an illustrative example from a complexly deformed terrain.

Regional and local tectonics at Erta Ale caldera, Afar (Ethiopia)

Erta Ale volcano lies along the on-shore Red Sea Rift (northern Afar, Ethiopia), separating the Nubia and Danakil plates. Erta Ale has a NNW–SSE elongated caldera, with a subvertical rim scarp, hosting a lava lake. Structural field work was aimed at defining the deformation pattern around the caldera. The caldera consists of along-rim and across-rim structures, resulting from local and regional (maximum extension ∼NE–SW) stress fields, respectively. These structures cross-cut each other at high angles, suggesting that the two stress fields remain distinct, each prevailing during rifting or caldera collapse. The local along-rim extensional fractures are gravity-driven structures that formed due to the retreat of the caldera wall after collapse, and are confined to the region of caldera subsidence. The across-rim structures are mainly located to the N and S of the caldera, where they form rift zones each accommodating a similar amount of extension (∼6.3 m), but displaying different trends and extension directions. Analogue models of interacting fractures are consistent with the Southern Rift being representative of the regional fault kinematics, while the Northern Rift is a local perturbation, resulting from the interaction between two right-stepping rift segments along the Erta Ale Range.