Catherine Homberg

Rift jump process in Northern Iceland since 10 Ma from 40Ar/39Ar geochronology

Due to the westward American–Eurasian plate boundary migration, the rift zone location in Northern Iceland imposed by the stable Icelandic hotspot has jumped eastward. The present-day North Volcanic Zone of Iceland is thus shifted about 100 km to the east with respect to the Kolbeinsey Ridge. A Miocene paleo-rift location was proposed along the Hunafloi-Skagi axis. Unconformities inside the lava pile on both sides of the present-day rift as a result of the new rift formation have been used to date the rift jump, but timing is still controversial. Whether extinction of the paleo-rift was synchronous with initiation of the new rift or the two rifts were active during some time period is unknown. Using the 40Ar/39Ar radiochronology method, 37 dykes were dated along a 350 km profile parallel to the divergent plate motion direction (i.e., ESE–WNW) crossing the present rift, the unconformities, and the inferred paleo-rift. Our results reveal that the paleo-rift axis was not located along the Hunafloi-Skagi axis, but 60 km to the east. This previously unknown paleo-rift is named the Skagafjordur paleo-rift. It remained active until approximately 3 Ma. The activity of the present North Volcanic Zone started about 8–8.5 Ma, intruding 9–9.5 Myr rocks located at least 10 km east of the Skagafjordur paleo-rift axis. Consequently, the two rifts were simultaneously active for 5–5.5 Myr. During this time period, their accretion rates were nearly equal but with asymmetrical opening, the largest rates occurring on external flanks. These results are coherent in the Icelandic context and permit a first reconstruction of the evolution of Northern Iceland since 10 Ma.

Tectonic analysis of an oceanic transform fault zone based on fault-slip data and earthquake focal mechanisms: the Húsavı́k–Flatey Fault zone, Iceland

Abstract The Husavik–Flatey Fault (HFF) is an oblique dextral transform fault, part of the Tjornes Fracture Zone (TFZ), that connects the North Volcanic Zone of Iceland and the Kolbeinsey Ridge. We carry out stress inversion to reconstruct the paleostress fields and present-day stress fields along the Husavik–Flatey Fault, analysing 2700 brittle tectonic data measured on the field and about 700 earthquake focal mechanisms calculated by the Icelandic Meteorological Office. This allows us to discuss the Latest Cenozoic finite deformations (from the tectonic data) as well as the present-day deformations (from the earthquake mechanisms). In both these cases, different tectonic groups are reconstructed and each of them includes several distinct stress states characterised by normal or strike-slip faulting. The stress states of a same tectonic group are related through stress permutations ( σ 1 − σ 2 and σ 2 − σ 3 permutations as well as σ 1 − σ 3 reversals). They do not reflect separate tectonic episodes. The tectonic groups derived from the geological data and the earthquake data have striking similarity and are considered to be related. The obliquity of the Husavik–Flatey Fault implies geometric accommodation in the transform zone, resulting mainly from a dextral transtension along an ENE–WSW trend. This overall mechanism is subject to slip partitioning into two stress states: a Husavik–Flatey Fault-perpendicular, NE–SW trending extension and a Husavik–Flatey Fault-parallel, NW–SE trending extension. These three regimes occur in various local tectonic successions and not as a regional definite succession of tectonic events. The largest magnitude earthquakes reveal a regional stress field tightly related to the transform motion, whereas the lowest magnitude earthquakes depend on the local stress fields. The field data also reveal an early extension trending similar to the spreading vector. The focal mechanism data do not reflect this extension, which occurred earlier in the evolution of the HFF and is interpreted as a stage of structural development dominated by the rifting process.

Structural inheritance and cenozoic stress fields in the Jura fold-and-thrust belt (France)

Abstract Based on an analysis of 8000 minor fault-slip data in the Jura Mountains (France), we discuss the influence of pre-existing discontinuities on the development of fold-and-thrust belts. We present palinspastic maps showing the stress fields and active faults during the Cenozoic pre-orogenic events in the Jura belt prior to the main Late Miocene fold-and-thrust tectonics. During the Eocene, a N–S strike-slip regime produced a few NNE–SSW sub-vertical strike-slip faults in the central external Jura and a few E–W reverse faults in the eastern Jura near the future frontal thrust. During the Oligocene, an average WNW–ESE extension, with irregular stress trajectories, resulted in normal faulting along N–S to NE–SW trends in the external part of the belt, WNW–ESE trends along the future northern and northeastern frontal thrust, and NW–SE trends in the internal Jura. The Late Miocene tectonics began with a strike-slip regime with a fan-shaped compressional trajectory. It was followed by a stress field with similar stress direction, but local σ 2 / σ 3 stress permutation resulted in strike-slip regime domains contrasting with reverse regime domains. Stress deflections and permutations occurred near inherited cover and basement discontinuities. Major deformation zones, like the Jura frontal thrust onto the foreland, the thrust of the internal central Jura onto the external Jura, and the narrow deformation bands within the flat-lying plateaus formed close to the inherited faults. The structural style of the Jura belt thus partly mimics the pre-orogenic fault pattern. Stress deflections point to the pre-orogenic faults, express the indentation process of the Jura by its hinterland, and highlight successive slip events along major faults during the fold-and-thrust tectonics. This case study illustrates the relevance of minor fault-slip studies for characterizing both the pre-orogenic tectonics and the kinematics of the deformation.

Perturbation of stress and oceanic rift extension across transform faults shown by earthquake focal mechanisms in Iceland

Abstract Major sources of stress perturbation are expected in the brittle crust, and documented by local studies in Iceland. Whether or not a significant average stress state may emerge from regional-scale inversion of very large sets of focal mechanisms is thus subject to doubt. We carried out stress inversion of double couple focal mechanisms recorded by the Icelandic seismological network within a set of 126 588 shallow earthquakes from July 1991 to July 1999. We performed mass inversion of two main data sets of 12 191 and 71 889 focal mechanisms in the transform zone areas of North and South Iceland respectively. The inversion reveals surprisingly high levels of consistency within these data sets, with regard to the expected dispersion. Adopting a threshold value of +40% in an individual fit scale from −100% (total misfit) to +100% (perfect fit), the proportion of acceptable data is as high as 78% for 65 571 focal mechanisms of the major regime in these two areas (53% for 18 519 mechanisms of the minor regime). The major stress regimes thus calculated show nearly vertical intermediate principal axes and the azimuths of extension (minimum stress σ 3 ) are 065° in the Tjornes Fracture Zone and 140° in the South Iceland Seismic Zone. With respect to the 105° azimuth of plate separation, these directions show nearly symmetrical angular deviations: 40° anticlockwise in the right-lateral transform zone area of North Iceland and 35° clockwise in the left-lateral one of South Iceland. Such large deviations reveal first-order stress perturbation in the areas where major rift offset resulted from Late Cainozoic rift jumps related to plate boundary migration with respect to the Iceland Mantle Plume.

New constraints for indentation mechanisms in arcuate belts from the Jura Mountains, France

The mechanism of indentation in arcuate belts is discussed using the Jura Mountains as an example. Using fault-inversion analysis from numerous sites, the Miocene-Pliocene stress pattern is reconstructed. Two superimposed compressional stress fields have been recognized. Considering the theoretical distribution of compression in front of a rigid indenter, we interpret the complex stress pattern in the Jura Mountains as a result of a major change in the mechanism of indentation of the Jura by its hinterland. Initially, the indenter was narrower and limited in the southwest by the Vuache transfer fault. With time, it expanded to the southwest and included the entire Molasse basin. This evolution highlights the importance of decoupling along faults, which here controlled the indenter shape.

Evolution of the Levant margin and western Arabia platform since the Mesozoic: introduction

Abstract The Levant area comprises the offshore Levant Basin (LB) (eastern corner of the Eastern Mediterranean) as well as the adjacent continental slopes and platforms of the African and Arabian plates. This area experienced major events of the geodynamical evolution of the Middle East, such as the Late Palaeozoic–Early Mesozoic Pangea break up, the Late Cretaceous–Cenozoic closure of the Neo-Tethys and individualization of the Arabian plate, as well as a set of external factors like global sea-level and climate changes. This volume combines original data from the offshore and onshore Levant in various fields, including sedimentology, palaeontology, sequence stratigraphy, geochemistry, structural geology, stress reconstitution and geophysics (seismic lines, palaeomagnetism). All together, these multidisciplinary approaches allow the review of the development of the LB and gain a better insight on the later geological history and deformation processes of the Levant provinces.