ISTeP

Traces of extraction in Delian marble quarries attest their exploitation during Antiquity. A preliminary non-destructive provenance study confirmed the presence of indigenous marble in Delos constructions. In contrast, Delos marble quarries have not been geochemically described so far. Therefore, a detailed (1/5000 scale) geological mapping and cross-sections were performed in the four Delian marble quarries in order to better determine their dimension and to estimate the volume of extracted marble. The surface of the quarries was revised into up to six times larger areas, increasing the extracted volume estimations. Quarries were sampled and studied with mineralo-petrographic (optical and electronic microscopy, X-Ray Diffraction) and isotopic (d 13 C and d 18 O) characterization. Three categories were observed, i.e., a coarse whitish to bluish marble, a fine yellowish dolomitic marble and marble with giant white and blue calcite crystals. The Maximum Grain Size associated with oxygen and carbon isotopic ratios showed a good potential to distinguish Delian marbles from most of the main Mediterranean marbles used during Antiquity. However, geochemical elemental analyses such as trace elements analysis could supplement Delian marble characterization which will benefit future provenance studies.

Traces of extraction in Delian marble quarries attest their exploitation during Antiquity. A preliminary non-destructive provenance study confirmed the presence of indigenous marble in Delos constructions. In contrast, Delos marble quarries have not been geochemically described so far. Therefore, a detailed (1/5000 scale) geological mapping and cross-sections were performed in the four Delian marble quarries in order to better determine their dimension and to estimate the volume of extracted marble. The surface of the quarries was revised into up to six times larger areas, increasing the extracted volume estimations. Quarries were sampled and studied with mineralo-petrographic (optical and electronic microscopy, X-Ray Diffraction) and isotopic (δ13C and δ18O) characterization. Three categories were observed, i.e., a coarse whitish to bluish marble, a fine yellowish dolomitic marble and marble with giant white and blue calcite crystals. The Maximum Grain Size associated with oxygen a...

The Vestfirdir Peninsula of northwestern Iceland mainly consists of tholeiitic lava flows, 8-14 Ma old, gently dipping to the southeast. A detailed study of strike-slip and normal faulting allowed identification of two main paleostress regimes. Two sets of normal faults were recognized. The largest set trends ENE-WSW to NNE-SSW; the minor set trends NW-SE to WNW-ESE. Concerning the major extension, the reconstruction of paleostress trajectories shows a gradual change in trend from ESE-WNW, in the northern half of the peninsula, to NNW-SSE to the south. The minor extension also shows a gradual change from NNE-SSW to ENE-WSW trends, from north to south. The nearly constant perpendicularity between the major and minor trends of extension is accounted for by permutation of stress axes within the general pattern of extension related to oceanic rifting. The progressive azimuthal change of the major extension trend, from northeast to southwest across the peninsula, is interpreted as the expression of a change in trend of the extinct Skagi-Snaefells rift, a structure that cannot be reconstructed directly through geological mapping. The average trends of extension in the southern Vestfirdir Peninsula, N1500E, suggest a N600E trend for the ancient Snaefells rift segment. Likewise, the N100-11OOE trends of extension in northern Vestfirdir suggest a NID-ZooE trend for the Skagi paleo-rift. ,© Elsevier, Paris Mid-Atlantic Ridge I Iceland I faults I Paleo-stresses I Miocene Resume-Systemes de failles et paleocontraintes dans Ia peninsule de Vestfirdir. Relations avec les paleo-rifts tertiaires de Skagi et du Snaefells (Nord-Ouest de l'Islande). La peninsule de Vestfirdir, dans Ie Nord-Ouest de I'Islande, est composee d'un empilement de laves essentieIlement tholeitiques agees de 8 a 14 Ma, plongeant faiblement vers Ie Sud-Est. Vne etude detaillee des decrochements et des * Correspondence and reprints failles normales permet de caracteriser les principaux paleoetats de contrainte. Deux populations de failles normales peuvent~tre distinguees : I'une, principale, est de direction ENE-WSW a NNE-SSW; I'autre est de direction NW-SE a WNW-ESE. La reconstruction des trajectoires de contraintes montre pour I'extension principale Ie passage graduel d'une direction proche de N-S au Nord a une direction proche de E-W au Sud. Pour I'extension secondaire, Ie passage se fait d'une direction NNE-SSW au Nord aune direction ENE-WSW au Sud. La quasi-perpendicularite des extensions majeure et mineure peut~tre expliquee par des permutations d'axes de contrainte. Le champ de contrainte majeur reflete I'extension liee aI'axe de rift ancien Snaefells-Skagi et temoigne des changements de direction de celui-ci. Les directions d'extension 03 calcuIees dans Ie Sud de la peninsule indiqueraient ainsi, pour Ie segment du Snaefells, une direction proche de N 60 0 E, celles calcuIees dans Ie Nord montreraient une direction de N 10°_ 20 0 E pour Ie paleo-rift de Skagi. © Elsevier, Paris rifting medio-atlantique I Islande I failles I paleo-eontraintes I Miocene

Tectonics, tectonophysics Architecture and development of (Pliocene to Holocene) faults and fissures in the East Volcanic Zone of Iceland Architecture et de´veloppement des failles et fractures (plioce`nes à actuelles) dans la zone volcanique orientale de l'Islande

This paper provides detailed structural data on the crustal deformation of the Esja peninsula, at the southwestern margin of the rift zone in SW Iceland. Forty percent of the faults are strike-slip with steep dips, 35% are normal dip-slip with dips ranging from 50 ° to 90*. For normal faults, E-W, NE-SW and NNE-SSW are the three major trends. The paleostress tensors determined using the fault data, indicate three main states of stress. Two extensional regimes (03 N180*E +10" and Nll0°E +100) are related to normal slip faults. The third is a compressive stress regime with ol trending N10*E-N30*E. Associations between strike-slip and dip-slip faults are often observed, resulting from both the geometrical requirements of interactions between faulted blocks and from local-regional permutations between Ol and 02. We note a constant obliquity of 10-2& between the trend perpendicular to the regional structures and the computed direction of extension. This conclusion is also supported by the major trends of dykes and tensional fractures. There are two mechanisms of volcanic injection (local sheets and regional dykes) resulting in power-law cumulative thickness distributions with exponents of 1.5 and 3.0, respectively.

We describe and compare the two transform zones that connect the Icelandic rift segments and the mid-Atlantic Ridge close to the Icelandic hot spot, in terms of geometry of faulting and stress fields. The E-W trending South Iceland Seismic Zone is a diffuse shear zone with a Riedel fault pattern including N0°-N20°E trending right-lateral and N60°-N70°E trending left-lateral faults. The dominant stress field in this zone is characterised by NW-SE extension, in general agreement with left-lateral transform motion. The Tjörnes Fracture Zone includes three major lineaments at different stages of development. The most developed, the Húsavík-Flatey Fault, presents a relatively simple geometry with a major fault that trends ESE-WNW. The stress pattern is however complex, with two dominant directions of extension, E-W and NE-SW on average. Both these extensions are compatible with the right-lateral transform motion and reveal different behaviours in terms of coupling. Transform motion has unambiguous fault expression along a mature zone, a situation close to that of the Tjörnes Fracture Zone. In contrast, transform motion along the immature South Iceland Seismic Zone is expressed through a more complicate structural pattern. At the early stage of the transform process, relatively simple stress patterns prevail, with a single dominant stress field, whereas, when the transform zone is mature, moderate and low coupling situations may alternate, as a function of volcanic-tectonic crises and induce changes in stress orientation.

We carry out mass inversion of focal mechanisms of earthquakes from 1991 to 2004 along the South Iceland Seismic Zone, within a quadrangle about 37 km wide and 65 km long, with 33,878 earthquakes illuminating a crust volume of nearly 35,000 km 3. Our inversion aims at identifying the seismotectonic stress regimes that best account for the whole set of data, not at examining local variations and stress perturbations in space and time. Regardless of magnitude selection, the inversion reveals consistent orientations, for both a primary regime with NE-SW compression and NW-SE extension, accounting for about three-fourths of the data, and a secondary regime with NW-SE compression and NE-SW extension. The corresponding earthquakes are highly intricate in space and time, suggesting that the secondary regime, which shows larger dispersion and slightly lower magnitudes as compared with the primary regime, mainly results from the effects of elastic rebound, stress drop and fluid migration. Because the data acquisition and our inversion technique are independent, this analysis provides confirmation that even the negative magnitude earthquakes recorded by the Icelandic seismic network (SIL) are reliable. Our analysis reveals high levels of consistency between the seismotectonic stress pattern and the transform kinematics of the South Iceland Seismic Zone. The proximity of the Icelandic Mantle Plume, with a positive thermal anomaly, modified the rheology of the upper lithosphere and hence influenced structural development. Simple shear probably prevails within a 20 km wide corridor in the underlying viscous layer, inducing particular block faulting at the surface. Both the fault distribution and the effective block dimensions are consistent with a simple structural evolution model.

Observations of ~crot~to~c faults in and around the P~non~an basin suggest that the Tertiary evolution of the area was controlled by large-scale strike-slip fault activity. Statistical analysis of the fault pattern for different localities makes it possible to separate several conjugate sets of strike-slip faults and related dilatational and compressive structures. 'l%ese stress fields have a distinct rotation pattern with time. Inversion of fault data suggests a characteristic rotation of the principal stress axes with time. Some rigid body rotations are indicated by paleomagnetic data. There are basically three contrasting mechanisms to explain this observation: (1) Appropriate rotation of the regional (e.g., European) stress field; (2) rotation of larger tectonic units (e.g., ~cr~ontinents) under a stable regional stress field; and (3) coherent rotation of smaller, detached continental blocks under a stable regional stress field. Our model for the Neogene tectonic evolution of the Pannonian area combines these mechanisms. Moderate microcontinent rotation may have occurred during the Paleogene-Early Miocene. In the early Middle Miocene, the microcontinents were dissected by a system of strike-slip and normal faults, created by a N-S compressive/E-W extensive stress field. Gravity spreading of the Pannonian units and block rotation within bounding strike-slip faults are suggested to explain rotated magnetic directions and fault patterns. In the Late Neogene, clockwise rotation of the general stress-field by at least 40* is thought to be responsible for the reactivations of older strike-slip faults and for the creation of new sets. This change in the orientation of the stress field is related to the eastward and then southward shift of the Carpathian subduction activity.

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 Hu ¤ naflo ¤ i-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 40 Ar/ 39 Ar 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 Hu ¤ naflo ¤ i-Skagi axis, but 60 km to the east. This previously unknown paleo-rift is named the Skagafjo « rdur 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 Skagafjo « rdur 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.

A new approach for identifying past slip events in strike-slip fault systems without geological markers is presented. A 2-D distinct element modeling code is used to simulate the stress distribution around a strike-slip fault zone after a major slip event. Mature and immature fault zones are investigated. Slip on the fault is responsible for stress rotations near the active fault strand. Series of experiments reveal that the stress deflections are qualitatively the same whatever the fault strike, fault friction, and far-field differential stresses. For a left-lateral fault, stresses are rotated clockwise in the two compressive quadrants near the fault tips and counterclockwise in the two extensive quadrants. The stress field signature of a slip event thus highlights the extent of the active fault strand. Conversely, the reconstruction of paleostress fields through inversion of minor striated faults that commonly follow slip on the master fault and the analysis of their deflections allow recognition of the fault strands that slipped in a close or remote past. Application to the left-lateral Pontarlier Fault Zone in the Jura fold-and-thrust belt (France), Late Cenozoic in age, shows that the fault activity has migrated northward, in agreement with the in-sequence development of the mountain belt.

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 3100% (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 c 3) are 065 ‡ in the Tjo « rnes 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.