Françoise BERGERAT Département de Géotectonique, Boite 129, Université Pierre et Marie Curie, 4, place Jussieu, 75252 , Paris cédex 05 (France)
e-mail :bergerat@lgs.jussieu.fr - tel: 33-1 44 27 34 43 - - fax: 33 1 44 27 50 85
Jacques ANGELIER Département de Géotectonique, Boite 129, Université Pierre et Marie Curie, 4, place Jussieu, 75252 , Paris cédex 05 (France)
e-mail :ja@lgs.jussieu.fr - tel: 33-1 44 27 58 57 - - fax: 33 1 44 27 50 85
Report n° 1
Year 1, november 1996
The aim of the subproject 6A was to study the paleo- and present-stress associated with the South Iceland Seismic Zone and the Tjörnes Fracture Zone, and to determine how these stress fields control the fault activity in theses zones. We plan to (1) reconstruct the paleostress tensors from fault-slip data sets, and (2) reconstruct the present stress field using focal mechanisms of earthquakes, and carry out seismotectonic analysis along active fault scarps. Topic 2 is studied in co-operation with other staffs of the Prenlab project.
These two targets are planned to be hit between months 5 and 16 for the first one, and between months 5 and 24 for the second one. Both are now in progress, in agreement with the milestones. The main results are given below (cf. C. Report Paper)
BERGERAT F., ANGELIER J., DAUTEUIL O. et VILLEMIN T. (1996). - Morphologie et déformation dans un rift océanique émergé : exemple du champ de fissures du Krafla (Nord-Est Islande). 16ème Réun. Sc. Terre, Orléans, 10-12 avril 1996, vol. résumés, p.20, 1 fig.
BERGERAT F., ANGELIER J., DAUTEUIL O. et VILLEMIN T. (1996) - Present-day tension-shear deformation and morphology in the Krafla fissure swarm, axial rift zone of northeastern Iceland. XXVe General Assembly of the European Seismol. Comm., Reykjavik, 9-14 septembre 1996, Abstracts volume, p. 53.
ANGELIER J., ROGNVALDSSON S. Th., BERGERAT F., GUDMUNDSSON A., JACOBSDOTTIR S. et STEFANSSON R. (1996) - Focal mechanisms of earthquakes and recent faulting : A seismotectonic analysis of the vordufell area, South Iceland Seismic Zone. XXVe General Assembly of the European Seismol. Comm., Reykjavik, 9-14 septembre 1996, Abstracts volume, p. 53 and Papers volume, P. 199-204, 2 fig., 4 tabl.
ANGELIER J., BERGERAT F., DAUTEUIL O. et VILLEMIN T. (1996). - Tension-shear relationships in extensional fissure swarms, axial rift of northeastern Iceland : morphological evidences. Journ. Struct. Geology, in press.
ANGELIER J., ROGNVALDSSON S. Th., BERGERAT F. et GUDMUNDSSON A. (1997). - Earthquake focal mechanisms and regional stress : the seismotectonic behaviour of the South Iceland Seismic zone revealed by the Vordufell earthquake activity. Communication à l'E.U.G.IX, Strasbourg, 23-27 mars 1997.
BERGERAT F., ANGELIER J., GUDMUNDSSON A. et ROGNVALDSSON S. Th. (1997) - Joint study of recent faulting and earthquake focal mechanisms in the South Iceland Seismic Zone : the Vordufell area as a case example. Communication à l'E.U.G. IX, Strasbourg, 23-27 mars 1997.
DAUTEUIL O., ANGELIER J., BERGERAT F., FERBER V., VERRIER S. et VILLEMIN T. (1997). - Deformation pattern and morphology in the northern Iceland rift. Communication à l'E.U.G. IX, Strasbourg, 23-27 mars 1997.
GUDMUNDSSON A., BERGERAT F. et ANGELIER J. (1997). Strike-slip faults in the central part of the South Iceland Seismic Zone. Communication à l'E.U.G. IX, Strasbourg, 23-27 mars 1997.
This chapter summarizes the main results. The abstracts and submitted papers listed above are available on request.
1) Deformation pattern and morphology in the Krafla fissure swarm: the Mofell area. (work carried out with O. Dauteuil and T. Villemin)
In the northern part of the rift, the deformation affects an area 60 km wide. In this area most of the active extensional deformation is localised into fissure swarms, 1 to 5 km wide. Major volcanic centers are present along swarm axes. We analysed the partitioning of extension into a major fissure swarm : the Krafla fissure swarm. The studied area was a 4,5 km2 area, near the Mofell Mtn, north of the Krafla volcano and composed of basaltic lava flows younger than 10,000 years.
We used two information sources to estimate the spatial distribution of strain in this area. The first data set was obtained from detailed mapping structures combining field measurements, SPOT satellite images and aerial photographs. Most faults and fissures trend N 010°-030°. The fissures width ranges from ten centimeters to two meters. The faults have a maximum vertical throw of 20 meters. The second data set used is a network of more than 450 geodetic points providing an accurate record of topography within the fissure swarm. This allows us to determine exactly the amount of tilt blocks (less than 2 degrees), in an area with a weak extensive rate since 10,000 years. This network was used to define blocks with planar upper surface. The plunge and the trend of each plane were estimated and analysed in compared to the fault network. The plunge values vary from 0.2° to 3°, with trend comprised between N 030° and N 100°. The tilt of the blocks was used to estimate the stretching accommodated by the block rotations.
A balance of the extension accommodated by block tilting and fissure dilation is discussed along E-W profiles and on maps. Assuming that the whole area is affected by the same stretching amount, important variations are thus highlighted inside the system.
2) Tension-shear deformation in the Krafla fissure swarm : the Leirhnjukur area (work carried out with O. Dauteuil and T. Villemin)
The geometry of the fracture pattern of a small graben (studied area : 280 m long and 150 m wide) in the Krafla fissure swarm was analysed in detail. Based on geodetic analysis of the present-day topography at the top of Holocene basaltic lava flows which fill the axial rift zone, the deformation of this initially horizontal surface can be reconstructed. Extensional deformation is localised at all scales and block tilting, albeit present, remains minor.
Using simple models of the surface expression of normal faults, the geometrical characteristics of the topographic features related to active deformation during tectonic-volcanic events are quantitatively analysed. At crustal depths of about one km, normal faults are present and have an average 70° dip. Comparison with the dip distribution of older normal faults observed in the uplifted and eroded shoulders of the rift zone, at palaeodepths of 1-2 km, indicates that this dip determination is valid. Comparisons between the local case study and structural analyses of active fissure swarms on a larger scale suggest that normal faulting plays a major role in the middle section of the thin, newly formed brittle crust of the rift zone. In the axial oceanic rift zone of NE Iceland, the extensional deformation in the upper crust is dominated by horizontal tension and normal shear, their relative importance depending on depth. Absolute tension dominates in the uppermost several hundred metres of the crust, resulting in the development of fissure swarms. Effective tension plays an important role at a deeper level (2-5 km), because of the presence of magmatic fluid pressure from magma chambers which feed dyke injections. At crustal depths of about 1 km, normal shear prevails along fault planes which dip 60°-75°. This importance of normal shear at moderate depth, between upper and lower crustal levels where tension prevails, is pointed out. Within the extensional context of rifting, these variations of tectonic behaviour with depth are controlled by both the lithostatic pressure and the effective tension induced by the presence of magmatic fluid pressure.
1) Focal mechanisms of earthquakes and recent faulting in the central part of the SISZ (work carried out with A. Gudmundsson and S. Th. Rögnvaldsson)
A seismotectonic analysis was carried out in the Vordufell Mountain (64.05-64.12°N and 20.5-20.6°W). Good outcrops permit geological study of the recent faults which affect the quaternary lava pile, and focal mechanisms of shallow earthquakes are available. Among the earthquakes recorded in southern Iceland by the SIL network (Stefansson et al., 1993) with determinations of earthquakes mechanisms (Rognvaldsson and Slunga, 1993 and 1994; Slunga et al., 1995), a data set of 68 earthquakes, occurring in this area from 1991 to 1995, was used. 18 data corresponding to quarry blasts were rejected, and 50 focal mechanisms of natural earthquakes were selected.
The variety of these focal mechanisms of shallow earthquakes shows that the whole set cannot be accounted for by a single tectonic stress state. Numerous mechanical analyses were done, based either on geometrical considerations or on stress-slip consistency. Within the range of acceptable misfits (defined as a function of both the assumptions about stress-slip relationships and the uncertainties of data), a separation of two main groups of data -and related stress regimes- accounts for the whole data set. First, using the P- and T- dihedra method (Angelier and Mechler, 1977), the general consistency within each group is highlighted. The largest subset includes 30 strike-slip, 4 reverse and 4 normal mechanisms. It is consistent with NW-SE compression and NE-SW extension ,in agreement with left-lateral shear along E-W trends and right lateral strike-slip on N-S trending faults. The smallest subset includes 8 strike-slip, 1 reverse and 3 normal mechanisms. Its indicates NE-SW extension and NW-SE compression, with more dispersion than for the main group. Second, numerical inverse methods were used in order to compute the average stress tensors which best fit the observed fault plane solutions. Two main methods were used: the 4-D search (R4DT-R4DS, Angelier 1975 and 1984) and the direct inversion method (INVD, Angelier 1984 and 1990). Contrary to the P- and T- dihedra method, these methods require a choice among nodal planes. Because of its arbitrary character in geological terms, the choice of the nodal plane which best fits an average stress tensor was not adopted as an unique criterion. The geological study in the field and from aerial photographs allowed identification of the orientations of faults, fractures and other zones of weakness at both the scale of outcrops and that of the Vordufell Mtn. A comparison between fault plane solutions of earthquakes and fault slip data observed in outcrops was carried out. Combining these three criteria resulted in the final selection. In terms of numerical estimators considered alone, one may simply choose the best fitting fault plane solution for each earthquake. This was not done because, dealing with shallow earthquakes, reasonable choices between nodal planes imply consideration of the geological structure.
For each subset, the orientation of stress axes, the ratios of principal stress differences and the misfit estimators depend relatively little on the method adopted. The results are quite significant for the main subset. For the secondary subset, the misfits are larger despite its smaller size, which indicates inhomogeneity. Weighting data according to the quality of individual determinations did not result in significant improvement which suggests that mechanisms with small weights are quite acceptable. The main difference between the subsets mainly results from a kind of permutation between extreme stress axes. The direction of the maximum stress average N60°E for the main subset and N120°E for the subsidiary one, while the directions of the minimum stress average average N150°E and N40°E, respectively. The main subset reflects regional tectonic mechanisms, whereas the secondary subset, mechanically less consistent, principally reflects fault rebound and local accommodation.
Before examining earthquake data, a surprising result of geological studies of fault slip data in the field was the identification of two opposite tectonic regimes, respectively characterized by a NW-SE extension (the major one, principally including strike-slip and normal faults) and a NW-SE compression (the minor one, principally including strike-slip and few reverse faults). We point out first that for most of geological and geophysical data independently collected, similar tectonic regimes dominated by NW-SE maximum stress and NE-SW minimum stress were identified, and second that both these studies revealed permutation of extreme stress axes for the remaining data.
We conclude that the Vordufell area is dominated by NW-SE extension, principally accommodated by strike-slip and normal faulting, in agreement with the general behaviour of the South Iceland Seismic Zone. Local stress permutations, however, play a major role, resulting in subsets of conflicting mechanisms for both the present-day shallow earthquakes and the quaternary fault movements.
2) Preliminary studies in the whole SISZ (work carried out with A. Gudmundsson and S. Th. Rögnvaldsson)
Field studies have been carried out in september 1996 in the SISZ in order to collect fault slip data measurements. The collection has been made in some selected sites in late Tertiary and Pleiostocene lavas and hyaloclastites and in postglacial lavas. The areas investigated were the Skardsfjall, Hestfjall and Burfell mountains, the Grimsnes area and the canyons of Stora Laxa and Stora Mastunga. The measurements are now being analysed in terms of stress tensors. The study will be completed in the next few months by analysis of focal mechanisms of earthquakes in the same areas.
New methods, especially aiming at solving the problem of the choice between nodal planes for each datum within a set of focal mechanisms of earthquakes, are in preparation. Such a methodology will be of particular interest in the SISZ, where large numbers of data are available, as well as accurate relocations of earthquakes by the Icelandic Meteorological Office..
The techniques being presently prepared give more weight to analytical resuloution of the system of equations involved in the inverse problem, relative to the numerical procedures. As a preliminary case study, the Vordufell analysis menntioned above provides a good example for methodological evaluation, in addition to its intrinsic seismotectonic interest.
In 1997, the selected areas of the SISZ will be studied in detail both in the field and for focal mechanisms of earthquakes. Some preliminary studies in the Tjörnes Fracture Zone will also be carried out (paleo-stress analyses, focal mechanisms of earthquakes).
The aims of our future project in the framework of the new proposal are consistent with the results of the preliminary studies, which revealed high potential for extensive seismotectonic studies in the SISZ. These future studies will focus on fault development and stress controlling this development related to transform faulting and rifting. For this reason, the main objectives will be located essentially in the South Iceland Seismic Zone but also in the Tjörnes Fracture Zone. The main objective of our staff is to carry out a detailed seismotectonic analysis of these two zones, in order to advance the understanding of their general evolution and to improve our understanding of the development of faults and fault populations. The prediction of their future evolution and of associated seismic risk is of course the ultimate goal of the project.