The Roermond earthquake of 13 April 1992, the Netherlands: geological aspects

The epicentre of the Roermond earthquake is located near the western boundary fault of the Roer valley trough, one of the deepest and the most active in the Quaternary part of the Lower Rhine graben. The Late Pleistocene and Holocene activity of the trough is manifested by offsets of the main (Mindel) and the lower (Riss and Late Pleistocene) terraces along the boundary faults.
Surface fractures have been observed in an area of more than 50 km²: 2.5–3.5 km to the north of the town of Roermond, at 0.8 km to the south of the village of Herkenbosch and in the southeastern part of the village of Montfort. Three types of ruptures were differentiated: scarps up to 50 cm high along open fractures near the Maas River; open fractures (continued by scarps in some places) and open fractures with a liquefaction of the Quaternary alluvium sands. The last type is predominant. The location of the ruptures depends on the landscape and water-table of the region. While they could be produced solely by hydraulic shock during the earthquake (increased by the wet spring season), the majority of the ruptures strike N50°W ± 10°, i.e. parallel to the main trough faults, or N55°E ± 10°, along 'neotectonic lines', parallel to the Maas valley and deduced from space imagery. Thus, the ruptures could be the secondary surficial effect of the earthquake, linked indirectly with the active tectonics of the region.     

The 1 October 1995 Dinar earthquake, SW Turkey

The Dinar earthquake (M_s= 6.1, USGS-PDE) of 1 October 1995 occurred on the NWSE-trending Dinar Fault. The earthquake is associated with a 10-km-long surface rupture with predominantly normal faulting. The mainshock was preceded by a series of foreshocks that started 6 days before the mainshock and included two M_d = 4.5 events. The mainshock source mechanism derived from the inversion of broad-band P waves revealed that two sub-events occurred on a NW-SE trending normal fault with a small strike-slip component. According to the source model estimated in this study, the first rupture started at a depth of about 8 km and reached to a depth of about 12 km propagating north-west. The total seismic moment found from the inversion of P waveforms is 2.0 times 10 ¹⁸ Nm. The seismic moment of the second sub-event was about four times larger than the first one. Field observations, GPS measurements and slip vector obtained from the inversion of broad-band P waveforms suggest that the NW-SE trending Dinar Fault is due to the internal deformation of SW Anatolia moving south-westwards.     

A second natural occurrence of yoderite

A second example of yoderite has been discovered in whiteschists from the Southern Chewore Hills of northern Zimbabwe. The mineral is pale green in colour and occurs in an equilibrium assemblage with talc+chlorite+kyanite+dravite+hematite. There is no quartz present. Recalculated microprobe analyses give a structural formula of Mg₂Al_5.7Fe_0.3Si₄O₁₈(OH)₂, similar to that obtained for the type locality at Mautia Hill, Tanzania, i.e. Mg₂Al_5.6Fe_0.4Si₄O₁₈(OH)₂. Textural relationships and relative proportions of minerals suggest that the yoderite was formed by reaction between talc, chlorite, kyanite and hematite. Experimental evidence suggests high-water-pressure metamorphic conditions at temperatures exceeding a reaction curve that extends between 13  kbar at 590  °C and 21  kbar at 650  °C. The yoderite-bearing whiteschist is associated with a 1.4  Ga dismembered ophiolite. It is proposed that this yoderite occurrence is associated with a relict subduction/suture zone.     

The 10 June 2012 Fethiye Mw 6.0 aftershock sequence and its relation to the 24–25 April 1957 Ms 6.9–7.1 earthquakes in SW Anatolia,Turkey

The 10 June 2012 M_w 6.0 aftershock sequence in southwestern Anatolia is examined. Centroid moment tensors for 23 earthquakes with moment magnitudes (M_w) between 3.7 and 6.0 are determined by applying a waveform inversion method. The mainshock is a shallow focus strike-slip with reverse component event at a depth of 30 km. The seismic moment (M_o) of the mainshock is estimated as 1.28 × 10¹⁸ Nm and rupture duration of the Fethiye mainshock is 38 s. The focal mechanisms of the aftershocks are mainly strike-slip faulting with a reverse component. The geometry of the focal mechanisms reveals a strike-slip faulting regime with NE–SW trending direction of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to obtain a more accurate picture of the Fethiye earthquake stress field. The stress tensor inversion results indicate a predominant strike-slip stress regime with a NW–SE oriented maximum horizontal compressive stress (S_H). According to variance of the stress tensor inversion, to first order, the Fethiye earthquake area is characterized by a homogeneous interplate stress field. The Coulomb stress change associated with the mainshock and the largest aftershock are also investigated to evaluate any significant enhancement of stresses along the Gulf of Fethiye and surrounding region. Positive lobes with stress more than 0.4 bars are obtained, indicating that these values are large enough to increase the Coulomb stress failure towards NNW–SSE and E–W directions.     

Observation of surface displacements from GPS analyses before and after the Jiashian earthquake (M = 6.4) in Taiwan

A blind thrust fault with a unique strike, which is orthogonal to the strike of most tectonic structures in Taiwan, triggered the Jiashian earthquake on March 4, 2010 (M = 6.4; 22.96°N, 120.70°E). This study utilizes 100 global positioning system stations to examine changes of surface displacements during the Jiashian earthquake. We mitigate effects of short-term noise and long-term plate movements from surface displacement data using a frequency dependent filter via the Hilbert–Huang transform and compute the horizontal azimuth (i.e. GPS-azimuth) using residual data at the NS component relative to residual data at the EW component. Analytical results show that orientations of horizontal azimuths were aligned and orthogonal to the strike of the blind thrust fault. Meanwhile, inverse orientations are observed before and after the earthquake that agrees well with the seismic rebound theory. As stress disturbed on strata a few days before the earthquake, an impeded region can be clearly identified by disordered orientations of horizontal azimuths for anticipating the mainshock. These results provide an additional view to explore stress disturbance associated with earthquakes and offer more information to examine diverse models of tectonic evolution in this region.     

Petrology and geothermometry of retrograded Hercynian charnockites and host gneisses, Agly Massif, French Pyrenees

Abstract A Hercynian charnockite occurs within high-grade gneisses in the Agly Massif, French Pyrenees. Its thermal history has been evaluated using the Fe-Mg distribution coefticient ( K _D) between garnet and biotite. These minerals have different origins but similar compositions in the charnockites and host gneisses. In the charnockite, the Bi–Ga pairs are the retrograde products of Opx alteration. This Opx reaction with feldspar can be written. Opx + PI + Fluid 1(H₂O + Al + K + Fe + Ti) = Bi + Ga + Q + Fluid 2(H₂O + Na). The garnets are relatively Ca poor (4–2.5% grossular); they are automorphic and zoned in the gneisses and poikiloblastic in the charnockites. Both types show a retrograde rim (of few hundred microns'width) across which Fe and Mn increase as Mg decreases. The biotites show a good correlation between the octahedral cations (Ti⁴⁺+ Fe²⁺) and (Mg²⁺+ Al³⁺_VI); Ti and Fe both increase, whereas Mg and Al_VI decrease. There is an inverse linear correlation between Fe²⁺ and Mg²⁺ and the Fe/Mg ratio increases as Ti increases. The relation between Ti and K ^Ga-Bi_DFe-Mg is less clear: it seems that K _D slightly decreases as Ti increases. The equilibration temperatures of Ga–Bi pairs are discussed: the charnockite Ga-Bi pairs have equilibrated between 550°C and 600°C; whereas those of the gneisses have equilibrated between 550°C and 650°C. Two main thermal steps appear: one in the gneisses between 600-650°C and a second one in both the gneisses and the charnockites between 550°C and 600°C.     

Low-thermal-gradient Hercynian metamorphism in the eastern Pyrenees

Abstract Fluids with compositions in the system CO₂-H₂O-NaCl were trapped in quartz veins enclosed in low-grade metamorphic rocks (chlorite zone) on the southern flank of the Canigó Massif, eastern Pyrenees. The veins, which also contain arsenopyrite crystals, were formed contemporaneously with the main Hercynian foliation and metamorphism. Volumetric properties of the fluid and the results of arsenopyrite geothermometry suggest P-T trapping conditions of 4.6–6 kbar and 450–530° C. This implies that an episode of metamorphism with an average geothermal gradient of 25° C km⁻¹ occurred during the main deformation event. This episode preceded the low- P /high- T metamorphism described around domes and to date considered as characteristic of the Hercynian orogeny in the Pyrenees.     

Plagioclase relations in pelites, central Menderes Massif, Turkey. I. The peristerite gap with coexisting kyanite

Abstract In pelites of the central Menderes Massif, albite and oligoclase with only slight chemical zoning coexist in apparent textural equilibrium in the garnet zone, staurolite zone, and staurolite + kyanite transition zone. The metamorphic temperature range is estimated as approximately 440–550°C (from the Hodges-Spear calibration of the garnet-biotite geo-thermometer), or 440–500°C (Ganguly-Saxena calibration). While oligoclase composition at the peristerite gap changes from An₂₂ to An₁₄, albite also becomes more sodic (An_1,5–An_0.6). The slope of the albite limb is thus the reverse of that reported in other areas, and may not be a true equilibrium feature. Occurrence of kyanite, at temperatures below the crest of the gap, is due to low water activity in the presence of graphite: a H_2o is estimated at approximately 0.1–0.2 from the Na content of muscovite coexisting with albite + kyanite + quartz.     

Exhumation during oblique transpression: The Feiran–Solaf region, Egypt

The Feiran–Solaf metamorphic complex of Sinai, Egypt, is one of the highest grade metamorphic complexes of a series of basement domes that crop out throughout the Arabian-Nubian Shield. In the Eastern Desert of Egypt these basement domes have been interpreted as metamorphic core complexes exhumed in extensional settings. For the Feiran–Solaf complex an interpretation of the exhumation mechanism is difficult to obtain with structural arguments as all of its margins are obliterated by post-tectonic granites. Here, metamorphic methods are used to investigate its tectonic history and show that the complex was characterized by a single metamorphic cycle experiencing peak metamorphism at ∼700–750 °C and 7–8 kbar and subsequent isothermal decompression to ∼4–5 kbar, followed by near isobaric cooling to 450 °C. Correlation of this metamorphic evolution with the deformation history shows that peak metamorphism occurred prior to the compressive deformation phase D ₂, while the compressive D ₂ and D ₃ deformation occurred during the near isothermal decompression phase of the P–T loop. We interpret the concurrence of decompression of the P–T path and compression by structural shortening as evidence for the Najd fault system exhuming the complex in an oblique transpressive regime. However, final exhumation from ∼15 km depth must have occurred due to an unrelated mechanism.     

Magmatic and hydrothermal processes in the Bouvet Triple Junction Region (South Atlantic)

Results of electron microprobe and microthermometric studies of samples collected from the Bouvet Triple Junction Region (BTJR) during a joint Russian-Italian geological expedition on the R/V Academician Nikolaj Strakhov (1994) have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes. Detailed analysis of basaltic glasses shows that the modem Mid-Atlantic Ridge (MAR) rift valley is composed of normal mid-ocean ridge basalts with low concentrations of K₂ O and TiO_z (N-MORB), while its flanks are more enriched with these components approaching E-MORB. A marked influence of the Bouvet hot spot volcanism on magma generation on the South-West Indian Ridge (SWIR) near Bouvet Island is observed. Basaltic melts in this area belong to alkalic and transitional series and have maximum contents of K₂O, TiO₂, H₂O.
Microthermometric analyses of fluid inclusions in the samples from the BTJR have revealed major differences in the oceanic hydrothermal fluid systems on the MAR and near SWIR, which depends on the peculiarities of magma. In the area of the MAR (with dry melts) only H₂O solution inclusions in quartz were found; thus, seawater is probably the only primary source of hydrothermal fluids (NaCl + MgCl₂+ H₂O; T = 170–200°C). In the SWIR area (with the high content of water in melts) syngenetic liquid CO₂ and H₂O solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore-forming water/carbon dioxide solutions (NaCl + CaC1₂+ H₂O + CO₂; T = 200–310 °C; P = 900–1700 bar).     

Low-latitude anhydrite from the Red Sea coast: palaeoclimaticand palaeohydrological implications

Recent supratidal anhydrite (CaS0₄) is reportedly confined to a latitudinal range of ˜ 24–30°N. Because of this apparent constraint, it is problematical to relate known distributions of ancient deposits to palaeomagnetically determined latitudes. Recent work on the Red Sea coast of the Sudan shows that anhydrite is present south of 18°N and possibly down to 15°N latitude. In Arabia and North America ancient sabkha anhydrites occupy near-equatorial positions, which imply climatic belts within subtropical high-pressure cells. There is no reason why anhydrite should not occur at lower latitudes, provided the depositional area is within an effective rain-shadow where evaporation far exceeds rainfall (as in the Red Sea region).     

Static stress changes induced by the 1924 Pasinler (M = 6.8) and 1983 Horasan-Narman (M = 6.8) earthquakes, Northeastern Turkey

The 1924 Pasinler & 1983 Horasan-Narman earthquakes which struck the Erzurum region occurred on the NE–SW-trending Horasan fault zone about 60 km east of Erzurum basin. The inversion of teleseismic seismograms, the aftershock pattern and the surface faulting of the 30 October 1983 ( M _s = 6.8) Horasan-Narman earthquake indicate that it had dominantly left-lateral motion. One moderately sized aftershock occurred 8 h after the main event and two others a year later on the NE extension of the fault zone. The aftershock distribution dominantly overlapped with the Horasan fault zone, and the aftershocks also migrated from south-west to north-east within the year following the mainshock. The results obtained from modelling of static stress changes caused by the 1983 earthquake are consistent with the spatial distribution of aftershocks. Macroseismic observations of the 1924 earthquake ( M _s = 6.8) indicated that this event occurred on the SW extension of the Horasan fault zone. Static stress modelling of the 1924 earthquake, by using the same input parameters of the 1983 event, has shown that its occurrence increased the stress in the region of the 1983 rupture zone. The static stress changes caused both by the 1924 and the 1983 earthquakes has increased the failure stress at the NE and SW extensions of the Horasan fault zone and in Narman area. Furthermore, the stress has decreased in the vicinity of the Erzurum fault zone, east of the city of Erzurum, the largest city in eastern Turkey, and in the populated Sarikamis area. This might delay the occurrence of a future probable damaging earthquake in these areas.     

Significance of earthquake-related anomalies in fluids of Val D'Agri (southern Italy)

Geochemical investigations carried out at the Campano–Lucano Apennine (Southern Italy) revealed the presence of fluids composed of a mixing between components of shallow and deep origin, where mantle-derived helium is also detectable. For the gas phase, the deep component is represented by both CH₄ and CO₂-rich gases, while the shallow one is N₂-dominated. Coinciding with the 3 April 1996  M _L=4.9 earthquake, the CH₄-rich component mixed with the shallow, N₂-dominated one at the Tramutola well (Val d'Agri), displaying wide variations in mixing proportions. In contrast, no significant modifications occurred in relation to the 1998 M _L=5.5 event. According to the collected data, an earthquake-related transient modification of local crustal permeability is suggested for the 1996 event. The different crustal response to the two events may be related to different stress distributions around the epicentres or may suggest a different tectonic connection between the Val d'Agri and the two earthquake locations.     

Aluminum silicates in the Mount Raleigh pendant, British Columbia

In regionally metamorphosed pelites of the Mount Raleigh pendant, the fibrolite isograd occurs 5km downgrade from the sillimanite isograd. Fibrolite formed from the decomposition of biotite, a reaction that probably resulted from the late-stage influx of acidic volatiles. In contrast, sillimanite formed by the direct,'volume-for-volume'replacement of andalusite. Andalusite and sillimanite coexist in a 3 km-wide zone above the sillimanite isograd. Electron probe analyses of these phases reveal low minor element contents and yield K _D [= X ] values close to unity; the low Fe₂O₃ contents are compatible with reducing conditions implied by the ubiquity of graphite. Because K _D → 1.0, the zone of coexisting andalusite + sillimanite cannot be attributed to multivariancy resulting from partitioning of minor elements between these phases. Rather, the metastable persistence of andalusite into the sillimanite P-T stability field is suggested. The modal proportions of sillimanite versus andalusite imply that minimal (<5%) and alusitesillimanite reaction occurred in a zone 1.5km above the sillimanite isograd; in contrast, there was a marked increase in reaction progress immediately above this zone. With an estimated thermal gradient (in the plane of exposure) of approximately 20°C/km, the 1.5 km-wide zone of nil reaction suggests that the andalusite-sillimanite equilibrium boundary was overstepped by about 30 °C before significant reaction occurred. Inclusion-rich areas in andalusite provided favourable sites for sillimanite nucleation ; however, the growth of sillimanite may have been impeded by'pinning'of sillimanite grain boundaries by inclusions.     

Chloritoid-bearing rocks associated with blueschists and eclogites, northern New Caledonia

Abstract Chloritoid-bearing metasedimentary rocks occur in close proximity to blueschists and eclogites in the Tertiary high-pressure metamorphic belt of northern New Caledonia. The typical assemblage of chloritoid-bearing rocks in the epidote zone is quartzchlorite-muscovite-garnet-chloritoid. In the omphacite zone, epidote is an additional member of the chloritoid-bearing assemblage. Paragonite is rare, plagioclase was not detected, and rutile and ilmenite are the Fe-Ti oxide phases. Chloritoid-glaucophane is not a common assemblage. Chloritoid-bearing rocks have relatively low (Ca+K+Na)/Al ratios and the chloritoids are relatively Mg-rich with Mg/ (Mg+Fe) up to about 0.4. A comparison of the mineral assemblages and mineral chemistry with experimental and computed phase equilibria suggest an upper temperature limit near 560° C in the omphacite zone and a minimum temperature limit near 450° C at 10 kbar. An empirical garnet-chlorite Fe-Mg exchange thermometer does not yield consistent results for the higher-grade rocks, suggesting T s ranging from 390 to 535° C in the omphacite zone and 420–465° C in the epidote zone. The distribution coefficient K _D = (Fe/Mg)_ctd/(Fe/Mg)_chl for chloritoid and chlorite ranges from 3.9 to 6.4, values which are lower than those (=10) from lower greenschist facies rocks, but are near those of upper greenschist facies and albite-epidote amphibolite facies.     

40Ar/ 39Ar dating of Penninic Front tectonic displacement (W Alps) during the Lower Oligocene (31–34 Ma)

Direct absolute dating of the Penninic Frontal Thrust tectonic motion is achieved using the ⁴⁰Ar/³⁹Ar technique in the Pelvoux Crystalline Massif (Western Alps). The dated phengites were formed syn-kinematically in shear zones. They underline the brittle-ductile stretching lineation, pressure-shadow fibres and slickensides consistent with underthrusting of the European continental slab below the propagating Penninic Thrust. Chlorite–phengite thermobarometry yields 10–15 km and T ∼280 °C, while ⁴⁰Ar/³⁹Ar phengite ages mainly range between 34 and 30 Ma, with one younger age at 27 Ma. This Early Oligocene age range matches a major tectonic rearrangement of the Alpine chain. Preservation of prograde ⁴⁰Ar/³⁹Ar ages is ascribed to passive exhumation of the Pelvoux shear zone network, sandwiched between more external thrusts and the Penninic Front reactivated as an E-dipping detachment fault. Partial resetting in the Low Temperature part of argon spectra below 24 Ma is ascribed to brittle deformation and alteration of phengites.     

Plagioclase relations in pelites, central Menderes Massif, Turkey. II. Perturbation of garnet-plagioclase geobarometers

Abstract Activity-composition relations in oligoclase near the peristerite gap are investigated in pelites from the Central Menderes Massif. The pressure of metamorphism is estimated independently, from garnet-rutile-ilmenite-kyanite-quartz, as being in the range 4–7 kbar. In the temperature range, 450–600°C approximately, both the Newton-Haselton calibration of the garnet-plagioclase-kyanite-quartz geobarometer and a related simple treatment of garnet-plagioclase-muscovite-biotite give a wide range of apparent pressures, correlated with plagioclase composition and ranging up to 11–12 kbar where the plagioclase is most sodic. This effect is attributed to failure of the activity model for plagioclase used in the Newton-Haselton treatment. It is inferred that, in the present area, γ^plag_An decreases with increasing X ^plag_An in the range An₁₅-An₂₅. The data can be interpreted in terms of high γ ^plag_An in the high-albite structure at these temperatures, modified to lower values by 'e'ordering in the more calcic oligoclases. The ordering appears to be independent of the peristerite gap, and the data do not support the interpretation of the gap as a solvus. Garnet-plagioclase assemblages are unreliable as geobarometers where the plagioclase is more sodic than approximately An₂₀ and T < 700°C, and should instead be used to investigate the γ -X behaviour of the plagioclase where independent geobarometry can be used as a constraint.     

Thermal evolution of the orogenic lower crust during exhumation within a thickened Moldanubian root of the Variscan belt of Central Europe

At the eastern margin of the Bohemian Massif (Variscan belt of Central Europe), large bodies of felsic granulite preserve mineral assemblages and structures developed during the early stages of exhumation of the orogenic lower continental crust within the Moldanubian orogenic root. The development of an early steep fabric is associated with east–west-oriented compression and vertical extrusion of the high-grade rocks into higher crustal levels. The high-pressure mineral assemblage Grt-Ky-Kfs-Pl-Qtz-Liq corresponds to metamorphic pressures of ∼18 kbar at ∼850 °C, which are minimum estimates, whereas crystallization of biotite occurred at 13 kbar and ∼790 °C during decompression with slight cooling. The late stages of the granulite exhumation were associated with lateral spreading of associated high-grade rocks over a middle crustal unit at ∼4 kbar and ∼700 °C, as estimated from accompanying cordierite-bearing gneisses. The internal structure of a contemporaneously intruded syenite is coherent with late structures developed in felsic granulites and surrounding gneisses, and the magma only locally explored the early subvertical fabric of the felsic granulite during emplacement. Consequently, the emplacement age of the syenite provides an independent constraint on the timing of the final stages of exhumation and allows calculation of exhumation and cooling rates, which for this part of the Variscan orogenic root are 2.9–3.5 mm yr⁻¹ and 7–9.4 °C Myr⁻¹, respectively. The final part of the temperature evolution shows very rapid cooling, which is interpreted as the result of juxtaposition of hot high-grade rocks with a cold upper-crustal lid.     

TEM observations of several spinel-garnet assemblies: toward the rheology of the transition zone

In order to better identify the mineral phase which controls the rheology of the transition zone (between 410 and 660 km depth) transmission electron microscopy observations were made on several coexisting spinel-garnet assemblies: alkremite xenolith; pyrope-rich – MgO:1.1Al₂O₃ spinel assembly deformed at 1173K, 800 MPa in a Griggs apparatus; (Mg,Fe)₃(Al,Mg,Si)₂Si₃O₁₂ majorite – (Mg,Fe)₂SiO₄ spinel assembly synthesized in a laser heated diamond anvil cell. It was found that garnet crystals systematically remain undeformed while spinel crystals are plastically deformed. These results are in accord with the assumption that the rheology of majorite is stronger than the rheology of spinel, in the conditions of the transition zone.     

Evolution of orthogonal sets of coeval extension joints

The formation of two orthogonal sets of extension joints either crossing or abutting each other is a typical product of brittle deformation. Such systems of joints, with the two joint sets being geologically coeval, have been called a fracture grid-lock. The two sets are of common genesis and thus a unique remote stress field can be inferred. This interpretation causes some perplexity if the two joint sets are purely extension fractures and formed perpendicular to the least principal stress. In the present paper a conceptual model to explain the origin and the evolution of such systems is proposed. In a volume of rock undergoing a tensional and uniform remote stress state, caused for example by a tectonic regime, two horizontal and negative (i.e. tensional) stress rates are assumed to exist. When the tensile strength of the rock is locally reached, failure occurs perpendicular to the least principal stress. Then, that direction locally experiences a positive stress drop due to the stress release. For this reason, the stress field, retaining the same principal directions, is locally distorted by a swap between the σ₃ and the σ₂ components in a volume of rock surrounding the fracture. As a consequence of the persisting remote stress rates, when elastic failure conditions are newly accumulated, a second fracture forms and propagates perpendicular to the previous one. Repeated failure events, stress-drops and stress swaps eventually generate a fracture grid-lock. The whole process is also described with a simplified analytical model by applying elasticity theory.