Cenozoic post-collisional brittle tectonic history and stress reorientation in the High Zagros Belt (Iran, Fars Province)

The Zagros fold-and-thrust belt of SW-Iran is among the youngest continental collision zones on Earth. Collision is thought to have occurred in the late Oligocene–early Miocene, followed by continental shortening. The High Zagros Belt (HZB) presents a Neogene imbricate structure that has affected the thick sedimentary cover of the former Arabian continental passive margin. The HZB of interior Fars marks the innermost part of SE-Zagros, trending NW–SE, that is characterised by higher elevation, lack of seismicity, and no evident active crustal shortening with respect to the outer (SW) parts. This study examines the brittle structures that developed during the mountain building process to decipher the history of polyphase deformation and variations in compressive tectonic fields since the onset of collision. Analytic inversion techniques enabled us to determine and separate different brittle tectonic regimes in terms of stress tensors. Various strike–slip, compressional, and tensional stress regimes are thus identified with different stress fields. Brittle tectonic analyses were carried out to reconstruct possible geometrical relationships between different structures and to establish relative chronologies of corresponding stress fields, considering the folding process. Results indicate that in the studied area, the main fold and thrust structure developed in a general compressional stress regime with an average N032° direction of σ₁ stress axis during the Miocene. Strike–slip structures were generated under three successive strike–slip stress regimes with different σ₁ directions in the early Miocene (N053°), late Miocene–early Pliocene (N026°), and post-Pliocene (N002°), evolving from pre-fold to post-fold faulting. Tensional structures also developed as a function of the evolving stress regimes. Our reconstruction of stress fields suggests an anticlockwise reorientation of the horizontal σ₁ axis since the onset of collision and a significant change in vertical stress from σ₃ to σ₂ since the late stage of folding and thrusting. A late right-lateral reactivation was also observed on some pre-existing belt-parallel brittle structures, especially along the reverse fault systems, consistent with the recent N–S plate convergence. However, this feature was not reflected by large structures in the HZB of interior Fars. The results should not be extrapolated to the entire Zagros belt, where the deformation front has propagated from inner to outer zones during the younger events.     

Seismotectonics of strike–slip earthquakes within the deep crust of southern Italy: Geometry, kinematics, stress field and crustal rheology of the Potenza 1990–1991 seismic sequences (Mmax 5.7)

We present a revision and a seismotectonic interpretation of deep crust strike–slip earthquake sequences that occurred in 1990–1991 in the Southern Apennines (Potenza area). The revision is motivated by: i) the striking similarity to a seismic sequence that occurred in 2002  140 km NNW, in an analogous tectonic context (Molise area), suggesting a common seismotectonic environment of regional importance; ii) the close proximity of such deep strike–slip seismicity with shallow extensional seismicity (Apennine area); and iii) the lack of knowledge about the mechanical properties of the crust that might justify the observed crustal seismicity. A comparison between the revised 1990–1991 earthquakes and the 2002 earthquakes, as well as the integration of seismological data with a rheological analysis offer new constraints on the regional seismotectonic context of crustal seismicity in the Southern Apennines. The seismological revision consists of a relocation of the aftershock sequences based on newly constrained velocity models. New focal mechanisms of the aftershocks are computed and the active state of stress is constrained via the use of a stress inversion technique. The relationships among the observed seismicity, the crustal structure of the Southern Apennines, and the rheological layering are analysed along a crustal section crossing southern Italy, by computing geotherms and two-mechanism (brittle frictional vs. ductile plastic strength) rheological profiles. The 1990–1991 seismicity is concentrated in a well-defined depth range (mostly between 15 and 23 km depths). This depth range corresponds to the upper pat of the middle crust underlying the Apulian sedimentary cover, in the footwall of the easternmost Apennine thrust system. The 3D distribution of the aftershocks, the fault kinematics, and the stress inversion indicate the activation of a right-lateral strike–slip fault striking N100°E under a stress field characterized by a sub-horizontal N142°-trending σ1 and a sub-horizontal N232°-trending σ3, very similar to the known stress field of the Gargano seismic zone in the Apulian foreland. The apparent anomalous depths of the earthquakes (> 15 km) and the confinement within a relatively narrow depth range are explained by the crustal rheology, which consists of a strong brittle layer at mid crustal depths sandwiched between two plastic horizons. This articulated rheological stratification is typical of the central part of the Southern Apennine crust, where the Apulian crust is overthrusted by Apennine units. Both the Potenza 1990–1991 and the Molise 2002 seismic sequences can be interpreted to be due to crustal E–W fault zones within the Apulian crust inherited from previous tectonic phases and overthrusted by Apennine units during the Late Pliocene–Middle Pleistocene. The present strike–slip tectonic regime reactivated these fault zones and caused them to move with an uneven mechanical behaviour; brittle seismogenic faulting is confined to the strong brittle part of the middle crust. This strong brittle layer might also act as a stress guide able to laterally transmit the deviatoric stresses responsible for the strike–slip regime in the Apulian crust and may explain the close proximity (nearly overlapping) of the strike–slip and normal faulting regimes in the Southern Apennines. From a methodological point of view, it seems that rather simple two-mechanism rheological profiles, though affected by uncertainties, are still a useful tool for estimating the rheological properties and likely seismogenic behaviour of the crust.     

Compressional- and transpressional-stress pattern for Pliocene and Quaternary brittle deformation in fore arc and intra-arc zones (Andes of Central and Southern Chile)

Kinematic analysis of fault slip data for stress determination was carried out on Late Miocene to Quaternary rocks from the fore arc and intra-arc regions of the Chilean Andes, between 33° and 46° south latitudes. Studies of Neogene and Quaternary infilling (the Central Depression), as well as plutonic rocks of the North Patagonian Batholith along the Liquiñe–Ofqui Fault Zone, have revealed various compressional and/or transpressional states of stress. In the Pliocene, the maximum compressional stress (σ₁) was generally oriented east–west. During the Quaternary, the deformation was partitioned into two coeval distinctive states of stress. In the fore arc zone, the state of stress was compressional, with σ₁ oriented in a N–S to NNE–SSW direction. In the intra-arc zone the state of stress was transpressional with σ₁ striking NE–SW. Along the coast, in one site (37°30′S) the Quaternary strain deformation is extensional, with an E–W direction, which can be explained by a co-seismic crustal bending readjustment.     

Constraining stress magnitudes using petroleum exploration data in the Cooper–Eromanga Basins, Australia

The magnitude of the in situ stresses in the Cooper–Eromanga Basins have been determined using an extensive petroleum exploration database from over 40 years of drilling. The magnitude of the vertical stress (S_v) was calculated based on density and velocity checkshot data in 24 wells. Upper and lower bound values of the vertical stress magnitude are approximated by S_v = (14.39 × Z)^1.12 and S_v = (11.67 × Z)^1.15 functions respectively (where Z is depth in km and S_v is in MPa). Leak-off test data from the two basins constrain the lower bound estimate for the minimum horizontal stress (S_hmin) magnitude to 15.5 MPa/km. Closure pressures from a large number of minifrac tests indicate considerable scatter in the minimum horizontal stress magnitude, with values approaching the magnitude of the vertical stress in some areas. The magnitude of the maximum horizontal stress (S_Hmax) was constrained by the frictional limits to stress beyond which faulting occurs and by the presence of drilling-induced tensile fractures in some wells. The maximum horizontal stress magnitude can only be loosely constrained regionally using frictional limits, due to the variability of both the minimum horizontal stress and vertical stress estimates. However, the maximum horizontal stress and thus the full stress tensor can be better constrained at individual well locations, as demonstrated in Bulyeroo-1 and Dullingari North-8, where the necessary data (i.e. image logs, minifrac tests and density logs) are available. The stress magnitudes determined indicate a predominantly strike-slip fault stress regime (S_Hmax > S_v > S_hmin) at a depth of between 1 and 3 km in the Cooper–Eromanga Basins. However, some areas of the basin are transitional between strike-slip and reverse fault stress regimes (S_Hmax > S_v ≈ S_hmin). Large differential stresses in the Cooper–Eromanga Basins indicate a high upper crustal strength for the region, consistent with other intraplate regions. We propose that the in situ stress field in the Cooper–Eromanga Basins is a direct result of the complex interaction of tectonic stresses from the convergent plate boundaries surrounding the Indo-Australian plate that are transmitted into the center of the plate through a high-strength upper crust.     

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

The Húsavík–Flatey Fault (HFF) is an oblique dextral transform fault, part of the Tjörnes 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 Húsavík–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 (σ₁−σ₂ and σ₂−σ₃ permutations as well as σ₁−σ₃ 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 Húsavík–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 Húsavík–Flatey Fault-perpendicular, NE–SW trending extension and a Húsavík–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.     

Variation with depth of the stress tensor anisotropy inferred from microfault analysis

In order to study the Pyrenean tectonic phase, a quantitative method of stress analysis using microfault measurements is used on a calcareous plateau located in southern France. The method developed here allows the determination of several tectonic events and the evaluation of (with σ σ₂ σ₃).The Pyrenean compression is seen to occur in two stages, confirming previous geological studies.A recent canyon allows the study of the variation with depth of the R ratio on a vertical cross-section (300 m). With a simple model of gravity and tectonic stresses, the vertical variation of R can be used to estimate quantitatively the Pyrenean tectonic stresses. For realistic values of the parameters in this model, the horizontal tectonic stresses are obtained in the following range: 50–200 bar for the maximum horizontal principal stress, 10–25 bar for the other horizontal stress.These results seem to be consistent with in situ stress measurements, but they are much lower than those predicted by experimental rock mechanics.     

The tectonic regime in Italy inferred from borehole breakout data

In Italy, the horizontal stress directions are well constrained in many regions, but the tectonic regime is not well known because the stress magnitudes are unknown. Our intention is to improve the knowledge of crustal stress in Italy, both at shallow depth and in low seismicity areas. Therefore, we inferred the tectonic regime from the comparison between the depth of breakout occurrence and the physical properties of the rocks in 20 boreholes. The critical value of the maximum horizontal stress, for which the effective tangential stress at the borehole wall overcomes the rock strength to form breakouts, could be computed from rock strength and density. Comparing the theoretical stress distributions for different tectonic regimes with the depth distribution of breakout occurrence, it is possible to infer the tectonic regime that fits best to the breakout depth distribution. We investigated boreholes up to 6 km deep located in different tectonic environments over the Italian peninsula: the Po Plain, the Apenninic chain, the Adriatic foredeep and the Tyrrhenian Quaternary volcanic region. These wells are characterised by breakout data of good quality (A, B and C, according to World Stress Map quality ranking system). The results are in general agreement with the style of faulting derived from earthquake focal mechanisms and other stress indicators. Our results show a predominance of a normal faulting (NF) regime in the inner Apennines and both normal faulting and strike–slip faulting (SS) style in the surrounding regions, possibly also associated with changes in the tectonic regime with depth.     

Focal mechanism inversion in the Giudicarie–Lessini seismotectonic region (Southern Alps, Italy): Insights on tectonic stress and strain

A set of 41 focal mechanisms (1989–2006) from P-wave first polarities is computed from relocated seismic events in the Giudicarie–Lessini region (Southern Alps). Estimated hypocentral depths vary from 3.1 to 20.8 km, for duration magnitudes (M_D) in the range 2.7–5.1. Stress and strain inversions are performed for two seismotectonic zones, namely G (Giudicarie) and L (Lessini). This subdivision is supported by geological evidence, seismicity distribution, and focal mechanism types. The available number of data (16 in G, 22 in L) does not make possible any further subdivisions. Seismotectonic zones G and L are undergoing different kinematic regimes: thrust with strike-slip component in G, and strike-slip in L. Principal stress and strain axes in each sub-region show similar orientations. The direction of maximum horizontal compressive stress is roughly perpendicular to the thrust fronts along the Giudicarie Belt in zone G, and compatible with right-lateral strike-slip reactivation of the faults belonging to the Schio-Vicenza system in zone L. On the whole, kinematic regimes and horizontal stress orientations show a good fit with other stress data from focal mechanisms and breakouts and with geodetic strain rate axes.     

The importance of the effective intermediate principal stress (σ′2) to fault slip patterns

In normal faulting regimes, the magnitudes and orientations of the maximum and minimum principal compressive stresses may be known with some confidence. However, the magnitude of the intermediate principal compressive stress is generally much more difficult to constrain and is often not considered to be an important factor. In this paper, we show that the slip characteristics of faults and fractures with complex or nonoptimal geometry are highly sensitive to variation or uncertainty in the ambient effective intermediate principal stress (σ′₂). Optimally oriented faults and fractures may be less sensitive to such variations or uncertainties. Slip tendency (T_s) analysis provides a basis for quantifying the effects of uncertainty in the magnitudes and orientations of all principal stresses and in any stress regime, thereby focusing efforts on the most important components of the system. We also show, for a normal faulting stress regime, that the proportion of potential surfaces experiencing high slip tendency (e.g., T_s ≥ 0.6) decreases from a maximum of about 38% where σ′₂ = σ′₃, to a minimum of approximately 14% where σ′₂ is halfway between σ′₃ and σ′₁, and increases to another high of approximately 29% where σ′₂ = σ′₁. This analysis illustrates the influence of the magnitude of σ′₂ on rock mass strength, an observation previously documented by experimental rock deformation studies. Because of the link between fault and fracture slip characteristics and transmissivity in critically stressed rock, this analysis can provide new insights into stress-controlled fault transmissivity.     

鄂尔多斯地块南缘地应力测量研究

利用水压致裂法得到的地应力测试数据对鄂尔多斯地块南缘地壳浅部地应力分布规律及断层活动性进行研究。结果表明:（1）两个水平主应力随深度线性增长,应力梯度分别为0.032和0.021,在测量深度域内水平和垂直应力的关系为S_H > S_h > S_V,该应力状态有利于断层发生逆断层活动,与1556年华山大地震的发震正断层的性质不同;（2）研究区的最大水平主应力方向为北南-北北西向,与区域速度矢量场方向一致,与其他资料解译的区域构造应力方向有一些差异,主要是受鄂尔多斯地块周缘断层活动的影响;（3）利用Mohr-Coulomb准则及Byerlee定律,摩擦系数取0.6~1.0,对研究区的地应力状态进行分析,发现鄂尔多斯地块南缘的测点未达到或超过地壳破裂极限状态,不存在断层失稳或地震等其他形式的地壳活动,处于较稳定地壳应力状态;（4）实测数据为该区补充了新的地应力测量资料,研究结果为该区工程设计及建设、构造应力场数值模拟提供了边界条件,对于该区地质灾害评价、地壳稳定性以及大陆动力学的研究具有重要意义。      

In Situ Stress Measurements in the Lhasa Terrane, Tibetan Plateau, China

Tectonic activities are frequent in the Lhasa terrane because of the ongoing collision between the India and Eurasia plates. Knowledge of the stress state is critical to evaluate the crustal stability and the design of underground excavations. Because of the limitations imposed by natural conditions,little research has been performed on the present crustal in situ stress in the Tibetan Plateau,and further study is imperative. In this study,hydraulic fracturing measurements were conducted in Nyching County(LZX) and Lang County(LX),Lhasa terrane to characterize the shallow crustal stress state. The results indicate that the stress state in the LZX borehole is markedly different from that in the LX borehole,in both magnitude and orientation. At the same measurement depths,the magnitudes of horizontal principal stresses in the LX borehole are 1.5–3.0 times larger than those in the LZX borehole. The stress regime in the LX borehole favors reverse faulting characterized by SHShSv,where SH,Sh,and Sv are maximum horizontal,minimum horizontal,and vertical principal stresses,respectively. The SH and Sh values are approximately three and two times greater than Sv. Fracture impression results reveal that SH in the LX borehole are predominantly N–S,while in the LZX borehole the maximum horizontal principal stress is mainly in the NNE-direction. The heterogeneity of the regional stress state might be a result of the population and distribution of local structures and seismic activities. The stress state in the LX borehole has exceeded the critical state of failure equilibrium,and there is an optimally orientated pre-existing fault near the borehole. It can be concluded that the optimally orientated fault is likely to be active when the stress has built up sufficiently to destroy the frictional equilibrium; it is suggested that research focus should be placed on this in future. The stress states in boreholes LZX and LX indicate uniformity of the regional stress field and diversity of the local stress fields resulting from the interactions among regional dynamic forces,tectonic stress field,and geological structures.     

Active faults, seismicity and stresses in an internal boundary of a tectonic arc (Campo de Dalías and Níjar, southeastern Betic Cordilleras, Spain)

The Betic-Rif Cordilleras, formed by the interaction of NW–SE convergence between the Eurasian and African plates and the westward motion of their Internal Zones, provide a good example of an active tectonic arc. The Campo de Dalías and Campo de Níjar constitute outcropping sectors of Neogene and Quaternary rocks located in the southeastern border of the Betic Cordilleras and allow us to study the recent deformations developed in the internal border of this tectonic arc.The main active faults with related seismicity, representing a moderate seismic hazard, associated to the southeastern Betic Cordilleras boundary, include high-angle NW–SE-oriented normal faults that affect, at least, the upper part of the crust, a main detachment located at 10 km depth, and probably another detachment at 20 km as well. Seismite structures, recent fault scarps with associated colluvial wedges that deform the drainage network and the alignment of the coastline, indicate that the high-angle faults have been active at least since the Quaternary.Paleostresses determined from microfault analysis in Quaternary deposits generally show an ENE–WSW trend of extension. Present-day earthquake focal mechanisms include normal, strike-slip and reverse faulting. Normal and strike-slip focal mechanisms generally indicate ENE–WSW extension, and strike-slip and reverse focal mechanisms are related to NNW–SSE compression.The maximum horizontal compression has a consistently NNW–SSE trend. The deep activity of detachments and reverse faults determines the NNW–SSE crustal shortening related to the Eurasian–African plate convergence. At surface, however, the predominance of normal faults is probably produced by the increase in the relative weight of the vertical stress axis, which in turn may be related to relief uplift and subsequent horizontal spreading. The internal mountain front boundary of the Betic Cordilleras developed through the activity of a set of structures that is more complex than a typical external mountain front, probably as a consequence of a vertical variable stress field that acted on previously deformed rocks belonging to the Internal Zone of the cordilleras.     

Strain partitioning between the Suruga Trough and the Zenisu Thrust: Neogene to present tectonic evolution in the onland and offshore Tokai district, Japan

The Tokai district in central Japan is located close to the convergent boundary between the Philippine Sea and Eurasian plates, and has experienced not only repeated large interplate earthquakes but also intense aseismic movement. In this paper, the spatial and temporal tectonic evolution of the Tokai district, particularly around the Omaezaki area, is discussed to assess whether the district has been and will be active or inactive. According to a geological survey, the horizontal crustal shortening strain can imply the hypothetical tectonic model that the area has been getting less active and the strain rate since the Neogene can be calculated as 12% and 2×10⁻⁶%/year, respectively. The present interseismic horizontal crustal strain and strain rate around the Omaezaki area are approximately 4×10⁻⁷% and 4×10⁻⁹%/year. By comparing these rates, the decrease since Neogene can imply the hypothetical tectonic model that the area has been getting less active influenced by the strain partitioning between the Suruga Trough and the Zenisu Thrust.     

Relocation of aftershocks, focal mechanisms and stress inversion: Implications toward the seismo-tectonics of the causative fault zone of Mw7.6 2001 Bhuj earthquake (India)

The HYPODD relocation of 1172 aftershocks, recorded on 8–17 three-component digital seismographs, delineate a distinct south dipping E–W trending aftershock zone extending up to 35 km depth, which involves a crustal volume of 40 km × 60 km × 35 km. The relocated focal depths delineate the presence of three fault segments and variation in the brittle–ductile transition depths amongst the individual faults as the earthquake foci in the both western and eastern ends are confined up to 28 km depth whilst in the central aftershock zone they are limited up to 35 km depth. The FPFIT focal mechanism solutions of 444 aftershocks (using 8–12 first motions) suggest that the focal mechanisms ranged between pure reverse and pure strike slip except some pure dip slip solutions. Stress inversion performed using the P and T axes of the selected focal mechanisms reveals an N181°E oriented maximum principal stress with a very shallow dip (= 14°). The stress inversions of different depth bins of the P and T axes of selected aftershocks suggest a heterogeneous stress regime at 0–30 km depth range with a dominant consistent N–S orientation of the P-axes over the aftershock zone, which could be attributed to the existence of varied nature and orientation of fractures and faults as revealed by the relocated aftershocks.     

Stress field in the Northern Rhine area, Central Europe, from earthquake fault plane solutions

Fault plane solutions (FPS) from 110 earthquakes in the northern Rhine area with local magnitudes, ranging from 1.0 to 6.1, and occurring between 1976 and 2002 are determined. FPS are retrieved from P-wave first motions using a grid search approach allowing a detailed exploration of the parameter space. The influence of the 1D velocity model on take-off angles and resulting FPS is examined. All events were relocated with a recently developed minimum 1D model of the velocity structure [J. Geophys. Res. (2003)]. Rose diagrams of the orientation of P, T and B axes show a clear preference of trends of P and T axes at N292°E and N27°E, respectively. The majority of B axes trend in northerly directions. Plunges of P and T axes are mostly around 45° while most B axes are subhorizontal. The main direction of the maximum horizontal stress directly inferred from the fault plane solutions is N118°E.To calculate the orientations of the principal stress axes and the shape of the stress tensor, the inversion method of Gephard and Forsyth [J. Geophys. Res. 89 (1984) 9305] was applied to the whole data set and to several subsets of data. The subsets were formed by grouping events from various geological and tectonic areas and by grouping events into different depth ranges. The subset areas include the Lower Rhine Embayment, the Rhenish Massif, the middle Rhine area, the Neuwied Basin and the area known as the Stavelot–Venn Massif. Inversion of the entire data set shows some ambiguity between a strike-slip and extensional stress regime, with a vertical axis for the medium principal stress and a trend of N305°E and N35°E for the σ₁ and σ₃ axis, respectively, as the best fitting tensor. Earthquakes from the Lower Rhine Embayment and, to some degree, from the middle Rhine area indicate an extensional stress regime. In the Lower Rhine Embayment, plunge and trend of the σ₁ axis are 76° and N162°E and for the σ₃ axis 7° and N42°E. The best fitting solution for the area of the Stavelot–Venn Massif is a strike-slip regime with subhorizontal σ₁ and σ₃ axes with a trend of N316°E and N225°E, respectively. Stress orientations found here agree overall with the results from earlier studies based on smaller data sets. The directions of the maximum and minimum horizontal stresses inverted from focal mechanisms agree well with the stress field predicted by the European Stress Map. This confirms earlier interpretations that the stress field of the Rhine Graben system is controlled by plate driving forces acting on the plate boundaries. However, amplitudes of the stresses change on a local scale and with depth. Estimates of the absolute magnitude of principal stresses favor a normal faulting regime in the shallow crust (above 12-km depth) and a strike-slip regime in the lower crust.     

Stress tensor analysis of the 1998–1999 tectonic swarm of northern Quito related to the volcanic swarm of Guagua Pichincha volcano, Ecuador

The phreatic activity and the subsequent dacitic dome growth in 1998–1999 at Guagua Pichincha volcano, Ecuador, were associated with two seismic swarms: one located in the northern part of Quito (population: 1,500,000) and another one, just below the active volcano, about 15–20 km SW from the first one. Quito swarm tectonic events have high frequencies (from 1 to 10–15 Hz). We registered more than 3200 events (among which 2354 events of 1.4≤M_L≤4.2) between June 1998 and December 1999 at the −2- and −17-km depth. The volcanic events below the Guagua Pichincha caldera have high (from 1 to 10–15 Hz) and low (less than 3 Hz) frequencies. Approximately, 130,000 events were registered between September 1998 and December 1999 at the +2.4- and −3.5-km depth. Here, we study the stress tensors of these two swarms deduced from the polarities of P first motions and compare them to the regional stress tensor deduced from CMT Harvard focal mechanisms. The Quito swarm stress tensor is relatively close to the regional stress tensor (the σ₁ axis was oriented N117°E close to the N102°E direction of the plate motion found by the GPS measurement, and σ₃ is nearly vertical). The difference may be due to the action of the closely active Guagua Pichincha volcano. The Guagua Pichincha stress tensor is very different from the regional tectonic one. The σ₁ axis of the volcano is oriented N214°E, almost perpendicular to the σ₁ of the swarm of Quito and σ₃ is almost horizontal. Even if these two tensors are different, they can be explained in a more general tectonic scheme. The almost horizontal direction of σ₃ just below the volcano is compatible with an extensional horizontal direction that may be expected in the shallow extrados part of a compressional region and consistent with an opening of the top of the Guagua Pichincha volcano. The movement of the fluids (magma, gas and/or groundwater) produced by the closely active Guagua Pichincha volcano seems to have an influence in the acceleration of the generation of seismic events.     

Timing of deformation in the Norseman-Wiluna Belt, Yilgarn Craton, Western Australia

Establishing relative and absolute time frameworks for the sedimentary, magmatic, tectonic and gold mineralisation events in the Norseman-Wiluna Belt of the Archean Yilgarn Craton of Western Australia, has long been the main aim of research efforts. Recently published constraints on the timing of sedimentation and absolute granite ages have emphasized the shortcomings of the established rationale used for interpreting the timing of deformation events. In this paper the assumptions underlying this rationale are scrutinized, and it is shown that they are the source of significant misinterpretations. A revised time chart for the deformation events of the belt is established. The first shortening phase to affect the belt, D₁, was preceded by an extensional event D_1e and accompanied by a change from volcanic-dominated to plutonic-dominated magmatism at approximately 2685–2675 Ma. Later extension (D_2e) controlled deposition of the ca 2655 Ma Kurrawang Sequence and was followed by D₂, a major shortening event, which folded this sequence. D₂ must therefore have started after 2655 Ma—at least 20 Ma later than previously thought and after the voluminous 2670–2655 Ma high-Ca granite intrusion. Younger transcurrent deformation, D₃–D₄, waned at around 2630 Ma, suggesting that the crustal shortening deformation cycle D₂–D₄ lasted approximately 20–30 Ma, contemporaneous with low-volume 2650–2630 Ma low-Ca granites and alkaline intrusions. Time constraints on gold deposits suggest a late mineralisation event between 2640–2630 Ma. Thus, D₂–D₄ deformation cycle and late felsic magmatism define a 20–30 Ma long tectonothermal event, which culminated with gold mineralisation. The finding that D₂ folding took place after voluminous high-Ca granite intrusion led to research into the role of competent bodies during folding by means of numerical models. Results suggest that buoyancy-driven doming of pre-tectonic competent bodies trigger growth of antiforms, whereas non-buoyant, competent granite bodies trigger growth of synforms. The conspicuous presence of pre-folding granites in the cores of anticlines may be a result from active buoyancy doming during folding.     

Stress field in the Ryukyu–Kyushu Wadati–Benioff zone by inversion of earthquake focal mechanisms

The study addresses the space distribution of the stress field in the Kyushu–Ryukyu Wadati–Benioff zone based on homogeneous data of earthquake focal mechanisms and the inverse technique by Gephart and Forsyth [J. Geophys. Res. 89 (1984) 9305]. The used data set consists of 148 Harvard CMT solutions and 22 earthquake focal mechanisms listed in previous studies. The stress field parameters are determined for 0–40, 41–100 and h>100 km depth ranges. The top 100-km layer of the Wadati–Benioff zone (WBZ) is characterized by strike normal maximum compression σ₁ and steeper than the slab minimum compression σ₃, the last indicating for unbalanced slab pull force. The Tokara channel ‘divides’ the subduction into two parts of different stress regime at depth greater than 100 km. To the south of the channel the slab is under slab parallel σ₁ and slab normal σ₃ while its northern part, beneath Kyushu, is under slab parallel extension and slab normal compression. The results of recent studies on the regional velocity structure and geochemistry of the volcanic lava indicate that the most plausible reason for the observed stress field difference below 100 km in the northern and rest part of the arc is the presence of hot low viscosity upper mantle west of Kyushu.The results of this study indicate that the forces involved in the contemporary subduction dynamics in the Ryukyu–Kyushu Wadati–Benioff zone are related to the convergence between the Philippine Sea Plate and the Eurasian plate, the trench suction force, slab pull, the slab anchor force and, in the southern-central part of the arc, mantle resistance.     

Transtensional deformation in the Lake Tahoe region, California and Nevada, USA

Dextral transtensional deformation is occurring along the Sierra Nevada–Great Basin boundary zone (SNGBBZ) at the eastern edge of the Sierra Nevada microplate. In the Lake Tahoe region of the SNGBBZ, transtension is partitioned spatially and temporally into domains of north–south striking normal faults and transitional domains with conjugate strike-slip faults. The normal fault domains, which have had large Holocene earthquakes but account only for background seismicity in the historic period, primarily accommodate east–west extension, while the transitional domains, which have had moderate Holocene and historic earthquakes and are currently seismically active, primarily record north–south shortening. Through partitioned slip, the upper crust in this region undergoes overall constrictional strain.Major fault zones within the Lake Tahoe basin include two normal fault zones: the northwest-trending Tahoe–Sierra frontal fault zone (TSFFZ) and the north-trending West Tahoe–Dollar Point fault zone. Most faults in these zones show eastside down displacements. Both of these fault zones show evidence of Holocene earthquakes but are relatively quiet seismically through the historic record. The northeast-trending North Tahoe–Incline Village fault zone is a major normal to sinistral-oblique fault zone. This fault zone shows evidence for large Holocene earthquakes and based on the historic record is seismically active at the microearthquake level. The zone forms the boundary between the Lake Tahoe normal fault domain to the south and the Truckee transition zone to the north.Several lines of evidence, including both geology and historic seismicity, indicate that the seismically active Truckee and Gardnerville transition zones, north and southeast of Lake Tahoe basin, respectively, are undergoing north–south shortening. In addition, the central Carson Range, a major north-trending range block between two large normal fault zones, shows internal fault patterns that suggest the range is undergoing north–south shortening in addition to east–west extension.A model capable of explaining the spatial and temporal partitioning of slip suggests that seismic behavior in the region alternates between two modes, one mode characterized by an east–west minimum principal stress and a north–south maximum principal stress as at present. In this mode, seismicity and small-scale faulting reflecting north–south shortening concentrate in mechanically weak transition zones with primarily strike-slip faulting in relatively small-magnitude events, and domains with major normal faults are relatively quiet. A second mode occurs after sufficient north–south shortening reduces the north–south S_hmax in magnitude until it is less than S_v, at which point S_v becomes the maximum principal stress. This second mode is then characterized by large earthquakes on major normal faults in the large normal fault domains, which dominate the overall moment release in the region, producing significant east–west extension.     

Plio-Quaternary stress states in NE Iran: Kopeh Dagh and Allah Dagh-Binalud mountain ranges

NE Iran, including the Kopeh Dagh and Allah Dagh-Binalud deformation domains, comprises the northeastern boundary of the Arabia–Eurasia collision zone. This study focuses on the evolution of the Plio-Quaternary tectonic regimes of northeast Iran. We present evidence for drastic temporal changes in the stress state by inversion of both geologically and seismically determined fault slip vectors. The inversions of fault kinematics data reveal distinct temporal changes in states of stress during the Plio-Quaternary (since ～ 5 Ma). The paleostress state is characterized by a regional transpressional tectonic regime with a mean N140 ± 10°E trending horizontal maximum stress axis (σ₁). The youngest (modern) state of stress shows two distinct strike-slip and compressional tectonic regimes with a regional mean of N030 ± 15°E trending horizontal σ₁. The change from the paleostress to modern stress states has occurred through an intermediate stress field characterized by a mean regional N trending σ₁. The inversion analysis of earthquake focal mechanisms reveals a homogeneous, transpressional tectonic regime with a regional N023 ± 5°E trending σ₁. The modern stress state, deduced from the youngest fault kinematics data, is in close agreement with the present-day stress state given by the inversions of earthquake focal mechanisms. According to our data and the deduced results, in northeast Iran, the Arabia–Eurasia convergence is taken up by strike-slip faulting along NE trending left-lateral and NNW trending right-lateral faults, as well as reverse to oblique-slip reverse faulting along NW trending faults. Such a structural assemblage is involved in a mechanically compatible and homogeneous modern stress field. This implies that no strain and/or stress partitioning or systematic block rotations have occurred in the Kopeh Dagh and Allah Dagh-Binalud deformation domains. The Plio-Quaternary stress changes documented in this paper call into question the extrapolation of the present-day seismic and GPS-derived deformation rates over geological time intervals encompassing tens of millions of years.