Maximum Horizontal Shortening Research Articles

Disagreements in Geodetically Inferred Strain Rates in the Western US With Stress Orientations and Geologic Moment Rates

AbstractI employ an elasticity‐based method to invert a geodetically derived surface velocity field in the western US using for present‐day surface strain rate fields with uncertainties. The method uses distributed body forces in a thin elastic sheet and allows for discontinuities in velocity across creeping faults using the solution for dislocations in a thin elastic plate. I compare the strain rate fields with previously published stress orientations and moment rates from geological slip rate data and previous geodetic studies. Geologic and geodetic moment rates are calculated using slip rate and off‐fault strain rates from the 2023 US National Seismic Hazard Model (NSHM) deformation models. I find that computed total geodetic moment rates are higher than NSHM summed moment rates on faults for all regions of the western US except the highest deforming rate regions including the Western Transverse Ranges and the northern and southern San Andreas Fault (SAF) system in California. Computed geodetic moment rates are comparable to the moment rates derived from the geodetically based NSHM deformation models in all regions. I find systematic differences in orientations of maximum horizontal shortening rate and maximum horizontal compressive stress in the Pacific Northwest region and along much of the SAF system. In the Pacific Northwest, the maximum horizontal stress orientations are rotated counterclockwise 40–90° relative to the maximum horizontal strain rate directions. Along the SAF system, the maximum horizontal stresses are rotated systematically 25–40° clockwise (closer to fault normal) relative to the strain rates.

Stress and strain analysis and induced seismicity in geological gas storage (Yela, Spain)

Geologic gas storage is a sustainable strategy to store energy in underground reservoirs in order to be used under demand. Injection and withdrawal gas operations could trigger induced seismicity according to the tectonic constrains of the reservoir. The study of the active stress/strain fields shows the properties of the fault patterns that can be involved in induced seismicity. In our work, we have determined the strain/stress field in the Yela underground gas storage, an Upper Cretaceous carbonate reservoir located in central Spain. Slickensides on fault planes were measured from Cretaceous, Miocene and Quaternary rocks. Results show a strain field with ey (maximum horizontal shortening) oriented NW-SE. This field was compressional during the early Miocene, switching off to extensional from late Miocene to present-day. Our results suggest that NNW-SSE and NW-SE oriented faults could act as an effective pathway for gas leakage and prone to trigger induced seismicity. Moreover, 141 small earthquakes (Mmax 2.3) were recorded during underground operations in Yela. Peaks of earthquakes appear seasonally from November to February, in coincidence with the injection operations prior to the winter withdrawal. Working at its maximum gas storage volume, earthquake peaks decreased in intensity and magnitude from 2017 to 2021, and show a time lag of 3 months from the month of maximum volume injection to the earthquake occurrence.

An active tectonic field for CO<sub>2</sub> storage management: the Hontomín onshore case study (Spain)

Abstract. One of the concerns of underground CO2 onshore storage is the triggering of induced seismicity and fault reactivation by the pore pressure increasing. Hence, a comprehensive analysis of the tectonic parameters involved in the storage rock formation is mandatory for safety management operations. Unquestionably, active faults and seal faults depicting the storage bulk are relevant parameters to be considered. However, there is a lack of analysis of the active tectonic strain field affecting these faults during the CO2 storage monitoring. The advantage of reconstructing the tectonic field is the possibility to determine the strain trajectories and describing the fault patterns affecting the reservoir rock. In this work, we adapt a methodology of systematic geostructural analysis to underground CO2 storage, based on the calculation of the strain field from kinematics indicators on the fault planes (ey and ex for the maximum and minimum horizontal shortening, respectively). This methodology is based on a statistical analysis of individual strain tensor solutions obtained from fresh outcrops from the Triassic to the Miocene. Consequently, we have collected 447 fault data in 32 field stations located within a 20 km radius. The understanding of the fault sets' role for underground fluid circulation can also be established, helping further analysis of CO2 leakage and seepage. We have applied this methodology to Hontomín onshore CO2 storage facilities (central Spain). The geology of the area and the number of high-quality outcrops made this site a good candidate for studying the strain field from kinematics fault analysis. The results indicate a strike-slip tectonic regime with maximum horizontal shortening with a 160 and 50∘ E trend for the local regime, which activates NE–SW strike-slip faults. A regional extensional tectonic field was also recognized with a N–S trend, which activates N–S extensional faults, and NNE–SSW and NNW–SSE strike-slip faults, measured in the Cretaceous limestone on top of the Hontomín facilities. Monitoring these faults within the reservoir is suggested in addition to the possibility of obtaining a focal mechanism solutions for micro-earthquakes (M<3).

Strain Field Associated With a Component of Divergent Motion Along the Southern Dead Sea Fault: Insights From Magnetic Fabrics

AbstractIn order to reconstruct the strain field along the southern segment of the Dead Sea Fault (DSF) plate boundary, we analyzed the magnetic fabrics of carbonate rocks, outcropping along it. The magnetic fabrics provide a microstructural indicator that help to approximate the principal strain directions in the rocks. Our analysis includes ~900 cores from 58 sampling localities, along ~400 km of the southern DSF. We measured the magnetic fabrics of (1) pure calcite‐bearing limestones that consist diamagnetic fabrics and (2) chalks with composite fabrics, which we further separated into diamagnetic and paramagnetic subfabrics, using measurements of Anisotropy of Magnetic Susceptibility at low temperatures. The results show that 87% of the diamagnetic fabrics and subfabrics are of tectonic origin. The orientations of the maximum Anisotropy of Magnetic Susceptibility axes (K3 axes) approximately align with the maximum horizontal shortening directions along the southern segment of the DSF, differ from the remote stress direction, and are largely parallel to the main trace of the DSF. This parallelism is not related to local variations in the geometry of the faults. We suggest that the deflection of the maximum horizontal shortening parallel to the transform plate boundary is a kinematic consequence of the Sinai‐Arabia relative plate motion, which expresses a component of divergence along the southern segment of the DSF. We conclude that magnetic fabrics of carbonate rocks are sensitive and reliable microstructural indicators for determination of the strain field along major fault systems.

Active mountain building along the eastern Colombian Subandes: A folding history from deformed terraces across the Tame anticline, Llanos Basin

Research Article| September 01, 2015 Active mountain building along the eastern Colombian Subandes: A folding history from deformed terraces across the Tame anticline, Llanos Basin Gabriel Veloza; Gabriel Veloza † 1Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA †E-mail: gvelozaf@ku.edu. Search for other works by this author on: GSW Google Scholar Michael Taylor; Michael Taylor 1Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA Search for other works by this author on: GSW Google Scholar Andrés Mora; Andrés Mora 2Ecopetrol–Instituto Colombiano Del Petroleo, Km7 Via Bucaramanga-Piedecuesta, Piedecuesta-Santander, Colombia Search for other works by this author on: GSW Google Scholar John Gosse John Gosse 3Department of Earth Sciences, Dalhousie University, Halifax B3H 4R2, Canada Search for other works by this author on: GSW Google Scholar GSA Bulletin (2015) 127 (9-10): 1155–1173. https://doi.org/10.1130/B31168.1 Article history received: 18 Jul 2014 rev-recd: 18 Dec 2014 accepted: 26 Jan 2015 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gabriel Veloza, Michael Taylor, Andrés Mora, John Gosse; Active mountain building along the eastern Colombian Subandes: A folding history from deformed terraces across the Tame anticline, Llanos Basin. GSA Bulletin 2015;; 127 (9-10): 1155–1173. doi: https://doi.org/10.1130/B31168.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Active crustal shortening and surface uplift are vividly expressed in the foothills of the Eastern Cordillera of Colombia. Quaternary alluvial-fan and terrace deposits are folded and uplifted tens to hundreds of meters above the local base level, recording deformation and incision of the landscape. We used geomorphic observations and structural analysis from a 300 km2 three-dimensional seismic-reflection survey over an actively growing anticline to investigate the relationship between finite shortening and surface uplift, and its effects on landscape evolution. A quantitative description of finite shortening of the pregrowth strata from excess-area and line-length calculations were compared with uplift and shortening estimates obtained from geomorphic strain markers dated using in situ 10Be cosmogenic nuclide depth profiles. This combined approach provides the first estimates of Quaternary surface uplift and horizontal shortening rates along the foothills of the northeastern Colombian Andes. Finite shortening for the Tame anticline was calculated using two different approaches. Excess-area shortening varies from 530 ± 36 m (2σ) at its southern end, decreasing northward to 404 ± 72 m (2σ), whereas line-length shortening is typically an order of magnitude smaller. Using the maximum horizontal shortening, our results for the Tame anticline reveal a Quaternary uplift rate of 2.8 ± 1.5 mm/yr, and a shortening rate of 2.1 ± 1.2 mm/yr (2σ), with the older terraces located at the highest elevations and recording the highest rotation from the horizontal, indicating growth by limb rotation. Based on these rates, the initiation of folding is estimated to be 0.25 +0.09/–0.05 Ma (2σ). We hypothesize that the discrepancy between shortening estimates from line-length and excess-area calculations can be reconciled by accounting for internal deformation, likely developed during the initial stages of folding, which can be represented by strain hardening, for example. The shortening rates we obtain are comparable to geodetically derived shortening rates but are slightly lower than long-term geologic shortening rates. Slower shortening rates and the presence of a youthful structure at the thrust front suggest that the orogen has been migrating eastward since the late Pleistocene, probably due to reorganization of a supercritical thrust wedge farther west in the hinterland, to attain critical taper. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Magnetic fabric of Pleistocene continental clays from the hanging-wall of an active low-angle normal fault (Altotiberina Fault, Italy)

Anisotropy of magnetic susceptibility (AMS) represents a valuable proxy able to detect subtle strain effects in very weakly deformed sediments. In compressive tectonic settings, the magnetic lineation is commonly parallel to fold axes, thrust faults, and local bedding strike, while in extensional regimes, it is perpendicular to normal faults and parallel to bedding dip directions. The Altotiberina Fault (ATF) in the northern Apennines (Italy) is a Plio-Quaternary NNW–SSE low-angle normal fault; the sedimentary basin (Tiber basin) at its hanging-wall is infilled with a syn-tectonic, sandy-clayey continental succession. We measured the AMS of apparently undeformed sandy clays sampled at 12 sites within the Tiber basin. The anisotropy parameters suggest that a primary sedimentary fabric has been overprinted by an incipient tectonic fabric. The magnetic lineation is well developed at all sites, and at the sites from the western sector of the basin it is oriented sub-perpendicular to the trend of the ATF, suggesting that it may be related to extensional strain. Conversely, the magnetic lineation of the sites from the eastern sector has a prevailing N–S direction. The occurrence of triaxial to prolate AMS ellipsoids and sub-horizontal magnetic lineations suggests that a maximum horizontal shortening along an E–W direction occurred at these sites. The presence of compressive AMS features at the hanging-wall of the ATF can be explained by the presence of gently N–S-trending local folds (hardly visible in the field) formed by either passive accommodation above an undulated fault plane, or rollover mechanism along antithetic faults. The long-lasting debate on the extensional versus compressive Plio-Quaternary tectonics of the Apennines orogenic belt should now be revised taking into account the importance of compressive structures related to local effects.

Structural evolution of Lavinia Planitia, Venus: Implications for the tectonics of the lowland plains

This work shows the results of a detailed structural analysis of the deformation belts of Lavinia Planitia. Ridge belts and graben and groove belts can be observed at the studied area, while wrinkle ridges and large individual grooves predominate in the smooth plains. Transcurrent components of displacement are commonly observed, and transpression and transtension zones are the rule rather than the exception at most of the studied belts. Along strike azimuth changes of deformation belts are accommodated by internal variations in the predominance of contractional, transcurrent or extensional structures. The material of the surrounding plains embays most of these deformation belts. The kinematic analysis of this complex network of tectonic structures suggests a broadly synchronous activity of contractional, transcurrent and extensional structures. The maximum horizontal shortening axis determined in this work describes a steady, semicircular pattern centered at Alpha Regio. This deformation continued, although with subdued activity, after embayment of the deformation belts by the material of the plains. Future study of the tectonic evolution of the lowland plains should take into account the importance of the coeval history of neighboring uplands and lowlands.

Present-day strain field on the South American slab underneath the Sandwich Plate (Southern Atlantic Ocean): a kinematic model

Abstract This work analyses the present-day principal strain orientation on the downgoing slab of the South America Plate (SAM) beneath the Sandwich Plate (SAND). The strain regime was deduced from the study of 331 earthquake focal mechanism solutions examined by fault population analysis methods. In the slab, the maximum horizontal shortening direction (ey) rotates in trend in a clockwise direction from NE in the north, to SE in the south. Based on this rotation, three different areas were defined according to the prevailing focal mechanism type: (1) the North Zone, with ey oriented N058°E and reverse and strike–slip focal mechanisms; (2) the Central Zone, with only reverse focal mechanisms and ey striking N080°E; and (3) the South Zone, with ey oriented N106°E and reverse and strike–slip focal mechanisms. The strain field in the North Zone of the SAND involves decoupling of the slab at approximately 70 km depth. In contrast, the South Zone edge slab exhibits no decoupling and it exhibits different geometry (hook-like shaped) from the North Zone. Finally, we define the dextral strike–slip component acting at the South Sandwich Fracture Zone (SSFZ), according to focal mechanism solutions and the regional tectonic configuration.

Paleogene building of the Bolivian Orocline: Tectonic restoration of the central Andes in 2‐D map view

Using available information on the magnitude and age of tectonic shortening, as well as paleomagnetically determined tectonic rotations, we have run a series of 2‐D map view restorations of the central Andes. Neogene shortening in the foreland belt induced only slight orogenic curvature of the central Andes. The constraints on the ages of the large observed fore‐arc rotations (average of 37° counterclockwise in southern Peru and 29° clockwise in northern Chile) indicate that the Bolivian Orocline formed during the Eocene‐Oligocene as a consequence of differential shortening focused in the Eastern Cordillera. To minimize local block rotations along the fore arc, the restoration that best fits the central Andean rotation pattern requires about 400 km of total (Paleogene plus Neogene) shortening near the Arica bend. This value corresponds to the upper bound of estimates of maximum horizontal shortening for the central Andes. Along‐strike variations in horizontal shortening in the back arc induced bending of the continental margin, block rotations with fore‐arc along‐strike extension, and/or orogen‐parallel transport of upper crustal material toward the symmetry axis of the orocline.

European continuous active tectonic strain–stress map

This paper shows a new continuous strain–stress map for Europe obtained from the direct inversion of earthquake focal mechanisms calculated from the centroid tensor method. A total of 1608 focal mechanisms have been selected with several quality criteria from different catalogues (CMT Harvard, ETH, Med-Net, I.G.N. and I.A.G.) from 1973 to the present day. Values for the maximum horizontal shortening direction and brittle strain–stress regime defined by the k′ ratio ( e y/ e z, horizontal maximum/vertical strain) have been calculated following in Europe and Pannonian Basin the slip model of tri-axial deformation. The individual results including Dey and the shape of the active brittle strain ellipsoid have been interpolated to a final 15′ regular grid taking into account the relationship between the tectonic horizontal strain–stress value and the vertical load. Both continuous strain regime and maximum horizontal shortening (Dey) maps show a good correlation with the primary tectonic forces generated along the plate boundaries, plate kinematics and also some local perturbations related with main crustal heterogeneities and topography, as well as significant spatial variations in integrated crustal strength.

Late Pleistocene/Holocene slip rate of the Zhangye thrust (Qilian Shan, China) and implications for the active growth of the northeastern Tibetan Plateau

We derive a slip rate for a thrust at the central Qilian Shan mountain front by combining structural investigations, satellite imagery, topographic profiling, luminescence dating, and 10Be exposure dating. The seismically active Zhangye thrust transects late Pleistocene alluvial fan deposits and forms a prominent north facing scarp. The fault consists of two segments that differ in orientation, scarp height, and age. A series of loess‐covered terraces records the uplift history of the western thrust segment. Loess accumulation on all terraces started at 8.5 ± 1.5 kyr and postdates terrace formation. Gravels from the highest terrace yielded a 10Be exposure age of 90 ± 11 kyr, which dates the onset of faulting. With a displacement of 55–60 m derived from fault scarp profiles, this yields a vertical slip rate of 0.64 ± 0.08 mm yr−1. Along the eastern thrust segment, three 10Be ages from the uplifted alluvial fan constrain that faulting started at ∼31 ± 5 kyr. Together with a displacement of 25–30 m this leads to a vertical faulting rate of 0.88 ± 0.16 mm yr−1. A dip estimate of 40° to 60° for the fault plane combined with lower and upper limits of ∼0.6 and ∼0.9 mm yr−1 for the vertical slip rate gives minimum and maximum horizontal shortening rates of 0.4 and 1.1 mm yr−1 across the Zhangye thrust. Our results are consistent with geologic and GPS constraints, which suggest that NNE directed shortening across the northeastern Tibetan Plateau is distributed on several active faults with a total shortening rate of 4 to 10 mm yr−1.

A kinematic model of the Scotia plate (SW Atlantic Ocean)

The focal mechanisms located around the Scotia plate were examined by fault population analysis to calculate both the trends of the maximum horizontal shortening on the plate and the orientations of its associated faults. The results show that both the northern and southern boundaries of the Scotia plate (the north and south Scotia ridges) are currently undergoing left-lateral movement. This movement induces a left-lateral shear couple in the whole plate, with a maximum horizontal NE–SW shortening direction. The established fault orientations and fault classes closely match the predicted structural pattern of a theoretical shear couple that includes the entire Scotia plate. The stress trajectories deduced from finite element modeling also agree with the proposed left-lateral shear couple model; the calculated stress trajectories are characterized by a homogeneous NE–SW compression direction over the whole Scotia plate. This shear model also explains the slip of recent tectonic structures found at the boundaries of the Scotia plate and suggests the possibility of reactivation of its former tectonic structures.

Tectonics of the Jurassic‐Early Cretaceous magmatic arc of the north Chilean Coastal Cordillera (22°–26°S): A story of crustal deformation along a convergent plate boundary

The tectonic evolution of a continental magmatic arc that was active in the north Chilean Coastal Cordillera in Jurassic‐Early Cretaceous times is described in order to show the relationship between arc deformation and plate convergence. During stage I (circa 195–155 Ma) a variety of structures formed at deep to shallow crustal levels, indicating sinistral arc‐parallel strike‐slip movements. From deep crustal levels a sequence of structures is described, starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions (Bolfin Complex). The high‐grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike‐slip and thrust kinematics with a NW–SE directed maximum horizontal shortening, i.e., parallel to the probable Late Jurassic vector of plate convergence. A kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds, thrusts, and high‐angle normal faults which formed during the same time interval as the discrete shear zones. During stage II (160–150 Ma), strong arc‐normal extension is revealed by brittle low‐angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels. During stage III (155–147 Ma), two reversals in the stress regime took place indicated by two generations of dikes, an older one trending NE–SW and a younger one trending NW–SE. Sinistral strike‐slip movements also prevailed during stage IV (until ∼125 Ma) when the Atacama Fault Zone originated as a sinistral trench‐linked strike‐slip fault. The tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates. The structures of stages I and IV suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations. However, decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages II and III.

Late Cenozoic slip on the Talas-Ferghana fault, the Tien Shan, central Asia

Although Cenozoic crustal shortening and thickening by thrust faulting have built the present Tien Shan, active right-lateral shear on the northwest-trending Talas-Ferghana fault appears to be the most rapid localized deformation in the belt. Ephemeral stream valleys have been offset right-laterally tens of metres. New and published radiocarbon dates of organic material deposited in depressions blocked by offset ridges place upper bounds on the average Holocene slip rate at 18 localities. Uncertainties allow 14 upper bounds to overlap the range of 8–16 mm/yr, and 95% confidence limits on such bounds at 11 sites are entirely within this range. We infer that the rate of ≈10 mm/yr is not simply an upper bound, but applies to the late Holocene Epoch. Although the bounds on rates permit more rapid slip in the northwest than the southeast, they do not place a useful constraint on variations in slip rate along the fault. Offsets of Paleozoic facies boundaries imply a total right-lateral shear of 180–250 km, but Early Cretaceous sedimentary rock appears to have been offset only 60 ± 10 km. Published paleomagnetic declinations of Cretaceous- Miocene rock demonstrate 20°–30° of counterclockwise rotation of the Ferghana Valley, which lies just west of the Talas-Ferghana fault, with respect to stable parts of Eurasia and 20° ± 11° with respect to the central Tien Shan east of the fault. These declinations are consistent with a maximum northwestward translation of 70–210 km of the Ferghana Valley at the Talas-Ferghana fault and, therefore, with a similar maximum horizontal shortening across the Chatkal Ranges, which lie between the Ferghana Valley and the Kazakh platform. Estimates of crustal thickness beneath the Chatkal Ranges, however, permit only 60–100 km of Cenozoic shortening. If <100 km of slip on the Talas-Ferghana fault accumulated at a constant rate of 10 mm/yr, it would imply an initiation of slip more recently than ca. 10 Ma, long after India collided with the rest of Eurasia.

Active faults, stress field and plate motion along the Indo-Eurasian plate boundary

The active faults of the Himalayas and neighboring areas are direct indicators of Recent and sub-Recent crustal movements due to continental collision between the Indian and Eurasian plates. The direction of the maximum horizontal shortening or horizontal compressive stress axes deduced from the strike and type of active faulting reveals a characteristic regional stress field along the colliding boundary. The trajectories of the stress axes along the transcurrent faults and the Eastern Himalayan Front, are approximately N-S, parallel to the relative motion of the two plates. However, along the southern margin of the Eurasian plate, they are NE-SW in the Western Himalayan Front and NW-SE to E-W in the Kirthar-Sulaiman Front, which is not consistent with the direction of relative plate motion. A simple model is proposed in order to explain the regional stress pattern. In this model, the tectonic sliver between the transcurrent faults and the plate margin, is dragged northward by the oblique convergence of the Indian plate. Thus, the direction of relative motion between the tectonic sliver and the Indian plate changes regionally, causing local compressive stress fields. Judging from the long-term slip rates along the active faults, the relative motion between the Indian and Eurasian plates absorbed in the colliding zone is about one fourth of its total amount; the rest may be consumed along the extensive strike-slip faults in Tibet and China.

Late Cenozoic deformation in the Matanuska Valley, Alaska: Three-dimensional strain in a forearc region

The Matanuska Valley is located in the forearc terrane of the Alaska-Aleutian volcanic arc. A period of extensive faulting occurred in the western Matanuska Valley during the Neogene. Faulting was a response to intraplate strain caused by subduction of the Pacific plate beneath the convergent margin. The Neogene fault system consisted of three sets of faults: two sets of north-trending strike-slip faults and a set of east-northeast–trending reverse faults. Three fault sets were required to accommodate the general three-dimensional deformation. Crustal shortening in a north-northwest direction was accompanied by lateral and vertical extension. The direction of maximum horizontal shortening strain has apparently been constant since the Neogene, although there is no evidence for extensive faulting of Quaternary deposits in the area we have studied. The western Matanuska Valley is located only 50 km above the Benioff zone, and the predicted maximum principal shortening direction is north-northwest, on the basis of the slip-vector azimuth between the North American and Pacific plates and the nature of surficial ground displacements during the 1964 (M = 8.4) subduction zone earthquake. Focal mechanisms for earthquakes in the vicinity of the Benioff zone and in the lower part of the North American plate indicate west-northwest extension and north-northeast shortening. This means that the strain field at depths below 20 km may differ significantly from the near-surface strain field.

Seismicity, crustal stresses and recent crustal movements in Switzerland

A seismotectonic map of Switzerland was recently completed. The map is a first attempt to present and combine the seismicity and neotectonics of Switzerland. Maximum compression in the upper crust inferred from fault-plane solutions of 23 earthquakes and micro-earthquake studies is oriented NNW—SSE in eastern Switzerland and NW—SE to WNW—ESE in western Switzerland, which corresponds well with the orientation of Neogene and Quaternary maximum horizontal shortening. Evidently the stress field which causes the present seismicity is very similar in its orientation to the stress field of the last 5 to 10 million years which produced the neotectonic deformation. Recent vertical movements and uplift rates in Switzerland are in close relation to the present and neotectonic stress field of the upper crust.

Reflections on the rigidity of the Lut Block and recent crustal deformation in eastern Iran

Imagery from the first Earth Resources Technology Satellite passes over east-central Iran are interpreted with special emphasis on geomorphology of the structural trends in the area. Several units of arcuate structure are named and discussed. Newly found dislocations and other lineaments within the Lut Block, in addition to its internal seismicity and Quaternary volcanic activity, support the argument against the rigidity of the block. Neotectonic and seismotectonic maps of the area are prepared and discussed. These maps reveal a complicated and mostly radial pattern of the pre-Neogene compressive directions. The post-Neogene directions of compression are, however, consistent with a NE—NNE trend throughout the area. These axes of maximum horizontal shortening are deduced from the trend of normal and en-echelon Neogene folds, the pattern and sense of movement of faults active in Quaternary, geodetic measurements, and the source mechanism of earthquakes. It is concluded that the plate-tectonic concept cannot readily be used to explain the recent deformation taking place in continental area structurally as complicated as eastcentral Iran.