S-trending Faults Research Articles

The 2022 Har Lake earthquake sequence highlights a complex fault system in the western Qilian Shan, northeastern Tibetan Plateau

SUMMARY The 2022 Har Lake earthquake sequence, which began in 2022 January and lasted for ∼70 d, jolted the Har Lake area, which is located in the western Qilian Shan, northeastern Tibetan Plateau. Two Mw > 5.5 earthquakes occurred during the earthquake sequence, among which the March 25 Mw 5.8 event is considered the largest event recorded in the area. However, determining the seismogenic faults of the earthquake sequence, as well as the detailed rupture features, is difficult due to the lack of geological data and near-field seismological observations. In this study, we use Sentinel-1 synthetic aperture radar (SAR) data to obtain the coseismic deformation field, identify possible ruptured faults and associated fault geometries, and further estimate detailed coseismic slip models of the two Mw > 5.5 earthquakes. The results show that the January 23 Mw 5.6 earthquake (Earthquake A) occurred on a N15° W-trending dextral-slip fault with a dip angle of ∼61°. For the March 25 Mw 5.8 earthquake (Earthquake B), the interferometric synthetic aperture radar (InSAR) data can be described by either an ∼N–S-trending dextral-slip fault or an ∼E–W-trending sinistral-slip fault. The ∼N–S-trending fault better describes the aftershock distribution, while the ∼E–W-trending model is more consistent with the regional geological setting. We suggest that the complex coseismic ruptures in the multiple-fault system are driven by widespread NE–SW-trending compression in the western Qilian Shan. This study demonstrates the importance of integrating geodetic and seismological observations to capture the full complexity of moderate earthquakes and further suggests potential seismic hazards in the Har Lake area.

Stepwise growth of the southeastern Tibetan Plateau: Structural and thermochronological evidence from the Panxi tectonic belt

The Panxi tectonic belt is located in the transition zone between the Tibetan Plateau and the Yangtze Block and is characterised by ~N–S-trending faults and active earthquakes, which have long been considered to be a result of the growth (expansion and uplift) of the Tibetan Plateau. However, the mismatch between the ~N–S-trending faults and top-to-the-SE arcuate thrust faults is problematic for understanding the mechanism of growth of the southeastern Tibetan Plateau. In this study, we integrate the results of structural analysis with new apatite fission track and (UTh)/He thermochronological data to constrain the cooling, exhumation, and uplift history of the Panxi tectonic belt. Thermochronological data reveal that widespread and rapid cooling was initiated during the late Eocene–Oligocene (36–26 Ma), with a change in cooling/exhumation rate (either increase or decrease, dependent on location) occurring at 13–8 Ma (Middle–Late Miocene). These two periods of cooling are attributed to two major tectonic deformation episodes, D1 and D2. The D1 deformation is characterised by top-to-the-east/southeast thrusting with back-thrusting along N–S-trending faults. The Middle–Late Miocene spatially differential exhumation was associated with D2 deformation involving strike-slip and local normal faulting that overprinted D1 and potentially accommodated accumulated strain. Integration of the new results with previous records of uplift and exhumation, sedimentation, magmatism, and fault activity, supports a diachronous staged uplift process of the southeastern Tibetan Plateau, with episodes during the late Eocene–Early Miocene and since the Middle–Late Miocene. Crustal deformation has played a significant role in promoting uplift through the generation of new faults and reactivation of pre-existing faults, whereas lower-crustal ductile channel flow might have driven the Middle–Late Miocene tectonic transformation in the upper crust.

Influence of multi-trend major fault reactivation during multiphase rifting: Beier Depression, Hailar Basin, NE China

AbstractComplex fault patterns associated with rift development in the Hailar Basin were largely influenced by the Mongolia–Okhotsk Ocean and Palaeo-Pacific tectonic regimes during the Late Jurassic to Early Cretaceous periods. Based on 3D seismic data from the Beier Depression in the Hailar Basin, we characterized the reactivation history of multi-trend major faults and examined the evolution of the Beier Depression during the Early Cretaceous period. NE–SW-, NW–SE- and ENE–WSW-oriented major faults originated from strike-slip-associated structures that were pre-existing fabrics and then were reactivated and propagated upward under extensional regimes in the Late Jurassic. During the syn-rift stage (K1t–K1n), the Hailar Basin was in a NNW–SSE- to NW–SE-oriented extensional setting, and major faults of all orientations were active. There was tectonic quiescence (K1n1L) between the syn-rift stages (a rifting transition stage). The short compression stage after the syn-rift stage caused regional compressional deformation. During the post-rift stage (K1d–K1y), the extension direction rotated to an E–W orientation, and a new population of N–S-trending faults formed together with the reactivation of NE–SW- and ENE–WSW-trending major faults. Structural analysis shows that the major ENE–WSW-trending major faults were polycyclic growth faults reactivated via an upward propagation mode and that the NE–SW-trending faults were dip linkage faults reactivated via a dip linkage mode. The reactivation intensity of the NE–SW-trending major faults was stronger than that of the ENE–WSW-trending major faults. These results demonstrate the differences in the evolution of the different trending faults in the same tectonic regime, and the complexity of the final fault patterns in the Beier Depression was produced by differences in the reactivation of major faults. The originate interpretation of the multi-trend major faults in the Hailar Basin provides new insights into fault generation, and the classification of fault growth also has useful implications for future research on multiphase rifts.

Genesis of the Yarikçi Hydrothermal Clay Deposit Within the Mesozoic Metamorphic Units, Mihaliççik, Eskişehir, Turkey

AbstractHydrothermal alteration led to development of the Yarıkçı clay deposit within the Mesozoic chlorite-, muscovite-, chlorite-muscovite-schist, and garnet-graphite phyllite units along NW–SE- and N–S-trending faults in Mihalıççık in western central Anatolia. The geological, mineralogical, and geochemical characteristics and genesis of this economically important clay deposit have not been examined in detail previously. The present study has attempted to fill this gap. Green smectitic and cream kaolinitic claystones are abundant with smaller amounts of gray illite, dark brown Fe oxides, and silica phases occurring as stockwork/fracture infill and stain/coating. These units are covered by a dark, hard, sharp-edged, and thick silica cap. Metamorphic units exhibit cataclastic texture due to tectonic activities. Muscovite is mostly degraded to kaolinite, and feldspars show sericitization and argillization. Kaolinite typically has a platy form with irregular margins and locally sub-rounded, book-like texture suggesting hydrolysis during the hydrothermal injections. The association of Fe oxides, cristobalite/tridymite/quartz, gypsum/anhydrite, and jarosite are indicative of intense hydrothermal activities and development of kaolinite under acidic geochemical conditions. The local enrichment of SiO2, Fe2O3, S, Cu, and Au also supports this suggestion. The leaching of Sr, Rb, Ba, and Zr, and the slight increase in LREE/MREE+HREE ratios together with the negative Eu and Ce anomalies suggest the selective dissolution of muscovite, garnet, feldspar, and pyroxene by the hydrothermal fluids. Thus, abundant claystones of smectite and kaolinite were formed via the increase in Al+Fe+Mg/Si and Al±Fe/Si ratios in the alkaline and acidic environment, respectively, under the tectonic control of hydrothermal activity as seen in the alteration of chlorite, muscovite, and feldspar in metamorphic units.

Linked thick- to thin-skinned inversion in the central Kirthar Fold Belt of Pakistan

Abstract. The Kirthar Fold Belt is part of the transpressive transfer zone in Pakistan linking the Makran accretionary wedge with the Himalaya orogeny. The region is deforming very obliquely, nearly parallel to the regional S–N plate motion vector, indicating strong strain partitioning. In the central Kirthar Fold Belt, folds trend roughly N–S and their structural control is poorly understood. In this study, we use newly acquired 2-D seismic data with pre-stack depth migration, published focal mechanisms, surface and subsurface geological data, and structural modelling with restoration and balancing to constrain the structural architecture and kinematics of the Kirthar Fold Belt. The central Kirthar Fold Belt is controlled by Pliocene to recent linked thick-skinned to thin-skinned deformation. The thick-skinned faults are most likely partially inverting rift-related normal faults. Focal mechanisms indicate dip-slip faulting on roughly N–S-trending faults with some dip angles exceeding 40∘, which are considered too steep for newly initiated thrust faults. The hinterland of the study area is primarily dominated by strike-slip faulting. The inverting faults do not break straight through the thick sedimentary column of the post-rift and flexural foreland; rather, the inversion movements link with a series of detachment horizons in the sedimentary cover. Large-scale folding and layer-parallel shortening has been observed in the northern study area. In the southern study area progressive imbrication of the former footwall of the normal fault is inferred. Due to the presence of a thick incompetent upper unit (Eocene Ghazij shales) these imbricates develop as passive roof duplexes. In both sectors the youngest footwall shortcut links with a major detachment and the deformation propagates to the deformation front, forming a large fault-propagation fold. Shortening within the studied sections is calculated to be 18 %–20 %. The central Kirthar Fold Belt is a genuine example of a hybrid thick- and thin-skinned system in which the paleogeography controls the deformation. The locations and sizes of the former rift faults control the location and orientation of the major folds. The complex tectonostratigraphy (rift, post-rift, flexural foreland) and strong E–W gradients define the mechanical stratigraphy, which in turn controls the complex thin-skinned deformation.

Indicator typomorphic features and potential sources of small-size placer gold from the Ebelyakh area (northeast Siberian platform)

«Вестник Санкт-Петербургского университета. Науки о Земле» — научно-теоретический рецензируемый журнал, обобщающий результаты исследований в области геологии, гидрогеологии и геохимии, петрологии и экологии, ландшафтного планирования и геоморфологии, экономической и социальной географии.

Review of Mesozoic multiple magmatism and porphyry Cu–Mo (W) mineralization in the Yidun Arc, eastern Tibet Plateau

The Yidun Arc was formed in response to the westward subduction of Garze–Litang Ocean (a branch of Paleotethys) in the Late Triassic, where abundant porphyry Cu–Mo deposits (221–213Ma) developed along the regional NW–SE sinistral faults and emplaced in the southern portion of the arc. The ore-related porphyries are mostly metaluminous or slightly peraluminous, belonging to shoshonitic high-potassium calc-alkaline I-type granites, with εHf(t) values of −6.64 to +4.12. The ore-bearing magmas were probably derived from the partial melting of subduction-metasomatic-enriched mantle, with the contamination of underplated mafic materials. The Late Cretaceous (88–80Ma) highly fractionated I-type granite belt and related porphyry Cu–Mo deposits and magmatic-hydrothermal Cu–Mo–W deposits occur along approximately N–S-trending faults in the Yidun Arc. This belt extended across the Yidun Arc and Garze–Litang suture zone to the north and across the Yangtze Craton to the south, intruding the Late Triassic porphyry belt. The ore-related porphyries are characterized by high silica and high total alkalis, with enrichment in large ion lithophile elements (LILEs; Rb, U and K) and depletion in high field strength elements (HFSE; Nb, Ta, P and Ti) and Ba. They have lower εHf(t) values varying from −9.55 to −2.75, and significant negative Eu anomalies, indicating that the ore-bearing porphyritic magmas originated from ancient middle-upper crust. Two-stage magmatism and mineralization were superimposed in the Xiangcheng-Shangri-La district. Some ore deposits comprise two episodes of magmatism and associated mineralization such as both 207±3.0Ma granodiorite and 82.1±1.2Ma monzogranite intruded in the Xiuwacu deposit, causing Cu–Mo–W polymetallic mineralization. To date, 11 Late Triassic porphyry Cu deposits (e.g. the Pulang giant deposit with 5.1 Mt Cu), and five Late Cretaceous porphyry Cu–Mo (W) deposits (e.g. Tongchanggou Mo deposit with 0.59 Mt Mo) have been evaluated in the Xiangcheng-Shangri-La district. The continuity and inheritance of multiphase magmatism and the new understanding of superimposed mineralization will help to guide future exploration.

Fault inference and boundary recognition based on near-bottom magnetic data in the Longqi hydrothermal field

Near-bottom magnetic prospecting, which provides useful information to study shallow geological structures, is an efficient method for investigating active and inactive hydrothermal fields and researching the structure of hydrothermal systems. We collected near-bottom magnetic data in the Longqi hydrothermal area on the Southwest Indian Ridge using the Autonomous Benthic Explorer in 2007 and set up a processing system for magnetic data calibration. By removing the influence of terrain on magnetic anomalies and using the intensity of the spatial differential vector (ISDV) method, we inferred the presence of an N–S-trending fault and estimated its crush zone to be about 120 m wide and >2 km long along the known hydrothermal vents. This inferred fault is consistent with the precise topography mapped during the ABE 201 dive. The fault may be connected to a known detachment fault and form part of a hydrothermal channel. We delineated the hydrothermal alteration zone using the ISDV method and conclude that demagnetization was induced by hydrothermal alteration.

Characteristics and origin of the Zhulazhaga gold deposit in Inner Mongolia, China

The Zhulazhaga gold deposit in the northern margin of the North China Craton (NCC) is a large tonnage hydrothermal deposit with a reserve of about 50t at about 4g/t Au. The deposit is characterized by an epigenetic style of mineralization in metasedimentary rocks dominated by slates, low-grade metamorphic clastic rocks and minor volcanic rocks. Gold mineralization is dominantly controlled by structures, manifested by stratabound orebodies located immediately to the east and west of a major N–S-trending fault and associated extensional fractures and NE-trending fault zones. The mineralization is also preferentially located in the fourth sub-member of first member of the Agulugou Formation. There are many auriferous quartz veins parallel to the bedding of the host rocks which contain the orebodies, and high-grade ore preferentially occurs near to these quartz veins. The structures provided pathways for transportation of gold-bearing fluids within the formation that appears to form a particularly favorable host rock and depositional setting.Hydrogen and oxygen isotope data indicate that ore-forming fluids were dominated by magmatic water mixed with minor meteoric water. Sulfur and lead isotope data indicate that some sulfur and lead came from the magmatic fluid. The metallogenic age (280Ma) of this deposit and the crystallization age (280–290Ma) of the granite porphyry in the district indicate a link between the Hercynian magmatism and gold mineralization. The Hercynian tectono-magmatism and associated hydrothermal activity remobilized gold and drove ore-forming fluids to the favorable depositional environment.The northern margin of the NCC is prospective for gold mineralization hosted in metasedimentary rocks which may be one of the most important deposit types for future gold exploration in northern China.

Løvehovden fault and Billefjorden rift basin segmentation and development, Spitsbergen, Norway

AbstractThe Carboniferous Billefjorden rift basin is a well-known example of a suite of Carboniferous basins on the Barents Shelf and NE Greenland. The basin has a clastic, carbonate and evaporite fill with complex and disputed stratigraphic relationships, especially regarding the Ebbadalen and Minkinfjellet formations. Geometrically, the basin is considered a simple half-graben. A N–S-trending fault and monocline structure within the northern portion of the basin, the Løvehovden fault, has lithological and thickness differences across it within the Minkinfjellet and possibly Ebbadalen formations. The fault shows W-side-down movement, defining a sub-basin within the larger half-graben. Significant along-strike changes occur. Down-throw to the west is at least 150 metres and possibly 400 metres, as shown by across-fault thickness differences of Ebbadalen and/or Minkinfjellet formations. To the east of the fault, the contact between the Ebbadalen and Minkinfjellet formations is a disconformity with significant local relief, and is interpreted to represent exposure from footwall uplift, and associated near- or at-surface solution, producing basal stratiform breccias. A similar contact is not exposed west of the fault. Monoclinal deformation and thickening of the younger Wordiekammen Formation above and across the monocline constrain a later movement component. Kinematic data and the structural style clearly indicate the Løvehovden fault is a normal fault with associated tri-shear zone development, consistent with the regional Carboniferous rift setting. Earlier interpretations describe the Løvehovden fault and monocline as Tertiary contractional features. In contrast, our work advocates that they are an important architectural basin element, defining a sub-basin within the Billeforden Trough during Minkinfjellet Formation deposition, with insignificant, if any, Tertiary reactivation. The Løvehovden fault is aligned with and represents the southern termination of the Lemströmfjellet fault to the north. Thus, the Billefjorden basin changes from a narrow graben to a broader half-graben to the south. These along-strike changes have important implications for the stratigraphic architecture of the basin, and for palaeogeographic reconstructions. These results and application of 3-D models for extension related tri-shear zones may help inform interpretation of other Carboniferous basins on the Barents Shelf.

Escape tectonism in the Gulf of Thailand: Paleogene left-lateral pull-apart rifting in the Vietnamese part of the Malay Basin

The Malay Basin represents one of the largest rift basins of SE Asia. Based on a comprehensive 2-D seismic database tied to wells covering mainly Vietnamese acreage, the evolution of the Vietnamese part of the basin is outlined and a new tectonic model is proposed for the development of the basin. The Vietnamese part of the Malay Basin comprises a large and deep Paleogene pull-apart basin formed through Middle or Late Eocene to Oligocene left-lateral strike-slip along NNW-trending fault zones. The Tho Chu Fault Zone constitutes a significant Paleogene left-lateral strike-slip zone most likely associated with SE Asian extrusion tectonism. The fault zone outlines a deep rift that widens to the south and connects with the main Malay Basin. In the central northern part of the basin, a series of intra-basinal left-lateral fracture zones are interconnected by NW to WNW-trending extensional faults and worked to distribute sinistral shearing across the width of the basin. Extensive thermal sagging throughout the Neogene has led to the accommodation of a very thick sedimentary succession. Moderate rifting resumed during the Early Miocene following older structural fabric. The intensity of rifting increases towards the west and was probably related to coeval extension in the western part of the Gulf of Thailand. Neogene extension culminated before the Pliocene, although faults in places remains active. Late Neogene basin inversion has been attributed to c. 70 km of right-lateral movement across major c. N–S-trending faults in the central part of the basin. However, the lack of inversion in Vietnamese territory only seems to merit a few kilometers of dextral inversion.

New insights into the tectonic evolution of the Andaman basin, northeast Indian Ocean

Multibeam swath bathymetry data acquired over an area of about 30 000 km 2, together with magnetic and single channel seismic data, have been analyzed to understand the tectonic evolution of the Andaman basin, northeast Indian Ocean. Swath bathymetry data revealed several morphotectonic features that divide the basin into a complex western part comprising arc-parallel seamount chains and N–S-trending fault systems and a relatively smooth eastern part. A SW–NE-trending spreading ridge bisects the basin. An offset of about 11.8 km at 94°21′E longitude divides the spreading ridge into SW and NE segments. High-resolution bathymetry and magnetic data enabled us to identify two finer-scale segments over the SW part of the ridge that display distinct topographic fabric compared to the NE segment. Thin sediment cover and presence of axial seamounts characterize SW segments of the ridge whereas the NE segment has thick sediment cover and well-maintained rift valley. Kinematics of basin evolution, derived from the identification of the seafloor spreading magnetic anomalies, suggest very slow initial opening rates of about 1.6 cm/yr and an increase in the spreading rate to about 3.8 cm/yr from anomaly 2 to present, although this interpretation relies only on magnetic anomalies in the two westernmost segments. It is suggested that the true seafloor spreading started about 4 Ma in the Andaman backarc basin, rather than the 11 Ma postulated previously. We infer based on the integrated analysis of swath bathymetry, magnetic and seismological data that the basin has evolved as a consequence of extrusive tectonics that prompted extension and rifting along the plane joining the Sagaing and Semengko fault systems. Extension by seafloor spreading during the past 4 Myr resulted in the formation of the deep Andaman backarc basin. This phase has also experienced westward propagation of the spreading center.

Middle Jurassic – Early Cretaceous rifting of the Danish Central Graben

During the Jurassic – Early Cretaceous, the Danish Central Graben developed as a N–S- to NNW– SSE-trending graben bounded by the Ringkøbing–Fyn High towards the east and the Mid North Sea High towards the west. The graben consists of a system of half-grabens and evolved by faultcontrolled subsidence; three main rift pulses have been recognised. The first pulse ranged from the Callovian to the Early Oxfordian, the second pulse was initiated in the latest Late Kimmeridgian and lasted for most of the Early Volgian, and the third and final pulse occurred within the Ryazanian in the Early Cretaceous. The first pulse was characterised by subsidence along N–S-trending faults. The most pronounced fault-controlled subsidence occurred in the east, especially along N–S-striking segments of the boundary fault to the Ringkøbing–Fyn High. During this period, minor salt movements occurred with the development of salt pillows. The activity along the N–S-trending faults ceased during the Oxfordian. During the second pulse, in Early Volgian times, subsidence was concentrated along new NNW–SSE-trending faults and the main depocentre shifted westward, being most marked within the Tail End Graben, the Arne–Elin Graben, and the Feda Graben. This tectonic event was accompanied by the accumulation of a relatively thick sediment load resulting in the development of salt diapirs, especially within the Salt Dome Province. The third tectonic pulse was essentially a reactivation of the NNW–SSE-trending structures and there is clear evidence of subsidence controlled by faulting and salt movements. Despite the overall extensional tectonic regime, local compressional tectonics resulted in thrusting. For instance, the Gert Ridge is interpreted to have formed by readjustment at the boundary fault between two subsiding blocks. The structural framework during graben evolution controlled, to some degree, the distribution of reservoir sandstones. Reservoir sandstones associated with periods of rotational tilt include Middle Jurassic deposits referred to the Bryne and Lulu Formations, and Upper Jurassic sandstones referred informally to the ‘Fife Sandstone Formation’. Sands deposited during tectonic relaxation are represented by the Heno Formation and Upper Jurassic turbidites interbedded in the Farsund Formation. Sea-level changes were probably most important during periods of tectonic relaxation, particularly with respect to the deposition of lowstand sandstones in basinal areas.

Characteristics of surface ruptures associated with the Chi-Chi earthquake of September 21, 1999

The Chi-Chi earthquake, which reactivated the Chelungpu fault in central Taiwan, generated surface ruptures over 100 km long. Surface ruptures of the Chi-Chi earthquake can be divided into the north–south and the east–west sections. The north–south section extends for about 84 km, and the northern E–W section 16 km. The north–south section consists of a continuous main rupture and some branches. These are characterized by thrust motion with some strike–slip components. Topographic surveys indicate that the ground commonly uplifted as a single, continuous ramp-step surface, and deformation is mainly concentrated on the hanging wall. Fault slip data and GPS measurements show that the hanging wall along the northern segment of the Chelungpu fault moved northwest, but that along the southern segment of the fault, the hanging wall shifted to the southwest. Sandbox models suggest that the above dissimilarity in hanging-wall movement direction is mainly due to the presence of the Peikang Basement High. In the east–west section, the ruptures comprise a series of en echelon steep faults with right-step or left-step geometry, and trend ENE–WSW to E–W. Near the Ta'an River, the surface rupture orientation shifts from NE–SW to NW–SE and terminates at the Shuangtung fault. Here, the ruptures appear to be controlled by the 10–15-km-wide, NW-trending Sanyi-Puli shear zone. Clay modeling displays the development of E–W-trending ruptures when a N–S-trending fault meets a N30°W-trending weak zone.

Emplacement of the Miocene west Naxos pluton (Aegean Sea, Greece): a structural study

Synmagmatic and solid-state structures within the Naxos pluton and its rim may provide insight into the interplay between plutonism and regional deformation at upper-crustal level. Within the hornblende–biotite granite of western Naxos, synmagmatic foliations display two distinct patterns, onion-skin in the north and tangential to the rim in the south. The two areas are separated by the NE-trending Glinadon fault. Deformed mafic enclaves in the pluton are prolate, with their long axes parallel to the synmagmatic lineation. In contrast, phenocryst distribution analysis, using the Fry method, defines an apparent oblate strain with a horizontal stretching lineation. Planar markers within the pluton progressively steepen through the vertical at the east pluton border. Several lines of evidence, such as dykes intruding axial areas of rim-parallel folds, foliated or folded aplite veins, folds and spaced cleavage in the mollase, and inverted stratigraphy, suggest pluton emplacement and deformation during transpressional deformation. A northward divergent flow regime with magma spreading out mainly from the Naxos fault, and the deflection of both the synmagmatic foliation pattern and the flow lines at the Glinadon fault, suggest that the NE–SW- and N–S-trending faults were active during pluton formation. In the south the pluton has grown by the expansion of dykes occupying P-shear positions with respect with the Naxos fault; in the north a piecemeal block down-drop complements this process and favours voluminous magma concentration. During the late evolutionary stages of pluton construction, the magma chamber was compartmentalized into NE-trending sectors affected by block rotation in an anticlockwise manner. Understanding the role of faults in the emplacement of the Naxos pluton is important for understanding emplacement of other plutons in the Aegean Sea region, since most of them are controlled by N–S- (Ikaria pluton) or NE- (Tinos, Serifos and Delos plutons) trending faults.

Tectonic development of North African basins

Abstract Mostly the Palaeozoic and Mesozoic basins of North Africa have generally followed, and reworked, earlier basement trends formed by: (1) the NW-SE accretion of continental and oceanic terranes onto a Pan-African nucleus in northeastern Africa, and (2) the collision of this amalgam of accretionary terranes with the West African Craton. During the Upper Precambrian Pan-African Orogeny, the West African Craton formed a rigid block which indented this amalgam of accreted mobile belts to form much of North Africa. Intrusion of this indentor into North Africa caused the expulsion of narrow, triangular-shaped blocks of lithosphere to the north and south in a tectonic style very similar to the Miocene-Pliocene deformation of Tibet. Expulsion reactivated the earlier shear zones to form an anastomosing pattern of steeply dipping shears with left and right lateral sense of displacement. Left lateral shear also affected the northern edge of the West African Craton during this process of indentation. Subsequent rifting of the Pan-African mountain belt resulted in a series of grabens, which were infilled with Upper Precambrian-Cambrian molasse. These are the precursor basins for the Palaeozoic sediments which cover much of North Africa. The effects of rifting continued into the Cambro-Ordovician in the western basins. During the Silurian-Devonian many of the rifts were reworked. A new basin formed in the Atlas and Anti-Atlas, related to the growth of the proto-Tethyan Ocean. Basin inversion characterizes the Palaeozoic structures of the western Atlas and Anti-Atlas, producing thickened crust and a large mountain belt during the Carboniferous. Foreland basins formed on either side of this mountain belt and both the mountains and the adjacent basins were compartmentalized by WNW—ESE-trending transfer zones. Pan-African structures, within the African Plate, were reworked with further indentation of the West African Craton into Pan-African crust. The craton was pushed eastward, generating a left lateral shear couple along its northern margin. NW—SE-trending faults were reworked as dominantly left lateral strike-slip faults and N—S-trending fault blocks were rotated slightly in a clockwise sense. There was probably further lateral expulsion of lithosphere, ahead of the NE—SW-trending front of the indentor, reworking earlier N—S-trending shear zones. The North African Palaeozoic basins were inverted during the Hercynian-Appalachian Orogeny. In the Ahnet Basin the shortening was approximately NNE-SSW, perpendicular to the trend of the structures. This inversion was particularly marked in the Ougarta-Ahnet Basin where it produced a series of open to closed, north-south to NW—SE-trending folds above reactivated basement faults. During the Mesozoic, the Hercynian-Appalachian mountain belt underwent extension to produce deep rift basins infilled with continental sediments and some volcanics. The High Atlas formed as an arm to the Atlantic Basin. Transfer zones have a WNW-ESE trend, indicating that this was the main extension direction, similar to that in western and southwestern Europe. In northeastern Algeria, the orientation of the Mesozoic grabens suggests reworking of the basement fabric formed by Pan-African accretionary tectonics. The structures appear to die out toward the southwest into a broad transfer zone with some NW—SE-trending faults. The northeastern edge of the basin is obscured by later rift basins in the eastern Mediterranean. The Palaeozoic faults of the Amguid Spur, overlying one of the major shear zones of the Hoggar, formed a structural high throughout the Mesozoic with probably several pulses of inversion. An important episode of inversion occurred during the Aptian-Albian with the development of anticlines and associated reverse faults. Crustal extension associated with block faulting occurred in the Sirte Basin of Libya during the Mid- and Late Cretaceous. The block faults trend NNW—SSE to NW—SE, cross-cutting earlier Palaeozoic fold structures at a high angle but possibly parallel to some of the basement shear zones. The faults form the tips of a rift basin which opened between Sicily and Tunisia in the central Mediterranean. The Cretaceous faults have a component of right lateral displacement as well as normal fault movements. The Mesozoic basins of the High and Middle Atlas were inverted during the Late Cretaceous-Early Oligocene. The displacement direction, as seen from the transfer systems, was NW—SE, almost perpendicular to the Middle Atlas, but at a lower angle with the High and Sahara Atlas, which must have had components of oblique or right lateral movement. Minor effects of this inversion are reported from the Saharan basins.

Seismicity around the Hekla and Torfajökull volcanoes, Iceland, during a volcanically quiet period, 1991-1995

The volcano Hekla in south Iceland had its latest eruption in January–March 1991. The eruption was accompanied and followed by considerable seismic activity. This study examines the seismicity in the Hekla region (63°42′–64°18′N, 18°30′–20°12′W) during a period when the high activity related to the eruption had ceased, from July 1991 to October 1995. The aim is to define the level of the normal background seismicity of the area that can be compared to the eruption-related activity. The Hekla Volcano proper was generally aseismic during the study period. The most prominent earthquake cluster is in the neighbouring Torfajokull Volcano. The epicentres are concentrated in the western part of the caldera and west of it. The hypocentres are located at all depths from the surface down to 14 km, with highest activity at 5–12 km. Inside this cluster, in the northwest part of the caldera, is a spherical volume void of earthquakes, approximately 4 km in diameter and centred at 8 km depth. This is interpreted as a cooling magma body. Small, low-frequency events of volcanic origin were occasionally recorded at Torfajokull. This activity has mainly occurred in swarms and was most abundant during the first year of the study period, presumably reflecting some kind of connection to the 1991 Hekla eruption. Our study area also includes the easternmost section of the South Iceland seismic zone, a transform zone characterized by bookshelf faulting on transverse faults. Two lineaments of epicentres were identified, roughly corresponding to mapped faults of the South Iceland seismic zone. The hypocentres are relatively deep, mainly at 6–12 km, matching the general trend of hypocentral depth increasing toward the east. The seismicity is highest in the area of the mapped faults. However, the epicentres extend beyond them and indicate greater width of the South Iceland seismic zone, or 20–30 km rather than approximately 10 km as indicated by the length of the surface faults. The seismicity in the volcanic systems of Hekla and Vatnafjoll shows some characteristics of the South Iceland seismic zone. Epicentres are concentrated into two N–S lineaments, one of which coincides with the location of the 1987 Vatnafjoll earthquake (Mw=5.9), a strike-slip event on a N- to S-trending fault. The hypocentres of the Hekla–Vatnafjoll events are mainly at 8–13 km depth, which indicates a continuation of the depth trend of the earthquakes of the South Iceland seismic zone. The events located at Hekla proper and immediately north of it are all of low-frequency character, which can be held as an indication of volcanic origin. On the other hand, they show clear S arrivals at observing stations like normal high-frequency tectonic earthquakes.