Complex geometry and segmentation of the surface rupture associated with the 14 November 2001 great Kunlun earthquake, northern Tibet, China

The 14 November 2001 Kunlun, China, earthquake with a moment magnitude (M_w) 7.8 occurred along the Kusai Lake–Kunlun Pass fault of the Kunlun fault system. We document the spatial distribution and geometry of surface rupture zone produced by this earthquake, based on high-resolution satellite (Landsat ETM, ASTER, SPOT and IKONOS) images combined with field measurements. Our results show that the surface rupture zone can be divided into five segments according to the geometry of surface rupture, including the Sun Lake, Buka Daban–Hongshui River, Kusai Lake, Hubei Peak and Kunlun Pass segments from west to east. These segments, each 55 to 130 km long, are separated by step-overs. The Sun Lake segment extends about 65 km with a strike of N45° 75°W (between 90°05′E 90°50′E) along the previously unrecognized West Sun Lake fault. A gap of about 30 km long exists between the Sun Lake and Buka Daban Peak where no obvious surface ruptures can be observed either from the satellite images or field observations. The Buka Daban–Hongshui River, Kusai Lake, Hubei Peak and Kunlun Pass segments run about 365 km striking N75° 85°W along the southern slope of the Kunlun Mountains (between 91°07′E 94°58′E). This segmentation of the surface rupture is well correlated with the pattern of slip distribution measured in the field. Detailed mapping suggest that these five first-order segments can be further separated into over 20 second-order segments with a length of 10–30 km, linked by smaller scale step-overs or bends.Our result also shows that the total coseismic surface rupture length produced by the 2001 Kunlun earthquake is about 430 km (excluding the 30-km-long gap), which is the longest coseismic surface rupture for an intracontinental earthquake ever recorded.Finally, we suggest a multiple bilateral rupture propagation model that shows the rupture process of the 2001 M_w 7.8 earthquake is complex. It consists of westward and eastward rupture propagations and interaction of these bilateral rupture processes.     

Internal deformation of Sikhote Alin volcanic belt, far eastern Russia: Paleocene paleomagnetic results

We present paleomagnetic results of Paleocene welded tuffs of the 53–50 Ma Bogopol Group from the northern region (46°N, 137°E) of the Sikhote Alin volcanic belt. Characteristic paleomagnetic directions with high unblocking temperature components above 560 °C were isolated from all the sites. A tilt-corrected mean paleomagnetic direction from the northern region is D=345.8°, I=49.9°, α₉₅=14.6° (N=9). The reliability of the magnetization is ascertained through the presence of normal and reversed polarities. The mean paleomagnetic direction from the northern region of the Sikhote Alin volcanic belt reflects a counterclockwise rotation of 29° from the Paleocene mean paleomagnetic direction expected from its southern region. The counterclockwise rotation of 25° is suggested from the paleomagnetic data of the Kisin Group that underlies the Bogopol Group. These results establish that internal tectonic deformation occurred within the Sikhote Alin volcanic belt over the past 50 Ma. The northern region from 44.6° to 46.0°N in the Sikhote Alin volcanic belt was subjected to counterclockwise rotational motion through 29±17° with respect to the southern region. The tectonic rotation of the northern region is ascribable to relative motion between the Zhuravlevka terrane and the Olginsk–Taukhinsk terranes that compose the basements of the Sikhote Alin volcanic belt.     

Spatial relationships between natural seismicity and faults, southeastern Ontario and north-central New York state

A statistical analysis was carried out to investigate spatial associations between natural seismicity and faults in southeastern Ontario and north-central New York State (between 73°18′ and 77°00′W and 43°30′ and 45°18′N). The study area is situated to the west of the seismically active St. Lawrence fault zone, and to the east of the Lake Ontario basin where recently documented geological and geophysical evidence points to possible neotectonic faulting. The weights of evidence method was used to judge the spatial associations between seismic events and populations of faults in eight arbitrarily defined orientation groups. Spatial analysis of data sets for seismic events in the periods 1930–1970 and post-1970 suggest stronger spatial associations between earthquake epicentres and faults with strikes that lie in the NW–SE quadrants, and weaker spatial associations of epicentres with faults that have strikes in the NE–SW quadrants. The strongest spatial associations were determined for groups of faults with strikes between 101° and 146°. The results suggest that faults striking broadly NW–SE, at high angles to the regional maximum horizontal compressive stress, are statistically more likely to be spatially associated with seismic events than faults striking broadly NE–SW. If the positive spatial associations can be interpreted as indicating genetic relationships between earthquakes and mapped faults, then the results may suggest that, as a population, NW–SE trending faults are more likely to be seismically active than NE–SW striking faults. Detailed geological studies of faults in the study area would be required to determine possible neotectonic displacements and the kinematics of the displacements.     

Thermal variations in the gregory rift of southern Kenya (?)

A compilation of thermal and seismic data collected over the last sixty years allows one to infer that tectonic phenomena and heat emanation could be linked in an oscillatory mode up and down the Kenyan part of the East African Rift. The seismic period is approximately 20–30 years during which time the loci of maximum intensity earthquakes move in a rhythmic pattern from south to north and back to south. Temperatures measured from hot springs also fluctuate over this time span increasing or decreasing in different sections of the rift.Spatial variations were measured by infrared radiometers from low altitude aircraft or high-altitude satellites. These reveal that individual thermal springs ranging from 35°C to 80°C, warm up greater than 5 km² of the lake bottom of Magadi (only a slightly active thermal region which, however, yields greater then 300 MW). The heated area is large enough to detect by satellite imagery, making it possible to monitor the heat budget and flux over time and relate it to tectonic activity in the rift.     

Immature and mature transform zones near a hot spot: The South Iceland Seismic Zone and the Tjörnes Fracture Zone (Iceland)

We describe and compare the two transform zones that connect the Icelandic rift segments and the mid-Atlantic Ridge close to the Icelandic hot spot, in terms of geometry of faulting and stress fields. The E–W trending South Iceland Seismic Zone is a diffuse shear zone with a Riedel fault pattern including N0°–N20°E trending right-lateral and N60°–N70°E trending left-lateral faults. The dominant stress field in this zone is characterised by NW–SE extension, in general agreement with left-lateral transform motion. The Tjörnes Fracture Zone includes three major lineaments at different stages of development. The most developed, the Húsavík–Flatey Fault, presents a relatively simple geometry with a major fault that trends ESE–WNW. The stress pattern is however complex, with two dominant directions of extension, E–W and NE–SW on average. Both these extensions are compatible with the right-lateral transform motion and reveal different behaviours in terms of coupling. Transform motion has unambiguous fault expression along a mature zone, a situation close to that of the Tjörnes Fracture Zone. In contrast, transform motion along the immature South Iceland Seismic Zone is expressed through a more complicate structural pattern. At the early stage of the transform process, relatively simple stress patterns prevail, with a single dominant stress field, whereas, when the transform zone is mature, moderate and low coupling situations may alternate, as a function of volcanic–tectonic crises and induce changes in stress orientation.     

Geometry and distribution of regional joint sets in a non-homogeneous stress field: case study in the Ebro basin (Spain)

Three regional joint sets striking N–S, E–W and WNW–ESE affect the Tertiary rocks of the central Ebro basin. From analysis of their chronological relationships and spatial distribution, it is concluded that they correspond to two different tectonic events. The N–S set (oldest) and the E–W set (younger) are present in the southern and central sectors, while the WNW–ESE joint set predominates in the northern one. The N–S joints propagated in response to joint-normal and fluid loads under an intraplate stress field with S_Hmax oriented near N–S (related to forces caused by the convergence of Africa, Iberia and Europe and rifting at the Valencia trough) during the sedimentary infilling of the basin. These joints are only present in the southern part of the area. The E–W joint set in the southern-central sector records the same fracturing event as the WNW–ESE set does in the northern one. Its orientation was modified by the presence of the older N–S set in the south, which perturbed the regional stress field. The younger WNW–ESE and E–W joint sets are interpreted as unloading joints. These propagated as a consequence of flexural uplift and exhumation related to isostatic rebound at the Pyrenees and the Ebro foreland basin. A numerical approach is used to explain the inhomogeneous distribution of the N–S joint set in terms of their absence being controlled by the depth of the water table at the time of their formation.     

Palaeomagnetic correlations of Precambrian formations of east-central Africa and their tectonic implications

Palaeomagnetic poles derived from Precambrian formations can be valuable for determining relative, and sometimes absolute, ages of the formations. In this paper palaeomagnetic results are presented from a variety of these formations in Tanzania and Zambia. The Ikorongo Group sediments of Tanzania give a pole at 80° E, 25° S commensurate with an age of 900–1000 m.y. The lower Buanji Series of southern Tanzania yields a pole at 263°E,87°N indicating an age of either Late Precambrian (c. 650 m.y.) or Early Cambrian. The Plateau Series outcrop at the southern end of Lake Tanganyika gives several poles falling on the Late Precambrian to Ordovician apparent polar wander loop recognized by McElhinny et al. (1974), and a small amount of evidence from the Abercorn Sandstone and southern part of the Plateau Series outcrop suggests an age of c. 900 m.y. for these rocks. Dating of formations at the southern end of the Lake Tanganyika depression gives an estimate of 1500 m for the minimum amount of downthrow at this end of the rift system. Five sites from the Mbozi gabbro—syenite complex of southern Tanzania give a pole at 68° E, 72° N and two sites from Mbala dolerites of Zambia yield a pole close to one from the Bukoban dolerites of Tanzania and a similar age (c. 806 m.y.) is suggested.Some palaeomagnetic information is now available from all the Proterozoic platform sediments margining the Tanganyika craton and a correlation scheme is given which incorporates this information together with geochronological data. These formations postdate geosynclinal sequences involved in the Kibaran (c. 1300 m.y.) and Irumide (c. 1100 m.y.) mobile belts, and geological environment and situation demonstrate that the Tanganyika craton was subject to intermittent uplift between about 1000 m.y. and Cambrian times.     

The April 2007 earthquake swarm near Lake Trichonis and implications for active tectonics in western Greece

We investigate the properties of the April 2007 earthquake swarm (Mw 5.2) which occurred at the vicinity of Lake Trichonis (western Greece). First we relocated the earthquakes, using P- and S-wave arrivals to the stations of the Hellenic Unified Seismic Network (HUSN), and then we applied moment tensor inversion to regional broad-band waveforms to obtain the focal mechanisms of the strongest events of the 2007 swarm. The relocated epicentres, cluster along the eastern banks of the lake, and follow a distinct NNW–ESE trend. The previous strong sequence close to Lake Trichonis occurred in June–December 1975. We applied teleseismic body waveform inversion, to obtain the focal mechanism solution of the strongest earthquake of this sequence, i.e. the 31 December 1975 (Mw 6.0) event. Our results indicate that: a) the 31 December 1975 Mw 6.0 event was produced by a NW–SE normal fault, dipping to the NE, with considerable sinistral strike-slip component; we relocated its epicentre: i) using phase data reported to ISC and its coordinates are 38.486°N, 21.661°E; ii) using the available macroseismic data, and the coordinates of the macroseismic epicentre are 38.49°N, 21.63°E, close to the strongly affected village of Kato Makrinou; b) the earthquakes of the 2007 swarm indicate a NNW–SSE strike for the activated main structure, parallel to the eastern banks of Lake Trichonis, dipping to the NE and characterized by mainly normal faulting, occasionally combined with sinistral strike-slip component. The 2007 earthquake swarm did not rupture the well documented E–W striking Trichonis normal fault that bounds the southern flank of the lake, but on the contrary it is due to rupture of a NW–SE normal fault that strikes at a  45° angle to the Trichonis fault. The left-lateral component of faulting is mapped for the first time to the north of the Gulf of Patras which was previously regarded as the boundary for strike-slip motions in western Greece. This result signifies the importance of further investigations to unravel in detail the tectonics of this region.     

An expression of Philippine Sea plate rotation: the Parece Vela and Shikoku Basins

The Philippine Sea plate, located between the Pacific, Eurasian and Australian plates, is the world's largest marginal basin plate. The motion of the Philippine Sea plate through time is poorly understood as it is almost entirely surrounded by subduction zones and hence, previous studies have relied on palaeomagnetic analysis to constrain its rotation. We present a comprehensive analysis of geophysical data within the Parece Vela and Shikoku Basins—two Oligocene to Miocene back-arc basins—which provide independent constraints on the rotational history of the Philippine Sea plate by means of their seafloor spreading record. We have created a detailed plate model for the opening of the Parece Vela and Shikoku Basins based on an analysis of all available magnetic, gravity and bathymetric data in the region. Subduction along the Izu–Bonin–Mariana trench led to trench roll-back, arc rupture and back-arc rifting in the Parece Vela and Shikoku Basins at 30 Ma. Seafloor spreading in both basins developed by chron 9o (28 Ma), and possibly by chron 10o (29 Ma), as a northward and southward propagating rift, respectively. The spreading orientation in the Parece Vela Basin was E–W as opposed to ENE–WSW in the Shikoku Basin. The spreading ridges joined by chron 6By (23 Ma) and formed a R–R–R triple junction to accommodate the difference in spreading orientations in both basins. At chron 6No (20 Ma), the spreading direction in the Parece Vela Basin changed from E–W to NE–SW. At chron 5Ey (19 Ma), the spreading direction in the Shikoku Basin changed from ENE–WSW to NE–SW. This change was accompanied by a marked decrease in spreading rate. Cessation of back-arc opening occurred at 15 Ma, a time of regional plate reorganisation in SE Asia. We interpret the dramatic change in spreading rate and direction from E–W to NE–SW at 20±1.3 Ma as an expression of Philippine Sea plate rotation and is constrained by the spacing between our magnetic anomaly identifications and the curvature of the fracture zones. This rotation was previously thought to have begun at 25 Ma as a result of a global change in plate motions. Our results suggest that the Philippine Sea plate rotated clockwise by about 4° between 20 and 15 Ma about a pole located 35°N, 84°E. This implies that the majority of the 34° clockwise rotation inferred to have occurred between 25 and 5 Ma from paleomagnetic data may have in fact been confined to the period between 15 and 5 Ma.     

Seismic strain rates and distributed continental deformation in the northern Andes and three-dimensional seismotectonics of northwestern South America

Remote sensing and field studies of several extensional basins along the northern margin of the Gulf of Aden in Yemen show that Oligocene–Miocene syn-rift extension trends N20°E on average, in agreement with the E–W to N120°E strike of main rift-related normal faults, but oblique to the main trend of the Gulf (N70°E). These faults show a systematic reactivation under a 160°E extensional stress that we interpret also as syn-rift. The occurrence of these two successive phases of extension over more than 1000 km along the continental margin suggests a common origin linked to the rifting process. After discussing other possible mechanisms such as a change in plate motion, far-field effects of Arabia–Eurasia collision, and stress rotations in transfer zones, we present a working hypothesis that relates the 160°E extension to the westward propagation since about 20 Ma of the N70°E-trending, obliquely spreading, Gulf of Aden oceanic rift. The late 160°E extension, perpendicular to the direction of rift propagation, could result from crack-induced extension associated with the strain localization that characterises the rift-to-drift transition.     

Travels of the Cache Creek Terrane: a paleomagnetic, geobarometric and Ar/Ar study of the Jurassic Fourth of July Batholith, Canadian Cordillera

The Middle Jurassic Fourth of July Batholith and cross-cutting mafic dikes have been studied geochronologically, geobarometrically and paleomagnetically to estimate subsequent tectonic motion of the Cache Creek Terrane (CCT) in the northern Canadian Cordillera. ⁴⁰Ar/³⁹Ar hornblende ages from a granodiorite phase are similar to U–Pb zircon ages and indicate rapid cooling of the batholith upon intrusion, suggesting that the magnetization age is coincident with the 173-Ma crystallization age. Argon ages of biotite from the granodiorite and two mafic dikes have similar ages of 165 Ma, which dates cooling through 280 °C.Aluminum-in-hornblende geobarometry indicates differential uplift of the batholith across a north–south fault zone along Atlin Lake with >6 km more uplift on its eastern side. Also, the eastern side has been tilted downward to the south–southwest by 9°.Combined paleomagnetic data from 20 granitoid and 11 mafic dike sites yield an in situ paleopole at 55°W, 63°N (d_p=5°, d_m=5°) and a tilt-corrected paleopole at 81°W, 55°N (d_p=5°, d_m=6°). Compared to the 173-Ma reference pole for the North American craton, the tilt-corrected pole suggests a significant southward translation of 16.1±3.7° and a significant clockwise rotation of 107±7°. The translation estimate is similar to the Jurassic Teslin Crossing pluton in the Stikine Terrane, however, the rotation estimate is very different. This could indicate that the Cache Creek Terrane was at a similar latitude of the Stikine Terrane, but the two were not yet amalgamated.     

Seismotectonics of Taiwan

High-quality seismicity data and focal mechanism solutions obtained during 1973–1983 by the permanent Taiwan Telemetered Seismographic Network and several temporary local seismographic networks are used for a detailed study of the seismotectonics of the Taiwan area. Seismicity distribution in southern Taiwan clearly reveals an east-dipping Benioff zone which has a thickness of about 30 km and begins to deepen along 121°E at a dip angle of 55°–60°. The leading edge of this Benioff zone reaches a depth of about 180 km between 21°N and 22°N, but tapers off to a shallower depth of about 100 km from 22°N to 23°N. The presence of this seismic zone implies that subduction of the South China Sea plate under the Philippine Sea plate extends from Luzon northward to about 23°N. The position of the northern boundary of the South China Sea plate, as tentatively defined according to the seismicity distribution, passes through southern Taiwan from the offshore area in the Taiwan Strait west of Kaohsiung in an east-northeast direction to the Taitung area where a triple junction probably lies. Seismicity is found to disappear abruptly below a certain depth in many parts of Taiwan. This phenomenon may be attributed to the frictional to quasiplastic transition in the crust or upper mantle. Comparison of shallow seismicity with surface faults and fractures shows that all areas of active shallow seismicity are marked by densely-developed faults and fractures. However, the converse is not necessarily true. This may be partly due to the relatively short duration of seismicity data and partly due to excessive weakening of some of the severely faulted and fractured areas. Finally, focal mechanism solutions for west central Taiwan and the Kuangfu-Fuli area in eastern Taiwan predominantly show a maximum horizontal compression in the SE-NW direction which can be related to collision between the Eurasian and Philippine Sea plates. However, focal mechanism solutions for both the Hualien area in eastern Taiwan and the Tainan area in southwestern Taiwan show remarkable irregularities which may result from local tectonic complexities.     

The polymetallic Au–Ag-bearing veins of Bou Madine (Jbel Ougnat, eastern Anti-Atlas, Morocco): tectonic control and evolution of a Neoproterozoic epithermal deposit

The Bou Madine ore deposit is located SW of Jbel Ougnat, the easternmost inlier of the Anti-Atlas Pan-African belt in Morocco. The host rocks are high-K calc-alkaline volcanic rocks, that are part of the Neoproterozoic Tamerzaga-Timrachine Formation (TTF, lower PIII). The TTF consists of ignimbrites of rhyolitic to dacitic compositions, andesite flows and hypovolcanic bodies (andesite dykes and rhyolite chonoliths) emplaced along N160°E tension gashes associated with a regional N30°E sinistral fault system. The mineralization is related to a high enthalpy geothermal system, eventually evolving into a low temperature epithermal system. A regional propylitisation (T around 260 °C) overprinted the TTF rocks prior to the emplacement of the mineralization. There were two main hydrothermal stages. During the first stage, massive veins with pyrite, arsenopyrite and minor pyrrhotite and cassiterite were formed. The veins were emplaced along N160°E-trending en echelon joints related to N120°E dextral arrays. A quartz-sericite-pyrite alteration overprinted the propylites around the veins (“bleached haloes”), at temperatures up to 300–310 °C. The second stage of mineralization was coeval with dextral re-activation of the N160°E veins, in relation with a NE-ward shift of the shortening direction. First, polymetallic sulphides (sphalerite, chalcopyrite, stannite, galena) were deposited at temperatures 260 °C. Younger quartz veinlets contain arsenopyrite and minor micrometre-size sulphides and sulpho-salts, hosting the precious metals. This was the low temperature epithermal stage (≈150 °C), in relation with invading meteoric water.     

Volcanic and tectonic evolution of the Northern Antarctic Peninsula—Late Cenozoic to recent

Field investigation together with a number of geochemical petrographical analyses, as well as absolute K-Ar age determinations and geophysical data, allow the recognition of an evolutionary sequence of geodynamic events which have affected the northern region of Antarctic Peninsula and the adjacent islands.A significant volcanic calc-alkaline belt, which developed on the northwestern margin of the Antarctic Peninsula during the Cretaceous to Middle Tertiary, is indicative of active subduction of the Antarctic plate in that area. This activity decreases during the Lower Miocene, giving way to an expansive phase represented by the Bransfield Rift. These extensional processes are dominant during the Pliocene, creating a rift system in southeastern Bransfield towards Larsen. Both the Bransfield and Larsen systems comprise one “fan-like rift system”, associated with the Prince Gustav Rift and the Scotia Arc micro-plate. Ejection of abundant pyroclastic material generated a large plateau of palagonite hyaloclastites of basaltic alkaline composition. During the Pleistocene-Recent, the extensional activity continued, as evidenced by the active volcanic fractures represented in Bransfield by the Deception, Penguin and Bridgeman volcanic centres; in the Prince Gustav Rift by Paulet Islands and others, and in Larsen by the Coley, Seal Nunatak and Argo volcanic centres. The latter is characterized by basaltic olivine-alkaline effusions. These rifts and the continental blocks are affected by a series of fractures with a N60°–70°W strike, which could be directly associated with the Hero Fracture Zone extending northwest of the South Shetland Islands Trench.     

Along strike changes in the structural evolution over a brittle detachment fault: Example of the Pleistocene Corinth–Patras rift (Greece)

The Patras, Corinth, and northern Saronic gulfs occupy a 200-km-long, N120° trending Pleistocene rift zone, where Peloponnese drifts away from mainland Greece. The axes of Patras and Corinth basins are 25 km apart and linked by two transfer-fault zones trending N040°. The older one defines the western slope of Panachaïkon mountain, and the younger one limits the narrow Rion–Patras littoral plain. Between these two faults, the ca. 4-km-thick Rion–Patras series dips 20–30° SSW. It is part of the Patras gulf synrift deposits, which pile in an asymmetric basin governed by a fault dipping ca. 25–35° NNE, located in the southern Gulf of Patras. Mapping of this fault to the east in northern Peloponnese shows that it is an inactive north-dipping low-angle normal fault (0° to 30°N), called the northern Peloponnese major fault (NPMF). The structural evolution of the NPMF was different in the gulfs of Patras and Corinth. In the Gulf of Patras, it is still active. In northern Peloponnese, footwall uplift and coeval southward tilting flattened the fault and locked its southern part. Steeper normal faults formed north of the locked area, connecting the still active northern part of the NPMF to the surface. After several locks, the presently active normal faults (Psathopyrgos, Aigion, Helike) trend along the southern shore of the Gulf of Corinth. This migration of faults caused the relative 25 km northward shift of the Corinth basin, and the formation of NE–SW trending transfer-faults between the Corinth and Patras gulfs.     

Ngurumanite from Southern Kenya and its Bearing on the Origin of Rocks in the Northern Tanganyika Alkaline District

A new alkaline rock, ngurumanite, is described, from the NgurumanEscarpment west of Lake Magadi in the Rift Valley of Kenya.This essentially nepheline-pyroxene rock is unique in containingiron-rich mesostasis and vug-like patches of primary calcite,analcime, and zeolites; it intrudes ankaratrite and representsthe hypabyssal equivalent of the lava. These rocks form part of a strongly alkaline series of the northernTanganyika alkaline district; a second series of mildly alkaline.Character is also recognized. The rocks of the strongly alkalineseries were evidently derived from a parental magma of ankaratriticcomposition, whilst mildly alkaline types are differentiatesfrom a magma of alkali olivine-basalt composition. The intimateassociation of rocks of the two series indicates derivationfrom a common source, and an ultimate peridotitic origin isenvisaged. Late enrichment in soda and lime during crystallization of ngurumaniteparallels the trend of differentiation shown by the stronglyalkaline rocks of the district, where the ultimate derivativeis carbonatite.     

Primary curvature in the Mid-Continent Rift: Paleomagnetism of the Portage Lake Volcanics (northern Michigan, USA)

Rocks of the North American Mid-Continent Rift (MCR) in the Keweenaw Peninsula of northern Michigan display a change in structural trend from east to west, varying in strike from 100° in the east to 35° farther west, before returning to a more east–west trend of 80° near Wisconsin. In general, curvature can be described either as of primary origin, meaning that the arc developed in its present curved state, or as of secondary origin, when the arc formed from an initially straighter geometry. A powerful tool in evaluating the origin and degree of curvature in any deformed belt is paleomagnetism, where coincident change in declination and strike is evidence for secondary curvature. Several paleomagnetic studies have been completed on rift-related sedimentary rocks, as well as lava flows, from the MCR in the Lake Superior region. Paleomagnetic directions for the sedimentary rocks vary much more in declination than in inclination, possibly reflecting a vertical axis rotation. If secondary rotation has affected the sedimentary rocks, then underlying volcanic rocks should also demonstrate a similar rotation.Thirty-one sites were collected from the Portage Lake Volcanics in the most highly curved part of the Keweenaw Peninsula in the Upper Peninsula of Michigan. Sites were chosen to maximize the variation in structural trend. Thermal demagnetization results showed two components in samples from most sites, a lower temperature A component (< 580 °C) as well as a higher temperature B component (> 580 °C). Both components were tested for primary remanence using a conglomerate test. The A component, carried by magnetite, passes the conglomerate test at a 95% confidence level, and is therefore considered a primary remanence. The B component, carried by hematite, fails at the 95% level and is considered a secondary magnetization. Most importantly, declination of the primary (A) magnetization between sites shows no correlation with strike, demonstrating that vertical axis rotations cannot explain the curvature of the Mid-Continent Rift. We conclude that curvature in the Lake Superior Region is a primary feature, likely reflecting a pre-existing zone of weakness that was exploited during rifting.     

Structural fabric of the Central Metasedimentary Belt of southern Ontario, Canada, from deep seismic profiling

The Central Metasedimentary Belt boundary tectonic zone (CMBbtz) is a 10–20-km-wide zone of intense structural deformation within the 1.3–1.0 Ga Grenville orogen of southeastern Canada. The crustal structure of the exposed CMBbtz has been well studied, but its sub-Phanerozoic location and geometry beneath the urban development and nuclear stations of the Toronto region are not well known. A new 75-km Lithoprobe reflection profile acquired close to Toronto provides a clear image of the CMBbtz as a panel of southeast-dipping reflections that extends with moderate dip (<25°) to mid-crustal depth (25 km). These dipping reflections truncate and (or) overprint a subhorizontal band of reflectivity at 21 km depth. The seismic line is oblique to the major structural trends; cross-dip analysis shows that the southeast-dipping reflections have a strike and dip of N13°E and 25°, whereas the “subhorizontal” reflections strike and dip at N65°E and 20°, respectively. Both of these bands of reflectivity can be correlated to magnetic anomalies in the CMBbtz or its immediate footwall. Magnetic anomalies with similar strike directions are well expressed within a distinct rhomboid-shaped region (106×109 km) in the subsurface of western Lake Ontario, herein named Mississauga domain. Taken together, the seismic and magnetic data are inconsistent with existing models, in which the CMBbtz is extrapolated beneath Lake Ontario along a linear magnetic anomaly. We propose a revised subsurface trace of the CMBbtz along the western edge of the Mississauga domain. Small earthquakes in western Lake Ontario appear to cluster along trends co-linear with ENE magnetic anomalies, suggesting a possible degree of basement tectonic control on local intraplate seismicity.     

The migration of plate boundaries in SE Sicily: Influence on the large-scale kinematic model of the African promontory in southern Italy

A detailed field mapping, coupled with structural analyses and morphological investigation, has been carried out along the northern and western borders of the Hyblean Plateau (SE Sicily), in order to define the nature and the kinematics of a major Quaternary fault belt. This, here designed as the Scicli Line Fault Belt, is composed of two N50 oriented extensional basins that, linked by a regional N10 trending transfer zone, originated during the Early Pleistocene and experienced, since the Late Quaternary, a positive tectonic inversion. In both the two stages of deformation, the Scicli Line Fault Belt has been characterised by displacement-rate comparable with the relative velocities measured between the distinct plates composing the central Mediterranean region. In the period going from 1.5–1.2 to 0.85 Ma, the fault belt accommodated the entire divergence between Adria and Nubia. At present, the Scicli Line Fault Belt absorbs most of the Nubia–Eurasia convergence, while the western divergent margin of the Adria microplate has jumped to the eastern and the southern margins of the Hyblean Plateau, along the Late Quaternary Siculo–Calabrian Rift Zone. The off-shore prolongation of the two tectonic boundaries of the Hyblean Plateau has been recognised in the Sicily Channel, where they are both interrupted by a WNW-ESE oriented dextral fault. According to our reconstruction, the Hyblean Plateau represents an isolated lithospheric block, whose evolution can be related to the propagation of the western divergent margin of the Adria microplate, accompanied with the southward migration of the triple junction between Eurasia, Nubia and Adria. In this new large-scale kinematic picture, the GPS velocity measured in the Hyblean region, at the permanent site of NOTO, is actually representative of the local kinematics, rather than of the entire African promontory of southern Italy. This implies a correction of previous regional kinematic models based on combination of GPS vectors. In particular, our data constrain a new interpretation both for the kinematics along the E–W oriented Nubia–Eurasia margin, dominated by prevalent dextral deformation rather than reverse motions, and for the intraplate deformation in the Sicily Channel, within the Africa promontory, which would be dominated by a roughly N110° oriented extension. This conclusion has implication also on the mechanism and the origin of the Pantelleria–Linosa–Malta Rift that is here interpreted as a transtensive feature developed along a major transform fault, rather than the result of passive rifting induced by the Nubia–Eurasia collision, as it is currently interpreted.     

Spatial and temporal geochemical variability in lacustrine sedimentation in the East African Rift System: Evidence from the Kenya Rift and regional analyses

Many previous studies on lacustrine basins in the East African Rift System have directed their attention to climatic controls on contemporary sedimentation or climate change as part of palaeoenvironmental reconstruction. In contrast, this research focuses on the impact of tectonism and volcanism on rift deposition and develops models that help to explain their roles and relative importance. The study focuses on the spatial and temporal variability in bulk sediment geochemistry from a diverse range of modern and ancient rift sediments through an analysis of 519 samples and 50 major and trace elements. The basins examined variously include, or have contained, wetlands and/or shallow to deep, fresh to hypersaline lakes. Substantial spatial variability is documented for Holocene to modern deposits in lakes Turkana, Baringo, Bogoria, Magadi and Malawi. Mio‐Pleistocene sediments in the Central Kenya Rift and Quaternary deposits of the southern Kenya Rift illustrate temporal variability. Tectonic and volcanic controls on geochemical variability are explained in terms of: (i) primary controlling factors (faulting, subsidence, uplift, volcanism, magma evolution and antecedent lithologies and landscapes); (ii) secondary controls (bedrock types, rift shoulder and axis elevations, accommodation space, meteoric and hydrothermal fluids and mantle CO ₂); and (iii) response factors (catchment area size, orographic rains, rain shadows, vegetation densities, erosion and weathering rates, and spring/runoff ratios). The models developed have, in turn, important implications for palaeoenvironmental interpretation in other depositional basins.