East Dipping Research Articles

Faulted and tilted Pliocene olivine-tholeiite lavas near Alturas, NE California, and their bearing on the uplift of the Warner Range

Late Miocene-Pliocene (8-3 Ma) olivine basalt lavas, dated in this study by the 40 Ar/ 39 Ar method, have been faulted and tilted on both the east and west sides of the Warner Range of NE California, which is itself a tilted block rising to 2960 m at its crest that is composed of Miocene-Oligocene lavas and volcaniclastic rocks. The late Miocene-Pliocene lavas, distinctively poor in K 2 O and rich in MgO, are called low-K olivine tholeiites and have a different mantle source region than that of the older subduction-related lavas of the main Warner Range. Hays Canyon Range (max. elev. 2400 m) lies to the east of the Wamer Range, and the broad Surprise Valley separates the two fault-bounded ranges. Middle Miocene (ca. 15 Ma) basic lavas, with a small easterly dip, cap the Hays Canyon Range and overlie Oligocene silicic ash-flow deposits and a basaltic andesite spatter volcano. Middle Miocene basic lavas also form the crest of the Warner Range and its westerly dip slope (-15°). Nearly horizontal basic lavas of the same age are also found on both sides of the Warner Range, and it is a plausible conclusion that these middle Miocene basalts were a contiguous group before faulting and uplift of the Warner Range. Derived estimates of uplift rates (∼1 mm/ yr) of the Warner Range indicate that uplift could have been initiated at ca. 4 Ma, a period of the most voluminous eruption of low-K olivine tholeiite lavas. If the slower Cretaceous exhumation rate of the Sierra Nevada (0.5-1.0 mm/yr) is applied to the total offset of the Warner Range (4270 m), and it did not vary with time, then the uplift of the Warner Range was initiated at ca. 8 Ma, which coincides with the age of the oldest low-K olivine tholeiite lava (8 Ma). Low-K olivine tholeiites require a hot shallow asthenospheric source, and it is the rise of this hot mantle that is presumed to have caused the uplift of the Warner Range. Whether or not the widespread eruption of small volumes of Pliocene low-K olivine tholeiites in central and eastern Oregon is associated with crustal uplift is unknown.

The Gibraltar Arc seismogenic zone (part 1): Constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by seismic data, gravity and thermal modeling

The Great Lisbon earthquake of 1755 with an estimated magnitude of 8.5–9.0 is the most destructive earthquake in European history, yet the source region remains enigmatic. Recent geophysical data provide compelling evidence for an active east dipping subduction zone beneath the nearby Gibraltar Arc. Marine seismic data in the Gulf of Cadiz image active thrust faults in an accretionary wedge, above an east dipping decollement and an eastward dipping basement. Tomographic and other data support subduction and rollback of a narrow slab of oceanic lithosphere beneath the westward advancing Gibraltar block. Although, no instrumentally recorded seismicity has been documented for the subduction interface, we propose the hypothesis that this shallow east dipping fault plane is locked and capable of generating great earthquakes (like the Nankai or Cascadia seismogenic zones). We further propose this east dipping fault plane to be a candidate source for the Great Lisbon earthquake of 1755. In this paper we use all available geophysical data on the deep structure of the Gulf of Cadiz–Gibraltar region for the purpose of constraining the 3-D geometry of this potentially seismogenic fault plane. To this end, we use new depth processed seismic data, have interpreted all available published and unpublished time sections, examine the distribution of hypocenters and perform 2-D gravity modeling. Finally, a finite-element model of the forearc thermal structure is constructed to determine the temperature distribution along the fault interface and thus the thermally predicted updip and downdip limits of the seismogenic zone.

The Gibraltar Arc seismogenic zone (part 2): Constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by tsunami modeling and seismic intensity

The Great Lisbon earthquake has the largest documented felt area of any shallow earthquake and an estimated magnitude of 8.5–9.0. The associated tsunami ravaged the coast of SW Portugal and the Gulf of Cadiz, with run-up heights reported to have reached 5–15 m. While several source regions offshore SW Portugal have been proposed (e.g.— Gorringe Bank, Marquis de Pombal fault), no single source appears to be able to account for the great seismic moment as well as all the historical tsunami amplitude and travel time observations. A shallow east dipping fault plane beneath the Gulf of Cadiz associated with active subduction beneath Gibraltar, represents a candidate source for the Lisbon earthquake of 1755. Here we consider the fault parameters implied by this hypothesis, with respect to total slip, seismic moment, and recurrence interval to test the viability of this source. The geometry of the seismogenic zone is obtained from deep crustal studies and can be represented by an east dipping fault plane with mean dimensions of 180 km (N–S) × 210 km (E–W). For 10 m of co-seismic slip an Mw 8.64 event results and for 20 m of slip an Mw 8.8 earthquake is generated. Thus, for convergence rates of about 1 cm/yr, an event of this magnitude could occur every 1000–2000 years. Available kinematic and sedimentological data are in general agreement with such a recurrence interval. Tsunami wave form modeling indicates a subduction source in the Gulf of Cadiz can partly satisfy the historical observations with respect to wave amplitudes and arrival times, though discrepancies remain for some stations. A macroseismic analysis is performed using site effect functions calculated from isoseismals observed during instrumentally recorded strong earthquakes in the region (M7.9 1969 and M6.8 1964). The resulting synthetic isoseismals for the 1755 event suggest a subduction source, possibly in combination with an additional source at the NW corner of the Gulf of Cadiz can satisfactorily explain the historically observed seismic intensities. Further studies are needed to sample the turbidites in the adjacent abyssal plains to better document the source region and more precisely calibrate the chronology of great earthquakes in this region.

Faulted and tilted Pliocene olivine-tholeiite lavas near Alturas, NE California, and their bearing on the uplift of the Warner Range

Research Article| September 01, 2006 Faulted and tilted Pliocene olivine-tholeiite lavas near Alturas, NE California, and their bearing on the uplift of the Warner Range Ian S.E. Carmichael; Ian S.E. Carmichael 1Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA Search for other works by this author on: GSW Google Scholar Rebecca A. Lange; Rebecca A. Lange 2Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA Search for other works by this author on: GSW Google Scholar Chris M. Hall; Chris M. Hall 2Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA Search for other works by this author on: GSW Google Scholar Paul R. Renne Paul R. Renne 3Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA, and Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Ian S.E. Carmichael 1Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA Rebecca A. Lange 2Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA Chris M. Hall 2Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA Paul R. Renne 3Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA, and Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA Publisher: Geological Society of America Received: 18 Oct 2005 Revision Received: 25 Mar 2006 Accepted: 04 Apr 2006 First Online: 08 Mar 2017 Online Issn: 1943-2674 Print Issn: 0016-7606 Geological Society of America GSA Bulletin (2006) 118 (9-10): 1196–1211. https://doi.org/10.1130/B25918.1 Article history Received: 18 Oct 2005 Revision Received: 25 Mar 2006 Accepted: 04 Apr 2006 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 Ian S.E. Carmichael, Rebecca A. Lange, Chris M. Hall, Paul R. Renne; Faulted and tilted Pliocene olivine-tholeiite lavas near Alturas, NE California, and their bearing on the uplift of the Warner Range. GSA Bulletin 2006;; 118 (9-10): 1196–1211. doi: https://doi.org/10.1130/B25918.1 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 Late Miocene–Pliocene (8–3 Ma) olivine basalt lavas, dated in this study by the 40Ar/39Ar method, have been faulted and tilted on both the east and west sides of the Warner Range of NE California, which is itself a tilted block rising to 2960 m at its crest that is composed of Miocene-Oligocene lavas and volcaniclastic rocks. The late Miocene–Pliocene lavas, distinctively poor in K2O and rich in MgO, are called low-K olivine tholeiites and have a different mantle source region than that of the older subduction-related lavas of the main Warner Range. Hays Canyon Range (max. elev. 2400 m) lies to the east of the Warner Range, and the broad Surprise Valley separates the two fault-bounded ranges. Middle Miocene (ca. 15 Ma) basic lavas, with a small easterly dip, cap the Hays Canyon Range and overlie Oligocene silicic ash-flow deposits and a basaltic andesite spatter volcano. Middle Miocene basic lavas also form the crest of the Warner Range and its westerly dip slope (∼15°). Nearly horizontal basic lavas of the same age are also found on both sides of the Warner Range, and it is a plausible conclusion that these middle Miocene basalts were a contiguous group before faulting and uplift of the Warner Range.Derived estimates of uplift rates (∼1 mm/yr) of the Warner Range indicate that uplift could have been initiated at ca. 4 Ma, a period of the most voluminous eruption of low-K olivine tholeiite lavas. If the slower Cretaceous exhumation rate of the Sierra Nevada (0.5–1.0 mm/yr) is applied to the total offset of the Warner Range (4270 m), and it did not vary with time, then the uplift of the Warner Range was initiated at ca. 8 Ma, which coincides with the age of the oldest low-K olivine tholeiite lava (8 Ma). Low-K olivine tholeiites require a hot shallow asthenospheric source, and it is the rise of this hot mantle that is presumed to have caused the uplift of the Warner Range. Whether or not the widespread eruption of small volumes of Pliocene low-K olivine tholeiites in central and eastern Oregon is associated with crustal uplift is unknown. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Marine self-potential gradient exploration of the continental margin

The self-potential (SP) method for mineral exploration is seldom used on land, primarily because of electrode noise problems and nonunique interpretations. Marine measurements of the horizontal gradient of the SP field, on the other hand, are relatively simple to make with an array of electrodes towed behind a ship. With low ship speeds of 5 to 10 km/hour, dense spatial sampling (∼1 m) can be obtained with resolution of better than 1 μV/m. In this paper we report on gradient SP data recorded on the continental shelf of South Australia by a horizontal array of towed electrodes approximately 20 m above the seafloor. Ocean waves and swells with periods of 5 to 15 s yielded large amplitude signals ±20 μV/m, but subseafloor mineralization produced SP gradient anomalies of ±50 μV/m and widths of 2 km or more in a number of parallel traverses. Integrating the observed SP gradients along each line delineated SP anomalies of amplitude up to −100 mV. Self-potential and magnetic anomaly data show limited spatial correlation and have different wavelengths, suggesting that SP sources are probably nonferrous minerals, such as graphite, and are deeper than the magnetic sources. The source of the SP signal is probably reduction-oxidation (redox) potential ([Formula: see text]) variations across a conducting body below the seafloor. We approximate the source as being two dimensional and find the most probable locations of line sources by an image reconstruction method. Numerical finite-element modeling of more realistic source regions suggests shallow, easterly dipping (∼15°) conductors of 1 Ω.m in the uppermost 2 km.

Distribution of aseismic slip rate on the Hayward fault inferred from seismic and geodetic data

We solve for the slip rate distribution on the Hayward fault by performing a least squares inversion of geodetic and seismic data sets. Our analysis focuses on the northern 60 km of the fault. Interferometric synthetic aperture radar (InSAR) data from 13 independent ERS interferograms are stacked to obtain range change rates from 1992 to 2000. Horizontal surface displacement rates at 141 bench marks are measured using GPS from 1994 to 2003. Surface creep observations and estimates of deep slip rates determined from characteristic repeating earthquake sequences are also incorporated in the inversion. The fault is discretized into 283 triangular dislocation elements that approximate the nonplanar attributes of the fault surface. South of the city of Hayward, a steeply, east dipping fault geometry accommodates the divergence of the surface trace and the microseismicity at depth. The inferred slip rate distribution is consistent with a fault that creeps aseismically at a rate of ∼5 mm/yr to a depth of 4–6 km. The interferometric synthetic aperture radar (InSAR) data require an aseismic slip rate that approaches the geologic slip rate on the northernmost fault segment beneath Point Pinole, although the InSAR data might be complicated by a small dip‐slip component at this location. A low slip rate patch of <1 mm/yr is inferred beneath San Leandro consistent with the source location of the 1868 earthquake. We calculate that the entire fault is accumulating a slip rate deficit equivalent to a Mw = 6.77 ± 0.05 per century. However, this estimate of potential coseismic moment represents an upper bound because we do not know how much of the accumulated strain will be released through aseismic processes such as afterslip.

Remote Sensing and Resistivity Image for the Tectonic Analysis of Biligirirangan Region of Peninsular India

Tectonic mapping and geologic evolution of a region were historically conducted on the basis of field observations. After the advent of remote sensing technology, tectonic mapping became much easier and structural mapping through imagery has gained credibility and suitability in regional mapping, tectonic evolution and modeling. In this regard, an attempt has been made to make use of the satellite technology to bring out the structural trends and the fracture/lineament pattern in Biligirirangan region of Northern Tamil Nadu through satellite remote sensing. The folded architecture in Biligirirangan region has evolved from structural trends. The subsurface structures of the region were derived through resistivity contours and resistivity images created by GIS software. The surficial information derived from the satellite imagery and subsurface structures derived from the resistivity contours and images were integrated with field observations. This integration has given a clear tectonic picture of the Biligirirangan region and the tectonic structures were then analysed for tectonic forces, evolution and modeling. The study determined that the Structural trends were mostly in a N‐S direction and the folds were of isoclinal type with alternating synclinal and anticlinal folds of an easterly dipping nature. These folds in conjunction with the lineaments have brought out the palaeostress pattern of the Biligirirangan region. The study further suggests that the regional compressive force trending in ENE‐WSW direction was responsible for the present configuration of folds. In addition, the study also focuses the efficacy of the satellite imagery in tectonic analysis and the resistivity contours and images for the subsurface structural analysis.

Source parameters of the May 29, 2004 South Korea earthquake (ML 5.2)

AbstractA moderate earthquake with local magnitude 5.2 (ML) occurred off the eastern coast of South Korea on May 29, 2004. This earthquake is one of the largest events near the Korean Peninsula since 1978 when instrumental observations began by the Korea Meteorological Administration. We carried out modeling of regional waveforms and deconvolutions using empirical Green functions to estimate source parameters. The results of the regional waveform modeling show a reverse fault mechanism with a hypocentral depth of 8 km and seismic moment of 5 × 1016N · m, which is equivalent to Mw 5.1. Performing the empirical Green function deconvolution, we estimated a fault radius of 1.1 to 1.4 km and an average stress drop of 13.5 MPa, assuming an east dipping fault plane. Assuming a west dipping fault plane, we estimated a fault radius of 1.3 to 2.9 km and an average stress drop of 6.6 MPa.

Zircon-Rich Ta-Nb-REE Mineralization in the McKeel Lake Pegmatite-Aplite System, Welsford Intrusion, Southwestern New Brunswick

The Ta-Nb-Zr-REE-rich McKeel Lake alkali granite pegmatite-aplite dike swarm is hosted within the late Silurian multiphase alkali granite intrusion (Welsford) near Browns Flats in southwestern New Brunswick. A steep easterly dipping series of pegmatite-aplite dikes trending 170° can be traced 150 m along strike; they are up to 40 cm wide, closely spaced (10–20 cm apart), and interconnected by narrow branching dikelets that generally trend perpendicular to the overall strike direction. The Welsford intrusion has mantle A-type affinities characterized by enrichment of Zr, Ta, Nb, Th, and rare earth elements (REE). Two separate sets of dikes have been identified: (1) an unmineralized fine-grained porphyry, which occurs at depth and seems to be the youngest intrusive phase; and (2) a mineralized pegmatite-aplite dike swarm (419 ± 20 Ma) that crops out at surface. The mineralized pegmatite-aplite dikes likely represent a late magmatic, highly fractionated phase of the Welsford intrusion. XRF and INAA data indicate that the Welsford intrusions and related pegmatite-aplite dikes show elevated levels of Ta (74–220 ppm), Nb (750–3,040 ppm), Zr (1,450–36,000 ppm), Y (540–3,070 ppm), Th (170–870 ppm), U (60–250 ppm), Ce (430–1,950 ppm) and Yb (110–290 ppm). Electron microprobe analysis of two samples identified Ta-Nb- and REE-rich phases: euxenite, fergusonite, aeschynite, (Y)-aeschynite, and a REE carbonate.

Crustal structure in the northern Fossa Magna region, central Japan, modeled from refraction/wide-angle reflection data

The northern Fossa Magna (NFM) is a back-arc rift basin filled with thick Tertiary sediments, which show strong NW-SE shortening deformation. In the NFM, there exist two major active fault systems, the Itoigawa-Shizuoka Tectonic Line active fault system (ISTL) and the Western Nagano Basin active fault system (WNB), both of which have great potentials to cause destructive earthquakes. By reanalyzing five sets of refraction/wide-angle reflection data, we successfully obtained detailed and consistent models of the crustal structure in the NFM region. It was a very effective modeling procedure to incorporate vicinal seismic reflection data and geologic information. The geometries of the active faults in the NFM region were revealed. The ISTL is east dipping, and the WNB is northwest dipping. The Tertiary sedimentary layer (<4.0 km/sec) west and adjacent to the ISTL extends to a depth of 4–5 km. The basement rocks below the Central Uplift Belt (CUB) form a wedge structure, which suggests the westward movement of the CUB basement rocks.

Pan-African high pressure granulites from SE-Kenya: Petrological and geothermobarometric evidence for a polycyclic evolution in the Mozambique belt

Two different Pan-African tectono-metamorphic events are recognised in the Taita Hill Tsavo East National Park/Galana river area, SE-Kenya (Mozambique belt) based on petrographic and geothermobarometric evidence. Structurally, this area can be subdivided into four units: (1) the easternmost part of the basement along the Galana river is characterized by subhorizontal slightly to the west and east dipping foliation planes. Migmatic paragneisses with intercalated marbles, calcsilicates and metapelites and bands of amphibolites are the dominant rock type. (2) The western part of the Galana river within the Tsavo East National Park is a ca. 25 km wide shear zone with subvertical foliation planes. The eastern part shows similar rocks as observed in unit 1, while towards west, metasedimentary units become rare and the main rock types are tonalitic gneisses with intercalated amphibolites. (3) A 10 km wide zone (Sagala Hills zone) between the strike slip zone (unit 2) and the Taita Hills (unit 4) is developed. This zone is characterized by elongated and folded felsic migmatic amphibole and garnet bearing orthogneiss bodies with intercalated bands of mafic rocks. (4) The Taita Hills are a slightly to the N dipping nappe stack. The main rock type in the Taita Hills are amphibole–biotite–plagioclase–quartz ± garnet ± clinopyroxene ± scapolite bearing migmatic gneisses with mafic bands. In the southern part, metapelites, marbles and some amphibolites are common. Although the geological structures are different in units 1 and 2, the calculated PT conditions are similar with peak PT of 760–820 °C and 7.5–9.5 kbar. Temperatures in unit 3 (Sagalla Hills zone) and unit 4 (Taita Hills) are slightly higher ca. 760–840 °C, but pressure is significantly higher, ranging from 10 to 12 kbar. Sillimanite growth around kyanite, garnet zonation pattern, mineral reaction textures, and PT calculations constrain a “clock-wise” PT-path with near isobaric cooling following the peak of metamorphism. The different PT conditions, tectonic setting, and a different age of metamorphism are evidence that units 1 and 2 (Galana river) belong to a different tectono-metamorphic event than unit 3 (Sagala Hills zone) and 4 (Taita Hills). The major shear zone (unit 2) marks a tectonic suture dividing the two different tectono-metamorphic domains. It is also likely that it played an important role during exhumation of the granulite facies rocks from units 3 and 4.

K–Ar dating of fault gouge in the northern Sydney Basin, NSW, Australia—implications for the breakup of Gondwana

The occurrence of synkinematic and authigenic clay minerals is a common feature in fault gouges. Few attempts have been made to date fault gouges. We present the first age data in Australia for synkinematic illite–smectite growth in two fault zones of the northern Sydney Basin, NSW. The faults occur at Burwood Beach, NSW in the northern part of the Sydney Basin and are hosted by Early Permian siltstones, tuffs and coals of the Lambton Formation, Newcastle Coal Measures. The faults are 1.5 m apart, show normal displacement and trend N–S with steep easterly dips. Foliated gouge zones, comminution and dilational breccias are developed along both fault surfaces. K–Ar ages extracted from samples in the gouge and tuffs in the damage zones are 172 (6–10 μm) to 119 Ma (<0.4 μm), respectively. Older ages of 272–281 Ma for the coarse fractions (>2 μm), 237–245 Ma for the <2 μm fraction, 218 Ma for the <0.4 μm fraction and 196 Ma for the <0.1 μm fraction have been obtained from siltstones within and outside the damage zone. We believe the younger ages of 196–237 Ma indicate the time at which diagenetic illite–smectite formed and the 122–150 Ma dates from the <2 μm fraction represent the maximum age of gouge formation. The younger ages are thought to reflect the last slip event occurring on the faults, which is related to the rifting and dispersal of the eastern margin of the Australian continent.

Deep seismic reflection profiling in the Archaean northeastern Yilgarn Craton, Western Australia: implications for crustal architecture and mineral potential

Deep seismic reflection data across the Archaean Eastern Goldfields Province, northeastern Yilgarn Craton, Western Australia, have provided information on its crustal architecture and on several of its highly mineralised belts. The seismic reflection data allow interpretation of several prominent crustal scale features, including an eastward thickening of the crust, subdivision of the crust into three broad layers, the presence of a prominent east dip to the majority of the reflections and the interpretation of three east-dipping crustal-penetrating shear zones. These east-dipping shear zones are major structures that subdivide the region into four terranes. Major orogenic gold deposits in the Eastern Goldfields Province are spatially associated with these major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world-class gold deposits plus many smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed metallogenically as the Laverton structure. The seismic reflection data have also imaged a series of low-angle shear zones within and beneath the granite–greenstone terranes. Where the low-angle shear zones intersect the major crustal-penetrating structures, a wedge shaped geometry is formed. This geometry forms a suitable fluid focusing wedge in which upward to subhorizontal moving fluids are focused and then distributed into the nearby complexly deformed greenstones.

Geoelectric structure of the Dharwar Craton from magnetotelluric studies: Archean suture identified along the Chitradurga-Gadag schist belt

SUMMARY Broad-band magnetotelluric data were collected at 50 stations over a 400 km long, approximately east‐west profile over the granite‐greenstone terrain of Dharwar, southern India. The tensor decomposed data were interpreted using a 2-D inversion scheme. The geoelectric model is suggestive of a suture along the Chitradurga‐Gadag schist belt, formed by the thrusting of the West Dharwar Craton beneath its eastern counterpart, with an easterly dip of 20‐30 ◦ . The thrust proposed here pre-dates the formation of these schists, which occurred during the Late Archean (2600 Ma). The accretionary wedge of the thrust and the depressed part of the West Dharwar Craton may have controlled the emplacement of the Chitradurga‐Gadag and Shimoga‐Dharwar schists. The subsequent weathering, the several episodes of tectonic activity witnessed during the Precambrian and the vertical block movements caused during the passage of the Indian Plate over the Reunion hotspot may have modified the crust, leading to the present-day geological configuration. Despite its age and several tectonothermal episodes, the signature of this thrust is adequately preserved in the Dharwar Craton. Several similarities with younger sutures, as is evident from the observed relics of the oceanic rocks present along the Chitradurga schist belt, suggest that the tectonic processes leading to this Archean event may have had a close resemblance to those witnessed in recent times. Magnetotelluric studies also image a zone of low resistivity at a depth of 40 km beneath the west Dharwar Craton. This seems to be a regional feature, extending to the north over a distance of at least 250 km beneath the Deccan volcanics. The low heat flow values and the high density associated with this feature make partial melting an unlikely explanation for the low resistivity. The grain boundary graphites and the sulphides deposited in the form of pyrites may have caused the low resistivity in the lithospheric mantle of the West Dharwar Craton, although the fluids generated and trapped in the mantle during the passage of the Indian Plate over the Reunion hotspot in the waning phase of its outburst could also be a possibility. The asthenosphere is delineated at a depth of about 100 km beneath the East Dharwar Craton.

Geochemistry of North Shore Hypabyssal dikes and sills in the Midcontinent Rift of Minnesota: an example — the 47th Avenue sill

This study presents geochemical data for several of the numerous small to large dikes and sills, including the 47th Avenue sill, exposed along the shore of Lake Superior in and north of Duluth, Minnesota. These intrusions are late magmatic features of the Proterozoic Midcontinent Rift System and together form the North Shore Hypabyssal Group. The dikes are geochemically distinct from the sills, and, when the two are exposed together, the younger dike intrudes the older sill. Dikes are primitive with Mg# up to 68, have positive εNd values, and are oriented approximately north–south with steep westerly or near vertical dips. The older sills are more evolved, usually have εNd values near or below 0, and have the same gentle easterly dip as the thick sequence of North Shore Volcanic Group flows they intrude. Dike compositions correlate best with a mixture of widespread basalt compositions types 4 and 5, with primitive geochemistry and positive εNd values, whereas sill compositions are similar to widespread basalt composition type 4 typical of most North Shore Volcanic Group flows. The 47th Avenue sill in Duluth is an evolved single intrusion North Shore Hypabyssal Group diabase sill with trough banding, sharp lower and upper contacts, and a spectacular fractured and undulating roof zone containing blocks of the overlying ferroandesite flow.

Seismic evidence for Late Proterozoic orogenic structures below the Phanerozoic sedimentary cover in the Kattegat area, SW Scandinavia

Commercial seismic reflection profiles from SW Scandinavia image Sveconorwegian orogenic structures in the late Precambrian basement beneath the thin Phanerozoic cover. We interpret (1) pronounced west to WNW dipping reflections as Sveconorwegian localized ductile shear which overprint; (2) weaker east dipping reflections, interpreted as Gothian or early Sveconorwegian compressional structures. Folded and laminar seismic fabrics may represent F and C type crystalline thrust sheets emplaced during the Gothian and Sveconorwegian events. Some of the dipping reflections are observed to sole into (3) an interval of subhorizontal reflections at 2.5–4.5 s two‐way travel time, which we interpret as a “fossil” ductile to brittle transition that was unroofed during the Gothian Orogeny. (4) NW‐SE striking thrusts cutting the Sveconorwegian structures may indicate a shift in the late Sveconorwegian stress field. They may represent an early stage in the subsequent Phanerozoic basin evolution in the Danish area.

Miocene extension and extensional folding in an anticlinal segment of the Black Mountains accommodation zone, Colorado River extensional corridor, southwestern United States

Recent studies demonstrate that rifts are characterized by linked tilt domains, each containing a consistent polarity of normal faults and stratal tilt directions, and that the transition between domains is typically through formation of accommodation zones and generally not through production of throughgoing transfer faults. The mid‐Miocene Black Mountains accommodation zone of southern Nevada and western Arizona is a well‐exposed example of an accommodation zone linking two regionally extensive and opposing tilt domains. In the southeastern part of this zone near Kingman, Arizona, east dipping normal faults of the Whipple tilt domain and west dipping normal faults of the Lake Mead domain coalesce across a relatively narrow region characterized by a series of linked, extensional folds. The geometry of these folds in this strike‐parallel portion of the accommodation zone is dictated by the geometry of the interdigitating normal faults of opposed polarity. Synclines formed where normal faults of opposite polarity face away from each other whereas anticlines formed where the opposed normal faults face each other. Opposed normal faults with small overlaps produced short folds with axial trends at significant angles to regional strike directions, whereas large fault overlaps produce elongate folds parallel to faults. Analysis of faults shows that the folds are purely extensional and result from east/northeast stretching and fault‐related tilting. The structural geometry of this portion of the accommodation zone mirrors that of the Black Mountains accommodation zone more regionally, with both transverse and strike‐parallel antithetic segments. Normal faults of both tilt domains lose displacement and terminate within the accommodation zone northwest of Kingman, Arizona. However, isotopic dating of growth sequences and crosscutting relationships show that the initiation of the two fault systems in this area was not entirely synchronous and that west dipping faults of the Lake Mead domain began to form between 1 m.y. to 0.2 m.y. prior to east dipping faults of the Whipple domain. The accommodation zone formed above an active and evolving magmatic center that, prior to rifting, produced intermediate‐composition volcanic rocks and that, during rifting, produced voluminous rhyolite and basalt magmas.

Paleomagnetic constraints on deformation models for uppermost oceanic crust exposed at the Hess Deep Rift: Implications for axial processes at the East Pacific Rise

Studies of oceanic crust exposed in tectonic windows and in ophiolites have revealed the importance of normal faulting and attendant tilting of upper crustal rock units in the accretion process at oceanic spreading centers. We present paleomagnetic remanence data from 45 fully oriented samples from dikes, gabbros and a small number of basaltic lavas from fast spread crust exposed along the Hess Deep Rift. Over ∼25 km along this escarpment, dikes and dike‐subparallel fault zones dip consistently away from the East Pacific Rise (EPR) while lava flows dip toward the ridge. Underlying gabbro is less deformed but contains widely spaced, low‐angle fractures, tentatively interpreted as shear zones. As expected from the crustal age (∼1.07–1.48 Ma), most remanence data indicate reversed polarity magnetization and are compatible with the expected range of secular variation at the site. Overly steep and directionally scattered gabbro remanence and observed low‐angle shear structures within this unit are tentatively interpreted as the manifestation of three‐dimensional strain along anastomosing shear zones. Although some remanence directions are incompatible with any plausible deformation history, and thus likely reflect orientation errors, the overall data set is consistent with a model involving sequential rotations on (1) outward dipping, EPR‐parallel (∼N‐S) normal faults and (2) Hess Deep Rift‐parallel (∼E‐W) normal faults Average rotations for these sequential events are 22° to the east (defined by the mean dike attitude) and 10° to the south (estimated by bathymetry), respectively. This model best explains the remanence data, observed dikes and lava orientations, presence of dike‐parallel fault zones, and the observation of steep, little deformed dikes cutting both east dipping dikes and faults. The data support a structural model for spreading at the EPR in which outcrop‐scale faulting and rotation is linked to subaxial subsidence and to consequent development of dominantly outward facing normal faults close to the spreading axis. Because these faults form within the neovolcanic zone, they are subject to burial and are expected to have subdued to little surface expression.

Seismic imaging of a convergent continental margin and plateau in the central Andes (Andean Continental Research Project 1996 (ANCORP'96))

A 400‐km‐long seismic reflection profile (Andean Continental Research Project 1996 (ANCORP'96)) and integrated geophysical experiments (wide‐angle seismology, passive seismology, gravity, and magnetotelluric depth sounding) across the central Andes (21°S) observed subduction of the Nazca plate under the South American continent. An east dipping reflector (Nazca Reflector) is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km. We interpret parts of the Nazca Reflector as a fluid trap located at the front of recent hydration and shearing of the mantle, the fluids being supplied by dehydration of the oceanic plate. Patches of bright (Quebrada Blanca Bright Spot) to more diffuse reflectivity underlie the plateau domain at 15–30 km depth. This reflectivity is associated with a low‐velocity zone, P to S wave conversions, the upper limits of high conductivity and high Vp/Vs ratios, and to the occurrence of Neogene volcanic rocks at surface. We interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during Neogene shortening and plateau growth. The imaging properties of the continental Moho beneath the Andes indicate a broad transitional character of the crust‐mantle boundary owing to active processes like hydration of mantle rocks (in the cooler parts of the plate margin system), magmatic underplating and intraplating under and into the lowermost crust, mechanical instability at Moho, etc. Hence all first‐order features appear to be related to fluid‐assisted processes in a subduction setting.

Transpressive duplex and flower structure: Dent Fault System, NW England

Revised mapping along the Dent Fault (northwest England) has improved the resolution of folds and faults formed during Variscan (late Carboniferous) sinistral transpression. A NNE-trending east-down monocline, comprising the Fell End Syncline and Taythes Anticline, was forced in Carboniferous cover above a reactivated precursor to the Dent Fault within the Lower Palaeozoic basement. The Taythes Anticline is periclinal due to interference with earlier Acadian folds. The steep limb of the monocline was eventually cut by the west-dipping Dent Fault. The hangingwall of the Dent Fault was dissected by sub-vertical or east dipping faults, together forming a positive flower structure in cross-section and a contractional duplex in plan view. The footwall to the Dent Fault preserves evidence of mostly dip-slip displacements, whereas strike-slip was preferentially partitioned into the hangingwall faults. This pattern of displacement partitioning may be typical of transpressive structures in general. The faults of the Taythes duplex formed in a restraining overlap zone between the Dent Fault and the Rawthey Fault to the west. The orientations of the duplex faults were a response to kinematic boundary conditions rather than to the regional stress field directly. Kinematic constraints provided by the Dent and neighbouring Variscan faults yield a NNW–SSE regional shortening direction in this part of the Variscan foreland.