Northeast Trending Research Articles

Ground Cracks Associated with the 1994 Double Spring Flat Earthquake, West-Central Nevada

The 1994 Double Spring Flat earthquake ( M W 5.8) occurred within a densely faulted step-over between the Genoa and Antelope Valley faults, two principal normal faults of the transition zone between the Basin and Range Province and the northern Sierra Nevada. The earthquake created zones of ground cracks from 0.1 to 2.8 km long along at least five northwest- to north-northwest-striking faults in the epicentral area. Individual cracks had extensional openings generally from 1 to 10 mm wide. No cracks displayed obvious vertical separation, and only one zone showed permissive evidence of right-lateral separation. Over the 8 days following the mainshock (the period over which the cracks were found), aftershocks formed a dominant northeast trend suggesting the earthquake occurred along a northeast-striking structure. However, no ground breakage was found along faults striking parallel to this northeast aftershock alignment, and subsequent aftershocks formed a conjugate northwest trend. Based on the location and character of the five zones, the observed cracks are attributed to secondary fault slip and shaking effects. The earthquake also created ground cracks along at least two faults 15-25 km from the epicenter. In both of these cases, the faults had documented histories of prior ground cracking, indicating that they are particularly susceptible to such triggered deformation. Manuscript received 21 August 2002.

U–Pb Geochronology of Paleoproterozoic Rocks in the Southern Part of the Kedougou-Kéniéba Inlier, Senegal, West Africa: Evidence for Diachronous Accretionary Development of the Eburnean Province

The Kedougou-Kéniéba Inlier (KKI) in eastern Senegal is the westernmost exposed part of the Paleoproterozoic (Birimian/Eburnean) Baoulé-Mossi domain on the West African craton. Regional mapping indicates that the southern, relatively well preserved part of the KKI contains two northeast trending volcanic belts, the Mako and Falémé belts with associated Na-rich granitoid stocks. These belts are separated by the Diale-Dalema metasedimentary basin, containing the extensive syn- to late-kinematic K-rich Saraya batholith and other basin-type plutons. A similar pattern of northeast trending volcanic belts and sedimentary basins is also seen in Ghana and eastern Côte d'Ivoire, i.e. in the eastern part of the Baoulé-Mossi domain, as well as in central Côte d'Ivoire where, however, primary lithological relationships seem to be more obliterated by later tectono-thermal overprint. U–Pb ages were determined on single zircon and monazite from rocks in the KKI to investigate its relationship with the rest of the Baoulé-Mossi domain. A rhyolite from the Falémé belt gave a zircon age of 2099±4 Ma. Granitoid rocks from the Falémé (2) and Mako (1) volcanic belts possess zircon ages of 2082±1, 2080±1 and 2076±3 Ma, respectively. Reversely discordant monazites from a muscovite-bearing phase of the Saraya batholith in the Diale-Dalema basin yielded an interpreted igneous age of 2079±2 Ma and a metamorphic age of 2064±4 Ma. Detrital zircons from a quartz wacke in the Diale-Dalema basin all gave the same age within error, defining an average age of 2165±1 Ma for the source. Inherited zircon, 2155±34 Ma in age, was seen in the felsic flow from the Falémé belt while the Mako belt granitoid rock contains inheritance at least as old as 2122±5 Ma. These data indicate that volcanic belts in the KKI are substantially younger than the 2150–2190 Ma Birimian volcanic and associated plutonic rocks of the eastern Baoulé-Mossi domain, despite their geological similarities. Syn- to late-kinematic basin-type plutons in the KKI are also younger than lithologically similar 2090–2115 Ma old plutons in Ghana and eastern Côte d'Ivoire (eastern part of the Baoulé-Mossi domain). These relationships are consistent with previous suggestions that Paleoproterozoic rocks of the Baoulé-Mossi domain may be divided into an older eastern subprovince and an approximately 50–100 Ma younger western subprovince. They also show that supracrustal and intrusive Paleoproterozoic rocks in both subprovinces as well as over the entire domain developed diachronously from east to west. The data support an accretionary tectonic model for Paleoproterozoic growth of the West African craton.

Relation of Carlin-type gold mineralization to lithology, structure and alteration: Screamer zone, Betze-Post deposit, Nevada

An evaluation of the relation between geologic features and gold grade has been undertaken in the Screamer zone of the Betze-Post deposit, the largest Carlin-type gold deposit in the world. Screamer, which forms the western end of Betze-Post, is a stratiform ore zone that is hosted almost entirely by the wispy member of the Popovich Formation. The wispy member is overlain by the planar member of the same formation and underlain by the Roberts Mountains Formation, both of which contain much smaller amounts of gold mineralization in the Screamer zone. Geologic features recognized at Screamer reflect the long history of events that have affected the area since mid-Paleozoic time. The two earliest events at Screamer were probably sulfidation and formation of flat fractures, and the two latest events were pyritization and formation of kaolinite-filled fractures, which took place at least partly during gold mineralization. Gold mineralization at Screamer shows ambiguous relations to most lithologic, alteration and structural features. For instance, gold grades in the wispy member of the Popovich Formation are slightly higher in rock units that contain the largest proportion of debris flow material, but this relation does not account for most ore. The correlation of gold grades with fracture density is also generally poor, but gold grades are slightly higher in samples containing vertical and/or kaolinite-bearing fractures, particularly those with a northeasterly trend. Most early veins at Screamer are horizontal and most later veins are vertical, suggesting that the long-term trend of fluid pressures in the area has been from lithostatic to hydrostatic. Finally, gold grades correlate poorly with silicification and decarbonatization, and although some gold appears to be associated with rock containing kaolinite, much is not. The only rock feature that shows a direct relation to gold grade is pyrite. Some pyrite formed by sulfidation of the original rock, but much of it formed by addition of pyrite to the rock. Stability limits for ore and alteration assemblages at Screamer indicate that pressures of mineralization were probably not less than about 300 bar, which require depths of about 3 km for the hydrostatic conditions that probably prevailed in the Betze-Post hydrothermal system.

Spatial association of mineral occurences and curvilinear geological features

Qualitative and quantitative knowledge about the spatial association between mineral occurrences and geological features are important in mineral potential mapping. Two existing methods for quantifying spatial association between mineral occurrences and curvilinear geological features are applied to the Baguio district of the Philippines. An experimental method is described and applied to the study area as well. The results of the three methods are highly similar, which demonstrates the effectiveness of the experimental method presented here for quantifying spatial association between mineral occurrences and curvilinear geological features. It is shown that gold occurrence in the Baguio district are strongly spatially associated with northeasterly trending faults/fractures rather than with northwesterly trending faults/fractures. It is also shown that the spatial association between the gold occurrence and older batholithic intrusives is stronger than the spatial association between gold occurrence and younger porphyry intrusives. These spatial geoinformation characteristics can be used as evidential data layers in GIS-based mineral potential mapping.

Pan-African intraplate deformation in the northern Prince Charles Mountains, east Antarctica

New structural and metamorphic data coupled with U–Pb SHRIMP zircon and Rb–Sr step-leach biotite ages help constrain a period of Early Palaeozoic (Pan-African) deformation recognised in the northern Prince Charles Mountains, east Antarctica. This period of deformation is accommodated along discrete northeast trending mylonites that preserve up-dip reverse kinematics with dominantly southeast over northwest vergence. Ambient P–T conditions of 524±20°C and 7.6±4 kbar accompanied deformation. This phase of deformation post-dated the intrusion of planar felsic dykes that yield ages of c. 550 Ma and pre-dated Rb–Sr biotite ages of c. 475 Ma that record cooling of the terrane below c. 300°C. These mylonites are identical in age to continental collisional events recognised in the southern Prince Charles Mountains and Prydz Bay, which lie to the south and east of the northern Prince Charles Mountains, and similar in age to orogenesis recognised to the west in Lützow-Holm Bay. These belts represent sutures between the component lithospheric blocks of east and west Gondwana. The northern Prince Charles Mountains lie between these sutures. Consequently, the mylonites we report here are interpreted to have formed in an intraplate setting and developed in response to stresses applied along the plated margins as a consequence of continental collision during the amalgamation of Gondwana.

Primary centers and secondary concentrations of tectonic activity through time in the western hemisphere of Mars

Five main stages of radial and concentric structures formed around Tharsis from the Noachian through the Amazonian as determined by geologic mapping of 24,452 structures within the stratigraphic framework of Mars and by testing their radial and concentric orientations. Tectonic activity peaked in the Noachian (stage 1) around the largest center, Claritas, an elongate center extending more than 20° in latitude and defined by about half of the total grabens which are concentrated in the Syria Planum, Thaumasia, and Tempe Terra regions. During the Late Noachian and Early Hesperian (stage 2), extensional structures formed along the length of present‐day Valles Marineris and in Thaumasia (with a secondary concentration near Warrego Vallis) radial to a region just to the south of the central margin of Valles Marineris. Early Hesperian (stage 3) radial grabens in Pavonis, Syria, Ulysses, and Tempe Terra and somewhat concentric wrinkle ridges in Lunae and Solis Plana and in Thaumasia, Sirenum, Memnonia, and Amazonis are centered northwest of Syria with secondary centers at Thaumasia, Tempe Terra, Ulysses Fossae, and western Valles Marineris. Late Hesperian/Early Amazonian (stage 4) structures around Alba Patera, the northeast trending alignment of Tharsis Montes, and Olympus Mons appears centered on Alba Patera. Stage 5 structures (Middle‐Late Amazonian) represent the last pulse of Tharsis‐related activity and are found around the large shield volcanoes and are centered near Pavonis Mons. Tectonic activity around Tharsis began in the Noachian and generally decreased through geologic time to the Amazonian. Statistically significant radial distributions of structures formed during each stage, centered at different locations within the higher elevations of Tharsis. Secondary centers of radial structures during many of the stages appear related to previously identified local magmatic centers that formed at different times and locations throughout Tharsis.

Velocity field across the Southern Caribbean Plate Boundary and estimates of Caribbean/South‐American Plate Motion using GPS Geodesy 1994–2000

Global Positioning System (GPS) observations between 1994 and 2000 at twenty‐two sites in the Lesser Antilles and northern South‐America indicate that the Caribbean plate, along its southern boundary, slips at a rate of 20.5±2 mm/a with an azimuth of N 84°±2°E at 65°W, relative to the South‐American plate. East of 68° W, 80% of the dextral slip is contained within a 80‐km wide shear zone centered on the El Pilar‐San Sebastián fault system. West of 68° W the plate boundary broadens to more than 300 km with dextral shear shared between the northeast trending Boconó fault (9–11 mm/a) in western Venezuelan, and an offshore system near the northern coast.

Interpretation of magnetic anomalies over the Waimangu geothermal area, Taupo volcanic zone, New Zealand

The study area is located on the eastern side of the Taupo volcanic zone in central North Island of New Zealand. It lies a few kilometres to the southwest of Mt Tarawera, the site of the biggest New Zealand volcanic eruption in historical times (the June 1886 Tarawera eruption). The study area includes the Waimangu geothermal field and a small part of Waiotapu and Waikite fields. The extensive surface thermal expressions (boiling springs, hot lakes, craters, and sinter terraces) occurring at the Waimangu field were all formed following the 1886 Tarawera eruption. Another large area of less intense thermal manifestations (thermal ground and hydrothermally altered rocks) exists about 5 km southwest of Waimangu, extending towards the Waiotapu field in the south. In 1993 an aeromagnetic survey was conducted over the study area at an average altitude of about 350 m above the ground. The results show a subdued negative residual anomaly (about −100 nT) over the Waimangu field, which can be interpreted by near-surface hydrothermal demagnetisation of rhyolitic host rocks. The lateral distribution of the demagnetised rocks is much greater than the thermal area of Waimangu, and is consistent with the extent of low resistivity rocks across the study area. The magnetic interpretation also shows that two-high standing dacite domes situated about 5 and 7 km to the southwest of Waimangu have been affected by hydrothermal demagnetisation. There are negative residual anomalies outside the low resistivity zone that could be associated with reversely magnetised rocks (age >0.78 Ma). A strong positive residual anomaly (up to 450 nT) occurs to the east of the Waimangu field. Results from 3-D magnetic interpretation indicate some alternative models for this positive anomaly: (1) southwest–northeast trending, vertical basalt dykes (magnetisation 10 A/m), tops between −0.1 and −0.65 km RL (reduced level=relative to sea level), (2) a thick (⩾ 1 km) sequence of rhyolites (magnetisation 2.5 A/m) extending from the surface down to about −0.8 km RL, and (3) a rather thin (0.35 km) sequence of rhyolites (from surface to sea level) underlain by basalt bodies similar to those of model (1).

A reappraisal of the Namibian Damara stratigraphy in part of the Southern Swakop Terrane and its implications to basin evolution

The basin evolution of the Panafrican belts in Namibia was controlled by major rift systems comprising the Kaoko-West Congolian and Damara Branches with their associated parallel rift zones. Rifting along the Kaoko Branch caused continental spreading, while the Damara Branch formed the terrestrial Otjiwarongo and Windhoek Rift Zones, separated by the Usakos Horst, in which the rift related coarse clastics (Etusis Formation) and associated basinal sediments (Khan Formation) were deposited. Thermal subsidence along the Otjiwarongo Rift Zone produced an oceanic basin with thinned continental crust. In the Windhoek Rift Zone, terrestrial conditions prevailed and the newly deposited rift sequences were subjected to erosion. Succeeding the primary rifting was the Chuos Formation glacial/marine event, which produced thick deposits along the northern and western margins of the Windhoek Rift Zone. After the glaciation, a shelf carbonate unit was deposited in the slow subsiding Otjiwarongo Rift Zone. In the Windhoek Rift Zone, however, northeast trending faulting along the Okahandja Lineament produced the submerged Okawakuejawa rift unit in which the Adler dolomite and Quelle calcareous turbidites were deposited. The Okawakuejawa rift unit also divided the Windhoek Rift Zone into a northern Usakos Horst and southern Uitdraai Block. Renewed tectonism and uplift produced rifts splaying from the Okahandja Lineament northward to trend parallel to the Abbabis-Welwitchia Lineament. On the Usakos Horst, basaltic volcanism was associated with the rifting, while those on the Uitdraai block was more fissure like, non fault-related extrusions. The graben floors were submerged while the scarp shoulders were still areal as water laid and arid, non-reworked successions were deposited on them respectively. Further south in the Windhoek Rift Zone, continental separation caused the cessation of the Omusema volcanism and thermal subsidence of the entire Windhoek Rift Zone with dolomite platforms being formed on the Usakos Horst and Uitdraai block terranes. Beneath the platform edges, continuing subsidence created peri-platforms and below the Usakos Horst influx of clastic material produced the Tinkas Formation comprising the alternating dolomite and pelite sequence of the Kudulecke Member and dolomite/pelite turbiditic flows and slump deposits, in incised canyons, of the Jacobs Member. On the Uitdraai block, the Flachen quartzites were deposited onto the platform dolomite, while the Lievental Member dolomitic turbidites were deposited on the slopes. Continuing subsidence caused the north and eastward transgression of the pelitic turbidites of the Fahlwater Formation, which eventually covered the entire Windhoek Rift Zone area. Rifting of a Kibaran-aged continent produced a spreading Kaoko Branch with a subsidiary splayed Damara Branch in which the Rift Zones developed at different rates. The northern Otjiwarongo Rift Zone produced a submarine basin underlain by continental crust, while the southerly Windhoek Rift Zone stayed terrestrial. In the Windhoek Rift Zone, rejuvenation along the Okahandja Lineament lead to renewed rifting ending in continental spreading and thermal subsidence forming a passive continental margin.

Quaternary stratigraphy and history of the Ootsa Lake - Cheslatta River area, Nechako Plateau, central British Columbia

Erosion along the Nechako Reservoir and Cheslatta River Spillway has resulted in unusually well-exposed late Quaternary and Holocene stratigraphy. Surficial sediments in the study area are mostly products of Late Wisconsinan glaciation. However, evidence for pre-Late Wisconsinan sedimentation has been found along the shores of the Nechako Reservoir, including till of an older glaciation and organic-bearing, blue-grey, lacustrine sediments of probable Middle Wisconsinan age. Stratigraphic correlation of the lake sediments suggests that an extensive lake system occurred in the region during the Olympia Nonglacial Interval. Late Wisconsinan ice initially moved along major valleys, with glaciofluvial outwash deposited in front of the advancing ice. Advance-phase glaciolacustrine sediments are rare but significant, as slope failures are spatially associated with areas where they are preserved. The distribution of these sediments and associated deltaic deposits indicates that advance-phase glacial lakes occurred up to approximately 855 m asl, at least several metres above the modern reservoir level. Sediments deposited in front of the ice margin were overridden during ice advance and are best preserved in large valleys. At the glacial maximum, ice flowed northeasterly throughout the study region. Crag and tails, flutings, and drumlinoid ridges with a generally consistent northeast trend are the dominant landforms. Till is the most common Pleistocene surficial sediment, covering approximately 80% of the area; large areas of exposed bedrock are rare. Late-glacial glaciofluvial and Holocene fluvial deposits are uncommon and occur mainly along the Cheslatta River valley.

Electron-microprobe monazite dating of ca. 1.71–1.63 Ga and ca. 1.45–1.38 Ga deformation in the Homestake shear zone, Colorado: Origin and early evolution of a persistent intracontinental tectonic zone

The Homestake shear zone, one of the principal Precambrian structures within the Colorado mineral belt, has a history of tectonism that extends from the Proterozoic to the Tertiary. New field mapping, microstructural analysis, and electron-microprobe U-Th-Pb monazite dates define the style, kinematics, and timing of multistage Proterozoic ductile deformation. Early subhorizontal shearing (D1) progressed to northwest-southeast crustal shortening (D2) during a protracted ca. 1710–1630 Ma orogeny involving high-temperature metamorphism, partial melting, and emplacement of the ca. 1.7 Ga Cross Creek batholith. The subvertical, northeast trend of the shear zone was established by D2 shortening. A major episode of simple-shear displacement occurred ca. 1.4 Ga during emplacement of the St. Kevin batholith. Complex kinematics are recorded by 1380 Ma, southeast- side-down, dextral mylonite zones (D3) and post–1380 Ma southeast-side-up, dextral ultramylonite-pseudotachylyte zones (D4) that were both localized in D2 foliation domains. The succession of ductile tectonites in the Homestake records >300 m.y. of episodic deformation at progressively shallower crustal levels as Colorado evolved from an active accretionary margin (1.8–1.7 Ga), through crustal assembly (1.7–1.6 Ga), to a tectonically active continental interior (post 1.45 Ga). The Homestake shear zone constitutes a classic example of ductile shear-zone reactivation and the development of persistent tectonic zones in the continental lithosphere.

A New Understanding of the Provinces of the Amazon Craton Based on Integration of Field Mapping and U-Pb and Sm-Nd Geochronology

New conventional and sensitive high-resolution ion microprobe zircon U-Pb dating has led to a new understanding of the subdivision and evolution of the Amazon Craton during Precambrian time, with major improvements and changes made to the previous Rb-Sr based model. The interpretation of U-Pb and Sm-Nd isotopic data identifies eight main Precambrian tectonic provinces in the Craton, with ages ranging from 3.1 to 0.99 Ga. Some of the provinces were generated by accretional, arc-related processes (Carajás, Transamazonic, Tapajós-Parima and Rondônia-Juruena) and others by recycling of continental crust (Central Amazon, Rio Negro and Sunsas). The exposed Archean crust is restricted to the east (Carajás and south Amapá in Brazil) and north (Imataca in Venezuela) of the craton, indicating that the Amazon Craton is largely a Proterozoic crust. The Carajás-Imataca (3.10–2.53 Ga) and Transamazonian (2.25–2.00 Ga) Provinces are composed predominantly of granite-greenstone terranes. The Tapajós-Parima (2.10–1.87 Ga) and Rondônia-Juruena (1.75–1.47 Ga) Provinces represent new crust added as orogenic belts, while the Rio Negro (1.86–1.52 Ga) and Sunsas (1.33–0.99 Ga) Provinces originated mainly by magmatic-tectonic recycling of the above two orogenic belts. The only zone with a prominent northeast trend is the poorly known K'Mudku Shear Belt, characterized by a ∼1.20 Ga shear zone which deforms the rocks of at least three different provinces (Rio Negro, Tapajós-Parima and Transamazonic). The Central Amazon Province comprises mostly Orosirian volcano-plutonic rocks (Uatumã Magmatism) and is a terrane in which the exposed crustal structure and deformation are pluton-related. The Sm-Nd T DM model ages and ɛ Nd suggest that the Central Amazon Province was generated by the partial melting of Archean continental crust (Carajás Province?), perhaps related to underplating that began at the end of the Tapajós-Parima Orogeny (1.88–1.86 Ga).

Three‐dimensional P wave velocity structure in the New Madrid seismic zone

A three‐dimensional P wave velocity model for the New Madrid seismic zone (NMSZ) has been developed using a nonlinear travel time tomography method. The inversion involved 709 earthquakes recorded by digital, three‐component Portable Array for Numerical Data Acquisition stations deployed in the NMSZ over the time period 1989 through 1992. Analysis of ray coverage and inversion of a synthetic data set showed that the model has high resolution to a depth of 11 km. Low‐velocity anomalies correlate with the prominent northwest and northeast trending arms of seismicity. Lowest‐velocity anomalies (−8%) occur at the intersection of these arms and are associated with shallow seismicity and a high swarm rate. These low‐velocity regions are interpreted to be the result of increased fluid pressure associated with structurally disrupted rocks beneath the Blytheville‐Pascola arch complex. Small regions of high velocity, limited to the upper few km of the crust, correlate with gravity and magnetic anomalies and are interpreted to be igneous intrusions. High‐velocity anomalies also parallel the edges of the Reelfoot rift margin and probably are associated with crystalline Precambrian rocks that form the margin of the Reelfoot rift graben. Most earthquakes are associated with low‐velocity regions and avoid regions of high velocity.

The 1998 edition of the National Geological Map of Botswana

A new National Geological Map of Botswana incorporates data acquired from a variety of sources; the map is produced as a 1:1 million hardcopy as well as in digital format. The new map shows the pre-Kalahari Group geology. The oldest rocks are exposed in eastern Botswana where three Archaean terranes are recognised: the western parts of the Kaapvaal and Zimbabwe Cratons and the western part of the Limpopo Mobile Belt. All three terranes are lithologically similar but differ in their structural styles and in the timing of major thermal events. The oldest (pre-3.0 Gal high-grade metamorphic rocks are found in the Kaapvaal Craton, and the youngest in the Limpopo Mobile Belt, which appears to record Palæoproterozoic ductile shearing. Proterozoic orogenic belts, mostly concealed beneath Karoo rocks, define the western limits of the Archaean terranes and pprogressively young westwards away from the Archaean rocks. The Palwoproterozoic Magondi and Kheis Belts are well-defined by regional magnetic maps, but both are very poorly exposed in Botswana. The Kheis Belt trends due north from South Africa into central Botswana to define the western edge of the Kaapvaal Craton. The western part of the Magondi Belt, as well as all of a Mesoproterozoic (Kibaran) belt and rift are overprinted by the Neoproterozoic Damara Belt; all have pronounced northeasterly trends. During the Palæoproterozoic, there was also significant intraplate magmatism, sedimentation and deformation within the Archæan terranes. Some of the magmatism (in southeastern Botswana) was contemporaneous with, and lithologically similar to, the Bushveld Igneous Complex of South Africa. The main feature of the Mesoproterozoic geology of Botswana is a northeast trending rift that extends right across the northwest of the country and which is partly infilled with ca 1 106 Ma volcanic rocks. Neoproterozoic sedimentary rocks overlie the volcanics within the rift. The various rocks are exposed along the Ghanzi Ridge and to the northeast in the Chobe District. New detailed airborne magnetic surveys in northwest Botswana (Ngamiland) show the detailed geology of the northeast trending inland branch of the Damara Belt and exactly define its northwestern and southeastern boundaries. The southeastern part of the Damara Belt comprises the Mesoproterozoic volcanics of the Kgwebe Formation and the Neoproterozoic Ghanzi Group sedimentary strata. The full extent of the volcanics, and of the three formations recognised in the Ghanzi Group, is shown on the new map. Deformation of these rocks increases to the northwest where they are bounded by the tectono-stratigraphical Roibok Group. To the northwest of the Roibok Group are poorly dated granitoid rocks separated into several units that are locally overlain by carbonate-dominated sequences. A cover sequence of metasedimentary rocks with northnorthwest trending folds lies northwest of the Damara Belt. These sediments may overlie the southernmost part of the Congo Craton in the extreme northwest of Botswana. Neoproterozoic/ Lower Palæozoic sediments of the Nama Group partly infill a foreland basin to the south of the Damara belt in western Botswana. Karoo strata deposited within the Kalahari Basin underlie central Botswana. The distribution of the four major sedimentary groups, as well as of the capping basalts, is shown. The total thickness of the sediments is < 2000 m and the basalts are up to 1000 m in thickness. The sediments comprise a lower sequence (Dwyka and Ecca Groups) related to regional sagging and an upper sequence (Beaufort and Lebung Groups) that succeeded regional uplift that created intra-Karoo unconformities. Karoo sedimentation commenced towards the end of the Carboniferous Period and the basalts were extruded at about 180 Ma before Present. Wherever there have been detailed studies undertaken on the Karoo rocks, they show intense faulting that may or may not mimic structures in the pre-Karoo bedrock. The faulting appears to be post-sedimentation. No evidence was found for growth faults producing abnormal thicknesses of Karoo sediments. It is always possible to correlate the internal stratigraphy, at least at the formational level across the faults. Abnormal thicknesses of the basalts are preserved on the downthrow sides of the major faults. A major dyke swarm coeval with the extrusive basalts trends east-southeast right across north-central Botswana to cut across older structural trends. Over 200 kimberlites are shown on the new map. The kimberlites are distributed throughout Botswana in a number of separate clusters. Most of the kimberlites are of Cretaceous age. Isopachs are shown of the Kalahari Group, which is generally < 180 m in total thickness.

Magnetic evidence for the geometry and evolution of the eastern margin of the East European Craton in the Southern Urals

The eastern margin of the East European Craton (EEC), adjacent to the Middle and Southern Urals, is characterised by broad, large amplitude magnetic anomalies due to magnetic rocks within the crystalline basement. These anomalies most commonly have east to north-east trends and contrast markedly with the north-trending, shorter wavelength anomalies over the Uralide orogen to the east. The trace of a line drawn along the truncation of the characteristic EEC anomalies is offset to the west of the Main Uralian Fault in the Southern Urals, crossing Precambrian units which were subject to relatively modest westward-directed thrusting during the Palaeozoic Uralian orogeny. In contrast, the magnetic models require either a major down-to-the-east displacement of an underlying magnetic basement or juxtaposition of dissimilar basement units. Integration of magnetic modelling of the EEC basement with structural mapping and seismic interpretation confirms that the truncation of the magnetic basement is difficult to explain in terms of structures developed during the Uralian orogeny. There is a correlation between the edge of the magnetic basement and the western limit of deformation associated with an earlier (Late Vendian) orogeny. The feature truncating the crystalline basement may have been initiated during the Vendian event, but our preferred interpretation is that it is an earlier structure, perhaps formed by rifting in Riphean times. The edge of the crystalline basement block subsequently acted as a buttress, significantly influencing later deformation. This margin of the EEC thus appears to have been the focus for rifting and orogenic episodes spanning a time interval of more than 1 Ga.

Petrotectonic evolution of the high- to ultrahigh-pressure Maksyutov Complex, Karayanova area, south Ural Mountains: structural and oxygen isotope constraints

The Maksyutov Complex consists of three fault-bounded lithologic units: a quartzofeldspathic gneiss containing mafic eclogite boudins (Unit #1); a metasedimentary blueschist-facies (Yumaguzinskaya) unit; and a meta-ophiolitic mélange (Unit #2). The geologic history of the high- to ultrahigh-pressure (HP–UHP) assembly of the Maksyutov Complex is complicated by several stages of prolonged retrograde metamorphism and deformation. The Sakmara River exposes all three units near the former village of Karayanova. A structural/petrologic cross-section through the area yields new quantitative data for the complex and, regionally, for the south Urals. Analysis of the Karayanova area has identified the major structures. Regional folding within the complex is parallel to the dominant foliation trending northeast–southwest. Stereonet data show that, during exhumation, this large-scale folding was refolded about axes trending southeast. Unit #1 and the Yumaguzinskaya are tectonically and petrologically distinct units juxtaposed by west-vergent thrusting and recrystallization within the same subduction zone. A shear zone developed later between Unit #2 and the Unit #1+Yumaguzinskaya tectonic package accompanying exhumation. Field relations and petrofabric demonstrate that blueschist-facies recrystallization overprinted an earlier eclogite-facies metamorphism. Thermobarometric measurements yield P– T values of 594–637°C, 1.5–1.7 GPa for eclogite, but these conditions may reflect annealing during the early-stage exhumation at ∼375 Ma. Cuboid graphite aggregates testify to precursor conditions for Unit #1 within the diamond stability field, if such textures are correctly interpreted. Measured 18 O/ 16 O partitioning between pairs of coexisting phases yield three main recrystallization temperature ranges: (1) 678±83°C, attending Unit #1 eclogite-facies metamorphism; (2) 453±17°C, during transitional blueschist/greenschist-facies metamorphism for the amalgamated Unit #1+Yumaguzinskaya+Unit #2 assembly; and (3) 250±68°C, reflecting late-stage hydrothermal alteration and exhumation. Oxygen isotope data for Units #1 and #2 indicate that garnet, blue amphibole, and pyroxene crystallized in isotopic equilibrium, validating previous thermobarometric calculations for a Unit #1 retrograde metamorphic event. Variations in δ 18 O values for phengites suggest the possibility of late metamorphic fluid infiltration. Retrograde recrystallization at high pressure in the presence of fluids and a calculated slow exhumation rate for the Maksyutov Complex account for the fact that inferred UHP coesite and diamond were completely back-reacted during decompression.

Overview of the Geology and Mineral Industry of Nicaragua

This paper is a synthesis and update of the geology of Nicaragua and a discussion of metallic and non-metallic mineral deposits. Nicaragua is divided into 15 departments and 2 autonomous territories, within an area of 130,700 km2. The geologic environments include Paleozoic crystalline basement rocks, Mesozoic sedimentary and intrusive rocks, nearly continuous sedimentary sequences of Late Cretaceous to Pleistocene age, a broad area of Tertiary volcanic rocks, and Quaternary sedimentary and volcanic rocks that include several active volcanoes. These rocks have a dominant continental crust affiliation and are assigned to the Chortis Block, although there is evidence of fragments of oceanic-crust material. The structural framework is controlled largely by the NW-trending Middle America Trench, which marks the boundary between the Cocos and Caribbean plates. This trend is manifest in the Nicaraguan Depression, the chain of volcanoes that extend from western Panama to El Salvador, as well as fold axes within a forearc depositional basin. East of the Nicaraguan Depression, a northeast structural trend becomes important. There are some 94 metallic and 46 nonmetallic mineral deposits within the various geologic environments. Gold is mined from three deposits—Limon, Bonanza, and La Libertad—and limestone, gypsum, aggregates, and clay minerals are mined at six quarries. Nicaragua is the leading gold-producing country in Central America and the Caribbean Basin. Since the late 1930s more than eight million ounces of gold have been mined, mainly from epithermal and mesothermal veins (Limon, Bonanza), skarn zones (Siuna, Rosita), and various placer deposits. Geologic reserves are estimated at 6.5 million ounces. Other mineral commodities include silver, copper, zinc, limestone, aggregates, gypsum, and clay minerals. Brief descriptions of these and several other commodities are provided along with production statistics and general location maps. As a result of political unrest and nationalization of many foreign assets during the 1970s and 1980s, little exploration was conducted. However, the election of a stable, democratic government in the early 1990s and a revision of foreign investment regulations has resulted in an increase in exploration activity; nevertheless, much of the country remains underexplored.

Correlating deformation and metamorphism around orogenic arcs

The timing of metamorphism and the P-T-t path the rocks have undergone commonly varies around orogenic arcs, hindering correlation along orogens. Isotopic dating enables broad correlations to be made, but detailed correlations can be difficult using this approach where several phases of metamorphism are present. This is compounded by the large size of oroclines, which hinders correlation of deformation and metamorphism using a succession of structures and associated metamorphism. However, foliation inflexion-intersection axes (FIAs) preserved in porphyroblasts provide a tool for correlation around oroclinal arcs. Consistent successions of FIAs that remain constantly oriented around folds and across large tracts of multiply ductilely deformed country rock allow correlation of periods of metamorphism taking place during the one direction of horizontal bulk shortening, rather than correlation of inferred growth events. Consequently, rocks that have undergone different P-T-t paths can be correlated. Measurement of FIAs around the spectacular Kimberley Arc in northwest Australia reveals that the first-formed set maintain a consistent average trend of 127° and a subhorizontal plunge. This FIA trend is parallel to the western arm of the arc, but lies at a very high angle to the eastern arm. Foliations preserved as inclusion trails in porphyroblasts are continuous with foliations in the matrix on the western arm, where the FIAs and structural grain are sub-parallel. However, inclusion trails in porphyroblasts defining FIAs with these same trends on the eastern arm are truncated by the matrix foliation. This indicates that foliations in the matrix on the eastern arm postdate all foliations in the matrix on the western arm. Two late sets of locally developed, small kinks of the matrix foliation on the western arm, which have axial planes parallel to foliations on the eastern arm, are weak expressions of the overprinting events that controlled the geometry of the latter arm. A second group of FIAs has been found in some samples on the eastern arm that lie sub-parallel to the northeast trend of this arm. They are defined by inclusion trails that are continuous with the matrix foliation and that occur in the rims of porphyroblasts. Such porphyroblasts generally contain a core with inclusion trails defining the first-formed FIA that lies sub-parallel to the western arm. The inclusion trails in porphyroblast cores are truncated completely by those in the rims, in the same manner that those without this rim-set of inclusion trails are truncated by the matrix foliation. This second set of FIAs has not been found in any samples from the western arm. FIAs thus provide a simple tool for correlating metamorphism around orogenic arcs. The porphyroblasts with consistently oriented FIAs around the arc can be readily correlated and grew during the same period of metamorphism that affected both arms. Those with northeast-trending FIA record a younger period of metamorphism that affected only the eastern arm. Indeed, until techniques and standards for dating porphyroblasts and their inclusions by both the microprobe and ionmicroprobe improve, FIAs may provide the only quantitative tool that integrates metamorphic and structural phenomena and thus allows detailed correlation of these events along an orogen.

ASPECTOS GEOLÓGICOS DE LAS ROCAS METAVOLCÁNICAS Y METASEDIMENTARIAS DEL GRUPO LAVALLEJA, SUDESTE DE URUGUAY

The Lavalleja Group consists on a volcanic-sedimentary sequence, lithologically represented by metapelites, metapsamites, metashales and by interestratified acidic and basic metavolcanic rocks. The Lavalleja Group presents a Ryocke-Abukuma-like metamorphism, varying from very low grade in the northwest to low amphibolite facies in the southeast; it has a northeast trend, showing an important shortening with western vergence. To the east, the Carapé Complex tectonically bound it. The western limit consists on an intrusive-like contact with the alkalic volcanic-subvolcanic Sierra de Las Animas Complex. The Lavalleja Group is part of the Dom Feliciano Belt. The Lavalleja Group can be divided in Minas, Fuente del Puma and Zanja del Tigre Formations.

Kinematics of basin development during the transition from terrane accretion to strike‐slip tectonics. Late Cretaceous‐early Tertiary Cantwell Formation, south central Alaska

The Cantwell basin was formed during Late Cretaceous time in the suture zone between the Wrangellia composite terrane and the former continental margin of southern Alaska. The Late Cretaceous (∼80–70 Ma) lower Cantwell Formation represents the initial fill of this basin and includes ∼4000 m of sedimentary rock. Between Maastrichtian and late Paleocene time (∼70–60 Ma), rocks of the lower Cantwell Formation were deformed by nearly eastwest trending folds and north vergent thrust faults. This deformation can be linked to the northward accretion and suturing of the Wrangellia composite terrane to southern Alaska. During late Paleocene to early Eocene time (∼60–55.5 Ma), at least 2750 m of volcanic rocks of the upper Cantwell Formation were deposited above the folded lower Cantwell Formation. New40Ar/39Ar ages for the upper Cantwell Formation range from 59.8+/−0.2 to 55.5+/−0.2 Ma and reveal that the unconformity between the lower and upper Cantwell Formations represents a 10–20 million year hiatus. Following volcanism the Cantwell Formation was crosscut by northeast trending folds, northwest trending normal faults, north trending left‐lateral slip faults, northeast trending reverse faults, and east trending right‐lateral slip faults. These structures are consistent with northwest‐southeast compression in a zone of right‐lateral simple shear along a fault with the same trend (east‐northeast) as that of the present McKinley strand of the Denali fault system. Cantwell basin kinematics are compatible with an escape tectonics model for south central Alaska whereby the Cantwell basin was formed, folded, and partially uplifted during northward terrane accretion and suturing and was deformed again during the westward transfer of terranes along right‐lateral strike‐slip fault systems.