Eastern Laurentia in Rodinia: constraints from whole-rock Pb and U/Pb geochronology

Whole-rock Pb isotopic signatures and U/Pb geochronology refute a Rodinian correlation of northeastern Laurentia and proto-Andean Amazonia. According to this previously proposed model, the Labrador–Scotland–Greenland Promontory (LSGP) of northeastern Laurentia collided with the proto-Andean margin of Amazonia, at the Arica Embayment, during the Grenville/Sunsás Orogeny (ca. 1.0 Ga). Links between the two margins were based upon the correlation of the LSGP with Arequipa-Antofalla Basement (AAB), a Proterozoic block along the proto-Andean margin of Amazonia adjacent to the Arica Embayment. Specifically, similarities in 1.8–1.0 Ga basement rocks in both regions suggested that the AAB was originally a piece of the LSGP. Furthermore, similarities in unique, post-collisional, but pre-rift, glacial sedimentary sequences also supported a link between the AAB and LSGP.Tests of these apparent similarities fail to support correlation of the AAB and the LSGP and, thus, eliminate a direct link between northeastern Laurentia and southwestern Amazonia in Rodinia. However, Pb isotopic compositions and U/Pb geochronology provide the basis for two new correlations, namely, (1) the ca. 1.3–1.0 Ga basement in the central and southern Appalachians may be an allochthonous block that was transferred to Laurentia from Amazonia at ca. 1.0 Ga, and (2) an allochthonous AAB may be a piece of the Kalahari Craton that was transferred to Amazonia at ca. 1.0 Ga. Based on these new correlations and a previously proposed Grenvillian connection between southern Laurentia (Llano) and Kalahari, we propose that Amazonia may have collided with a contiguous southeastern Laurentia/Kalahari margin at ca. 1.0 Ga.     

Fifty-thousand-year vegetation and climate history of Noel Kempff Mercado National Park, Bolivian Amazon

Pollen and charcoal records from two large, shallow lakes reveal that throughout most of the past 50,000 yr Noel Kempff Mercado National Park, in northeastern lowland Bolivia (southwestern Amazon Basin), was predominantly covered by savannas and seasonally dry semideciduous forests. Lowered atmospheric CO₂ concentrations, in combination with a longer dry season, caused expansion of dry forests and savannas during the last glacial period, especially at the last glacial maximum. These ecosystems persisted until the mid-Holocene, although they underwent significant species reassortment. Forest communities containing a mixture of evergreen and semideciduous species began to expand between 6000 and 3000 ¹⁴C yr B.P. Humid evergreen rain forests expanded to cover most of the area within the past 2000 ¹⁴C yr B.P., coincident with a reduction in fire frequencies. Comparisons between modern pollen spectra and vegetation reveal that the Moraceae-dominated rain forest pollen spectra likely have a regional source area at least 2-3 km beyond the lake shore, whereas the grass- and sedge-dominated savanna pollen spectra likely have a predominantly local source area. The Holocene vegetation changes are consistent with independent paleoprecipitation records from the Bolivian Altiplano and paleovegetation records from other parts of southwestern Amazonia. The progressive expansion in rain forests through the Holocene can be largely attributed to enhanced convective activity over Amazonia, due to greater seasonality of insolation in the Southern Hemisphere tropics driven by the precession cycle according to the Milankovitch Astronomical Theory.     

Late pleistocene temperature depression and vegetation change in Ecuadorian Amazonia

Paleoecological (pollen, phytolith, and wood) analyses of sediments, radiocarbon dated 33,000 to 26,000 yr B.P., from two sites in Ecuadorian Amazonia provide data that suggest a cooling of ca. 7.5°C below present in equatorial lowlands from 33,000 to 30,000 yr B.P. A period of warning followed in which novel species assemblages, a blend of montane and lowland floral components, persisted for at least 4000 years. These data of forest community change, from sites lying within the postulated glacial rain forest Napo refugium, provide the strongest paleoecological refutation of the refugial hypothesis yet obtained. The large temperature depression at ca. 30,000 yr B.P. allows the possibility that if maximum cooling at the equator was synchronous with the last glacial maximum (LGM) of the northern hemisphere, freezing temperatures would have been experienced in parts of lowland Amazonia between 25,000 and 18,000 B.P.     

Age and autochthonous evolution of the Sunsás Orogen in West Amazon Craton based on mapping and U–Pb geochronology

The West Amazon Craton consists of rocks of the Sunsás Orogen and the Rondônia-Juruena Province. The Sunsás Orogen comprises the western part of the Amazon Craton in South America and is best exposed in eastern Bolivia and western Rondônia and Mato Grosso states of Brazil. The integration of available maps and isotopic data together with new U–Pb and Sm–Nd analyses from 20 samples (plus 55 earlier dates), establish the timing of geologic events in the West Amazon Craton from 1840 to 1110 Ma. To unravel the complex geologic history of the study area, we primarily sampled granitoids and gneisses to develop a better stratigraphy and secondarily to narrow the age gaps between known discordances. Four periods of orogenic activity are identified within the Sunsás Orogen: 1465–1427 Ma (Santa Helena orogeny), 1371–1319 Ma (Candeias orogeny), ca. 1275 Ma (San Andrés orogeny), and 1180–1110 Ma (Nova Brasilândia orogeny). Notable is the absence of an Ottawan orogeny (1080–1020 Ma) equivalent. In the Rondônia-Juruena Province three main orogenies are recognized: the Juruena (1840–1780 Ma), the Jamari (1760–1740 Ma) and the Quatro Cachoeiras (1670–1630 Ma). Post-Sunsás rocks include Rondônia tin granites, Palmeiral sandstones, Nova Floresta basalt, and alkalic pipes.All inherited U–Pb ages of zircon and all exposed pre-Sunsás rocks in Bolivia have ages that correlate well to the neighbouring Rondônia-Juruena Province. This fact, together with the absence of fragments of older, Archean and Trans-Amazonian crust, suggests that the Sunsás Orogen is autochthonous and evolved over a continental margin formed dominantly by rocks of the Jamari (1760–1740 Ma) and Quatro Cachoeiras (1670–1630 Ma) orogenies plus rocks of the post-tectonic Serra Providência Suite (1560–1540 Ma). Almost all granulites known in Eastern Bolivia and in neighbouring area in Brazil are not basement rocks, but were formed during the Mesoproterozoic and are mainly associated with the Candeias orogeny (1371–1319 Ma). Dated samples of the Chiquitania and Lomas Manechi Complexes in Bolivia revealed a variety of ages and types of ages (metamorphic, magmatic, and inherited) indicating that those two units require more study. There is no evidence for the existence of a Paraguá Craton or Paraguá Block, which is almost totally composed of arc-related granites also formed during the Candeias orogeny.The main difference between the Sunsás Orogen and the Grenville Orogen of Laurentia is the absence in Amazonia of an Ottawan-equivalent orogeny (1080–1020 Ma). The existence of age-equivalents of the Candeias and Santa Helena orogenies in Laurentia (Pinwarian orogeny and rocks of the Eastern Granite-Rhyolite Province and the Composite Arc Belt) indicates that the connection of the two continents may have started from about 1450 Ma. In addition, the two belts may not have been directly juxtaposed, but instead, that one may have been the extension of the other during the Mesoproterozoic. The possibility that Amazonia joined the southwestern part of Laurentia also provides a good fit for the Hudson-Tapajós and Mazatzal-Yapavai-Rondônia-Juruena Provinces. This possible link to Laurentia may have started during the formation of the Trans-Hudson Orogen and its correlative Rondônia-Juruena and Tapajós provinces from about 1900 Ma.     

Forebulge dynamics and environmental control in Western Amazonia: The case study of the Arch of Iquitos (Peru)

The Iquitos Arch corresponds to a broad topographic high in the Western Amazonia. Morphostructural and geophysical data and flexural modeling show that the Iquitos Arch is the present-day forebulge of the Northwestern Amazonian foreland basin. A detailed tectono-sedimentary study of the Neogene and Quaternary deposits of the Iquitos area has been carried out in order to circumscribe the timing of the forebulge uplift and its environmental consequences. The Neogene and Quaternary sedimentary succession of the Iquitos Arch consists of six formations that evolved from tidal to fluvial environments. The first three formations exhibit Late Miocene gliding features and synsedimentary normal faults. Such soft-sediment deformations bear witness to tectonic activity ascribed to the growth of the forebulge. Regional erosive surfaces that separate the Neogene and Quaternary formations recorded the progressive forebulge emersion and the evolution of Amazonian drainage system. This uplift is related to an increase in tectonic activity within the Andes, which has provoked the eastern propagation of the orogenic wedge and caused an orogenic loading stage in the Amazonian foreland basin system. The emersion of the forebulge induced the retreat of the Pebas “marine megalake” nearby the Iquitos area and consequently caused important environmental changes in the Amazonian basin. From the end of the Late Miocene to the Pliocene, the forebulge acted as a barrier inducing the deposition of fluvial deposits in the forebulge depozone and the deposition of the “White Sand” deposits in the backbulge depozone. Since about 6 Ma, the forebulge is incised and crossed over by the modern Amazon River. The Iquitos forebulge is still growing as shown by the faulted Holocene terrace deposits.     

Geochronology and settlement disposition in the early Palaeo-Indian occupation of southern Ontario, Canada

Examination of a sample of 150 fluted-point localities from southern Ontario, Canada in relation to datable features of Late Wisconsinan ice retreat discloses maximum possible ages for early Palaeo-Indian occupation and reveals selection of specific physiographic situations. General relationship to maximum ice-advance positions suggests occupation during the Two Creeks Interstade after Port Huron ice retreat about 12,300 yr B.P. Specific relationship to ¹⁴C-dated proglacial Great Lake strands favors occupation during the North Bay Interstade after Greatlakean ice retreat about 11,500 yr B.P. Locality frequency on Lake Algonquin strands suggests contemporaneity with the main stage of this lake about 11,500 to 10,400 yr B.P., with a small number of lake-plain localities indicating minor post-Algonquin persistence. Radiocarbon dates for fluted-point sites elsewhere in the glaciated northeast place occupation coeval with the southwestern Folsom fluted-point tradition of about 10,850 to 10,200 yr B.P. Locality situation in regions dominated by proglacial sediments, on lake-edge features adjacent to strand-dissecting tributaries, within major river valleys, implies selectivity reflecting primary adaptation. Fluted-point associations with caribou and elk remains suggest that “focal” adaptation to cervids, comparable to southwestern Folsom bison exploitation, might underlie the homogeneous nature and distribution of early Palaeo-Indian localities throughout the northeast.     

Superposed deposits of thick coal on the eastern edge of the Illinois Basin and their association with underlying geologic features

Comparison of maps produced from publicly available data (drillers' logs, electrical logs and mine maps) provides a basis for inferring a deep-seated influence on the distribution of superposed deposits of thick coal (>1.4 m) in four Middle Pennsylvania (Desmoinesian Series) coal beds in three mining districts of west-central Indiana. Thick sandstone (>18 m) is common in areas between and around the mining districts, but less than 3 percent of the study area (consisting of 3200 km²) is underlain by both thick coal and thick sandstone. Only thick sandstone associated with the Survant Coal Member (Linton Formation), and informally referred to by us “Survant sandstone”, exists in all of the thin-coal areas. After comparison with published maps by other authors, it is inferred that distribution of the Survant sandstone, which was deposited immediately after a long period of slow deposition associated with the Colchester Coal Member (Linton Formation), may reflect topographic expression of long-term subsidence associated with differential thinning of much deeper Silurian strata (580 m below).Although the findings of this study provide the basis for a conceptual geologic model with a hypothetical structure that is amenable to statistical testing, such analysis should be undertaken only after the data are analyzed for randomness, spatial autocorrelation, linearity and normality.     

Holocene displacement along branch and secondary faults in the San Andreas fault zone, southern California

Detailed geologic mapping of the San Andreas fault zone in Los Angeles County since 1972 has revealed evidence for diverse histories of displacement on branch and secondary faults near Palmdale. The main trace of the San Andreas fault is well defined by a variety of physiographic features. The geologic record supports the concept of many kilometers of lateral displacement on the main trace and on some secondary faults, especially when dealing with pre-Quaternary rocks. However, the distribution of upper Pleistocene rocks along branch and secondary faults suggests a strong vertical component of displacement and, in many locations, Holocene displacement appears to be primarily vertical. The most recent movement on many secondary and some branch faults has been either high-angle (reverse and normal) or thrust. This is in contrast to the abundant evidence for lateral movement seen along the main San Andreas fault. We suggest that this change in the sense of displacement is more common than has been previously recognized.The branch and secondary faults described here have geomorphic features along them that are as fresh as similar features visible along the most recent trace of the San Andreas fault. From this we infer that surface rupture occurred on these faults in 1857, as it did on the main San Andreas fault. Branch faults commonly form “Riedel” and “thrust” shear configurations adjacent to the main San Andreas fault and affect a zone less than a few hundred meters wide. Holocene and upper Pleistocene deposits have been repeatedly offset along faults that also separate contrasting older rocks. Secondary faults are located up to 1500 m on either side of the San Andreas fault and trend subparallel to it. Moreover, our mapping indicates that some portions of these secondary faults appear to have been “inactive” throughout much of Quaternary time, even though Holocene and upper Pleistocene deposits have been repeatedly offset along other parts of these same faults. For example, near 37th Street E. and Barrel Springs Road, a limited stretch of the Nadeau fault has a very fresh normal scarp, in one place as much as 3 m high, which breaks upper Pleistocene or Holocene deposits. This scarp has two bevelled surfaces, the upper surface sloping significantly less than the lower, suggesting at least two periods of recent movement. Other exposures along this fault show undisturbed Quaternary deposits overlying the fault. The Cemetery and Little Rock faults also exhibit selected reactivation of isolated segments separated by “inactive” stretches.Activity on branch and secondary faults, as outlined above, is presumed to be the result of sympathetic movement on limited segments of older faults in response to major movement on the San Andreas fault. The recognition that Holocene activity is possible on faults where much of the evidence suggests prolonged inactivity emphasizes the need for regional, as well as detailed site studies to evaluate adequately the hazard of any fault trace in a major fault zone. Similar problems may be encountered when geodetic or other studies, Which depend on stable sites, are conducted in the vicinity of major faults.     

Use of two-dimensional electric resistivity and ground penetrating radar for archaeological prospecting at the ancient capital of Egypt

Two-dimensional (2-D) resistivity and ground penetrating radar (GPR) surveys facilitate tracing of buried archaeological relics at the archaeological site of the Tell El Rabi'a, southwestern Cairo. These surveys also provide information on variations in depth of the groundwater table, which affect both exhumed and buried archaeological remains. The 2-D electrical resistivity profiles obtained show some high anomalies (up to 6900 Ω m). Based upon on-site calibration at partly exhumed sites, such anomalies are interpreted as pillar crowns and wall-like structures of hard limestone of the Hathour temple. The low background resistivities are interpreted to represent riverine deposits with differing moisture (or saturation) characteristics. The results of the GPR survey show three forms of high-amplitude radar anomalies, denoted as “P, H and R”. The “P” anomalies have semi-hyperbolic shapes, which suggest the presence of thin buried walls. The second, near-horizontal group of anomalies, “H”, may be either due to the presence of thick buried walls or arise from coincidental parallel alignment of survey lines with buried linear structures. The third group of GPR anomalies “R” is composed of chaotic reflections, and interpreted to depict buried stone-filled chambers. Some un-exhumed parts of the study area show weak radar reflections with occasional highly attenuated radar signals. These phenomena are ascribed to differing compaction and moisture characteristics of riverine soil layers. Recommendations with regard to archaeological site-excavation, preservation of archaeological relics, and soil conservation are submitted.     

The partitioning of copper among selected phases of geologic media of two porphyry copper districts, Puerto Rico

In experiments designed to determine the manner in which copper is partitioned among selected phases that constitute geologic media, we have applied the five-step sequential extraction procedure of Chao and Theobald to the analysis of drill core, soils, and stream sediments of the Rio Vivi and Rio Tanama porphyry copper districts of Puerto Rico. The extraction procedure affords a convenient means of determining the trace-metal content of the following fractions: (1) Mn oxides and “reactive” Fe oxides; (2) “amorphous” Fe oxides; (3) “crystalline” Fe oxides; (4) sulfides and magnetite; and (5) silicates. An additional extraction between steps (1) and (2) was performed to determine organic-related copper in stream sediments.The experimental results indicate that apportionment of copper among phases constituting geologic media is a function of geochemical environment. Distinctive partitioning patterns were derived from the analysis of drill core from each of three geochemical zones: (a) the supergene zone of oxidation; (b) the supergene zone of enrichment; and (c) the hypogene zone; and similarly, from the analysis of; (d) soils on a weakly leached capping; (e) soils on a strongly leached capping; and (f) active stream sediment.The experimental results also show that geochemical contrasts (anomaly-to-background ratios) vary widely among the five fractions of each sampling medium investigated, and that at least one fraction of each medium provides substantially stronger contrast than does the bulk medium. Fraction (1) provides optimal contrast for stream sediments of the district; fraction (2) provides optimal contrast for soils on a weakly leached capping; fraction (3) provides optimal contrast for soils on a strongly leached capping.Selective extraction procedures appear to have important applications to the orientation and interpretive stages of geochemical exploration. Further investigation and testing of a similar nature are recommended.     

Late Miocene continental sedimentation in southwestern Amazonia and its regional significance: Biotic and geological evidence

Fossil content (vertebrate paleofauna and palynology) indicates that the sediments of the Solimões Formation in Acre (SW Brazilian Amazonia) are continental, having been deposited by avulsive fluvial belts in a floodbasin–floodplain environment. The main source area was the Andes chain. Widespread lacustrine swampy deposits, stacked channel deposits, and paleosoils are typical elements that characterize the Solimões Formation sediments that outcrop in southwestern Brazilian Amazonia. New data on fossil vertebrate assemblages and palynology corroborate the Late Miocene age suggested previously and assign the fossils to the Huayquerian mammalian biozone, spanning 9–6.5 Ma. These geological and paleontological data show that the existence of an intracontinental seaway through SW Amazonia during the Late Miocene (11–10 Ma), connecting the Caribbean Sea with the Parana Basin as previously proposed is unsustainable, because the sediments used by previous authors to propose the seaway were deposited in a continental environment and are younger than 11–10 Ma.     

Effects of Climate Change on Soils in Glacial Deposits, Wind River Basin, Wyoming

Soil chronofunctions are often quasilinear, but those relationships may be only gross, fortuitous generalizations that ignore changes in soils resulting from climatic and other cyclic external influences that govern soil formation or soil degradation. In the Rocky Mountains, soil development has been extensively used to estimate the age of sediments, but the number of soils is usually few, and large ranges in values for soil data may be dismissed as “normal variation.” In a detailed study of soils in the Wind River Range and Wind River Basin, characteristics of near-surface horizons do not follow age trends. However, the underlying carbonate-bearing horizons do have age-related characteristics. This “soil paradox” may be related to glacial–interglacial cycles in which (1) wind erodes near-surface horizons and then provides new parent material and (2) cryoturbation disrupts carbonate horizons, remobilizing carbonate and changing carbonate morphology but with only minimal loss of carbonate from the soil.     

Spatial data availability and its implications for sustainable development of the Brazilian Amazon

In the Amazonia region, general policy based on sustainable development must be adopted to counterbalance natural conservation and the needs of people. To discuss and monitor the regional development requires a significant variety of spatial data. However, making data available is not simple and various solutions usually are presented in non-standardized and non-interchangeable databases. This paper presents an overview of spatial data available for the Amazonia region, and initiatives of data dissemination systems, considering various geographical scales and data access. From this general perspective, we discuss some ideas about the importance of spatial data for sustainability and suggest interactive systems that could contribute to local development and environmental conservation. The limitation on the promotion of spatial information and data sharing in Amazonia is neither computational nor technological. Rather, it is a matter of informing those who generate the data about the benefits of knowledge as a public good and promoting collaborative work. Creating a convergent spatial consciousness about the territory, and bringing together every environmental player, is a step towards the discussion and planning of the sustainable development of the Amazonia.     

Transient and steady-state deformation of synthetic rocksalt

Synthetic rocksalt with a porosity less than 2.5% and an average grain size of about 0.35 mm was made by warm-pressing at 100°C, 150 MPa and for 15 min. Cylinders of this material, 25 mm in diameter by 50 mm long, were deformed at strain rates of 0.1 ksec⁻¹ at confining pressures of 20, 50, 100 and 200 MPa and at temperatures of 100, 200 and 300°C. The resulting stress-time data show transient-stress behavior before steady-state stress occurs. Very little variation in the stress/time data occurs for the above confining pressures at a constant temperature. Many of the tests reach steady state at 10% strain, where all the experiments were terminated. The differential stress at 10% varies from about 22 MPa at 100°C to about 6 MPa at 300°C. These “strengths” are slightly less than those measured by Heard (1972) (also on synthetic, polycrystalline rocksalt) and are similar to those measured by Wawersik and Hannum (1980) and by Hansen and Mellegard (1980) in modified creep tests on coarse-grained, natural rocksalt under similar pressure and temperature conditions. Activation energies computed from these steady-state stresses vary between 7.5 and 25 kcal/mole and are consistent with those obtained by Heard (1972) and by Parrish and Gangi (1977, 1981). The stress/time data were fit using one- and two-mechanism, transient-stress functions which assume independent mechanisms or processes. The rms errors decrease from about 1 MPa for the one-mechanism fits to about 0.2 MPa for the two-mechanism fits, indicating at least two mechanisms are operative in these tests. Similar one- and two-mechanism fits were made to creep tests performed by W. Wawersik et al. on New Mexico bedded rocksalt. Similar improvements in the fits were obtained for those tests that lasted long enough so that the effect of a second mechanism could be noted. It was found that the “steady-state” strain rate found in creep tests could be interpreted as the beginning of another mechanism. This raises the question of whether the “steady-state” phenomena exist at all or whether it is just an approximation to a mechanism with a time constant that is long compared to the length of the test.     

The correlation between the macroseismic attenuation trend and the geo-structural framework; The calabropeloritan arc as an example

After describing attempts at perfecting a methodology for studying isotropic and anisotropic macroseismic fields in previous works, the authors here try to identify the causes of anisotropy in the context of the “new basement tectonics”.The seismic data are taken both from reconstructions of the macroseismic fields of historic events, by means of a critical analysis of the data, and from macroseismic fields of recent events surveyed by the authors. These data are correlated to the structural framework obtained through recent neotectonic studies and the lineament distribution traced on satellite images and using the “shadow” method. Generally the direction of elongation of the mesoseismic area is closely dependent on the source parameters and can be associated with recent and present-day systems outlined by the latest neotectonic studies.The best correlation is observed, however, with the lineament pattern obtained using the “shadow” method: the domains of the lineaments associated with the preferential trend of the macroseismic field show, in the rose diagram of cumulative number, values of prevalence and kurtosis higher than average; in the cumulative lengths diagram, on the other hand, they show prevalence maxima and, in particular, kurtosis maxima which are all the higher the more the anisotropic trend of field is accentuated. Using the “Giant Griffith Cracks” model for the lineaments, it can be deduced that the swarms refer to fracture systems with greater vertical development generated during the most recent tectonic phases.Finally, from a study of the dynamic characteristics of the elastic waves, that are the main agents responsible for macroseismic effects, it can observed that the wavelength order of magnitude is comparable with that of the linear parameters in the “warp” formed by the “Giant Cracks”. It can, thus, be deduced that the strong absorption of energy can be determined by the fracture swarm when the wave propagation occurs orthogonally to the swarm.     

Integration of P- and SH-wave high-resolution seismic reflection and micro-gravity techniques to improve interpretation of shallow subsurface structure: New Madrid seismic zone

Shallow high-resolution seismic reflection surveys have traditionally been restricted to either compressional (P) or horizontally polarized shear (SH) waves in order to produce 2-D images of subsurface structure. The northernmost Mississippi embayment and coincident New Madrid seismic zone (NMSZ) provide an ideal laboratory to study the experimental use of integrating P- and SH-wave seismic profiles, integrated, where practicable, with micro-gravity data. In this area, the relation between “deeper” deformation of Paleozoic bedrock associated with the formation of the Reelfoot rift and NMSZ seismicity and “shallower” deformation of overlying sediments has remained elusive, but could be revealed using integrated P- and SH-wave reflection. Surface expressions of deformation are almost non-existent in this region, which makes seismic reflection surveying the only means of detecting structures that are possibly pertinent to seismic hazard assessment. Since P- and SH-waves respond differently to the rock and fluid properties and travel at dissimilar speeds, the resulting seismic profiles provide complementary views of the subsurface based on different levels of resolution and imaging capability. P-wave profiles acquired in southwestern Illinois and western Kentucky (USA) detect faulting of deep, Paleozoic bedrock and Cretaceous reflectors while coincident SH-wave surveys show that this deformation propagates higher into overlying Tertiary and Quaternary strata. Forward modeling of micro-gravity data acquired along one of the seismic profiles further supports an interpretation of faulting of bedrock and Cretaceous strata. The integration of the two seismic and the micro-gravity methods therefore increases the scope for investigating the relation between the older and younger deformation in an area of critical seismic hazard.     

The geologic history of the passive margin off new england and the canadian maritime provinces

The geologic history of the passive continental margin off the east coast of North America from New England to Newfoundland is described using all available geological and geophysical information. “Rift” and “drift” phases of the margin's evolution are recognized, with rifting initiated in Late Triassic and completed by Early Jurassic. The plate decoupling process created a complex block-faulted terrain as a result of uplift and tensional fracturing. The approximate plane of continental separation is marked by a “hinge zone” characterized by a pronounced steepening of basement gradients. Since the Early Jurassic, the margin has undergone continual subsidence in response to cooling and sediment loading. This “drift” sequence attains its maximum thickness in the vicinity of the continental slope and thins both landward and seaward. On the shelf, this unit consists of Mesozoic evaporites, carbonates, and deltaic deposits. Overlying these sediments is a prograding wedge of Cenozoic elastics. On the rise, the Mesozoic sediments are evaporites, hemipelagic limestones and shales and carbonaceous clays. The Cenozoic is dominantly terrigenous material. Separating these two sedimentary provinces is the continental slope, a site of major facies changes and a Mesozoic reef complex.     

Significance of ~ 1.5 Ga zircon and monazite ages from charnockites in southern Lithuania and NE Poland

The presence of 1.52–1.50 Ga charnockites from the anorthosite–mangerite–charnockite–granite (AMCG) Mazury complex in southern Lithuania and NE Poland, in the western East European Craton (EEC) is revealed by secondary ion mass-spectrometry (SIMS) and EPMA geochronology. Early 1.85–1.82 Ga charnockites are related to major orogeny in the region whereas the newly studied charnockites intrude the already consolidated crust. The 1.52–1.50 Ga charnockite magmatism (SIMS data on zircon) was followed by high-grade metamorphism (EPMA data on monazite), which strongly affected the surrounding rocks. The 1.85–1.81 Ga zircon cores in Lazdijai and 1.81 Ga monazite domains in the Lanowicze charnockites represent the protolith age of a volcanic island arc. The 1.52–1.50 Ga charnockite magmatism and metamorphism are likely related to the distal, Danopolonian, orogeny further to the west, at the margin of Baltica. The c.1.52–1.50 Ga AMCG magmatism and metamorphism in the western EEC as well as the paired accretionary-rapakivi suites in Amazonia, may be the inboard manifestations of the same early Mesoproterozoic orogeny associated with the juxtaposition of Amazonia and Baltica during the amalgamation of the supercontinent Columbia.     

Radiocarbon Age Constraints on Rates of Advance and Retreat of the Puget Lobe of the Cordilleran Ice Sheet during the Last Glaciation

Calibrated radiocarbon dates of organic matter below and above till of the last (Fraser) glaciation provide limiting ages that constrain the chronology and duration of the last advance–retreat cycle of the Puget Lobe in the central and southeastern Puget Lowland. Seven dates for wood near the top of a thick proglacial delta have a weighted mean age of 17,420 ± 90 cal yr B.P., which is the closest limiting age for arrival of the glacier near the latitude of Seattle. A time–distance curve constructed along a flowline extending south from southwestern British Columbia to the central Puget Lowland implies an average glacier advance rate of ca. 135 m/yr. The glacier terminus reached its southernmost limit ca. 16,950 yr ago and likely remained there for ca. 100 yr. In the vicinity of Seattle, where the glacier reached a maximum thickness of 1000 m, ice covered the landscape for ca. 1020 yr. Postglacial dates constraining the timing of ice retreat in the central lowland are as old as 16,420 cal yr B.P. and show that the terminus had retreated to the northern limit of the lowland within three to four centuries after the glacial maximum. The average rate of retreat was about twice the rate of advance and was enhanced by rapid calving recession along flowline sectors where the glacier front crossed deep proglacial lakes.