The determination of resistivity and porosity of the sediment and fractured basalt layers near the Juan de Fuca Ridge

Summary. In the summer of 1984 an electrical survey using magnetometric off-shore electrical sounding (MOSES) was conducted at two sites in Middle Valley, part of the northern Juan de Fuca Ridge complex. MOSES has been designed to minimize the difficulties inherent in electrical surveys of the crust below the electrically conductive sea layer. Site 1, at 48°32N, 128°42W, is in the central part of the turbidite-filled basin. Using a two-layer model of conductive sediments overlying a fractured basalt basement, the sediment resistivity and thickness were found to be 0.82 ± 0.06 Ωm and 1800 ± 300 m, respectively. The basement resistivity, although not well constrained by the data is consistent with the results obtained at site 2.
Site 2, located at 48°10N, 128°50W, has a thinner sediment layer, which appears to vary with position. The sediment conductivity—thickness product is the parameter determined by the data. If the sediment resistivity were the same as at site 1, the sediment thickness would be 140 ± 30 m to the SE of site 2, and 240 ± 55 m to the NW. The fractured basalt basement has a resistivity of 8.5 ± 3.4 Ωm and is at least 1000 m thick.
Using temperature-corrected pore fluid resistivity, the calculated porosity is found to vary from 62 per cent at the top to 21 per cent at the base of the sediments and is 8 per cent in the basement. These values are in good agreement with estimates from seismic velocities for a thick turbidite sequence in a nearby sediment-filled basin and determined for layer 2A/B basalts in DSDP hole 504B, respectively.     

Characterization of the depocenters and the basement structure,below the central Chile Andean Forearc: A 3D geophysical modelling in Santiago Basin area

Since the last century, several geological and geophysical studies have been developed in the Santiago Basin to understand its morphology and tectonic evolution. However, some uncertainties regarding sedimentary fill properties and possible density anomalies below the sediments/basement boundary remain. Considering that this is an area densely populated with more than 6 million inhabitants in a highly active seismotectonic environment, the physical properties of the Santiago Basin are important to study the geological and structural evolution of the Andean forearc and to characterize its seismic response and related seismic hazard. Two and three‐dimensional gravimetric models were developed, based on a database of 797 compiled and 883 newly acquired gravity stations. To produce a well‐constrained basement elevation model, a review of 499 wells and 30 transient electromagnetic soundings were used, which contribute with basement depth or minimum sedimentary thickness information. For the 2‐D modelling, a total of 49 gravimetric profiles were processed considering a homogeneous density contrast and independent regional trends. A strong positive gravity anomaly was observed in the centre of the basin, which complicated the modelling process but was carefully addressed with the available constrains. The resulting basement elevation models show complex basement geometry with, at least, eight recognizable depocenters with maximum sedimentary infill of ~ 500 m. The 3‐D density models show alignments in the basement that correlates well with important intrusive units of the Cenozoic and Mesozoic. Along with interpreted fault zones westwards and eastwards of the basin, the observations suggest a structural control of Santiago basin geometry, where recent deformation associated with the Andean contractional deformation front and old structures developed during the Cenozoic extension are superimposed to the variability of river erosion/deposition processes.     

The Acre Basin basement (NW Brazil) and the transition from the intracratonic to retroarc foreland basin system

The understanding of the crustal transition between orogenic zones and cratonic portions in distal regions of foreland basins has received increasing attention, but the analysis is often hampered by the sedimentary cover. Despite the peculiar location of the Acre Basin, specifically between the Amazonian Craton and the sub-Andean zone, local basement studies are still scarce due to lacking seismic data and exploratory wells. Therefore, this work aims to map basement depths, estimate crustal compositions and identify the main depocenters, structures and limits of Acre Basin using an integrated analysis to understand better the region lithospheric evolution, its relationship with the Amazonian Craton and its positioning within the Andean orogeny. For that, we used well, 2D seismic reflection, airborne and ground gravity and magnetic data as well as the EMG2008. Tilt Depth estimates indicate basement depths between 500 and 7800 m and larger sedimentary thicknesses in the northern portion. Additionally, we modelled groups of potential sources between 0.1 and 22 km and Moho depths between 26 and 37 km. Compositionally, the upper crust consists dominantly of meta-sedimentary and low-grade metamorphic rocks and granites, indicating that the sub-Andean and Acre Basins share a similar basement. Thus, there are indications that the basement of the Acre Basin is essentially formed by the Sunsás province in the Amazonian Craton. However, local differences in basement depth, magnetic susceptibility and exploratory potential led to the subdivision into Divisor and Xapuri sub-basins, north and south of the Fitzcarrald Arch, respectively. Finally, it was possible to establish the limits of the Andean orogeny influence in the Acre Basin and delimit the area of the Western Amazon Foredeep installed during the Neogene.     

H<Subscript>2</Subscript>-rich and Hydrocarbon Gas Recovered in a Deep Precambrian Well in Northeastern Kansas

In late 2005 and early 2006, the WTW Operating, LLC (W.T.W. Oil Co., Inc.) #1 Wilson well (T.D. = 5772 ft; 1759.3 m) was drilled for 1826 ft (556.6 m) into Precambrian basement underlying the Forest City Basin in northeastern Kansas. Approximately 4500 of the 380,000 wells drilled in Kansas penetrate Precambrian basement. Except for two previous wells drilled into the arkoses and basalts of the 1.1-Ga Midcontinent Rift and another well drilled in 1929 in basement on the Nemaha Uplift east of the Midcontinent Rift, this well represents the deepest penetration into basement rocks in the state to date. Granite is the typical lithology observed in wells that penetrate the Precambrian in the northern Midcontinent. Although no cores were taken to definitively identify lithologies, well cuttings and petrophysical logs indicate that this well encountered basement metamorphic rocks consisting of schist, gneiss, and amphibolitic gneiss, all cut by aplite dikes. The well was cased and perforated in the Precambrian, and then acidized. After several days of swabbing operations, the well produced shows of low-Btu gas, dominated by the non-flammable component gases of nitrogen (20%), carbon dioxide (43%), and helium (1%). Combustible components include methane (26%), hydrogen (10%), and higher molecular-weight hydrocarbons (1%). Although Coveney and others [Am. Assoc. Petroleum Geologists Bull., v. 71, no, 1, p. 39–48, 1987] identified H₂-rich gas in two wells located close to the Midcontinent Rift in eastern Kansas, this study indicates that high levels of H₂ may be a more widespread phenomenon than previously thought. Unlike previous results, the gases in this study have a significant component of hydrocarbon gas, as well as H₂, N₂, and CO₂. Although redox reactions between iron-bearing minerals and groundwater are a possible source of H₂ in the Precambrian basement rocks, the hydrocarbon gas does not exhibit the characteristics typically associated with proposed abiogenic hydrocarbon gases from Precambrian Shield sites in Canada, Finland, and South Africa. Compositional and isotopic signatures for gas from the #1 Wilson well are consistent with a predominantly thermogenic origin, with possible mixing with a component of microbial gas. Given the geologic history of uplift and rifting this region, and the major fracture systems present in the basement, this hydrocarbon gas likely migrated from source rocks and reservoirs in the overlying Paleozoic sediments and is not evidence for abiogenic hydrocarbons generated in situ in the Precambrian basement.     

Basement-controlled deformation of sedimentary sequences,Anadarko Shelf,Oklahoma

Structures rooted in the crystalline basement frequently control the deformation of the host bedrock and the overlying sedimentary sequences. Here, we elucidate the structure of the c. 2-km deep Precambrian granitic basement in the Anadarko Shelf, Oklahoma, and how the propagation of basement faults deformed the sedimentary cover. Although the basin is foreland in origin, the gently dipping shelf sequences experienced transpressional deformation in the Late Palaeozoic. We analyse a 3-D seismic reflection data set and basement penetrating well data in an area of 824 km². We observe: (a) pervasive deformation of the basement by basement-bounded interconnected mafic sills, and a system of subvertical discontinuity planes (interpreted as faults) of which some penetrate the overlying sedimentary cover; (b) three large (>10 km-long) through-going faults, with relatively small (<100 m) vertical separation (Vsep) of the deformed stratigraphic surfaces; (c) upward propagation of the large faults characterized by faulted-blocks near the basement, and faulted-monoclines in the deeper sedimentary units that transition into open monoclinal flexures up-section; (d) cumulative along-fault deformation of the stratigraphy exhibits systematic trends that varies with offset accrual; (e) two styles of Vsep—Depth distribution which include a unidirectional decrease of Vsep from the basement through the cover rocks (Style-1) and a bidirectional decrease of Vsep from a deep sedimentary unit towards the basement and shallower sequences (Style-2). We find that the basement-driven propagation (Style-1) shows greater efficiency of driving the fault deformation to shallower depths compared to the intrasedimentary-driven fault nucleation and propagation (Style-2). Our study demonstrates an evolution of cumulative Vsep trends with offset accrual on the faults, and the partial inheritance of the heterogeneous intra-basement deformation by the sedimentary cover. This contribution provides important insight into the upward propagation of basement-driven faulting associated with structural inheritance in contractional sedimentary basins.     

马祖列岛海岸阶地研究

距今二亿年前的三迭纪晚期一系列之大地构造运动,形成福建沿海的平潭-东山褶皱带,以及长樂-诏安断裂带,又因为构造运动以及岩浆活动的影响,在褶皱带与断裂带的东缘形成一系列的岛屿,马祖列岛的形成,与中国东南沿海的造山运动有密切关系,皆受到影响而产生变动。马祖地区的海阶,共有9 段阶地,各段阶地的海拔高度依序为0~10 m、20~38 m、40~56 m、58~78 m、80~97 m、100~128 m、142~160 m、172~182 m、238~248 m。依据晚更新世海阶序列与隆升率关系对比基图迭合法的推估,各段阶地生成年代分别为6 ka BP、46 ka BP、55 ka BP、76 kaBP、79 ka BP、94 ka BP、105 ka BP、119 ka BP、175 ka BP,除了第一级阶地为全新世时期所形成,其余皆为更新世时期的产物,比对马祖列岛海阶的高度与间距,测得整个地区的基盘平均隆升速率为1.6 mm/a。     

Geomorphological evidence of the influence of pre-volcanic basement structure on emplacement and deformation of volcanic edifices at the Cofre de Perote–Pico de Orizaba chain and implications for avalanche generation

Pre-volcanic structure of the basement influences volcanism distribution and avalanche generation in volcanic edifices. Therefore, systematic studies of basement structure below volcanic chains are necessary to understand the deformation effects observed in the surface and vice versa. Based on a compilation of pre-existing data, interpretation of aerial photographs and satellite images, and a collection of structural data we analyzed morphological and structural features of the Cofre de Perote–Pico de Orizaba (CP–PO) volcanic chain and its basement. We have identified three sets of regional lineaments that are related to basement trends. (1) NW 55° SE fractures are parallel to anticline folds observed in Cretaceous rocks that originated during Laramide shortening. These folds present an abrupt morphology observed only in the eastern flank but that is likely to continue below the volcanic chain. (2) NE 55° SW fractures are parallel to normal faults at the basement. We infer that these basement faults confine the CP–PO chain within a stepped graben with a total normal displacement of about 400 m. These faults have been active through time since they have affected volcanic deposits and induced the emplacement of monogenetic vents. Notably, lineaments of monogenetic vents concentrate where the basement is relatively shallow. (3) Another set of faults, oriented N–S, has been observed affecting the scarce basement outcrops at the western flank of the chain covered by lacustrine deposits. Lineaments measured in the volcanic edifice of Pico de Orizaba correlate with the regional trends.In particular, the NE 55° SW alignment of monogenetic vents and fractures at Pico de Orizaba suggest that the same dike trend exists within the volcanic edifice. A normal fault with similar orientation was documented at the NE continuation of an alignment crossing the volcanic edifice along the Jamapa canyon. In the absence of magmatic activity related to collapses, the displacement of NE 55° SW faults represents a potential triggering mechanism for generating avalanches at Pico de Orizaba volcano. Instability is enhanced by the presence of N–S trending fractures crossing the entire volcanic edifice and E–W fractures affecting only the present day cone. We conclude that mechanical instability of the volcanic chain is influenced by the basement structure heterogeneity, but further detailed studies are necessary at individual volcanoes to evaluate their effects on volcano deformation.     

The Elk Creek Carbonatite,Southeast Nebraska–An Overview

A framework geophysical program in southeastern Nebraska during 1970 identified a near-circular feature having gravity relief of about 8 mgal and a magnetic anomaly of about 800 gammas. Analysis of the geophysical data provided a model of a cylindrical mass of indefinite length with a radius of 5500 ft (1676 m) and beveled at the basement surface at about 600 ft (183 m). At the approximate depth at which Precambrian rocks were expected, the initial test hole (2-B-71) encountered an iron-rich weathered zone overlying carbonate-rich rock. The carbonate rocks consist essentially of dolomite, calcite, and ankerite and lesser amounts of hematite, chlorite, phlogopite, barite, serpentine, pyrochlore, and quartz and contain barium, strontium, and rare earths. Total REE, P2O5, and 87Sr/86Sr ratios confirm the carbonatite identification. Texturally, the rocks range from fragmental to contorted to massive. Associated with the carbonatite are lesser amounts of basalt, lamprophyre, and syenite. Additional exploratory drilling has provided about 80,000 ft (24,384 m) of rock record and has penetrated about 3400 ft (1038 m) of carbonatite. The carbonatite is overlain by marine sediments of Pennsylvanian (Missourian) age. The surrounding Precambrian basement rocks are low-to medium-grade metamorphic gneiss and schist of island arc origin and granitic plutons. The Elk Creek carbonatite is located near the boundary between the Penokean orogen created at about 1.84 Ga (billion years) and the Dawes terrane (1.78 Ga) of the Central Plains orogen. This boundary strongly influenced the geometry of both the Midcontinent Rift System (1.1 Ga) and the Nemaha uplift (0.3 Ga). It is assumed that the emplacement of the Elk Creek carbonatite (0.5 Ga) was influenced similarly by the pre-existing tectonic sutures.     

金门地区海岸阶地研究

海岸阶地的形成通常是海平面变动、地壳变动或两者共同作用之结果．对于海阶的研究除了可印证推论古气候、往昔海平面变化及地壳变动状况,还可藉以了解各区域间变动的差异,全盘了解大地构造的意义;小规模的海阶变动,时常与地震活动伴生,因此,研究海阶更可作为判读地震周期的依据之一．世界各地位于地壳活动带的国家对海阶的形态与演育过程均作详细的调查及研究．距今2亿年前的三叠纪晚期的一系列构造运动,形成福建沿海地区的区域变质带和长乐一南澳断裂带,由于构造运动的影响,于断裂带东缘形成了一系列岛屿,金门列岛的形成,与中国东南沿海的造山运动有着密切关系,皆受到影响而产生变动．金门地区的海阶,共有11段阶地,海拔依序为0～10m、15～25m、30～45m、50—65m、70—80ITI、90～120in、130—154m、160～165m、190～208m、215～225m、230～250m,依据晚更新世海阶序列与隆升率关系对比基图迭合法的推估,各段阶地的年代分别为6kaBP、46kaBP、57kaBP、64kaBP、80kaBP、100kaBP、103kaBP、120kaBP、176kaBP、190kaBP、202kaBP,除了第一阶为全新世时期所形成,其余皆为更新世时期的产物,针对金门列岛海阶的高度与间距做对比,得知该区的平均隆升速率为1．3mm／a．     

Shallow velocity structure along the Hirapur–Mandla profile using traveltime inversion of wide-angle seismic data, and its tectonic implications

In order to investigate the velocity structure, and hence shed light on the related tectonics, across the Narmada–Son lineament, traveltimes of wide-angle seismic data along the 240 km long Hirapur–Mandla profile in central India have been inverted. A blocky, laterally heterogeneous, three-layer velocity model down to a depth of 10 km has been derived. The first layer shows a maximum thickness of the upper Vindhyans (4.5 km s⁻¹ ) of about 1.35 km and rests on top of normal crystalline basement, represented by the 5.9 km s⁻¹ velocity layer. The anomalous feature of the study is the absence of normal granitic basement in the great Vindhyan Graben, where lower Vindhyan sediments (5.3 km s⁻¹ ) were deposited during the Precambrian on high-velocity (6.3 km s⁻¹ ) metamorphic rock. The block beneath the Narmada–Son lineament represents a horst feature in which high-velocity (6.5 km s⁻¹ ) lower crustal material has risen to a depth of less than 2 km. South of the lineament, the Deccan Traps were deposited on normal basement during the upper Cretaceous period and attained a maximum thickness of about 800 m.     

Uplift, exhumation and precipitation: tectonic and climatic control of Late Cenozoic landscape evolution in the northern Sierras Pampeanas, Argentina

Deciphering the evolution of mountain belts requires information on the temporal history of both topographic growth and erosion. The exhumation rate of a mountain range undergoing shortening is related to the erodability of the uplifting range as well as the efficiency of erosion, which partly depends on the available precipitation. Young, rapidly deposited sediments have low thermal conductivity and are readily eroded, in contrast to underlying resistant basement rocks that have a higher thermal conductivity. Apatite fission‐track thermochronology can quantify cooling; thermal models constrain the relationship between this cooling and exhumation. By utilizing geological relations for a datum, we can examine the evolution of rock uplift, surface uplift and exhumation. In the northern Sierras Pampeanas of Argentina, a young sedimentary basin that overlay resistant crystalline basement prior to rapid exhumation provides an ideal setting to examine the effect of contrasting thermal and erosional regimes. There, tectonically active reverse‐fault‐bounded blocks partly preserve a basement peneplain at elevations in excess of 4500 m. Prior to exhumation, the two study areas were covered by 1000 and 1600 m of recently deposited sediments; this sequence begins with shallow marine deposits immediately overlying the regional erosion surface. Apatite fission‐track data were obtained from vertical transects in the Calchaquíes and Aconquija ranges. At Cumbres Calchaquíes, erosion leading to the development of the peneplain commenced in the Cretaceous, probably as a result of rift‐shoulder uplift. In contrast, Sierra Aconquija cooled rapidly between 5.5 and 4.5 Myr. At the onset of this rapid exhumation, the sediment was quickly removed, causing fast cooling, but relatively slow rates of surface uplift. Syntectonic conglomerates were produced when faulting exposed resistant bedrock; this change in rock erodability led to enhanced surface uplift rates, but decreased exhumation rates. The creation of an orographic barrier after the range had attained sufficient elevation further decreased exhumation rates and increased surface uplift rates. Differences in the magnitude of exhumation at the two transects are related to both differences in the thickness of the sedimentary basin prior to exhumation and differences in the effective precipitation due to an orographic barrier in the foreland and hence differences in the magnitude of headward erosion.     

3-D magnetotelluric inversion and model validation with gravity data for the investigation of flood basalts and associated volcanic rifted margins

20 magnetotelluric (MT) soundings were collected on the Isle of Skye, Scotland to provide a high-resolution three-dimensional (3-D) electrical resistivity model of a volcanic province within the framework of a project jointly interpreting gravity, seismic, geological and MT data. The full 3-D inversion of the MT data jointly interpreted with gravity data reveals upper crustal structure. The main features of the model are interpreted in conjunction with previous geological mapping and borehole data. Our model extends to 13 km depth, several kilometres below the top of the Lewisian basement. The top of the Lewisian basement is at approximately 7–8 km depth and the topography of its surface was controlled by Precambrian rifting, during which a 4.5 km thick sequence of Torridonian sediments was deposited. The Mesozoic sediments above, which can reach up to 2.2 km thick, have small-scale depocentres and are covered by up to 600 m of Tertiary lava flows. The interpretation of the resistivity model shows that 3-D MT inversion is an appropriate tool to image sedimentary structures beneath extrusive basalt units, where conventional seismic reflection methods may fail.     

Re-exposed basement landforms in the Disko region, West Greenland — disregarded data for estimation of glacial erosion and uplift modelling

Classifications of large-scale landscapes in Greenland have traditionally been based on type and intensity of glacial erosion, with the general idea that present landforms are mainly the result of erosion from ice sheets and glaciers. However, on southern Disko and in areas offshore in Disko Bugt, a basement surface has preserved remnants of weathered gneiss and pre-Paleocene landforms, recently exhumed from Paleocene basalt. Isolated hills and lineaments have been mapped in a digital terrain model and aerial photographs. Offshore have hills been mapped from seismic lines. The medium size bedrock forms on southern Disko as tors, clefts and roche moutonées have been studied in the field. Remnant saprolites were inventoried, sampled and analysed according to grain size and clay mineralogy. The basement surface retains saprolites up to 8 m thick in close relation to the cover rocks. The landforms in the basement rocks belong essentially to an etched surface only slightly remodelled by glacial erosion and, below the highest coastline, also by wave action. The outline of hills is governed by two lineament directions, ENE–WSW representing the schistocity of the gneiss and NW–SE fracture zones. These structures are thus interpreted to have been exploited by the deep weathering while the frequent N–S lineaments have not and thus might be younger. Main ice-flow has been from the NE and has resulted in plucking of SW facing lee sides, however the resulting bedrock forms are mainly controlled by structures and orientation of joints. The identification of re-exposed sub-Paleocene etch forms on Disko and the hills of similar size offshore, forming a hilly relief, have implications for identification of a hilly relief south of Disko Bugt, its relation to younger planation surfaces as well as for conclusions of uplift events.     

Deformation of a young salt giant: regional topography of the Red Sea Miocene evaporites

The deformational behaviour of ‘salt giants’ during and shortly after their deposition is difficult to decipher in ocean margin settings where the original evaporites have been deeply buried and strongly mobilized. Here, we examine seismic reflection data from the Red Sea, where evaporites deposited until the end of the Miocene (~5.3 Ma), are generally covered by only 200–300 m of low‐density sediments and where the presence of an axial spreading centre allows us to observe how they have responded to a varied configuration of underlying basement. The regional morphology of the S‐reflection, representing the evaporite surface, is mapped out from seismic data from 13 cruises. The S‐reflection is locally rugged and commonly angular. It is either underlain by layered reflectivity, suggestive of layered evaporite beds, or by more transparent seismic character, suggestive of massive halite. On average, the depth of the reflection on the flanks of the axial rift systematically declines from 700 to 1100 m below sea level (mbsl) going northwards from 16 to 23°N. In the central Red Sea, the S‐reflection has 100‐ to 200‐m‐deep depressions, extending towards the coasts in places. In the southern Red Sea, the S‐reflection forms a surface at 300–800 mbsl that appears less disrupted. We suggest that the evaporites originally had a flat, horizontal surface at the end of the Miocene and have subsequently been distorted by isostatic effects and axial rifting, which in turn promoted evaporite flowage. Off‐axis evaporite depressions correspond with flows identified with multibeam sonar. Furthermore, across‐rift lows in Bouguer gravity anomalies represent valleys in the underlying basement. The off‐axis evaporite depressions overlie those valleys, as would be expected if halokinetic movements were greatest where the evaporites are locally thick, leading to deflation of the evaporite surface. The thickness of post‐Miocene sediment, also mapped out as part of this procedure, confirms the generally pelagic nature of this interval and increases on average from ~250 to 300 m from the central to the southern Red Sea, mimicking the variation in pelagic productivity observed in the present water column.     

A Neoproterozoic glacially influenced basin margin succession and 'atypical' cap carbonate associated with bedrock palaeovalleys, Mirbat area, southern Oman

The Ayn Formation of the Neoproterozoic Mirbat Group comprises <400 m of little‐deformed, glacially influenced basin margin deposits. These deposits are preserved in several palaeovalleys eroded in crystalline basement and overlain by a discontinuous cap carbonate. The Ayn Formation and the cap carbonate, which are superbly exposed along a 20 km SW–NE‐striking escarpment in south Oman, provide important insights into the processes operating on a basin margin during a Neoproterozoic glaciation and its demise. The Ayn Formation comprises units of glacimarine rain‐out diamictite and sediment gravity flow deposits, alternated with units of fluvial and deltaic sandstones and conglomerates, which may have formed by proglacial outwash. The stratigraphic evolution of the Ayn Formation indicates a highly active hydrological cycle during a phase of overall (glacio‐eustatic?) low stand when glaciers advanced into and receded upon bedrock valleys. The transgressive cap carbonate was deposited primarily in shallow marine or shallow lacustrine environments over palaeohighs during the deglaciation, and was partly reworked into deeper parts of the basin through sediment gravity flow processes. Locally, the cap carbonate transgresses over crystalline basement containing a network of fissures filled with carbonate originating from the cap. The δ¹³C isotopic composition of the cap carbonate varies systematically between ?3.5 and +5.8‰ Pee Dee Belemnite standard, in common with other older Cryogenian examples.     

Tectonic controls on a large landslide complex: Williams Fork Mountains near Dillon, Colorado

An extensive ( 25 km²) landslide complex covers a large area on the west side of the Williams Fork Mountains in central Colorado. The complex is deeply weathered and incised, and in most places geomorphic evidence of sliding (breakaways, hummocky topography, transverse ridges, and lobate distal zones) are no longer visible, indicating that the main mass of the slide has long been inactive. However, localized Holocene reactivation of the landslide deposits is common above the timberline (at about 3300 m) and locally at lower elevations. Clasts within the complex, as long as several tens of meters, are entirely of crystalline basement (Proterozoic gneiss and granitic rocks) from the hanging wall of the Laramide (Late Cretaceous to Early Tertiary), west-directed Williams Range thrust, which forms the western structural boundary of the Colorado Front Range. Late Cretaceous shale and sandstone compose most footwall rocks. The crystalline hanging-wall rocks are pervasively fractured or shattered, and alteration to clay minerals is locally well developed. Sackung structures (trenches or small-scale grabens and upslope-facing scarps) are common near the rounded crest of the range, suggesting gravitational spreading of the fractured rocks and oversteepening of the mountain flanks. Late Tertiary and Quaternary incision of the Blue River Valley, just west of the Williams Fork Mountains, contributed to the oversteepening. Major landslide movement is suspected during periods of deglaciation when abundant meltwater increased pore-water pressure in bedrock fractures.A fault-flexure model for the development of the widespread fracturing and weakening of the Proterozoic basement proposes that the surface of the Williams Range thrust contains a concave-downward flexure, the axis of which coincides approximately with the contact in the footwall between Proterozoic basement and mostly Cretaceous rocks. Movement of brittle, hanging-wall rocks through the flexure during Laramide deformation pervasively fractured the hanging-wall rocks.     

Groups, algebras, and the non-linearity of geophysical inverse problems

Mathematical methods from the theory of continuous groups are used to determine whether a non-linear inverse problem, in the form of a functional, can be transformed into a linear inverse problem. If such transformations exist they can be constructed from the solutions of a linear system of differential equations. An illustration of the methodology is given by the linearization of the functional relating basement topography to observed surface gravity. The linearized inversion of gravity data for basement topography is applied to observations from Yucca Mountain, Nevada. A 2.0 km step in the basement to the west of Yucca Mountain, corresponding to the Bare Mountain fault, matches the Bouguer gravity anomaly. The resolution and uncertainty associated with the estimates of basement topography indicate that the structure directly beneath the gravity line is well constrained.     

Distinguishing fault reactivation from flexural deformation in the distal stratigraphy of the Peripheral Blountian Foreland Basin, southern Appalachians, USA

Reactivation of intraplate structures and weak zones within the foreland lithosphere disrupt the modelled geometry and pattern of migration of the flexural wave in foreland basins. In the southern Appalachians (USA), the Middle Ordovician unconformity, irregular Middle Ordovician distal foreland deposition and backstepping of Middle–lower Upper Ordovician carbonate strata have been related to migration of the flexural wave. However, integration of stratigraphy, tectonic subsidence history and composition of palinspastically restored distal foreland strata, using a map of subsurface basement structures as reference, allows us to distinguish an early event of inversion from two events of flexural migration. Sections restoring at very short distances outside the boundaries of a former basement graben have the youngest passive‐margin strata preserved beneath Middle Ordovician (～466 Ma) peritidal to deep lagoonal carbonates with gravel‐size chert clasts. In contrast, sections restoring inside the graben record >470 m of truncation of pre‐Middle Ordovician passive‐margin strata, late onset of deposition (～456 Ma), and subaerial features in carbonate and siliciclastic strata. The lacuna geometry and early patterns of distal foreland uplift and carbonate deposition indicate that inversion of a basement graben in response to Middle Ordovician convergence, rather than a migrating or semi‐fixed forebulge, was the primary control on the early evolution of the distal foreland. Drowning of the carbonate platform in more proximal settings, northeastward onset of deposition on upthrown blocks, and thick accumulation of carbonates in downthrown blocks record northwestward and northeastward flexural wave migration at the Middle–Late Ordovician boundary. In early Late Ordovician, the overall shoaling of carbonate and siliciclastic depocentres and the rise of tectonic subsidence curves indicate hinterlandward migration of flexural uplift. Both events of flexural migration were accompanied by influx of volcanic ash and synorogenic sediments.     

Cenozoic tectonic activity in a Variscan basement: Evidence from geomorphological markers and structural mapping (NW Iberian Massif)

The NW Iberian Massif is part of an ancient basement that has been considered a seismically stable area with no outstanding Cenozoic tectonics. However, recent seismic activity revealed the need for better knowledge of the Cenozoic structures in the area. Because of the lack of Mesozoic deposits and the scarcity of Cenozoic sediments, as well as the intense deformation of the Pre-Mesozoic Variscan basement, it is difficult to study the Cenozoic tectonic structures. In this work, the combination of detailed structural mapping and study of geomorphological markers in the Variscan basement has allowed recognition of Cenozoic tectonic structures, kinematics and processes that otherwise would not have been identified. The identified structures have been gathered into three groups: a) NE–SW-trending strike-slip faults, mainly sinistral, b) NNW-vergent thrusts that uplift the Caurel Mountains and Galaico-Leoneses Mountains, and c) E–W and ENE–WSW thrusts that uplift the Ancares Mountains in a pop-up structure. The structures cut the Pre-Cenozoic erosion surface and affect the drainage network that shows patterns characteristic of tectonic activity. The three groups of structures define sectors with different relief showing a strong link between geomorphological elements and tectonic structures. The intense drainage reorganisations observed in the area and the deformation of Miocene–Pliocene deposits, point out to a significant Late Miocene tectonic activity in the region. Thus, the Cenozoic tectonic activity in the northwest of the Iberian Peninsula takes place during an extensive period of time which started with the episodes of compression in the Cantabrian Margin and it is identified nowadays by the recent seismic activity recorded in the north-western Iberian Peninsula (1995 and 1997 Lugo events). The seismicity is related to the Cenozoic structures identified in the area, which move under the present SE–NW horizontal maximum compression and coincide with the proposed seismogenic faults.