Fault Wedge Research Articles

English

Reinforcement of loose soils with geosynthetics in order to achieve a strengthened soil-reinforcement system, with enhanced tensile strength and reduced settlement is an important subject in the field of geotechnical engineering. In this study, a soil reinforcement system was used to increase the bearing capacity of sandy soil beneath a strip foundation. A small-scale laboratory model was built to investigate the behavior of geosynthetic reinforced sandy soil. The displacement vectors of the soil and bearing capacity of the strip foundation, in reinforced and non-reinforced states were measured to investigate the impact of several parameters, including the type of reinforcement, number of the reinforcement layers, depth of first layer of the reinforcement, and width of the reinforcement. Compared to non-reinforced and geotextile-reinforced models, the geogrid-reinforced model generally had large mass of reinforced soil with higher resistance to loading, which resulted in higher bearing capacity. Similar impacts were observed after increasing the number and width of reinforcement layers. Evaluation of the performance of reinforcements and sandy soil in the physical model with the PIV method and in the numerical simulation showed an increase in the volume of fault wedge and consequently an increase in the bearing capacity of the strip foundation. Key words: Strip foundation, reinforced sandy soil, laboratory model (PIV). &nbsp

Extensional episodes in the Paleoproterozoic Capricorn Orogen, Western Australia, revealed by petrogenesis and geochronology of mafic–ultramafic rocks

Extensional episodes in Precambrian orogens are often difficult to decipher because of subsequent orogenesis and intracontinental reworking. Here, we use geochemical and geochronological constraints of a suite of preserved mafic–ultramafic rocks in the Paleoproterozoic Capricorn Orogen of Western Australia to reveal ophiolites, continental ribbons and aborted rifts. The Capricorn Orogen separates the Archean Yilgarn and Pilbara cratons and includes mafic–ultramafic rocks of the fault-bounded Trillbar Complex and Bryah Sub-basin. The Trillbar Complex is situated within a fault wedge between the Yilgarn Craton and a reworked portion of the craton to the north (Yarlarweelor Gneiss Complex), and has been variously interpreted as an obducted ophiolite, oceanic plateau or continental rift-related magmatic suite. In this study, a new U–Pb zircon age of 2069 ± 9 Ma from the Trillbar Complex indicates that it is at least 40 Myr older than the mafic and ultramafic rocks of the Bryah Sub-basin, with which it has previously been linked. The Trillbar Complex is characterised by E-MORB-like signatures, hydrous crystallization and a lack of crustal contamination, and probably formed in a mid-ocean ridge or, alternatively, an oceanic intraplate setting. Conversely, the 2030–1990 Ma mafic and ultramafic rocks in the Bryah Sub-basin show evidence of crustal contamination and are interpreted to have formed in a continental rift setting. Moreover, there is no evidence for boninites in the Bryah Sub-basin and, therefore, no justification for invoking a fore-arc setting. Thus, these pieces of evidence reveal a different tectonic and geodynamic origin for the Trillbar Complex compared to the Bryah Sub-basin rocks. The tectonic setting for the Trillbar Complex requires oceanic crust to have existed between the Yilgarn Craton and the Yarlarweelor Gneiss Complex. However, almost identical Archean histories of these crustal blocks support a proximal origin for the Yarlarweelor Gneiss Complex and a likely origin as a microcontinental ribbon. Farther east (in present-day coordinates), the Bryah Sub-basin and other sedimentary basins record punctuated rifting that never led to the formation of oceanic crust. Therefore, rigorous evaluation of high quality geochemical data coupled to geochronology from mafic–ultramafic rocks is able to provide valuable constraints on extensional episodes, where other evidence has since been erased from the rock record.

Late Miocene–Quaternary fault evolution and interaction in the southern California Inner Continental Borderland

Changing conditions along plate boundaries are thought to result in the reactivation of preexisting structures. The offshore southern California Borderland has undergone dramatic adjustments as conditions changed from subduction tectonics to transform tectonics, including major Miocene oblique extension, followed by transpressional fault reactivation. However, consensus is still lacking about stratigraphic age models, fault geometry, and slip history for the near-offshore area between southern Los Angeles and San Diego (California, USA). We interpret an extensive data set of seismic reflection, bathymetric, and stratigraphic data from that area to determine the three-dimensional geometry and kinematic evolution of the faults and folds and document how preexisting structures have changed their activity and type of slip through time. The resulting structural representation reveals a moderately landward-dipping San Mateo–Carlsbad fault that converges downward with the steeper, right-lateral Newport-Inglewood fault, forming a fault wedge affected by Quaternary contractional folding. This fault wedge deformed in transtension during late Miocene through Pliocene time. Subsequently, the San Mateo–Carlsbad fault underwent 0.6–1.0 km displacement, spatially varying between reverse right lateral and transtensional right lateral. In contrast, shallow parts of the previously identified gently dipping Oceanside detachment and the faults above it appear to have been inactive since the early Pliocene. These observations, together with new and revised geometric representations of additional steeper faults, and the evidence for a pervasive strike-slip component on these nearshore faults, suggest a need to revise the earthquake hazard estimates for the coastal region.

Lidar reveals uniform Alpine fault offsets and bimodal plate boundary rupture behavior, New Zealand: COMMENT

De Pascale et al. (2014) present four dextral offsets (fluvial terrace risers and channels) identified from an unnamed creek 800 m southwest of Gaunt Creek (one of the best-studied outcrops on the Alpine fault). They interpret these offsets to record three cumulative single-event displacements associated with the last three major surface-rupturing earthquakes on the central Alpine fault and use these data to draw conclusions about the seismogenic behavior of the Alpine fault overall. Herein I argue (1) their apparent dextral offsets occur on a steep dextral-normal fault subsidiary to the main dextral-thrust trace of the Alpine fault at this location, (2) that the subsidiary fault in question is kinematically incapable of producing single-event displacements on the order of 6–8 m, and thus (3) that their discussions and conclusions regarding more complex or bimodal behavior on the Alpine fault are unfounded in light of their data. The nature of complex near-surface fault segmentation on the Alpine fault in the Gaunt Creek–Darnley Creek area has been well established by previous workers. Serially partitioned dextral-thrust and linking strike-slip slip surfaces segment the main Alpine fault plane at a map scale of 1–10 km and to a depth of ~1–2 km (e.g., Norris and Cooper, 2007, and references therein). Superimposed on this serial partitioning are parallel partitioned dextral-thrust and dextral-normal faults defining 100–500-m-wide fault wedges in the near surface (Barth et al., 2012). All of the measurements by De Pascale et al. are clearly made on a subsidiary dextral-normal fault (Southern Alps-side-down motion) and not the main dextral-thrust Alpine fault plane at this location (Fig. 1; cf. their figure 3). This particular subsidiary fault had previously been observed in the field and three-dimensionally modeled by Easterbrook (2010). Barth et al. (2012) used the same 2010 lidar dataset as the study in question to show that these southeast-facing, dextral-normal sense fault scarps are ubiquitous along the length of the 34-km-long central Alpine fault lidar swath, and when tied to outcrop observations (e.g., Norris and Cooper, 2007; Easterbrook, 2010), indicate the presence of parallel partitioning between a dominant (but poorly preserved) outboard dextral-thrust and a subsidiary steep dextral-normal fault (better preserved because the fault plane is steeper and its scarp is inboard of the rangefront). Structural contouring of the Alpine fault in this region by Norris et al. (2012) indicates the fault trace in question is subsidiary to the main Alpine fault plane and likely terminates into it at an intersection angle greater than 50 and at a shallow depth of ~100 m. Barth et al. (2012, their figure 9A) used a simple vector summation of fault plane/slickenline measurements and a plate boundary vector of 251 to show that the (frontal) basal dextral-thrust plane is expected to accommodate greater than three times the displacement of the corresponding fault wedge-bounding dextral-normal fault. A single-event dextral displacement of 7 m on one of these subsidiary dextral-normal faults would kinematically require there to be greater than three times more displacement on the corresponding dextral-thrust trace, a value wholly unrealistic with known Alpine fault behavior. With no direct evidence for the timing of their apparent dextral offsets, De Pascale et al. compile measured offset data along the Alpine fault and near-fault/on-fault paleoseismic proxies to fit their measurements to. Note that all of their compiled offsets occur in locations where the surface geometry of the Alpine fault is considerably less complex than the area in question and strike-slip motion is more dominant overall. Their catalog of paleoseismic events relies on how they chose to define their 20–220 yr uncertainty range for events, yet no explanation of the necessary interpretations of the various proxies and how they were selectively weighted was presented in the paper, or as a supplement in the GSA Data Repository, so readers are left unknowing.

Two-phased evolution of the Suşehri Basin on the North Anatolian Fault Zone, Turkey

This study has aimed to evaluate the current tectonic structure of the Suşehri Basin located on the eastern part of the North Anatolian Fault Zone (NAFZ), one of the most important active faults in Turkey. The work extends earlier investigations of offset and seismicity on the NAFZ and tests a range of evolutionary models. In this study, buried faults have been determined from Ground penetrating radar and magnetic anomalies and possible discontinuities identified by interpolating these data in a region between Gölova and Suşehri. The discontinuities are shown to be linked to negative flower structures formed within the strike-slip fault zone. Quickbird satellite images have been used to map faults and produce kinematic analyses which show that the active stress regime is dominantly strike-slip. However, normal faults and oblique-slip faults are also observed in the basin together with strike-slip faults and the stress regime creating the strike-slip faults is shown to have formed under NW-SE directed transtension. In addition, oblique faults formed under an extensional regime with NNE-SSW direction also occur in the Suşehri Basin as subsets formed under the constraining strike-slip regime. We conclude that the Suşehri Basin started to grow as a fault wedge basin following which it transformed into a pull-apart basin by a south splay on the NAFZ so it is now dominantly a transtensional pull-apart feature.

Scale dependence of oblique plate-boundary partitioning: New insights from LiDAR, central Alpine fault, New Zealand

We combine recently acquired airborne light detection and ranging (LiDAR) data along a portion of the Alpine fault with previous work to define the ways in which the plate-boundary structures partition at three different scales from 6 to 10 0 m. At the first order ( 6 –10 4 m), the Alpine fault is a remarkably straight and unpartitioned structure controlled by inherited and active weakening processes at depth. At the second order (10 4 –10 3 m), motion is serially partitioned in the upper ∼1–2 km onto oblique-thrust and strike-slip fault segments that arise at the scale of major river valleys due to stress perturbations from hanging-wall topographic variations and river incision destabilization of the hanging-wall critical wedge, concepts proposed by previous workers. The resolution of the LiDAR data refines second-order mapping and reveals for the first time that at a third order (10 3 –10 0 m), the fault is parallel-partitioned into asymmetric positive flower structures, or fault wedges, in the hanging wall. These fault wedges are bounded by dextral-normal and dextral-thrust faults rooted at shallow depths (

The kinematic significance of rotation-related deformation features in a fault-defined wedge associated with the North Anatolian Fault, central Turkey

Along most of its 1150km length the North Anatolian Fault Zone (NAFZ) of northern Turkey separates the Anatolian platelet from the Eurasian plate. In its eastern sector this fault zone displays several major splays, including the Kırıkkale-Erbaa (Sungurlu) Fault Zone. The region investigated in this study forms a wedge delimited by the NAFZ, the Kırıkkale-Erbaa splay and the Elmadag Imbricate Thrust Zone and this tectonically defined wedge also hosts the greater part of an important Cenozoic sedimentary trough, the Çankiri-Çorum Basin. Detailed study of an area in the southern part of this basin, between Delice and Salmanli, has provided evidence for the complex neotectonic evolution of this region, including details of the nature and behaviour of the Kırıkkale-Erbaa Fault (KEF). Evidence of significant neotectonic deformation has been observed in basin-fill strata of Eocene to Late Pliocene age, mostly within a zone of high strain adjacent to the KEF. These features include young thrusting of Eocene limestones on to Miocene units, tight upright and overturned folds within the fault zone that involve Eocene–Pliocene strata, and kinematic indicators from the KEF master fault plane. These and related morpho-tectonic features demonstrate that this young phase of deformation is predominantly compressive/transpressive in character in this area.It is concluded that the local compressive/transpressive stress field has been created by anticlockwise rotation of a wedge-shaped block (fault wedge) lying between the North Anatolian Fault Zone and its splay, the KEF, with compression maximised in a narrow zone close to the latter. Moreover, the north–south trending Elmadağ Imbricate Thrust Zone together with a buried structural high within the local basement has blocked the westwards translation of this tectonic wedge, thus creating further localised compression and inducing southwestwards ‘escape’ of the fault wedge block. Previously recorded differences in strain rates associated with the North Anatolian Fault Zone and the KEF, together with the north-convex curvature of the North Anatolian Fault Zone in this region are considered to be the factors primarily responsible for this anticlockwise rotation. This study demonstrates that previous geotectonic history plays a determinant role in the neotectonic behaviour of the individual structural elements present within the Anatolian collage, a conclusion that may be relevant to the understanding of strike-slip dominated domains in comparable geotectonic settings elsewhere.

Evolution of the Gölbaşı basin and its implications for the long-term offset on the East Anatolian Fault Zone, Turkey

The left lateral East Anatolian Fault Zone (EAFZ) is one of the most active major neotectonic structures of the Eastern Mediterranean region. The fault zone runs for a distance of about 550km between Karlıova in northeast and Mediterranean Sea in southwest. Several fault-parallel basins (such as Hazar and Gölbaşı basins) have been forming along the fault zone. The Gölbaşı basin is the largest basin along the EAFZ and it is located near the junction of the Çelikhan–Erkenek and Gölbaşı–Türkoğlu segments of the EAFZ. Different interpretations including pull-apart, fault wedge and fault ramp basin were made about the evolution of the basin in previous studies. Detailed mapping shows that the Çelikhan–Erkenek and Gölbaşı–Türkoğlu segments are connected by a releasing bend around Gölbaşı Lake. Our study also suggests that Gölbaşı basin was a wide river valley in which the Aksu River flowed and occupied by a large lake. The valley was blocked by a large landslide at least 31,600±500 years ago in the northeastern corner of the basin and as a result, the Aksu River was captured to the SW corner of the basin. Our scenario implies that the Aksu River valley is left laterally offset by the EAFZ about 16.5±0.5km, which is the largest documented morphological offset on the EAFZ.

Structural collapse of a transpressive hanging‐wall fault wedge, Charwell region of the Hope Fault, South Island, New Zealand

The northeast‐trending dextral‐reverse oblique slip Hope Fault is one of the major structures of the Marlborough Fault System and the Australia‐Pacific plate boundary zone in the South Island of New Zealand. This study presents an analysis of the structural and tectonic geomorphic development of the Hope Fault Zone in the vicinity of the Charwell River to understand the near‐surface temporal and spatial structural style of deformation and fault zone kinematics along a 10 km section of the fault. Significant fault‐related landscape units include: (1) flights of aggradation‐degradation terraces in the footwall forming an extensive piedmont; (2) fault‐dissected, sloping topography in the hanging wall containing 95% of all the faults; and (3) eroded subhorizontal piedmont terrace remnants that indicate the range front has repeatedly propagated to the southeast into the footwall block. We recognise four distinct types of fault scarps: (1) the main range‐front trace of the Hope Fault defines a releasing bend geometry, with a projected step‐over width of c. 1000 m; (2) at the foot of the range front, two thrust faults ramp over the aggradational surfaces in the footwall block; (3) in the toe of the hanging‐wall block, c. 20 normal fault scarps are mapped near‐parallel to the main Hope Fault; and (4) more than 100 late normal faults are oblique to the main Hope Fault and cut obliquely across all other faults. The overall fault pattern outlines an initial fault wedge between the thrust and early normal faults that is 5 km in length and 1 km at its widest point. A secondary wedge defined by the late normal faults is 7 km in length, 2 km at its widest, and it overprints the initial wedge. The structural/geomorphic interactions between the initial and secondary fault wedges developed in a series of at least four successive stages. These spatial and temporal changes in the structural geometry and style of deformation of the Hope Fault Zone at Charwell are interpreted to reflect the role of topographic loading in adjusting the fault zone break‐out along the range front, and is recorded by the tectonic geomorphic landforms there.

Surface rupture of the Poulter Fault in the 1929 March 9 Arthur's Pass earthquake, and redefinition of the Kakapo Fault, New Zealand

The 1929 March 9 Arthur's Pass earthquake of MS7.1 occurred on a newly mapped fault in the Arthur's Pass region, which we name the Poulter Fault. Surface rupture of at least 16 km and possibly as much as 36 km occurred with 4 m of dextral displacement at one site. The extent of fault rupture coincides very closely with a narrow, elongate zone of intense landsliding. A best estimate of the dip‐slip component of faulting is 1–2 m (north side up), making the 1929 rupture a dextral to oblique‐dextral fault displacement, in keeping with earthquake first motion studies. The Poulter Fault is mapped from the South Hurunui River in a WSW direction to Williams Saddle near the confluence of the Mingha and Edwards Rivers, a distance of nearly 50 km. The 1929 earthquake was not on the Kakapo Fault as previously proposed by Yang. No active fault has been found along the line proposed by Yang, and the Kakapo Fault is here redefined as the southern element of a rhomboid fault wedge formed with the Hope Fault between Kakapo Brook and MacKenzie Stream in the upper Hurunui valley.

Geodynamics along an increasingly curved convergent plate margin: Late Miocene‐Pleistocene Rhodes, Greece

Neogene‐Holocene outward migration of the absolute position of the convergent Hellenic plate boundary produced simultaneous increased curvature of the plate boundary, changing obliquity of plate convergence vectors and boundary‐parallel stretching of the forearc region. To study the effects of the plate boundary migration and curvature, a tectonostratigraphy is constructed from the middle Miocene–Pleistocene Apolakkia basin on Rhodes, whose easternmost location makes it a key island to assess the inner forearc's kinematic response to expansion of the overriding Aegean‐Anatolian block and thus obliquity of convergence with the African plate. The basin fill provides temporal and paleogeographic control to interpret its syndepositional and postdepositional structural assemblages. Five fault populations in the Apolakkia basin record two neotectonic deformation phases separated by a kinematic change at ∼4.5 Ma, both of which are consistent with outward expansion of the Aegean‐Anatolian block. The Apolakkia basin originated as a late Miocene fault wedge basin in response to syndepositional southwest‐northeast D1 extension with similar strain patterns in the adjacent offshore Hellenic inner forearc. The kinematic change at ∼4–5 Ma is attributed to a threshold of obliquity whereby the inner forearc started to experience sinistral‐oblique divergence. The Plio‐Pleistocene D2 transtensional phase reoriented the basin and resulted in combined syndepositional west‐northwest‐east‐southeast extension (283°) and 070° sinistral shear, orientations that are best attributed to simultaneous outward expansion of the Hellenic forearc, increasing curvature of the plate boundary and associated boundary‐parallel stretching of the forearc. Principal shear zones offshore also occur consistently at ∼070°, mimicking the D2 kinematic history of the Apolakkia basin and suggesting a consistent geodynamic regime throughout the inner eastern Hellenic forearc. Effects of sinistral‐oblique plate convergence along the subduction zone appear confined to the outer forearc and the accretionary prism of the Mediterranean Ridge. Thus the upper crust of the expanding Aegean‐Anatolian block behaved independently at its leading edge, and the Pliny “trench” constitutes a major boundary separating partitioned forearc slivers, a post‐4.5 Ma partitioning recorded reliably by the Pliocene fill of the Apolakkia basin.

Transtensional deformation in the evolution of the Bohai Basin, northern China

Abstract Extensional basins with an element of strike-slip deformation can form because of a perturbation in a strike-slip fault zone (pull-apart and fault wedge basins), or where extension is oblique to the margins of the deforming zone (transtensional basins). Transtensional basins are characterized by en echelon arrays of normal faults which are individually oblique to the basin margins. The Bohai Basin, northern China, has previously been modelled as either (1) a giant pull-apart between NNE-SSW trending dextral strike-slip faults, or (2) a rift basin caused by WNW-ESE extension, without significant strike-slip deformation. We present a model for the Bohai Basin’s rift history in which the basin formed as a result of dextral transtension. The Bohai Basin is one of a family of early Tertiary extensional basins present within eastern Asia from northeastern Russia to southeast China. The structural grain in this basin was inherited from a phase of late Mesozoic sinistral transpression. Tertiary extension began in the Paleocene. Most half-grabens in the eastern and western regions of the Bohai Basin have master faults with a NE-SW or NNE-SSW orientation. Secondary normal faults strike oblique to the main structures, in en echelon arrays which indicate a component of dextral transtension. The central part of the basin, the Bozhong Depression, became a significant depocentre for the first time in the middle Eocene. It formed when activity on transtensional zones to its east and west created an extensional overlap between them. Thus the basin as a whole resembles a giant pull-apart basin, with the Bozhong Depression as its central depocentre, but dextral transtension rather than simple strike-slip controlled the deformation. The component of dextral deformation in the Bohai Basin is shared by other early Tertiary east Asian extensional basins, and is consistent with the sense of shear implied by the oblique convergence of the Pacific and Asian plates: an east-west convergence vector applied to a NE-SW trending plate boundary. The consistency of this dextral shear along the Asian margin, and the fact that several of these basins pre-date the India-Asia collision, supports an origin by subduction roll-back of the oceanic Pacific plate from Asia. The extrusion model for east Asian basin formation, whereby extension was caused by lateral transport of lithospheric blocks out of India’s northward path following the India-Asia collision, is not applicable to major basins east and northeast of the Red River Fault.

Geospatial Modeling: A Breakthrough 3-D Technology for Understanding Complexly Faulted Geologic Structures: ABSTRACT

Today's geoscientists are required to find hydrocarbons in increasingly complicated geologic structures. These structures are usually faulted and contain complex geometric relationships. Their complexity requires modeling and mapping capabilities beyond traditional surface mapping tools. Geospatial modeling enables geoscientists to create models of complex geologic structures, calculate distributions of properties therein, visualize models in three dimensions, and analyze models through volumetric calculations. Faults and horizons are treated as individual surfaces in three-dimensional space. Fault relationships are controlled by hierarchies, and relationships between stratigraphic layers are specified by simple geologic rules. The following features can thus be accommodated: depositional layers; layers truncated by unconformities; layers cut by channels; channel in-fill; and salt diapirs. Although the resulting model is calculated from a fixed set of geological relationships, input data may change as data are added or existing interpretations altered, allowing for quick update. Property models may be calculated for each layer and/or each fault block. The three-dimensional property modeling utilizes fault and horizon surfaces as boundaries. Properties are correctly calculated near faults and show appropriate displacements across faults. Unique three-dimensionally consistent contour maps and cross sections are extracted from the models. The three-dimensional contouring process determines the spatial extent of layers, defines geometriesmore » of fault and unconformity truncation wedge zones, and accurately portrays fault gaps. Increased success is important in today's market. Geospatial modeling technology can address the sizeable problems facing geoscientists working with complexly faulted geologic structures, and can significantly increase overall understanding in a timely manner.« less

Mechanics of wedge‐shaped fault blocks: 2. An elastic solution for extensional wedges

Listric, planar, low‐angle, and high‐angle normal faults are common in hanging walls of detachment faults. An elastic model has been developed to evaluate the role of basal friction, wedge geometry, pore fluid pressure within the wedge, and boundary conditions applied along the wedge rear in controlling the stress distribution in an extensional fault wedge. This model assumes a stress‐free condition on the top and fractional sliding on the base of the wedge, respectively, a linear variation of stress components as a function of depth along the wedge rear, and a uniform horizontal stress applied on the wedge toe. The model predicts that (1) for the surface slope equal to zero, a thin wedge favors development of high‐angle and planar faults, whereas a thick wedge favors development of low‐angle and listric normal faults; variation of pore fluid pressures along the basal detachment fault hardly affects the predicted fault geometry; pore fluid pressure within the wedge is critical in controlling the state of stress in the wedge: higher values of the internal pore fluid pressure promote low‐angle normal faulting and locally high‐angle reverse faulting; (2) variation of surface slope and the uniform horizontal normal stress applied at the wedge toe does not affect the predicted fault geometry appreciably, although the distribution of deviatoric stress magnitude changes for different cases; and (3) listric normal faults are predicted in all computations and the fault curvature increases as the vertical gradient of the horizontal normal stress along the wedge rear, the wedge angle (surface slope + dip angle), and the internal pore fluid pressure increase. The model provides a simple conceptual guide to deciphering the formation of complex fault geometries and cross cutting relationships as a function of mechanical parameters related to geologic processes and settings. For example, it provides an explanation for why low‐angle normal faults cut high‐angle normal faults in hanging walls of some detachment fault systems in the U.S. Cordillera.

Geochemistry of the Cambrian volcanic-hosted massive sulfide-rich Mount Read Volcanics, Tasmania, and some tectonic implications

The Cambrian Mount Read Volcanics belt in western Tasmania hosts five major gold-rich massive sulfide deposits and remains the focus of intense exploration activity and geologic research. A geochemical study of least altered lavas and shallow intrusions, with emphasis on basaltic to andesitic compositions, was carried out to determine the primary magmatic affinities and tectonic setting of eruption of the Mount Read Volcanics, to aid in internal correlations within the belt, and to provide starting-material estimates for mineralization-related alteration studies.Three calc-alkaline suites (one extending to shoshonitic compositions) and two tholeitiic suites have been distinguished within the Mount Read belt. Suite I is voluminous and includes the Eastern sequence, Central Volcanic Complex, Tyndall Group, the intrusive quartz-feld-spar porphyries and granitoids, and the andesitic lavas of the Que-Hellyer footwall sequence. Suite I andesites are transitional, medium to high K calc-alkaline rocks ((La/Yb) N = 5-12, avg 8.1) and are the least light REE-enriched calc-alkaline lavas in the Mount Read Volcanics.Suite II comprises intrusive and extrusive, often hornblende-porphyritic andesites and dacites mainly from the upper part of the southern Central Volcanic Complex. They are more P 2 O 5 and light REE enriched ((La/Yb) N = 10-26, avg 16.7) than suite I and have high K calc-alkaline affinities.Suite III includes basaltic and andesitic lavas from the Que-Hellyer hanging-wall sequence in the northern part of the belt, the Lynch Creek basalts near Queenstown, and intrusive basalts in the Howards Plains area northwest of Queenstown. Suite III rocks show a striking compositional range, from low TiO 2 (0.4-0.5%), low P 2 O 5 (<0.1%) basalts with (La/Yb) N values from 8 to 12 and almost flat heavy REE patterns, to low TiO 2 (0.4-0.8%) but strongly to exceptionally P 2 O 5 and light REE-enriched basalts with up to 350 times chondritic La and (La/Yb) N values up to 34. The latter are regarded as shoshonites.Tholeiitic pillow basalts of the Henty fault wedge, and the closely related dikes of the Henty dike swarm, constitute suite IV: these have TiO 2 contents from 1.0 to 1.6 percent, very low Nb contents (<3 ppm), and only slightly light REE-enriched REE patterns ((La/Yb) N = 1.4-3.4). They are considered to be most similar to suprasubduction zone basalts erupted during the early phase of arc splitting and back-arc basin development and indicate a period of tension and rifting of the Mount Read belt in post-Central Volcanic Complex but pre-Tyndall Group time.Suite V is constituted by the Miners Ridge basalts and includes low TiO 2 (<0.7%) basalts with strong light REE depletion ((La/Yb) N <0.5) and higher TiO 2 (to 2.6%) lavas with only slight light REE depletion ((La/Yb) N = 0.9). Suite V basalts are tentatively correlated with rift tholeiite sequences associated with the Crimson Creek Formation, and therefore, are regarded as part of the pre-Mount Read Volcanics basement.The three calc-alkaline suites occur in a general stratigraphic order (with suites I and II being more or less coeval near Queenstown); they indicate a magmatic evolution from transitional medium to high K rocks through high K rocks, to strongly enriched shoshonitic rocks. Closest modern analogues are postcollisional volcanics in regions of arc-continent collision (e.g., East Papua and the Roman province).The occurrence of boninite-derived chromites (probably derived from the allochthonous mafic-ultramafic complexes outcropping just west of the Mount Read belt) in two sedimentary units within the Mount Read Volcanics sequence, below suite III basalts, supports a postcollisional origin for at least the major part of the Mount Read Volcanics. Other similar postcollisional sequences, particularly if formed in a submarine setting, must be considered highly prospective for VHMS deposits.

Massive sulfide deposits of the Noranda area, Quebec. I. The Horne mine

The Horne massive sulfide deposits occur within volcanic rocks of the Blake River Group of the Archean Abitibi greenstone belt. The orebodies dip subvertically within rhyolitic flows, breccias, and tuffs that are bounded by the Andesite and the Horne Creek faults. Least-altered rhyolites have low K2O contents and other geochemical features that place them within the FII tholeiitic series. Graded volcaniclastic beds, metal zoning in the orebodies, and locations of chloritized–mineralized rhyolites indicate that the volcanic sequence youngs to the north. The volcanics in the fault wedge are variably silicified and sericitized, and local zones in the orebody sidewalls and footwall are chloritized.The H orebodies formed podiform masses up to 120 m wide, 100 m thick, and 300 m in downplunge extent, consisting of chalcopyrite–pyrrhotite–pyrite Au ore. Between 1927 and 1976, 54 × 106 t of ore were recovered, grading 2.2% Cu, 6.1 g/t Au, and 13.0 g/t Ag (Zn and Pb are &amp;lt0.1% and &lt;0.01%, respectively). A semicontinuous Cu-rich base (up to ~15 m thick) exists above the footwall and adjacent to the sidewalls of the orebodies. The ore changes stratigraphically upwards from a chalcopyrite-rich base, through middle pyrrhotite–pyrite-rich zones, to upper pyrite-rich zones. Au enrichments occur in some of the Cu-rich ores but also in overlying pyritic ores and in adjacent host volcanics. Cu–Au-bearing chloritized rhyolites occur mainly in the western and eastern sidewalls and at downplunge terminations of the H orebodies.The No. 5 zone occurs at lower mine levels and consists of numerous, partly overlapping Zn-bearing pyritic lenses up to 30 m thick, within mineralized rhyolitic breccias and tuffs. The No. 5 zone extends up to 750 m along strike and at least 1500 m downdip, with high-pyrite reserves of ~22 × 106 t between the 21st and 39th levels, grading 1.2% Zn, 0.15% Cu, and 1.4 g/t Au. Massive pyritic lenses are richer in Zn (&gt; 50 ×) and Pb, Ag, As, Cd, and Sb relative to the H orebodies but are low in Cu and Au.The restored stratigraphic level of the H orebodies and No. 5 zone was dominated from south to north by rhyolite flows and breccias, then rhyolite breccias and tuffs. The volcanic rocks are interpreted as proximal to distal facies on a volcanic edifice that was affected by widespread silicification and sericitization. A graben system on the flank of the edifice became the depositional site of the H orebodies. High-temperature fluid discharge occurred along the fault-bounded graben margins, producing zones of chloritization and stringer-type Cu mineralization ± Au in rhyolites, and infilling the grabens with Cu-bearing massive sulfides. Lower on the edifice, in the No. 5 zone, Zn-bearing pyritic sulfide lenses accumulated within broader, breccia-based depressions roughly on strike with the H orebodies. Mineralization in the No. 5 zone may reflect lower temperature, more diffuse fluid discharge through a permeable sequence of volcaniclastic rocks.

Paleoecology and Biogeography of Late Cretaceous Molluskan Assemblages Near Loma Prieta, California: ABSTRACT

Deformed, fossilliferous turbidite deposits resembling the Great Valley sequence are present in a fault wedge just east of the San Andreas fault near Loma Prieta Santa Cruz Mountains, California. Near the top of these deposits late Campanian to early Maastrichtian Baculites rex is found along with late Campanian B. inornatus and B. aff B. anceps. Turbidite deposits associated with a conglomerate near the base of the Upper Cretaceous strata contain abundant exogyrid oysters and many rare or uncommon taxa including Amphidonte parasitica, Lyriochlamys traskii, Spondvlus subnodosus, and Hipponix dichotomus, which also imply a late Campanian age. Composition of the oyster-rich assemblage indicates a relatively high-energy, nearshore source area for those turbidite deposits. This source area consisted of mixed sand and rock or shell bed substrates. Fossils found in the overlying Upper Cretaceous turbidites are indicative of a more offshore shelf habitat, suggesting a seaward shift in sediment source area or transgression through time. Molluskan assemblages include North Pacific species having tectonically transported distributions ranging from Baja California to Vancouver Island. Some species in the oyster beds have been previously reported only to the north and some only to the south of Loma Prieta; thus, the unusual faunal composition is apparently more » due to the preservation of a very nearshore assemblage, which is rare in strata of this age in California, rather than to biogeographic constraints. The apparent absence of rudists in this very nearshore assemblage, where they would be expected if present suggests significant displacement of the Upper Cretaceous rudist-bearing rocks found to the west of the San Andreas fault relative to Loma Prieta. « less

Tectonic wedges and offset Laramide structures along the Polochic Fault of Guatemala and Chiapas, Mexico: Reaffirmation of large Neogene displacement

Fault wedges along the Polochic fault of northern Guatemala and Chiapas, Mexico record left‐lateral slip of from 20 to 65 km that accompanied documented Neogene slip of 130 km across this North American—Caribbean plate boundary fault. Wedges can be correlated to source areas consistent with left‐lateral displacement along the Polochic fault. They occur adjacent to restraining bends in three areas where changes in strike of the fault have been mapped. These wedges, which have been previously interpreted as allochthons emplaced along low‐angle reverse faults, are bounded by high‐angle faults. Folds that terminate against the Polochic fault are part of a regional fold belt that originated during the Campanian through Paleocene (Laramide) orogenic event. A previous contention that some of these major folds are secondary and related to fault movement is shown to be incorrect. The 130 km model of left‐lateral offset across the Polochic fault is reviewed along with evidence for and against a Neogene time of major displacement. Additional evidence for the model is presented: (1) Late Cretaceous granitoid rocks from near Motozintla, Chiapas, Mexico (northern fault block), are correlated with granitic rocks of the Santa Maria Batholith in the region of Huehuetenango, Guatemala (southern block), (2) mineralization at the contacts of the granitoid intrusives of the two areas is correlated, (3) points of deflection of Laramide structural axes are correlated across the fault, (4) a major thrust sheet similar to that of the Sierra de Santa Cruz in eastern Guatemala is reassembled by reversing 130 km of left‐lateral slip which has offset serpentinites north of the fault in western Guatemala and those south of the fault in central Guatemala.

Basement balancing of Rocky Mountain foreland uplifts

Restorable cross sections of foreland basement uplifts must contain faults whose curvatures are consistent with the relative slip and tilt between adjacent basement blocks. Cylindrical fault surfaces can explain the uniform dip of strata on the back side of foreland uplifts; local zones of shortening and extension occur in hinge zones above transitions in fault curvature. High-curvature fault splays form fault wedges of basement which ease the transition from thrust and reverse faulting of basement blocks to folding in the sedimentary cover. 20 references, 4 figures.

El Pilar fault zone, northeastern Venezuela: Brief review

Abstract The El Pilar fault zone extends for about 700 km in an approximately E—W direction, from the Cariaco Trench to a point about 200 km northeast of Trinidad. It marks the southern boundary of the Araya—Paria peninsulas (eastern Caribbean Mts.) and of the Northern Range of Trinidad. It is characterized along its length by straight valleys, fault wedges, fumaroles, thermal springs, and sulfur deposits. The displacement along the El Pilar fault zone has been the subject of much controversy. Various authors recognize the following types of displacement: (1) southward thrust; (2) normal or graben faulting; (3) right-lateral strike-slip. The El Pilar fault zone probably represents a transform fault between the Caribbean and South America plates, and a hinge fault at its contact with the subduction zone east and south of the Lesser Antilles. It is planned to investigate the present-day movement along the El Pilar fault zone by high-precision geodetic methods, at the following localities: across the Gulf of Cariaco and at Casanay (central-southern Araya—Paria peninsulas).