Regional NW Research Articles

Effect of thermal refraction on heat flow near the San Andreas Fault, Parkfield, California

Heat flow data near the San Andreas Fault (SAF) do not reveal a near‐fault anomaly as expected from frictional heat generation, an observation interpreted to indicate that the fault slips at a depth‐averaged shear stress <20 MPa. The data also contain large unexplained scatter, which has been a separate major issue in the analysis of heat flow within the California Coast Ranges. Here we use numerical models of heat conduction to evaluate the hypothesis that thermal refraction, due to contrasts in thermal conductivity in the subsurface, both produces the observed scatter in heat flow and as a result obscures the thermal signature from frictional heating on a fault that supports large shear stress during slip. Our study focuses on the region around the San Andreas Fault Observatory at Depth (SAFOD) near Parkfield, California. Our results show that surface heat flow is most sensitive to the contrast between Tertiary sediments and basement rocks and to wavelengths of basement topography of ∼10 km. With realistic thermal conductivity contrasts and a reasonable interpretation of this geologic contact, we show that thermal refraction is a plausible explanation for the observed heat flow scatter. However, refraction effects are unable to mask frictional heat generation in a manner consistent with observations. We show that even with large refraction effects, low background heat flow, a regional NW–SE decrease in heat flow, or nonsteady state heat conduction, the data are most consistent with a fault that produces little to no frictional heat.

Volcanic and structural history of the rocks exposed at Pickering Crater (Daedalia Planum, Mars)

In the western hemisphere of Mars Amazonian volcanism from Arsia Mons produced the smooth surfaces of Daedalia Planum and masks older rocks. Close to the southern termination of Daedalia Planum basement rocks are exposed in which are preserved craters that escaped or were only partially filled by this most recent volcanism. Pickering Crater is an approximately 130 km diameter crater. The youngest lavas flowed into this crater from Daedalia Planum by way of a NW rim breach, covering its western part. East of a well-defined flow front an older lava sequence with a distinctive platy surface and derived from a more proximal unestablished source to the northeast is exposed. Several units are identified within this sequence on the basis of surface texture, which is more subdued in progressively older rocks. Only local mapping of the flow front boundaries of these units is possible because of incomplete coverage by high resolution imagery. During emplacement of the older lavas a NE–SW striking en echelon graben system and parallel smaller troughs and dikes formed under inferred regional NW–SE extension. A much earlier strike-slip regime pre-dating the lavas exposed in the crater floor is postulated, based on the highly fretted nature of the rim of Pickering Crater and an elongated smaller crater to its northeast, approximately 40 km long in the NE–SW direction. The rims of these craters contrast with that of a smoother rimmed impact crater in the southeast that was excavated subsequent to strike-slip deformation but prior to the emplacement of platy surfaced lavas.

Geochemical mapping of the deeply weathered western Yilgarn Craton of Western Australia, using laterite geochemistry

Multi-element analysis of ferruginous nodules and pisoliths from lateritic residuum, derived lag and ferruginous gravel, selected from locally derived colluvium (laterite) sampled at an approximate 9 km interval (triangular grid) over the western Yilgarn Craton, shows regional geochemical trends, major lithologies and dispersion halos around significant bedrock mineralization. The sample density (one sample per 60–100 km 2 , depending on sample availability) and extent of the coverage ( c . 400 000 km 2 , including large unsampled areas in drainage) mean that the data are potentially valuable for both exploration and environmental purposes. More than 3100 samples have been analysed for 53 elements by XRF, ICP-AES and ICP-MS, with selected samples also analysed for PGE. Elevated Au abundances in the NE of the survey area not only cluster around known gold deposits but extend beyond them, indicating the likelihood of more widespread mineralization in these areas. A chalcophile element index illustrates potential for Au and base metal mineralization in the westernmost part of the Yilgarn Craton, whereas a pegmatophile index shows a regional NW trend parallel to regional structures. Abundant chromium in granite-dominated areas might indicate mafic-ultramafic remnants (some with anomalous Au) beyond known greenstone belts. A newly discovered regional Hg anomaly trends NW for more than 500 km. Anomalous As, Bi, Mo and Sb along the southern margin of the craton may be related to Au mineralization. The spatial and geochemical consistency of the dataset means that it is well suited to multivariate statistics.

Recent large fold nucleation in the upper crust: Insight from gravity, magnetic, magnetotelluric and seismicity data (Sierra de Los Filabres–Sierra de Las Estancias, Internal Zones, Betic Cordillera)

Rheological heterogeneities in the upper-crust have a close relationship with the fold position where rigid bodies could constitute initial perturbations that allow the nucleation of folds. Consequently, establish the position and geometry of anomalous rocks located in the upper-crust by geophysical studies help to understand the folded structure observed on surface. New geological observations in the field, along with gravity, magnetic, magnetotelluric and seismicity data, reveal the subsurface structure in the Sierra de Los Filabres–Sierra de Las Estancias folded region part of the Alpine belt in southern Spain. The geometry of the upper crust is determined by geological field data, 2D gravity models, 2D magnetic models and 2D MT resistivity model, while seismicity evidences the location of the deep active structures. These results allow us to propose that a basic rock body at 4 to 9 km depth has determined the nucleation and development of the Sierra de Los Filabres kilometric antiform. N-vergent large late folds are subjected to a variable present-day stress field. Earthquake focal mechanisms suggest the presence in depth of a regional NW–SE compressive stress field. However, most of the seismogenetic structures do not extend up to the surface, where NW–SE and WNW–ESE outcropping active normal faults are observed, thus indicating a NE–SW extension in the upper crust simultaneous to orthogonal NW–SE compression related to reverse faults and minor folds developed in the Eastern Almanzora Corridor and in the nearby Huércal–Overa Basin. The recent and active tectonic studies of cordilleras hinterland subjected to late folding greatly benefits from the integration of surface observations together with geophysical data.

The Anti-Atlas chain (Morocco): the southern margin of the Variscan belt along the edge of the West African craton

Abstract Broadly synchronous circum-Atlantic Variscan–Alleghanian orogenic belts developed during the Late Palaeozoic Gondwana–Laurentia collision. In the northern part of the West African craton (WAC), the Variscan orogeny produced basement-controlled structures in the Anti-Atlas, which represents the pericratonic foreland, now located south of the Variscan domains of Morocco and north of the Mauritanides belt. New structural field observations document the strong involvement of the basement and the inversion and folding of the Palaeozoic sedimentary basins at the edge the WAC. Two contrasting domains differently responding to regional NW–SE shortening are recognized: (1) a narrow belt along the Atlantic coast characterized by thin-skinned folding and ESE-vergent thrusting (para-autochthonous Anti-Atlas); (2) a large area between the WAC sensu stricto and the South Atlas front showing huge basement uplifts amidst a folded Palaeozoic cover with upright polyharmonic folds (autochthonous Anti-Atlas). The structural trend of the basement inliers is inherited at least in some case from previous Proterozoic fractures. Compressional reactivations led to basement uplift and concomitant folding of the Palaeozoic cover. Cover series are horizontally shortened by mostly upright symmetrical buckle folds of various wavelengths in response to thickness variations between abundant incompetent silt and shale horizons and rare competent carbonate and quartzite beds. Deformation is greatest near the borders of and between closely spaced basement uplifts. Regionally, deformation intensity decreases, either abruptly or progressively, towards the SE and it vanishes within the undeformed Tindouf basin.

Present‐day stress field in the Gibraltar Arc (western Mediterranean)

The Gibraltar Arc in the western Mediterranean consists of the Betic and Rif Alpine chains and the Alboran Sea Basin. Four types of stress indicators (wellbore breakouts, earthquake focal plane mechanisms, young geologic fault slip data, and hydraulic fracture orientations) indicate a regional NW–SE compressive stress field resulting from Africa‐Eurasia plate convergence. In some particular regions, deviations of SHmax are observed with respect to the regional stress field. They are gentle‐to‐moderate (22°–36°) anticlockwise rotations located along the North Alboran margin and moderate‐to‐significant (36°–78°) clockwise rotations around the Trans‐Alboran Shear Zone (TASZ). This is a broad fault zone composed of different left‐lateral strike‐slip fault segments running from the eastern Betics to the Alhoceima region in the Rif and resulting in a major bathymetric high in the Alboran Sea (the Alboran Ridge fault zone). Some of these stress rotations appear to be controlled by steep gradients of crustal thickness variation across the North Alboran margin and/or differential loading imposed by thick sedimentary accumulations in basin depocenters parallel to the shoreline. Other stress perturbations may be related to active left‐lateral, strike‐slip deformation within the TASZ that crosscuts the entire orogenic arc on a NE–SW trend and represents a key element to understand present‐day deformation partitioning in the western Mediterranean.

Lithospheric and sublithospheric anisotropy beneath central-southeastern Brazil constrained by long period magnetotelluric data

Electric anisotropy calculated from geoelectric strikes of magnetotelluric (MT) data and seismic anisotropy derived from shear-wave splitting parameters are jointly analyzed to estimate the degree and orientation of strain in the subcontinental mantle of central-southeastern Brazil. High-quality long-period MT soundings are available at 90 sites concentrated along four profiles at the southwestern and southern borders of the Paleoproterozoic-Archean São Francisco craton. This data set is complemented by a previous study of SKS and SKKS splitting measurements available at more than 40 sites covering a slightly wider region. For this study, the MT data were processed with modern techniques, including recovery of the undistorted EM field polarizations through correction of static shift and phase mixing due to galvanic distortions. Three-dimensional forward modelling of MT and GDS (geomagnetic depth soundings) transfer functions supports interpretation of deep electrical anisotropy in the region. Magnetotelluric phase responses of orthogonal propagation modes present slight splitting at long periods for most of the sites, indicative of overall electrically low mantle anisotropy to depths greater than 250 km. The electrical strike azimuths are parallel to the fast NW directions of shear-wave splitting along the southern borders of the São Francisco craton, suggesting that the seismic anisotropy also resides within the same depth range. Since this direction is very different from that of present-day South American westward absolute plate motion, it is inferred that no significant lateral mantle flow or deformation related to the plate motion is observed under the study area, either because it is absent or the rigid lithosphere is much thicker than expected. To explain the prevailing electric and seismic common direction it is suggested that a mechanical coupling between lithospheric and sublithospheric mantle exists. A relic strain, possibly resulting from ancient continental collision processes, is interpreted to have induced a general alignment of olivine down to mantle depths beneath the continental rigid plate. The observed slightly enhanced conductive texture could be associated with hydrogen diffusion along the aligned olivine a-axis, especially within mantle patches subjected to metasomatic processes. The coincidence of mantle strikes with the trend of surface deformation pattern also suggests that crust, lithospheric and sublithospheric upper-mantle have deformed and translated coherently, preserving the regional NW direction since the tectono-thermal event responsible for the deformation. Distinct electric azimuths from the general NW trend are observed in regions probably perturbed by Cretaceous rift-related intraplate magmatism and in zones of complex deep structures produced by superposed nearly simultaneous oblique Neoproterozoic collisions in the southeast of the São Francisco craton.

Magnetic fabrics and paleomagnetism of the Storsjön–Edsbyn deformation zone, central Sweden

A regional deformation zone, separating tectonomagmatic blocks in the central part of the Fennoscandian Shield, has been studied using anisotropy of magnetic susceptibility and paleomagnetism in order to define the tectonic character and age of the zone and to put the deformation into a plate tectonic context. The deformation zone (the Storsjön–Edsbyn Deformation Zone; SEDZ) separates the Rätan granite of the Trans Scandinavian Igneous Belt (TIB) and the southern Svecofennian subprovince in the southwest from older Svecofennian granitoids of the central Svecofennian subprovince in the northeast. The SEDZ is the most prominent deformation zone in a regional pattern of NW–SE trending shear zones. On basis of AMS data the width of the zone, which cuts through 1.7–1.9 Ga old rocks, is estimated at 10–12 km in its central–southern part and 20–25 km in its northern part. The orientations of the susceptibility axes vary systematically across the zone, in a way that suggests that the SEDZ is a pure shear dominated transpression zone caused by a compressive stress from the present SSW. Paleomagnetic data from a dolerite sill (Plat=10°, Plon=166°) and from a dyke (Plat=25°, Plon=190°) that cut the SEDZ indicate that no plastic deformation has occurred along the zone during the past 1.25 Ga, and probably not for the past 1.5–1.6 Ga. AMS data do not point to any deformation of the 1.70 Ga old Rätan granite, which further limits the time span of deformation. The main phase of deformation most likely took place between ∼1.80 and 1.70 Ga ago. The geological evolution of this part of the Fennoscandian Shield at the time between 1.80 and 1.70 Ga ago is very complex and in part still enigmatic. This time period involves magmatism of the TIB, which is related to eastward subduction. It also comprises the beginning of the Gothian orogeny and the development of a regional system of transpressive deformation zones. The compressive stress field from the present SSW indicated in this study is interpreted to be related to an initial stage of plate collision between the southwestern Scandinavian subprovince and older parts of the Fennoscandian Shield. This collision resulted in a system of regional NW–SE trending intra-plate deformation zones, one of which is the SEDZ

Land–offshore tectonic links in western Norway and the northern North Sea

Aeromagnetic and marine gravity data sets for the northern North Sea and western Norway have been interpreted in terms of land to offshore tectonic links. The data allow identification of linking-structures both in crystalline basement and overlying sedimentary rocks and consequently facilitate a more comprehensive interpretation of regional structural features and trends. With the help of bathymetric data in the inshore region, major fault structures can be traced between western Norway and the offshore region west of the Øygarden Fault Zone. In this way spatial and chronological links have been established between tectonic events recognized on land and the development of the North Sea Basin. An interpretation of basement geology is proposed for the region between western Norway and the Uer Terrace. This is complemented with a depth to top crystalline basement map covering the North Sea Basin between the Uer terrace and the East Shetland Basin. Two regional NW–SE lineaments were recognized offshore western Norway and given the names Marflo Lineament and Troll Lineament. The Marflo Lineament is interpreted to be a Caledonian or older zone of weakness that accommodated development of a 20–30 km offset in the Jurassic Viking Graben axis. The lineament can be traced into the North Atlantic where it parallels oceanic fracture zones and transform faults. The Troll Lineament relates to a deep geological discontinuity crossing the Uer and Lomre Terraces. A grouping of earthquake epicentres along part of the lineament suggests that the discontinuity is tectonically active.

Patterns of tectonic stress in Sicily from borehole breakout observations and finite element modeling

The orientation of in situ tectonic stress was deduced from borehole breakout analysis of 22 wells from onshore Sicily. The results allow us to distinguish the stress field of different geological units: (1) A nearly NNW (148°) orientation is detected in the Hyblean Plateau. (2) A NNE SHmax direction characterizes the Gela area. To the north, within the thrust belt (around Mount Judica), the SHmax direction swings to NE. In the northeastern segment of the foredeep, the Catania Plain, the direction of SHmax is roughly parallel to the NE trending grabens that mark the northern margin of the Hyblean Plateau. (3) The southwestern segment of the foredeep has no preferential SHmax orientation and may act as a transition zone with isotropic horizontal stresses. (4) In western Sicily a SHmax orientation from N–S to NW–SE is observed. This fits well the kinematics of the SE migrating Egadi and Adventure thrust belts and the direction of shortening inferred from the modern seismicity of the area. A three‐dimensional finite element modeling, including the most important tectonic features, was performed. Modeling results of the stress field indicate that the NNW trending SHmax is locally influenced by the variation of crustal thickness and local zones of weakness. In addition, the Pantelleria Rift causes a rotation of the regional NW–SE stress orientation to NNE along the northern rim of the rift system, including large on‐shore areas in the adjacent central south Sicily.

Active tectonism in the central Alps: contrasting stress regimes north and south of the Rhone Valley

ABSTRACTAn integrated interpretation of seismicity, fault plane solutions and deep seismic reflection data suggests that the NE–SW to NW–SE trending Rhone–Simplon fault zone and the gently S‐dipping basal Penninic thrust separate fundamentally different stress regimes in the western Swiss Alps. North of the Rhone‐Simplon fault zone, strike‐slip earthquakes on steep‐dipping faults within the Helvetic nappes are a consequence of regional NW–SE compression and NE–SW extension. To the south, vertical maximum stress and N–S extension are responsible for normal mechanism earthquakes that occur entirely within the Penninic nappes above the basal Penninic thrust. Such normal faulting likely results from extension associated with southward movements (collapse) of the Penninic nappes and/or continued uplift and relative northward displacements of the underlying Alpine massifs. Geological mapping and fission‐track dating suggest that the two distinct stress regimes have controlled tectonism in the western Swiss Alps since at least the Neogene.

The mother lode mine in British Columbia: R. H. Allen. Eng. Min. J., lxxxviii, 1101

Copper–gold–bismuth–tellurium mineralization in the Stanos area, Chalkidiki Peninsula, Greece, occurs in the Proterozoic- to Silurian-aged Serbomacedonian Massif, which tectonically borders the Mesozoic Circum-Rhodope metamorphic belt to the west and crystalline rocks of the Rhodope Massif to the east. This area contains the Paliomylos, Chalkoma, and Karambogia prospects, which are spatially related to regional NW–SE trending shear zones and hosted by marble, amphibolite gneiss, metagabbro, and various muscovite–biotite–chlorite–actinolite–feldspar–quartz schists of the Silurian Vertiskos Unit. Metallic minerals occur as disseminated to massive aggregates along foliation planes and in boudinaged quartz veins. Iron-bearing sulfides (pyrite, arsenopyrite, and pyrrhotite) formed prior to a copper-bearing stage that contains chalcopyrite along with galena, sphalerite, molybdenite, and various minerals in the system Bi–Cu–Pb–Au–Ag–Te. Fluid inclusion homogenization temperatures of primary aqueous liquid–vapor inclusions in stage I quartz veins range from 170.1 °C to 349.6 °C (peak at ~ 230 °C), with salinities of 4.5 to 13.1 wt.% NaCl equiv. Calculated isochores intersect P–T conditions associated with the upper greenschist facies caused by local overpressures during late-stage tectonic movement along the shear zone in the Eocene, which produced stretching and unroofing of rocks in the region. Values of δ34S for sulfides in the Stanos shear zone range from 2.42 to 10.19‰ and suggest a magmatic sulfur source with a partially reduced seawater contribution. For fluids in equilibrium with quartz, δ18O at 480 °C varies from 5.76 to 9.21‰ but does not allow for a distinction between a metamorphic and a magmatic fluid.A 187Re–187Os isochron of 19.2 ± 2.1 Ma for pyrite in the Paliomylos prospect overlaps ages obtained previously from intrusive rocks spatially-related to the Skouries porphyry Cu–Au, the Asimotrypes Au, and the intrusion-related Palea Kavala Bi–Te–Pb–Sb ± Au deposits in northern Greece, as well as alteration minerals in the carbonate-replacement Madem Lakkos Pb–Zn deposit. Ore-forming components of deposits in the Stanos area were likely derived from magmatic rocks at shallow depth that intruded an extensional shear environment at ~ 19 Ma.