Shallow Crustal Rocks Research Articles

The formation of orthogonal joint systems and cuboidal blocks: New insights gained from flat-lying limestone beds in the region of Havre-Saint-Pierre (Quebec, Canada)

Vertical orthogonal joints are a common feature in shallow crustal rocks. There are several competing theories for their formation despite the ubiquity. We examined the exceptional exposures of orthogonal joints in flat-lying Ordovician limestone beds from the Havre-Saint-Pierre Region in Quebec, Canada (north shore of Saint-Lawrence River) to test conceptual models of joint formation in a natural setting. In the region, the spacing of cross-joints is consistently larger than the spacing of systematic joints by a factor of 1.5 approximately. The joint-spacing-to-bed-thickness ratios (s/t) are much larger in these beds (s/t = 4.3 for systematic joints, and 6.4 for cross-joints) than those in higher strained strata along the south shore of the Saint-Lawrence River (s/t = 1), highlighting the effect of tectonic strain in decreasing fracture spacing and block size. The high values of s/t indicate that cross-joint formation was unlikely caused by a switch from compression to tension once a critical s/t ratio for systematic joints was reached (as hypothesized in previous studies). We proposed a new model for the formation of orthogonal joint systems where the principal stress axes locally switch during the formation of systematic fractures. The presence of ladder-shaped orthogonal joints suggests a state of effective stress with σ1∗≫0 > σ2∗>σ3∗ and where σ2∗-σ3∗ is within the range of fracture strength variability at the time of fracture. This research provides a new mechanical model for the formation of orthogonal joint systems and cuboidal blocks.

An integrated remote sensing, aeromagnetic and field study of the Ablah shear zone (Asir Terrane, Saudi Arabia): Structural styles and implications for the tectonic evolution

• Surface and subsurface structural mapping was carried out for the Ablah shear zone. • Northerly striking thrust faults define the positive flower structure that impacts the shallow rocks in the zone. • Shallow thrusting ends at 1.5 to 2 km depth and is converted into single thrust shear. • Contraction tectonic regime initiated the shear zone and was later superposed by dextral-transpression regime. An integration of merged remote-sensing data having different spatial/spectral resolutions with aeromagnetic data and detailed field investigation is used to identify the composite Neoproterozoic rock complexes and define the structural architecture of the Ablah Shear Zone (ASZ). The SPOT and Landsat-8 data are processed as color composite images allowed the separation of different lithologic units and associated structures. Aeromagnetic data were analyzed using the Total Magnetic Intensity (TMI) and Reduced to the Pole (RTP) methods. The resulted anomalies show seven N-S opposite-dipping thrust faults that affect the shallow crustal rocks and coalesce together at about 1.5 to 2 km depth where the rock magnetic polarities inversed. This thin-skinned thrusting was initiated by a contractional tectonic regime in the E-W direction and superposed later by a transpresssion tectonic regime in the ENE-WSW direction. This study deals with the thin-skinned thrusting deformation in the ASZ that could be occurred during the deformation of the other shear zones that affect the internal part of the Asir tectonic terrane. Two controversial tectonic meanings describe the tectonic evolution of the ASZ, the first one is the thin-skinned thrusting tectonic that indicating shallow earth s crust deformation and the second one is the occurrence of some ophiolitic slices that indicating suturing tectonic deformation. The absent of the complete ophiolitic section encourages us regarding the ASZ as a structural shearing zone rather than a suture zone.

In situ Th/Pb dating of monazite in fibrous veins: Direct dating of veins and deformation in the shallow upper crust of the Mexican Orogen

Few methods exist for directly dating orogenic events in the upper crust. Here we report in-situ dates from (∼10 μm diameter) monazite grown in quartz-calcite fibrous veins, which precipitated at very low-grade metamorphic conditions (about 250 °C) in volcaniclastic and calcareous rocks of the Early Cretaceous Trancas Fm. in the Zimapán Basin, Central Mexico. Our integrated study includes regional and local structural and kinematic analysis of vein and folds, detailed textural and mineralogical characterization of vein-bearing minerals, and texturally-controlled Th/Pb geochronologic analysis of monazite by LA-MC-ICPMS. We find that monazite in the veins preferentially crystallized at the interface between vein-forming calcite and quartz fibers, as well as at the vein-host rock interface. Monazite ages range from 154 to 68 Ma, with the youngest sub-population yielding a weighted mean age of 76.8 ± 0.8 Ma, coincident with the age of basin-wide folding as earlier constrained by Ar/Ar illite dating. The oldest monazite ages coincide with the timing of deposition of the volcaniclastic host-rock. Given that veins are ubiquitous in shallow crustal rocks, this approach to dating of veins with enclosed monazite has great potential to improve the constraints on temporal resolution of fluid-rock interaction during of deformation.

Effective Physical Parameter Evaluation of Shallow Crustal Rocks by Lattice Element Method

Abstract Behavior of materials at high temperature and pressure is of interest in many engineering and scientific applications, such as, Borehole Heat Exchanger, Ground Source Heap Pump, Deep and shallow geothermal energy generation and deep cast mining. A direct measurement of the buried rock mass is not fissile in most of the cases and correlations based of seismic wave velocities are developed. However, these correlations are affected largely by many physio-chemical anomalies and micro and meso scale effects in the rock mass and are developed for a chosen set of rocks. Here, we propose a lattice based meso-scale model to capture the behavior of shallow crustal rocks at high temperature and pressure and calculate the effective parameters, such as, effective shear moduli and Poisson’s ratio with change in the applied thermal and/or mechanical loads. The computed values by lattice element method are validated against the experimental results and are found to be in acceptable error range.

Intrusion of shoshonitic magmas at shallow crustal depth: T–P path, H2O estimates, and AFC modeling of the Middle Triassic Predazzo Intrusive Complex (Southern Alps, Italy)

The multi-pulse shoshonitic Predazzo intrusive complex represents an ideal igneous laboratory for investigating the chemical and physical conditions of magma emplacement in a crustal context, since numerical models can be constrained by field evidence. It constitutes the most intriguing remnant of the Middle Triassic magmatic systems of the Dolomitic Area (Southern Alps), preserved by the Alpine tectonics. Predazzo Intrusive Complex comprises silica saturated (pyroxenites/gabbros to syenites), silica undersaturated (gabbros to syenites), and silica oversaturated (granites and syenogranites) rock suites. In this paper, we modeled its emplacement and evolution with a multiple thermo-/oxy-barometric, hygrometric, and EC-AFC approach. At odds with what proposed in literature but according to the field evidence, the emplacement of the Predazzo Intrusive Complex occurred at shallow depth (< 6 km). In this context, the different pulses differed slightly in bulk water content, but shared a common thermal regime, with temperatures between 1000 and 1100 °C and ~ 600 °C at low-to-moderate oxidizing conditions (− 0.1 to + 0.7 ΔFMQ). The interaction between the intrusion and the shallow crustal rocks was minimal, with Sr and Nd isotopic compositions indicating an average of 5–6% assimilation of crust. A thermo- and oxy-barometric comparison with the nearby Mt. Monzoni also enabled to speculate about the solidification time of the intrusion, which we infer took place over about 700 ka.

Facies analysis of tuffaceous volcaniclastics and felsic volcanics of Tadpatri Formation, Cuddapah basin, Andhra Pradesh, India

The felsic volcanics, tuff and volcaniclastic rocks within the Tadpatri Formation of Proterozoic Cuddapah basin are not extensively studied so far. It is necessary to evaluate the extrusive environment of felsic lavas with associated ash fall tuffs and define the resedimented volcaniclastic components. The spatial and temporal bimodal association were addressed, but geochemical and petrographic studies of mafic volcanics are paid more attention so far. The limited exposures of eroded felsic volcanics and tuffaceous volcaniclastic components in this terrain are highly altered and that is the challenge of the present facies analysis. Based on field observation and mapping of different lithounits a number of facies are categorized. Unbiased lithogeochemical sampling have provided major and selective trace element data to characterize facies types. Thin-section studies are also carried out to interpret different syn- and post- volcanic features. The facies analysis are used to prepare a representative facies model to visualize the entire phenomenon with reference to the basin evolution. Different devitrification features and other textural as well as structural attributes typical of flow, surge and ash fall deposits are manifested in the middle, lower and upper stratigraphic levels. Spatial and temporal correlation of lithologs are also supportive of bimodal volcanism. Felsic and mafic lavas are interpreted to have erupted through the ~ N-S trending rift-associated fissures due to lithospheric stretching during late Palaeoproterozoic. It is also established from the facies model that the volcaniclastics were deposited in the deeper part of the basin in the east. The rifting and associated pressure release must have provided suitable condition of decompression melting at shallow depth with high geothermal gradient and this partial melting of mantle derived material at lower crust must have produced mafic magmas. Such upwelling into cold crust also caused partial heat transfer and associated melting of the surrounding shallow crustal rocks to generate felsic magmas.

Late Neoproterozoic to Carboniferous genesis of A-type magmas in Avalonia of northern Nova Scotia: repeated partial melting of anhydrous lower crust in contrasting tectonic environments

Avalonian rocks in northern mainland Nova Scotia are characterized by voluminous 640–600 Ma calc-alkalic to tholeiitic mafic to felsic magmas produced in a volcanic arc. However, after the cessation of arc activity, repeated episodes of felsic magmatism between ca. 580 Ma and 350 Ma are dominated by A-type geochemical characteristics. Sm–Nd isotopic data, combined with zircon saturation temperature estimates, indicate that these magmas were formed by high temperature (800–1050 °C) melting of the same anhydrous crustal source. Regional tectonic considerations indicate that A-type felsic magmatism was produced (1) at 580 Ma in a San Andreas-type strike slip setting, (2) at 495 Ma as Avalonia rifted off Gondwana, (3) at 465 and 455 in an ensialic island arc environment and (4) at 360–350 Ma during post-collisional, intra-continental strike-slip activity as Avalonia was translated dextrally along the Laurentian margin. These results attest to the importance of crustal source, rather than tectonic setting, in the generation of these A-type magmas and are an example of how additional insights are provided by comparing the geochemical and isotopic characteristics of igneous suites of different ages within the same terrane. They also suggest that the shallow crustal rocks in northern mainland Nova Scotia were not significantly detached from their lower crustal source between ca. 620 Ma and 350 Ma, a time interval that includes the separation of Avalonia from Gondwana, its drift and accretion to Laurentia as well as post-accretionary strike-slip displacement.

K–Ar illite dating to constrain multiple events in shallow crustal rocks: Implications for the Late Phanerozoic evolution of NE Asia

A recently developed illite-age-analysis (IAA) approach was applied to determine the multiple events for the Chugaryeong fault belt, Korea. Each event was determined by a combined approach of the optimized illite-polytype quantification and the K–Ar age-dating of clay fractions separated from the fault clays. The Late Cretaceous to Paleogene events (76.5±0.8, 69.1±0.6, 59.3±0.7, and 48.2±0.7Ma) were recognized by calculating the authigenic 1M/1Md illite ages on the IAA plots of the fault clays. The Early Cretaceous ages (121.7–124.7 and 112.4±1.5Ma) were also obtained from the convergent intercepts of 100% 2M1 illite on the IAA plots. The absence of the 2M1 illites in the host-rock indicates that the Early Cretaceous ages represent the timings of high-T hydrothermal events of >280°C. The 2M1 illites in the fault clays should be pre-formed by a fluid-rock interaction under a relatively high-T subsurface condition, and be mechanically reworked into the near surface along the fault by post-tectonic events. This is the first report determining the absolute age constraints of multi-activated tectonic events from a fault. These geochronological determinations of the multiple events recorded in the Chugaryeong fault belt are crucial to establish the tectonic evolution of the Korean Peninsula since the Late Cretaceous.

Fold dating: A new Ar/Ar illite dating application to constrain the age of deformation in shallow crustal rocks

We propose a deformation dating method that combines XRD quantification and Ar chronology of submicroscopic illite to determine the absolute ages of folds that contain clay-bearing layers. Two folds in the frontal segment of the Mexican Fold-Thrust Belt (MFTB), which was deformed from Late Cretaceous to Eocene, are used to illustrate the method and its future potential.Variations in mineral composition, illite-polytype, crystallite-size (CS) and Ar total gas ages were analyzed in the limbs and hinge of two mesoscopic folds. This analysis examines potential effects of strain variation on illitization and the Ar isotopic system along folded layers, versus possible regional thermal overprints. The Ar total-gas ages for 9 samples in Fold 1 vary between 48.4 and 43.9 Ma. The % of 2M1 (detrital) illite vs. Ar total-gas ages tightly constrains the age of folding at 43.5 ± 0.3 Ma. Nine ages from three samples in Fold 2 range from 76.2 to 62.7 Ma, which results in a folding age of 63.9 ± 2.2 Ma. Both ages are in excellent agreement with more broadly constrained stratigraphic timing. The method offers a novel approach to radiometric dating of clay-bearing folds formed at very low-grade metamorphic conditions, and has the potential to constrain dates and rates of regional and local deformation along and across foreland orogenic belts.

Why joints are more abundant than faults. A conceptual model to estimate their ratio in layered carbonate rocks

It is a commonplace field observation that extension fractures are more abundant than shear fractures. The questions of how much more abundant, and why, are posed in this paper and qualitative estimates of their ratio within a rock volume are made on the basis of field observations and mechanical considerations. A conceptual model is also proposed to explain the common range of ratios between extension and shear fractures, here called the j/ f ratio. The model considers three major genetic stress components originated from overburden, pore-fluid pressure and tectonics and assumes that some of the remote genetic stress components vary with time ( i.e. stress-rates are included). Other important assumptions of the numerical model are that: i) the strength of the sub-volumes is randomly attributed following a Weibull probabilistic distribution, ii) all fractures heal after a given time, thus simulating the cementation process, and therefore iii) both extensional jointing and shear fracturing could be recurrent events within the same sub-volume. As a direct consequence of these assumptions, the stress tensor at any point varies continuously in time and these variations are caused by both remote stresses and local stress drops associated with in-situ and neighbouring fracturing events. The conceptual model is implemented in a computer program to simulate layered carbonate rock bodies undergoing brittle deformation. The numerical results are obtained by varying the principal parameters, like depth ( viz. confining pressure), tensile strength, pore-fluid pressure and shape of the Weibull distribution function, in a wide range of values, therefore simulating a broad spectrum of possible mechanical and lithological conditions. The quantitative estimates of the j/ f ratio confirm the general predominance of extensional failure events during brittle deformation in shallow crustal rocks and provide useful insights for better understanding the role played by the different parameters. For example, as a general trend it is observed that the j/ f ratio is inversely proportional to depth ( viz. confining pressure) and directly proportional to pore-fluid pressure, while the stronger is the rock, the wider is the range of depths showing a finite value of the j/ f ratio and in general the deeper are the conditions where extension fractures can form. Moreover, the wider is the strength variability of rocks ( i.e. the lower is the m parameter of the Weibull probabilistic distribution function), the wider is the depth range where both fractures can form providing a finite value of the j/ f ratio. Natural case studies from different geological and tectonic settings are also used to test the conceptual model and the numerical results showing a good agreement between measured and predicted j/ f ratios.

Continuous near‐bottom gravity measurements made with a BGM‐3 gravimeter in DSV Alvin on the East Pacific Rise crest near 9°31′N and 9°50′N

A Bell BGM‐3 gravimeter has been used to collect continuous, underway, nearbottom (3‐ to 10‐m altitude) gravity measurements from the deep‐diving submersible DSV Alvin during surveys on the East Pacific Rise (EPR) crest near 9°31′N and 9°50′N. Closely spaced (20‐to 30‐m) gravity measurements were made along transects up to 8 km long in both regions. Repeatability of measurements made at the same location on different dives is ∼0.3 mGal. Along‐track spatial resolution of anomalies is ∼130–160 m, with the limiting factors being precision and sampling rate of the pressure gauge depth data used to calculate vertical accelerations of the submersible. The average upper crustal density of the ridge crest determined from the relationship between depth and free‐water gravity anomalies varies greatly between 9°31′N and 9°50′N. Average upper crustal densities of 2410 kg/m3 for the 9°50′N area and 2690 kg/m3 for the 9°31′N area were calculated. The different densities are not due to differing geometry of the Layer 2A‐2B boundary or a regional cross‐axis gravity gradient. Differences in porosity of the shallow crustal rocks, or a difference in the proportion of low density extrusives to higher‐density dikes and sills within Layer 2A in these two areas, are the likely causes of the different upper crustal densities. Bouguer gravity anomalies near the EPR axis are primarily small amplitude (0.5–2 mGal), are a few hundred meters across, and appear to be lineated parallel to the axis. Larger‐amplitude Bouguer anomalies of up to 4 mGal were found at a few locations across the crestal plateau and are associated with pillow ridges composed of lavas which are clearly younger than the surrounding seafloor. These ridges have distinct chemical compositions compared to lavas from the axial summit collapse trough (ASCT) at the same latitude. Probable sources of the 0.5‐ to 2‐mGal anomalies observed on the summit plateau include areas of collapsed and fissured terrain and dike swarms feeding melt through Layer 2A to the surface. A grid survey of the ridge axis near 9°50′N shows Bouguer anomalies lineated along the axis, suggesting that dike swarms do contribute to the observed Bouguer anomalies. The along‐axis continuity of the gravity anomalies is disrupted at a 75‐m offset of the ASCT, suggesting that shallow feeders of lava to the surface may be segmented on a finer scale than the deeper crustal magmatic system. This initial study confirms the ability to conduct high‐resolution, near‐bottom, continuous gravity measurements from Alvin. It also provides important information on how the shallow crustal structure of a fast spreading mid‐ocean ridge develops and how it varies with the surface morphology.

Fluid evolution in submarine magma‐hydrothermal systems at the Mid‐Atlantic Ridge

Fluid inclusions in a suite of gabbro, quartz‐breccia, and metabasalt samples recovered from the MARK area on the Mid‐Atlantic Ridge are the product of a complex hydrothermal history involving late stage magmatic fluids at temperatures &gt;700°C and penetration by modified seawater at 300–400°C. The evolution of volatiles during the early stages of solidification and cooling of magma bodies near the ridge‐transform intersection is marked by exsolution of a CO2 fluid, entrapped within primary inclusions in fluorapatites. Attendant with progressive melt fractionation, residual evolved melts reached water saturation, and locally, supercritical CO2+H2O+NaCl±Fe brines (&gt;50 wt % NaCl) and cogenetic H2O+CO2‐rich vapors (1–2 wt % NaCl) were exsolved as immiscible phases. Concomitant or subsequent fracturing, perhaps in response to volatile exsolution from the melts, allowed migration of these fluids along microfracture networks at &gt;700°C. Trondhjemitic‐hosted inclusions, which homogenize by halite dissolution, indicate that the last fluids exsolved from the melts may have been 35–40 wt % brines. The transition from magmatic to seawater‐dominated hydrothermal conditions in the gabbros is marked by initial penetration of lower salinity fluids (1–7 wt % NaCl) at temperatures in excess of 400°C, with the general cessation of fluid flow occurring at minimum temperatures of ≈ 250°C. The relative enrichment and depletion of NaCl with respect to seawater in these fluids may record supercritical phases separation of seawater or boiling of hydrothermal fluids enriched in NaCl. Migration along microfracture networks of CH4‐rich, 350°C fluids, may reflect deeper seated hydrothermal processes involving hydration of underlying mantle material in response to fluid flow along deeply penetrating fault systems. In shallow crustal rocks, circulation of seawater‐derived fluids occurred at temperatures up to 400°C, with subsequent collapse of the active hydrothermal system at minimum temperatures of 200–250°C. In fault‐related upflow zones, multiple hydrothermal pulses involving 180–340°C and 3.5–10 wt % NaCl fluids, pervasively altered bounding wall rocks, forming chlorite‐rich, pyrite‐ and chalcopyrite‐bearing breccias. At shallow crustal depths, fluids reached temperatures of 150–300°C and contained salinities of 3.8–6.9 wt % NaCl. Following collapse of the axial‐related hydrothermal system, the plutonic and shallow crustal rocks were uplifted and emplaced as allocthonous blocks attending formation of the ridge‐transform intersection massif.

Comment and Reply on "Seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern Alaska"

Research Article| August 01, 1991 Comment and Reply on "Seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern Alaska" Stuart Crampin; Stuart Crampin 1British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, Scotland Search for other works by this author on: GSW Google Scholar Thomas M. Brocher; Thomas M. Brocher 2U.S. Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Nikolas I. Christensen Nikolas I. Christensen 3Department of Earth & Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907 Search for other works by this author on: GSW Google Scholar Author and Article Information Stuart Crampin 1British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, Scotland Thomas M. Brocher 2U.S. Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, California 94025 Nikolas I. Christensen 3Department of Earth & Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (8): 859–860. https://doi.org/10.1130/0091-7613(1991)019<0859:CAROSA>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Stuart Crampin, Thomas M. Brocher, Nikolas I. Christensen; Comment and Reply on "Seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern Alaska". Geology 1991;; 19 (8): 859–860. doi: https://doi.org/10.1130/0091-7613(1991)019<0859:CAROSA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No abstract available This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Characteristics and Formation Mechanism of the Geothermal Field in the North China Downfaulted Basin

Abstract The geothermal field is mainly controlled by the regional tectonic framework characterized by alternations of uplifted and depressed basement, and exhibits a similar zoned distribution of temperatures. In the uplifted area the geothermal gradient (G) and terrestrial heat flow value(q) of the Cenozoic sedimentary cover are relatively high, with G = 3.5 − 5.0°C / 100 m and q = 63‐84 mW / m2; whereas in the depressions they are relatively low, with G = 2.7−3.5°C / 100 m and q = 46‐59 mW / m2. In the whole region, Ḡ = 3.581°C / 100 m and q = 61.5± 13.4 nW/ m2, indicating a comparatively high geothermal background and the presence of local geothermal anomalies. A comparison of the results of mathematical simulation of the geothermal field with the measured values shows a good agreement between them. The geothermal difference between various tectonic units is caused chiefly by the lateral and vertical variation of thermal properties of shallow crustal rocks. This phenomenon can be regarded as the result of redistribution of relatively uniform heat flows from the deep crust in the surficial part of the crust in the process of their upward conduction.

Structural geology of the upper plate of the Bullfrog Hills detachment fault system, southern Nevada

An extremely distended terrane containing two detachment faults and an overlying complex of normal faults is exposed in the Bullfrog Hills, southern Nevada. Shallow crustal rocks have been extended along the detachment faults by listric and planar-rotational normal faults. The detachment faults define three structurally discordant plates. The lower detachment fault separates a lower plate of metamorphosed Late Proterozoic rocks from an overlying middle plate, composed of slivers of lower and middle Paleozoic clastic and carbonate rocks. The middle-plate rocks are brecciated and essentially unmetamorphosed, and the stratigraphic succession is incomplete and highly attenuated. The upper detachment fault separates the middle-plate rocks from an upper-plate succession of block-faulted Miocene volcanic, volcaniclastic, and sedimentary rocks. Miocene rocks of the upper plate dip at moderate to steep angles into the upper detachment fault, or, where the middle plate has been tectonically removed, into the lower detachment fault. The rocks are broken, tilted, and repeated in blocks bounded by normal faults that terminate against, or flatten and merge into, the upper detachment fault or, where the middle plate has been removed, the lower detachment fault. The normal faults in the upper plate are (1) planarrotational faults that form imbricate map patterns and (2) listric faults that are characterized by oval and horseshoe map patterns. These fault map patterns may be due to (1) curvilinear intersection of a listric or planar fault with an antithetic fault, (2) complex intersection of two or more faults of different ages, (3) rotated listric faults, (4) extremely rotated planar normal faults that resemble listric faults, (5) nearly flat-lying normal faults, or (6) topographic and erosional effects. Attenuation of the Late Proterozoic and Paleozoic strata indicates large movement on the detachment faults; the upper plate has been extended more than 100% and possibly more than 275% locally. The geometry of the normal faults and the repetition and dip direction of the Miocene rocks indicate that major extension, at least of the upper plate, was west-northwest-east-southeast and occurred mostly between about 10 and 8 Ma.

Seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern Alaska

Laboratory velocity measurements and structural field relations explain the observation of a significant seismic anisotropy within highly foliated rocks in southern Alaska. The orientation of the principal compressive stress indicates that this anisotropy is not related to extensive dilatancy, but can be satisfactorily explained by the foliation of the metasedimentary rocks. Laboratory measurements indicate a significant anisotropy within these rocks even at pressures well above those thought to close microcracks. These observations document that anisotropy related to preferred mineral orientation may be widespread in continental rocks. Thus, pervasive foliation can play a more important role in causing anisotropy than dilatancy, and inferences of principal stress orientations solely from anisotropy may be erroneous.