Pre-thrust normal faults and post-tectonic micas in the Taconic Range of west-central Vermont

New geologic maps show that the Champlain Thrust System traces continuously between Shoreham and Benson, Vermont. The Champlain Thrust System consists of at least three internally imbricated thrust slices in continental shelf facies quartzites and carbonates. From north to south there is a structurally controlled thinning of the Champlain Thrust System, and a climbing in stratigraphic level by the thrusts. Within the Champlain Thrust System are a set of across-strike structures which create offsets in the thrusts and the surrounding lithic map unit boundaries. These structures function as lateral ramps in the thrust system geometry and bound thrust duplexes. Often there are changes in the stratigraphic level across these offsets which cannot be explained by the thrust geometry. These are interpreted to be to be pre-thrust normal faults, and a subsidiary (trench normal) set to the normal faults predicted by the model of Bradley and Kidd (1991), wherein normal faulting occurs in the continental crust in response to lithospheric flexure with the onset of collision. Trench parallel normal faults bounded grabens containing shales, and horsts of carbonate rocks. These horsts and grabens are now reflected in the thrust system\u27s geometry. The Mettawee River Fault, an east-side-down normal fault, juxtaposes an intermixed belt of Middle Ordovician shelf facies shale and Pre-Cambrian continental rise facies slates and arenites, and Middle Ordovician flysch and melange, against the Champlain Thrust System. This structure could be late Taconic (orogenic) to post-Taconic in age, and have amounts of throw of tens to hundreds of meters. East of the Mettawee River Fault is the westernmost Taconic allochthon, the Sunset Lake Slice. This is a roughly north-south trending belt of green slates interspersed with lenses of continental rise facies quartzites. The Sunset Lake Slice is bound to the east by the Taconic Frontal Thrust, a late, out of sequence thrust which transports an eastern belt of intermixed Ordovician and Taconic shales and melange, as well as an eastern thrust belt of shelf facies carbonates and shales. This belt of carbonates and shales is litholologically identical to the rocks found in the Champlain Thrust System but structurally dissimilar. The separation of these rocks from their facies equivalents in the Champlain Thrust System is suggested to be due to their topographically higher position on the paleo-continental shelf, the topography being largely (pre-thrust) normal fault controlled. Cross-micas are observed mica grains from the slates at the Cedar Point Quarry, W. Castleton, Vt.. The Cedar Point cross-micas cross-cut the boundaries of a late crenulation cleavage which has the morphology of micro-kinks at the microscopic scale. This cross-cutting relationship suggests that these grains grew later than the deformation which produced the micro-kinks, and therefore later than the deformation which produced the slaty cleavage. If this is the case, then perhaps many, or all of the mica grains in the slate grew at this later time. The investigation to gain insight into the timing of the development of the cross-micas included producing a structural map and cross-section of the Cedar Point Quarry which may be useful for related future studies. Field based observations provide insight into the deformation history recorded at Cedar Point. The cross-micas, and the surrounding micro-structure of the slate are documented with photomicrographs. The observation that some micro-kinks are enriched with opaque minerals (oxides and sulfides), whereas other, adjacent, micro-kinks are enriched with phyllosilicates, is evidence that there was some amount of mass transfer, presumably due to solution processes, after the formation of the micro-kinks. Electron microprobe data was attained from the slates in the hopes of confirming whether the cross-micas represent mica growth which was localized along the micro-kinks (and thus in the absence of a widespread metamorphic/mica growth event), or mica growth which was widespread throughout the slates. It was found that there are two groups of muscovite in the slate, a high K and a low K muscovite. There is also a group of interlayered muscovite and chlorite. There is no obvious relationship between structural setting, such as cross-micas, and the mineralogical variation, though with future work, such a relationship may be established

CAYSEIS - magma-starved oceanic crustal accretion and transform margin formation in the Cayman Trough revealed by seismic and seismological data - Cruise No. M115, April 1 - April 28, 2015 - Kingston (Jamaica) - Pointe-à-Pitre (Guadeloupe)

About 57% of the Earth’s outer surface is oceanic crust and new ocean floor is continuously created along the 55,000-60,000 km long mid-ocean ridge (MOR) system. About 25% of MORs spread at an ultra-slow spreading rate of < 20 mm/yr. Most ultra-slow spreading ridges occur in areas of the world that are difficult to reach, like the Gakkel Ridge in the Arctic Ocean and the Southwest Indian Ridge in the Indian Ocean. It has long been recognized that crustal accretion at ultra-slow spreading rates is fundamentally different from crust generated at faster spreading rates. However, due to the remoteness of ultra-slow ridges, the formation of crust at these magma-starved centres is yet not well understood. During the CAYSEIS cruise we surveyed lithospheric formation at ultra-slow spreading rates at the Mid-Cayman spreading centre (MCSC) in the Caribbean Sea, where oceanic crust is formed at a full rate of ~17 mm/yr. To the northeast and southwest, the MCSC is bound by two major transform faults. Using active-source wide-angle seismic imaging and passive local earthquake monitoring we, studied the balance between magmatic accretion and tectonic stretching (and hence oceanic core complex formation) and the relationship between faulting and hydrothermal activity at ultra-slow spreading rates. In addition, we explored transform margin formation at a unique setting, occurring at the southern terminus of the MCSC. In total, six seismic lines surveyed crust formed at the MCSC, two of these profiles also crossed the Swan Island transform fault. The project was a collaboration between German, British and American scientists

Magnetic and Clast Fabrics as Measurements of Grain-Scale Processes Within the Death Valley Shallow Crustal Detachment Faults

The rock product of shallow-crustal faulting includes fine-grained breccia and clay-rich gouge. Many gouges and breccias have a fabric produced by distributed deformation. The orientation of fabric elements provides constraints on the kinematics of fault slip and is the structural record of intrafault strain not accommodated by planar and penetrative surfaces. However, it can be difficult to quantify the deformational fabric of fault rocks, especially the preferred orientations of fine-grained minerals, or to uniquely determine the relationship between fabric geometry and finite strain. Here, we present the results of a fabric study of gouge and breccia sampled from low-angle normal (detachment) faults in the Black Mountains, Death Valley, CA. We measured a preferred orientation of the long axes of the clasts inherited from the crystalline footwall of the fault and compared the shape preferred orientation to the anisotropy of magnetic susceptibility of the fault rocks. The two measurements of fabric exhibit systematic similarities and differences in orientation and anisotropy that are compatible with the large-scale kinematics of fault slip. The dominant carriers of the magnetic susceptibility are micron- and sub-micron scale iron oxides and clay minerals. Therefore even the finest grains in the fault rock were sensitive to the distributed deformation and the micro-mechanics of particle interaction must have departed from those assumed by the passive-marker kinematic model that best explains the fabric

Magmatic accretion versus serpentinized mantle exhumation at ultraslow spreading rates: constraints from seismic data and Vp/Vs ratios, Mid-Cayman Spreading Centre, Caribbean Sea

About 57% of the Earth’s surface is covered by oceanic crust and new ocean floor is continuously created along the ~60.000 km long mid-ocean ridge (MOR) system. About 25% of the MOR spread at an ultra-slow spreading rate of 1.9. Here, we report results from a seismic refraction survey from the ultra-slow spreading Cayman Spreading Centre in the Caribbean Sea, sampling mature crust along a flowline from both conjugated ridge flanks. The ocean-bottom-seismometer and hydrophones provide both P-wave and S-wave refracted arrivals. Travel time data were inverted using seismic tomography. Resulting Vp/Vs ratios suggest that up to 25% of the lithosphere have high ratios of >1.9, supporting serpentinization and exposure of hydrated mantle at the seafloor. Further, the mode of accretion has changed over time, supporting both areas of mantle exposure and magmatic crust. Magmatic crust has a typical layer 2 and layer 3 velocity structure and a thin crust of 3 to 5 km thickness. However, a well-defined Moho boundary was not observed. Thus, crustal rocks are characterized by typical crustal-velocities (7.6 km/s. Domains of un-roofed mantle have high Vp/Vs ratios and velocities gradually increasing to 7.4-7.6 km/s. In addition, we will use our results to re-assess the depth distribution of local earthquakes at ultra-slow spreading ridges, including the Cayman Trough and the Southwest Indian Ridge. Most importantly, the high Vp/Vs ratio of >1.9 characterizing serpentinized mantle causes earthquakes to focus at much shallower depth when compared to location procedures using a global average for Vp/Vs of 1.73; the bias in depth might be in the order of 10 km

Investigation of wing crack formation with a combined phase-field and experimental approach

Fractures that propagate off of weak slip planes are known as wing cracks and often play important roles in both tectonic deformation and fluid flow across reservoir seals. Previous numerical models have produced the basic kinematics of wing crack openings but generally have not been able to capture fracture geometries seen in nature. Here we present both a phase-field modeling approach and a physical experiment using gelatin for a wing crack formation. By treating the fracture surfaces as diffusive zones instead of as discontinuities, the phase-field model does not require consideration of unpredictable rock properties or stress inhomogeneities around crack tips. It is shown by benchmarking the models with physical experiments that the numerical assumptions in the phase-field approach do not affect the final model predictions of wing crack nucleation and growth. With this study, we demonstrate that it is feasible to implement the formation of wing cracks in large scale phase-field reservoir models

Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid-Atlantic Ridge 30°N

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): B07103, doi:10.1029/2010JB007931.Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100–220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45° rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge

Phenotypic Characterization of EIF2AK4 Mutation Carriers in a Large Cohort of Patients Diagnosed Clinically With Pulmonary Arterial Hypertension.

BACKGROUND: Pulmonary arterial hypertension (PAH) is a rare disease with an emerging genetic basis. Heterozygous mutations in the gene encoding the bone morphogenetic protein receptor type 2 (BMPR2) are the commonest genetic cause of PAH, whereas biallelic mutations in the eukaryotic translation initiation factor 2 alpha kinase 4 gene (EIF2AK4) are described in pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis. Here, we determine the frequency of these mutations and define the genotype-phenotype characteristics in a large cohort of patients diagnosed clinically with PAH. METHODS: Whole-genome sequencing was performed on DNA from patients with idiopathic and heritable PAH and with pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis recruited to the National Institute of Health Research BioResource-Rare Diseases study. Heterozygous variants in BMPR2 and biallelic EIF2AK4 variants with a minor allele frequency of <1:10 000 in control data sets and predicted to be deleterious (by combined annotation-dependent depletion, PolyPhen-2, and sorting intolerant from tolerant predictions) were identified as potentially causal. Phenotype data from the time of diagnosis were also captured. RESULTS: Eight hundred sixty-four patients with idiopathic or heritable PAH and 16 with pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis were recruited. Mutations in BMPR2 were identified in 130 patients (14.8%). Biallelic mutations in EIF2AK4 were identified in 5 patients with a clinical diagnosis of pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis. Furthermore, 9 patients with a clinical diagnosis of PAH carried biallelic EIF2AK4 mutations. These patients had a reduced transfer coefficient for carbon monoxide (Kco; 33% [interquartile range, 30%-35%] predicted) and younger age at diagnosis (29 years; interquartile range, 23-38 years) and more interlobular septal thickening and mediastinal lymphadenopathy on computed tomography of the chest compared with patients with PAH without EIF2AK4 mutations. However, radiological assessment alone could not accurately identify biallelic EIF2AK4 mutation carriers. Patients with PAH with biallelic EIF2AK4 mutations had a shorter survival. CONCLUSIONS: Biallelic EIF2AK4 mutations are found in patients classified clinically as having idiopathic and heritable PAH. These patients cannot be identified reliably by computed tomography, but a low Kco and a young age at diagnosis suggests the underlying molecular diagnosis. Genetic testing can identify these misclassified patients, allowing appropriate management and early referral for lung transplantation

Phenotypic Characterization of <i>EIF2AK4</i> Mutation Carriers in a Large Cohort of Patients Diagnosed Clinically With Pulmonary Arterial Hypertension

Background: Pulmonary arterial hypertension (PAH) is a rare disease with an emerging genetic basis. Heterozygous mutations in the gene encoding the bone morphogenetic protein receptor type 2 ( BMPR2 ) are the commonest genetic cause of PAH, whereas biallelic mutations in the eukaryotic translation initiation factor 2 alpha kinase 4 gene ( EIF2AK4 ) are described in pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis. Here, we determine the frequency of these mutations and define the genotype-phenotype characteristics in a large cohort of patients diagnosed clinically with PAH. Methods: Whole-genome sequencing was performed on DNA from patients with idiopathic and heritable PAH and with pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis recruited to the National Institute of Health Research BioResource–Rare Diseases study. Heterozygous variants in BMPR2 and biallelic EIF2AK4 variants with a minor allele frequency of &lt;1:10 000 in control data sets and predicted to be deleterious (by combined annotation-dependent depletion, PolyPhen-2, and sorting intolerant from tolerant predictions) were identified as potentially causal. Phenotype data from the time of diagnosis were also captured. Results: Eight hundred sixty-four patients with idiopathic or heritable PAH and 16 with pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis were recruited. Mutations in BMPR2 were identified in 130 patients (14.8%). Biallelic mutations in EIF2AK4 were identified in 5 patients with a clinical diagnosis of pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis. Furthermore, 9 patients with a clinical diagnosis of PAH carried biallelic EIF2AK4 mutations. These patients had a reduced transfer coefficient for carbon monoxide (K co ; 33% [interquartile range, 30%–35%] predicted) and younger age at diagnosis (29 years; interquartile range, 23–38 years) and more interlobular septal thickening and mediastinal lymphadenopathy on computed tomography of the chest compared with patients with PAH without EIF2AK4 mutations. However, radiological assessment alone could not accurately identify biallelic EIF2AK4 mutation carriers. Patients with PAH with biallelic EIF2AK4 mutations had a shorter survival. Conclusions: Biallelic EIF2AK4 mutations are found in patients classified clinically as having idiopathic and heritable PAH. These patients cannot be identified reliably by computed tomography, but a low K co and a young age at diagnosis suggests the underlying molecular diagnosis. Genetic testing can identify these misclassified patients, allowing appropriate management and early referral for lung transplantation. </jats:sec