Browsing by Subject "Structural geology"

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Cenozoic deformation history of the Andean plateau in southern Peru : stratigraphic, structural, and geochronologic constraints
(2015-08) Perez, Nicholas David; Horton, Brian K., 1970; Gulick, Sean; McQuarrie, Nadine; Steel, Ron; Stockli, Daniel F
Cenozoic shortening in the central Andes of southern Peru was accommodated by thin- and thick-skinned deformation that governed hinterland/foreland basin dynamics, the timing and location of exhumation, and development of modern high topography. A new line length balanced cross section estimates 130 km of shortening (38%) across the Eastern Cordillera and Subandean Zone. I propose the location of a pre-Andean graben in the Eastern Cordillera, and a kinematic model that links selectively inverted basement-involved normal faults to shallow detachments that accommodate thin-skinned deformation across the orogen. New U-Pb zircon geochronology from synrift deposits establishes Triassic age deposition, and suggests compartmentalized rift basins were filled with local Eastern Cordillera sediment sources. Eocene exhumation in the Eastern Cordillera represents reactivation of Triassic normal faults and the onset of Andean deformation. In-sequence deformation was transferred from the Eastern Cordillera to the Altiplano by the thin-skinned Central Andean Backthrust Belt and induced flexural subsidence in the Ayaviri hinterland basin beginning at ~30 Ma. Facies analyses, sediment provenance, geochronology, and structural mapping define multiple phases of basin reorganization that are temporally correlative with motion along basin margin thrust faults. Major middle Miocene reorganization of the Ayaviri basin is linked to ~17 Ma out-of-sequence thrust fault motion in the Western Cordillera. Oligocene-Miocene hinterland basin evolution in the northern Altiplano was driven by thrust tectonics. U-Pb detrital zircon geochronology from Cretaceous through Cenozoic stratigraphy in hinterland and foreland basins record distinct provenance differences since the Cretaceous. This is the detrital record of either an inherited structural high in the Eastern Cordillera that predated Eocene shortening and created two depocenters with distinct provenance, or lateral provenance variations across a large retroarc foreland basin. Existing K/Ar, ⁴⁰Ar/³⁹Ar, and new zircon (U-Th)/He thermochronology suggest Eocene-Oligocene exhumation in the Eastern Cordillera was synchronous ~400 km along strike. New apatite (U-Th)/He data from the Eastern Cordillera demonstrate a change to localized, diachronous exhumation and uplift events in the Miocene-Pliocene. Apatite (U-Th)/He thermochronology demonstrates onset of deformation in the Subandean Zone by ~15 Ma, after shortening and exhumation in the Eastern and Western Cordillera ceased.
Chronostratigraphy, depositional rates, continental margin progradation, and growth-fault dynamics within the tertiary wedge, San Marcos arch, northwest Gulf of Mexico
(1988) Travis, Deborah Sue, 1963-; Galloway, William E.
Sedimentation, growth fault slip, and shelf-margin progradation rates were determined for a transect of Tertiary sediments along the axis of the San Marcos arch in the northwestern Gulf of Mexico basin. This low relief area was the site of shorezone depositional systems throughout most of the Tertiary and has not been affected by salt mobilization or the sedimentological overprint of large delta systems. Consequently, rates calculated in the area represent average rather than extreme values. Detailed correlation of 142 well logs and the utilization of paleontologic data from 17 wells made possible the construction of a chronostratigraphic cross section which divided the Tertiary section into 1 or 2 million year increments. A decompaction computer program, which utilizes published compaction curves and distinguishes between normally pressured shale, underconsolidated overpressured shale, and sandstone, calculated original sediment thickness and sedimentation rates for each of the chronostratigraphic layers. Regional sedimentation rates varied almost two orders of magnitude during the Tertiary. These findings seriously challenge the common assumption of relatively uniform sediment supply over geological time spans. Within individual chronostratigraphic layers, sedimentation rates ranged from 4 cm/1000 yrs in the fluvial and transgressive facies to 130 cm/1000 yrs in the lower Miocene outer shelf and slope facies. Decompaction of the section also allowed the calculation of growth fault slip rates and of the percent increase in accommodation on the downthrown side of the faults relative to their upthrown equivalents. Slip rates during times of maximum movement ranged from 250-600 m/my, agreeing with rates calculated for salt dome growth. Percent increase in accommodation on the downthrown side of the faults ranged from 20-83%. Shelf-margin progradation rates were calculated and ranged from -1.5 mi/my (2.4 km/my) during brief times of temporary retrogradation (Queen City and Yegua Formations) to 17.5 mi/my (28.2 km/my) during the Early Miocene
Cretaceous to Neogene tectonic control on sedimentation : Maracaibo Basin, Venezuela
(1991) Lugo Lobo, Jairo Miguel, 1955-; Salvador, Amos
The Maracaibo basin records a complex tectonic and depositional history. Compilation of seismic and well data allows me to distinguish six tectonostratigraphic episodes. The first episode is characterized by Jurassic rifting along north-northeast-trending half-grabens filled with continental red beds and volcanics rocks. The second episode is characterized by the deposition of shallow to deep marine carbonates and clastic rocks in which subsidence rates increase away from the northwest-trending Mérida arch, a mid-Late Paleozoic northwest-trending range anchored to the Guayana Shield. The third episode is distinguished by oblique collision of south-southwest-verging Caribbean terranes toward the continental platform. As collision evolved, the tectonically carried crustal flexure and associated turbiditic basin migrated south-southeastward. Evidences from geohistory analysis shows the shift of the axis of subsidence in that direction. During this episode six unconformity-bounded deltaic wedges marked an equal number of regressive stages caused by thrust propagation above the flysch basin. Seismic clinoform orientations and sandstone composition document the regional northern provenance for such paralic wedges in the northeastern portion of the basin. The fourth episode developed from Late Eocene to Middle Miocene in which transpressive tectonism reactivated the structural weaknesses developed in the earlier rift. Left-lateral north-northeast-trending strike-slip faults and related en echelon secondary structures were built in two main active phases: one during Late Eocene and the other in Middle Miocene time. The reactivation of the main Icotea and Pueblo Viejo faults influenced the distribution of intrabasinal restraining bends, pull-apart basins, and associated sediment infill. The fifth episode is characterized by diachronous orogeny that began with Oligocene uplift along the Sierra de Perijá and continued southeastward toward the Mérida Andes. These uplifts created a closed depositional basin and converted the depositional environment from marine to continental. The six episode is characterized by compression evidenced by north-striking east-vergent reverse fault in the Sierra de Perijá
Epithermal vein and carbonate replacement mineralization related to caldera development, Cunningham Gulch, Silverton, Colorado
(1984-05) Hardwick, James Fredrick, 1955-; Kyle, J. Richard
Epithermal vein and carbonate replacement deposits in Cunningham Gulch are located within the western San Juan Tertiary volcanic field in southwestern Colorado. The Pride of the West epithermal vein system is hosted within the intracaldera facies of the Sapinero Mesa Tuff, a voluminous ash-flow tuff that erupted from and resulted in the formation of the San Juan Caldera at 28 mybp. The Pride of the West vein system is developed along a radial fracture formed during resurgence of the San Juan Caldera prior to eruption of the Crystal Lake Tuff (27.5 mybp). This eruption led to the concomitant collapse of the Silverton Caldera, nested within the larger San Juan Caldera. The Pride of the West, Osceola, and Little Fanny mines are positioned near the intersection of the Pride radial fracture system and the buried structural margin of the San Juan Caldera, suggesting that ore concentration was controlled by this structural setting. Large limestone blocks of the Mississippian Leadville Formation are incorporated into the intracaldera fill volcanics in the mine area. These blocks appear to have been engulfed within mudflow breccias of the Tertiary San Juan Formation (32.1 mybp). They were then emplaced in their present structural position within a caldera-collapse breccia which caved from the oversteepened margin of the San Juan Caldera. Regional propylitic alteration of the hosting volcanics to a chlorite-calcite-pyrite assemblage preceded vein-associated alteration and mineralization. The veins are enveloped by a narrow phyllic alteration assemblage of quartz, sericite, illite, kaolinite, and pyrite. The veins are comprised of sphalerite, galena, chalcopyrite, pyrite, hematite, magnetite, quartz, pyroxmangite, calcite, and minor barite. Substantial bodies of replacement ore are present where the vein structures intersect the limestone blocks; the mineral assemblages of the replacement deposits are identical to those of the feeding vein structures. Commonly, replacement textures are spectacular concentrations, especially the "zebra ore" which primarily consists of regularly spaced, alternating bands of sulfides and quartz. These "zebra" laminations are stratigraphically controlled and appear to represent replacement of a depositional or diagenetic fabric. Main ore-stage mineralization began with widespread deposition of quartz with or without pyrite, followed by sphalerite, chalcopyrite, and galena. Post ore-stage brecciation and silicification events are evident and were followed by deposition of calcite and minor barite during the waning stages of the hydrothermal system. The distributions of Fe, Mn, Pb, and Ca suggest a lateral component of fluid flow from northwest the southeast, away from the structural margin of the Silverton Caldera. Fluid inclusion data from both vein and replacement-type sphalerite and quartz indicate that mineral deposition occurred over a range of 200 to 312°C (mean 243°C) from solutions containing 1 to 5% total salts. The high base metal to precious metal content of the ore, the phyllic alteration assemblage, and the temperature and composition of the ore-forming fluid indicate that the mine workings are within the lower portion of a fossil geothermal system.
Fold-related brittle structures and associated strain in a limestone bed of the Carmel Formation, San Rafael Swell, Utah
(2015-12) Laciano, Peter Joseph; Marrett, Randall; Cloos, Mark; Ukar, Estibalitz
The San Rafael Swell (SRS) is a basement-cored Laramide uplift located in central-eastern Utah. The SRS is bounded on the east by a 70 km long monocline, a fault-propagation fold, with excellent exposure of sedimentary strata including the Carmel Formation. This monocline is an ideal natural laboratory for studying brittle deformation associated with folding. Qualitative and quantitative observations for brittle structures in a limestone bed near the base of the Carmel Fm. were made in a wide range of bedding dip, curvature, and fold domains. Kinematic data was collected for 2942 structures (1865 veins, 746 stylolites, 314 faults) in 30 locations in order to calculate principal directions of strain. Additionally, data was collected along 71 scanlines at 19 of those locations in order to estimate structure intensities and strain magnitudes. Dekameter-displacement thrust faults, acting as ramps between inferred layer-parallel faults, accommodate orders of magnitude more strain than all other observed brittle structures. These faults are only found in segments of the monocline where bedding dip is high, but curvature is low, which provides strong evidence that limb rotation more strongly controls strain magnitudes than layer bending in the SRS. The trishear model effectively predicts SRS monocline geometry, specifically observed limb thickening, broad, curved hinges, and progressively rotating limb. This is likely due to the dominance of thick, homogeneous rock packages, such as the Navajo Sandstone, in the SRS monocline. In contrast, strain localization within the Carmel Fm. is poorly predicted by trishear: there is strong evidence of flexural slip, and folding induced structure orientations and calculated principal strain directions remain consistent relative to bedding. These strain directions are inconsistent with trishear forward models produced by workers such as Zuluaga et al. (2014) that do not stay consistent relative to bedding. These divergences are likely due to the fact that trishear is a kinematic model that assumes rock homogeneity, while the Carmel Fm. is stratigraphically and mechanically heterogeneous. Because this heterogeneity appears to have a strong effect on strain localization, kink band models likely better estimate strain localization in the Carmel limestone bed as well as other layers in folded heterogeneous strata. The monocline’s interpreted transition from layer-parallel shortening to extension at the steepest locations in the monocline, and thus at most advanced stage of folding, enabled estimation of the dip of the basement fault beneath the SRS as ~30°. This shallow dip contrasts with the steep dip (~60°) assumed for the SRS by Zuluaga et al. (2014) and observed in the Kaibab uplift (Huntoon and Sears, 1975; Tindall, 2000), but is consistent with a recent estimation of 20-40° for the SRS by Davis and Bump (2009) using trishear modeling.
Fracture spatial arrangement in the context of diagenesis
(2023-07-28) Wang, Qiqi; Laubach, Stephen E. (Stephen Ernest), 1955-; Fall, Andras; Bonnell, Linda; Tisato, Nicola; Mosher, Sharon; Gale, Julia F. W.
During the past decade, the need for meaningful documentation fracture spatial arrangement has become increasingly important as this is a key aspect of structural heterogeneity and anisotropy in the upper crust. Uncertainty about fundamental fault and fracture spatial patterns impacts engineering operations such as fluid injection underground, management of induced seismicity and the efficiency and success of fluid extraction. Spatial arrangement is an essential element in developing and managing unconventional and deep hydrocarbon reservoirs, CO2 sequestration, and geothermal systems. My dissertation research focuses on the challenge of describing and understanding fracture spatial arrangement in space. Specifically, my aim is to improve spatial characterization methods and use those methods together with other approaches to understand how the fracture patterns arise. My strategy explicitly links structure and diagenesis: the chemical/diagenetic aspects of fractures are an underutilized source of information on when and under what conditions fracture patterns arise. Fracture patterns comprise the spatial arrangements, sizes, and orientations of fractures. Moreover, cement deposits in fractures may themselves influence fracture arrangements. The central hypothesis guiding my research, is that the mechanical effects of progressive diagenesis—as manifest as deposits within fractures or as mechanical properties or property changes in the host rock—govern the types of patterns that arise. In this dissertation, Chapter 1 primarily explores and explains fracture spatial quantification methods and developing fracture pattern interpretation, but also corroborates that clustering can form independently of diagenesis (or cement deposits) within the fractures themselves. Chapter 2 explores fracture spatial patterns in a folded naturally fractured sandstone hydrocarbon reservoir using horizontal image logs and an outcrop analogue, leveraging the spatial quantification method explained in chapter 1. We discovered spatial patterns associated with high fracture intensities reflect shear on preexisting fractures, possibly resulting in more numerous but less spatially correlated open fractures. Furthermore, production data suggests that clustered but sparse quartz-lined open fractures are more effective fluid conduits than closely spaced partly damaged/sheared arrays. Chapter 3 show that due to different diagenetic histories and quartz accumulation amounts, rocks in outcrops with low thermal exposure and in the deep subsurface with extensive thermal exposure have fractures that differ in terms of their size, shape, and fracture porosity despite forming in the same sandstone host rock and in a common regional structural setting. Previous chapters set up the stage for Chapter 4, in which I address how diagenesis affects rock property evolution through time with diagenesis and rock physics modelling, in situations where timing of fractures is well attested. My results indicate that considerable magnitude of property changes can occur during fracture pattern development, and thus should be taken into consideration in fracture pattern interpretation and modeling processes.
From mountain belts to continental margins : causes and controls of rheological inheritance during extension
(2018-05) Lima, Rodrigo Dias; Hayman, Nicholas W.; Lavier, Luc Louis; Stockli, Daniel F; Van Avendonk, Harm A.; Behr, Whitney M.
Continents typically thin and break apart on sites that underwent a previous orogenic history. Geodynamic and conceptual models explain the causal link between orogenesis and extension via the influence of inherited hot geotherms and reactivation of weak structures. In many regions, however, extension post-dates orogeny by 10’s to 100’s million of years implying that the orogen’s thermal structure has been long re-equilibrated, and deformation is not everywhere accommodated by kinematic reactivation. In contrast, preexisting basement fabrics often influence the locus and propagation of deformation, suggesting a mechanical control. It has remained unexplored how preexisting orogenic fabrics affect the crustal rheology that governs extension. This dissertation documents such rheological inheritance from orogenesis, and explores its mechanical consequences on continental extension. This study is focused on the Death Valley region of the North American Basin and Range province, a hallmark example of continental extension preceded by orogenesis. The ranges that border Death Valley expose a basement complex that preserves the geologic record of the Mesozoic Sevier-Laramide deep crustal processes and later overprinting and exhumation by Basin-and-Range extension in the Cenozoic. Here, field observations, microstructural analysis (including EBSD), thermodynamic modeling of metamorphic reaction textures, and geochronology all document rheological inheritance in the region. The thermal evolution of the Mesozoic orogeny is recorded by zircon U-Pb geochronology (Chapter 1); magmatic age modes reveal that crustal heating in the Cordillera interior was sustained for ca. 36 m.y. (Late Cretaceous through Eocene). Such heating reached temperatures in excess to produce crustal melting, enabling deep crustal flow that affected the dynamic of the orogen (e.g., stability of surface topography). Chapter 2 explores how partial melting produced fine-grained fabrics via melt-rock reactions that favored textural softening along shear zones that accommodated syn-convergent extension. Importantly, the orogenic thermal evolution resulted in petrological transformations that included the segregation of melt products into compositional domains, resulting in deep crustal rheological heterogeneities that influenced softening mechanisms during later extension. The mechanical consequences of the inheritance of rheological heterogeneities on continental extension are then explored in thermomechanical experiments (Chapter 3). In particular, the experiments do not include weakening effects in order to evaluate the effect of the mid-crustal heterogeneities on localization. The results show that at the crustal-scale, the inherited heterogeneities favor unstable extensional regime and boudinage, and at the meso-scale nucleate localization by the growth of strain instabilities within the mid-to-lower crust. The linkage of the mid-crustal shear zone with upper crustal faulting results in detachments systems that can either exhume the middle crust into core complexes or concentrate thinning into necking zones, depending on the flowing behavior of the lower crust. The geological observations and thermomechanical experiment detailed in this dissertation led to a conceptual model wherein the heterogeneous distribution of peak, orogenic fabrics (including peak-metamorphic textures and segregation of melt products) impose a mechanical control, the rheological inheritance, on continental extension.
From rifting to collision : the evolution of the Taiwan Mountain Belt
(2013-05) Lester, William Ryan; McIntosh, Kirk D.; Lavier, Luc Louis
Arc-continent collisions are believed to be an important mechanism for the growth of continents. Taiwan is one of the modern day examples of this process, and as such, it is an ideal natural laboratories to investigate the uncertain behavior of continental crust during collision. The obliquity of collision between the northern South China Sea (SCS) rifted margin and Luzon arc in the Manila trench subduction zone allows for glimpses into different temporal stages of collision at different spatial locations, from the mature mountain-belt in central-northern Taiwan to the 'pre-collision' rifted margin and subduction zone south of Taiwan. Recently acquired seismic reflection and wide-angle seismic refraction data document the crustal-scale structure of the mountain belt through these different stages. These data reveal a wide rifted margin near Taiwan with half-graben rift basins along the continental shelf and a broad distal margin consisting of highly-extended continental crust modified by post-rift magmatism. Magmatic features in the distal margin include sills in the post-rift sediments, intruded crust, and a high-velocity lower crustal layer that likely represents mafic magmatism. Post-rift magmatism may have been induced by thermal erosion of lithospheric mantle following breakup and the onset of seafloor spreading. Geophysical profiles across the early-stage collision offshore southern Taiwan show evidence the thin crust of the distal margin is subducting at the Manila trench and structurally underplating the growing orogenic wedge ahead of the encroaching continental shelf. Subduction of the distal margin may induce a pre-collision flexural response along the continental shelf as suggested by a recently active major rift fault and a geodynamic model of collision. The weak rift faults may be inverted during the subsequent collision with the continental shelf. These findings support a multi-phase collision model where the early growth of the mountain belt is driven in part by underplating of the accretionary prism by crustal blocks from the distal margin. The wedge is subsequently uplift and deformed during a collision with the continental shelf that involves both thin-skinned and thick-skinned structural styles. This model highlights the importance of rifting styles on mountain-building.
Geologic evolution of the Sierra Madre Oriental between Linares, Concepción del Oro, Saltillo, and Monterrey, Mexico
(1982) Padilla y Sánchez, Ricardo José; Muehlberger, William R.
The Sierra Madre Oriental between Saltillo, Monterrey, and Linares shows a bend in structures that strike from approximately N 35° E to about N 35° W. Most of the rocks involved in the Curvature of Monterrey are Mesozoic in age and range from Late Triassic to Late Cretaceous. Large amplitude folds and thrust faults contribute to the structural complexity of this region. The structural trends present in northeast Mexico are the result of the Late Paleocene-Early Eocene Laramide Orogeny, and their different styles of folding are intimately related to the fundamental landforms of Early Mesozoic paleogeography. Relatively mild deformation is shown in the Mesozoic sedimentary cover that overlies the stable paleocontinental basement highs of the Coahuila, La Mula and Monclova Islands, Tamaulipas Archipelago, and El Burro-Peyotes Peninsula. The tight folding observed in the Sierra Madre Oriental is the result of regional northeastward décollement blocked by the basement highs. Two prominent west-northwest-trending lineaments transect the region: herein named the "Boquillas-Sabinas" and "Sierra Mojada-China" Lineaments. Prominent paleogeographic highs lie on the outside of the area bounded by these lineaments, with the area between occupied by major basins and small "islands". Recurrent motion along these lineaments seems likely and movement along them in Early Mesozoic time blocked out the paleogeographic elements discussed in this dissertation. It is proposed in this study that the structural features of northeast Mexico are the result of a sinistral relative movement of southern United States (westward) with respect to northern Mexico (eastward) during the Laramide Orogeny, contemporaneously with a regional décollement event produced by the tilting toward the northeast of the so-called "Unnamed Occidental Continent" . Thus from the detailed study of the mapped structures and stratigraphic sequences at the Curvature of Monterrey, and from detailed interpretations of satellite photographs, the model presented here for the mechanism of deformation of northeast Mexico explains not only the bend in structures at the Curvature of Monterrey, but also most of the structural trends in northeast Mexico, including the en e[]chelon folds in the Sabinas Gulf
High-temperature carbonate replacement mineralization, metamorphism, deformation, and intrusion in the Bryant District, Beaverhead County, Montana
(2003) McGuire, James B.; Kyle, J. Richard
The Bryant District, in the southwest Montana fold-and-thrust belt, contains many structurally and lithologically controlled high-temperature carbonate replacement Pb-Zn-Cu-Ag-Au deposits. Laramide crustal shortening prepared Cambrian and Devonian carbonate strata for fluid circulation through thrusting and folding. Thrust-controlled mineralization is present in the Lion Mountain mines, whereas fold-controlled chimney-style mineralization is present in the Cleve-Avon mines. Pb isotopic evidence indicates that base and precious metals in the district were not directly sourced from phases of the adjacent Late Cretaceous Pioneer Batholith. Ore Pb was probably scavenged from the Middle Proterozoic Belt Supergroup that underlies the district. A small pluton, satellitic to the Pioneer Batholith, underlies the Hecla Dome and may have set up a hydrothermal system which produced carbonate replacement as well as Mo skarn mineralization. Fluid inclusions in ore-associated quartz from the Bryant District are CO2-rich and have salinities from 3 to 8 wt% NaCl equivalent with homogenization temperatures from 260 to 330°C. Fluorine-deficient porphyry Mo deposits in Montana and Idaho, most notably Cannivan Gulch 5 km northwest of the Bryant District, have remarkably similar fluid inclusion characteristics, suggesting that the Bryant District carbonate replacement deposits may be the distal portion of a porphyry Mo system. The mineralization in the Bryant District appears to be related to a late satellitic intrusion of the Pioneer Batholith. Such an intrusion was hypothesized (Winchell, 1914; Karlstrom, 1948) and recently proven by exploratory drilling on the Hecla Dome. Field evidence suggests that Pb-Zn-Cu-Ag-Au mineralization took place after most Laramide shortening, intrusion of the bulk of the Pioneer Batholith, regional metamorphism, and contact metamorphism related to the emplacement of an intrusion below the Hecla Dome
Joints in igneous rocks
(2009-03) Barker, Daniel S.
Letter to E.H. Sellards from H.B. Stenzel on 1933-05-18
(1933-05-18) Stenzel, Henryk B.
Natural fractures in mudrocks and top seal integrity : insights from diagenesis, rock mechanics, and modeling applied to CO₂ sequestration and hydrocarbon exploration
(2018-08) Major, Jonathan R., 1984-; Eichhubl, Peter; Barnes, Jaime D; Gale, Julia F; Behr, Whitney; Hesse, Marc A
The viability of carbon sequestration for climate change mitigation depends on both the short and long-term security of injected CO₂, which may be impacted by the coupled chemical and mechanical properties of reservoir and seal rocks. Analogs such as the Crystal Geyser/Little Grand Wash fault field site near Green River, Utah allow investigation on longer time scales than laboratory or numerical experiments and was studied to assess the potential for leakage via fracturing or capillary failure of reservoir and seal rocks altered by natural, long-term CO₂-water-rock interactions. Fracture mechanics testing using the double torsion method was first performed on a suite of naturally altered and unaltered rocks exposed at Crystal Geyser. CO₂-related alteration measurably changed fracture toughness (K [subscript IC]) and subcritical index (SCI). A schematic model based on measured K [subscript IC] and SCI values and their predicted influence on fracture pattern development, and their chemical and spatial context relative to the main fault, was developed that qualitatively matches three distinct fracture network patterns observed. Fracture toughness and subcritical index (SCI) are also sensitive to chemical environment and temperature, for example, decreasing by up to 60% and 90%, respectively, in five different sandstone samples immersed in water versus ambient conditions. Sensitivity is controlled by rock composition, grains, cements, and fabric. Aztec Sandstone, a silica-cemented subarkose is relatively insensitive to pH and salinity compared to other sandstones such as the chlorite-cemented Tuscaloosa Sandstone, a CO₂ sequestration reservoir. In general, inert grains and cements such as quartz were less sensitive to the changing chemistry than carbonates and clays. The potential for capillary failure or enhancement of top seals over long (> 10³ years) scales was also studied by mercury intrusion capillary pressure (MICP) analyses on altered shale samples from Crystal Geyser Relatively low capillary seal capacity was measured < 5 m from the fault where CO₂-alteration is most intense but then increases by over an order of magnitude before gradually declining to background levels > 100 m from the fault. Systematic variations in the petrophysical properties are largely explained by changes in pore networks due to matrix replacement with calcite observed by SEM imaging.
Structural and petrographic analysis of Cambrian rocks at Beavertail State Park, Narragansett Basin, Rhode Island
(2008) Carter, Matthew James; Mosher, Sharon, 1951-
Structural Mapping at 1:1000 scale of the Cambrian rocks within Beavertail State Park, Narragansett Basin, Rhode Island has revealed an early open to isoclinal fold generation (F₁) and axial planar foliation (S₁) followed by a later, more prominent, Evergent, N-trending non-coaxial fold generation (F₂) and associated foliation (S₂). A third map-scale folding phase is inferred from the NNE-trending, broad warping of S₂. Quartz veins are folded by and cross cut F₂ folds. N-S to NE-SW extension resulted in 1-10 m scale boudins that deform S₂. All of these structures are cross cut by NNE to ENEstriking faults and joints. Kinematic indicators show that the dominant motion on these faults was normal with minor sinistral and/or dextral motions. The Beaverhead Shear Zone that juxtaposes the Cambrian rocks with Pennsylvanian rocks of the Narragansett Basin deflects S₂ in a dextral sense, consistent with recorded motion elsewhere. Kink bands associated with faulting trend NNE to ENE and display WNW to NNW side up. Petrographic work reveals S₁ is a penetrative foliation defined by alignment of white micas. S₂ is a crenulation cleavage of S₁ that is enhanced by pressure solution. S₂ is the dominant foliation expressed in the field; however there are areas where S₁ is locally better developed. Siderite and quartz nodules overgrow S₁ and are wrapped by S₂. Mica, quartz and iron carbonate pressure shadows on these nodules are parallel to S₂. Fabric development and orientation, style, and sequence of structures found in the lower greenschist Cambrian rocks within and up to 4 km north of Beavertail State Park, RI are dissimilar to those associated with the Taconic and Acadian events, but identical to Alleghanian structures in Pennsylvanian aged rocks of the Narragansett Basin. These similarities indicate that the Cambrian rocks have exclusively recorded the Pennsylvanian-Permian Alleghanian orogeny, and could not have been in place in North America during the Ordovician Taconic and Devonian Acadian orogenies. Timing constraints indicate the collision of the Avalon microcontinent occurred during the late Pennsylvanian, whereas progressive deformation of the Pennsylvanian rocks of the Narragansett Basin and the Cambrian rocks took place during the Permian
Structural and rheological evolution of subduction interface shear zones : insights from exhumed rocks
(2019-12-04) Kotowski, Alissa Jeanne; Behr, Whitney M.; Stockli, Daniel F.; Cloos, Mark; Barnes, Jaime; Soukis, Konstaninos; Wallace, Laura
The subduction interface is an inherently heterogeneous distributed shear zone occupying the boundary between a down-going (i.e., subducting) plate and overriding crust and mantle. The sinking of cold, dense oceanic lithosphere creates a slab pull force, which is a driving force for Plate Tectonics. Interface rheology (i.e., deformation or flow) exerts a first-order control on plate boundary strength, seismic style, and the propensity for rocks to exhume, or return to the surface of the Earth from mantle depths. When subducted rocks are exhumed, they provide rare snapshots of the rheological behavior of these complex plate boundary shear zones, and how rocks are subsequently modified by brittle and semi-brittle processes in the upper crust. In this dissertation, I combine a variety of observational and analytical techniques to investigate exhumed subduction-type rocks from Syros Island (Cyclades, Greece) and the sub-ophiolite metamorphic sole in Oman. This work provides constraints on the rheological behavior of subduction plate boundary shear zones, to better inform geodynamic models and to contextualize geophysical observations of active subduction zones. Rheological heterogeneities (i.e., outcrop- to regional-scale features producing strain gradients and/or significant differences in deformation mode or mechanism) are thought to be directly related to the seismic style that occurs along the subduction interface. At relevant depth and temperature conditions for Syros (~50-60 km, 500°C), the important seismic style is an enigmatic, coupled seismic-aseismic phenomena deemed Episodic Tremor and Slow Slip (ETS). ETS involves accelerated – but aseismic – slip over ~10’s-100’s km² of the interface, in conjunction with swarms of micro-seismicity, or tremor, and seems to occur nearly ubiquitously in subduction zones regardless of thermal structure, predicted depths of metamorphic dehydration reactions, or subducting rock type. In Chapter 2, I use exhumed blueschist- and greenschist-facies rocks on Syros to characterize the length scales, types, sources, and deformation mechanisms of rheological heterogeneities that occupy the deep subduction interface, and how they may contribute to enigmatic seismicity like ETS. Partial eclogitization of subducting rocks sets up stark rheological contrast across shear zones, which results in coupled brittle-viscous behavior assisted by near-lithostatic pore fluid pressures. Geologic observations are consistent with a mechanical model of ETS in which the deep interface comprises transiently brittle, potentially tremorgenic sub-patches, within a larger viscously creeping interface patch. These observations scale appropriately with geophysical constraints of tremor source areas and seismic moments. During a subduction-exhumation cycle, the length scales of mixing along the interface, maximum pressures (i.e., depths) that rocks reach, and mechanisms of rock exhumation depend in part on interface rheology. In Chapters 3 and 4, I combine structural and microstructural analysis, novel techniques in thermobarometry, and new interpretation of published metamorphic geochronology on Syros to understand bulk interface deformational style, progressive metamorphism, and rheology. The results are all consistent with a model of coherent subduction and underplating (i.e., transfer of subducting material to the overriding forearc), as opposed to large-scale, chaotic mixing in a mega-mélange. Exhumation of rocks on Syros occurred nearly entirely by buoyancy- and viscosity-driven subduction channel return flow, which accommodated vertical translations of ~40 km from peak depths (60 km) to the middle-lower crust. These inferences are consistent with calculated estimates of shear zone viscosity, and the balance between buoyancy forces and shear tractions at peak subduction depths. While exhumation mechanisms and mechanical behavior of thermally mature subduction zones are particularly important to an understanding of subduction tectonics, subduction initiation is even more enigmatic and poorly understood. The only geologic record of rocks deformed and metamorphosed in infant subduction zones are present as tectonic slivers beneath the world’s ophiolite sequences, deemed metamorphic soles. In Chapter 5, I investigate the structural and petrologic signatures of subduction, return flow, and ophiolite emplacement in a 100 m section of ”low-temperature” metamorphic sole acquired during Phase I of the Oman Drilling Project (Site BT-1B). The ”low-temperature” sole rocks acted as a mechanically coherent slice preserving various evidence for subduction and return flow prior to obduction. Intraoceanic subduction initiation is characterized by rapid cooling of the plate interface, which exerts primary control on metamorphic grade and changes in deformation mechanisms. Exhumation in the subduction channel accommodated at least 15 km of vertical translation, and occurred concurrently with early stages of ophiolite emplacement.
Structural evolution and metamorphism of mid-Proterozoic basement in the northwest Van Horn Mountains, Trans-Pecos, Texas
(1987) Bristol, David Arthur, 1961-; Mosher, Sharon, 1951-
Psammitic, pelitic and mafic schists in a NNW-trending horst in the NW Van Horn Mountains of west Texas show evidence of a multiphase, synmetamorphic, ductile Grenville-age deformation. Other basement exposures in the Van Horn area record a later phase of Grenville-age brittle deformation that has wholly or partly obscured the earlier-formed ductile features. Detailed mapping indicates that basement rocks exposed in the NW Van Horn Mountains underwent three phases of progressive isoclinal folding followed by two later, less intense, nearly coaxial folding episodes. The second phase of folding produced the dominant foliation and foliation intersection (S₁/S₂) lineation observed throughout the area. This deformation is inferred to have resulted from a large scale folding event, at or very near, peak metamorphic conditions, possibly associated with nappe formation. Garnet-biotite and garnet-hornblende Fe-Mg exchange thermometry indicate maximum temperatures of 640±50°C, corresponding to mid-amphibolite facies conditions, for this event. The intrusion of late syn- to post-orogenic pegmatite sills and dikes was widespread across the area. The associated pegmatitic fluids either facilitated or became the principal catalysts of a static recrystallization event. This event altered previously formed deformational fabrics and produced diagnostic strain-free textures in many minerals. The Grenville-age, polydeformational history recorded by the basement rocks of the NW Van Horn Mountains is similar to that noted in portions of the Llano Uplift of central Texas, about 650 km to the east. The strong similarity of deformational styles in these two areas suggests that large-scale folds with localized shear zones were produced in this region of the Grenville Province
Structure and tectonics of the Puerto Rico-Virgin Islands platform and multi-configuration ground penetrating radar data
(2000) Van Gestel, Jean-Paul; Stoffa, Paul L., 1948-
The Puerto Rico-Virgin Islands platform was deposited from early Oligocene to Holocene on top of an inactive and subsiding Cretaceous-earliest Oligocene island arc. Regional single- and multi-channel seismic reflection lines presented in this study provide the first information on the regional stratigraphy and structure of this platform. Integration of seismic reflection, well, and outcrop data indicates three major tectonic phases. The first tectonic phase consists of Cretaceous to Eocene formation and sedimentary infilling of a forearc basin. During the second tectonic phase from Oligocene to Pliocene the ~1600-m-thick, northward-thickening Puerto Rico-Virgin Islands platform was formed. In the final tectonic phase, from Pliocene to Holocene, the Puerto Rico-Virgin Islands platform tilted northward, whereby the northern edge of the platform submerged to a depth of 4 km. This tilting occurred on the northern limb of a large arch formed parallel to the long axis of the island of Puerto Rico. The arch formed in response to a post-Pliocene convergence between the North America and Caribbean plates. Traditionally Ground Penetrating Radar measurements are conducted using two co-polarized antennas oriented perpendicular to the line of data collection. Configurations, where the antennas are rotated to the line of data collection or are held cross-polarized, are rarely used. In this study it is shown that collection of this kind of data can be used in several methods to improve the final radargram. By recording the reflected field using four different configurations, and applying Alford rotation, information can be extracted about the orientation of objects that have angle-dependent reflectivity. Combination of Ground Penetrating Radar data collected in different polarized configurations using a weighted migration approach does not only improve the signal to noise ratio of the final image, but also results in a more uniform distribution of the radiated energy in the target zone. Alford rotation can also be implemented in a faster and more accurate migration algorithm. All these methods are proven to work on synthetic Ground Penetrating Radar data. They are also successfully applied on field data collected at a controlled Ground Penetrating Radar testing site in Scheveningen, The Netherlands
Tectonic geomorphology of the eastern Trinidad shelf : implications for influence of structure on reservoir distribution and nature in older basin fill
(2008) Alvarez, Tricia Grier; Wood, Lesli J.
Few deltaic systems in tectonically active areas of the world exhibit more data for detailed study than the Orinoco Delta extending along the hydrocarbon-rich, narrow, eastern shelf of the island of Trinidad. The entire region has undergone tectonic extension, compression and transpression during the late Tertiary and into the Quaternary. Paralic and shelf reservoir sand distribution and geometry have been significantly influenced by both structuring and strong offshore current activity, as well as large forced and unforced sealevel regressions. These sedimentary deposits hold significant shallow gas resources across the region, but the complexity of their distribution and architecture is poorly understood. A large merged 3D seismic survey (~9,000 sq km) was integrated with well penetrations across the modern shelf to examine the influence of structuring on near-modern basin fill depositional morphology and architecture. Key reflecting horizons were mapped across the area and these document the structural opening of the basin by extension superimposed over ongoing uplift of compressional anticlines. Seismic attributes image reservoir elements such as channels and channel belts, tidally inundated interfluves and interdistributary areas which facilitate analysis of the scale and form of these features as well as enable evaluation of the influence of structure on deposition. Both larger channels (1-2 km wide) and smaller channels (less than 100 meters) show patterns of avulsion and lateral migration and appear to be ubiquitous in some areas of the shelf during lowstand times. The largest major sediment fairway (valley) is long-lived (~1.0 million years) and structurally confined by east-west trending anticlinal uplifts, funneling sediments down the axis of the basin. Alternations between surfaces with well defined depositional elements interpreted to be indicative of subaerial conditions, and surfaces devoid of such features which have been interpreted to be submarine in nature suggest large scale fluctuations in the depositional environment over time under the influence of changes in sea level. Extensional faults, which show a complex pattern of displacement, both spatially and temporally; appear to remain active up to near present day time. Coupled with the apparent long-lived nature of the tectonic uplifts in this region, observations suggest that the structure is controlling sedimentation, as opposed to sedimentation driving deformation. Structure is playing a significant role in accommodation creation and therefore the location of the axis of sediment transport and accumulation throughout the Pleistocene history of the basin. This conclusion would be in keeping with the regional structural history of the basin showing transpression between the east-southeastward moving Caribbean Plate and stationary South American plate, initiated in the early Tertiary and continuing to the present day. Quantitative data on systems tract architectural elements, including spatial orientation and distribution should significantly improve 3D modeling of these reservoirs and improve understanding of sand distribution and the processes of sediment transfer from proximal sources to shelf staging areas
Veins and alteration envelopes in the Grasberg Igneous Complex, Gunung Bijih (Ertsberg) District, Irian Jaya, Indonesia
(1997) Penniston-Dorland, Sarah C.; Cloos, Mark
The Grasberg Igneous Complex (GIC) consists of three main phases of igneous activity: the Dalam Igneous Complex, the Main Grasberg Intrusion, and the Kali Intrusion. Each contains veins revealing a history of fluid flow that has concentrated minerals of economic value. A generalized sequence of early magnetite ± quartz veins followed by quartz ± sulfide/oxide veins followed by late chalcopyrite/pyrite veins is observed in the Dalam Igneous Complex and Main Grasberg Intrusion. The youngest igneous body, the Late Kali Intrusion, cross-cuts the older igneous bodies as well as their veins, and has biotite ± quartz, quartz ± pyrite and pyrite ± quartz veins. Pyrite ± quartz veins with alteration envelopes up to 14 cm total width are found in regions of the complex that are higher in elevation and distant from the center of copper mineralization. Geochemical analyses of wall rock and alteration envelopes from eleven samples are compared to determine which components were added to the altered samples and which were removed by the fluids. Most major components were removed by fluids (Na₂O, MgO, SiO₂, CaO, FeO, and TiO₂) along with many trace elements (Cu, Cl, Ga, Rb, Sr, Nb, Ba, Y, Zr, Eu, Yb, Tl, Au, La, Th, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Tm, and Lu). Gain or loss of K₂O and P₂O₅ vary depending on the sample. H₂O and S were added to the altered wall rock. Typical host-rock mineral assemblages include plagioclase, biotite, quartz, alkali feldspar, pyrite, chalcopyrite, and magnetite. Typical alteration envelope mineral assemblages include muscovite, alkali feldspar, pyrite, and quartz. Balanced reactions between wall rock minerals and fluid to produce alteration minerals typically involve the consumption of HCI, indicating that the altering fluids had a low pH. The alteration envelopes are believed to be the result of changes as fluids flowed through the complex, including decreasing temperature, generation of HCI by the precipitation of pyrite and chalcopyrite from copper and iron chlorides in the lower and central parts of the complex, and/or the decrease in the fluid prewall rock fluid pressure surrounding veins. Scanned cathodoluminescence of quartz in quartz-sulfide veins reveals detailed textures on the scale of tens to hundreds of microns including concentric growth zoning and fractures. Growth textures indicate that the quartz grew into open space, so these veins remained open during infilling. Vein growth is believed to have occurred from fluids that flowed through the veins. Microfracturing occurred after the veins began to close. Experimental studies of Cline and Bodnar (1991) applied to fluid exsolution from magma chambers are used as a basis to explain the sequence of veining. Fluid separating from a magma at low pressure (<1 kilobar) has initially low concentrations of copper, whereas fluid separating from a magma at high pressure (≥2 kilobars) has initially high concentrations of copper. Crystallization from a deep batholithic magma chamber at depths greater than 6 km with a molten stock reaching up to shallower depths (less than 3 km) can account for the changes in copper precipitation observed in the GIC system over time. Early crystallization in a stock at shallow depths led to exsolution of an early fluid that was relatively copper-poor. This resulted in early magnetite ± quartz veins. Deeper-seated crystallization eventually generated copper-rich fluids forming chalcopyrite/pyrite veins. Finally, the latest stages of veining following the Late Kali Intrusion were relatively copper-poor due to the last fluids exsolving from the deeper copper-depleted part of the magma chamber