Robert J. Varga

Spreading structure of the Troodos ophiolite, Cyprus

Orientations of dikes within the sheeted dike complex of the Troodos ophiolite reveal primary spreading structure produced at a complex ridge/transform intersection. Three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes. Faults flatten at depth into a detachment within the upper parts of the plutonic complex. Large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids. We suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north-trending (present coordinates), slow-spreading ridge crest. A simple model for ridge migration indicates eastward jumps of 8–13 km; changes in ridge orientation are suggested by changes in trends of dikes and graben axes. Comparison of the pattern of dikes near the Arakapas fault zone with the structure of active ridge/transform intersections suggests that the fault is a right-offset (sinistral) transform, in contrast to earlier models in which a ridge to the west of the exposed Troodos complex was proposed.

Dike surface lineations as magma flow indicators within the sheeted dike complex of the Troodos Ophiolite, Cyprus

Mesoscopic flow lineations and anisotropy of magnetic susceptibility (AMS) have been measured for dikes within the Cretaceous-age Troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth. Dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite pseudostratigraphy and ridge-and-groove structures termed hot slickenlines found throughout the complex. A unique flow direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin. Significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or back-flow of magma during the waning stages of intrusion. Surface lineations generally lie subparallel to the direction of flow inferred from AMS determinations on cores within 5 cm of dike margins. Surface lineations also lie subparallel to the long axis (e1) of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from AMS cores. Correlation of surface lineations with interior indicators of flow (AMS, plagioclase trachytic texture) indicate that the surface features are good proxies for grain-scale magma flow directions during dike propagation in Troodos dikes. Orientations of surface flow features in the dikes of the Troodos ophiolite indicate an approximately equal mix of subhorizontal to near-vertical magma flow, contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers. Our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest.

Shallow intrusive directions of sheeted dikes in the Troodos ophiolite: Anisotropy of magnetic susceptibility and structural data

Sheeted dikes play a central role in the formation of oceanic crust. It is commonly assumed that sheeted dikes intrude vertically upward, from elongated mid-ocean ridge (MOR) magma chambers, but there are no direct observational data bearing on this hypothesis. This assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes. The authors have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust, the Troodos ophiolite. Structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow (< 20[degree]) to nearly vertical, upward magma-transport directions. These data suggest that horizontal emplacement has to be considered for sheeted dikes at MORs, implying more centralized MOR plumbing systems than previously thought. Such plumbing systems provide ample opportunity for complex mixing, fractionation, and contamination of MOR lavas in magma chambers and tabular magma-storage volumes. Whether the MOR magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems.

Modes of extension at oceanic spreading centers: evidence from the Solea graben, Troodos ophiolite, Cyprus

Abstract The Solea graben in the Troodos ophiolite, Cyprus, represents a preserved Cretaceous-age axial valley formed during a brief period of amagmatic seafloor spreading. Structural analysis of all crustal levels of the graben reveals two periods of deformation; an extensive early phase of extensional deformation associated with seafloor spreading and a later phase of minor deformation associated with obduction and doming of the complex during the Tertiary-Recent. Amagmatic extensional deformation produced networks of normal faults that dip toward the graben axis and sole into a regional detachment fault located at the sheeted dike-plutonic contact and oriented subparallel to ophiolite pseudostratigraphy. Individual fault segments are essentially planar but link to approximate listric geometries locally. Bulk structural rotation of fault blocks bounding the graben occurred about a NNW-trending axis, parallel to the present graben trend. Average dips in the sheeted complex within the central portions of the graben suggest 40–50° of structural rotation, although this is a minimum figure because present dike dips are not a simple function of rotation magnitude. Tensor analysis of fault populations associated with extensional deformation shows that σ 3 was oriented approximately perpendicular to the graben axis. Late, near-axis extensional deformation segmented the detachment fault along NNW-trending normal faults. Obduction-related uplift and doming of the Troodos ophiolite is expressed in the Solea graben by minor thrust, reverse and strike-slip faults principally developed during reactivation of pre-existing low-angle fault zones formed during seafloor deformation. Tensor analysis of this later phase of faulting shows that σ 1 was oriented approximately N—S and that σ 2 and σ 3 alternated position during the deformation. The style of deformation documented in the Solea graben supports recent speculation for Basin and Range-style detachment faulting at slow-spreading ridge crests with low magma budgets, such as parts of the Mid-Atlantic Ridge near large-offset transform faults. Natural cross-sectional exposure of the Solea graben shows how extensional structures might link at depth beneath such slow-spreading ridge segments and demonstrates the importance of extensive brittle deformation of the crust prior to establishment of large-scale (ore-forming) hydrothermal circulation cells.

Rocky Mountain foreland uplifts: Products of a rotating stress field or strain partitioning?

Stress-tensor analysis of minor faults near the structural range fronts of diversely oriented Laramide foreland uplifts in the central Rocky Mountains, Wyoming, indicates that paleo-σ 1 was horizontal and oriented nearly perpendicular to uplift trends. Trend directions for σ 1 vary from east-northeast in the north-trending Laramie Range, to northeast in the northwest-trending Bighorn Mountains, to north at east-trending Casper Mountain and in the east-trending central Owl Creek Mountains. Although these compression directions are compatible with models invoking temporally changing stress directions during the Laramide orogeny, the apparent similarity in timing of uplift in these ranges suggests that the computed stress tensors may not reflect regional stress conditions. Recent stress determinations in transpressional regimes suggest that strain partitioning may be an important factor in considering local stress patterns in the vicinity of mountain belt structures oblique to regional trends. In light of these studies, caution should be used in interpreting regional paleostress from structures measured close to the tectonic fronts of Laramide uplifts.

EARLY ESTABLISHMENT OF SEAFLOOR HYDROTHERMAL SYSTEMS DURING STRUCTURAL EXTENSION : PALEOMAGNETIC EVIDENCE FROM THE TROODOS OPHIOLITE, CYPRUS

Abstract Paleomagnetic data from the Troodos ophiolite are used to help constrain models for the relationship between extensional normal faulting and hydrothermal alteration related to production of large-tonnage sulfide deposits at oceanic ridges. We have sampled dikes from the Troodos sheeted complex that have been subjected to variable hydrothermal alteration, from greenschist alteration typical of the low water/rock mass ratio interactions outside of hydrothermal upflow zones as well as from severely recrystallized rocks (epidosites) altered within high water/rock mass ratio hydrothermal upflow zones in the root zones beneath large sulfide ore deposits. These dikes are moderately to highly tilted from their initial near-vertical orientations due to rotations in the hangingwalls of approximately dike-parallel, oceanic normal faults. Comparison of characteristic remanence directions from these dikes with the Late Cretaceous Troodos reference direction, therefore, allows a tilt test to determine whether remanent magnetizations were acquired prior to or subsequent to tilting. Remanence directions for both greenschist and epidosite dikes show similar magnitudes of tilting due to rotational normal faulting and restore to the Late Cretaceous Troodos reference direction upon restoration of dikes to near-vertical positions about a NNW-trending, horizontal axis. These data, along with field observations of focused alteration along normal faults, suggest that epidosite alteration occurred during the early stages of extensional tilting and prior to significant rotation. This sequence of events is similar to that observed for creation of large-tonnage sulfide bodies at intermediate to slow spreading centers which form soon after cessation of magmatism and during the early stages of structural extension. We suggest that the dike-parallel normal faults were initiated as extensional fractures during this early stage of crustal extension, thus providing the necessary permeability for focused fluid flow, and that later slip along these structures during rotational-planar normal faulting caused reduction in permeability due to gouge formation.

Structural and geophysical expression of the Solea graben, Troodos Ophiolite, Cyprus

Field studies show that extensional structures in the north-central part of the Troodos Ophiolite, including steep normal faults, grabens, dikes, and low-angle detachment faults are related to E-W (present coordinates) spreading. The Solea graben, directly north of the mafic-ultramafic rock outcrops around Mount Olympus, consists of a 10- to 15-km-wide western portion that is underlain by a low-angle detachment fault between the sheeted dike complex and the plutonic section. Structural and paleomagnetic evidence indicates that the sheeted dikes above this detachment have been rotated 40° or more about subhorizontal axes. The eastern portion of the graben contains sheeted dike blocks with varying axes of rotation and significantly different style of intrusive activity. Most structural features in the graben formed simultaneously with or shortly after cessation of spreading at the graben axis based on cross-cutting ophiolitic dikes and hydrothermal mineralization. Timing of the uplift of the central part of the Troodos Ophiolite is uncertain but is probably related to the tectonic thinning of the upper crust by low-angle normal faulting. The Solea graben spreading center provides a model for the structure of modern mid-ocean ridges that are spreading amagmatically.

Isotopic age constraints on middle Paleozoic deformation in the northern Sierra Nevada, California

Allochthons of the lower Paleozoic Shoo Fly Complex in the northern Sierra Nevada were assembled and internally deformed prior to formation of a Devonian-Permian island-arc sequence. U/Pb data on zircons indicate ages of 423 +5/−15 Ma for a submarine tuff within the uppermost thrust slice of the Shoo Fly Complex and 378 +5/−10 Ma for a granitic intrusion that may be cogenetic with the lower part of the arc sequence. These data indicate late Early Silurian Shoo Fly deposition and proximity to active volcanism, as well as late Middle Devonian initiation of arc-related magmatism.

Relation between ore-forming hydrothermal systems and extensional deformation in the Solea graben spreading center, Troodos ophiolite, Cyprus

Field relations indicate that high-temperature hydrothermal circulation and accumulation of massive sulfide deposits within the Solea graben of the Troodos ophiolite, Cyprus, followed extreme crustal attenuation. Zones of pervasive, massive epidosite strike parallel to the axis of the Solea graben and to the strike of extensional normal faults. Initial fluid flow, evidenced by preferential epidotization in weakly altered areas surrounding massively altered regions, was focused along joints, microfractures, and (now) low-angle normal-fault zones related to graben formation. Permeability within the sheeted-dike section was enhanced by brittle deformation related to extensional structures as well as through volume reduction inherent in the diabase to epidosite mineralogic phase transformations. Intrusion of high-level gabbros into epidosite zones occurred both before and after significant amagmatic tectonic extension. Structural control on epidotization suggests that intrusion of late stocks into attenuated and highly deformed crust is necessary to drive the vigorous hydrothermal circulation that produced the epidosites and ore bodies of the Solea graben. A similar sequence of events is more likely to occur in the modern oceans along ridge crests with ephemeral magmatism, especially at intermediate- to slow-spreading ridges near transform faults.

Direct evidence from anisotropy of magnetic susceptibility for lateral melt migration at superfast spreading centers

[1] Rare, fault-bounded escarpments expose natural cross sections of ocean crust in several areas and provide an unparalleled opportunity to study the end products of tectonic and magmatic processes that operated at depth beneath oceanic spreading centers. We mapped the geologic structure of ocean crust produced at the East Pacific Rise (EPR) and now exposed along steep cliffs of the Pito Deep Rift near the northern edge of the Easter microplate. The upper oceanic crust in this area is typified by basaltic lavas underlain by a sheeted dike complex comprising northeast striking, moderately to steeply southeast dipping dikes. Paleomagnetic remanence of oriented blocks of dikes collected with both Alvin and Jason II indicate clockwise rotation of 61 related to rotation of the microplate indicating structural coupling between the microplate and crust of the Nazca Plate to the north. The consistent southeast dip of dikes formed as the result of tilting at the EPR shortly after their injection. Anisotropy of magnetic susceptibility of dikes provides well-defined magmatic flow directions that are dominantly dike-parallel and shallowly plunging. Corrected to their original EPR orientation, magma flow is interpreted as near-horizontal and parallel to the ridge axis. These data provide the first direct evidence from sheeted dikes in ocean crust for along-axis magma transport. These results also suggest that lateral transport in dikes is important even at fast spreading ridges where a laterally continuous subaxial magma chamber is present.