ALBERT

Description: Constraining the timing of brittle faulting is critical in understanding crustal deformation and fluid flow, but many regional-scale fault systems lack readily available techniques to provide absolute chronological information. Calcite mineralization occurs in crustal faults in many geological settings and can be suitable for U-Pb geochronology. This application has remained underutilized because traditional bulk dissolution techniques require uncommonly high U concentration. Because U and Pb are distributed heterogeneously throughout calcite crystals, high-spatial-resolution sampling techniques can target domains with high U and variable U/Pb ratios. Here we present a novel application of in-situ laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) to basaltic fault rock geochronology in the Faroe Islands, northeast Atlantic margin. Faults that are kinematically linked to deformation associated with continental break-up were targeted. Acquired ages for fault events range from mid-Eocene to mid-Miocene and are therefore consistently younger than the regional early Eocene onset of ocean spreading, highlighting protracted brittle deformation within the newly developed continental margin. Calcite geochronology from LA-ICP-MS U-Pb analysis represents an important and novel method to constrain the absolute timing of fault and fluid-flow events.

Description: Igneous sills represent an important contribution to upper crustal magma transport and storage. This study focuses on an exemplary 20–50-m-thick transgressive sill in the Faroe Islands on the European Atlantic passive margin, which is hosted in layered lavas (1–20 m thick) and basaltic volcaniclastic units (1–30 m thick). Preserved steps in the sill, and offset intrusive segments, are consistent with initial propagation via segmented fractures followed by inflation to create a through-going sheet. Although steps correspond to the position of some host rock interfaces and volcaniclastic horizons, most interfaces are bypassed. Transgressive sill contacts are subparallel to thrust faults that record ENE-WSW shortening, which are observed within the surrounding country rock and within the sill. Remnant sill segments are elongate along a NNW-SSE axis, parallel to the derived intermediate stress axis for thrust faults. The overall transgressive geometry is consistent with regional horizontal shortening, with steps indicating transitions between transgressive and lateral sill propagation are controlled locally by layering. This work emphasizes the importance of scale of observation in considering the controls on sill emplacement, and in particular, that layering is not the primary control on geometry.

Description: Exhumed magma conduits provide important evidence of the development and evolution of subvolcanic plumbing systems. We use a 5–14-m-thick flow-banded rhyolite dike in Arran (Scotland) to present the first reconstruction of the directions and styles of initial propagation and subsequent magma flow, based on mesoscale kinematic indicators. The dike has concave-inward dike-margin segments with plumose-like structures that record vertical and horizontal propagation of lobes, which inflated and linked to form a through-going sheet. Devitrified rhyolite zones at the dike margins show gentle to open folds. In contrast, glassy central parts of the dike are flow laminated and preserve folded and refolded isoclinal, curvilinear folds and sheath folds that record sustained progressive deformation. The inner interface between the glassy and devitrified facies is abrupt and marked by elongation lineations and mullions. In the dike center, fold axes plunge 27°NE along the dike, and parallel to elongation lineations. Combined with shear sense indicators (- and -objects, sheared vesicles, and asymmetric folds), these features indicate that magma flow was obliquely upward, to the southwest, and locally ≤60° to the propagation direction of the dike. The distribution of structures within the rhyolite indicates local accretion of the (now) devitrified material to the margins, with localization of flow into the center of the dike. We find that the initial magma flow direction was controlled by fracture propagation and interaction, with the subsequent flow record controlled by accretion and flow localization in the conduit. This study demonstrates that analysis of mesoscopic structural and kinematic features (several of which have not previously been reported from dikes) is a powerful tool that can be used to reconstruct the complex evolution of conduit initiation and magma flow processes.

Description: Zealandia is a largely submerged, continental fragment in the southwest Pacific, generally considered to be derived from East Gondwana, but whose origins, age, structure, and relationships with other continental masses are poorly known. To explore the development of this microcontinent, a suite of mantle xenoliths was assembled from 12 localities throughout New Zealand, an emergent part of Zealandia. The 187 Re- 188 Os isotopic systematics of the xenoliths yield model ages (T RD2 ) between 0 and 2.3 Ga. Six samples from the newly defined Waitaha domain, South Island, have a narrow range of T RD2 ages from 1.6 to 1.9 Ga, in agreement with an aluminochron model age for this mantle domain of ca. 1.95 Ga, and with a three-point Re-Os isochron age of 2.26 ± 0.10 Ga. These ages are 〉500 m.y. older than T RD2 ages preserved in other regions of mantle lithosphere from the eastern margin of Gondwana (e.g., southeastern Australia and Marie Byrd Land, Antarctica) and 〉1 b.y. older than the oldest crustal rocks exposed in New Zealand. Thus, the lithospheric mantle of Zealandia has a complex age structure, including a region of Paleoproterozoic cratonic mantle with a minimum extent of ~45,000 km 2 . This ancient mantle resided at the margins of several supercontinents during the past ~2 b.y., attesting to the durability of subcontinental lithospheric mantle domains, even when decoupled from overlying contemporaneous crust and in an oceanic setting distanced from stable cratonic nuclei.

Description: A combination of field, microstructural, and experimental static permeability characterization is used to determine fault permeability structure evolution in upper crustal basalt-hosted fault zones in the Faroe Islands. The faults comprise low-strain fracture networks to high-strain breccias that form tabular volumes around a principal slip zone hosting gouge or cataclasite. Samples representative of these fault zone components are used for static experimental permeability measurement. Results indicate that within the appropriate effective pressure (depth) range (10–90 MPa; ~0.3–3.0 km), basalt-hosted faults evolve from relatively low-permeability (〈10 –17 m 2 ) structures with 〈1 m displacement to relatively high-permeability (〉10 –17 m 2 ) structures with ≥1 m displacement. Sample analyses reveal that static permeability is controlled by the development of: (1) fault-parallel clay growth (decreasing permeability), and (2) porous zeolite vein connectivity due to hydrofracture (increasing permeability). Fault-parallel permeability is increased relative to the host rock, while fault-normal permeability is low throughout fault rock evolution. This configuration will tend to promote across-fault compartmentalization and along-fault fluid flow, facilitating migration between relatively high-permeability horizons (e.g., vesicular flow-unit tops and siliciclastic horizons), bypassing the bulk of the stratigraphy.