Microseismicity and lithosphere thickness at a nearly amagmatic mid-ocean ridge

Successive flip-flop detachment faults in a nearly-amagmatic region of the ultraslow-spreading Southwest Indian Ridge (SWIR) at 64°30'E accommodate ~100% of plate divergence, with mostly ultramafic seafloor. As magma is the main heat carrier to the oceanic lithosphere, the nearly-amagmatic SWIR 64°30'E is expected to have a very thick lithosphere. Here, our microseismicity data shows a 15-km thick seismogenic lithosphere, actually thinner than the more magmatic SWIR Dragon Flag detachment with the same spreading rate. This challenges current models of how spreading rate and melt supply control the thermal regime of mid-ocean ridges. Microearthquakes with normal focal mechanisms are colocated with seismically imaged damage zones of the detachment and reveal hanging-wall normal faulting, which is not seen at more magmatic detachments at the SWIR or the Mid-Atlantic Ridge. We also document a two-day seismic swarm, interpret as caused by an upward-migrating melt intrusion in the detachment footwall (6-11 km), triggering a sequence of shallower (~1.5 km) tectonic earthquakes in the detachment fault plane. This points to a possible link between sparse magmatism and tectonic failure at melt-poor ultraslow ridges.

Cenozoic structural evolution of the Catalina metamorphic core complex and reassembly of Laramide reverse faults, southeastern Arizona, USA

This study investigates the Late Cretaceous through mid-Cenozoic struc­tural evolution of the Catalina core complex and adjacent areas by integrating new geologic mapping, structural analysis, and geochronologic data. Multiple generations of normal faults associated with mid-Cenozoic extensional deformation cut across older reverse faults that formed during the Laramide orogeny. A proposed stepwise, cross-sectional structural reconstruction of mid-Cenozoic extension satisfies surface geologic and reflection seismologic constraints, balances, and indicates that detachment faults played no role in the formation of the core complex and Laramide reverse faults represent major thick-skinned structures. The orientations of the oldest synextensional strata, pre-shortening nor­mal faults, and pre-Cenozoic strata unaffected by Laramide compression indicate that rocks across most of the study area were steeply tilted east since the mid-Cenozoic. Crosscutting relations between faults and synextensional strata reveal that sequential generations of primarily down-to-the-west, mid- Cenozoic normal faults produced the net eastward tilting of ~60°. Restorations of the balanced cross section demonstrate that Cenozoic normal faults were originally steeply dipping and resulted in an estimated 59 km or 120% extension across the study area. Representative segments of those gently dipping faults are exposed at shallow, intermediate (~5–10 km), and deep structural levels (~10–20 km), as distinguished by the nature of deformation in the exhumed footwall, and these segments all restore to high angles, which indicates that they were not listric. Offset on major normal faults does not exceed 11 km, as opposed to tens of kilometers of offset commonly ascribed to “detachment” faults in most interpretations of this and other Cordilleran metamorphic core complexes. Once mid-Cenozoic extension is restored, reverse faults with moderate to steep original dips bound basement-cored uplifts that exhibit significant involvement of basement rocks. Net vertical uplift from all reverse faults is estimated to be 9.4 km, and estimated total shortening was 12 km or 20%. This magnitude of uplift is consistent with the vast exposure of metamorphosed and foliated cover strata in the northeastern and eastern Santa Catalina and Rincon Mountains and with the distribution of subsequently dismembered mid-Cenozoic erosion surfaces along the San Pedro Valley. New and existing geochronologic data constrain the timing of offset on local reverse faults to ca. 75–54 Ma. The thick-skinned style of Laramide shortening in the area is consistent with the structure of surrounding locales. Because detachment faults do not appear to have resulted in the formation of the Catalina core complex, other extensional systems that have been interpreted within the context of detachments may require further structural analyses including identification of crosscutting relations between generations of normal faults and palinspastic reconstructions.

The role of extensional detachment systems in thinning the crust and exhuming granulites: analogies between the offshore Le Danois High and the onshore Labourd Massif in the Biscay/Pyrenean rifts

Large uncertainties remain about the architecture, timing and role of the structures responsible for high degrees of crustal thinning and the exhumation of mid-crustal granulites in the Pyrenean and Biscay rift systems. Both, Le Danois High in the North Iberian margin and the Labourd Massif in the Western Pyrenees preserve evidence of extensional detachment faults and include exhumed granulites, which are locally reworked in syn-rift sediments. In this study, we compare the crustal structure and their link to the overlying sediments at the two sites based on the interpretation of high quality 2D seismic reflection profiles offshore and field observations and published geological cross-sections onshore. New reported seismic and field observations support the interpretation that the Le Danois High and the Labourd Massif are capped by extensional detachment systems, advocating for a similar tectonic evolution of the two sites. We propose that the two detachment systems were responsible for high degrees of crustal thinning and the exhumation of the pre-rift brittle-ductile transition and associated mid-crustal granulites during Aptian to Cenomanian extension, leading to the formation of the Le Danois and Labourd crustal tapers. Tilted and uplifted during the Alpine convergence, the two basement blocks lay at present in the hanging-wall of major Alpine thrusts. Their position at overlapping, en-echelon hyperextended rift segments at the end of rifting, and the occurrence of shortcutting structures linking neighbouring rift segments, can explain the preservation of the rift-related detachment systems. This study not only proposes for the first time analogies between the offshore Le Danois High and the onshore Labourd Massif, but it also demonstrates the importance of extensional detachment systems in thinning the crust and exhuming mid-crustal granulites at the seafloor in the Biscay and Pyrenean domains during Aptian to Cenomanian extension.

Pervasive detachment faults within the slow spreading oceanic crust at the poorly coupled Antilles subduction zone

Oceanic crust formed at slow-spreading ridges is currently subducted in only a few places on Earth and the tectonic and seismogenic imprint of the slow-spreading process is poorly understood. Here we present seismic and bathymetric data from the Northeastern Lesser Antilles Subduction Zone where thick sediments enable seismic imaging to greater depths than in the ocean basins. This dataset highlights a pervasive tectonic fabric characterized by closely spaced sequences of convex-up Ridgeward-Dipping Reflectors, which extend down to about 15 km depth with a 15-to-40° angle. We interpret these reflectors as discrete shear planes formed during the early stages of exhumation of magma-poor mantle rocks at an inside corner of a Mid-Atlantic Ridge fracture zone. Closer to the trench, plate bending could have reactivated this tectonic fabric and enabled deep fluid circulation and serpentinization of the basement rocks. This weak serpentinized basement likely explains the very low interplate seismic activity associated with the Barbuda-Anegada margin segment above.

Gravitational sliding or tectonic thrusting?: Examples and field recognition in the Miura-Boso subduction zone prism

ABSTRACT Discrimination between gravity slides and tectonic fold-and-thrust belts in the geologic record has long been a challenge, as both have similar layer shortening structures resulting from single bed duplication by thrust faults of outcrop to map scales. Outcrops on uplifted benches within the Miocene to Pliocene Misaki accretionary unit of Miura-Boso accretionary prism, Miura Peninsula, central Japan, preserve good examples of various types of bedding duplication and duplex structures with multiple styles of folds. These provide a foundation for discussion of the processes, mechanisms, and tectonic implications of structure formation in shallow parts of accretionary prisms. Careful observation of 2-D or 3-D and time dimensions of attitudes allows discrimination between formative processes. The structures of gravitational slide origin develop under semi-lithified conditions existing before the sediments are incorporated into the prism at the shallow surfaces of the outward, or on the inward slopes of the trench. They are constrained within the intraformational horizons above bedding-parallel detachment faults and are unconformably covered with the superjacent beds, or are intruded by diapiric, sedimentary sill or dike intrusions associated with liquefaction or fluidization under ductile conditions. The directions of vergence are variable. On the other hand, layer shortening structure formed by tectonic deformation within the accretionary prism are characterized by more constant styles and attitudes, and by strong shear features with cataclastic textures. In these structures, the fault surfaces are oblique to the bedding, and the beds are systematically duplicated (i.e., lacking random styles of slump folds), and they are commonly associated with fault-propagation folds. Gravitational slide bodies may be further deformed at deeper levels in the prism by tectonism. Such deformed rocks with both processes constitute the whole accretionary prism at depth, and later may be deformed, exhumed to shallow levels, and exposed at the surface of the trench slope, where they may experience further deformation. These observations are not only applicable in time and space to large-scale thrust-and-fold belts of accretionary prism orogens, but to small-scale examples. If we know the total 3-D geometry of geologic bodies, including the time and scale of deformational stages, we can discriminate between gravitational slide and tectonic formation of each fold-and-thrust belt at the various scales of occurrence.

Coalescing upper-crust detachment faults as a major structural style in the Great Basin: Evidence from the White Pine and Horse Ranges, east-central Nevada, USA

ABSTRACT Determining the origin and evolution of basin-and-range geomorphology and structure in the western United States is a fundamental problem with global implications for continental tectonics. Has the extensional tectonic development of the Great Basin been dominated by steeply dipping (horst and graben) faulting or detachment faulting? The purpose of this paper is to provide evidence that attenuation due to multiple coalescing detachment faults has been a significant or dominant upper-crustal process in at least some areas of the Great Basin. We present mapping at a scale of 1:3000 and seismic refraction profiling of an area at the discontinuity between the White Pine and Horse Ranges, east-central Nevada, USA, which indicate the existence of a detachment rooted in an argillaceous ductile unit. This fault, which we call the Currant Gap detachment, coalesces with the previously mapped regional White Pine detachment. Our data suggest that the Currant Summit strike-slip fault at the surface, previously proposed to explain a nearly 2500 m east-west surface offset between the two ranges, likely does not exist. If a discontinuity exists at depth, it could be manifested at the surface by the undulating topography of the two coalescing detachments. On the other hand, offset domal uplifts in the two ranges would obviate the need for any lateral discontinuity at depth to explain the observed surface features. Our previous mapping of the White Pine detachment showed that it extends over the White Pine, Horse, and Grant Ranges and into Railroad Valley (total of 3000 km2). Accordingly, we propose a model of stacked, coalescing detachments above the metamorphic infrastructure; these detachments are regional and thus account for most of the basin-range relief and upper-crust extension in this area. An essential feature of our model is that these detachments are rooted in ductile units. Detachments that have been observed in brittle units could have initiated at a time when elevated temperatures or fluid flow enhanced the ductility of the rocks. The Currant Gap and White Pine detachments exhibit distinctive types of fluid-genetic silicified rocks. Study of such rocks in fault contacts could provide insights into the initiation and early history of detachment faulting as well as the migration of fluids, including petroleum.

Permian rifting and detachment faults and their role in Alpine collisional tectonics

<p>The Permian was a time of strong crustal extension in the area of the later-formed Alpine orogen. This involved extensional detachment faulting and the formation of metamorphic core complexes. We describe (1) an area in the Southern Alps (Valsassina, Orobic chain) where a metamorphic core complex and detachment fault have been preserved and only moderately overprinted by Alpine collisional shortening, and (2) an area in the Austroalpine (Schneeberg) where Alpine deformation and metamorphism are intense but a Permian low-angle normal fault is reconstructed from the present-day tectonometamorphic setting. In the Southern Alps case, the Grassi Detachment Fault represents a low-angle detachment capping a metamorphic core complex in the footwall which was affected by upward‐increasing, top‐to‐the‐southeast mylonitization. Two granitoid intrusions occur in the core complex, c. 289 Ma and c. 287 Ma, the older of which was syn-tectonic with respect to the extensional mylonites (Pohl, Froitzheim, et al., 2018, Tectonics). Consequently, detachment‐related mylonitic shearing took place during the Early Permian and ended at ~288 Ma, but kinematically coherent brittle faulting continued. Considering 30° anticlockwise rotation of the Southern Alps since Early Permian, the extension direction of the Grassi Detachment Fault was originally ~N‐S and the sense of transport top-South. In this area, there is no evidence of Permian strike-slip faulting but only of extension. In the Schneeberg area of the Austroalpine, a unit of Early Paleozoic metasediments with only Eoalpine (Cretaceous) garnet, the Schneeberg Complex, overlies units with two-phased (Variscan plus Eoalpine) garnet both to the North (Ötztal Complex) and to the South (Texel Complex). The basal contact of the Schneeberg Complex was active as a north-directed thrust during the Eoalpine orogeny. It reactivated a pre-existing, post-Variscan but pre-Mesozoic, i.e. Permian low-angle normal fault. This normal fault had emplaced the Schneeberg Complex with only low Variscan metamorphism (no Variscan garnet) on an amphibolite-facies metamorphic Variscan basement. The original normal fault dipped south or southeast, like the Grassi detachment in the Southern Alps. As the most deeply subducted units of the Eoalpine orogen (e.g. Koralpe, Saualpe, Pohorje) are also the ones showing the strongest Permian rift-related magmatism, we hypothesize that the Eoalpine subduction was localized in a deep Permian rift system within continental crust.</p>

Rift evolution and structural style in the Orphan Basin: results from regional structural restorations and crustal area balancing

<p>The Orphan Basin on the eastern edge of the Newfoundland continental margin formed as a Mesozoic rift basin prior to continental breakup associated with the opening of the North Atlantic. Few exploration wells exist in the basin, and until recently regional interpretations have been based on sparse seismic data coverage - because of this the structural evolution of the Orphan Basin has historically not been well understood. Key uncertainties include the timing and amount of rift-related extension, dominant extension directions, and the structural styles that accommodated progressive rift development in the basin.     </p><p>Interpretation of newly acquired modern broadband seismic data and structural restoration of three regional, WNW-ESE oriented cross-sections across the Orphan Basin and Flemish Cap provide new insights into rift evolution and structural style in the area. Our results show that major extension in the basin occurred between 167 Ma and 135 Ma, with most extension occurring prior to 151 Ma. We show that extension after 135 Ma largely occurred east of Flemish Cap due to a shift in the locus of rifting from the Orphan Basin to east of Flemish Cap. We find no evidence for discrete rifting events in the Orphan Basin, as has been suggested by other authors.  Kinematic restoration and associated heave measurements for the Orphan Basin show that extension was both widespread and relatively evenly distributed across the basin from Middle-Late Jurassic to Early Cretaceous.</p><p>We provide evidence for more widespread deposition of Jurassic strata throughout the Orphan Basin than previously interpreted, and show that Jurassic deposition was controlled by the occurrence and displacement of crustal-scale extensional detachment faults.  Structure in the three regional cross sections is dominated by large-scale, shallowly dipping extensional detachment faults. These faults mainly dip to the northwest and control the geometry and position of extensional basins – grabens and half-grabens – which occur at a range of scales. Stacked detachment surfaces, hyperextension, and attenuation of the crust are observed in central and eastern parts of the Orphan Basin. Zones of extreme crustal attenuation (to ca. 3.7 km) are interpreted to be coincident with large-displacement (up to 60 km) low-angle detachments. Results from crustal area balancing suggest that up to 41% of extension is not recognized through structural seismic interpretation, which we attribute to subseismic-scale ductile and brittle deformation, and uncertainties in the identification of detachment surfaces or complex structural configurations (e.g., overprinting of early extensional deformation).</p><p>Rifting style in the central, northern, and eastern parts of the Orphan Basin is dominated by low-angle detachment faulting with maximum extension perpendicular to the incipient rift axis. In contrast, structural geometries in the southwestern part of the basin are suggestive of transtensional deformation, and interplay of normal and strike-slip faulting.  Results from map-based interpretation show that strike-slip faults within this transtensional zone are associated with displacement transfer between half-grabens of opposing polarity, rather than regional strike-slip displacement.  These structures are interpreted as contemporaneous and kinematically linked to displacement along low-angle detachment surfaces elsewhere, and are not attributed to distinct episodes of oblique extension.       </p>