An intermittent detachment faulting system with a large sulfide deposit revealed by multi-scale magnetic surveys

Magmatic and tectonic processes can contribute to discontinuous crustal accretion and play an important role in hydrothermal circulation at ultraslow-spreading ridges, however, it is difficult to accurately describe the processes without an age framework to constrain crustal evolution. Here we report on a multi-scale magnetic survey that provides constraints on the fine-scale evolution of a detachment faulting system that hosts hydrothermal activity at 49.7°E on the Southwest Indian Ridge. Reconstruction of the multi-stage detachment faulting history shows a previous episode of detachment faulting took place 0.76~1.48 My BP, while the present fault has been active for the past ~0.33 My and is just in the prime of life. This fault sustains hydrothermal circulation that has the potential for developing a large sulfide deposit. High resolution multiscale magnetics allows us to constrain the relative balance between periods of detachment faulting and magmatism to better describe accretionary processes on an ultraslow spreading ridge.

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.

Detachment Faulting, Successive Incision and Controls on Supradetachment Basin Formation at the Mid-Norwegian Rifted Margin

<p>Detachment fault systems recording displacements in the order of 10s to 100s of km remain poorly understood compared to smaller scale normal faults. The evolutionary models developed for the growth and interaction of Andersonian-type faults are not fully applicable to these large-magnitude systems. Consequently, the associated basins - the so-called supradetachment basins - are still poorly understood compared to extensional half-graben basins.</p><p>Numerical and analogue 2D modelling have shed light on the mechanisms of footwall back-rotation during progressive extension (rolling hinge model; e.g. Lavier et al., 1999) but the along-strike evolution of such large-scale detachment systems remain poorly understood. It has been proposed that with increasing amounts of extension, detachment faulting favors formation of isostatically induced, longitudinal and transverse folds and consequently basin inversion in the area of maximum displacement (e.g. Kapp et al., 2008; Osmundsen & Péron-Pinvidic, 2018). The 4D configuration of the associated supradetachment basins is then controlled by the growth and (potential) lateral linkage of such faults - which may result in complex geometries.</p><p>In this study, we use interpretation of 3D- and 2D seismic reflection data from the necking domain of the Mid-Norwegian rifted margin to discuss the effects of lateral interaction and linkage of extensional detachment faults. The study area demonstrates how successive incision of such master faults may induce a complex structural relief in response to extensional detachment faulting and folding. In the inner parts of the south Vøring and northeastern Møre basins, the Klakk and Main Møre Fault Complexes form the outer necking breakaway complex and the western boundary of the Frøya High. The central Frøya High contains remnants of a metamorphic core complex, which we interpret as an extension parallel turtleback-structure. The turtleback is flanked two main synclinal depocenters constituting a supradetachment basin, whose location corresponds to the crustal taper break associated with the outer necking domain. We attribute the turtleback exhumation to Late Jurassic-Early Cretaceous detachment faulting along the Klakk and Main Møre Fault Complexes. Southwest of the Frøya High, the supradetachment basin links the Frøya High Turtleback with the core complex previously interpreted for the Gossa High, near where the Main Møre Fault Complex incises the Slørebotn detachment. The Slørebotn Subbasin consequently forms a synclinal keel basin with rafted blocks, a structural configuration which is recognizable also north of the ‘Frøya High Turtleback’ towards the Halten Terrace. We find that the pre-rift structural template and crustal heterogeneity facilitated differential supradetachment basin configuration during and after Late Jurassic-Early Cretaceous rifting, and that the supradetachment basin architecture was likely controlled by localized isostatic uplift, lateral linkage and successive incision of large-magnitude normal faults.</p>

3D structure of detachment faulting and related tectono-sedimentary processes in the SE South China Sea

<p>Before Break-Up, the opening of the South China Sea Passive Margin (SCS) was characterized by a wide rift mode during Cenozoic rifting. Such wide extensional margin (>600 km wide) is controlled by a set of hyper-extended sub-basins separated by basement highs.</p><p>These basins infill recorded a polyphased extensional deformation hence resulting in complex 3D sedimentary evolution. Based on a recent industrial 3D seismic reflection survey along the Sabah area (southern margin of the SCS), this contribution aims to investigate the detailed 3D geometries of extensional structures as well as their control on the overlying successive sedimentary sequences and relation to crustal deformation.</p><p>We mapped and analyzed several crustal-scale rolling hinge structures controlled by a series of low-angle normal faults. Deeper crustal levels are likely exhumed along the core of these rolling hinge structures, separated by extensional allochthones blocs of upper continental crust. Our structural analysis enables us to identify three main extensional phases corresponding to distinct sedimentary packages: (1) a synrift sequence 1 controlled by small offset normal faults formed during incipient rifting; (2) an intermediate synrift sequence 2 recording the development of extensional detachment faults. (3) a thick syn-rift sequence 3 recording a continuation of extension along the detachment faults resulting in the dismembering of the syn-<br>rift sequence 2. Intra-basement seismic reflectors dipping towards the north-west are observed, onto which extensional structures often seem to root. Some of these reflectors are interpreted as interleaved thrust sheets from a dismantled accretionary wedge of the former Mesozoic active margin (Yenshanian magmatic Arc).</p><p>Our results provide new key observations on the 3D mechanisms of detachment faulting and its control on sedimentary evolution as well as coeval crustal deformation. 3D approach throw some light on the detailed geometries of a metamorphic core-complex in relation with crustal boudinage, shear zones and lower/middle crust exhumation below the syn- rift sediments. These geometries can be compared to those described in the Basin and Range province or the Aegean Sea. Consequently, our results have implications for our understanding of rift and breakup mechanisms of marginal basins as a whole.</p>

Extensional detachments related to extreme crustal thinning and their fate during contractional reactivation: the Le Danois-North Mauléon offshore-onshore examples in north Iberia

<p>Extensional detachment faults accommodate high degrees of crustal thinning and exhumation, shaping largely the final architecture of magma-poor rifted margins. Great efforts have been directed to study extensional detachments based on offshore seismic surveys and onshore field analogues. However, little is known about the breakaway of these structures as well as their role and evolution during rifting and subsequent contractional reactivation. </p><p>In this work, we use the Le Danois-Labourd offshore-onshore natural laboratory (northern Spain) to explore the features characterising major Mesozoic extensional detachment faults and their fate during subsequent Alpine contractional reactivation. Both sites keep evidence of Mesozoic extensional detachment faulting and high degrees of crustal thinning, including exhumed mid-crustal granulites reworked as clasts into Apto-Albian syn-rift sediments, and show mild Alpine reactivation, corresponding at present-day to structural highs. Relying on the interpretation of high quality 2D seismic reflection profiles offshore and on field-based cross-sections onshore, we describe and compare the former rift architecture associated with these major detachment faults and the distribution of contractional structures at the two sites.</p><p> This combined study enable us to evidence strong structural similarities between the two sites and to propose that the Le Danois and the North Mauléon extensional detachment systems are major rift structures within the North Iberian rift system. We propose that they were responsible for high degrees of crustal thinning and the exhumation of mid-crustal rocks during the Late Aptian to Albian N-S directed extension. Major thrusts truncated the two extensional detachments during subsequent Alpine reactivation, leading to the uplift and tilting of the Le Danois and the Labourd rift-inherited crustal blocks. We suggest that the location of the two blocks at the termination of offset/overlapped hyperextended rift segments allowed for their preservation as mildly inverted structural highs, including rift-related structures.</p>

Microstructures reveal brittle and viscous flow during exhumation of the high-temperature lower oceanic crust from Site U1473A, Atlantis Bank, Southwest Indian Ridge (IODP Expedition 360)

<p>Detachment faulting has been hypothesized as the main process of tectonic spreading in mid-ocean ridges. The ongoing faulting leads to exhumation of oceanic core complexes (OCC) through large-scale normal faults, exposing heterogeneous sectors of the mylonitic lower crust, locally interlayered with pristine upper-mantle rocks. However, the mechanisms involved in this process – and the interplay between magmatism, deformation and fluid-rock interaction – are still debatable. To address these issues, we performed a quantitative microstructural analysis and thermodynamic modelling on mafic shear zones that occur in the lower section (≥ 600 meters below sea-floor) of Site U1473A (Atlantis Bank OCC, SW Indian Ridge), the target of IODP Expedition 360, to constrain deformation conditions and strain localization mechanisms during detachment faulting. The gabbroic shear zones consist of large (up to 5 mm in size) porphyroclasts of clinopyroxene, orthopyroxene, plagioclase and olivine embedded in a fine-grained (≤ 30 µm), polyphase matrix composed of plagioclase, clinopyroxene, orthopyroxene, amphibole, ilmenite, magnetite and olivine. Plagioclase-rich layers (~ 80 µm) are in abrupt contact with the fine-grained mixture, which define the mylonitic foliation. The porphyroclasts have undulose extinction, subgrains and are surrounded by fine-grained recrystallized grains (core-mantle structure) showing internal lattice distortion. Microfractures are common in orthopyroxene porphyroclasts. Amphibole replaces clinopyroxene and orthopyroxene porphyroclasts at their margins and fills cleavage planes. The plagioclase-rich layers show undulose extinction and subgrain boundaries in the larger grains within the layers. Mechanical twin lamellae occur in some grains regardless of grain size. Plagioclase grains show a weak shape preferred orientation with their long axes parallel to the main planar fabric of the shear zone. The grains in the polyphase matrix are mostly strain free. EBSD data in clinopyroxene clasts indicate activation of (010)[001] slip system and twinning along (001)[100]. Plagioclase-rich layers deforms by slip along the (010)[100] system. The polyphase matrix has a very weak but non-random CPO pattern. #Mg and Al content in the recrystallized clinopyroxene and orthopyroxene grains are lower compared to the porphyroclasts. Plagioclase has similar An content in both porphyroclasts and recrystallized grains. Amphibole has low concentrations of Cl and high content of F. The content of #Mg, Al and Si is similar in amphibole grains replacing pyroxene and in the polyphase matrix. Thermodynamic modelling indicates that the gabbroic shear zones formed at 820-870 °C and 2.0-2.8 kbar. Our results suggest that deformation in the porphyroclasts was accommodated by combined mechanical fragmentation and intracrystalline plasticity, which resulted in fractured grains of orthopyroxene, and clasts rimmed by recrystallized neoblasts. Plagioclase-rich layers formed mainly through dislocation creep. Phase mixing and weak CPO in the polyphase matrix point to oriented-growth during diffusion-assisted grain boundary sliding, mainly in the presence of melt, as evidenced by amphibole formed at the expense of pyroxene. Magmatic fluids are the possible source of reactant amphibole. Such mechanisms effectively resulted in strain localization in fine-grained, polyphase shear zones that contributed to the weakening of the ocean crust during detachment faulting and subsequent exhumation of the Atlantis Bank OCC.</p>

An intermittent detachment faulting system with a large sulfide deposit revealed by multi-scale magnetic surveys

Magmatic and tectonic processes can contribute to discontinuous crustal accretion and play an important role in hydrothermal circulation at ultraslow-spreading ridges, however, it is difficult to accurately describe the processes without an age framework to constrain crustal evolution. Here we report on a multi-scale magnetic survey that provides constraints on the fine-scale evolution of a detachment faulting system that hosts hydrothermal activity at 49.7°E on the Southwest Indian Ridge. Reconstruction of the multi-stage detachment faulting history shows a previous episode of detachment faulting took place 0.76~1.48 My BP, while the present fault has been active for the past ~0.33 My just in the prime of life. This fault sustains hydrothermal circulation that has the potential for developing a large sulfide deposit. High resolution multiscale magnetics allows us to constrain the relative balance between periods of detachment faulting and magmatism to better describe accretionary processes on an ultraslow spreading ridge.