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2019
Olive, JA, Parnell-Turner R, Escartin J, Smith DK, Petersen S.  2019.  Controls on the seafloor exposure of detachment fault surfaces. Earth and Planetary Science Letters. 506:381-387.   10.1016/j.epsl.2018.11.001   AbstractWebsite

While oceanic detachment faults have been proposed to account for the accretion of similar to 40% of new seafloor in the North Atlantic ocean, clear exposures of large-offset, often-corrugated fault surfaces remain scarce and spatially limited. To help resolve this paradox, we examine the conditions under which detachment fault growth may or may not lead to extensive exposure of corrugated fault planes at the seafloor. Using high-resolution bathymetry from four detachment faults at the northern Mid-Atlantic Ridge, we investigate the rafting of hanging wall-derived debris over emerging fault scarps, which can lead to covering shallow-dipping corrugated fault surfaces. We model this process using critical taper theory, and infer low effective friction coefficients (similar to 0.2) on the shallowest portion of detachment faults. A corollary to this result is that detachments emerging from the seafloor at angles <13 degrees are more likely to become blanketed under an apron of hanging wall material. We generalize these findings as a simple model for the progressive exposure and flexural rotation of detachment footwalls, which accounts for the continued action of seafloor-shaping processes. Our model suggests that many moderate-offset, hidden detachment faults may exist along slow mid-ocean ridges, and do not feature an exposed fault surface. (C) 2018 Elsevier B.V. All rights reserved.

2018
Parnell-Turner, R, Escartin J, Olive JA, Smith DK, Petersen S.  2018.  Genesis of corrugated fault surfaces by strain localization recorded at oceanic detachments. Earth and Planetary Science Letters. 498:116-128.   10.1016/j.epsl.2018.06.034   AbstractWebsite

Seafloor spreading at slow and ultraslow rates is often taken up by extension on large-offset faults called detachments, which exhume lower crustal and mantle rocks, and in some cases make up domed oceanic core complexes. The exposed footwall may reveal a characteristic pattern of spreading-parallel corrugations, the largest of which are clearly visible in multibeam bathymetric data, and whose nature and origin have been the subject of controversy. In order to tackle this debate, we use available near bottom bathymetric surveys recently acquired with autonomous deep-sea vehicles over five corrugated detachments along the Mid-Atlantic Ridge. With a spatial resolution of 2 m, these data allow us to compare the geometry of corrugations on oceanic detachments that are characterized by differing fault zone lithologies, and accommodate varying amounts of slip. The fault surfaces host corrugations with wavelengths of 10-250 m, while individual corrugations are finite in length, typically 100-500 m. Power spectra of profiles calculated across the corrugated fault surfaces reveal a common level of roughness, and indicate that the fault surfaces are not fractal. Since systematic variation in roughness with fault offset is not evident, we propose that portions of the exposed footwalls analyzed here record constant brittle strain. We assess three competing hypotheses for corrugation formation and find that the continuous casting and varying depth to brittle-ductile transition models cannot explain the observed corrugation geometry nor available geological observations. We suggest a model involving brittle strain localization on a network of linked fractures within a zone of finite thickness is a better explanation for the observations. This model explains corrugations on oceanic detachment faults exposed at the seafloor and on normal faults in the continents, and is consistent with recently imaged corrugations on a subduction zone megathrust. Hence fracture linkage and coalescence may give rise to corrugated fault zones, regardless of earlier deformation history and tectonic setting. (C) 2018 Elsevier B.V. All rights reserved.

Parnell-Turner, RE, Mittelstaedt E, Kurz MD, Jones MR, Soule SA, Klein F, Wanless VD, Fornari DJ.  2018.  The final stages of slip and volcanism on an oceanic detachment fault at 13°48′N, Mid-Atlantic Ridge. Geochemistry, Geophysics, Geosystems.   10.1029/2018GC007536   Abstract

While processes associated with initiation and maintenance of oceanic detachment faults are becoming better constrained, much less is known about the tectonic and magmatic conditions that lead to fault abandonment. Here we present results from near-bottom investigations using the submersible Alvin and autonomous underwater vehicle Sentry at a recently extinct detachment fault near 13°48′N, Mid-Atlantic Ridge, that allow documentation of the final stages of fault activity and magmatism. Seafloor imagery, sampling, and near-bottom magnetic data show that the detachment footwall is intersected by an ~850 m-wide volcanic outcrop including pillow lavas. Saturation pressures in these vesicular basalts, based on dissolved H2O and CO2, are less than their collection pressures, which could be explained by eruption at a shallower level than their present depth. Sub-bottom profiles reveal that sediment thickness, a loose proxy for seafloor age, is ~2 m greater on top of the volcanic terrain than on the footwall adjacent to the hanging-wall cutoff. This difference could be explained by current-driven erosion in the axial valley or by continued slip after volcanic emplacement, on either a newly formed or pre-existing fault. Since current speeds near the footwall are unlikely to be sufficient to cause significant erosion, we favor the hypothesis that detachment slip continued after the episode of magmatism, consistent with growing evidence that oceanic detachments can continue to slip despite hosting magmatic intrusions.

2017
Craig, TJ, Parnell-Turner R.  2017.  Depth-varying seismogenesis on an oceanic detachment fault at 13 degrees 20 ' N on the Mid-Atlantic Ridge. Earth and Planetary Science Letters. 479:60-70.   10.1016/j.epsl.2017.09.020   AbstractWebsite

Extension at slow- and intermediate-spreading mid-ocean ridges is commonly accommodated through slip on long-lived faults called oceanic detachments. These curved, convex-upward faults consist of a steeply-dipping section thought to be rooted in the lower crust or upper mantle which rotates to progressively shallower dip-angles at shallower depths. The commonly-observed result is a domed, sub horizontal oceanic core complex at the seabed. Although it is accepted that detachment faults can accumulate kilometre-scale offsets over millions of years, the mechanism of slip, and their capacity to sustain the shear stresses necessary to produce large earthquakes, remains subject to debate. Here we present a comprehensive seismological study of an active oceanic detachment fault system on the Mid-Atlantic Ridge near 13 degrees 20'N, combining the results from a local ocean-bottom seismograph deployment with waveform inversion of a series of larger teleseismically-observed earthquakes. The unique coincidence of these two datasets provides a comprehensive definition of rupture on the fault, from the uppermost mantle to the seabed. Our results demonstrate that although slip on the deep, steeply-dipping portion of detachment faults is accommodated by failure in numerous microearthquakes, the shallow, gently-dipping section of the fault within the upper few kilometres is relatively strong, and is capable of producing large-magnitude earthquakes. This result brings into question the current paradigm that the shallow sections of oceanic detachment faults are dominated by low-friction mineralogies and therefore slip aseismically, but is consistent with observations from continental detachment faults. Slip on the shallow portion of active detachment faults at relatively low angles may therefore account for many more large-magnitude earthquakes at mid-ocean ridges than previously thought, and suggests that the lithospheric strength at slow-spreading mid-ocean ridges may be concentrated at shallow depths. (C) 2017 Elsevier B.V. All rights reserved.

Parnell-Turner, R, White N, Henstock TJ, Jones SM, Maclennan J, Murton BJ.  2017.  Causes and consequences of diachronous V-shaped ridges in the North Atlantic Ocean. Journal of Geophysical Research-Solid Earth. 122:8675-8708.   10.1002/2017jb014225   AbstractWebsite

In the North Atlantic Ocean, the geometry of diachronous V-shaped features that straddle the Reykjanes Ridge is often attributed to thermal pulses which advect away from the center of the Iceland plume. Recently, two alternative hypotheses have been proposed: rift propagation and buoyant mantle upwelling. Here we evaluate these different proposals using basin-wide geophysical and geochemical observations. The centerpiece of our analysis is a pair of seismic reflection profiles oriented parallel to flow lines that span the North Atlantic Ocean. V-shaped ridges and troughs are mapped on both Neogene and Paleogene oceanic crust, enabling a detailed chronology of activity to be established for the last 50 million years. Estimates of the cumulative horizontal displacement across normal faults help to discriminate between brittle and magmatic modes of plate separation, suggesting that crustal architecture is sensitive to the changing planform of the plume. Water-loaded residual depth measurements are used to estimate crustal thickness and to infer mantle potential temperature which varies by 25 degrees C on timescales of 3-8Ma. This variation is consistent with the range of temperatures inferred from geochemical modeling of dredged basaltic rocks along the ridge axis itself, from changes in Neogene deep-water circulation, and from the regional record of episodic Cenozoic magmatism. We conclude that radial propagation of transient thermal anomalies within an asthenospheric channel that is 150 50km thick best accounts for the available geophysical and geochemical observations.Plain Language Summary In the North Atlantic Ocean, immense amounts of hot material rises up beneath Iceland from deep within Earth's mantle, forming a gigantic pancake-shaped upwelling. This upwelling, known as the Iceland mantle plume, is the largest on Earth and plays a key role in determining the depth and shape of the North Atlantic Ocean over thousands of kilometers. A pattern of distinctive V-shaped ridges and troughs that are hundreds of kilometers long and tens of kilometers wide occur on the seabed south of Iceland. These V-shaped ridges are thought to have been generated by waxing and waning of the plume, but their precise origin is hotly debated. Here we use an acoustic (i.e., seismic) survey, spanning the North Atlantic Ocean to image these features. We assess competing hypotheses for their formation and argue that they are indeed an indirect record of plume activity through time. Pulses of hot material appear to be generated every 3 to 8Ma. As they spread beneath adjacent tectonic plates, these pulses cause vertical movements that trigger changes in ancient oceanic circulation.

Parnell-Turner, R, Sohn RA, Peirce C, Reston TJ, MacLeod CJ, Searle RC, Simao NM.  2017.  Oceanic detachment faults generate compression in extension. Geology. 45:923-926.   10.1130/G39232.1   AbstractWebsite

In extensional geologic systems such as mid-ocean ridges, deformation is typically accommodated by slip on normal faults, where material is pulled apart under tension and stress is released by rupture during earthquakes and magmatic accretion. However, at slowly spreading mid-ocean ridges where the tectonic plates move apart at rates <80 km m.y.(-1), these normal faults may roll over to form long-lived, low-angled detachments that exhume mantle rocks and form corrugated domes on the seabed. Here we present the results of a local micro-earthquake study over an active detachment at 13 degrees 20'N on the Mid-Atlantic Ridge to show that these features can give rise to reverse-faulting earthquakes in response to plate bending. During a 6 month survey period, we observed a remarkably high rate of seismic activity, with >244,000 events detected along 25 km of the ridge axis, to depths of similar to 10 km below seafloor. Surprisingly, the majority of these were reverse-faulting events. Restricted to depths of 3-7 km below seafloor, these reverse events delineate a band of intense compressional seismicity located adjacent to a zone of deeper extensional events. This deformation pattern is consistent with flexural models of plate bending during lithospheric accretion. Our results indicate that the lower portion of the detachment footwall experiences compressive stresses and deforms internally as the fault rolls over to low angles before emerging at the seafloor. These compressive stresses trigger reverse faulting even though the detachment itself is an extensional system.

2016
Parnell-Turner, R, Schouten H, Smith DK.  2016.  Tectonic structure of the Mid-Atlantic Ridge near 16 degrees 30 ' N. Geochemistry Geophysics Geosystems. 17:3993-4010.   10.1002/2016gc006514   AbstractWebsite

The 16 degrees 30'N area of the Mid-Atlantic Ridge represents an area of present-day detachment faulting. Here we present shipboard bathymetric, magnetic and gravity data acquired up to 65 km from the ridge axis that reveal a varied tectonic history of this region. Magnetic data are used to calculate spreading rates and examine spreading rate variability along and across the axis. Bathymetric and gravity data are used to infer the crustal structure. A central magnetic anomaly 40% narrower than expected is observed along much of the study area. Misalignment between modern-day spreading center and magnetic anomalies indicates tectonic reorganization of the axis within the past 780 ka. Observed magnetic anomalies show a pattern of anomalous skewness consistent with rotation of magnetic vectors probably associated with detachment faulting. Relatively thin crust north of a small (similar to 7 km) nontransform offset coincides with a weakly magmatic spreading axis. In contrast, to the south a robust axial volcanic ridge is underlain by thicker crust. Variations in crustal structure perpendicular to the axis occur over tens of kilometers, indicating processes which occur over timescales of 1-2 Ma.

2015
Parnell-Turner, R, White NJ, McCave IN, Henstock TJ, Murton B, Jones SM.  2015.  Architecture of North Atlantic contourite drifts modified by transient circulation of the Icelandic mantle plume. Geochemistry Geophysics Geosystems. 16:3414-3435.   10.1002/2015gc005947   AbstractWebsite

Overflow of Northern Component Water, the precursor of North Atlantic Deep Water, appears to have varied during Neogene times. It has been suggested that this variation is moderated by transient behavior of the Icelandic mantle plume, which has influenced North Atlantic bathymetry through time. Thus pathways and intensities of bottom currents that control deposition of contourite drifts could be affected by mantle processes. Here, we present regional seismic reflection profiles that cross sedimentary accumulations (Bjorn, Gardar, Eirik, and Hatton Drifts). Prominent reflections were mapped and calibrated using a combination of boreholes and legacy seismic profiles. Interpreted seismic profiles were used to reconstruct solid sedimentation rates. Bjorn Drift began to accumulate in late Miocene times. Its average sedimentation rate decreased at approximate to 2.5 Ma and increased again at approximate to 0.75 Ma. In contrast, Eirik Drift started to accumulate in early Miocene times. Its average sedimentation rate increased at approximate to 5.5 Ma and decreased at approximate to 2.2 Ma. In both cases, there is a good correlation between sedimentation rates, inferred Northern Component Water overflow, and the variation of Icelandic plume temperature independently obtained from the geometry of diachronous V-shaped ridges. Between 5.5 and 2.5 Ma, the plume cooled, which probably caused subsidence of the Greenland-Iceland-Scotland Ridge, allowing drift accumulation to increase. When the plume became hotter at 2.5 Ma, drift accumulation rate fell. We infer that deep-water current strength is modulated by fluctuating dynamic support of the Greenland-Scotland Ridge. Our results highlight the potential link between mantle convective processes and ocean circulation.

2014
Parnell-Turner, R, Cann JR, Smith DK, Schouten H, Yoerger D, Palmiotto C, Zheleznov A, Bai HL.  2014.  Sedimentation rates test models of oceanic detachment faulting. Geophysical Research Letters. 41:7080-7088.   10.1002/2014gl061555   AbstractWebsite

Long-lived detachment faults play an important role in the construction of new oceanic crust at slow-spreading mid-oceanic ridges. Although the corrugated surfaces of exposed low-angle faults demonstrate past slip, it is difficult to determine whether a given fault is currently active. If inactive, it is unclear when slip ceased. This judgment is crucial for tectonic reconstructions where detachment faults are present, and for models of plate spreading. We quantify variation in sediment thickness over two corrugated surfaces near 16.5 degrees N at the Mid-Atlantic Ridge using near-bottom Compressed High Intensity Radar Pulse (CHIRP) data. We show that the distribution of sediment and tectonic features at one detachment fault is consistent with slip occurring today. In contrast, another corrugated surface 20km to the south shows a sediment distribution suggesting that slip ceased similar to 150,000years ago. Data presented here provide new evidence for active detachment faulting, and suggest along-axis variations in fault activity occur over tens of kilometers.

Parnell-Turner, R, White N, Henstock T, Murton B, Maclennan J, Jones SM.  2014.  A continuous 55-million-year record of transient mantle plume activity beneath Iceland. Nature Geoscience. 7:914-919.   10.1038/Ngeo2281   AbstractWebsite

In the North Atlantic Ocean, a mid-ocean ridge bisects the Icelandic mantle plume, and provides a window into its temporal evolution(1-3). V-shaped ridges of thick oceanic crust observed south of Iceland are thought to record pulses of upwelling within the plume(4-7). Specifically, excess crust is thought to form during the quasi-periodic generation of hot solitary waves triggered by thermal instabilities in the mantle(8). Here we use seismic reflection data to show that V-shaped ridges have formed over the past 55 million years-providing the longest record of plume periodicity of its kind. We find evidence for minor, but systematic, asymmetric formation of crust, due to migration of the mid-ocean ridge with respect to the underlying plume. We also find changes in periodicity: from 55 to 35 million years ago, the V-shaped ridges form every 3 million years or so and reflect small fluctuations in plume temperature of about 5-10 degrees C. From35 million years ago, the periodicity changes to about 8 million years and reflects changes in mantle temperature of 25-30 degrees C. We suggest that this change in periodicity is probably caused by perturbations in the thermal state at the plume source, either at the mantle-transition zone or core-mantle boundary.

Smith, DK, Schouten H, Dick HJB, Cann JR, Salters V, Marschall HR, Ji FW, Yoerger D, Sanfilippo A, Parnell-Turner R, Palmiotto C, Zheleznov A, Bai HL, Junkin W, Urann B, Dick S, Sulanowska M, Lemmond P, Curry S.  2014.  Development and evolution of detachment faulting along 50 km of the Mid-Atlantic Ridge near 16.5 degrees N. Geochemistry Geophysics Geosystems. 15:4692-4711.   10.1002/2014gc005563   AbstractWebsite

A multifaceted study of the slow spreading Mid-Atlantic Ridge (MAR) at 16.5 degrees N provides new insights into detachment faulting and its evolution through time. The survey included regional multibeam bathymetry mapping, high-resolution mapping using AUV Sentry, seafloor imaging using the TowCam system, and an extensive rock-dredging program. At different times, detachment faulting was active along approximate to 50 km of the western flank of the study area, and may have dominated spreading on that flank for the last 5 Ma. Detachment morphologies vary and include a classic corrugated massif, noncorrugated massifs, and back-tilted ridges marking detachment breakaways. High-resolution Sentry data reveal a new detachment morphology; a low-angle, irregular surface in the regional bathymetry is shown to be a finely corrugated detachment surface (corrugation wavelength of only tens of meters and relief of just a few meters). Multiscale corrugations are observed 2-3 km from the detachment breakaway suggesting that they formed in the brittle layer, perhaps by anastomosing faults. The thin wedge of hanging wall lavas that covers a low-angle (6 degrees) detachment footwall near its termination are intensely faulted and fissured; this deformation may be enhanced by the low angle of the emerging footwall. Active detachment faulting currently is limited to the western side of the rift valley. Nonetheless, detachment fault morphologies also are present over a large portion of the eastern flank on crust >2 Ma, indicating that within the last 5 Ma parts of the ridge axis have experienced periods of two-sided detachment faulting.

2013
Parnell-Turner, RE, White NJ, Maclennan J, Henstock TJ, Murton BJ, Jones SM.  2013.  Crustal manifestations of a hot transient pulse at 60 degrees N beneath the Mid-Atlantic Ridge. Earth and Planetary Science Letters. 363:109-120.   10.1016/j.epsl.2012.12.030   AbstractWebsite

Since its inception at 62 Ma, mantle convective upwelling beneath Iceland has had a significant influence on Cenozoic vertical motions, magmatism and paleoceanography in the North Atlantic Ocean. Crucially, intersection of the Reykjanes Ridge with the Icelandic Plume provides us with a useful window into the transient activity of this plume. Here, the spreading ridge acts as a linear sampler of plume activity, which is recorded as a series of time-transgressive V-shaped ridges and troughs. We present the results of a detailed study of the spreading ridge close to 60 degrees N, where the youngest V-shaped ridge of thickened oceanic crust is forming today. A combination of multibeam bathymetry and seismic reflection profiles, acquired along and across the ridge axis, is used to map the detailed pattern of volcanism and normal faulting. Along the ridge axis, the density of volcanic seamounts varies markedly, increasing by a factor of two between 59 degrees N and 62 degrees N. Within this zone, seismic imaging shows that there is enhanced acoustic scattering at the seabed. These observations are accompanied by a decrease in mean fault length from similar to 12 km to similar to 6 km. A 1960-2009 catalog of relocated teleseismic earthquake hypocenters indicates that there is a pronounced gap in seismicity between 59 degrees N and 62 degrees N where the cumulative moment release is two orders of magnitude smaller than that along adjacent ridge segments. A steady-state thermal model is used to show that a combination of increased melt generation and decreased hydrothermal circulation accounts for this suite of observations. The predicted decrease in the thickness of the brittle seismogenic layer is consistent with geochemical modeling of dredged basaltic samples, which require hotter asthenospheric material beneath the spreading axis. Thus, along-axis variation in melt supply caused by passage of a pulse of hot material modulates crustal accretion processes and rheological properties. (C) 2013 Elsevier B.V. All rights reserved.