Semibrittle seismic deformation in high-temperature mantle mylonite shear zone along the Romanche transform fault

©2018. The Authors. Transform faults are a fundamental tenet of plate tectonics, connecting offset extensional segments of mid-ocean ridges in ocean basins worldwide. The current consensus is that oceanic transform faults initiate after the onset of seafloor spreading. However, this inference has been difficult to test given the lack of direct observations of transform fault formation. Here we integrate evidence from surface faults, geodetic measurements, local seismicity, and numerical modeling of the subaerial Afar continental rift and show that a proto-transform fault is initiating during the final stages of continental breakup. This is the first direct observation of proto-transform fault initiation in a continental rift and sheds unprecedented light on their formation mechanisms. We demonstrate that they can initiate during late-stage continental rifting, earlier in the rifting cycle than previously thought. Future studies of volcanic rifted margins cannot assume that oceanic transform faults initiated after the onset of seafloor spreading.

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Serpentinized peridotite versus thick mafic crust at the Romanche oceanic transform fault

Geology ◽

10.1130/g49097.1 ◽

2021 ◽

Author(s):

Emma P.M. Gregory ◽

Satish C. Singh ◽

Milena Marjanović ◽

Zhikai Wang

Keyword(s):

Mantle Peridotite ◽

Transform Fault ◽

Equatorial Atlantic ◽

Low Velocity ◽

Transform Faults ◽

Velocity Anomaly ◽

The North ◽

Serpentinized Peridotite ◽

Equatorial Atlantic Ocean ◽

Alteration Processes

The crust beneath transform faults at slow-spreading ridges has been considered to be thin, comprising a thin mafic layer overlying serpentinized peridotite. Using wide-angle seismic data, we report the presence of a Moho at ~6 km depth and a low-velocity anomaly extending down to 9 km beneath the 20-km-wide Romanche transform valley floor in the equatorial Atlantic Ocean. The low crustal velocities above the Moho could be due to either highly serpentinized mantle peridotite or fractured mafic rocks. The existence of clear Moho reflections and the occurrence of a large crustal-depth rupture during the 2016 magnitude 7.1 earthquake suggest that the crust likely consists of fractured mafic material. Furthermore, the presence of low velocities below the Moho advocates for extensive serpentinization of the mantle, indicating that the Moho reflection is unlikely to be produced by a serpentinization front. The crust to the north of the transform fault likely consists of mafic material, but that in the south appears to be more amagmatic, possibly containing serpentinized peridotite. Our results imply that the transform fault structure is complex and highly heterogeneous, and thus would have significant influence on earthquake rupture and alteration processes.

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Initiation of a Proto-transform Fault Prior to Seafloor Spreading

10.26686/wgtn.13089095.v1 ◽

2020 ◽

Author(s):

Finnigan Illsley-Kemp ◽

JM Bull ◽

D Keir ◽

T Gerya ◽

C Pagli ◽

...

Keyword(s):

Plate Tectonics ◽

Seafloor Spreading ◽

Continental Rift ◽

Formation Mechanisms ◽

Transform Fault ◽

Transform Faults ◽

Local Seismicity ◽

Ocean Ridges ◽

Rifted Margins ◽

Direct Observations

©2018. The Authors. Transform faults are a fundamental tenet of plate tectonics, connecting offset extensional segments of mid-ocean ridges in ocean basins worldwide. The current consensus is that oceanic transform faults initiate after the onset of seafloor spreading. However, this inference has been difficult to test given the lack of direct observations of transform fault formation. Here we integrate evidence from surface faults, geodetic measurements, local seismicity, and numerical modeling of the subaerial Afar continental rift and show that a proto-transform fault is initiating during the final stages of continental breakup. This is the first direct observation of proto-transform fault initiation in a continental rift and sheds unprecedented light on their formation mechanisms. We demonstrate that they can initiate during late-stage continental rifting, earlier in the rifting cycle than previously thought. Future studies of volcanic rifted margins cannot assume that oceanic transform faults initiated after the onset of seafloor spreading.

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Mapping of a segment of the Romanche Fracture Zone: A morphostructural analysis of a major transform fault of the equatorial Atlantic Ocean

Geology ◽

10.1130/0091-7613(1991)019<0795:moasot>2.3.co;2 ◽

1991 ◽

Vol 19 (8) ◽

pp. 795 ◽

Cited By ~ 15

Author(s):

José Honnorez ◽

Jean Mascle ◽

Christophe Basile ◽

Pierre Tricart ◽

Michel Villeneuve ◽

...

Keyword(s):

Atlantic Ocean ◽

Fracture Zone ◽

Transform Fault ◽

Equatorial Atlantic ◽

Equatorial Atlantic Ocean ◽

Romanche Fracture Zone

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Microseismicity constrains on the lithospheric structure at the ridge-transform intersection at the Romanche Transform Fault and Mid-Atlantic Ridge

10.5194/egusphere-egu2020-6006 ◽

2020 ◽

Author(s):

Zhiteng Yu ◽

Satish C. Singh ◽

Emma Gregory ◽

Wayne Crawford ◽

Marcia Maia ◽

...

Keyword(s):

Thermal Structure ◽

P Wave ◽

Seismogenic Zone ◽

Depth Range ◽

Thermal Variation ◽

Transform Fault ◽

Pull Apart Basin ◽

Mid Atlantic Ridge ◽

Southern Border ◽

Slow Spreading

The Romanche Transform Fault (TF) in the equatorial Atlantic Ocean is the largest oceanic transform fault on Earth, offsetting the slow-spreading (2 cm/ yr) Mid-Atlantic Ridge (MAR) by 900-km and producing a maximum age contrast at the Ridge-Transform Intersection (RTI) of 45 Myr. This offset could cause a large thermal variation in the lithosphere around the RTI, but it is not known how this thermal variation would manifest itself. Here we present a ~21-day-long micro-earthquake study using a temporary deployment of 19 ocean-bottom seismometers (OBSs) during the 2019 SMARTIES cruise. 1363 earthquakes were detected on at least three OBSs and 622 could be located, of which 351 have high location accuracy (mean semi-major-axis of 3.9 km).Linear (HYPOSAT) and non-linear (NonLinLoc) location algorithms reveal a similar earthquake distribution. Two event groups cluster at depths of 1) 0 km to ~18 km and 2) ~20 km to 30 km. Along the Romanche TF, micro-earthquakes are located beneath the southern border of the 30 km wide transform valley; no events are observed beneath the central or northern sections of the valley. These events' depths increase rapidly and linearly from a few km at the RTI to 30 km at 40 km along the transform fault, indicating a rapid increase in the thickness of the seismogenic zone (and lithosphere) along the transform fault. The presence of earthquakes on the southern border of the transform fault, which is younger and hence warmer, suggests that these events, and hence the seismogenic zone, follow an isotherm separating the brittle-ductile boundary. The absence of seismicity beneath the centre and northern boundary of the transform fault could be due to a much colder lithosphere and hence deeper ductile-brittle boundary. &#160;An aseismic gap exists beneath the pull-apart basin observed on bathymetry data. Beneath the RTI, earthquakes mainly occur in&#160;the&#160;0-18 km depth range. Eight well-constrained focal mechanisms, derived from P-wave polarities, suggest that strike-slip faulting dominates along the transform fault. Normal faults are also observed, which may be attributed to an active detachment fault or pull-apart basin formation.From the RTI to the tip of the southern MAR segment, micro-earthquakes show an undulating focal depth distribution from north to south. They can be summarized into three clustering groups: the RTI, the 16.6&#176;W group, and the 16.2&#176;W group. Micro-earthquakes beneath the MAR are mainly located in the axial valley. Events in the 16.6&#176;W group mainly occur in the mantle at depths of 12-20 km, whereas those in the 16.2&#176;W group are located at shallow depths of 2-12 km, which is similar to that observed along other slow-spreading Mid-Ocean Ridges. This evidence indicates that there are significant variations in the along-axis thermal structure of the lithosphere along the rift axis.ZY acknowledges the China Postdoctoral Science Foundation (2019M652041, BX20180080); DB acknowledges funding PRIN2017KY5ZX8.

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Gravimetric structure for the abyssal mantle massif of Saint Peter and Saint Paul peridotite ridge, Equatorial Atlantic Ocean, and its relation to active uplift

Anais da Academia Brasileira de Ciências ◽

10.1590/0001-37652014117712 ◽

2014 ◽

Vol 86 (2) ◽

pp. 571-588 ◽

Cited By ~ 2

Author(s):

KENJI F. MOTOKI ◽

AKIHISA MOTOKI ◽

SUSANNA E. SICHEL

Keyword(s):

Atlantic Ocean ◽

Saint Paul ◽

Tectonic Uplift ◽

Plate Motion ◽

Ultramafic Rocks ◽

Transform Fault ◽

Equatorial Atlantic ◽

Data Set ◽

Equatorial Atlantic Ocean ◽

Saint Peter

This paper presents gravimetric and morphologic analyses based on the satellite-derived data set of EGM2008 and TOPEX for the area of the oceanic mantle massif of the Saint Peter and Saint Paul peridotite ridge, Equatorial Atlantic Ocean. The free-air anomaly indicates that the present plate boundary is not situated along the longitudinal graben which cuts peridotite ridge, but about 20 km to the north of it. The high Bouguer anomaly of the peridotite ridge suggests that it is constituted mainly by unserpentinised ultramafic rocks. The absence of isostatic compensation and low-degree serpentinisation of the ultramafic rocks indicate that the peridotite ridge is sustained mainly by active tectonic uplift. The unparallel relation between the transform fault and the relative plate motion generates near north-south compression and the consequent tectonic uplift. In this sense, the peridotite massif is a pressure ridge due to the strike-slip displacement of the Saint Paul Transform Fault.

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All at Sea

10.1093/oso/9780198717867.003.0009 ◽

2018 ◽

Author(s):

Roy Livermore

Keyword(s):

Plate Tectonics ◽

First Generation ◽

Basaltic Magma ◽

Ocean Floor ◽

Tennis Ball ◽

Magma Chambers ◽

Transform Faults ◽

Ocean Ridges ◽

Sea Floor Spreading ◽

Ocean Ridge

According to first-generation plate tectonics, sea-floor spreading was nice and simple. Plates were pulled apart at mid-ocean ridges, and weak mantle rocks rose to fill the gap and began to melt. The resulting basaltic magma ascended into the crust, where it ponded to form linear ‘infinite onion’ magma chambers beneath the mid-ocean tennis-ball seam. At frequent intervals, vertical sheets of magma rose from these chambers to the surface, where they erupted to form new ocean floor or solidified to form dykes, in the process acquiring a magnetization corresponding to the geomagnetic field at the time. Mid-ocean ridge axes were defined by rifted valleys and divided into segments by transform faults with offsets of tens to hundreds of kilometres, resulting in the staircase pattern seen on maps of the ocean floor. All mid-ocean ridges were thus essentially identical. Such a neat and elegant theory was bound to be undermined as new data were acquired in the oceans.

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Geothermal asymmetry in transform faults of the equatorial Atlantic Ocean

Doklady Earth Sciences ◽

10.1134/s1028334x17070212 ◽

2017 ◽

Vol 475 (1) ◽

pp. 836-839 ◽

Cited By ~ 2

Author(s):

M. D. Khutorskoi ◽

E. A. Teveleva ◽

L. V. Podgornykh

Keyword(s):

Atlantic Ocean ◽

Equatorial Atlantic ◽

Transform Faults ◽

Equatorial Atlantic Ocean

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Uppermost Mantle Velocity beneath the Mid-Atlantic Ridge and Transform Faults in the Equatorial Atlantic Ocean

Bulletin of the Seismological Society of America ◽

10.1785/0120200248 ◽

2020 ◽

Author(s):

Guilherme W. S. de Melo ◽

Ross Parnell-Turner ◽

Robert P. Dziak ◽

Deborah K. Smith ◽

Marcia Maia ◽

...

Keyword(s):

Atlantic Ocean ◽

Upper Mantle ◽

Equatorial Atlantic ◽

Uppermost Mantle ◽

Transform Faults ◽

Equatorial Atlantic Ocean ◽

Mid Atlantic Ridge ◽

Mantle Velocity ◽

Ray Paths

ABSTRACT Seismic rays traveling just below the Moho provide insights into the thermal and compositional properties of the upper mantle and can be detected as Pn phases from regional earthquakes. Such phases are routinely identified in the continents, but in the oceans, detection of Pn phases is limited by a lack of long-term instrument deployments. We present estimates of upper-mantle velocity in the equatorial Atlantic Ocean from Pn arrivals beneath, and flanking, the Mid-Atlantic Ridge and across several transform faults. We analyzed waveforms from 50 earthquakes with magnitude Mw>3.5, recorded over 12 months in 2012–2013 by five autonomous hydrophones and a broadband seismograph located on the St. Peter and St. Paul archipelago. The resulting catalog of 152 ray paths allows us to resolve spatial variations in upper-mantle velocities, which are consistent with estimates from nearby wide-angle seismic experiments. We find relatively high velocities near the St. Paul transform system (∼8.4 km s−1), compared with lower ridge-parallel velocities (∼7.7 km s−1). Hence, this method is able to resolve ridge-transform scale velocity variations. Ray paths in the lithosphere younger than 10 Ma have mean velocities of 7.9±0.5 km s−1, which is slightly lower than those sampled in the lithosphere older than 20 Ma (8.1 km±0.3 s−1). There is no apparent systematic relationship between velocity and ray azimuth, which could be due to a thickened lithosphere or complex mantle upwelling, although uncertainties in our velocity estimates may obscure such patterns. We also do not find any correlation between Pn velocity and shear-wave speeds from the global SL2013sv model at depths <150 km. Our results demonstrate that data from long-term deployments of autonomous hydrophones can be used to obtain rare and insightful estimates of uppermost mantle velocities over hundreds of kilometers in otherwise inaccessible parts of the deep oceans.

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Structural Evidences for Present-day Compressive Tectonics at the St. Peter and St. Paul Archipelago (Equatorial Atlantic Ocean)

10.20944/preprints202105.0645.v1 ◽

2021 ◽

Author(s):

Thomas Campos ◽

Kenji Motoki ◽

Susanna Sichel ◽

Leonardo Barão ◽

Marcia Maia ◽

...

Keyword(s):

Atlantic Ocean ◽

Average Rate ◽

High Angle ◽

Transform Fault ◽

Equatorial Atlantic ◽

Joint System ◽

Conjugate System ◽

The North ◽

Equatorial Atlantic Ocean ◽

Load Release

This paper discusses the tectonics of the St. Peter and St. Paul Archipelago (SPSPA) in the Equato-rial Atlantic Ocean, based on the joint-system geometry which show a North-South shorten-ing/transpressional uplift tectonism, is active leading to exhumation of the sub-oceanic mantle. These islets are the summits of a sigmoidal submarine ridge formed by mantle ultramafic rocks. The ridge is crossed by the principal transform deformation zone of the northern transform fault of the St. Paul Multifault System. The South flank ridge exposes serpentinized mantle perido-tites, while the North flank exposes strongly deformed/fractured ultramylonites, recording duc-tile and brittle deformation at lithospheric conditions. The SPSPA show multiple joint systems cutting mylonitic foliation of the exposed rocks, forming three main families: high-angle paral-lel joints of tectonic origin, serpentinization-related joints with random direction and load-release low-angle parallel joints. The tectonic joints show an average direction of N31°E and N28°W, forming a conjugate system with a N1ºW compression axes, coherent with a trans-pressive stress field. Accordingly, the earthquakes focal mechanism close to the islets also shows N-S compression. The previously reported active uplift with an average rate of 1.5 mm/year and the directions of the joint system here reported agreeing with a present-day active N-S compres-sive field at a high angle with the direction of the transform fault.

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