Supplemental Material: The ubiquitous creeping segments on oceanic transform faults

Major transform fault zones link extensional segments of the North American – Eurasian plate boundary as it transects the Iceland Hotspot. Changes in plate boundary geometry, involving ridge jumps, rift propagation, and related transform fault zone migration, have occurred as the boundary has moved relative to the hotspot. Reconfiguration of transform fault zones occurred at about 6 Ma in northern Iceland and began about 3 Ma in southern Iceland. These systems show a range of different types of transform fault zones, ranging from diffuse, oblique rift zones to narrower, well-defined, transform faults oriented parallel to current plate motions. Crustal deformation structures correlate with the inferred duration and magnitude of strike-slip displacements. Collectively, the different expressions of transform zones may represent different stages of development in an evolutionary sequence that may be relevant for understanding the tectonic history of plate boundaries in Iceland as well as the structure of transform fault zones on more typical parts of the mid-ocean ridge system.

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A new approach to the opening of the eastern Mediterranean Sea and the origin of the Hellenic Subduction Zone. Part 1: The eastern Mediterranean Sea

Canadian Journal of Earth Sciences ◽

10.1139/cjes-2018-0128 ◽

2019 ◽

Vol 56 (11) ◽

pp. 1119-1143 ◽

Cited By ~ 2

Author(s):

Xavier Le Pichon ◽

A.M. Celâl Şengör ◽

Caner İmren

Keyword(s):

Mediterranean Sea ◽

Eastern Mediterranean ◽

Transform Fault ◽

Northern Boundary ◽

Transform Faults ◽

Southern Boundary ◽

New Approach ◽

Eastern Mediterranean Sea ◽

Lateral Shearing ◽

Central Atlantic

We identify long transform faults that frame the eastern Mediterranean Sea and that were active during Jurassic and probably the Early Cretaceous, during the opening of the central Atlantic Ocean. We show that the African margin of the eastern Mediterranean Sea is an 1800 km long transform fault that absorbed the Africa/Eurasia Jurassic left-lateral motion during the opening of the central Atlantic. We call this transform fault the Eastern Mediterranean South Transform fault (EMST). We identify two other transform faults that were active simultaneously and framed the eastern Mediterranean Sea during its formation. These are the Apulia Transform fault (AT) and the Eastern Mediterranean North Transform fault (EMNT). The AT, three hundred km north of the EMST, followed the southern boundary of the Apulia block. Still 300 km farther north, the EMNT formed the northern boundary of this eastern Mediterranean shear zone. This last fault has been destroyed over a large portion by the Hellenic subduction. We relate these transform faults to the kinematics of the Jurassic Africa/Eurasia motion. We conclude that the eastern Mediterranean Sea is a long pull-apart created by left-lateral shearing of the Adria block as it was structurally linked to Africa.

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Petrology of a transform fault zone and adjacent ridge segments

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1971.0005 ◽

1971 ◽

Vol 268 (1192) ◽

pp. 423-441 ◽

Cited By ~ 122

Keyword(s):

Oceanic Crust ◽

Fault Zone ◽

Fracture Zone ◽

Volcanic Eruptions ◽

Spreading Ridge ◽

Transform Fault ◽

Transform Faults ◽

Near Surface ◽

The North ◽

Transform Fault Zone

The Verna Fracture Zone in the North Atlantic (9 to 11° N), which has been identified as a transform fault zone, contains exposures of serpentinized peridotites, while its adjacent ridge segments are floored mainly by typical abyssal ocean ridge basalts. This petrologic contrast correlates with the greater frequency of volcanic eruptions along the actively spreading ridge segments compared to the transform fault zone. Where rifting components occur across transform faults, exposures of the deeper zone of oceanic crust may result. The bathymetry of the Verna Fracture Zone suggests that some uplift parallel to the fracture zone as well as rifting led to exposures of deeper rocks. The basalts from the adjacent ridge axes contain ‘xenocrysts’ of plagioclase and olivine and more rarely of chromite. These appear to have a cognate origin, perhaps related to cooling and convection in near surface magma chambers. The basalts from the ridge axes, offset and on opposite sides of the transform fault, have similar features and compositions. The plagioclase peridotites have mineralogical features which indicate equilibration in the plagioclase pyrolite facies, suggesting maximum equilibration depths of around 30 km for a temperature of around 1200 °C. The chemical characteristics of the Vema F.Z. peridotites suggest that they may be undifferentiated mantle, emplaced as a subsolidus hot plastic intrusion or as a crystal mush. The abundance of peridotites and serpentinized peridotites is believed to reflect their abundance in seismic layer three of the oceanic crust.

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RELIEF-FORMING FACTORS OF ATLANTIC OCEAN TRANSFORM FAULTS

Bulletin of Kamchatka Regional Association «Educational-Scientific Center» Earth Sciences ◽

10.31431/1816-5524-2022-3-51-41-57 ◽

2021 ◽

pp. 41-57

Author(s):

V.A. Bogolyubsky ◽

◽

E.P. Dubinin ◽

S.Yu. Sokolov ◽

◽

...

Keyword(s):

Atlantic Ocean ◽

Deep Structure ◽

Morphometric Parameters ◽

Active Part ◽

Transform Fault ◽

Transform Faults ◽

Bathymetric Data ◽

Analogue Modeling ◽

Mid Atlantic Ridge ◽

Axis Offset

Transform faults are widespread within the Atlantic Ocean. Their relief is determined by a variety of factors related mainly to the peculiarities of the deep structure of the lithosphere and regional geodynamics. The degree of their influence changes when passing from one morphotectonic province of the Atlantic Ocean to another. The differences are manifested in the morphology of the main elements of the transform fault and the correlation of their morphometric parameters with the length of the active part, which was shown earlier by analogue modeling. The dependence between the depth of the transform valley and the axis offset of the Mid-Atlantic Ridge along the transform fault has been revealed. Variations in the values of morphometric parameters are interpreted as a consequence of different duration of fault development, as well as different degrees of influence of secondary factors within each of the provinces. Based on the analysis of the bathymetric data on the Atlantic transform faults, five main groups of relief-forming factors are identified, and the relative degree of importance of the factors is determined for each of them. It is assumed that the identified dependences are preserved for the transform faults in other oceans.

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Initiation of a Proto-transform Fault Prior to Seafloor Spreading

10.26686/wgtn.13089095 ◽

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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The Southwell Topple – reassessment of a very large coastal toppling failure on the Isle of Portland, UK

Quarterly Journal of Engineering Geology and Hydrogeology ◽

10.1144/qjegh2020-146 ◽

2021 ◽

pp. qjegh2020-146

Author(s):

Alan P. Dykes ◽

Edward N. Bromhead

Keyword(s):

South Coast ◽

Mass Movements ◽

Transform Fault ◽

The South ◽

Transform Faults ◽

Bathymetric Data ◽

Central Depression ◽

Toppling Failure

The Southwell Topple is a spectacular example of a toppling failure on the southeastern coastline of the Isle of Portland, on the south coast of England. Types of mass movements, which occur around almost the entire coastline of Portland and include some other much smaller but well-known topples, vary depending on local geological and topographic contexts. The ‘Southwell Landslide’ of 1734 (i.e. the Southwell Topple), differs in most respects from all the others, not least in its size. We examine the historical and geological contexts of the Southwell Topple in order to explain its origins and characteristics. The recently published bathymetric data from the DORIS project reveals the tectonic context for the landslide, particularly the frequent transform faults parallel to the southeastern coastline of Portland and the axis of the Shambles Syncline forming Portland's ‘central depression’. It appears that the Southwell Topple resulted from coast-parallel tectonic discontinuities – probably a single joint and/or transform fault – through the Portland Stone combined with preferential marine erosion of the underlying weaker Portland Sand.

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A discussion on the structure and evolution of the Red Sea and the nature of the Red Sea, Gulf of Aden and Ethiopia rift junction - The existence of transform faults in the Red Sea depression

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1970.0047 ◽

1970 ◽

Vol 267 (1181) ◽

pp. 407-408 ◽

Cited By ~ 3

Keyword(s):

Red Sea ◽

Gulf Of Aden ◽

Transform Fault ◽

Basement Complex ◽

Transform Faults ◽

Left Handed ◽

The World ◽

History Of ◽

First Motion ◽

Earthquake Mechanism

Girdler (1968, pp. 1102-1105) has suggested that transform faults may exist in the Red Sea depression. A possible left-handed transform fault, trending at 050°, and centred on 19°N 39°E was plotted (Girdler 1968, fig. 1). This is supposed to offset the axial trough, and run on to the land, where it is shown displacing the ‘marginal structure lines’ and the Mesozoic and Tertiary sedimentary/Basement Complex contact by about 50 km. Girdler (1968) cites Sykes (in History of the Earth's crust , Nasa, Symposium) as providing supporting first motion evidence of transform movement along this proposed fault. However Isaacs, Sykes & Oliver (1968, fig. 5) show a left-handed transform ‘earthquake mechanism’ bearing 020° for a centre associated with the axial trough at 17°N, 40° 30' E. Because of the proximity of these centres, the 30° difference in trend, and the scarcity of Red Sea earthquake data, the question arises are Girdler and Isaacs et al . dealing with the same centres ? More data and an explanation are required as there are few places in the world where undisputed transform faults have been described from the land and sea. Girdler’s proposed transform fault is therefore extremely important both from a local Red Sea and a global structural view.

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Imposed strain localization in the mantle section of an oceanic transform zone revealed by microstructural and stress variations

10.5194/egusphere-egu2020-11897 ◽

2020 ◽

Author(s):

Vasileios Chatzaras ◽

Basil Tikoff ◽

Seth C. Kruckenberg ◽

Sarah J. Titus ◽

Christian Teyssier ◽

...

Keyword(s):

Shear Zone ◽

Upper Mantle ◽

Temporal Variations ◽

Coarse Grained ◽

Transform Fault ◽

Differential Stress ◽

Transform Faults ◽

Fine Grained ◽

Micro Deformation ◽

Mantle Section

Mantle earthquakes that occur deeper than the 600 &#176;C isotherm in oceanic transform faults indicate seismic rupturing at conditions where viscous deformation (bulk ductile behavior) is dominant.&#160; However, direct geological evidence of earthquake-related deformation at ambient upper mantle conditions is rare, impeding our understanding of earthquake dynamics in plate-boundary fault systems.&#160; The Bogota Peninsula Shear Zone (BPSZ), New Caledonia, is an ancient oceanic transform fault exhumed from upper mantle depths.&#160; Ductile structures in the BPSZ formed at temperatures > 800 &#176;C and microstructures indicate that differential stress varies spatially and temporally.&#160; Spatial variation is observed as an increase in differential stress with strain toward localized zones of high strain; stress increases from 6&#8211;14 MPa in coarse grained tectonites to 11&#8211;22 MPa within 1&#8211;2 km wide mylonite zones.&#160; Temporal stress variation is observed by the formation of micro-deformation zones that seem to have brittle precursors, are filled with fine-grained recrystallized olivine grains and crosscut the background fabrics in the harzburgites that host them.&#160; The micro-deformation zones are not restricted to the mylonite zones, but rather are located throughout the BPSZ, having affected the protomylonites and the coarse grained tectonites. &#160;The micro-deformation zones record stresses of 22&#8211;81 MPa that are 2&#8211;6 times higher than the background, steady-state stresses in the surrounding mantle rocks.&#160; We interpret the observed spatial and temporal variations in microstructures and stresses in the upper mantle to demonstrate the influence of seismic events in the upper part of the oceanic transform fault system.&#160; We attribute the increase in stress with strain to be the result of imposed localization induced by downward propagation of the seismic rupture into the underlying mantle.&#160; The micro-deformation zones could result from brittle fractures caused by earthquake-related deformation in the mantle section of the transform fault, which are in turn overprinted by ductile deformation.&#160;Synthesizing the spatial and temporal variations in stresses and microstructures in the Bogota Peninsula Shear Zone we propose a conceptual model where brittle fracturing and shearing take place during coseismic rupture at increased stress, ductile flow at decaying stress is concentrated in the micro-deformation zones during postseismic relaxation, and uniformly distributed creep at low stress occurs in the host-rocks of the micro-deformation zones during interseismic deformation.&#160; The critical result from the studied paleotransform zone is that the fine-grained micro-deformation zones and the mylonites do not represent weak zones. &#160;Instead, they form by dislocation creep at transient high-stress deformation during the seismic cycle. &#160;The spatial distribution of the micro-deformation zones also suggests that repeated stress cycles in oceanic transform faults may not localize strain in pre-existing shear zones but disperse strain across the structure.

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

Science Advances ◽

10.1126/sciadv.abf3388 ◽

2021 ◽

Vol 7 (15) ◽

pp. eabf3388

Author(s):

Zhiteng Yu ◽

Satish C. Singh ◽

Emma P. M. Gregory ◽

Marcia Maia ◽

Zhikai Wang ◽

...

Keyword(s):

High Temperature ◽

Plate Tectonics ◽

Thermal Structure ◽

Depth Range ◽

Transform Fault ◽

Equatorial Atlantic ◽

Transform Faults ◽

Ocean Ridges ◽

Equatorial Atlantic Ocean ◽

Large Earthquakes

Oceanic transform faults, a key element of plate tectonics, represent the first-order discontinuities along mid-ocean ridges, host large earthquakes, and induce extreme thermal gradients in lithosphere. However, the thermal structure along transform faults and its effects on earthquake generation are poorly understood. Here we report the presence of a 10- to 15-kilometer-thick in-depth band of microseismicity in 10 to 34 kilometer depth range associated with a high-temperature (700° to 900°C) mantle below the brittle lithosphere along the Romanche mega transform fault in the equatorial Atlantic Ocean. The occurrence of the shallow 2016 moment magnitude 7.1 supershear rupture earthquake and these deep microearthquakes indicate that although large earthquakes occur in the upper brittle lithosphere, a substantial amount of deformation is accommodated in the semibrittle mylonitic mantle that resides at depths below the 600°C isotherm. We also observe a rapid westward deepening of this band of seismicity indicating a strong lateral heterogeneity.

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