Spreading rate dependence of gravity anomalies along oceanic transform faults

Magmatism controls global oceanic transform fault topography

Oceanic transform faults play an essential role in plate tectonics. Yet to date, there is no unifying explanation for the global trend in broad-scale transform fault topography, ranging from deep valleys to shallow topographic highs. Using three-dimensional numerical models, we find that spreading-rate dependent magmatism within the transform domain exerts a first-order control on the observed spectrum of transform fault depths. Low-rate magmatism results in deep transform valleys caused by transform-parallel tectonic stretching; intermediate-rate magmatism fully accommodates far-field stretching, but strike-slip motion induces across-transform tension, producing transform strength dependent shallow valleys; high-rate magmatism produces elevated transform zones due to local compression. Our models also address the observation that fracture zones are consistently shallower than their adjacent transform fault zones. These results suggest that plate motion change is not a necessary condition for reproducing oceanic transform topography and that oceanic transform faults are not simple conservative strike-slip plate boundaries.

Mechanism of progressive broad deformation from oceanic transform valley to off-transform faulting and rifting

Oceanic transform faults (TFs) are commonly viewed as single, narrow strike-slip seismic faults that offset two mid-ocean ridge segments. However, broad zones of complex deformation are ubiquitous at TFs. Here, we propose a new conceptual model for the progressive deformation within broad zones at oceanic TFs through detailed morphological, seismic, and stress analyses. We argue that, under across-transform extension due to a change in plate motion, plate deformation occurs first along high-angle transtensional faults (TTFs) within the transform valleys. Off-transform normal faults (ONFs) form when across-transform deviatoric extensional stresses exceed the yield strength of the adjacent oceanic lithosphere. With further extension, these normal faults can develop into off-transform rift zones (ORZs), some of which can further develop into transform plate boundaries. We illustrate that such progressive complex deformation is an inherent feature of oceanic TFs. The new conceptual model provides a unifying theory to explain the observed broad deformation at global transform systems.

•

Systematic progression of complex deformation near transform faults is revealed

•

Progressive complex broad deformation is an inherent feature of oceanic transform faults

•

TTFs on transform wall and off-transform rifting formed in response to plate rotation

•

Off-transform rift zones can develop into new transform plate boundaries

Extensional tectonics and two-stage crustal accretion at oceanic transform faults

Oceanic transform faults are seismically and tectonically active plate boundaries¹ that leave scars-known as fracture zones-on oceanic plates that can cross entire ocean basins². Current descriptions of plate tectonics assume transform faults to be conservative two-dimensional strike-slip boundaries^1,3, at which lithosphere is neither created nor destroyed and along which the lithosphere cools and deepens as a function of the age of the plate⁴. However, a recent compilation of high-resolution multibeam bathymetric data from 41 oceanic transform faults and their associated fracture zones that covers all possible spreading rates shows that this assumption is incorrect. Here we show that the seafloor along transform faults is systemically deeper (by up to 1.6 kilometres) than their associated fracture zones, in contrast to expectations based on plate-cooling arguments. Accretion at intersections between oceanic ridges and transform faults seems to be strongly asymmetric: the outside corners of the intersections show shallower relief and more extensive magmatism, whereas the inside corners have deep nodal basins and seem to be magmatically starved. Three-dimensional viscoplastic numerical models show that plastic-shear failure within the deformation zone around the transform fault results in the plate boundary experiencing increasingly oblique shear at increasing depths below the seafloor. This results in extension around the inside corner, which thins the crust and lithosphere at the transform fault and is linked to deepening of the seafloor along the transform fault. Bathymetric data suggest that the thinned transform-fault crust is augmented by a second stage of magmatism as the transform fault intersects the opposing ridge axis. This makes accretion at transform-fault systems a two-stage process, fundamentally different from accretion elsewhere along mid-ocean ridges.

Seismic Crustal Structure and Morphotectonic Features Associated With the Chain Fracture Zone and Their Role in the Evolution of the Equatorial Atlantic Region

Oceanic transform faults and fracture zones (FZs) represent major bathymetric features that keep the records of past and present strike-slip motion along conservative plate boundaries. Although they play an important role in ridge segmentation and evolution of the lithosphere, their structural characteristics, and their variation in space and time, are poorly understood. To address some of the unknowns, we conducted interdisciplinary geophysical studies in the equatorial Atlantic Ocean, the region where some of the most prominent transform discontinuities have been developing. Here we present the results of the data analysis in the vicinity of the Chain FZ, on the South American Plate. The crustal structure across the Chain FZ, at the contact between ∼10 and 24 Ma oceanic lithosphere, is sampled along seismic reflection and refraction profiles. We observe that the crustal thickness within and across the Chain FZ ranges from ∼4.6-5.9 km, which compares with the observations reported for slow-slipping transform discontinuities globally. We attribute this presence of close to normal oceanic crustal thickness within FZs to the mechanism of lateral dike propagation, previously considered to be valid only in fast-slipping environments. Furthermore, the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale. Our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower-order Mid-Atlantic Ridge segmentation around the equator.

Marine Geophysical Investigation of the Chain Fracture Zone in the Equatorial Atlantic From the PI-LAB Experiment

The Chain Fracture Zone is a 300-km-long transform fault that offsets the Mid-Atlantic Ridge. We analyzed new multibeam bathymetry, backscatter, gravity, and magnetic data with 100% multibeam bathymetric data over the active transform valley and adjacent spreading segments as part of the Passive Imaging of the Lithosphere Asthenosphere Boundary (PI-LAB) Experiment. Analyses of these data sets allow us to determine the history and mode of crustal formation and the tectonic evolution of the transform system and adjacent ridges over the past 20 Myr. We model the total field magnetic anomaly to determine the age of the crust along the northern ridge segment to better establish the timing of the variations in the seafloor fabric and the tectonic-magmatic history of the region. Within the active transform fault zone, we observe four distinct positive flower structures with several en échelon fault scarps visible in the backscatter data. We find up to -10 mGal residual Mantle Bouguer Anomaly in the region of the largest positive flower structure within the transform zone suggesting crustal thickening relative to the crustal thinning typically observed in fracture zones in the Atlantic. The extensional/compressional features observed in the Chain Transform are less pronounced than those observed further north in the Vema, St. Paul, and Romanche and may be due to local ridge segment adjustments.

The Role of Oceanic Transform Faults in Seafloor Spreading: A Global Perspective From Seismic Anisotropy

Mantle anisotropy beneath mid-ocean ridges and oceanic transforms is key to our understanding of seafloor spreading and underlying dynamics of divergent plate boundaries. Observations are sparse, however, given the remoteness of the oceans and the difficulties of seismic instrumentation. To overcome this, we utilize the global distribution of seismicity along transform faults to measure shear wave splitting of over 550 direct S phases recorded at 56 carefully selected seismic stations worldwide. Applying this source-side splitting technique allows for characterization of the upper mantle seismic anisotropy, and therefore the pattern of mantle flow, directly beneath seismically active transform faults. The majority of the results (60%) return nulls (no splitting), while the non-null measurements display clear azimuthal dependency. This is best simply explained by anisotropy with a near vertical symmetry axis, consistent with mantle upwelling beneath oceanic transforms as suggested by numerical models. It appears therefore that the long-term stability of seafloor spreading may be associated with widespread mantle upwelling beneath the transforms creating warm and weak faults that localize strain to the plate boundary.

Seismic imaging of mantle wedge corner flow and arc magmatism

I reviewed studies on the inhomogeneous seismic structure of the mantle wedge in subduction zones, in relation to corner flow and its implications for arc magmatism. Seismic studies in Tohoku clearly imaged the descending flow portion of the corner flow as a thin seismic low-velocity layer right above the slab. Slab-derived H₂O is fixed to the layer as hydrous minerals, which are brought down by the slab and eventually decompose. The released H₂O rises and encounters the ascending flow, formed to fill the gap caused by the descending flow. The combination of H₂O addition and adiabatic decompression causes partial melting within the ascending flow. For many subduction zones, seismic tomography has distinctly imaged the ascending flow of the corner flow as a seismic low-velocity and/or high-attenuation layer in the mantle wedge inclined nearly parallel to the slab. These observations indicate that the volcanic front in subduction zones is formed both by the ascending flow and the addition of slab-derived H₂O.

Reaction-induced rheological weakening enables oceanic plate subduction

Earth is the only terrestrial planet in our solar system where an oceanic plate subducts beneath an overriding plate. Although the initiation of plate subduction requires extremely weak boundaries between strong plates, the way in which oceanic mantle rheologically weakens remains unknown. Here we show that shear-enhanced hydration reactions contribute to the generation and maintenance of weak mantle shear zones at mid-lithospheric depths. High-pressure friction experiments on peridotite gouge reveal that in the presence of hydrothermal water, increasing strain and reactions lead to an order-of-magnitude reduction in strength. The rate of deformation is controlled by pressure-solution-accommodated frictional sliding on weak hydrous phyllosilicate (talc), providing a mechanism for the 'cutoff' of the high peak strength at the brittle-plastic transition. Our findings suggest that infiltration of seawater into transform faults with long lengths and low slip rates is an important controlling factor on the initiation of plate tectonics on terrestrial planets.

Fractal density and singularity analysis of heat flow over ocean ridges

Peak heat flow occurs at mid-ocean ridges and decreases with the age of the oceanic lithosphere. Several plate models, including the Parsons and Sclater (PSM) model, Global Depth and Heat (GDH1) model and Constant Heat flow Applied on the Bottom Lithospheric Isotherm (CHABLIS) model, have been used to predict heat flow in the ocean lithosphere. The discrepancy between the predicted and measured heat flow in the younger lithosphere (i.e. younger than 55 Myr) influenced by local hydrothermal circulation has been used to estimate hydrothermal heat flux and investigate hydrothermal processes. We can modify the cooling models by substituting the ordinary mass density of lithosphere by fractal density with singularity. This new model provides a modified solution to fit the observed heat flow data used in other models in the literature throughout the age range. This model significantly improves the results for prediction of heat flow that were obtained using the PSM, GDH1 and CHABLIS models. Furthermore, the heat flow model does not exhibit special characteristics around any particular age of lithosphere. This raises a fundamental question about the existence of a “sealing” age and accordingly the hydrothermal flux estimation based on the cooling models.

Dynamical instability produces transform faults at mid-ocean ridges

Transform faults at mid-ocean ridges--one of the most striking, yet enigmatic features of terrestrial plate tectonics--are considered to be the inherited product of preexisting fault structures. Ridge offsets along these faults therefore should remain constant with time. Here, numerical models suggest that transform faults are actively developing and result from dynamical instability of constructive plate boundaries, irrespective of previous structure. Boundary instability from asymmetric plate growth can spontaneously start in alternate directions along successive ridge sections; the resultant curved ridges become transform faults within a few million years. Fracture-related rheological weakening stabilizes ridge-parallel detachment faults. Offsets along the transform faults change continuously with time by asymmetric plate growth and discontinuously by ridge jumps.