Crustal deformation can occur via stick‐slip events, viscous creep, or strain transients at variable rates. Here we explore such strain transients with physical experiments comprising a ...quasi‐two‐dimensional shear zone with elastic, acrylic discs and interstitial viscous silicone. Experiments of solely elastic discs produce stick‐slip events and an overall (constant volume) strengthening. The addition of the viscous silicone enhances localization but does not greatly change the overall pattern of strengthening. It does, however, damp the stick‐slip events, leading to transient, creep‐like behavior that approaches the behavior of a Maxwell body. There is no gradual transition from frictional to viscous deformation with increasing amounts of silicone, suggesting that the mixed rheology is in effect as soon as an interstitial fluid is present. Our experiments support the hypothesis that a possible cause for strain transients in nature is an interstitial viscous phase in shear zones.
Key Points
Frictional shear zones with a weak viscous phase can cause strain transients
Deformation localization is strong in a lubricated granular media
The viscosity of the weak phase governs the damping of the stick‐slip behavior
Numerical experiments have been used to relate the range in the distribution and the style of deformation observed in rifted margins to localizing/delocalizing thermomechanical processes. The ...experiments give rise to four end‐members of margins for varying initial lithospheric strength and extension rates. The first two end‐members are narrow and asymmetric and narrow and near‐symmetric, conjugate margins. The third end‐member is asymmetric conjugate margins, wherein one side is <100 km wide and the other is >100–300 km wide. Lastly, we explore wide rift systems that may form very asymmetric conjugate margins with one narrow margin and a very wide conjugate, 200 km to > 350 km across. With initial and boundary conditions close to that inferred from the North and South Atlantic margins, we find that not all margins experience a polyphase rifting history of stretching‐thinning‐exhumation. Instead, the stretching mode can be very short or protracted, and the thinning or the exhumation modes can be incomplete or absent. The deformation localization of the thinning mode is in places associated with the formation of a keystone block or “block H.” A new mechanism for the formation of the unstable crustal root under block H is described, wherein the bounding border faults lead to differential thinning of the crust and mantle lithosphere. Nonuniform extension also occurs in both types of wide rift systems and is related to the sequential deformation migration outward of an initial graben, associated with effective lithospheric strengthening that occurs during crustal thinning and bending.
Key Points
Lithospheric strength interplay with dynamic processes controls strain evolution
Lithospheric hardening causes lateral deformation propagation and rift widening
The transition from distributed to localized deformation is not always clear
Roughly a third of the global mid‐ocean ridge system spreads at <20 mm/yr (full rate) with predicted low crustal thicknesses, great axial depths, end‐member basalt compositions, and prominent axial ...faults. These predictions are here further investigated along the ultraslow (15–17 mm/yr) Mid‐Cayman Spreading Center (MCSC) through a compilation of both previously published and unpublished data. The MCSC sits along the Caribbean‐North American plate boundary and is one of the world's deepest (>6 km) spreading centers, and thought to accrete some of the thinnest (∼3 km) crust. The MCSC generates end‐member mid‐ocean ridge basalt compositions and hosts recently discovered hydrothermal vents. Multibeam bathymetric data reveal that axial depth varies along the MCSC with intraridge rift walls defined by kilometer‐scale escarpments and massifs. Dredging and near‐bottom work has imaged and sampled predominantly basaltic lavas from the greatest axial depths and ∼15% peridotite surrounded by gabbroic rocks from the prominent massifs. The gabbroic rocks exhibit wide compositional variation (troctolites to ferrogabbros) and in many places contain high‐temperature (amphibolite to granulite facies) shear zones. Gabbroic compositions primarily reflect the accumulation of near‐liquidus phases that crystallized from a range of basaltic melts, as well as from interactions with interstitial melts in a subaxial mush zone. Magnetization variations inverted from aeromagnetic data are consistent with a discontinuous distribution of basaltic lavas and structurally asymmetric spreading. These observations support an oceanic core complex model for MCSC seafloor spreading, potentially making it a type example of ultraslow seafloor spreading through mush zone and detachment fault crustal processes.
Early forearc basin sedimentation and subsidence appear to be linked with convergent margin evolution, yet they do not follow predictable spatiotemporal patterns. In the Kumano basin of southwest ...Japan, 3‐D seismic mapping of major unconformities, combined with biostratigraphic age constraints from two Integrated Ocean Drilling Program (IODP) drill sites illustrates the development of several tectonostratigraphic packages during early evolution and initial creation of accommodation space in the forearc basin. Between ∼3.8 Ma and ∼2.06 Ma, a series of slope basins developed between thrust‐anticlines oriented along a plate convergence‐perpendicular axis. A thin and sheet‐like interval, likely slope‐cover, unconformably overlies the slope basin sediments. Deposition of this interval at IODP Site C0009 lasted from ∼2.06 Ma until between ∼1.24 Ma and ∼1.34 Ma and was partly coeval with the early upper Kumano basin sedimentary packages. Landward, postdating the slope‐cover sediments, a third sedimentary package was deposited before ∼0.9 Ma. This section correlates with an interval rich in terrigenous material (e.g., woody debris), indicating a possible distinct source or sediment routing system. Our work supports the idea that early forearc basin sediments may be deposited in an actively deforming outer wedge environment and that the outer‐to‐inner wedge transition of the lower forearc basin is likely a multistage process. The unsteady spatiotemporal nature of inner‐outer wedge coupling may lead to erratic stratigraphic patterns, such as have been observed in forearc basins worldwide.
Key Points:
Early forearc basin sedimentation occurred in a slope basin environment
The inner Kumano accretionary wedge stabilized in a polyphase manner
Changes in inner‐outer wedge coupling influenced accommodation space generation
The Black Mountain detachments denuded crystalline footwalls and extended sedimentary hanging walls from late Pliocene to Recent time. Fault rocks include gouges that crosscut breccias, and are in ...turn cut by compositionally and texturally distinct shear bands. Breccias have cataclastic textures, noteworthy for abundant transgranular fracture and power-law particle size distributions (
D) of 2.77–2.79. Gouges have granular textures, noteworthy for grains with abraded boundaries surrounded by a clay-rich matrix and
D
=
2.86–3.31. Matrix minerals include phyllosilicates, clay minerals, and oxide aggregates that serve as crude strain indicators. Geochemical data indicate that there was abundant water within the fault zone, but that the water was not plumbed from deeper crustal sources. There are systematic geochemical variations between fault-rock samples, but the inferred mass changes were minor, <10–30%. It is proposed that the fault rocks developed during exhumation since the late Pliocene from ≥3
km to near-surface conditions. Exhumation coincided with the development of granular textures and strain localization. The protracted history of the fault rocks involved multiple deformation mechanisms and authigenic mineral assemblages that hypothetically influenced the frictional properties of the detachment shear zones.
A 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction ...zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling‐induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike‐slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism.
We report the results of a two‐dimensional tomographic inversion of marine seismic refraction data from an array of ocean‐bottom seismographs (OBSs), which produced an image of the crustal structure ...along the axial valley of the ultraslow spreading Mid‐Cayman Spreading Center (MCSC). The seismic velocity model shows variations in the thickness and properties of the young oceanic crust that are consistent with the existence of two magmatic‐tectonic segments along the 110 km long spreading center. Seismic wave speeds are consistent with exhumed mantle at the boundary between these two segments, but changes in the vertical gradient of seismic velocity suggest that volcanic crust occupies most of the axial valley seafloor along the seismic transect. The two spreading segments both have a low‐velocity zone (LVZ) several kilometers beneath the seafloor, which may indicate the presence of shallow melt. However, the northern segment also has low seismic velocities (3 km/s) in a thick upper crustal layer (1.5–2.0 km), which we interpret as an extrusive volcanic section with high porosity and permeability. This segment hosts the Beebe vent field, the deepest known high‐temperature black smoker hydrothermal vent system. In contrast, the southern spreading segment has seismic velocities as high as 4.0 km/s near the seafloor. We suggest that the porosity and permeability of the volcanic crust in the southern segment are much lower, thus limiting deep seawater penetration and hydrothermal recharge. This may explain why no hydrothermal vent system has been found in the southern half of the MCSC.
Key Points
Large variability in crustal structure along the axial valley of the Mid‐Cayman Spreading Center
North of Mt Dent, a seismic low‐velocity zone underlies the Beebe hydrothermal vent field
Magmatism, faulting, and high porosity in the upper crust may facilitate high‐temperature venting at an ultraslow spreading center
Core recovered during Integrated Ocean Drilling Program (IODP) Expedition 319 from below the Kumano forearc basin of Japan's Nankai margin provides some of the only in situ samples from an inner ...accretionary wedge, and sheds light on the tectonic history of a seismically hazardous region. The 84m of core comprises Miocene-age well-bedded muds, silts, and volcaniclastic sediments. Beds increase in dip with depth, and are cut by (i) soft-sediment deformation bands (“vein structures”), (ii) ∼1-cm thick shear zones within ∼10-cm thick regions of high shear strain, and (iii) <1-mm thick slickensided faults which are the youngest structures in the core and highly localized. Microstructural analyses of the shear zones suggest that they formed via multiple increments of shear localization and a mixed granular and cataclastic flow. Kinematic analysis of slip indicators in shear zones further reveals that they formed via north–south shortening. In contrast, the faults cut the shear zones with mixed slip kinematics, and accommodated northwest–southeast shortening, roughly parallel to the modern shortening direction. The entire section was also rotated ∼15° counterclockwise about a roughly vertical axis. Therefore the principle strain axes and stratigraphic section rotated during or postdating development of the major sub-basin (∼5.6–3.8Ma) unconformity, a time that generally coincides with a change in the Philippine Sea plate convergence direction. Forearc basin development therefore postdates a protracted geologic evolution of shear-zone development, tectonic rotations, and inner-wedge development, the last of which coincides with a rheological evolution toward localized frictional faulting.
► First academic coring of an inner accretionary wedge and sub-forearc basin. ► Multiscale study of accretionary wedge shear zones and faults. ► Insight into depositional and deformational environment of Miocene Nankai margin. ► A change in mechanics from granular-cataclastic shear zones to localized faults. ► NantroSEIZE efforts to probe below the Kumano forearc basin.
Modeling studies suggest that fluid permeability is an important control on the maintenance and distribution of pore fluid pressures at subduction zones generated through tectonic loading. Yet, to ...date, few data are available to constrain permeability of these materials, at appropriate scales. During IODP Expedition 319, downhole measurements of permeability within the uppermost accretionary wedge offshore SW Japan were made using a dual‐packer device to isolate 1 m sections of borehole at a depth of 1500 m below sea floor. Analyses of pressure transients using numerical models suggest a range of in‐situ fluid permeabilities (5E‐15–9E‐17 m2). These values are significantly higher than those measured on core samples (2E‐19 m2). Borehole imagery and cores suggests the presence of multiple open fractures at this depth of measurement. These observations suggest that open permeable natural fractures at modest fracture densities could be important contributors to overall prism permeability structure at these scales.
Key Points
We present in‐situ measurements of permeability in the active interior wedge
In‐situ permeability is 3 orders of magnitude larger than core‐based measures
Fractures and faults in the interior wedge material are fluid conduits
Integrated Ocean Drilling Program (IODP) Hole 1256D successfully sampled a complete section of an intact oceanic crustal sheeted dike complex (SDC) (from 1061 to 1320 meters below seafloor; mbsf) on ...a 15 Ma old Cocos Plate. A series of rock magnetic measurements were carried out to understand the magmatic processes that accreted this end‐member, superfast‐spread (200 mm/yr full rate) oceanic crust. Results indicate that main ferromagnetic minerals are predominantly pseudo single‐domain (titano)magnetite crystals, responsible for both anisotropy of magnetic susceptibility (AMS) and magnetic remanence signals. AMS fabrics were reoriented into a geographic reference frame using magnetic remanence data, and corrected for a counterclockwise rotation of the Cocos Plate relative to the East Pacific Rise (EPR) ca. 15 Ma. Corrected AMS fabrics were then compared with the orientations of chilled margins previously obtained from Formation MicroScanner (FMS) images of the SDC at Hole 1256D. For some samples taken from close to dike margins, a dike‐normal orientation of the minimum AMS axes (Kmin) of prolate AMS ellipsoids mean that the long axis (Kmax) can be used to infer magma flow directions. Subvertical Kmin orientations in the interior of the dikes, however, may have required settling or compaction of the magma shortly after intrusion, thus rearranging the AMS fabric. Despite this orientation of Kmin axes, orientation of Kmax axes indicate a rather constant subhorizontal paleo‐flow direction, suggesting that magmas most probably traveled to the surface considerable distances from source regions within the EPR system.
Key Points
Melts were transported laterally during intrusion at EPR
Clustering of Kmax axes and prolate fabrics were used to obtain melt directions
Kmin axes indicate that secondary processes affected the petrofabrics
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