Description: 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.

Description: Hydrothermal venting, an important cooling mechanism of the Earth, supports a diverse array of seafloor and sub-seafloor ecosystems that are sustained by large thermal and chemical fluxes. Vents have been found along even the slowest and coldest spreading centers, calling into question the driving heat source for these vents. The ultraslow-spreading Mid-Cayman Spreading Center in the Caribbean Sea, which hosts the axial-flank Von Damm Vent Field (VDVF), provides an opportunity to probe the mechanisms for venting at ultraslow spreading rates. Using active-source seismic data from the 2015 CaySeis (Cayman Seismic) experiment, we determined the seismic velocities in the large massif beneath the VDVF. We propose that this massif was produced by a pulse of on-axis magmatism at ca.2 Ma, which was then followed by exhumation, cooling, and fracturing. A low seismic velocity anomaly 5 km below the VDVF is evidence for either a cracking front mining lithospheric heat or intrusive magmatic sills, both of which could drive ongoing deep hydrothermal fluid circulation. We conclude that the transient magmatism and variable crustal thickness at ultraslow-spreading centers create conditions for long-lived hydrothermal venting that may be widespread, and other VDVF-like vents may be common in these areas.

Description: Mid-ocean ridges spreading at ultraslow rates of less than 20 mm yr−1 can exhume serpentinized mantle to the seafloor, or they can produce magmatic crust. However, seismic imaging of ultraslow-spreading centres has not been able to resolve the abundance of serpentinized mantle exhumation, and instead supports 2 to 5 km of crust. Most seismic crustal thickness estimates reflect the depth at which the 7.1 km s−1 P-wave velocity is exceeded. Yet, the true nature of the oceanic lithosphere is more reliably deduced using the P- to S-wave velocity (Vp/Vs) ratio. Here we report on seismic data acquired along off-axis profiles of older oceanic lithosphere at the ultraslow-spreading Mid-Cayman Spreading Centre. We suggest that high Vp/Vs ratios greater than 1.9 and continuously increasing P-wave velocity, changing from 4 km s−1 at the seafloor to greater than 7.4 km s−1 at 2 to 4 km depth, indicate highly serpentinized peridotite exhumed to the seafloor. Elsewhere, either magmatic crust or serpentinized mantle deformed and uplifted at oceanic core complexes underlies areas of high bathymetry. The Cayman Trough therefore provides a window into mid-ocean ridge dynamics that switch between magma-rich and magma-poor oceanic crustal accretion, including exhumation of serpentinized mantle covering about 25% of the seafloor in this region.

Description: About 57% of the Earth’s outer surface is oceanic crust and new ocean floor is continuously created along the 55,000-60,000 km long mid-ocean ridge (MOR) system. About 25% of MORs spread at an ultra-slow spreading rate of 〈 20 mm/yr. Most ultra-slow spreading ridges occur in areas of the world that are difficult to reach, like the Gakkel Ridge in the Arctic Ocean and the Southwest Indian Ridge in the Indian Ocean. It has long been recognized that crustal accretion at ultra-slow spreading rates is fundamentally different from crust generated at faster spreading rates. However, due to the remoteness of ultra-slow ridges, the formation of crust at these magma-starved centres is yet not well understood. During the CAYSEIS cruise we surveyed lithospheric formation at ultra-slow spreading rates at the Mid-Cayman spreading centre (MCSC) in the Caribbean Sea, where oceanic crust is formed at a full rate of ~17 mm/yr. To the northeast and southwest, the MCSC is bound by two major transform faults. Using active-source wide-angle seismic imaging and passive local earthquake monitoring we, studied the balance between magmatic accretion and tectonic stretching (and hence oceanic core complex formation) and the relationship between faulting and hydrothermal activity at ultra-slow spreading rates. In addition, we explored transform margin formation at a unique setting, occurring at the southern terminus of the MCSC. In total, six seismic lines surveyed crust formed at the MCSC, two of these profiles also crossed the Swan Island transform fault. The project was a collaboration between German, British and American scientists.

Description: About 57% of the Earth’s surface is covered by oceanic crust and new ocean floor is continuously created along the 60.000 km long mid-ocean ridge (MOR) system. About 25% of the MOR spread at an ultra-slow spreading rate of 〈20 mm/yr. At ultra-slow spreading rates the melt supply to the ridge is thought to dramatically decrease and crustal thickness decreases to a thickness of 〈6 km. However, we know little about the processes shaping crust at reduced spreading rates. A formation of crust from a magma chamber would suggest the creation of a well stratified crust, with an extrusive upper crust (layer 2) and a lower gabbroic crust (lower 3) and a well-defined crust-mantle boundary and hence a seismic Moho. In contrast, decompressional melting without formation of a magma chamber would support a crustal structure where seismic velocities change gradually from values typical of crustal rocks to mantle rocks. Here, we report results from a survey from the ultra-slow spreading Cayman Spreading Centre in the Caribbean Sea, sampling mature crust along a flowline from both conjugated ridge flanks. The seismic refraction and wide-angle survey was conducted using ocean-bottom-seismometers from Germany, the UK, and Texas and a 5500 cubic-inch airgun-array source towed by the German research vessel METEOR in April 2015. Typical crustal P-wave velocities support a thin crust of 3 to 5 km thickness. However, a well-defined Moho boundary was not observed. Thus, velocities change gradually from crustal-type velocities (〈7.2 km/s) to values of 7.5-7.8 km/s, supporting mantle rocks. In addition to P-waves, we have sampled S-waves along the profile, yielding for the first time Vp/Vs ratios for lithosphere emplaced at ultra-slow spreading rates. Interestingly, about 15 to 20% of the lithosphere has Vp/Vs ratos of 〉1.9, supporting serpentine. Domains of high Vp/Vs ratio also occur right at the seafloor, supporting large-scale exposure of mantle as proposed by geological evidence from ultra-slow spreading ridges. Ingo Grevemeyer (1), Michaela Merz (1), Anke Dannowski (1), Cord Papenberg (1), Nicholas Hayman (2), Harm van Avendonk (2), and Christine Peirce (3)

Description: About 57% of the Earth’s surface is covered by oceanic crust and new ocean floor is continuously created along the ~60.000 km long mid-ocean ridge (MOR) system. About 25% of the MOR spread at an ultra-slow spreading rate of 〈20 mm/yr. At ultra-slow spreading rates the melt supply to the ridge is thought to dramatically decrease and crustal thickness decreases to a thickness of 〈6 km. Further, geological evidence suggests wide-spread un-roofing of mantle. Yet, seismic data provide little evidence for amagmatic lithospheric emplacement away from oceanic core complexes. Formation of crust from a magma chamber would suggest the creation of a well stratified crust, with an extrusive upper crust (layer 2) and a lower gabbroic crust (lower 3) and a well-defined crust-mantle boundary and hence a seismic Moho. In contrast, un-roofing of mantle would support a crustal structure where seismic velocities change gradually from about 4.5 km/s at the seabed to velocities of mantle rocks at depth. In addition, exposure of mantle to seawater would cause serpentinization. Serpentine, in turn, would support high Vp/Vs ratios of 〉1.9. Here, we report results from a seismic refraction survey from the ultra-slow spreading Cayman Spreading Centre in the Caribbean Sea, sampling mature crust along a flowline from both conjugated ridge flanks. The ocean-bottom-seismometer and hydrophones provide both P-wave and S-wave refracted arrivals. Travel time data were inverted using seismic tomography. Resulting Vp/Vs ratios suggest that up to 25% of the lithosphere have high ratios of 〉1.9, supporting serpentinization and exposure of hydrated mantle at the seafloor. Further, the mode of accretion has changed over time, supporting both areas of mantle exposure and magmatic crust. Magmatic crust has a typical layer 2 and layer 3 velocity structure and a thin crust of 3 to 5 km thickness. However, a well-defined Moho boundary was not observed. Thus, crustal rocks are characterized by typical crustal-velocities (〈7.2 km/s) and mantle has velocities of 〉7.6 km/s. Domains of un-roofed mantle have high Vp/Vs ratios and velocities gradually increasing to 7.4-7.6 km/s. In addition, we will use our results to re-assess the depth distribution of local earthquakes at ultra-slow spreading ridges, including the Cayman Trough and the Southwest Indian Ridge. Most importantly, the high Vp/Vs ratio of 〉1.9 characterizing serpentinized mantle causes earthquakes to focus at much shallower depth when compared to location procedures using a global average for Vp/Vs of 1.73; the bias in depth might be in the order of 10 km.

Description: Variations in trench and forearc morphology, and lithospheric velocity structure are observed where the Louisville Ridge seamount chain subducts at the Tonga-Kermadec Trench. Subduction of these seamounts has affected arc and back-arc processes along the trench for the last 5 Myr. High subduction rates (80 mm/yr in the north, 55 mm/yr in the south), a fast southwards migrating collision zone (~180 km/myr), and the obliquity of the subducting plate and the seamount chain to the trench, make this an ideal location to study the effects of seamount subduction on lithospheric structure. The “before and after”ù subduction regions have been targeted by several large-scale geophysical projects in recent years; the most recent being the R/V Sonne cruise SO215 in 2011. The crust and upper mantle velocity structure observed in profiles along strike of the seamount chain and perpendicular to the trench from this study, are compared to a similar profile from SO195, recorded ~100 km to the north. The affects of the passage of the seamounts through the subduction system are indicated by velocity anomalies in the crust and mantle of the overriding plate. Preliminary results indicate that in the present collision zone, mantle velocities (Pn) are reduced by ~5%. Around 100 km to the north, where seamounts are inferred to have subducted ~1 Myr ago, a reduction of 7% in mantle P-wave velocity is observed. The width of the trench slope and elevation of the forearc also vary along strike. At the collision zone a 〉100 km wide collapse region of kilometre-scale block faults comprise the trench slope, while the forearc is elevated. The elevated forearc has a 5 km think upper crust with a Vp of 2.5-5.5 km/s and the collapse zone also has upper crustal velocities as low as 2.5 km/s. To the east in the Pacific Plate, lower P-wave velocities are also observed and attributed to serpentinization due to deep fracturing in the outer trench high. Large bending faults permeate the crust and the Osbourn Seamount, currently on the verge of subduction, is fractured stepwise down into the trench. Pn velocities in the hinge zone of the Pacific Plate are as low as 7.3 km/s indicating that fracturing and serpentinization may also extend to sub-crustal depths. Finally, trench-parallel variations in subduction zone velocity structure are used to infer the degree to which seamount subduction has altered the physical state of the Pacific and Indo-Australian plates both pre- and post subduction.