Ultraslow-spreading Mid-ocean Ridges Research Articles

Seismic Arrays on Drifting Ice Floes: Experiences from Four Deployments in the Arctic Ocean

In the last three decades our knowledge about the seismic structure and the seismicity of the world's oceans has increased tremendously through the common availability of ocean bottom seismometers for active and passive seismic studies ( e.g. , Orcutt et al. 1976; Toomey et al. 1985; Tolstoy et al. 2008). The safe recovery of the instruments is essential to access the acquired data. Comparable studies of the polar oceans do not exist, because the perennial sea-ice cover in polar regions prevents a safe recovery of ocean bottom instruments. Hence, the seismic structure and the earthquake activity of large parts of the Arctic Ocean, including the Gakkel Ridge, one of the most prominent members of ultraslow-spreading mid-ocean ridges (< 20 mm/y full rate; Dick et al. 2003), are largely unknown. Mounting conventional land seismometers on drifting sea ice is the only way to acquire seismic data in the Arctic Ocean. The difficult access to the survey area, which requires icebreaking vessels and helicopters, and the cold temperatures and a constantly moving seismometer platform, represent highly unsuitable survey conditions. However, Kristoffersen et al. (1982) made first attempts at recording regional earthquakes in the Arctic by attaching a sonobuoy array to an ice floe. Jokat et al. (2003) demonstrated that the acquisition of refraction seismic data is possible with seismic stations mounted on drifting ice floes. Okal and MacAyeal (2006) report on a deployment of seismometers on icebergs in Antarctica recording a variety of ice-induced (cryogenic) signals (MacAyeal et al. 2009). We deployed seismic arrays on drifting sea ice during four expeditions in 2001, 2007, 2008, and 2009, recording local and regional earthquakes along Gakkel Ridge. In addition, the records contained many sorts of cryogenic signals (“icequakes”) stemming possibly from collisions between ice floes, crack formation, or disintegration of ice floes …

Seismicity and active tectonic processes in the ultra-slow spreading Lena Trough, Arctic Ocean

With its remote location in the ice-covered Fram Strait, Lena Trough is a poorly known segment of the global mid-ocean ridge system. It is a prominent member of the ultra-slow spreading mid-ocean ridges but its spreading mechanisms are not well understood. We relocalized teleseismically recorded earthquakes from the past five decades to identify tectonic processes in Lena Trough and the adjacent Spitsbergen Fracture Zone (FZ). During two cruises with RV Polarstern in 2008 and 2009 we deployed seismic arrays on ice floes to record the local seismicity of Lena Trough. We could identify and localize microseismic events which we assume to be present in the entire rift valley. In contrast, our relocalization of teleseismically recorded earthquakes shows an asymmetric epicentre distribution along Lena Trough with earthquakes occurring predominately along the western valley flanks of Lena Trough. In 2009 February/March, several high-magnitude earthquakes peaking in an Mb 6.6 event occurred in an outside-corner setting of the Spitsbergen FZ. This is the strongest earthquake which has ever been recorded in Fram Strait and its location at the outside-corner high of the ultra-slow spreading ridge is exceptional. Comparing the seismicity with the magnetic anomalies and high-resolution multibeam bathymetry, we divide Lena Trough in a symmetrically spreading northern part and an asymmetrically spreading southern part south of the South Lena FZ. We propose that a complex interaction between the former De Geer Megashear zone, which separated Greenland from Svalbard starting at Late Mesozoic/Early Cenozoic times, and the developing rift in the southern Lena Trough resulted an increasing eastward dislocation towards the Spitsbergen FZ between older spreading axes and the recent active spreading axis which we believe to be located west of the bathymetric rift valley flanks in a wide extensional plain.

On the occurrence, trace element geochemistry, and crystallization history of zircon from in situ ocean lithosphere

We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for a TiO2 ≈ 0.7, a SiO2 ≈ 1.0, pressure ≈ 2 kbar); intra-sample variation is typically ~60–150°C. Decreasing Ti correlates with enrichment in Hf to ~2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ~680–740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ~25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La)N ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD, n = 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ~80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance.

OCCURRENCE AND SIGNIFICANCE OF SERPENTINITE-HOSTED, TALC- AND AMPHIBOLE-RICH FAULT ROCKS IN MODERN OCEANIC SETTINGS AND OPHIOLITE COMPLEXES: AN OVERVIEW

This paper reviews the occurrence and significance of talc- and amphibole-rich fault rocks developed in mafic-ultramafic sequences and evaluates their role in deformation and alteration of the oceanic lithosphere from different tectonic settings (from spreading mid-ocean ridges up to orogenic belts). Recently, talc and amphibole-rich fault rocks have been sampled and studied from detachment fault surfaces along slow and ultra-slow spreading mid-ocean ridges, and constraining the conditions of deformation and strain localization during the evolution of oceanic core complexes. These rocks are documented not only in oceanic core complexes, but also in other oceanic fracture zones where ultramafic rocks are exposed on the seafloor, while only few occurrences have been reported in ophiolite sequences. Samples recovered in situ in oceanic settings record heterogeneous deformation (crystal-plastic to cataclastic) under greenschist- facies conditions and are commonly restricted to localized shear zones (< 200 m) and are associated with intense talc-amphibole metasomatism. The presence of mechanically weak minerals, such as talc, serpentine and chlorite, may be critical to the development of such fault zones and may enhance unroofing of upper mantle peridotites and lower crustal gabbroic rocks during seafloor spreading. Talc in particular may be influential in lubricating and softening mylonitic shear zones and can lead to strain localization and focused hydrothermal circulation along such faults. The rheology of these rocks, and its evolution during dehydration reactions could play an important role also in subduction-zone processes and during the formation of ultramafic orogenic belts. Here, we review the occurrence and significance of talc- and amphibole-rich fault rocks in different tectonic settings on the seafloor and evaluate their role in deformation and alteration of the oceanic lithosphere.

FIRE AND ICE: MANTLE MELTING BENEATH THE HIGH ARCTIC MID-OCEAN RIDGES

FIRE AND ICE: MANTLE MELTING BENEATH THE HIGH ARCTIC MID-OCEAN RIDGES