Yongshun John Chen

Oceanic crustal thickness versus spreading rate

A compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness, away from plateaus, is 6 km; no systematic increase of crustal thickness with spreading rate is observed. Instead, the data show large variations in crustal thickness at slow spreading ridges (3 – 8 km for half rates 30 mm/yr). The large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid-ocean ridges. Both data sets support the speculation of a transition from a 3-D structure of crustal accretion at slow ridges to a 2-D accretion pattern at fast ridges.

First active hydrothermal vents on an ultraslow-spreading center: Southwest Indian Ridge

The ultraslow-spreading Southwest Indian Ridge is a major tectonic province, representing one of the important end-member mid-ocean-ridge types for its very slow and oblique spreading, and providing the only known route for migration of chemosynthetic deep-sea vent fauna between the Atlantic and Indian Oceans. We report the investigation of the first active high-temperature hydrothermal field found on any ultraslow mid-ocean ridge worldwide. Located on Southwest Indian Ridge at 37°47′S, 49°39′E, it consists of three zones extending ∼1000 m laterally, and it is one of four recently discovered active and inactive vent sites within a 250-km-long magmatically robust section. Our results provide the first direct evidence for potentially widespread distribution of hydrothermal activity along ultraslow-spreading ridges—at least along magmatically robust segments. This implies that the segment sections with excess heat from enhanced magmatism and suitable crustal permeability along slow and ultraslow ridges might be the most promising areas for searching for hydrothermal activities. It is surprising that the special vent fauna appear to indicate some complex affinity to those on the Central Indian Ridge, southern Mid-Atlantic Ridge, and the southwest Pacific Ocean.

Thermal effects of gabbro accretion from a deeper second melt lens at the fast spreading East Pacific Rise

Motivated by recent observations, a ridge model has been developed which includes both a shallow melt lens and a second deeper melt lens above the Moho to assess the thermal effect of the gabbro accretion from the lower melt lens at the fast spreading East Pacific Rise. Modeling results argue for a limited role of the lower melt lens in constructing the layer 3 gabbros. If significant (>10%) amount of the layer 3 gabbros is emplaced near the Moho, then the liberated latent heat during crystallization would generate a large “molten” section of the lower crust due to the inability of cold seawater to penetrate into this hot/ductile region. A large molten section of the lower crust is not consistent with recent seismic tomographic results of the East Pacific Rise. Thus it is likely that the amount of the gabbro being emplaced from the lower melt lens at the EPR is significantly less than that suggested by Oman ophiolite studies, such a lower melt lens is short-lived, or a large portion of the sill intrusions observed at Oman was emplaced at a few kilometers off axis (>3–4 km).

High sensitivity of ocean ridge thermal structure to changes in magma supply: the Galápagos Spreading Center

Abstract We explore the physical mechanisms for the observed apparent sensitivity of ridge axis topography and crustal magma systems to small changes in magma supply at intermediate spreading rates. Numerical experiments were carried out to simulate crustal temperature structure of the Galapagos Spreading Center, which spreads at intermediate spreading rates with its various sections appear to have been influenced to different degrees by the nearby Galapagos hotspot. Model results show a strong ‘threshold’ effect: as the crustal thickness decreases from 7.4 km at the 92°W area with an axial high westward to 6.0 km at the 94°W area with a transitional topography, the depth to the top of a magma lens is calculated to increase from 1.7 to 2.5 km. In contrast, at the 97°W area, where crustal thickness is only 5.6 km and a rift valley is present, the model results predict no steady-state magma lens in the crust. These model calculations provide a simple physical explanation for the recent observations along the Galapagos Spreading Center, where abrupt changes in both magma lens and axial morphology occur within a short distance but crustal thickness changes only modestly. Results of this investigation illustrate the critical importance of hotspots in influencing mid-ocean ridge crustal thickness and the associated changes in thermal structure, especially for ridges that spread at the sensitive range of intermediate spreading rates.

Seismic observation of an extremely magmatic accretion at the ultraslow spreading Southwest Indian Ridge

The oceanic crust is formed by a combination of magmatic and tectonic processes at mid-ocean spreading centers. Under ultraslow spreading environment, however, observations of thin crust and mantle-derived peridotites on the seafloor suggest that a large portion of crust is formed mainly by tectonic processes, with little or absence of magmatism. Using three-dimensional seismic tomography at an ultraslow spreading Southwest Indian Ridge segment containing a central volcano at 50°28′E, here we report the presence of an extremely magmatic accretion of the oceanic crust. Our results reveal a low-velocity anomaly (−0.6 km/s) in the lower crust beneath the central volcano, suggesting the presence of partial melt, which is accompanied by an unusually thick crust (~9.5 km). We also observe a strong along-axis variation in crustal thickness from 9.5 to 4 km within 30–50 km distance, requiring a highly focused melt delivery from the mantle. We conclude that the extremely magmatic accretion is due to localized melt flow toward the central volcano, which was enhanced by the significant along-axis variation in lithosphere thickness at the ultraslow spreading Southwest Indian Ridge.

Crustal thickness anomalies in the North Atlantic Ocean basin from gravity analysis

Gravity-derived crustal thickness models were calculated for the North Atlantic Ocean between 76°N and the Chain Fracture Zone and calibrated using seismically determined crustal thickness. About 7% of the ocean crust is 7 km thick and is interpreted to have been affected by excess magmatism. Thin crust probably reflects reduced melt production from relatively cold or refractory mantle at scales of up to hundreds of kilometers along the spreading axis. By far the most prominent thick crust anomaly is associated with Iceland and adjacent areas, which accounts for 57% of total crustal volume in excess of 7 km. Much smaller anomalies include the Azores (8%), Cape Verde Islands (6%), Canary Islands (5%), Madeira (<4%), and New England–Great Meteor Seamount chain (2%), all of which appear to be associated with hot spots. Hot spot–related crustal thickening is largely intermittent, suggesting that melt production is episodic on time scales of tens of millions of years. Thickened crust shows both symmetrical and asymmetrical patterns about the Mid-Atlantic Ridge (MAR) axis, reflecting whether melt anomalies were or were not centered on the MAR axis, respectively. Thickened crust at the Bermuda and Cape Verde rises appears to have been formed by isolated melt anomalies over periods of only ∼20–25 Myr. Crustal thickness anomalies on the African plate generally are larger than those on the North American plate; this most likely results from slower absolute plate speed of the African plate over relatively fixed hot spots.

Correlated patterns in hydrothermal plume distribution and apparent magmatic budget along 2500 km of the Southeast Indian Ridge

Multiple geological processes affect the distribution of hydrothermal venting along a mid-ocean ridge. Deciphering the role of a specific process is often frustrated by simultaneous changes in other influences. Here we take advantage of the almost constant spreading rate (65–71 mm/yr) along 2500 km of the Southeast Indian Ridge (SEIR) between 77°E and 99°E to examine the spatial density of hydrothermal venting relative to regional and segment-scale changes in the apparent magmatic budget. We use 227 vertical profiles of light backscatter and (on 41 profiles) oxidation-reduction potential along 27 first and second-order ridge segments on and adjacent to the Amsterdam-St. Paul (ASP) Plateau to map ph, the fraction of casts detecting a plume. At the regional scale, venting on the five segments crossing the magma-thickened hot spot plateau is almost entirely suppressed (ph = 0.02). Conversely, the combined ph (0.34) from all other segments follows the global trend of ph versus spreading rate. Off the ASP Plateau, multisegment trends in ph track trends in the regional axial depth, high where regional depth increases and low where it decreases. At the individual segment scale, a robust correlation between ph and cross-axis inflation for first-order segments shows that different magmatic budgets among first-order segments are expressed as different levels of hydrothermal spatial density. This correlation is absent among second-order segments. Eighty-five percent of the plumes occur in eight clusters totaling ∼350 km. We hypothesize that these clusters are a minimum estimate of the length of axial melt lenses underlying this section of the SEIR.

Evidence of an axial magma chamber beneath the ultraslow-spreading Southwest Indian Ridge

Ultraslow-spreading ridges are a novel class of spreading centers symbolized by amagmatic crustal accretion, exposing vast amounts of mantle-derived peridotites on the seafloor. However, distinct magmatic centers with high topographies and thick crusts are also observed within the deep axial valleys. This suggests that despite the low overall melt supply, the magmatic process interacting with the tectonic process should play an important role in crustal accretion; however, this has been obscured due to the lack of seismic images of magma chambers. Using a combination of seismic tomography and full waveform inversion of ocean bottom seismometer data from the Southwest Indian Ridge at 50°28′E, we report the presence of a large low-velocity anomaly (LVA) ∼4–9 km below the seafloor, representing an axial magma chamber (AMC) in the lower crust. This suggests that the 9.5-km-thick crust here is mainly formed by a magmatic process. The LVA is overlain by a high-velocity layer, possibly forming the roof of the AMC and defining the base of hydrothermal circulation. The steep velocity gradient just below the high-velocity layer is explained by the ponding of magma at the top of the AMC; this could provide the overpressure for lateral dike propagation along the ridge axis, leading to a complex interaction between magma emplacement, tectonic, and hydrothermal processes, and creating a diversity of seafloor morphology and extremely heterogeneous crust.

Modeling the Thermal State of the Oceanic Crust

New ocean crust is created every year along the mid-ocean ridge system, which is one of the most active plate boundaries on the Earths surface. Studying the magmatic and tectonic processes of the construction of new oceanic crust at mid-ocean ridges is important for understanding the structure of oceanic crust and the evolution of the oceanic lithosphere. This review focuses on modeling the thermal state of the oceanic crust and its links to various seafloor observations such as ridge axis topography, gravity, and seismic crustal structure. Thermal modeling is important because the crustal thermal structure of a spreading ridge not only controls the rheology of the oceanic lithosphere and the ridge topography but also determines the style of oceanic crustal genesis, by a long-lived magma lens or by episodic magma intrusions.

Lithospheric Structure of the Northern Ordos From Ambient Noise and Teleseismic Surface Wave Tomography

We constructed a high-resolution 3-D Vsmodel of the northern Ordos block and its surrounding areas by surface wave tomography, which reveals significant intracratonic heterogeneities. In the western Ordos, the lithosphere thickness is ~200 km with shear velocities comparable to the high velocities of other Archean cratons worldwide. However, the lithosphere thins gradually toward the east and Vs drops by 2–3% in the uppermost mantle beneath the eastern Ordos, coincident with the high surface heat flow (~68 mW/m in average) there. This observation suggests that the thick, cratonic keel is only locally preserved beneath the western Ordos, and the eastern part of the Ordos seems to undergo local rejuvenation. At greater depths (>180 km), a low-velocity channel is observed beneath the high-velocity keel of the Ordos. Beneath the Datong volcanoes, a low-velocity anomaly is observed, dipping westward with depth and closely following the slope of the lithosphere beneath the northern Ordos. This prominent low velocity is connected with the low-velocity zone beneath the northern Ordos, which is further connected with the low-velocity zone beneath the northeastern Tibetan Plateau (NET). We propose that the asthenosphere beneath the NET flows toward the northern Ordos in response to the continuous northward convergence of the Indian-Eurasian continents, and the asthenosphere flows upward following the eastward thinning lithosphere which leads to decompression partial melting, which migrates upward to feed the Datong volcanoes. The significant variations of the lithospheric thickness of the Ordos block may control the distribution of the asthenospheric flow.