Calcium isotope systematics of altered oceanic crust at IODP site 1256: Insights into the hydrothermal alteration
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Seafloor Spreading
Dike
Isotopic signature
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Much new information exists on the long‐term geologic motions of the ocean floor. Horizontal motions, when averaged over a few million years, are approximately 10 to 100 km/m.y. Relative motions across transform faults are twice these values. Uplift of the ocean crust in the rift mountains may be at a rate of 40 km/m.y. for short periods of time. Broader thermal cooling drops the ridge at a rate of about .175 km/m.y. for 1 m.y. old crust, and at a rate of about .02 km/m.y. for 80 m.y. old crust. Near subduction zones the ocean crust subsides at a rate of 2–4 km/m.y. near the oceanic trench axis and 40–60 km/m.y. on a Wadati‐Benioff zone which dips at 45°. Seamounts subside at a rate of .02‐.06 km/m.y. which is comparable to the rate of subsidence due to cooling of the oceanic crustal plates, but two or three orders of magnitude slower than seafloor spreading rates or the subsidence in some Wadati‐Benioff zones.
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The concept that the rock formations which make up the ocean floor can change with time is relatively recent and is an idea that has itself undergone considerable evolution. Prior to the hypothesis of plate tectonics, the seafloor was thought of as unchanging and essentially ageless. The identification of a specific “age” for a particular portion of oceanic basement developed naturally from the concept of seafloor spreading early in the 1960s. Field studies during this period began to show that in fact, the upper oceanic crust was far from uniform and that some of this variability could be associated with systematic changes in crustal age.
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Earth crust
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The magmatically accreted oceanic crust contains two distinct layers, the upper and the lower crust, whereas the tectonically controlled crust may have gabbros and serpentinite close to the seafloor. Using full waveform inversion applied to ocean bottom seismometer data, we reveal the presence of a strong lateral variability in the 40 – 48 Ma old oceanic crust in the slow-spreading equatorial Atlantic. Over a 120 km-long section we observe four distinct 20-30 km long crustal segments. The segment affected by the St Paul FZ consists of three layers, 2 km thick layer with velocity <6 km/s, 1.5 km thick middle crust with velocity 6-6.5 km/s, and an underlying layer with velocity ~7 km/s in the lower crust. The segment associated with an abyssal hill morphology contains high velocity ~7 km/s from a shallow depth of 2 – 2.5 km below the basement, indicating the presence of either serpentinized peridotite or primitive gabbro close to the seafloor. The segment associated with a low basement morphology has 5.5 – 6 km/s velocity starting near the basement extending down to a depth of 4 km, indicating chemically distinct crust. The segment close to the Romanche transform fault, a normal oceanic crust with velocity 4.5-5 km/s near the seafloor indicates a magmatic origin. The four distinct crustal segments have a good correlation with the overlying seafloor morphology features. These observed strong crustal heterogeneities could result from alternate tectonic and magmatic processes along the ridge axis, possibly modulated by chemical variations in the mantle.
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Mid-Atlantic Ridge
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