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Magmatism in the Context of the Present-Day Tectonic Settings
Published in O.A. Bogatikov, R.F. Fursenko, G.V. Lazareva, E.A. Miloradovskaya, A. Ya, R.E. Sorkina, Magmatism and Geodynamics Terrestrial Magmatism Throughout the Earth’s History, 2020
O.A. Bogatikov, V.I. Kovalenko, E.V. Sharkov, V.V. Yarmolyuk
Gabbroids from the lower oceanic crust are very similar to MORB in their geochemistry and isotopic composition, as evidenced by samples from Site 735 (Kempten et al., 1991a). However, locally these rocks are intensely deformed and altered and their isotope ratio implies that metamorphic and hydrothermal processes have not strongly affected the Sr-, Nd- and Pb-isotopes. These rocks have higher 143Nd/144Nd (0.51301–0.50319) and lower 206Pb/204Pb (17.35– 17.67) ratios as compared with those of MORB. The 87Sr/86Sr ratio (0.7025– 0.7030) is higher than that of MORB from the West Indian Ridge and the Rodriguez triple junction. Strong variations in these ratios suggest that the chamber is fed by fresh batches of melt. The secondary processes affect only the O-isotopes; the pattern of the process suggests that the reaction took place at high temperatures and low sea-water content.
Illumination of deformation by bending stresses and slab pull within the Southern Hikurangi Double Benioff Zone
Published in New Zealand Journal of Geology and Geophysics, 2019
Dominic Evanzia, Simon Lamb, Martha K. Savage, Tim Stern
Benioff zones are the most direct evidence for the existence of subducting slabs. The nature and distribution of seismicity within Benioff zones provide constraints on the rheology and stresses in a subducting slab, enhancing our understanding of deformation, strain partitioning, and the generation of megathrust subduction earthquakes. Hasegawa et al. (1978) were first to characterize two distinct bands of seismicity within a Benioff zone, referred to as a Double Benioff Zone (DBZ), located in northeastern region of Honshu, Japan. DBZs are ubiquitous within subduction zone systems worldwide (Brudzinski et al. 2007). The upper band of seismicity has been interpreted as the faulting of dehydrated and/or metamorphosed upper oceanic crust, whereas the lower band is thought to be faulting in dehydrating lower oceanic crust and/or upper mantle (i.e. serpentinized peridotite and antigorite) (Kirby et al. 1996; Peacock 2001; Yamasaki and Seno 2003; Zhang et al. 2004; Brudzinski et al. 2007).
The Alpha-Mendeleev ridge, a large igneous province with continental affinities
Published in GFF, 2019
Upper, middle and lower oceanic crust with P-wave velocities of 6.0, 6.7, 7.1 km/s have a Poisson’s ratio of 0.30, 0.28 and 0.31 respectively based on laboratory measurements (Hyndman 1979; Holbrook et al. 1992). For comparison, drilled and logged basalts from the Faroe Islands have an average Poisson’s ratio based on the sonic logs of 0.28 or a Vp/Vs 1.84 (Christie et al. 2006). In contrast, laboratory measurements of mid crustal granites have an average Poisson’s ratio of 0.24 or a Vp/Vs 1.72 (Holbrook et al. 1992). Laboratory measurements on serpentinites have an average Poisson’s ratio of 0.34 (Vp/Vs 2.20).
Cenozoic continental rifting in the north-western Ross Sea
Published in New Zealand Journal of Geology and Geophysics, 2022
Fred J. Davey, Stephen Cande, Joann Stock
The earlier rifting episode occurred from 61 to 53 Ma (Chron 27–24; Cande et al. 2000b; Cande and Stock 2004; Wilson and Luyendyk 2009), and formed the Central Basin and Central Trough. Cande and Stock (2004) suggest about 100 km of NE-SW extension occurred. Interpretation of seismic reflection data across the Central Trough is consistent with an early Tertiary age for the oldest sediments there (Decesari et al. 2007a, 2007b). The Central Basin in the north is a major structural and bathymetric feature with water depths in excess of 2000 m and containing over 4000 m of sediments, suggesting that its crust may be oceanic in character (Wilson and Luyendyk 2009). This inference is supported by the continuity of high Bouguer gravity anomalies through to the south of the Central Basin – similar to that over Northern Basin to the west (Figure 3). However, it is poorly defined magnetically with no obvious linear magnetic anomalies within the basin, although the data base is sparse. Geophysical data recorded by USGS S P Lee (SP Lee profile 416–418; Cooper et al. 1987; Davey and Cooper 1987) provide a profile across the central part of the basin (A in Figure 3). Seismic reflection data (Figure 4A) show normal faulted margins to the basin with up to 4300 m of sediments underlying the basin. A basement with seismic velocity of 5.4 km/s at a depth of 6.5 km was measured (Cooper et al. 1987). The measured gravity anomalies along the seismic profile have been used to derive a crustal model for the basin (Figure 4B). Parameters used were: unrifted Ross Sea continental crustal thickness 21 km (after Davey et al. 2016); densities – water 1.03 Mg/m3, sediment 1.9, 2.25, and 2.6 Mg/m3, continental basement 2.67 Mg/m3, lower/oceanic crust 2.9 Mg/m3, mantle 3.3 Mg/m3. Relatively steep gravity gradients towards the margins of the deeper part of the basin indicate a thick, steep sided, high density (2.9 Mg/m3) lower crust, inferred to be oceanic crust, at a similar depth and thickness to that derived for Northern Basin and Adare Basin immediately to the west (Mueller et al. 2005; Davey et al. 2016). The modelled oceanic crust is about 70 km wide. With a total extension of 100 km from oceanic magnetic anomaly analysis (Cande and Stock 2004), this indicates about 30 km of extension associated with continental extension and thinning before rupture.