Nonvolcanic tectonic islands in ancient and modern oceans

Abstract

[1] Most oceanic islands are due to excess volcanism caused by thermal and/or compositional mantle melting anomalies. We call attention here to another class of oceanic islands, due not to volcanism but to vertical motions of blocks of oceanic lithosphere related to transform tectonics. Sunken tectonic islands capped by carbonate platforms have been previously identified along the Vema and Romanche transforms in the equatorial Atlantic. We reprocessed seismic reflection lines, did new facies analyses and 87 Sr/ 86 Sr dating of carbonate samples from the carbonate platforms. A 50 km long narrow paleoisland flanking the Vema transform, underwent subsidence, erosion, and truncation at sea level; it was then capped by a 500 m thick carbonate platform dated by 87 Sr/ 86 Sr at $11-10 Ma. Three former islands on the crest of the Romanche transverse ridge are now at $900 m bsl; they show horizontal truncated surfaces of oceanic crust capped by $300 m thick carbonate platforms, with 10-6 Ma Sr isotopic ages. These sunken islands formed due to vertical tectonics related to transtension/transpression along long-offset slow-slip transforms. Another tectonic sunken island is Atlantis Bank, an uplifted gabbroic block along the Atlantis II transform (SW Indian Ridge) $700 m bsl. A modern tectonic island is St. Peter and St. Paul Rocks, a rising slab of upper mantle located at the St. Paul transform (equatorial Atlantic). ''Cold'' tectonic islands contrast with ''hot'' volcanic islands related to mantle thermal and/or compositional anomalies along accretionary boundaries and within oceanic plates, or to supra-subduction mantle melting that gives rise to islands arcs.

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“…The absence of the low-temperature fault rocks on the flat surface of the Atlantis Bank platform compared to its flanks suggests they may have been removed, a deduction entirely consistent with the long-held supposition that Atlantis Bank was once an ocean island and that its flat top results from erosion at sea level followed by subsidence to its present level (Dick et al, 1991b;Palmiotto et al, 2013). Shallow drilling and dredging on the summit and uppermost flanks of the Atlantis Bank platform have amply verified this supposition.…”

“…Shallow drilling and dredging on the summit and uppermost flanks of the Atlantis Bank platform have amply verified this supposition. Indurated carbonates were drilled at 24 sites from the periphery of the platform and were also recovered in dredges from its flanks (MacLeod et al, 1998(MacLeod et al, , 2000Palmiotto et al, 2013). Although some (recent) carbonate sand was recovered, most of the sediments are indurated bioclastic limestone (skeletal packstone to wackestone; MacLeod et al, 2000) of Miocene to Pleistocene age (Palmiotto et al, 2013).…”

“…Indurated carbonates were drilled at 24 sites from the periphery of the platform and were also recovered in dredges from its flanks (MacLeod et al, 1998(MacLeod et al, , 2000Palmiotto et al, 2013). Although some (recent) carbonate sand was recovered, most of the sediments are indurated bioclastic limestone (skeletal packstone to wackestone; MacLeod et al, 2000) of Miocene to Pleistocene age (Palmiotto et al, 2013). The sediments contain abundant macrofauna, primarily bryozoans, mollusks, algal nodules, and echinoids, but also some solitary corals.…”

“…Benthic and, in some instances, large planktonic foraminifers are common. Palmiotto et al (2013) found a water depth of ~100-200 m based on assemblages in dredged carbonates from the flanks of the platform, whereas green algal (dasyclad) assemblages in drill cores from the platform summit (G. Della Porta and V.P. Wright, pers.…”

Intra-skeletal calcite in a live-collected Porites sp.: Impact on environmental proxies and potential formation process

“…Sr/ 86 Sr ratio)were measured withthe remaining of the seven annual Porites powders studied here. The measurements were performed at the LSCE,on the same MC-ICP-MS Neptune Plus as for δ 11 B,using the analytical protocol previously described by Palmiotto et al (2013).…”