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Seaward dipping reflectors (SDRs) are large piles of seaward thickening volcanic wedges imaged seismically along most rifted continental margins. Despite their global ubiquity, it is still debated whether the primary cause of SDR formation is tectonic faulting or magmatic loading. To study how SDRs might form, we developed the first two‐dimensional thermomechanical model that can account for both tectonics and magmatism development of SDRs during rifting. We focus here on the magmatic loading mechanism and show that the shape of SDRs may provide unprecedented constraints on lithospheric strength at volcanic rifting margins. For mapping SDRs geometries to lithospheric strength, a sequence of model lithospheric rheologies are treated, ranging from analytic thin elastic plates to numerical thick elasto‐visco‐plastic crust and mantle layers with temperature and stress‐dependent viscosity. We then analyzed multichannel seismic depth‐converted images of SDRs from Vøring and Argentinian rifted margins in terms of geometric parameters that can be compared to our model results. This results in estimates for the lithospheric thickness during rifting at the two margins of 3.4 and 5.7 km. The plate thickness correlates inversely with mantle potential temperature at these margins during rifting, as estimated by independent studies.
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Magmatism During the Continent – Ocean Transition
As continents break apart, the dominant mechanism of extension transitions from faulting and lithospheric stretching to magma intrusion and oceanic crust formation in a new ocean basin. A common feature of this evolution preserved at magmatic rifted margins worldwide are voluminous lava flows that erupted close to sea level during the final stages of development of the continent-ocean transition (COT). The mechanisms responsible for the generation of the melts that contribute to these voluminous flows, the so-called seaward dipping reflectors (SDR), and their significance in the context of COT development, are relatively poorly understood; they lie deep below post-rift strata along submarine rifted margins where they cannot be studied directly. Extensive coring of the Afar Stratoid Series - an areally-extensive sequence of Pliocene-aged basalts and intercalated sediments that lie atop the developing COT in the sub-aerial Afar Depression, northern Ethiopia - offers fresh scope to address this issue. We present a numerical model simulating the formation of enriched metasomes within the continental lithospheric mantle by the passage of magmas resembling modern axial basalts. Thermal destabilization of the metasome, caused by plate stretching, initiates melt formation within the metasome. These melts, when mixed with a depleted lithospheric mantle component, closely match the range of compositions of the Afar Stratoid Series lavas in this study. Metasomatic re-enrichment and subsequent melting of the lithospheric mantle during the COT may contribute to further plate thinning. These results demonstrate a novel mechanism by which large-volume flows may be erupted during the COT.
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- Award ID(s):
- 1850606
- PAR ID:
- 10408569
- Date Published:
- Journal Name:
- Earth and planetary science letters
- ISSN:
- 0012-821X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Seaward dipping reflectors (SDRs) are large piles of seaward thickening volcanic wedges imaged seismically along most rifted continental margins. Despite their global ubiquity, it is still debated whether the primary cause of SDR formation is tectonic faulting or magmatic loading. To study how SDRs might form, we developed the first two‐dimensional thermomechanical model that can account for both tectonics and magmatism development of SDRs during rifting. We focus here on the magmatic loading mechanism and show that the shape of SDRs may provide unprecedented constraints on lithospheric strength at volcanic rifting margins. For mapping SDRs geometries to lithospheric strength, a sequence of model lithospheric rheologies are treated, ranging from analytic thin elastic plates to numerical thick elasto‐visco‐plastic crust and mantle layers with temperature and stress‐dependent viscosity. We then analyzed multichannel seismic depth‐converted images of SDRs from Vøring and Argentinian rifted margins in terms of geometric parameters that can be compared to our model results. This results in estimates for the lithospheric thickness during rifting at the two margins of 3.4 and 5.7 km. The plate thickness correlates inversely with mantle potential temperature at these margins during rifting, as estimated by independent studies.more » « less
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null (Ed.)International Ocean Discovery Program (IODP) Expeditions 367 and 368 will address the mechanisms of lithosphere extension during continental breakup. State of the art deep reflection seismic data show that the northern South China Sea (SCS) margin offers excellent drilling opportunities that can address the process of plate rupture at a magma-poor rifted margin. The SCS margin shows similarities to the hyperextended Iberia-Newfoundland margins, possibly including exhumed and serpentinized mantle within the continent-ocean transition (COT). However, recent modeling studies suggest that mechanisms of plate weakening other than serpentinization of the subcontinental lithospheric mantle exist. Two competing models for plate rupture (in the absence of excessively hot asthenospheric mantle) have widely different predictions for (1) the crustal structure across the COT, (2) the time lag between breakup and formation of igneous ocean crust, (3) the rates of extension, and (4) the subsidence and thermal history. Proposed drilling will core through thick sedimentary sections and into the underlying basement to firmly discriminate between these models. We plan to occupy four sites across a 150-200 km wide zone of highly extended seaward-thinning crust with a well-imaged COT zone. Three sites will determine the nature of critical crustal entities within the COT and constrain postbreakup crustal subsidence. These three sites will also help constrain how soon after breakup igneous crust started to form. A fourth site on the continental margin landward of the COT will constrain the timing of rifting, rate of extension, and crustal subsidence. If serpentinized mantle is found within the COT, this will lend support to the notion that the Iberia-type margin is not unique, and hence that weakening of the lithosphere by introducing water into the mantle may be a common process during continental breakup. If serpentinite is not found, and alternatively, scientific drilling results for the first time are gained in support of an alternative model, this would be an equally important accomplishment. Constraints on SCS formation and stratigraphy, including industry drilling, Ocean Drilling Program Leg 184 and IODP Expedition 349 drilling, the young (Paleogene) rifting of the margin, and absence of excessively thick postrift sediment allow us to effectively address these key topics by drilling within a well-constrained setting. An initial spreading rate of ~2 cm/y half-rate reduces the potential complexity of magma-starved, slow-spreading crust forming after breakup. Drilling, coring, and logging to address these SCS rifted margin science objectives will be undertaken during Expeditions 367 and 368, which will be implemented as a single science program.more » « less
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A tenet of plate tectonics is that divergent plates cause the asthenospheric mantle to ascend, decompress, and melt, producing new magmatic crust. However, drilling west of Iberia in the 1980s discovered a continent–ocean transition (COT) made of exposed mantle, revising models of lithospheric thinning and melt generation and defining magma-poor margins. A long-standing argument about mantle in COTs concerns its nature as either subcontinental or being exhumed during ultraslow seafloor spreading. Additionally, two models attribute the apparent lack of melts either to slow extension resulting in low ascent rates with enhanced asthenospheric cooling and reduced melt production or to upwelling mantle originally too depleted to produce a significant melt fraction. The debate on COT models is limited by the scarce evidence obtained in ultra-deepwater drilling, restricted to a few basement highs. Thus, 30 y after its discovery, the nature and genesis of COTs is still controversial. The comparatively shallow water depth and thin sediment cover of the Tyrrhenian Sea provide an optimal location to test COT formation models by drilling. The Tyrrhenian is the only example where extensive modern geophysical data has accurately mapped basement domains of a conjugate pair of COTs. They can be characterized with unprecedented detail in a single drilling expedition to study the time and space evolution of COT processes. Expedition 402 will drill two perpendicular transects. An east–west transect will target the progression from magmatic crust to exhumed mantle; a north–south transect will map the fault zone that exhumed the mantle. Drilling will sample the complete sediment section including Messinian deposits, the sediment/basement interface, the mantle, the associated magmas, and the products of syntectonic, and possibly ongoing, fluid-rock interactions to evaluate the hydrosphere–lithosphere geochemical exchange and potential related ecosystems.more » « less
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In the classical view of tectonic rifting, divergent lithospheric plates cause the asthenospheric mantle to ascend, decompress, and melt, eventually producing new magmatic crust. This view has been updated by drilling results that found exhumed mantle at the continent–ocean transition (COT), leading to the definition of magma-poor rifted margins. Obtaining geologic samples from COTs to directly constrain the diversity of rifting processes is a challenge because the igneous crust and mantle rocks are typically buried under a thick sediment cover. The Tyrrhenian Sea provides an optimal location to test COT formation models by drilling because it has a comparatively thin sediment cover, allows for studying a conjugate pair of COT margins in a single drilling expedition, and has been mapped in unprecedented detail with recent geophysical measurements. The key objective of International Ocean Discovery Program Expedition 402 was to determine the nature of the geologic basement in the central Vavilov Basin, where exhumed mantle peridotites were expected, and in the conjugate margins to the west (Cornaglia Terrace) and east (Campania Terrace). In the Vavilov Basin, Sites U1614 and U1616 recovered an exceptional variety of mantle rocks, including lherzolites, harzburgites, plagioclase-bearing lherzolites and harzburgites, dunites, and minor amounts of pyroxenites and magmatic intrusions. The mantle peridotites are significantly hydrated and weathered, resulting in the formation of serpentine and carbonate veins. In contrast, Site U1612 recovered at the sediment/basement interface an unconsolidated breccia with clasts of basalt, peridotite, and granite, followed by variably deformed mylonitic gneisses that transition downhole to granitoid quartz-diorite rocks. On the western Tyrrhenian margin (Cornaglia Terrace), Site U1613 sampled a sediment sequence dating back to the Messinian (Late Miocene), resting on much older sedimentary rocks akin to the Triassic–Paleozoic successions outcropping in Sardinia, supporting the hypothesis that the margin consists of extended continental crust. On the conjugate margin to the east (Campania Terrace), Site U1617 did not reach the basement but recovered a complete sequence of Messinian evaporites, including halite. The samples and data collected during Expedition 402 provide an extensive new data set to determine the heterogeneity of the mantle, the nature and history of melt production and impregnation, and the extent and evolution of mantle serpentinization and carbonation; to constrain the geometry and timing of the deformation that led to mantle exhumation; to study the fluid-rock interactions between seawater, sediment, and mantle peridotites; and to constrain geodynamic models of rifting and COT formation.more » « less
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