Eocene Oceanic Crust Research Articles

Seismic Structure of the St. Paul Fracture Zone and Late Cretaceous to Mid Eocene Oceanic Crust in the Equatorial Atlantic Ocean Near 18°W

AbstractPlate tectonics characterize transform faults as conservative plate boundaries where the lithosphere is neither created nor destroyed. In the Atlantic, both transform faults and their inactive traces, fracture zones, are interpreted to be structurally heterogeneous, representing thin, intensely fractured, and hydrothermally altered basaltic crust overlying serpentinized mantle. This view, however, has recently been challenged. Instead, transform zone crust might be magmatically augmented at ridge‐transform intersections before becoming a fracture zone. Here, we present constraints on the structure of oceanic crust from seismic refraction and wide‐angle data obtained along and across the St. Paul fracture zone near 18°W in the equatorial Atlantic Ocean. Most notably, both crust along the fracture zone and away from it shows an almost uniform thickness of 5–6 km, closely resembling normal oceanic crust. Further, a well‐defined upper mantle refraction branch supports a normal mantle velocity of 8 km/s along the fracture zone valley. Therefore, the St. Paul fracture zone reflects magmatically accreted crust instead of the anomalous hydrated lithosphere. Little variation in crustal thickness and velocity structure along a 200 km long section across the fracture zone suggests that distance to a transform fault had negligible impact on crustal accretion. Alternatively, it could also indicate that a second phase of magmatic accretion at the proximal ridge‐transform intersection overprinted features of starved magma supply occurring along the St. Paul transform fault.

Investigation of geothermal structure of the Sulawesi, using gravity and magnetic method

The Sulawesi Sea and Sulawesi Island are located in the western Pacific area where volcanic activity, plate subduction, and seismic activity are very active. The Sulawesi basin formed during the Middle Eocene-Late Eocene and nearly half of the Eocene oceanic crust has subducted below the North Sulawesi Trench. The Sulawesi Island was spliced and finalized in the Early Pliocene-Pleistocene during volcanic activity and is recently very active. This area is an optimal location to study volcanic geothermal conditions and subduction initiation mechanisms in the southern part of the western Pacific plate margin, which are important in geothermal and geodynamic research. In this study, we combined 133 heat flow data with gravity and magnetic data to calculate the Moho structure and Curie point depth of the Sulawesi Sea and periphery of the Sulawesi Island, and analyze the distribution characteristics of the geothermal gradient and thermal conductivity. The results show that the average depths of the Moho and Curie surfaces in this area are 18.4 and 14.3 km, respectively, which is consistent with the crustal velocity layer structure in the Sulawesi Basin previously determined by seismic refraction. The average geothermal gradient is 4.96°C (100 m)−1. The oceanic area shows a high geothermal gradient and low thermal conductivity, whereas the land area shows a low geothermal gradient and high thermal conductivity, both of which are consistent with statistical results of the geothermal gradient at the measured heat flow points. The highest geothermal gradient zone occurs in the transition zone from the Sulawesi Sea to Sulawesi Island, corresponding to the spreading ridge of the southward-moving Sulawesi Basin. Comprehensive gravity, magnetic, and geothermal studies have shown a high crustal geothermal gradient in the study area, which is conducive to the subduction initiation. The northern part of the Palu-koro fault on the western side of Sulawesi is likely the location where subduction initiation is occurring. During the process of moving northwest, the northern and eastern branches of Sulawesi Island have different speeds; the former is slow and the latter is fast. These branches also show different deep tectonic dynamic directions; the northern branch tilts north-up and the eastern branch tilts north-down.

The prospectivity of the Cape Vogel Basin, Papua New Guinea

The frontier and underexplored Cape Vogel Basin (CVB), north of the Papuan Peninsula, is thought to be underlain by Late Palaeocene–Eocene oceanic crust and overlain by Cenozoic sediments. Several impartial data provide evidence of working petroleum system(s) including a flow of oil from a 1920s well, and two 1970s wells that encountered minor hydrocarbon traces and good source material. The 1970s wells chased Miocene reef plays (like the discoveries in the Gulf of Papua). No Miocene reefs were encountered, with both wells terminating in volcanics. Integration of open-file 2D seismic, modern 2D PSDM seismic and shipborne gravity and magnetic data improves the subsurface imaging and thus understanding of prospectivity. The data reveal a significant sedimentary section (including Mesozoic sediments) and that the volcanics are not laterally continuous (i.e. products of short periods of volcanism). The data also suggests several Mesozoic–Cenozoic plays (e.g. carbonate reefs, incised canyons). Repeatable sea surface slicks, and observable bottom-simulating reflectors and direct hydrocarbon indicators, also provide evidence of working petroleum system(s). It is hypothesised that the CVB has affinities with the Gulf of Papua with the extension of the Australian craton north of the Papuan Peninsula, with widespread deposition in the Mesozoic–Cenozoic, and with source rocks estimated to be within the hydrocarbon generative window. With incorporation of onshore data and presence of significant gravity low, it is postulated that the central and north-west were less susceptible to Late Cretaceous and Palaeocene differential uplift and erosion (related to Coral Sea breakup and extension), and thus have a higher chance of Late Mesozoic preservation.

Post-emplacement history of the Zambales Ophiolite Complex: Insights from petrography, geochronology and geochemistry of Neogene clastic rocks

The Zambales Ophiolite Complex in Luzon, Philippines is made up of two blocks with differing geochemical signatures and ages – the Middle Jurassic to Early Cretaceous Acoje Block-San Antonio Massif that is of island arc tholeiite composition and the Eocene Coto Block-Cabangan Massif which is of transitional mid-ocean ridge basalt-island arc tholeiite affinity. These ophiolitic bodies are overlain by Miocene to Pliocene sedimentary units whose petrochemistry are reported here for the first time. Varying degrees of influences from ophiolitic detritus and from arc volcanic materials, as shown by petrography and indicator elements including Cr, Co and Ni, are observed in these sedimentary formations from north to south and from the oldest to the youngest. The Early to Middle Miocene Cabaluan Formation, whose outcrops are found to overlie only the Acoje Block, registers a more dominant ophiolitic signature as compared to the Late Miocene to Pliocene Santa Cruz Formation. The Santa Cruz Formation is generally characterized by fewer ophiolitic clasts and higher amounts of felsic components. Additionally, within this formation itself, a pronounced compositional change is observed relative to its spatial distribution. From the south to the north, an increase in ophiolitic components and a relative decrease in felsic signature is noted in units of the Santa Cruz Formation. It is therefore inferred that changes in the petrochemistry of rocks from the older Cabaluan to the younger Santa Cruz sedimentary formations record a decline in the influx of ophiolitic detritus or, conversely, the introduction of more diverse sediment sources as the deposition progressed. Detrital zircon U–Pb ages from the Santa Cruz Formation, with peaks at 46.73±0.94 and 5.78±0.13Ma, reflects this change in provenance from the unroofing of an Early Eocene oceanic crust to fresh contributions from an active volcanic arc during the Late Miocene. The contrast in compositions of the southern and northern Santa Cruz Formation also indicates a closer proximity of the southern units to the source of these non-ophiolitic sources, which most likely corresponds to the Pliocene volcanoes of the West Luzon Arc.

From Devonian extensional collapse to early Eocene continental break-up: an extended transect of the Kejser Franz Joseph Fjord of the East Greenland margin

SUMMARY Seismic investigations along East Greenland’s Fjord Region completed during the last decade provide fundamental insights into the region’s crustal structure and tectonic history. A summary of models along a transect through the Kejser Franz Joseph Fjord provides a view from the Precambrian Shield to the Eocene oceanic crust. We conclude that a change of rifting geometry from an upper- to a lower-plate-style margin occurred in early Mesozoic times and formed the >350-km-wide rift zone. Despite the demonstrated asymmetry of the northeast Greenland and conjugate Voring margins, the change of rift geometries and the direction of rift jumps remain debatable. A combined model for productivity and duration of magmatism is proposed for the northeast Greenland fjord region. We suggest that magmatism started slowly at 58.8 ± 3.6 Ma with a production rate of 1.5 × 10 −4 km 3 km −1 a −1 , which is similar to the productivity of onshore upper and lower lava sequences on the Geikie Plateau. A peak of 9.4 × 10 −4 km 3 km −1 a −1 for 0.5 Myr, and a subsequent productivity of 4.4 ± 0.3 × 10 −4 km 3 km −1 a −1 for 2.5 Myr between 53.3 and 50.8 Ma, produced the majority of melt, but break-up did not occur immediately afterwards. Continuous production of melt, similar to the rate of ocean spreading until C22 (∼50 Ma), contributed to massive magmatic underplating until eventual break-up at 50 Ma. The volumes and production rates show similarities to those obtained from a profile off the southeast Greenland margin but with a major difference in a smaller regional spatial extent.

Ancient Fungal Life in North Pacific Eocene Oceanic Crust

We present evidence that eukaryotic life has existed in an extreme environment, inside the oceanic crust. Up to now only prokaryotes have been discovered within deep marine sediments and glass-rims of pillow basalts, no higher life forms are described as yet. This study demonstrates unique filamentous fossil structures observed within carbonate-filled vesicles of a massive lava flow unit from the upper oceanic crust in the North Pacific (ODP Site 1224). Based on morphological traits including branching, septa and central pores, the filaments are interpreted as fungi. The chemical composition of the fungal structures differs from the surrounding crystalline carbonate matrix in the deep basaltic rocks. Small open space between the fungi and the carbonate cement and undisturbed filamentous growth through different calcite crystals indicate endolithic fungal growth after the calcium carbonate filling. The presence of euhedral pyrite crystals within the carbonate cements points out anaerobic conditions in this habitat. Our results provide for the first time evidence for eukaryotic, fungal life in deep ocean basaltic rocks.

Structural and Tectonic Evolution of the Eastern Cayman Trough (Caribbean Sea) From Seismic Reflection Data

The eastern Cayman Trough preserves a record of the Late Cretaceous to Paleogene Caribbean history that is largely affected by Neogene strike-slip tectonics of the current plate boundary. We conducted an analysis of seismic data within the eastern Cayman Trough, based upon single and multichannel seismic reflection profiles collected during the Seacarib II cruise in 1987 and the Casis cruise in 1992. These data show that the basement of the eastern Cayman Trough can be divided into four domains from east to west, with distinct morphologic and sedimentary character and inferred older to younger ages: (1) a province of rifted Mesozoic continental crust exhibiting seven parallel horst blocks striking northeast-southwest; (2) a continent-ocean transition bet een provinces 1 and 3 that exhibits seamounts, small hills, and sedimentary basins; (3) an Eocene oceanic crust with rough basement but smoother relief than the rifted crust; basement trends are roughly north-south and oblique to the northwest trend in domain 1, and (4) the northern Jamaica slope, which forms an east-west-trending slope, with northward-dipping strata that flank the three deeper water domains of the Cayman Trough. The domains are interpreted to be the product of the Eocene east-west opening of the Cayman Trough as a pull-apart basin in a left-lateral strike-slip setting. Closure of the 1100 km of Eocene and younger oceanic crust of the Cayman Trough places the fault-block province adjacent to the Belize margin of Central America. Restoration of normal faults in the fault-block province places an additional 190 km of left-lateral offset along the Cayman Trough. A Neogene phase of transpression has reactivated structures in the four domains, along with on-land structures described by previous authors in Jamaica. The proximity of the eastern margin of the Cayman Trough to petroliferous, continental rocks in Central America suggests an improved possibility of hydrocarbon potential. Unfortunately, s diment thicknesses of less than 1 km probably are not conducive to hydrocarbon formation.

The Banda–Celebes–Sulu basin: a trapped piece of Cretaceous–Eocene oceanic crust?

The Banda, Celebes and Sulu basins are poorly understood marginal seas located at the junction of the Eurasian, Indian–Australian and Pacific–Philippine Sea plates. The incompleteness of the data sets, the complex geology of the surrounding islands and the late Cenozoic evolution involving subduction, rifting, transform faulting and island arc collision have complicated tectonic interpretation of the region. The Banda basin is underlain by oceanic crust1–3. Bowin et al.2 and Lapouille et al.3 have suggested that it is a trapped oceanic basin which was once continuous with the late Jurassic Argo Abyssal Plain off north-west Australia. Similarly, the Celebes and Sulu basins are also underlain by oceanic crust4–6. On the basis of newly identified magnetic reversal ages (Fig. 1), heat-flow data and on-land geological studies, we suggest that the Celebes and Sulu seas may have been continuous with the Banda basin. This Cretaceous–Eocene basin has been dissected into its present configuration by Tertiary tectonic processes.

Evidence for Late Paleocene to Late Eocene seafloor in the southern New Hebrides basin

We interpret marine magnetic anomaly data from the southern part of the New Hebrides basin as showing that a fragment of Late Paleocene-Late Eocene oceanic crust is preserved between the New Caledonia-Loyalty Islands platform and the New Hebrides island arc-trench system. The magnetic lineations which trend about NE-SW, are interpreted as anomalies 18 through 23 (42–55 m.y. B.P.) with the inferred crustal ages increasing to the north. Drilling at DSDP site 286, which is located on oceanic crust slightly older than 55 m.y. according to our magnetic anomaly identifications, revealed mid-Eocene sediments overlying igneous basement.