[1] Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100–220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45° rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.
Summary We develop this advanced analysis tool to get more precise results include the long axis length, area, sphericity and roundness. Moreover, the large/flatten patch can be used as paleocurrent analysis from the azimuth of long axis.
Processes that build continental-margin stratigraphy on time-scales of > 20 kyr have been investigated. Eustatic sea-level exerts a major influence on sedimentation, but the Eel River margin shows that its effects can be interwoven with those of tectonism. Rapid Oligocene subsidence along the Cascadia subduction zone resulted in a foundered forearc basin. Regression and sedimentary reconstruction began in the Pliocene, and up to 1 km of sediment has accumulated since then, with rotating faults, synclines, anticlines and regional uplifts marking plate interactions. Fourteen seismic unconformities along structural highs can be traced into synclines. Many are ravinements formed during rising sea level, and ∼70-100 kyr cyclicity suggests a glacio-eustatic signal. Incised channels formed during regressions over the past ∼360 kyr, when rivers drained into Eel Canyon. In contrast, the New Jersey margin has long been dormant tectonically, providing clearer access to a eustatic imprint. Lack of Paleogene sediment supply resulted in a carbonate ramp prior to development of Oligocene deltas. With little accommodation space to allow aggradation, clinoforms prograded ∼100 km seaward, reaching the shelf break by Late Pleistocene. Coastal-plain drilling recovered ∼15 Oligocene and Miocene highstand deposits, which correlate with glacio-eustatic oscillations. Beneath the mid-to-outer shelf, incised valleys have been preserved, and clinoform strata suggest reworking of lobate deposits. Four Late Pleistocene sequences reveal no hiatuses at sequence boundaries, and no correlations between glacio-eustatic oscillations and stratal architecture. Stratal discontinuities are a common feature in margin sediments and provide objective means of interpreting the geological record. Continuous coring is essential to understand the processes that create stratal architecture.
Abstract Well logging has become a standard method in the oil industry for the investigation of subsurface geology. Accordingly, interpretation techniques have been mainly developed for use in sedimentary rocks, and the log responses of sediments are well known. However, this is not the case for igneous and metamorphic rocks. We present a compilation of log responses for mafic rocks from drill-holes in oceanic and continental basement. The holes cover a variety of mafic rocks: mid-ocean ridge basalt (MORB), gabbro, basalt and andesitic basalt from back-arc basins, flood basalt from large igneous provinces (LIPs), and continental metamorphic rocks. The comparison of log responses shows that rocks from the same geological setting have similar in situ physical properties. Differences in physical properties between rocks from different geological settings are mainly related to variations in the structure of the rocks, while variations in composition have only a minor effect on the in situ physical properties. In volcanic rocks, variations in fracturing and vesicularity related to cooling of the lava strongly influence log responses. Mafic rocks from continental drill-holes were enriched in radioactive elements during regional metamorphism, resulting in higher values in the total gamma-ray compared to the oceanic rocks.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)