Three-quarters of the ocean crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the seafloor.Despite the significance of these rocks, sampling them in situ is extremely challenging due to the overlying dikes and lavas. This means that our models for
The interpretation of uppermost crustal deformation near oceanic transform faults is based on bathymetric lineaments and earthquake focal mechanisms, and relatively little is known about the detailed kinematics within the transform tectonized zone. The Tjörnes Fracture Zone is a broad zone of deformation produced by right‐lateral transform shearing in north Iceland and is partly exposed on land providing the opportunity to study shallow‐level crustal structure of mid‐Miocene, thick, oceanic‐like crust formed by subaerial spreading. A pronounced structural curvature of lava and dike orientations near the Húsavík‐Flatey Fault within the transform zone is well documented, yet of controversial origin. In order to develop an assessment of deformation near the transform zone, samples of lavas and dikes were collected from 182 paleomagnetic sites within eight structural localities across the deformation zone on the Flateyjarskagi Peninsula. A progressive clockwise increase in locality mean remanence declinations over more than 10 km south of the fault broadly mimics the structural curvature of lava and dike orientations. Rotation estimates based on inclined rotation axes indicate significant clockwise rotation (74° ± 7° to 96° ± 9°) of multiple crustal blocks. When combined, all data from 108 sites within the deformed zone <12 km to the Húsavík‐Flatey Fault yield a best fit inclined axis rotation of 55° ± 7°. The paleomagnetic data and field relationships are consistent with a modified bookshelf faulting model, with relatively small (~1 km across) independently rotated crustal blocks with variable, and in some cases large‐magnitude rotations found within 10 km to the transform fault zone. Similar crustal deformation and comparable amounts of rotation may be present near other oceanic transforms, where accessibility and surficial deposits may limit documentation of more complex fault structures.
The right-lateral Húsavík-Flatey fault is part of the Tjörnes Fracture Zone, which links the offshore and onshore rift axes in northern Iceland. There has been debate about whether rocks near this fault have accommodated distributed off-fault deformation, which is testable using paleomagnetic data. Recent studies from Flateyjarskagi show clockwise declination deflections that are largest near the fault. We augment these data with new structural and paleomagnetic measurements from 106 lava flows across three peninsulas-Flateyjarskagi, Tröllaskagi, and Tjörnes-also finding clockwise deflections that vary with distance from the fault. To test whether the deflections could be caused by off-fault deformation, we combine our measurements with other regional data sets, applying several statistical tools including regressions of structural or paleomagnetic directions versus fault-normal distance. To evaluate the significance and uncertainties of the regressions, we use permutation tests and bootstrapping. For Flateyjarskagi, our analysis suggests that lavas and dikes were deformed together; the regression results predict 4 ∘ -6 ∘ of rotation per kilometer about a steep, but not vertical, axis. Rocks on Tröllaskagi hint at similar spatial patterns with fault distance, but the data quality precludes a full analysis. Rocks on Tjörnes show no spatial patterns, but they preserve a temporal history, where rotation seems to have ceased after deposition of the Pliocene-age Tjörnes beds. Using constraints from our statistical analyses, geochronology, and comparisons with the transform system in southern Iceland, we propose several modifications to models for the evolution of axial rift zones in northern Iceland.
Integrated Ocean Drilling Program (IODP) Hess Deep Expedition 345 was designed to sample lower crustal primitive gabbroic rock that formed at the fast-spreading East Pacific Rise (EPR) in order to test competing models of magmatic accretion and the intensity of hydrothermal cooling at depth. The Hess Deep Rift was selected to exploit tectonic exposures of young EPR plutonic crust, building upon results from Ocean Drilling Program Leg 147 as well as more recent submersible, remotely operated vehicle, and nearbottom surveys. The primary goal was to acquire the observations required to test end-member crustal accretion models that were in large part based on relationships from ophiolites, in combination with mid-ocean ridge geophysical studies. This goal was achieved with the recovery of primitive layered olivine gabbro and troctolite with many unexpected mineralogical and textural relationships, such as the abundance of orthopyroxene and the preservation of delicate skeletal olivine textures. Site U1415 is located within the Hess Deep Rift along the southern slope of the intrarift ridge between 4675 and 4850 m water depths. Specific hole locations were selected in the general area of the proposed drill sites (HD-01B-HD-03B) using a combination of geomorphology, seafloor observations, and shallow acoustic subbottom profiling data. A total of 16 holes were drilled. The primary science results were obtained from coring of two ~110 m deep reentry holes (U1415J and U1415P) and five single-bit holes (U1415E and U1415G-U1415I). Despite deep water depths and challenging drilling conditions, reasonable recovery for hard rock expeditions (15%-30%) was achieved at three 35-110 m deep holes (U1415I, U1415J, and U1415P). The other holes occupied during this expedition included three failed attempts to establish reentry capability (Holes U1415K, U1415M, and U1415P) and six jet-in tests to assess sediment thickness (Holes U1415A-U1415D, U1415F, and U1415L). Olivine gabbro and troctolite are the dominant plutonic rock types recovered at Site U1415, with minor gabbro, clinopyroxene oikocryst-bearing gabbroic lithologies, and gabbronorite. These rocks exhibit cumulate textures similar to those found in layered mafic intrusions and some ophiolite complexes. All lithologies are primitive, with Mg# between 76 and 89, falling within the global range of primitive oceanic gabbros. Spectacular modal and grain size layering was prevalent in >50% of the recovered core, display
[1] Rare, fault-bounded escarpments expose natural cross sections of ocean crust in several areas and provide an unparalleled opportunity to study the end products of tectonic and magmatic processes that operated at depth beneath oceanic spreading centers. We mapped the geologic structure of ocean crust produced at the East Pacific Rise (EPR) and now exposed along steep cliffs of the Pito Deep Rift near the northern edge of the Easter microplate. The upper oceanic crust in this area is typified by basaltic lavas underlain by a sheeted dike complex comprising northeast striking, moderately to steeply southeast dipping dikes. Paleomagnetic remanence of oriented blocks of dikes collected with both Alvin and Jason II indicate clockwise rotation of $61°related to rotation of the microplate indicating structural coupling between the microplate and crust of the Nazca Plate to the north. The consistent southeast dip of dikes formed as the result of tilting at the EPR shortly after their injection. Anisotropy of magnetic susceptibility of dikes provides well-defined magmatic flow directions that are dominantly dike-parallel and shallowly plunging. Corrected to their original EPR orientation, magma flow is interpreted as near-horizontal and parallel to the ridge axis. These data provide the first direct evidence from sheeted dikes in ocean crust for along-axis magma transport. These results also suggest that lateral transport in dikes is important even at fast spreading ridges where a laterally continuous subaxial magma chamber is present.Components: 2965 words, 5 figures.
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