Abstract The crust beneath transform faults at slow-spreading ridges has been considered to be thin, comprising a thin mafic layer overlying serpentinized peridotite. Using wide-angle seismic data, we report the presence of a Moho at ∼6 km depth and a low-velocity anomaly extending down to 9 km beneath the 20-km-wide Romanche transform valley floor in the equatorial Atlantic Ocean. The low crustal velocities above the Moho could be due to either highly serpentinized mantle peridotite or fractured mafic rocks. The existence of clear Moho reflections and the occurrence of a large crustal-depth rupture during the 2016 magnitude 7.1 earthquake suggest that the crust likely consists of fractured mafic material. Furthermore, the presence of low velocities below the Moho advocates for extensive serpentinization of the mantle, indicating that the Moho reflection is unlikely to be produced by a serpentinization front. The crust to the north of the transform fault likely consists of mafic material, but that in the south appears to be more amagmatic, possibly containing serpentinized peridotite. Our results imply that the transform fault structure is complex and highly heterogeneous, and thus would have significant influence on earthquake rupture and alteration processes.
We report 18 new conductive heat flow measurements collected from a sediment pond located in the inactive part of the Ecuador Fracture Zone in the Panama Basin. The data were collected along an east–west transect coincident with a multi-channel seismic reflection profile that extends from ODP Hole 504B to west of the sediment pond. Conductive models indicate that heat flow should decrease from ≈400 mW m−2 on the 1.5 Ma western plate to ≈200 mW m−2 on the 6 Ma eastern plate; however the observed heat flow increases nearly linearly toward the east from approximately 140 mW m−2 to 190 mW m−2. The mean value of 160 mW m−2 represents an average heat flow deficit of ≈50%, which we attribute to lateral advective heat transfer between exposed outcrops on the western and eastern margins of the sediment pond. We apply the well-mixed aquifer model to explain this eastwardly flow, and estimate a volumetric flow rate per unit length in the north–south direction of ≈400±250m2yr−1 through the basement aquifer. Using a Darcy flow model with the mean flow rate, we estimate permeabilities of ∼10−11 and 10−12m2 for aquifer thicknesses of 100 and 1000 m, respectively. The estimated permeabilities are similar to other estimates in young oceanic upper crust and suggest that vigorous convection within the basement significantly modifies the thermal regime of fracture zones. Additional heat flow data are needed to determine the prevalence and importance of advective heat transfer in fracture zones on a global scale.
Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Journal of Geophysical Research - Solid Earth. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing an older version [v2]Go to new versionSeismic structure of the St. Paul Fracture Zone and Late Cretaceous to Mid Eocene oceanic crust in the equatorial Atlantic Ocean near 18°WAuthorsKevinGroweiDIngoGrevemeyeriDSatish ChandraSinghMilenaMarjanovicEmma PMGregoryCordPapenbergiDVenkata AbhishikthVaddineniLauraGómez de la PeñaiDZhikaiWangiDSee all authors Kevin GroweiDCorresponding Author• Submitting AuthorGEOMAR Helmholtz Centre for Ocean Research KieliDhttps://orcid.org/0000-0002-0822-3562view email addressThe email was not providedcopy email addressIngo GrevemeyeriDGEOMAR Helmholtz Centre for Ocean Research KieliDhttps://orcid.org/0000-0002-6807-604Xview email addressThe email was not providedcopy email addressSatish Chandra SinghInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressMilena MarjanovicInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressEmma PM GregoryInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressCord PapenbergiDGeomar, Kiel, GermanyiDhttps://orcid.org/0000-0001-8790-558Xview email addressThe email was not providedcopy email addressVenkata Abhishikth VaddineniInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressLaura Gómez de la PeñaiDGEOMAR Helmholtz Centre of Ocean ResearchiDhttps://orcid.org/0000-0001-7443-0503view email addressThe email was not providedcopy email addressZhikai WangiDInstitut De Physique Du Globe De ParisiDhttps://orcid.org/0000-0003-0852-2658view email addressThe email was not providedcopy email address
<p> Large-offset transform faults (TFs) in the Atlantic juxtapose hot spreading segments against older, colder oceanic lithosphere, leave permanent traces as fracture zones in ageing oceanic crust and represent a significant proportion of the plate boundary along the Mid-Atlantic Ridge (MAR). The manifestation of the thermal contrast and the structure and composition of TFs however, are not well understood. The Romanche TF, situated in the Equatorial Atlantic, offsets the MAR by ~950&#160;km, has a slip of ~1.7&#160;cm/yr, and divides the northern MAR from its equatorial and southern spreading systems. Close to the eastern ridge-transform intersection (RTI), shallowing of the seafloor from north to south across the TF reflects the change from old, cold African lithosphere to the warmer and younger South American plate close to the MAR axis, however the bathymetry and structures across the fault itself are complex. Over 100&#160;km distance, a large northern transverse ridge reaches depths of <1000&#160;m and contains a fossil transform trace, before steeply descending into a 45&#8209;km wide transform valley containing ~7000&#160;m&#8209;deep basins, which is bounded to the south by a further shallow structure reaching ~2500&#160;m&#8209;depth. Previous studies using seafloor sampling, seismic reflection and bathymetry data have suggested these features comprise a mix of uplifted magmatic crustal blocks and serpentinized mantle peridotites. However, these studies cannot effectively determine the sub&#8209;seafloor structure.</p><p>The ILAB-SPARC experiment in 2018 obtained an active-source wide-angle refraction profile across the eastern Romanche TF, consisting of twenty-eight ocean-bottom seismometers spaced at ~14 km. We present a P-wave velocity model produced by the inversion of seismic travel time picks which reveals variations in crustal structure from ~40 My lithosphere to the north to ~7 My lithosphere to the south. Within the TF, a ~15&#160;km-wide low-velocity anomaly extends from the top basement through to >10&#160;km below basement. A lack of Moho reflections suggests no abrupt crust/mantle boundary exists beneath the TF, likely indicating the presence of a deep column of fractured and sheared basalts, breccias and peridotites. Low mantle velocities suggest faulting and water penetration to depths of ~16 km, causing widespread and extensive serpentinization. The crust to the south of Romanche is relatively thin (~5&#160;km&#8209;thick) compared to north of Romanche (~6&#160;km&#8209;thick), and contains areas of high velocity indicative of a predominantly gabbroic crust. This may be attributed to the irregularity of the MAR segment as it approaches the RTI, as it jumps to the west in several non-transform discontinuities and exhibits seafloor fabric indicative of magma-starved, tectonic spreading with exhumation along detachment faults.</p><p>These results suggest the shearing and transtensional/transpressional forces present at large-offset transform faults result in mantle exhumation and form deep conduits for fluid circulation. At Romanche, these tectonic forces combined with the thermal contrast and magma-starved ridge axis, stretch and deform magmatic oceanic crust within the TF such that it is thin and patchy. This may suggest that crustal structure within transforms is linked to the fault offset, valley width, and the magma supply at the closest ridge segment.</p>
Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Journal of Geophysical Research - Solid Earth. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing an older version [v1]Go to new versionSeismic structure of the St. Paul Fracture Zone and Late Cretaceous to Mid Eocene oceanic crust in the equatorial Atlantic Ocean near 18°WAuthorsKevinGroweiDIngoGrevemeyeriDSatish ChandraSinghMilenaMarjanovicEmma PMGregoryCordPapenbergiDVenkata AbhishikthVaddineniLauraGómez de la PeñaiDZhikaiWangiDSee all authors Kevin GroweiDCorresponding Author• Submitting AuthorGEOMAR Helmholtz Centre for Ocean Research KieliDhttps://orcid.org/0000-0002-0822-3562view email addressThe email was not providedcopy email addressIngo GrevemeyeriDGEOMAR Helmholtz Centre for Ocean Research KieliDhttps://orcid.org/0000-0002-6807-604Xview email addressThe email was not providedcopy email addressSatish Chandra SinghInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressMilena MarjanovicInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressEmma PM GregoryInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressCord PapenbergiDGeomar, Kiel, GermanyiDhttps://orcid.org/0000-0001-8790-558Xview email addressThe email was not providedcopy email addressVenkata Abhishikth VaddineniInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressLaura Gómez de la PeñaiDGEOMAR Helmholtz Centre of Ocean ResearchiDhttps://orcid.org/0000-0001-7443-0503view email addressThe email was not providedcopy email addressZhikai WangiDInstitut De Physique Du Globe De ParisiDhttps://orcid.org/0000-0003-0852-2658view email addressThe email was not providedcopy email address
Abstract Heat flow across oceanic transform faults (TFs) and fracture zones (FZs) has rarely been studied in detail, despite these features representing distinct thermal boundaries within the oceanic lithosphere. Here, we present heat flow measurements across the St Paul fracture zone (SPFZ) in the equatorial Atlantic Ocean, from 48 Ma crust in the south to 71 Ma in the north. To the north of the FZ we find a basal heat flow of 63 mWm −2 , and to the south a basal heat flow of 79 mWm −2 , both in agreement with plate cooling models. However, within the SPFZ we find a heat flow of 83 mWm −2 , greater than the values of the adjacent crust and 10–15 mWm −2 higher than predicted from conductive cooling models, suggesting that the thermal structure of the FZ has been modified. Evidence from seismic and sub‐bottom profiler data indicate recent active deformation within the SPFZ, potentially driven by lithospheric flexure across the FZ or temporal changes in TF configuration. We propose that this deformation may enable fluid circulation and heat advection within the basement, creating the seafloor heat flow anomaly within the FZ. These findings suggest that FZs may remain important zones predisposed to host deformation and fluid flow in the oceanic lithosphere, despite not being active plate boundaries.
We presented results from a new seismic refraction experiment at the Semenov hydrothermal area at 13°30’ N on the western flank of Mid-Atlantic Ridge. The survey was carried out on Cruise JC254 on RRS James Cook in November 2023. Semenov is a typical ultramafic-hosted field consisting of five active and extinct hydrothermal sites (Semenov-1 to 5) associated with massive sulphide mounds (SMS), hosted on a 20-km-long oceanic core complex (OCC). The OCC offers an exceptional opportunity to observe deep-seated ultramafic rocks exposed on the seafloor by detachment faulting. Semenov field is an ideal location to investigate the link between OCC-related detachment and SMS deposit formation. Here, we aim to determine the Semenov OCC crustal structure, geometry, and extent of subseafloor SMS mineralisation. Our seismic refraction survey revealed a detailed 2-D P-wave velocity structure beneath Semenov. We target the Semenov-3 and Semenov-4 hydrothermal sites sitting at either side of the seabed termination of the detachment fault. We focused on profiles crossing the OCC in E-W and N-S directions, shot with two GI guns (250G and 105I cubic inch) every 30 m. The data were recorded by a network of 18 ocean bottom nodes (OBX) at 0.4 to 1 km spacing, showing clear first-arrival refractions from beneath the OCC and the hanging wall. We expect to define the seismic structure down to 2 km beneath the seabed. The derived velocity model could give information to the lithology beneath the Semenov OCC-related detachment and possibly driving source for the hydrothermal circulation. Lastly, we compare our result with another detachment-related hydrothermal system at TAG, where the OCC is thought to be at the initial stage and the hydrothermal system is basalt-hosted.
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