Structural and petrophysical characterization of the upper basement crustal section at ODP/IODP Site 1256 (East Pacific Ocean)

Abstract

[1] New ocean crust is constantly being formed from mid-ocean ridge axis. Voluminous flows of lava are emplaced also away from the ridge axis, adding off-axis crustal layers to the crustal pile. Research on deep sea is of great importance to better understand the mechanisms and the nature of the crust forming and evolution. In this work, we decipher the first stages of the postmagmatic evolution of an intact volcanic section from the upper oceanic crust at ocean drilling program/integrated ocean drilling program (ODP/IODP) Site 1256 (Eastern Pacific Ocean). Using for the first time an innovative core-log integration technique to match direct (core-related) and indirect (borehole-related) data by depth shifting and reorienting individual core pieces recovered by drilling, we are able to identify the clusterization of structures and physical properties within distinct downdeep ''strong'' and ''weak'' lava zones, reflecting the cooling and tectonic evolution of lavas rather than lithological variations. We define the evolution of the structural zones that typically affect lava flows: colonnades and entablature zones, studying an offaxis lava flow encountered in present-day upper ocean crust. For the same off-axis flow, we are also able to suggest the lava flow direction (NW-SE) and its relationships with the paleoridge axis. Despite the environmental difficulties in the study of the subseafloor under deep water and using only onedimensional data deriving from ocean drilling, this work shows how an array of diverse data can be integrated into a coherent interpretation of lava flow history obtaining detailed information on the mechanism of submarine lava emplacement and flow.

Submarine lava flow direction revealed by neutron diffraction analysis in mineral lattice orientation

“…The combination of neutron data with microstructural shape preferred orientation data indicates a NW‐SE lava flow direction. Such an orientation is compatible with the NW‐SE trend of the lava body deduced by the seismic velocity contour line >4.82 km s −1 for the lava field boundary [ Teagle et al ., ; Fontana and Tartarotti , ] that are, in turn, almost parallel to the orientation of magnetic anomaly pattern at Site 1256 (i.e., to the paleo‐ridge axis of EPR). The existence of a valley or negative topography may explain the preferred orientation of the lava field that is consistent with a model of an off‐axis eruption dammed by abyssal hill faults.…”

“…The presence of steep slopes influences the flow direction and the preferred orientations of the flowing particles, both in terms of lattice and shape preferred orientations. Moreover, the reconstructed fabric types profile in depth is consistent with the interpretation of a secondary injection of hot lava flow within the molten lava flow between 280 and 290 mbsf, as suggested by structural and geochemical studies [ Fontana and Tartarotti , ; Umino ; Wilson et al ., ].…”

“…This “lava field” according to Panseri et al . [] and this paper, displays distinctive structural features that support the hypothesis of an off‐axis origin [ Wilson et al ., ; Tartarotti and Crispini , ; Tartarotti et al ., ; Fontana and Tartarotti , ]. Seismic velocities in this area reach 5.5 km s −1 [ Wilson et al ., ; Swift et al ., ; Gilbert and Salisbury , ], e.g., they are relatively high for the uppermost basement of fast spreading ocean crust (upper Layer 2 velocities at Site 1256 are 4.5–5 km s −1 ) and consistently higher than those of the extrusive magma just below the lava field [ Crispini et al ., ].…”

“…[], the shape of the lava field has a NW‐SE elongation. A NW‐SE lava flow direction (i.e., almost parallel to the elongation direction) for Hole 1256D lava field has been postulated on the basis of postmagmatic structures orientation [ Fontana and Tartarotti , ].The area delineated by the 4.82 km s −1 contour line encompasses approximately 70 km 2 and the minimum inferred volume is approximately 3 km 3 (ranging from 3 to 10 km 3 according to Teagle et al . []) when assuming an average thickness of 40 m [ Teagle et al , ].…”

“…In the bottom part of the field, microstructural observations suggest that the lava flow was subjected to the turbulence that was likely controlled by the solid substrate [ Crispini et al ., ]. Moreover, Crystal Size Distribution (CSD) [ Umino , ] and geochemical analyses [ Wilson et al ., ], as well as studies of structural and physical properties patterns conducted by Fontana and Tartarotti [] suggest that lava with Enriched Mid‐Ocean Ridge Basalt (EMORB) composition was injected into the molten interior of the solidifying body between 280 and 290 mbsf (of the present day), changing the thermal state and varying the postmagmatic structural behavior and the physical property response of the lava body.…”