Scientific Drilling

Donna Blackman,Gilles Guerin,Alistair Harding,Andrew Mccaig,Angela Slagle

doi:10.2204/iodp.sd.15.04.2013

Abstract

Abstract. Integrated Ocean Drilling Program (IODP) Expedition 340T returned to the 1.4-km-deep Hole U1309D at Atlantis Massif to carry out borehole logging including vertical seismic profiling (VSP). Seismic, resistivity, and temperature logs were obtained throughout the geologic section in the footwall of this oceanic core complex. Reliable downhole temperature measurements throughout and the first seismic coverage of the 800–1400 meters below seafloor (mbsf) portion of the section were obtained. Distinct changes in velocity, resistivity, and magnetic susceptibility characterize the boundaries of altered, olivine-rich troctolite intervals within the otherwise dominantly gabbroic se-quence. Some narrow fault zones also are associated with downhole resistivity or velocity excursions. Small deviations in temperature were measured in borehole fluid adjacent to known faults at 750 mbsf and 1100 mbsf. This suggests that flow of seawater remains active along these zones of faulting and rock alteration. Vertical seismic profile station coverage at zero offset now extends the full length of the hole, including the uppermost 150 mbsf, where detachment processes are expected to have left their strongest imprint. Analysis of wallrock properties, together with alteration and structural characteristics of the cores from Site U1309, highlights the likely interplay between lithology, structure, lithospheric hydration, and core complex evolution. doi: 10.2204/iodp.sd.15.04.2013

Highlights

Integrated Ocean Drilling Program (IODP) Expedition 340T returned to the 1.4-km-deep Hole U1309D at Atlantis Massif to carry out borehole logging including vertical seismic profiling (VSP)
Hole U1309D was found to be in good condition, and an initial, minimal-disturbance borehole fluid temperature measurement was achieved on the first logging run into the borehole
Electrical resistivity was measured with the High-Resolution Laterolog Array (HRLA) during Exp. 340T, a newer resistivity tool than that used during Exp. 304/305

Summary

Geologic Setting and Motivation

The Atlantis Massif oceanic core complex (OCC) was drilled during IODP Expedition 304/305 (Blackman et al, 2006). It formed due to spatial and temporal variability in magmatism and faulting within the axial zone of the slow-spreading Mid-Atlantic Ridge at 30°N (Cann et al, 1997). Long-lived strain localization along select axial faults allowed development of a detachment zone that unroofed intrusive crust 0.5–1.5 Ma (Grimes et al, 2008). High fracture intensity in the detachment zone enabled circulation of seawater (McCaig et al, 2010), and associated alteration included formation of low-strength minerals that enhanced strain localization. Geophysical studies (Blackman et al, 2008; Canales et al, 2008; Collins et al, 2009; Blackman and Collins, 2010; Henig et al, 2012), seafloor mapping and sampling (Blackman et al, 2002; Schroeder and John, 2004; Boschi et al, 2006; Karson et al, 2006), and deep basement drilling and logging (Blackman et al, 2011; Ildefonse et al., A4 Mid-Atlantic Ri 4d000ge (MAR)

Logging Operations

Highlighted Initial Results

Stoneley waveforms