Seismic structure of 15 Ma oceanic crust formed at an ultrafast spreading East Pacific Rise: Evidence for kilometer‐scale fracturing from dipping reflectors

Abstract

[1] A grid of seismic reflection data from 15 Ma Cocos Plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading ($200 mm/yr, full rate) East Pacific Rise (EPR). The data contain an unexpected quantity of bright reflectors throughout the crust, generally <5 km in length. Significant reflection dip angles ($10-30°) are seen only on the isochron profiles, and several reflectors have a confirmed isochron dip orientation. Certain upper crustal reflectors (UCRs) appear to form a single network with isochron dip. The most prominent example (apparent reflection coefficient $0.2) projects toward a $70 m disruption in the igneous seafloor, and waveform analysis demonstrates that the reflector is likely the product of a low-velocity zone on the order of tens of meters thick. Our observations suggest that the isochron-dipping UCRs represent small-slip, kilometer-scale faults/ fractures, and none are primarily related to the seismic 2/3 boundary or to the sheeted dike/gabbro transition. Lower crustal reflectors (LCRs) have no preferred orientation or regular pattern on the 30 km scale of this survey. Certain LCRs may be similar to the UCRs in origin, while others may originate from near-axis ductile flow, lithologic banding, off-axis magmatism, or some combination thereof. Overall, reflectors in this area are abundant and complex, consistent with previous off-axis studies, but at odds with observations of young crust (<5 Ma) near the EPR. This dichotomy may be resolved by a delay in reflector formation, through some combination of postaccretionary tectonics, magmatism and hydrothermal alteration.

Site 1256

“…Relief to the northeast is lower and less organized. In the northeastern sector of the GUAT-3B region, there is evidence for a basement thrust fault with a strike approximately orthogonal to the regional fabric (Wilson et al, 2003;Hallenborg et al, 2003). This feature dips gently to the northwest (~15°) and is clearly discernible to a depth of ~1.3 km on seismic Line EW9903-28 (Wilson et al, 2003), but the feature is less pronounced on seismic Line EW9903-27, indicating that the offset on the thrust decreases to the southwest.…”

“…F2, F4). Three regions were surveyed during the 1999 Maurice Ewing site survey cruise (Wilson et al, 2003) with Site C in the GAUTB-03 region selected as the preferred location for deep drilling ( Fig. F5) (Wilson, Teagle, Acton, et al, 2003).…”

“…F5B). Using the site survey multichannel seismic data (Wilson et al, 2003), we constructed a geological sketch map of the uppermost basement in the GUATB-03 survey region (Fig. F8).…”

“…Multichannel seismic reflection data ( Fig. F6) (Hallenborg et al, 2003) show several nearly horizontal reflections with kilometers of horizontal extent at nearly 5.5 s traveltime, or roughly 800-900 msb. For the upper ~800 m, the relatively lower velocities and horizontal reflection character suggest that flows constitute a substantial fraction of the uppermost crust at Site 1256.…”

“…Depth to axial low-velocity zone plotted against spreading rate (modified from Purdy et al, 1992, andCarbotte et al, 1997b Figure F6. Site survey MCS data (Hallenborg et al, 2003) Figure F7. One-dimensional velocity model based on inversion of refraction data.…”