Results of SCS Line No.10 and 11 (Southwest Oahu volcanic field)

                    　　 Eiichi Takahashi

The SCS Line-10 has been carried out in order to solve the following two problems; 1) structure and origin of the low-shield like (or dome like) topography on the northeast side of the SW Oahu volcanic field、and 2) structure and origin of the circular collapsed feature on its NE flank. The SCS line started from a mid slope bench on the Waianae slump and traversed to nearly the top of the SW Oahu volcanic filed. The location of the Line-10 was chosen to cross the center of the circular collapsed region on the NE flank.

   Due to a mechanical failure of the compressor, the shooting interval of the air gun was changed from 20 to 30 seconds around the middle of the Line-11.  Structure of the sediments in the upper 200-300 m are imaged throughout the SCS Line-11.

Collapsed Structure

   The cross section of the collapsed structure indicates that the tectonic block fault motion postdates the youngest sediments.  It is possible that the structure is still tectonically active.  The sediments inside the collapsed structure are block faulted perhaps due to the extension by dome uplift.

Sediment strata across the NE flank

   Near the base of the sediment strata, two parallel strong reflections are present. These reflectors can be traced all the way from the top of the SW Oahu volcanic field to the topographic low adjacent to the Waianae slump. Although the thickness of the sediments between the two is virtually identical throughout SCS Line-11, thickness of the sediments on top of them varies significantly, with the thickest sediment closest to the Waianae slump.   

   The wedge shaped sediments on top of the two reflectors appear to be young sediments supplied from the Waianae slope. The maximum thickness just off Waianae slump is 150 m but it decreases to less than 50 m near the top of the SW Oahu volcanic field.  The change in thickness indicates that the upper sediments were deposited after the dome (or shield like bulge) was formed.

   If the sediment comprising the two reflectors was deposited after the dome was formed, then they should thin near the summit of the dome. The constant thickness of the strata the interpretation that the dome was uplifted after deposition of these continuous sediment layers, perhaps the dome uplift was related to emplacement of the SW Oahu volcanic field. An alternate hypothesis that the dome was formed from voluminous debris avalanche deposits caused by Waianae slump (ca.3 Ma, Pressley et al., 1998) is inconsistent with the apparent young age of the dome uplift. 

Presley, T., J. Sinton, N, Pringle, 1998, Post-shield volcanism and catastrophic mass wasting of the Waianae Volcano, Oahu, Hawaii: Bull. Vol., v. 58, p. 597-616.