New Multichannel seismic resutlts. Multichannel seismic [MCS] reflection profiles, acquired in Dixon Entrance by R/V EWING in 1994 as part of the ACCRETE project, imaged crustal reflectors and Moho across the fundamental pair of structural boundaries separating the Alexander, Wrangellia, and North America terranes. The eastern margin of the Alexander terrane features strong crustal reflectors, mostly dipping gently (ca. 12 degrees) WSW. Moho reflections within this zone are stronger and more continuous than those seen in the terranes to the east. A broad Moho arch, about 3 km high, 100 km wide, and striking NNE, is defined by two E-W profiles, 45 km apart. These two profiles are tied by a third, arch-parallel line. Minimum Moho two-way time is 8 seconds, corresponding to a Moho depth of approximately 26 km. It is likely that this crustal arching is the result of the Tertiary extension forming Queen Charlotte Basin. The overall strike of the arch is oblique to nearby terrane boundaries, to graben and half-graben structures previously mapped in Dixon Entrance and Hecate Strait, and to the trend of Queen Charlotte Basin, which lies to the south. The zone of thin crust defined by this Moho arch is apparently discontinuous with similar zones mapped in Hecate Strait and Queen Charlotte Basin, suggestiong their creation by oblique extension (transtension).

Figure 2. See discussion first abstract under "SNORCLE '97" button.

• FIGURE 2a -- Geologic map of portion of Central Gneiss Complex, after Hutchison (1982). The seismic section was made along the ship track. The north end of the line of cross section (Fig. 2b) corresponds to a point at end of arrow of "ship track".

 
• FIGURE 2b -- Crossection showing projection of surface geology to depth, and correspondance of some seismic features to the geology.

 
• FIGURE 3a -- P-wave velocity structure (km/sec) to the depth of about 12 km obtained by tomographic inversion of the first breaks picked from 32 receiver gathers. Before the inversion, corrections for shot statics, for crooked line geometry, and for non-surface consistent receiver statics were applied. Special measures were taken to ensure the consistency of the travel times at 292 reciprocal points.


 Vertical exaggeration is 3:1. Areas outside of dotted line correspond to the parts of the subsurface not penetrated by refracted waves.

 Zero km is at west end of Dundas island, at Reftek station 1. 186 km is at station 53, near Stewart, BC.

 Contact Igor Morozov (morozov@uwyo.edu) for more information.
 
 

• FIGURE 3b -- Horizontal scale same as for Fig. 3a.


 Top: Crustal section along the reversed part of the ACCRETE wide-angle line obtained using a combination of travel-time inversion techniques. The top 12 km are obtained using the time-field tomography (Fig. 3a), the middle and lower crust and the Moho imaged using iterative forward ray tracing. The locations of recording stations are marked along the top of the section.

The Moho between 30 - 130 km of the profile is densely and multiply sampled by PmP reflections. NE of km 130, the Moho profile is extended on the basis of comparison with MCS interpretation. Note a series of NE-dipping reflectors identified in the wide-angle data, and several prominent reflectors in the lower crust. Only reflectors imaged by several recording stations are shown.

Bottom: Comparison of the wide-angle reflectivity (top plot) to the most prominent reflectors picked from the MCS stack. The wide-angle model is converted to vertical travel times, MCS picks are shown in red. Note the correspondence of the picks, while the wide-angle data are able to resolve more detail in the upper- to middle crust. 
 

• FIGURE 4 -- Fence diagrams constructed from multichannel seismic reflection data (see ACCRETE figures 3a and b for locations) and, for a portion of line 1251, from wide angle data (Fig. 3b). Lines on diagrams are reflecting zones. M, Moho. See second abstract under "SNORCLE '97" button for discussion.

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