The Geometry of Accreted "Packets" in Subduction Zones; Examples from the Eastern Belt of the Franciscan in California and the Torlesse Terrane in New Zealand
Thomas C. MacKinnon, 20 Tara Road, Orinda, CA 94563
Accretionary complexes contain rocks with a variety of structural styles ranging from melange to relatively intact beds. This study focuses on some lesser deformed accreted rocks in areas with exceptional exposures: glaciated rocks of the Torlesse near Arthur's Pass, New Zealand, and stream-cut exposures of the Franciscan Eastern Belt in Grindstone and Thomes Creeks, California. The structural style in both areas consists of relatively intact, mappable "packets" of rock separated by faults. This structural style appears to be common in accretionary complexes but the geometry of the packets can only be documented with confidence where exposures are nearly 100%.
In both areas studied, rocks are mainly turbidites of prehnite-pumpellyite to lower blueschist grade, described as "broken formation" with no melange or exotic blocks present. The dominant structural features are steeply dipping beds, cut by thrust faults typically oriented at a modest angle to bedding. Fault spacing ranges from ~100 to 600 meters with the angle between bedding and faults usually ranging from 0° to 35°. Between faults, bedding continuity/stratigraphy is generally well-preserved. However, local disruption and folding, including isoclinal folds, may be present.
Deformation associated with the fault traces is variable. In some cases fault contacts are sharp and there is little deformation of adjacent beds. In other cases, small folds, boudinage, and intense fracturing and veining define fault zones. Intensity of deformation varies along the fault plane and is commonly more intense on one side than the other.
The faults described above represent primary surfaces along which "packets" of relatively intact rocks were accreted. This study and others show that these fault-bounded packets can be traced along strike for a few km to at least 10 km or more where not cut by younger faults. The faults appear to form after a period of diffuse, largely extensional shear in semi-consolidated sediments; as rigidity increases, shear becomes localized along fault surfaces.
Deformation associated with the faults is roughly an order of magnitude less than that described in the literature for "megathrusts." It is therefore inferred that the megathrust boundary driving the accretionary process is located some distance structurally below the accreted packets and is not preserved in the areas studied.