Strain Variation between the Monterey and Sisquoc Formations, Southern Santa Maria Basin, California, USA: Implications for Structural Assessment of Fold and Thrust Belts

Analysis of variation of fold strain at map-scale and outcrop-scale of the siliceous Monterey and Sisquoc formations in the southern Santa Maria basin, California provides insight into limitations of strain determination by construction of area-conservative balanced cross-sections. Diagenetic modification of these rocks allows strain quantification of rock intervals with high competence contrasts. Detailed strain analysis at map-scale shows significant variation in fold strain between rock types with shortening values ranging from 5.5% to 21.1%. Apparent shortening in the competent Monterey Formation is twice as high as in the overlying highly porous Sisquoc Formation. The large difference in apparent shortening suggests that the same amount of actual strain was accommodated by folding in the Monterey and horizontal compaction in the Sisquoc Formation, since there is no evidence of a detachment fault or major unconformity between the units. Strain analysis at outcrop-scale provides insight into how both units express such different shortening ratios without having an unconformity, or detachment fault between them