Active Tectonics Impact on Society (1986) / Chapter Skim
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4. Active Faults Related to Folding
4. Active Faults Related to Folding
Pages 63-79
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From page 63... ...
These faults are upthrown toward synclinal axes and die out in fold hinges. Bending-moment faults are produced because the convex side of a folded layer is lengthened normal to the fold axis and placed in tension, forming normal faults and extension fractures, whereas the concave side is shortened and placed in compression, forming reverse faults.
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From page 64... ...
Because fold-and-thrust belts terminate downward at a basal decollement over an undeformed rigid basement, the question of their seismotectonic signature is an important one. Where the decollement contains rocks of such low strength that deformation may occur plastically under low confining pressure, as in the Zagros Mountains and the Pakistan Salt Range, internal deformation is probably not accompanied by large earthquakes (Berberian, 1981; Seeber et al., 1981; Seeber, 1983)
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From page 65... ...
Transverse Ranges, California Flexural-slip faulting is also documented in the Ventura Basin, California, northeast of Santa Paula, where Pliocene-Pleistocene strata are strongly folded and locally overturned on the north flank of the Santa Clara syncline and are overlain by alluvial fans derived from mountains to the north (Figure 4.3) (Keller et al., 1982; Rockwell, 1983)
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From page 66... ...
' ' d. °~_ e FIGURE 4.3 Diagrammatic cross section across north flank of Santa Clara syncline east of Santa Paula, Ventura Basin, California.
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From page 67... ...
. The Rotokohu and Rough Creek Faults were formed during this earthquake by slip parallel to bedding on the north flank of a syncline in folded Neogene strata, producing scarps in late Pleistocene outwash gravels that overlie the folded strata unconformably (Lensen and Otway, 1971~.
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From page 68... ...
The flexural-slip fault scarps northeast of Santa Paula, California, face up the depositional slope of the fans that they cut (Figure 4.3) , but the high initial slope of the fan surface and the highvolume stream flow under the occasional torrential rainfall experienced in southern California result in gullying of the newly formed scarps, even if they do form suddenly.
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From page 69... ...
Bending-moment faults are also found at Toppenish Ridge, an east-trending anticline in Miocene basalts and Quaternary sediments in south-central Washington State (Campbell and Bentley, 1981; Figure4.9~. On the north side of Toppenish Ridge, surface ruptures near the hinge of the overturned Satus Peak anticline in basalt cut late Quaternary alluvium and landslide material.
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From page 70... ...
In both trenches, fractures are concentrated where strata are flexed. Most fractures are normal faults or soil-filled extension fractures, as would be expected if fractures are related to bending moment.
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From page 71... ...
Camarillo Fault is presumed to control steep south flank of this ridge. D, Sketch of presumed relation between bending-moment faults in trenches and subjacent Camarillo Fault.
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From page 72... ...
Near Ventura, California, the Montalvo Mounds are late Quaternary anticlinal ridges that mark the surface expression of the Oak Ridge high-angle reverse fault that does not itself reach the surface (Yeats et al., 1981~. The May 1983 earthquake at Coalinga, California (Ms = 6.5)
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From page 73... ...
Suppe and Witthe, 1977; Suppe and Namson, 1979; Suppe, 1980a,b, 1981; Namson, 1982~. The active-tectonic setting of fold-and-thrust belts is illustrated in this paper by a discussion of the active foreland thrust belt of northern Pakistan and India, part of the Himalayan convergence zone between the Indian and Eurasian plates, and the central Ventura Basin of California, where the convergence zone is limited in length, apparently controlled by the big bend of the San Andreas Fault.
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From page 74... ...
YEATS S TRUNK STREAM SHIELD DERIVED PARALLEL TO DERIVED ALLUVIAL MOUNTAIN FRONT ALLUVIAL ~_* SANDS FAR-DERIVED SANDS DECOLLEMENT ~- : : = THRUST SHEETS N MIGRATES SOUTH AT 60-78 MM/YR - I I I time lines 40 - 65 MM / YR ~ CONVERGENCE INDIAN SHIELD DEPOCENTER AND FAC IES BOUNDARIES MIGRATE SOUTH AT S AGE OF BASAL MOLASSE SEDIMENTS DECREASES SOUTHWARD AT 10 - 15 MM/YR (LYON-CAEN AND MOLNAR, 1985)
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From page 75... ...
The limbs may still be steepening, but an alternative way to accommodate contraction may be the Ventura Fault (Figure 4.7~. The south-facing fault scarp occurs at the sharp boundary between flat-lying strata of the Santa Clara syncline and south-dipping strata of the south flank of the Ventura Avenue anticline.
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From page 76... ...
that cut late Quaternary deposits may be used to monitor the recurrence of stick-slip faulting on subjacent seismogenic faults that may not cut late Quaternary deposits. The relations between coseismic flexural-slip and bending-moment faults to the seismogenic E1 Asnam thrust are instructive to this point.
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From page 77... ...
Late Quaternary deformation of the Toppenish Ridge uplift in south-central Washington, Geology 9, 519-524. Chapple, W
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From page 78... ...
Tectonic landforms of several active faults in the western Nepal Himalayas, J Nepal Geol.
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From page 79... ...
Triggered reverse fault and earthquake due to crustal unloading, northwest Transverse Ranges, California, Geology 11, 287-291. Yoshida, M., and Y
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