User:Shearzone6

Eastern California Shear Zone

The Eastern California Shear Zone (ECSZ) is a collection of strike-slip faults containing both the Mojave Desert Block and the Death Valley region; this area is considered a right shear region. This zone 'accounts for...9-14% of the total shear...along the Pacific North American transform boundary since ~ 10.6 Ma'(4). However, the entire area east of the San Andreas Fault accounts for one fourth of the relative motion between the Pacific and North American plates (1). This shear zone begins near the San Gorgonio Pass and has a 1000km spread Northward. This area is effected by the North Frontal fault system and the San Andreas Fault which results in polyphase deformation. (5)

Mojave Desert Block Studies conclude that this region experienced two major earthquakes during the Cenozoic era. This region has also rotated 30-50 degrees in a right axial direction due to the strain between the strike-slip and Northwestern striking faults in this area; the net rotation for this area is approximately 11-15 degrees of offset. Faults of the Mojave desert are approaching the Garlock Fault and produces patterns irregular of simple shear faulting areas. The general right shearing movement of the area is believed to have occurred 6-10 Ma. All major faults striking northwest in the Mojave Desert Block exhibit discontinuity in a northern central area of 'an irregular line extending eastward from near Barstow to near Ludlow and continuing north-northeast to Soda Lake' (4). Polyphase deformation likely occurred in this area, including: denudation and relative uplift. Marble, schist, and granite can be found both along the Mojave Desert Block, and nearly 20 km away in the Ord Mountain District, which suggests uplifting in the region.

ECSZ and the North Frontal fault system NFTS spans 80 km in length and has an estimated vertical offset of about 1.6 km. However, the uplift rate has been less than .5 mm/yr over the past few million years, suggesting greatly reduced activity. Although activity has been reduced, this area is still considered active because it experienced disturbance during the Holocene epoch. The ECSZ and NFTS interact towards the East and are interrupted by dextral faults. These faults could potentially effect these two zones, but they haven't caused any disruption yet. James A. Spotila and Kevin B. Anderson, co-authors of the article 'Fault Interaction at the junction of the Transverse Ranges and Eastern California shear zone: a case study of intersecting faults,' believe that the triple junction between these two faults has relocated to the NFTS and Helendale fault. Under this assumption, these faults are moving westward and were previously more towards the east. Dokka and Travis support this conclusion with their proposition that 'deformation within the ECSZ has migrated westward since the middle Pleistocene [epoch].'(5) Strike-slip faults to the south of the NFTS with Northern orientations may have been formed initially, when the eastern segment of the fault system was active. (5)

Offsetting of the Moho Part of what may account for the motion patterns in this area may have to do with the assumed offsetting of the Moho under the San Andreas Fault and the ECSZ. When earthquakes occur in this area, they usually do not exceed 15 km due to brittle deformation. Under the Mojave Desert, the Moho converts P waves to S waves; the amount of time it takes to convert to an S wave can be measured and tells us about how far the waves traveled before the change where the Moho is present. The P-S wave conversion is fairly shallow, while it deepens to 35 km as it spans into the San Andreas Fault. As you head North, the conversion happens more quickly in the San Gabriel Mountains (25 km). This disruption is possibly caused by the movement of the San Andreas Fault and the ECSZ right-lateral strike-slip movement. However, horizontal compression may have vertically stretched the crust or folded it upward in some areas. The orientation of the strike-slip faults in the ECSZ are parallel to the relative motion of the Pacific and North American plates, which makes this area very prone to earthquakes with a slip rate of 8-12 mm/yr. That requires 20% of the relative motion in the area and suggests that 'the ECSZ might be a newly developing branch of the Pacific-North American plate boundary'(3). During the Tertiary period this boundary shifted Eastward, so it is probable that the boundary is entering the ECSZ.(3)

1 McClusky, S.C., Bjornstad, S.C., Hager, B.H., King, R.W., Meade, B.J., Miller, M.M., Monastero, F.C., Souter, B.J. (2001). Present Day Kinematics of the Eastern California Shear Zone from a Geodetically Constrained Block Model 28, pg. 3369-3372. Geophysical Research Letters. Retrieved April 17, 2011 from http://topex.ucsd.edu/sio239/10_11_05/McClusky.pdf. 2 Jones, Laura E., Helmberger, Donald V. (1998). Earthquake source parameters and fault kinematics in the Eastern California Shear Zone 1, pg. 1-44. Retrieved April 17, 2011 from http://arxiv.org/PS_cache/physics/pdf/9806/9806031v1.pdf. 3 Zhu, Lupei. (2000). Crustal Strength across the San Andreas Fault, southern California from teleseismic converted waves, 179, pg. 183-190. Earth and Planetary Science Letters. Retrieved April 17, 2011 from http://www.colby.edu/personal/w/wasulliv/GE331%20Papers/Strike-slip%20tectonics/Zhu,%202000.pdf. 4 Dokka, Roy K., Travis, Christopher, J. (1990). Late Cenozoic Strike-Slip Faulting in the Mojave Desert, California, 9, pg. 311-340. Tectonics. Retrieved April 17, 2011 from http://www.mojavewater.org/Reports/RegionalandLocalizedStudies/RegionalandLocalizedDocuments/LateCenozoicStrike-SlipFaultingintheMojaveDesertCA.pdf. 5 Spotila, James A., Anderson, Kevin B. (2004). Fault Interaction at the Junction of the Transverse Ranges and Eastern California Shear Zone: a case study of intersecting faults, 379, pg. 43-60. Tectonophysics. Retrieved April 17, 2011 from http://www.geo.tu-freiberg.de/tektono/downloadfiles/Spotila%20and%20Abderson,%20paleoseismology%20Transverse%20Ranges%20Tectonophysics,%20379,%2043-60,%202004.pdf.