2018-04-02_REVISION - M1977342 - Laserfiche WebLink

Home Browse Search

This zone of faults forms a northwest-trending graben on the northeast flank of Greenhorn Mountain.The main fault forms a 25-m-high northeast-facing escarpment (Kirkham and Rogers,1981),but associated scarps on Quaternary deposits are typically less than 3 in high(Unruh et A,1993}However,Unruh et al.suggested that the scarps and other associated geomorphic features(saddles and depressions) developed in response to salt tectonism(migration and dissolution of evaporate deposits in the area)rather than fault slip related to earthquakes.On this basis, 0.006(02) Widmann(1997a)classified the Greenhorn Mountain fault as Class B(possibly non- Greenhorn 6.5(0.2) 0.02(0.6) Kirkham and Rogers(1981); seismogenic)and we assigned a lower probability of activity. Little is known about Mountain fault 6.8(0.6) 35°E(02) 0.1(0.2) Unruh et al.(1993);Widmann et possible rates of activity and our slip rates assume: (1)3 in of slip occurred since 130 (Q462297) 7.1(02) 50°E(0.6) WM:0.03 al.(1999,2002);Widmann ka(preferred rate);(2)3 in of slip occurred since 30 ka(maximum rate);and(3)3 in Independent 1.0 0.6 1 21 N 1 65° 02 1997a of slip occurred since 500 ka minimum rate). Recent detailed mapping(McCalpin et al.,20112a;Bohannon and Ruleman,2013)is consistent with our previous reconnaissance study(Wong et al.,2007),in that they found no evidence of post-Piinedale(younger than 40 ka)movement on the Mosquito fault,but did find some evidence for older Quaternary faulting(such as linear alignment of springs,scarps in colluvium and prominent bedrock scarps),although ages remain unconstrained. Apparent influence of pre-existing structures and multiple periods of deformation has resulted in complex geometries that have resulted indifferent interpretations as to what constitutes the Quaternary-active portion of the Mosquito fault. To address this uncertainty we include three rupture scenarios in this Bohannon and Ruleman(2013); study. Rupture scenario A includes the entire fault as mapped in our previous 6.9(0.2) McCalpin et al.(1012a,2012b); investigation(Plate 1 of Wong et al.,2007)and is the same as our previous longer Rupture Scenario A 62 72(0.6) Wong et al.(2007);Bergendshl rupture scenario.Rupture Scenario B is the same as our previous shorter scenario (025) 7.5(0.2) (1963,1%9);Tweto(1973); (Wong et al.,2007)and stops near the small step-over at Copper Mountain,excluding Tweto and Reed(1973);Tweto the portion to the north that shows reverse topography and generally appears to be 0.01(02) et al.(1978);Kirkham and limited to offsetting Precambrian rocks against Precambrian rocks(Bergendshl,1969; 6.8(0.2) 0.03(0.6) Rogers(1981);Widmann et al. Bergendah and Koschmann,1971;Widmann et al.,2004}Rupture Scenario C has a 42 7.1(0.6) 60°W(02) 0.17(02) (1998,2004);Smith and similar northern end to scenario B,but assumes the southern portion(south of Empire Mosquito fault Rupture Scenario B(03) 7.4(0.2) 75°W(0.6) WM:0.05 Utterback(1977),Wallace et al. Gulch)merges with the Union fault after Bohann and Ruleman(2013),which is also 522303 1.0 N/SS I 90° 02 1968 supported 6.8(0.2) Rupture Scenario C 7.1(0.6) 0.45 47 7.4(0.2) by the mapping of McCalpin et al.(2012b;see also comments below on Northeastern Boundary fault system). Our preferred slip rate is based on 4.0 f 1.2 in of net vertical displacement on a moraine in Iowa Gulch that is Bull Lake age or older(k130 to 160 ka)(Wong et al.,2007). This appears generally consistent with new studies where Bohannon and Ruleman(2013)measured 1.8 in of surface offset across one of two scarps in colluvium near the intersection of the Mosquito and Union faults(south of Iowa Gulch). They interpret the scarp as fault-related,but the age of displacement is unconstrained. Assuming it occurred since 40 ka yields a maximum rate of about 0.05 mm/yr,whereas assuming it occurred since 300 ka yields a minimum rate of—0.006 mm/yr.In contrast,looking longer term,Wallace et al.(1968)estimated 8500 to 9500 feet of dip slip on the Ceresco Ore Body,and Smith and Utterback(1977)estimated 3000 feet of left-lateral slip on a sericitized porphyry dike in the Climax Pit Based on these data,we assumed 3037 in of net slip occurred since 18 Ma for our maximum rate.Our minimum rate assumes 2.9 in ofvertical slip occurred since 300 ka(using the minimum slip and a pre-Bull Lake age for the moraine at Iowa Gulch}The structural setting,mine exposures,and linear fault trace all support steeper dips.