Paper No. 16-5
Presentation Time: 9:45 AM
PALEOMAGNETIC DATA FROM THE MIOCENE TUFF OF HOOVER DAM STRONGLY SUPPORT ERNIE ANDERSON'S ARGUMENT HE PRESENTED TO ME IN MY CSM OFFICE IN EARLY 1981 AS A VERY WET BEHIND THE EARS NEW MEMBER OF THE PROFESSORIATE
In early spring, 1981, Ernie Anderson knocked on my office door, in the Department of Geological Engineering, at CSM, my second semester on the faculty there. It was the first time that I had met Ernie; he mentioned that he went to my talk at the Colorado Scientific Society and that he had a quandary he was wrestling with. He was working with Jacques Angelier and Bernard Colletta on a fault-slip inversion study in the Hoover Dam area (published in the GSA Bulletin in 1985) and he was concerned about whether part of the study area, west or east of the dam, had experienced a counterclockwise or clockwise, respectively, rotation. He asked if it would be ok if he collected a suite of oriented samples for a pilot study, to see if more detailed work would be warranted. How is that for excitement, coming from a kind and passionate soul! So, here is an excerpt from abstracts of the work Ernie stimulated: “At and near Hoover Dam, late Cenozoic left-slip offset along the NE-SW trending Lake Mead fault system (LMFS) has resulted in the apparent rotation of structures and formation contacts and total displacement of up to 65 km. The mid-Miocene Tuff of Hoover Dam (THD), well-exposed on both sides of the Colorado River and sampled at over 90 sites, yields a well-grouped characteristic remanent magnetization. About 5 km south and east of the dam, gently east-dipping, north-striking THD rocks yield a magnetization with corrected direction D=324.8o,I=+27.4o (a95=10.7o, k=24, N=9 sites). The anomalous shallow inclination for the ChRM of the THD suggests that the ash-flow tuff was emplaced over a short period of time. Contact and conglomerate tests show that the THD ChRM is primary. Structurally corrected data from sites north and west of the dam (D=289.7o, I=30.2o,a95=8.6o,k=32, N=10) are interpreted to indicate, overall, about 35o of CCW rotation (R= -35.1o, delR= 12.4o, F= -2.8o, delF = 10.8o), relative to data from south of the dam, of crust across the dam site, as suggested by changes in strike of tilted fault blocks. The transition from unrotated crust to rotated crust is over a narrow zone, where blocks of THD and older strata have been tilted up to 50o, probably during rotation.” I remain very fortunate to have known and worked with Ernie and am very grateful and appreciative of his kindness and friendship, both to me and the many students who interacted with him.