ESTIMATING SLIP RATES FOR THE LAKE MARY FAULT SYSTEM USING GEOMORPHIC RELATIONSHIPS AND VOLCANIC FLOWS

The Lake Mary Fault System (LMFS) is a 45km long normal fault system located in Flagstaff, Arizona. We used volcanic petrology, lava flow mapping, ⁴⁰Ar/³⁹Ar, and geomorphic mapping to determine fault activity and slip rates for different sections of the LMFS. The LMFS has segments that have been active for several million years and have a complex faulting history that has resulted in dense fracturing of bedrock, reactivation of older reverse and normal faults, much of which have little vertical offset. In addition, fault movement for different segments of the LMFS varies along strike. For example, the northcentral segment, called the Lake Mary fault appears to be an active, normal fault with near vertical dip. Its “active” designation is based on mapping, dating and correlation of basalt flows and clasts found high on the fault scarp. The clasts were analyzed using inductively coupled plasma-mass spectrometry (ICP-MS) for Rare Earth Elements (REEs). ICP-MS data infer 3 different rock types for the 12 samples. Multiple volcanic rock samples were found on the main Lake Mary fault scarp adjacent to Walnut Creek, some of which were over 40 meters above the correlative basaltic sheet flow on the down thrown side of the main fault. The southern portion of the LMFS had several mapped lava flows such as Tby that did not appear to be faulted, but likely filled in a preexisting graben. Using these relationships and K/Ar dates from another study, we determined that the active length of the fault system is shorter than previously estimated by approximately 10km. Clasts found on the scarp correlate to a Quaternary basaltic lava flow (we call Qbwc) along Walnut Canyon that was dated for this study using ⁴⁰Ar/³⁹Ar at 1.19Ma. Walnut Creek, a major tributary to the Little Colorado River, was already incised and was part of an intermediate canyon system that developed prior to the eruption and flow of Qbwc. This flow filled in an ancestral Walnut Canyon, was faulted and subsequently incised by Walnut Creek as it is today. We used the ⁴⁰Ar/³⁹Ar dates for the three faulted basaltic sheet flows to derive a range in slip rates for different sections of the LMFS. The youngest flow (Qbwc) yielded an age of 1.17ma while the oldest flow (Tob) was dated at 5.9Ma. Given their respective vertical offsets, slip rates ranged from 0.024to 0.036mm/yr.