Open-file Report - 527
Additional analyses and mapping of the Hubbell Spring and other intrabasin faults south of Albuquerque, New Mexico

The Hubbell Spring fault system (HSFS) is an active, normal fault system near the eastern margin of the Albuquerque-Belen basin in the central Rio Grande rift. Previous mapping suggested that the HSFS includes 3-4 major splays. Trenching studies indicated that multiple late Quaternary events had large displacements ranging from 1.7 to 4.7 m, implying longer rupture lengths than previously mapped (∼43 km). We conducted additional mapping to the south on the southern Llano de Manzano surface and found that several previously unnamed faults connect with the HSFS, resulting in a ≈ 74-km long fault system. We also identified numerous previously unmapped late Quaternary fault splays, some of which are tens of kilometers long and have scarps over 10 m high. Almost all of the splays show down-to-the west displacement except a couple of prominent faults to the south near Abo Arroyo. Based on our mapping, the HSFS is characterized by an unusually broad deformation zone (as wide as 18 km, but typically 10-14 km) of subparallel, anastomozing and branching normal faults that strike north-south overall, typically dip west, and offset late Quaternary eolian and piedmont deposits down to the west. The geometry of fault traces and the along-strike displacement variation patterns suggest complex patterns of slip transfer between fault splays of the HSFS, with northwest- and northeast-striking sections serving as relay ramps between principal north-south striking faults. Additional study is needed to better quantify these deformation patterns.

Unlike typical normal faults, the HSFS generally lacks antithetic faults or backtilting in the hanging wall. Rather, many splays are characterized by backtilt to the east of the footwall resulting in closed depressions developing on the upthrown side of the scarp. We measured two long topographic profile transects across the HSFS. Backtilting in the footwall results in large uncertainties for displacement estimates. Vertical surface displacements across individual scarps range from 0 to 31 meters. The cumulative vertical surface displacement is about 28-54 m and 41-83 m across the northern and southern transects, respectively. Assuming an age of 80-130 ka for the faulted surface, we obtain cumulative average vertical slip rates of 0.2-0.7 mm/yr for the northern transect and 0.3 to 1.0 mm/yr for the southern transect, making the HSFS the longest, most active fault system near Albuquerque. These slip rates are much higher than previous estimates due to the newly discovered fault traces. Several other fault systems in the Rio Grande rift show similar distributed, anastomozing geometries, but the footwall backtilting is unusual and requires further investigation.

As a minor component of this study we also analyzed three additional luminescence samples (CHSF02-2, CHSF02-9, and CHSF02-11) previously collected from the Carrizo Spring trench on splay L, or the central HSFS (Olig et al., 2004). Results of the additional analyses are consistent with the previous results from 8 samples and help substantiate previous age estimates for the youngest and oldest faulting events, which respectively occurred shortly before 5 to 6 ka, and between 65 and 84 ka. These additional age analyses also help substantiate the average late Quaternary slip rate estimate of 0.09 ± 0.01 mm/yr for splay L of the HSFS (Olig et al., 2004).

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