Arizona Geological Society Digest, Volume XII, 1980 63 Tectonic Geomorphology of the Luke Air Force Range, Arizona by William C. Tucker, Jr.1 Abstract In arid and semiarid regions the relative rates of tectonic activity and the approximate age of the last major tectonic pulse can be ascertained by analyzing the landforms of the mountain fronts and their adjacent piedmonts. Tectonic activity triggers a change in the base level of fluvial systems that causes the development of a characteristic suite of land­ forms. It is these landform characteristics that are examined in making a tectonic geomor­ phic analysis of an area. The Luke Air Force Range, an Air Force missile and gunnery training range in south­ western Arizona, is an area of typical basin-and-range topography of generally linear mountain ranges separated by broad, flat valleys. While overall the ranges are quite linear, the mountain fronts are, for the most part, highly sinuous, pedimented, and lacking a thick accumulation of recent alluvial material on the piedmont. A tectonic geomorphic analysis of the Luke Air Force Range shows that the area has been almost entirely free of tectonic activity for at least the Holocene and Pleistocene epochs and probably for part of the Pliocene. A few mountain-front segments show signs of slight tec­ tonic activity during the Pleistocene. One mountain- front segment displays landforms indica­ tive of tectonism continuing into the Holocene. Introduction The major portion of the basin-and- range tectonism can be dated with fair reliability us­ Southwestern Arizona has had a long history ing the volcanic rocks and their relationship to of tectonic and igneous activity . Large bodies the faulting as a guide. Volcanic rocks that of granit e were intruded during the late Meso­ are clearly pre- faulting, being either ruptured zoic which metamorphosed the preexisting sedi­ by the basin-and-range faults or uplifted to mentary and metamorphic rocks. In the early form mountain ranges, or both, fall generally and middle Tertiary violent volcanic eruptions into the 12-to-15- m. y. -age range (Eberly and poured out small, steep-sided flows of andesite Stanley, 1978). The volcanic rocks that are and latite and blasted pyroclastic materials onto not uplifted and show no apparent evidence of the adjacent landscape. A quieter phase of faulting have ages from 3 to 1.75 m. y. in the basaltic volcanism followed and covered a wider Sentinel basalt flows (Eberly and Stanley, 1978) area than the previous activity . Middle to late and from 1. 0 m. y . to very recent in the Pina­ Tertiary high-angle normal faulting broke cate volcanic field (Lynch, pers. comm., 1979) . southwestern Arizona into horsts and grabens, This brackets the major, widespread, basin­ which resulted in linear, northwest-trending and-range faulting fairly well as being within mountain ranges separated by broad, down­ the 3-to-15-m. y. age range. However, the in­ faulted basins. There were some minor basalt dicators for the cessation of regional faulting flows along some of these faults, apparently do not eXclude the possibility that localized toward the end of the period of basin- and­ faulting could have occurred later t han approx­ range faulting. Additional basalt flows were imately 3 m. y. B. P . poured out during the late Tertiary and early Quaternary. These flows were widely sepa­ The area of the Luke Air Force Range, rated and apparently unrelated by either loca­ which is the area being considered in this tion or orientat ion to the basin- and- range tectonic geomorphic analysis, has had five faulting. seismic events greater than magnitude 4 re­ corded in recent times. The dates, locations, and magnitudes are shown on Figure 1. The lSouthwestern Exploration Associates, sources of these recent events and the exis­ Tucson , Arizona 85710 . tence of surface ruptures are presently un- 64 William C. Tucker, Jr. , 10 2'0 Kilometers \ ~ ~ 0 ? \ rt:)r~ ;il "\ (0,1/: ~~: Fig. 1. Recent seismic events within the Luke Air Force Range. Year and magnitude of the event are given at each approximate epicenter location. -- Source: Bureau of Reclamation (1976). known, but their occurrence, together with assess the relative tectonic activity of mountain the proximity of known active tectonism along fronts or other structural elements during the the nearby San Andreas fault-Salton trough­ Quaternary. In particular, it evaluates the Gulf of California rifting, provides evidence areal variations in relative uplift of mountain for the possibility of tectonic activity in the fronts within the Luke Air Force Range portion area. of southwestern Arizona and applies this eva1u­ ation of late Cenozoic tectonism of the area to Many variables act together to form arid determine its effect on the distribution and fluvial systems. Tectonic activity is the trig­ nature of fluvial sediments. The analysis ger that sets off the independent variable of covers the entire 13,500 km 2 (5,200 mi 2 ) area tectonic base-level change. If tectonic activity of the Luke Air Force Range. causes a stream's base level to fall along part of its reach, the stream will erode more aggres­ Evaluation of past tectonic activity has two sively upstream from the pOint or points of important applications: seismic and sedimen­ ground rupture, will create fresh deposits of tologic. Tectonic geomorphic analysis has the coarse clastics downstream from that point, and potential for indicating which faults within a will transport these coarser clastics a longer given study area have been active, including distance downstream than would otherwise be faults that have not been mapped by previous possible with all other independent variables workers. being the same. The presence of tectonic ac­ tivity has partly controlled sedimentation, both Rate of Tectonic Activity from the standpoint of particle sizes as well as The rate of tectonic activity within a given their areal distribution. mountain range is an important factor in deter­ mining the size and rate of production of sedi­ This report applies the concepts of tectonic ment fr om fluvial systems that originate within geomorphology through landform analysis to the mountain range and deposit their sedimen- William C. Tucker, Jr. 65 tary load within the adjacent basin. Consider two different tectonic settings. The first is tectonically active: rapid uplift at the mountain front has resulted in rugged mountains with steep V -shaped canyons plunging down to the BASIN stream courses that transport sediment. In such a setting, coarse-grained sediments are produced in abundance and delivered directly from the hillslopes into the stream subsystem where they are immediately available for trans­ port to the basin. In a second setting of tectonic quiescence, the hills lopes are steep, but embayed mountain fronts and broad valley floors are typical. In 0. this setting, gravel brought down from the 6u / 61 ~ 6w/61 + 6s/61 hillsides to those reaches of the streams that • 0 .' o · 0 " • 10 • 0 have aU-shaped cross-valley profile is not • • r::1 • • generally delivered directly to the basin. In­ • o . 0 stead, it remains on the flat valley floor and only the gradual weathering of these boulders Fig. 2. Interrelations of local base-level reduces the particle size to that which can be processes conducive for the accumulation of transported from the mountain subsystem to thick alluvial-fan deposits next to a mountain the depositional basin. Within a region tecton­ front (Bull, 1973) ically active in the past and generally inactive during the Quaternary such as the Luke Air shown by Equation 1: Force Range, one should expect to find fine­ grained basin fills overlying coarser basin fills t:; u/!::.t ~ t:;w / t:; t + t:; s/ t:; t (1) that were deposited at a time when the mountain fronts were tectonically active. Equation 1 is one of five equations interrelat­ ing local base processes for the three different Local Base-level Processes tectonic environments. The five equations A process that changes the altitude of a shown in Table 1 form the basis of the three point on a streambed is a local base-level proc­ classes of relative tectonic activity of mountain ess. Base-level processes include stream­ fronts within a given study area during the channel downcutting in the mountains (w) and Quaternary. erosion (e) or deposition (s) on the piedmont. These three processes are interdependent and Tectonic Activity Classes are affected by the relative uplift (u) of the The assignment of a tectonic activity class mountain front. These four base-level proc­ for a given mountain front is based on param­ esses, since they affect the activity and form eters that describe diagnostic landform mor­ of the stream, the hillslope development, and phologies within both the erosional and deposi­ the location of deposition and erosion, are re­ tional subsystems. The landforms described sponsible for the topography of the basins in Table 1 are only a few of the many diagnos­ (Bull, 1973). tic landforms for a given class. For example, Class 1 landscapes have more convex ridge The effects of continued and rapid uplift of crests, steeper footslopes, narrower and steep­ mountains relative to the adjacent basins, er canyons, less sinuous mountain fronts, either by continuous or pulsatory uplift, result thicker basin deposits next to the mountains, in a distinctive suite of landforms. Figure 2 better preserved spur facets, and less soil­ depicts the accumulation of thick alluvial-fan profile development on the piedmont than Class deposits adjacent to a faulted mountain front. 3 landscapes (Bull, 1973). Thus, the tectonic Channel downcutting in the mountains will tend geomorphology model defines the interrelations to cause the stream channel to become en­ of tectonic base-level change (uplift) to other trenched into the fan apex, which will cause base-level processes by the equations of Table the locus of deposition to be shifted downslope 1 and allows the assignment of an appropriate on the fan.