Evidence for a 1400 Km Long Socorro Fracture Zone

Allan R. Sanford and Kuo-wan Lin

 

Geophysics Open-File Report 89
Earth and Environmental Science Department
New Mexico Institute of Mining and Technology
Socorro, New Mexico 87801

December, 1998


Abstract. Presented is evidence for a 1400 km long fracture zone of probable Precambrian origin extending ENE from southwestern Arizona to the Texas Panhandle-Oklahoma border. The ~85 km wide track of the fracture zone is defined by a clear lineation of many topographic features such as rivers, elongate depressions, and faults. A 85 km wide zone of contemporary earthquake activity straddles a 500 km long segment of the topographic lineation and low sun-angle maps of Bouguer anomalies reveal significant structural changes across and along much of its course in New Mexico. At the intersection of the fracture zone with the Rio Grande rift, basin extension across the rift changes from an estimated 30% to the north to 50% to the south. Also at this intersection is located a very extensive midcrustal magma body. The latter suggests similarities with the Jemez lineament, a long broad leaky crustal flaw in northern New Mexico. A very large volcanic complex has been built at the junction of the Jemez lineament with the Rio Grande rift.

1. Introduction
We define the Socorro Fracture Zone (SFZ) as an ~85 km wide and ~1400 km long ENE trending crustal lineament extending from southwestern Arizona through New Mexico and across the Texas Panhandle to the Oklahoma border (Figure 1). The evidence for this major crustal flaw comes from topography, earthquake activity, gravity, and unusual geologic and geophysical structures along its path. In this paper we concentrate on the ~1000 km segment between 101°W and 111°W longitude that lies to the south of the well-known Jemez lineament in northern New Mexico [Aldrich and Laughlin, 1984]. The latter structure acquired its name from a large volcanic complex located where it intersects the Rio Grande rift (RGR). The SFZ derives its name from a very extensive midcrustal magma body located beneath the Socorro, New Mexico, area where the fracture zone crosses the RGR [Balch et al., 1996].

2. Topography
On their digital shaded relief map of the contiguous United States, Thelin and Pike [1991] called attention to a prominent topographic lineation the extends ENE from approximately 32.6°N and 114.3°W in southwestern Arizona to 36.0°N and 99.0°W near the Texas Panhandle-Oklahoma border. Included along its path are sections of the Gila and Salt rivers in Arizona, the San Augustin Plain in western New Mexico, the course of the Canadian River and other major drainages (e.g. Arroyo Pintada) in eastern New Mexico, and the continuation of the Canadian River through the Texas Panhandle into Oklahoma. The above topographic features are particularly well-defined on digital shaded relief maps with a low sun-angle and an azimuth approximately normal to the strike of the topographic lineation (Figure 2).
Although the ENE topographic lineation described here is seen most clearly on digital shaded relief maps, it is an easily identified feature on earlier generated physical maps, particularly the western half of the lineation (see for example, National Geographic Society, Atlas of North America [1985], p. 13). Accidental alignment of unrelated topographic and structural features is always a possible explanation for lineations present on these maps. However, in this case a number of geophysical and geologic observations suggest this lineation is controlled by a major crustal flaw.

3. Earthquake Activity
3.1 Instrumental Data
In the period from 1962 through July 1998, several organizations used instruments to locate and determine strengths of earthquakes in New Mexico and bordering areas; notably New Mexico Tech (NMT), Los Alamos National Laboratory (LANL), U.S. Geological Survey (USGS), and the University of Texas at El Paso (UTEP). The periods of operation, number and sensitivity of instrument, and procedures for locating and assigning magnitudes were highly variable amongst these organizations during the 36.6 year period from 1962 through July, 1998. For this reason, Sanford et al. [1997] spent two years collating data from all organizations into a comprehensive and consistent earthquake catalog for New Mexico and bordering areas. A major effort was made to have all magnitudes in the catalog based on or tied to a New Mexico duration magnitude scale [Ake et al., 1983]. This duration magnitude scale is tied to the local magnitude scale which Hanks and Kanamori [1979] have demonstrated is equivalent to the moment magnitude. In addition, a large number of earthquakes were relocated using new procedures, primarily a fuzzy logic algorithm [Lin and Sanford, 1994] that greatly improved locations of earthquakes only recorded by small aperture arrays, a frequent occurrence in the over three and one-half decades of instrumental recording.
The NMT catalog contains epicenters and magnitudes for over 2000 earthquakes with duration magnitudes greater or equal to 1.3; 78.6% from NMT, 13.3% from LANL, 6.7% from USGS, and 1.2% from UTEP. Tests indicate that a lower cut-off magnitude of 2.0 assures completeness of the earthquake data throughout the study area since 1962. Presented in Figure 3 is the geographical distribution of earthquakes with duration magnitudes of 3.0 or greater from 1962 through July 1998. Of the 117 earthquakes plotted on this map, 36 are located in a tight cluster centered near Socorro. The cluster, which we call the Socorro Seismic Anomaly (SSA) [Sanford et al., 1995], occupies about 5000 km2 or 1.1% of the area covered by instrumental data, but it accounts for 31% of the earthquakes in Figure 3. The SSA is believed to be the result of crustal extension over an inflating midcrustal magma body. The magma body is ~150 m thick, ~l9 km deep, and has a minimum lateral extent of 3400 km2 [Ake and Sanford, 1988; Hartse et al., 1992; Balch et al., 1996]. Level-line data indicate that the surface above the magma body is undergoing uplift at a maximum rate of ~1.8 mm/year [Larsen et al., 1986] (Figure 7).
Extending ENE from the SSA through the Great Plains of eastern New Mexico and into the Texas Panhandle is a clearly defined zone of seismicity. Approximately 30% of the earthquakes located outside the SSA occur in this ~85 km wide and ~ 500 km long region. The N68°E strike of the zone is unusual because it is at a high angle to the generally north trending basins and ranges of the Rio Grande Rift (RGR). However the zone of seismicity straddles the topographic lineation (Figure 2) which also has an approximate N68°E orientation.
We were concerned that the alignment of epicenters that overlies the topographic lineation might be accidental. We tested this possibility using a Monte Carlo technique. New Mexico and bordering areas were divided into small squares 10 km on a side. The earthquakes of magnitude 3.0 or greater located outside the SSA were randomly distributed over the region with no restrictions on the number of events in each block. This procedure was repeated nearly 1000 times without reproducing the distribution in Figure 3. Alignments of epicenters were generated but they were not as narrow nor did they contain as many earthquakes as appear along the SFZ.

3.2 Preinstrumental Data
We have compiled a list of felt earthquakes prior to 1962 for the region 31°N to 38°N and from 101°W to 111°W [Sanford, 1995]. In order to minimize bias towards populated regions, the list was restricted to felt events with maximum reported intensities of VI or greater (Modified Mercalli-1931) and/or felt areas greater than 100,000 km2. An empirical relation between magnitude and maximum intensity for the New Mexico area [Sanford, 1998] yields magnitude 4.5 for an event with a maximum reported intensity of VI.
The list of felt earthquakes, Imax > VI, contains 39 events in the 132 year interval from 1830 through 1961. As shown in Figure 4, 11 earthquakes are associated with the Socorro Seismic Anomaly, and considering the large uncertainties in the locations, eight could have occurred along the Socorro Fracture Zone. Of particular interest are the five earthquakes in the Texas Panhandle on the eastern end of the fracture zone. These events had felt areas as great as 520,000 km2 [Shurbet, 1969; Coffman et al., 1982] which suggests magnitudes as large as ~5.2 [Reiter, 1990]. The apparently higher level of activity in the Texas Panhandle may arise because of the intersection of the SFZ with another fracture zone associated with the Ouchita gravity high extending ESE from the panhandle to the Ouchita Mountains [Needs reference to maps on the Internet].

4. Gravity
We have generated low sun-angle maps based on digital (2.5’ x 2.5’) terrain corrected Bouguer gravity anomaly data obtained from NOAA [Hittelman et al., 1994]. A sun inclination of 45° and azimuths of 135° and 315° were used (Figure 5). These maps depict the first derivative of the Bouguer anomalies in the direction of the azimuth of the sun and therefore cannot be easily correlated with contour maps of gravity anomaly values. They do emphasize existing structural trends at large angles to the sun azimuth that may not be apparent on contour maps.
Both maps in Figure 5 show a marked change in texture across the SFZ, particularly from the RGR westward. Relative to the region north of the SFZ, the region to the south has a rough fabric dominated by narrow alternating light and dark bands with a dominant NW trend upon which the north trending signature of the RGR structures are superposed. The contrast is so clear west of the RGR that it suggests the SFZ may define the southern boundary of the Colorado Plateau in that region. East of the RGR the SFZ includes a prominent 80 km long ENE trending lineation (Figure 5a) that is probably related to the Pedernal Uplift and the western end of the Tucumcari Basin [Suleiman and Keller, 1985]. Eastward of this feature the SFZ cannot be traced on the basis of gravity data even with the most favorable sun angles.

5. Geology
5.1 Rio Grande Rift Morphology
The postulated SFZ intersects the RGR ~80 km south of Albuquerque at a location where the morphology of the rift undergoes a major change (Figure 6). North of this juncture the rift consists of the single 50 km wide Albuquerque-Belen basin, and south of the juncture the rift is a 70 km wide zone of three parallel basins and intermediary ranges [Lewis and Baldridge, 1994]. Basin extension north of the intersection of the SFZ and the RGR is estimated to be ~30%, whereas to the south the extension is believed to be 50% [Chapin and Cather, 1994; Cather et al., 1994]. The large change in extension in a short distance appears to require major differential movements along a deeply penetrating fracture zone.
5.2 Accommodation Zones
A feature of the RGR are half-graben basins which switch symmetry across accommodation zones [Chapin and Cather, 1994]. These transverse structures have the same general NE strike as the SFZ. Chapin and Cather [1994] believe they formed when the rift opened across preexisting lineaments of probable Precambrian age. One of the best documented of the transverse structures is the Socorro accommodation zone [Chapin et al., 1978; Chapin,1989] located 30 km south of the center of the SFZ. The Socorro accommodation zone extends 50 km WSW from the RGR and may define the southern margin of the SFZ in this region. It is not observed directly on geologic maps except as a 2 km wide margin between west-tilted strata to the north and east tilted strata to the south [Chapin and Cather, 1994].
5.3 Faulting in the Socorro Fracture Zone
A major geologic structure within the SFZ is the ENE trending San Augustin Plain located 50 km west of the RGR (Figure 6). Normal faulting has been inferred along the sharp boundaries of the 85 km long and 10 to 20 km wide depression [Woodward et al., 1978]. Detailed mapping by Baldridge et al. [1983] suggests very complex systems of faults around the perimeter of the San Augustin Plain but with EW and NE strikes dominant. Faults with orientations parallel or sub-parallel to the SFZ are (1) mapped from the western end of the plain as far as the Arizona border [Baldridge et al., 1983], and (2) inferred and mapped from the eastern end of the plain to the eastern boundary of the RGR (Figure 6) [Woodward et al., 1978; Baldridge et al., 1983]. In the latter area a large number of north trending faults cross the SFZ. On the basis of detailed mapping of a part of the region Lewis and Baldridge [1994] attribute the high-angle north-striking normal faults to a zone of left-lateral shear along a NE-SW oriented accommodation zone which separates the Colorado Plateau from rift basins to the south. The Lewis and Baldridge [1994] accommodation zone also has ENE and NE trending left-lateral strike-slip faults with up to 1 km of offset. The Socorro accommodation zone about 50 km to the south has no evidence of strike-slip movement [Chapin and Cather, 1994].
East of the boundary ranges of the RGR, the faulting within the SFZ is sparse compared to the same region of the SFZ to the west [Woodward et al. 1978]. However, if only faults that cut middle to late Quaternary deposits are considered, the fault occurrences in both regions is comparable [Callender et al., 1983]. Furthermore, for faults in this age bracket, the eastern branch of the SFZ has four fault segments with NE strikes; the western branch has one. The significance of these observations may be open to question because neither area may have been adequately studied to obtain an accurate inventory of faults that cut middle to late Quaternary deposits. However, in the absence of additional information, the existing data on faulting suggests that contemporary earthquake activity is as likely east of the RGR as to the west.

6. Jemez Lineament
The Jemez lineament is a broad northeast trending tectonically active zone which intersects the RGR 185 km north of the junction of the SFZ with the rift (Figures 2 and 3). This lineament was originally identified on the basis of an alignment of late Tertiary-Quaternary volcanic centers most of which can be encompassed in a 50 km wide zone extending 370 km N52°E from the White Mountains in Arizona to the RGR [Aldrich and Laughlin, 1984], and a 80 km wide zone extending 310 km N72°E from the RGR to the New Mexico border with Texas and Oklahoma [Callender et al., 1983]. The volcanic rocks along the lineament are dominantly basaltic and range in age from a few thousand years to 13 Ma with no progression in age from one end or the other. There is general agreement that the Jemez lineament is simply a long leaky flaw in the crust [Aldrich and Laughlin, 1984].
6.1 Differences from the Socorro Fracture Zone
The SFZ maintains nearly the same bearing, ~N70E, for its entire 1400 km length whereas the Jemez lineament changes orientation at its intersection with the RGR. East of the rift the strike of the lineament is nearly the same as the SFZ, but west of the rift its strike is 16 more to the southwest than the SFZ. The two structures converge rather rapidly to the west and intersect near the major White Mountain volcanic center in eastern Arizona [Aldrich and Laughlin, 1984]. An obvious difference between the SFZ and the Jemez lineament east of that intersection is the very small number and size of late Cenozoic volcanic centers along the SFZ compared to the prolific outpourings along the Jemez lineament. The major volcanic centers at the White, Mt. Taylor and Jemez mountains are connected by several hundred smaller volcanic centers [Callender et al., 1983]. Collectively the large and small volcanic structures produce a topographic trace which is very different from the SFZ which is primarily an alignment of different topographic and structural features. Despite the strong and clear topographic and geologic features along its entire length, the Jemez lineament is essentially invisible on the low sun-angle Bouguer anomaly maps in Figure 5.
6.2 Similarities with the Socorro Fracture
East of the RGR earthquakes occur in broad zones along the traces of the Jemez linament and the SFZ, but west of the rift neither structure has a good earthquake signature, particularly the SFZ (Figure 3). Collectively, the earthquakes in the Jemez lineament and the SFZ account for nearly 50% of the seismic activity outside the Socorro Seismic Anomaly. The width of the SFZ seismicity is in good agreement with the 50 km to 80 km width of the Jemez lineament as defined by volcanic centers.
At the intersection of the broad tectonically active Jemez lineament with the RGR is located the very large Jemez Mountain volcanic center, the largest such center anywhere along the lineament [Aldrich and Laughlin, 1984]. At the intersection of the SFZ and the RGR is located a very extensive midcrustal magma body [Balch et al., 1996]. The relationship of the geographical boundaries of the SFZ to the lateral extent of the midcrustal magma body and surface uplift is shown in Figure 7. We do not believe that the positions of the large Jemez Mountain volcanic complex and the extensive midcrustal magma body near Socorro are accidental. At both locations upward migration of magma has been enhanced by the intersection of major crustal flaws. Perhaps what is occurring along the SFZ in New Mexico at this time is the early stages of a structural feature which will ultimately have many of the characteristics of the Jemez lineament.

7. Conclusions
Summarized below are observations which appear to support the existence of the Socorro Fracture Zone; a major zone of crustal weakness of probable Precambrian origin extending 1400 km ENE from southwestern Arizona through New Mexico and the Texas Panhandle to the Oklahoma border.
1. On digital shaded relief maps the SFZ is a clearly defined ENE trending topographic feature that incorporates from west to east segments of the Gila and Salt rivers in Arizona, the long narrow San Augustin Plain structural depression in western New Mexico, and the course of the Canadian River in eastern New Mexico and the Texas Panhandle.
2. A 85 km wide band of contemporary earthquake activity straddles the trace of the topographic lineation eastward from the RGR for a distance of 500 km.
3. Low sun-angle maps of Bouguer anomalies show significant structural changes across and along the topographic lineation, particularly west of the RGR in New Mexico.
4. The morphology of the RGR undergoes a major change at its intersection with the SFZ; basin extension north of the junction is estimated to be ~30%, whereas to the south it is believed to be ~50%.
5. Accommodation zones of probable Precambrian origin occur within the SFZ and elsewhere in the RGR with orientations paralleled or sub-paralleled to the track of the fracture zone.
6. In marked contrast to the NS structural grain of the RGR, faults with ENE and NE strikes have been mapped or inferred along the SFZ in western and eastern New Mexico.
7. At the intersection of the RGR and the Jemez lineament, a long leaky crustal flaw in northern New Mexico, is located the very large Jemez Mountain volcanic complex, and at the intersection of the RGR and the SFZ is located an extensive 3400 km2 midcrustal magma body.

Acknowledgments. We wish to extend our special thanks to Lawrence Jaksha who for over 20 years has been responsible for the installation, calibration and maintenance of seismograph stations that have provided a majority of the earthquake data appearing in this paper. We would also like to acknowledge the contributions to our earthquake catalog from Los Alamos National Laboratory [Ken Olsen, Dan Cash, and Leigh House], the U.S. Geological Survey, and the University of Texas at El Paso [Diane Doser and G. Randy Keller]. Finally, our thanks to I-ching Tsai for her help in preparing a comprehensive and consistent earthquake catalog for New Mexico and bordering areas.


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Figure 1. Color shaded relief map of Arizona, New Mexico, and West Texas [Sterner, 1995]. The track of the topographic signature of the Socorro Fracture Zone is indicated with arrows in the margins of the figure. Other arrows indicate the eastern and western ends of the Jemez lineament.

Figure 2. Digital shaded relief map of New Mexico and bordering areas. Upper arrows mark the center line of the Jemez lineament, and lower arrows the center line of the Socorro Fracture Zone. The cities of Santa Fe, Albuquerque, Socorro, Las Cruces, and El Paso are located in the Rio Grande rift.


Figure 3. Epicenters of earthquakes in the New Mexico region with magnitudes greater or equal to 3.0 from January 1962 through July 1998. The total number of earthquakes plotted on the map is 117. Also shown are the boundaries of the Jemez lineament (upper set), the Socorro Fracture Zone (lower set), and the Rio Grande rift (generalized). The cities of Santa Fe, Albuquerque, and Las Cruces are located within the Rio Grande rift.

Figure 4. Epicenters for felt earthquakes in the New Mexico region from 1830 through 1961. The map is restricted to shocks with maximum intensity (Modified Mercalli-1931) of VI or greater, and/or a felt area greater than 100,000 km2. Also shown are the boundaries of the Socorro Fracture Zone. The cities of Santa Fe, Albuquerque, Las Cruces, and El Paso are located in the Rio Grande rift.

Figure 5. Low sun-angle maps of Bouguer anomalies for inclinations of 45 and azimuths of 135 (A.) and 315 (B.). Upper arrows mark the center line of the Jemez lineament, the lower arrows the center line of the Socorro Fracture Zone. Data are digital (2.5’x2.5’) terrain corrected Bouguer anomalies [Hittelman et al., 1994].

Figure 6. Tectonic map of the central Rio Grande rift [Woodward et al., 1978]. Sedimentary rocks of Miocene to Holocene age are shown in orange. Miocene and younger volcanic rocks are shown in pink. Sedimentary rocks of Cambrian to early Miocene age are shown in green. Crystalline rocks of Precambrian age are shown in light brown.


Figure 7. Geographic extent of the Socorro Magma Body with respect to the boundaries of the Socorro Fracure Zone [Balch et al., 1994]. The dashed contours are surface uplift from 1911 to 1980 [Larsen et al., 1986]; with the outer contour equal to 0 mm and the inner contour 120 mm.