Robert Reilinger

Modern uplift associated with a proposed magma body in the vicinity of Socorro, New Mexico

Analysis of repeated levelings in and near a part of the Rio Grande rift in New Mexico reveals a zone of anomalous relative uplift in the vicinity of Socorro. Maximum uplift relative to nearby bench marks was about 20 cm from 1911 to 1951, corresponding to an average relative velocity of 5 mm/yr. This feature is one of the most clear-cut and striking anomalies of this scale ever detected by leveling in the United States. The principal data come from a north-south profile extending from El Paso, Texas, to Albuquerque, New Mexico. The primary anomaly extends approximately 70 km north from a point about 15 km south of Socorro; the maximum observed uplift occurs approximately 25 km north of Socorro. This zone of anomalous relative uplift overlies and closely correlates spatially with the unusual crustal discontinuity that was detected by Sanford and others and which, on the basis of a variety of evidence, was tentatively identified by them as the upper boundary of a magma body at a depth of about 18 km beneath Socorro. A simple elastic model for surface deformation associated with subsurface magmatic activity indicates that the observed movement in the Socorro area could result from the expansion of a magma chamber at a depth approximately equal to the depth of the discontinuity reported by Sanford and others, but the model is nonunique, and an elastic model may not be realistic.

New measurements of crystal doming over the Socorro magma body, New Mexico

Recent releveling measurements conducted in the Socorro area of the Rio Grande rift specifically to study possible crustal deformation indicate uplift of the central part of the rift (Socorro) relative to bench marks to the west (Magdalena). Total relative uplift measured along the newly releveled line between 1934 and 1978.7 reached 10 ± 1 cm. Combining this new information with previously reported releveling data suggests a roughly elliptical uplift affecting at least 7,000 km 2 . Maximum observed uplift near the center of this area reaches about 20 cm relative to the periphery. The uplift occurs directly above an unusual crustal discontinuity at a depth of about 20 km below this part of the rift. This discontinuity, discovered on the basis of seismic evidence, has been inferred to be the upper boundary of an extensive magma body. The spatial coincidence of the zone of uplift and the presumed magma body, the persistent microearth-quake activity, and the modeling results strongly suggest that the observed movements result from expansion of the Socorro magma body. Although available data are not sufficient to determine uniquely the temporal behavior of the doming, the observations are consistent with more or less continuous uplift averaging about 5 mm/yr during the time the movements were monitored (1909 to 1979). The average inflation rate within the magma body required to produce the observed uplift is on the order of 1 to 2 × 10 −2 km 3 /yr and is consistent with measurements in volcanic regions. Although less clearly defined, the leveling data also suggest relative uplift within the Albuquerque-Belen Basin well beyond the presumed northern boundary of the Socorro magma body. If this uplift is associated with magmatic activity, the Socorro magma body extends considerably farther north than previously expected, or another magma body lies beneath the Albuquerque-Belen Basin.

Relative crystal subsidence from leveling data in a seismically active part of the Rio Grande rift, New Mexico

Analysis of repeated leveling surveys in the Rio Grande rift shows a pronounced zone of relative subsidence north of Espanola, New Mexico, which appears to be associated spatially with anomalous seismic activity. Maximum subsidence relative to nearby bench marks was 4.9 cm between September 1934 and March 1939. Observed subsidence occurs over 19 km and extends from 4 km north to 23 km north-northwest of Espanola. The leveling anomaly does not appear to be associated with either ground-water effects or leveling errors and thus most likely represents crustal deformation. The zone of relative subsidence occurs near some of the few late Tertiary dikes mapped in the rift and lies within the zone of high heat flow (> 2.5 HFU) along the western part of the rift. The crustal movement anomaly appears qualitatively similar but opposite in sign to movements associated with a crustal magma body that have been observed in the Socorro, New Mexico, area of the Rio Grande rift. The movements reported here appear consistent with either deflation of a shallow (

Tilt from releveling: An overview of the U.S. data base

Abstract Because of existing uncertainties as to the influence of errors and non-tectonic movements on releveling estimates of vertical crustal movement, we have taken an empirical approach to investigate some characteristics of the U.S. releveling data base as a whole. Our primary purpose is to establish typical magnitudes of apparent tilt which can serve as a basis for defining anomalous apparent tilts (the term “apparent” is used since differences between levelings can result from errors as well as surface movement). Our analysis is sensitive to apparent tilts with characteristic lengths (i.e. lengths over which tilt is coherent) of 10 to about 70 km, a range which is relevant to many tectonic processes. The apparent tilts developed over a given characteristic length tend to decline as the characteristic length increases. That is the “apparent tilt spectrum” has most “energy” at small spatial wavelengths. For the entire data base the typical magnitudes of apparent tilt for characteristic lengths between 20 and 30 km and between 60 and 70 km are 2.0 and 0.7 μrad respectively. These values are approximately three times larger than the magnitude of apparent tilt expected from random errors. The typical magnitude of apparent tilt averaged over all lengths in the western U.S. (∼- 2.6 μrad) is about 1 μrad higher than that in the east (∼- 1.6 μrad). The largest observed apparent tilts for the entire data base occur along approximately flat routes. For the most part, these large apparent tilts result from real surface movement and are directly associated with sediment compaction due to water withdrawal in flat lying sedimentary areas. While a number of tectonically active areas are characterized by high average apparent tilts, the average magnitude of apparent tilt in releveling data defining the southern California uplift, a feature of much current debate, is not significantly different from that for the entire western U.S. This analysis emphasizes that real surface movements (tectonic and nontectonic) are a significant factor contributing to the high magnitude of apparent tilt relative to random error estimates.

Land subsidence due to water withdrawal in the vicinity of Pecos, Texas

Abstract Analysis of repeated levelings conducted by the National Geodetic Survey indicates a zone of anomalous relative subsidence in the vicinity of Pecos, Texas. The subsidence appears to have resulted from the decline of water level due to pumping from unconsolidated stream-deposited alluvium of Cenozoic age. Maximum observed vertical ground movement relative to benchmarks outside the zone of pronounced subsidence exceeded 200 mm between 1934 and 1956. The decline in the water table was as much as 60 m during approximately the same period. The subsiding region is about 30 km wide and 50 km long.

Releveling Evidence for Crustal Deformation in the United States

Abstract We have reduced and analyzed over 90% of existing U.S. National Geodetic Survey releveling observations, in order to evaluate the implications of these measurements for contemporary crustal movements. The data are displayed both in profile form (movement versus distance along route) and, where adequate coverage is available, as maps of apparent elevation change. Our analysis involves: 1. (1) evaluating the reliability of the leveling measurements using modified geodetic checks as well as through comparison with independent measurements of crustal movement (e.g., tide and lake level measurements, seismological evidence, tilt and strain observations); 2. (2) interpreting what appear to be real movements in light of other geological and geophysical information on crustal dynamics. Empirical analysis of the U.S. releveling data base indicates that apparent tilts of less than 3 microradians are pervasive and must be regarded as typical signal levels and/or as typical levels of systematic error. Systematic leveling errors are suggested in some cases by close correlations between apparent elevation change and topography, disagreements between leveling and tide gauge measurements, and internal inconsistencies in leveling circuits. Apparent movements which correlate with topography are observed in many areas of the U.S. Such apparent movements often reach 35 mm per 100 m of relief. Disagreements between leveling and tide gauge measurements along the east and west coasts and internal inconsistencies in leveling circuits suggest errors reaching 1 mm/km which can accumulate monotonically for distances exceeding 1000 km. Regional maps of apparent elevation change for the Eastern U.S. show serious inconsistencies with similar maps for adjacent portions of Canada suggesting that such maps may be more indicative of systematic leveling errors than real crustal movements. The largest signals (apparent tilts greater than 4.5 microradians) are for the most part associated with: 1. (1) coseismic and postseismic movements of major (M>6) dip-slip earthquakes, 2. (2) regions of contemporary magmatic activity, 3. (3) subsidence due to fluid withdrawal, 4. (4) topography-correlated apparent movements. Coseismic movements reported for eleven U.S. earthquakes are roughly consistent with simple models of elastic rebound. At least some postseismic movements appear consistent with after-slip on the fault or an extension of the fault that ruptured during the earthquake, although other explanations have been proposed (e.g., viscoelastic relaxation of the lithosphere-asthenosphere). Deformations possibly associated with contemporary magmatic processes are not restricted to volcanically active areas (e.g. Hawaii) having been observed in the Rio Grande rift and in Yellowstone National Park as well. Subsidence due to fluid withdrawal, well known in areas of intense pumping, is considerably more widespread than previously reported occuring throughout much of the Atlantic and Gulf coastal plains, along the Mississippi Valley and within many smaller sedimentary basins which have been exploited for groundwater. The association of many of the largest signals with real surface movements further demonstrates the ability of historic leveling to detect relatively subtle deformation.

Recent vertical crustal movements from precise leveling data in southwestern montana, western Yellowstone National Park and the Snake River Plain

Abstract Repeated levelings in southwestern Montana, the western portion of Yellowstone National Park and the Snake River Plain provide information on the pattern of relative vertical crustal movement throughout this region. Except for the coseismic deformation associated with the 1959 Hebgen Lake earthquake, the most outstanding and best-defined feature of the data is contemporary doming at a rate of 3–5 mm/yr, involving approximately 8000 km 2 including the epicentral area and after-shock zone of the 1959 Hebgen Lake earthquake. Based on observations over different time intervals, doming appears to have continued throughout the time the movements were monitored, beginning at least 25 years prior to the 1959 earthquake and continuing for at least 1 year after the earthquake. The character of the coseismic deformation associated with the 1959 earthquake and the high regional elevation are consistent with the observed doming. It is suggested that doming preceded the earthquake for a considerable time (on the order of hundreds to thousands of years, perhaps longer), giving rise to tensional stresses in the upper crust. When these stresses exceeded some critical value, faulting and collapse in response to gravity occurred, resulting in the 1959 earthquake. The voluminous Tertiary and younger volcanics in the vicinity of the doming region suggest that magma intrusion into the crust is the most likely cause of the observed uplift. The proximity of the doming region to the thermally active Yellowstone area supports this suggestion. Secondary features of the data include: 1. (1) A spatial correlation between tilting and historic seismic activity. 2. (2) Uplift within the Norris—Mammoth corridor in Yellowstone National Park relative to nearby bench marks to the north and south. 3. (3) Regional subsidence of the eastern Snake River Plain relative to points north and west of this physiographic province, including subsidence of the Pleistocene Island Park caldera floor relative to its rim fractures. 4. (4) Rapid tilting in the vicinity of 4.1. (a) the Continental fault east of Butte, 4.2. (b) the intersection of the Gardiner, Mammoth and Reese faults just .north of Yellowstone National Park, and 4.3. (c) the Madison Range fault in eastern Idaho.