175 m, or about halfway between the 0.14 and eruptions that formed the Toledo and Valles Abstracts 0.25 s reflectors. Based on this geophysical evi- calderasin the JemezMountains. Most of the dence, we believe that the gravity anomaly is eruptions over the past 5 m.y. havebeen smaller the result of brecciation along a fault passing (VEI:3-4) and mainly basaltic. Simple linear New Mexico GeologicalSociety through the 3,650-m/s layer and that shallow, regression of age vs. frequenry data seemsto The New Mexico GeologicalSociety an- lower velocity layers were draped passively over show that the eruption frequenry is increasing nual spring meeting was held at New deeper faults to form a small monocline along over time. There is an averageeruption prob- Mexico Institute of Mining and Technol- each side of the graben. Location of the fissure ability of 0.0002per year over the next million ogy (Socorro)on April 6, 1990.Following within the graben contradicts existing models years, equivalent to 200 eruptions. The poten- for the formation of fissures by concave-down- tial activity includes lava, cinder cone, maar, are abstractsthat were omitted inadvert- ward bending of thin alluvium layers over bed- dome, and plinian eruptions. All of thesehave ently from thosepublished in the August rock projections. These models predict that the capacity for minor to severeimpact on the and November 1990issues and the Feb- fissures should form by propagating down- state'seconomy, transportation, agriculhrre, and ruary 1991issue of Nno MexicoGeology. ward from the ground surface in areas of con- communications. cave-downward topography where the surface Geophysics, petrology, and is in tension. In contrast, the San Marcial fis- Lers Pnorenozorc ANDCauarunN-Onoovrcnru structural geology session sure is located in an area where topography, ALKAUCPLUTONISM INWEST TEXAS AND SOI,JTHERN seismic reflectors, and the gravity profile are Nrw Mgxtco: A REACTIvATEDsHEAR-zoNE (Continued from November 1990issue) all gently concave-upward to straight, where MoDEL,by M. H. Sandidge.Department of the ground surface should be in compression, GeologicalSciences, University of Texasat El GpopnysrceL coNsrutNTs oN A MEcHANTCAL and the fissure appears to have propagated up- Paso,El Paso,TX79924 MoDEL FoR THE oRtctN or rue Seru Menctal ward. Analysis of elastic stresses and displace- Two parallel, linear, alkalic igneous belts have EARTHFrssuRE/ by W. C. Haneberg, New Mex- ments developed in a two-dimensional gravity- been recognizedin west Texasand southwest- ico Bureau of Mines and Mineral Resources, loaded span, which we use as an analog for a ern New Mexico. The northernmost of them New Mexico Institute of Mining and Tech- soil layer lying over a bedrock graben, shows measures240 kms in length while the southem nology, Socorro, NM 87801; and C. B. Rey- that minimum principal stress (taking com- belt extends for some 100 kms. Zircon U-Pb nolds, Charles B. Reynolds and Associates, pressive stresses to be negative) will be tensile ages for the northern belt range from 970 Ma 4409 San Andres NE, Albuquerque, NM 87110 to neutral throughout the lower portion of the to 7MS Ma from east to west. However, ages The San Marcial earth fissure, which ap- span. Magnitude of the tension decreaseswith of 495-507Ma (Clemons, 1989)along the same peared during a heavy rainstorm in August 1981, layer thickness and will increase if outward- trend have been obtained. Ages for the south- crosses I-25 along the western edge of the Rio directed shear is applied to the base of the layer. ern belt range from 623 Ma to 1400Ma from Grande valley about 50 km south of Socorro For layers with length:thickness ratios greater east to west. Theseages and geochemicaldata The fissure was widened by erosion immedi- than about 10:1, which can be analyzed using point toward an anorogenic magmatic event ately upstream of an embankment for south- elementary beam theory, the absolute value of and/or events rather than an orogenic event or bound I-25, then under construction, suggesting fiber stress along the upper and lower bound- events.Three possibletectonic processes may that blockage of the arroyo may have contrib- aries will decrease with the cube of thickness. have been operating during the Proterozoic and uted to its growth. On a large scale the trace Tensile to neutral normal stress would decrease Cambrian-Ordovician; they are: 1) plutonism of the fissure is straight, following a small un- soil shear strength-as well as capacity to resist associatedwith pre-rift or pre-ridge extension, named arroyo, and is nearly 1,500 m long. Strike seepage forces-and we speculate that the San 2) plutonism associatedwith a mantle-derived of the trace, which is not affected by topogra- Marcial fissure must have originated in a zone plume, and 3) plutonismassociated with shear phy, is northwest-southeast. Closer examina- of tension at depth and grew upward by pip- zones. The fact that both belts exhibit varia- tion of 1:250 aerial photographs, taken one tng. As piping continued, layer thickness would tions in ageswould seem to rule out an exten- month after the fissure appeared, shows that have decreased continuously, thereby increas- sional process.Associated ring complexeswould the trace is composed of many short segments ing tensile stresses at shallower and shallower illustrate no disparity of ages.A plume model meters to tens of meters long. Some segments depths until the ground surface was broken also must be rejectedbecause the northern belt are crudely en echelon but with no consistent and surface runoff continued to enlarge the fis- exhibits an older-youngest<ldest east-west sense of stepping along the entire length of the sure. progression;one would expectit to be younger trace In places where erosion widened the fis- in one of the two directions. Plutonism asso- sure before the photographs were taken, non- Rpctxr AND porENTrALvoLCANlsM oF NEw ciated with shear zones appearsto be the most overlapping en echelon segments seem to have MEXIco, by E. M. Limburg, Department of viable model. Such a system may function at coalesced to form larger segments. In places Geoscience, New Mexico Institute of Mining any time when the shearzones are active and, where erosion was not as severe and finer fea- and Technology, Socorro, NM 87801 therefore, initiation of similar magmatism in The State of New Mexico has experienced the same shear zones separatedby large time more than 400 volcanic events over ihe past 5 intervals can be explained m.y., ranging from small basalt to large plinian eruptions. The eruptive vents are grouped into DtscruvnattNc pyRoclAslc sURGEoR EoLIAN volcanic fields roughly aligned with two zones CENESIS FOR CROSSBEDDED TUFFS, 'EMEZ of structural weakness that cross New Mexico: MouxrerNs, New Mexco, by G. A. Smithand the Colorado Plateau transition zone (TZ\, and D. Kntzman, Department of Geology, Uni- the Rio Grande rift (RGR). The most recent versity of New Mexico, Albuquerque, NM activity was the eruption of two relatively large 87131 (>4 km3) basalts at approximately the same time The late Miocene PeraltaTuff Member of the define layers with P-wave velocities, inferred (ca. I ka): the Carrizozo and McCarty's basalt. BearheadRhyolite, in the southeasternJemez by previous investigators using seismic refrac- Seismic and geodetic evidence suggest that volcanic field, contains crossbeddedtuffs of both tion surveys, of approximately 800 m/s from 0 magmatic activity of New Mexico has not ceased pyroclastic surge and eolian origins. Study of to 48 m in depth, 1,900 m/s from 48 to 67 m in Leveling measurements indicate an inflation rate textural characteristics,stratification, and flow depth, and 3,650 m/s below 67 m in depth. of approximately 1.8 mm/yr over the Socorro directions was undertaken to discriminate de- There is no evidence of recent fault scarps albng magma body (SMB, a sill at 19 km depth). This posits produced by these very different pro- either of the graben margins, and weak reflecl research assesses both the recent and potential cesses.Surge deposits examined in this study tors at two-way times less than 0 10 s do not volcanism of New Mexico in light of the pos- are part of a phreatomagmatic tuff-ring ex- appear to be broken by faults. The average Bou- sibility for volcanic activitv and a lack of infor- posed at the Tent Rocks; their characteristics guer gravity profile, calculated from five read- mation about future volcinism and its effects were also compared to others previously stud- ings taken over two days at each seismic here. While volcanic eruptions may not present ied by the senior author and to published tex- - shotpoint, shows a 1 mgal anomaly over the an immediate threat to the population of New tural data. Other crossbeddedtuffs are northeastern boundary of the graben, which Mexico, they clearly pose a risk that should be commonly found overlying pyroclastic-fall tuffs coincides with both an 0.04 s vertical offset (to clarified when planning long-term develop- in the Peralta Tuff. Strongest evidence for eolian 0.29 s) of the 0.25 s reflector and a topographic ment in the state. The lareest volume of ma- origin is measured paleocurrent directions in- depression within the arroyo. Half-width anal- terial was erupted between 1-2 Ma. The maiority dicating flow from the south and southwest. ysis of the anomaly gives a maximum depth of of that came from the large (VEI = 6-Z) rhyolitic All known BearheadRhyolite vents are located May 1991 Nm Merico Geology to the north and the fluvial paleoslopewas in- of high accretion rates and fast-moving cur- tures include: sandwave, planat and massive clined to the southeast.Although theie is some rents and are rarely observedin the eolianites.