Jorge Nieto-Obregon | Universidad Nacional Autónoma de México (original) (raw)

Papers by Jorge Nieto-Obregon

Boletin De Mineralogia, 1992

Geosphere, 2013

Peña de Bernal is a natural monument located near the town of Bernal, in Querétaro State, central... more Peña de Bernal is a natural monument located near the town of Bernal, in Querétaro State, central Mexico. It is one of the tallest monoliths of the world, with a maximum height of 433 m. Peña de Bernal was recently declared Intangible Cultural Heritage of Humanity Patrimony by United Nations Educational, Scientifi c, and Cultural Organization (UNESCO). In spite of being both a natural and cultural monument, little is known about its origin, physical characteristics, and chemical composition. It is a leucocratic-igneous rock intruding marine Mesozoic sedimentary rocks and has been misinterpreted as a pluton of Eocene or older age. However, this study shows that Peña de Bernal is a dacitic dome with SiO 2 = 67 wt% and an age of 8.7 ± 0.2 Ma. The complete Peña de Bernal body includes three plugs that crop out in an ~3.5 × 1.5 km area elongated N40°E. Texture of the rock is porphyritic, nearly holocrystalline (80 vol% crystals and 20 vol% glass), with a mineral assemblage of pyroxene, hornblende, biotite, plagioclase, and quartz, plus accessory apatite and zircon. Peña de Bernal dacite is a spine-type endogenous dome that was forcefully intruded through the Mesozoic sequence practically as a solid plug.

Journal of Geoscience Education, 2001

In order to help students assimilate the concept of geologic time, I have designed a circular dev... more In order to help students assimilate the concept of geologic time, I have designed a circular device to compare any span of time to an interval based on a period of 24 hours. The device, called a chronoscalimeter, has an outer circle with the scale of time one chooses to consider, and in the inner part, about a third of the diameter, another circle is drawn subdivided in 24 intervals (i.e. the length of a day). Both scales are arranged in a counterclockwise manner to emphasize the way in which earth scientists measure time, i.e. towards the past. The age of the Earth (4,500 Ma), when compared with a 24 hour scale, allows the users to evaluate the duration of major aeons or the evolution of life. The evolution of continental and oceanic crust is, in this framework, easily understood and assimilated. The Pleistocene, the rise of humankind, or the way humanity has modified the environment of the planet in the last 200 years might serve as a basis for comparison of long and short events in Earth history. The device is particularly useful to evaluate time intervals when considering the history of a mapped area.

Geología de Depósitos Minerales acid-sulfate alteration: alteración ácido-sulfatada albitization:... more Geología de Depósitos Minerales acid-sulfate alteration: alteración ácido-sulfatada albitization: albitización alunitization: alunitización argillic alteration, argillization: alteración argílica, argilización authigenesis: autigénesis bacterial sulfate reduction: sulfatorreducción bacteriana, reducción bacteriana del sulfato. banded iron formation: formación de hierro bandeada. baroque dolomite: dolomita barroca, dolomita en silla de montar bladed calcite: calcita hojosa boiling -~ zone; ~ curve: ebullición -zona de ~; curva de ~ conductive cooling, heat loss: enfriamiento conductivo, pérdida conductiva de calor daughter mineral, daughter crystal: mineral hijo , cristal hijo decalcification: descalcificación decrepitation: decrepitación

Faults and fractures northeasterly oriented dipping NW and SE, with slips mainly normal with a sl... more Faults and fractures northeasterly oriented dipping NW and SE, with slips mainly normal with a slight left lateral component, affect a suite of rocks of Mesozoic to Pleistocene age, in the area of El Bajio, in the states of Queretaro, Guanajuato, Michoacan, and Aguascalientes. The faults and fractures have affected the infrastructure of the cities and surroundings of Queretaro, Celaya, Salamanca, Irapuato, Silao, Leon and Aguascalientes. In the city of Queretaro, the Tlacote-Balvanera active fault has developed a scarp and its motion may potentially affect life lines of great importance. In Celaya City a N-S trending fault traverses the city and has produced a step wise scarp more than 1.80 m high, damaging houses, streets and life lines. In Salamanca, a fault trending N 60oE, dipping to the SE extends from Cerro Gordo to the SW traversing the city and affecting with a varying degree its infrastructure. Displacements observed within the urban area reach as much as 50 cm. Close to Ir...

Revista mexicana de …, 1998

Revista Mexicana de Ciencias Geológicas, volumen 15, número 2, 1998. p. 115 131. 115 Universidad ... more Revista Mexicana de Ciencias Geológicas, volumen 15, número 2, 1998. p. 115 131. 115 Universidad Nacional Autónoma de México, Instituto de Geología; y Sociedad Geológica Mexicana, México, DF GEOLOGY OF XITLE VOLCANO IN SOUTHERN MEXICO CITYA ...

The Sierra Madre Occidental (SMO) covers the NW portion of Mexico and it is the host for several ... more The Sierra Madre Occidental (SMO) covers the NW portion of Mexico and it is the host for several important precious metal mine operations, such as Tayoltita, Cienega, Topia, Fresnillo, Zacatecas, Guanajuato and Bolaños, just to mention a few. The southern part of the Basin and Range extension affected also NW Mexico and formed NW- to NE-trending normal faults that bound

A distinct variation in the age of the ignimbrites of the Sierra Madre Occidental (SMO) is observ... more A distinct variation in the age of the ignimbrites of the Sierra Madre Occidental (SMO) is observed in the southern portion, which includes the area between Tepic, Nayarit (-105° W) and Aguascalientes, Ags (-102° W). Older, high-grade ignimbrites are Eocene and occur as scattered outcrops. These are in turn covered by a widespread and voluminous sequence of high-grade ignimbrites and

Special Paper 402: Neogene-Quaternary Continental Margin Volcanism: A perspective from México, 2006

The Valle de Bravo Volcanic Field is one of four monogenetic volcanic fi elds identifi ed in the ... more The Valle de Bravo Volcanic Field is one of four monogenetic volcanic fi elds identifi ed in the central sector of the Mexican Volcanic Belt. Michoacán-Guanajuato, Jilotepec, and Chichinautzin are the other three. The Valle de Bravo Volcanic Field is located at the southern front of the belt where it covers the western fl ank of the Pleistocene-Holocene Nevado de Toluca volcano. It covers an area of 3703 km 2 and includes at least 120 cinder cones, one shield volcano, a few lava domes, and two lava dome complexes. It overlies a rough paleotopography of Mesozoic metamorphic rocks (schists, metalimestones, and pillow lavas), Paleocene-Eocene granitic rocks, and Eocene-Oligocene ignimbrites. Based upon morphometric parameters that were calibrated with reported isotopic ages, four groups of cinder cones were identifi ed, older than 40 ka, 40-25 ka, 25-10 ka, and younger than 10 ka. Lava domes occur sporadically as high domes, low domes, and coulees, with ages between Pliocene to Pleistocene. We also observed several mafi c lava fl ows that lack a cone source, suggesting that they erupted from fi ssures. A geomorphologic analysis of the cinder cones indicates a relatively young age for most of them, since craters are still evident and fl anks are little eroded. Many lava fl ows still show levees and some of them are little vegetated and lack soil, which is signifi cant for this densely forested and humid area. An analysis of the distribution of the cinder cones shows that most vents follow a NE alignment. By contrast, the domes tend to be aligned in a NW trend. This suggests that emplacement of cinder cones follows the maximum horizontal compressional stress direction, parallel to the Cocos-North America plate convergence (NE), whereas the lava domes are better developed along the minimum horizontal stress direction, perpendicular to convergence (i.e., NW).

Geological Journal, 2005

North-northwest normal faults intersect ENE normal faults in the vicinity of Querétaro City, in c... more North-northwest normal faults intersect ENE normal faults in the vicinity of Querétaro City, in central México, affecting the Miocene-Pliocene northern-central sector of the Mexican Volcanic Belt province. This intersection produced an orthogonal arrangement of grabens, half-grabens and horsts that include the Querétaro graben. The NNW faults are part of the Taxco-San Miguel de Allende fault system, which is proposed here as part of the southernmost Basin and Range province in México. The ENE to E-W faults are part of the E-W oriented Chapala-Tula fault zone, which has been interpreted as an active intra-arc fault system of the Mexican Volcanic Belt. Seventy-four normal faults were mapped, of which the NNW faults are the largest and have the best morphological expression in the region. More numerous, although shorter, are the ENE faults. Total length of the ENE faults is greater than the total length of the NNW faults. Both sets are dominantly normal faults, indicating ENE extension for the NNW set and NNW extension for the ENE set. Field data indicate that displacement on the two fault sets has overlapped in time, as some NNW faults are younger than some ENE faults, which are supposed to be the younger ones. Seismicity in 1998 on a NNW fault indicates ENE active extension on the NNW faults. These observations support our interpretation that the northern Mexican Volcanic Belt lies on the boundary between the Basin and Range province, which is undergoing ENE extension, and the central Mexican Volcanic Belt province, which is undergoing northerly extension. The apparent overlap in space and time of displacements on the two fault sets reflects the difference in stress regime between the two provinces.

Geotechnical and Geological Engineering, 1989

Various kinds of geological evidence has recently been used to show the presence of a master faul... more Various kinds of geological evidence has recently been used to show the presence of a master fault along the course of the Rio Grande de Santiago (RGS). Triangulation data of small motions of survey monuments located in the neighbourhood of the Santa Rosa Dam for the period 1964–1981, show a pattern that closely matches the shape and orientation of the strain ellipsoid deduced previously from the regional structural framework. These and other observations on the mode of fracturing of both margins, and the instability of the wall rocks, indicate that the fault is active and therefore potentially dangerous to civil constructions. Petrologic parameters, sustained by major element chemical analysis and Strontium isotope data support the hypothesis of emplacement of nepheline bearing mildly alkaline basalts through fractures controlled by a complicated pattern of synthetic and antithetic faults, and a pull-apart basin development along the right lateral fault of the RGS. The area does not have a seismic history like the southern continental margin of Mexico (i.e. southern Jalisco, Guerrero, Michoacan, Oaxaca and Chiapas), yet in February 11, 1875, an earthquake of estimated magnitude 7.5, had its epicentre located in Zapopan, Jalisco, a village immediately adjacent to Guadalajara City, and only a few kilometres to the southeast of the study area. After that the region has remained relatively free of seismic activity. An important conclusion is that if the motion of two large blocks sliding past each other along the surface of the fault has been aseismic, it implies the possible accumulation of stress at depth. Seismic risk estimation should therefore be considered at a yellow alert, based on the following features: Probability of occurrence is high, but at the moment the data is too ambiguous to permit a short term prediction. Vulnerability of human settlements, production industries and civil infrastructures includes the villages of Santiago Ixcuintla, Villa Hidalgo, Villa Juarez, and small ranches along the course of the river, and access roads to the San Pedro Analco Mine, to the Dam of Santa Rosa, and of course the Dam itself. Value of social and economic infrastructures is therefore very high. It is recommended to establish a close survey of microseismic activity, a high precision triangulation and studies of paleoseismicity. A civil protection plan should therefore be carefully carried out to prevent a major disaster.

Journal of South American Earth Sciences, 2000

The contact between the Guerrero and Sierra Madre tectonostratigraphic terranes has been proposed... more The contact between the Guerrero and Sierra Madre tectonostratigraphic terranes has been proposed to lie in the Mesa Central, east of the city of Zacatecas. Marine Triassic units have been assigned to the Guerrero Terrane. It is here proposed that this contact occurs to the west of the city of Zacatecas and the Triassic marine sequence assigned to the Sierra Madre Terrane.

Developments in Volcanology, 2008