T51A-01
The Half-Graben Structure of Bahia de Banderas, Mexico
Bahia de Banderas contains important records of the detachment and initial separation of Baja California from mainland Mexico; however, its relief and structure are poorly known. The structure has been identified with a canyon associated with a fault striking E-W, named Banderas fault; it has also been proposed that the canyon may be a graben but no structural model was presented. To help in the description of its relief, a digital elevation model of the bay is constructed with individual soundings, ships soundings from 1970 to date, and satellite- derived depths, which shows an irregular topography of the canyon along the fault. The deepest portion of the bay reaches 1600 m; the south flank of the canyon dips at angles ranging from 15° to 22°, while the north flank ranges from 5° to 9°. This asymmetry between the flanks first suggested that the structure of the canyon might be that of a half-graben. A model is presented based on previous developments for half-graben structures of the fault growth type with reverse drag geometry; the model is based on the lengthening of the fault through seismically induced slip events; a seismic study of the area reported elsewhere complements the model and shows that the region is active. For Banderas canyon the footwall corresponds to the south flank, and the hanging wall corresponds to the north flank, including the shallow platform to the north of the bay. A fault length of 63 km is inferred and pertinent parameters are derived for the model calculations. Theoretical profiles are superposed and compared to actual topographic profiles of the canyon, concluding that the model describes well the central part of the structure, within 14 km of its mid-point in either direction, reproducing with less accuracy the geometries at the ends of the active portion of the fault, which is attributed to the perturbing effects of additional faults acting on those regions. The model allows for calculations of the age of the half-graben; however the parameters required for such a calculation are not well constrained at present and assumptions are made regarding the frequency of slip events along the fault. The maximum displacement of the fault at its center is determined as 2800 m, indicating an accumulation of sediments and slumped materials of 1200 m thickness, with respect to the oeanic depth of 1600 m reported above; assuming a rate of slip events of one every 5000 years the age of the fault is close to 10.6 M years. If this rate were changed to one slip event every 3000 or 7000 years, the corresponding ages would be of 6.36 and 14.8 M years. These results support the hypothesis that the structure of Banderas canyon is a half-graben, and strengthen the idea that it is the limit between the region to the north that underwent extension in the Miocene, and the region to the south that did not experienced it.
T51A-02
The Deep Seismic Structure of Volcan de Colima, Mexico
We present early-stage results from a novel seismic investigation at Volcan de Colima. The project is a collaboration between the Observatorio Vulcanologico de la Universidad de Colima and the University of Alaska Fairbanks. In January 2006, twenty broadband seismometers were deployed in a wide-aperture array around the volcano as part of the IRIS/PASSCAL-supported Colima Volcano Deep Seismic Experiment (CODEX). They are scheduled to be in the field for twenty-four months. Data from the first several months of the deployment, integrated with data from the Mapping of the Rivera Subduction Zone (MARS) project (see abstract by Grand et al.), have been used to characterize both the regional seismicity and the seismicity of the volcano. Overall patterns of regional seismicity agree well with the real-time RESCO network (see abstract by Dominguez). Colima volcano has an unusually well-distributed suite of earthquakes on the local, regional and teleseismic scale. Data recorded close to the edifice provide an opportunity to explore the daily explosive activity exhibited by the volcano. The diversity of regional and teleseismic earthquake source regions make Colima an ideal place to probe the deep magmatic structure of a prodigous volcanic center. Striking differences in attenuation of some body wave phases at stations east of the volcano versus those west of the volcano due to travel path differences hint at the geological complexity of the area in the deep crust. Results from the seismic investigation will be interpreted in the context of pre-existing petrologic models to address the relative role of crust and mantle in governing the evolution of an andesitic arc volcano.
T51A-03
Seismic Structure of the Half-Graben of Santiaguillo, Durango, Mexico
The Santiaguillo half-graben is part of the San Luis-Tepehuanes fault system, which is a major structure separating two physiographic provinces, the Mesa Central and the Sierra Madre Occidental. The younger movement of the faults is Quaternary, which is affecting the rocks of the Durango volcanic field. In this work, we study the faults and grabens forming the complex structure of the Santiaguillo half-graben. These structures result from active extensional tectonics since the Oligocene. The contemporary tectonic deformations have been manifested in the last 50 years by a number of earthquakes occurred in the region (1.2 < M < 4.5, epicenter depths < 10 km). The most recent event occurred on July 29, 2003, is a small-sized earthquake M4.5 reported by the Servicio Sismologico Nacional (SSN) that struck the middle of the basin. Some other small-sized earthquakes, microseismicity and swarms occurred around the basin. However, the lack of permanent seismic stations has prevented a recorded history of this activity. We report the preliminary results from the Durango network, which consists of an 8-station passive short-period array deployed around the Laguna de Santiaguillo. This temporal and portable network has been installed for a period of roughly 12 months starting in April 2006, over an area of about 80 km length and 40 km width. The overall aim of our experiment is to understand the driven forces controlling the tectonics of the western side of the Mesa Central in western Mexico. We combine structural observations and recorded seismicity to locate the potential seismogenic structures. Another objective is characterizing some of the crustal properties in the region. Results show a sparsed and scattered seismic activity. We recorded about 50 microearthquakes, half of them were located out side of the array. Bulk of this activity does not coincide with previously reported activity, which implies a more difficult definition of the seismogenic zones.
T51A-04
B-Be-Li Systematics in the Trans-Mexican Volcanic Belt: Along-Strike Consistencies and Variations
We are examining Li, Be and B abundance variations in mafic lavas along strike in the Trans-Mexican Volcanic Belt (TMVB), building on previous undergraduate research efforts focused on the Colima Graben and two sites in the Michoacan-Guanajuato Volcanic Field (MGVF; Fuller et al 2003; Ryan et al 2005). Our past work indicated that both differentiated calc-alkaline and alkaline (i.e., lamprophyric) TMVB lavas show evidence for assimilation of crustal materials variably enriched in boron (i.e., Hochstaedter et al 1996), while more mafic lavas appear to preserve signatures reflecting their magma sources. We have thus far examined basaltic lavas from the Mascota volcanic field and Volcan San Juan to the west; calc-alkaline lavas from monogenetic centers of the Michoacan- Guanajuato and Chichinautzin volcanic fields, and the Palma Sola volcanic field in the easternmost TMVB. In general, B, Be, and Li systematics for basalts in other TMVB centers are similar to those at Colima: Be is higher in alkaline lavas than calc-alkaline lavas, while B contents are low relative to other arcs. All the lavas show elevated Li/Y and Li/Yb, and K/Li-La/Yb trends indicate slab-derived alkali enrichments and amphibole crystallization. However, along-arc changes are evident in mean B/Be (~7 in west TMVB centers to ~20 in the east) and Li/Yb (~11 in the west, ~6 in the east). Be/Nd ratios are essentially constant along the TMVB, but mean Be/Ti declines from ~1.3 in the west to ~0.5 in the east, implying increases in the mean extent of partial melting from W to E. Be/Ti varies inversely with the age of the downgoing plate, and positively with inferred slab temperatures (Curry et al, 2002), so it is possible that deeper transport of slab fluids to arc depths in E-TMVB leads to higher degrees of mantle melting, an inference consistent with B/Be variations. That Li/Yb and Be/Ti correlate positively may mean that the slab flux of Li changes minimally along the arc and changing melt extents dominate the signature; or that the role of amphibole is greater in W-TMVB than in the east, due perhaps to greater magmatic water contents (Kelley et al, 2007).
T51A-05
Recent Tectonics in the Continental Slope Along the Gulf of Tehuantepec Derived From Seismic Reflection and Multibeam Data
During May 2000, the NEMO Expedition, Leg III on board the RV Melville was carried out at the Gulf of Tehuantepec with the goals to locate undisturbed sediment sites for a climate change study along depth transects at the eastern tropical Pacific margin. One of the depth transect was done across the continental slope in front the west coast of Mexico. To carry this transect, the seafloor relief was surveyed with a SEABEAM 2000 multibeam, and the subbottom sediment layers were digital seismic imaged with a 2 GI AIRGUN and 4 multichannel system. After the cruise, the multibeam bathymetric map clearly shows evidence of massif slumps disturbing the slope, one large surface fault and multiple long canyons transversal to the slope, between 200 m and 2,500 m depth. A standard post-processing of sixteen seismic reflection profiles were done at the UNAM Marine Geophysics Lab for the analysis of the tectonics structures at the continental margin, near the location where the Tehuantepec Ridge converges into the Middle America Trench and the proposed North American- Caribbean plate boundary intersecting the trench. Mayor recent tectonic structures (faults and filled sediment basins) identified in the seismic sections are well correlated with the surface evidence of the continental slope failure at the Gulf of Tehuantepec. This examination evidences a recent large submarine landslide that was originated by a slope failure in the upper slope, near the foot of the shelf, forming a horse-shoe shape basin (extending 23 km crosswise and 11 km parallel to the slope trend). This massif landslide carried out 23 cubic km of sediments and fill completely the horse-shoe basin. Also this survey provide evidences of recent active faulting across the upper slope, indicating that the slope stability in the Gulf of Tehuantepec is largely affected by the tectonics adjacent to the marine intersection of the Caribbean-North America-Pacific plate boundary. This slope stability should be considered as a potential geohazardous zone that could generate regional or local tsunamis, affecting the coasts of Mexico and Central America.