T41A-01
Crustal Structure Across the Three Gorges Dam from Wide-angle Seismic Data
The Three Gorges Dam (TGD) is one of the biggest engineering projects in the world. A wide-angle seismic profile from Fengjie, Sichuan Province, to Guanyindang, Hubei Province, was acquired in order to understand the crustal structure across TGD. The crustal velocity and reflectivity cross-section across TGD shows lateral velocity variation and northward-slipping, crustal scale, strong reflection. TGD is located at the crustal thickness transition belt from about 45 km in the northwest to about 35 km in the southeast. The crustal thickening and steep topography gradients across TGD are resultant from agents of the channel flow in the middle and lower crust, indented by the strong foreland of the Jianghan basin. Due to the fault, which is similar to Longmenshan in eastern Tibet, the earthquake occurrence in the TGD area is low.
T41A-02
Seismic Characteristics of Central Brazil Crust and Upper Mantle: A Deep Seismic refraction Study
A two-dimensional model of the Brazilian central crust and upper mantle was obtained from the travel-time interpretation of deep seismic refraction data from the Porangatu and Cavalcante lines, each approximately 300 km long. When the lines were deployed, they overlapped by 50 km, forming an EW transect approximately 530 km long across the Tocantins Province and western Sao Francisco Craton. The Tocantins Province formed during the Neoproterozoic when the Sao Francisco, the Paranapanema and the Amazon cratons collided, following the subduction of the former Goias Ocean basin. Average crustal VP and VP/VS ratios, Moho topography, and lateral discontinuities within crustal layers suggest that the crust beneath central Brazil can be associated with major geological domains recognized at the surface. The Moho is an irregular interface, between 36 and 44 km deep, that shows evidences of first order tectonic structures. 8.0 km/s and 8.3 km/s P-wave velocities identify the upper mantle beneath the Porangatu and Cavalcante lines, respectively. The observed seismic features allow for the identification of: i) the crust has largely felsic composition in the studied region, ii) the absence of the mafic- ultramafic root beneath the Goias magmatic arc; and iii) block tectonics in the foreland fold-and-thrust belt of the northern Brasilia Belt during the Neoproterozoic. Seismic data also suggested that the Bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle. Finally, the Goias-Tocantins seismic belt can be interpreted as a natural seismic alignment related to the Neoproterozoic mantle domain.
T41A-03
The Amount and Preferred Orientation of Simple-shear in a Deformation Tensor: Implications for Detecting Shear Zones and Faults with GPS
At the 2005 Fall Meeting of the American Geophysical Union, Griffiths and Johnson [2005] introduced a method of
extracting from the deformation-gradient (and velocity-gradient) tensor the amount and preferred orientation of
simple-shear associated with 2-D shear zones and faults. Noting the 2-D is important because the shear zones
and faults in Griffiths and Johnson [2005] were assumed non-dilatant and infinitely long, ignoring the scissors-
like action along strike associated with shear zones and faults of finite length. Because shear zones and faults
can dilate (and contract) normal to their walls and can have a scissors-like action associated with twisting about
an axis normal to their walls, the more general method of detecting simple-shear is introduced and called
MODES "method of detecting simple-shear." MODES can thus extract from the deformation-gradient (and velocity-
gradient) tensor the amount and preferred orientation of simple-shear associated with 3-D shear zones and
faults near or far from the Earth's surface, providing improvements and extensions to existing analytical methods
used in active tectonics studies, especially strain analysis and dislocation theory.
The derivation of MODES is based on one definition and two assumptions: by definition, simple-shear
deformation becomes localized in some way; by assumption, the twirl within the deformation-gradient (or the spin
within the velocity-gradient) is due to a combination of simple-shear and twist, and coupled with the simple-
shear and twist is a dilatation of the walls of shear zones and faults. The preferred orientation is thus the
orientation of the plane containing the simple-shear and satisfying the mechanical and kinematical boundary
conditions. Results from a MODES analysis are illustrated by means of a three-dimensional diagram, the cricket-
ball, which is reminiscent of the seismologist's "beach ball."
In this poster, we present the underlying theory of MODES and illustrate how it works by analyzing the three-
dimensional displacements measured with the Global Positioning System across the 1999 Chi-Chi earthquake
ground rupture in Taiwan. In contrast to the deformation zone in the upper several meters of the ground below the
surface detected by Yu et al. [2001], MODES determines the orientation and direction of shift of a shear zone
representing the earthquake fault within the upper several hundred or thousand meters of ground below the
surface. Thus, one value of the MODES analysis in this case is to provide boundary conditions for dislocation
solutions for the subsurface shape of the main rupture during the earthquake.
http:home.comcast.net/~jhpg/phd.htm
T41A-04
Using Taylor Series for Analyzing Non-uniform Deformation in a GPS Network
We have found a way of using Taylor series to distinguish domains of deformation within a Global Positioning System (GPS) or other geodetic network. Taylor's theorem states that any function, f(x,y), possessing continuous derivatives in the domain defined by [xr,xb] and yr yi yb can be expanded for all points (xi, yi) within the domain. The object of Taylor's formula is to express, via Taylor series, the value of the function, f(x,y)i, at some point, (xi, yi), in terms of the values of the function, f(x,y)r, and its derivatives at the reference point, (xr, yr). Our use of Taylor series is rather non-traditional; we use it to define and describe domains of deformation within a GPS network. Our approach consists of six basic steps: 1) Write the first-order Taylor series for the displacements (or velocities) at some point in terms of the diplacements (or velocities) and their derivatives at the reference point 2) Use displacement data from three GPS sites and solve the first-order series at the centroid of the triangle. 3) Store the solution. 4) Add data from a fourth site and re-solve the Taylor series at the same reference point to obtain a new set of first order terms 5) Compare the first-order terms from the two solutions, if the two solutions are indistinguishable at the 95% confidence-level, then the domain is expanded to include the fourth site. 6) Repeat until the network has been analyzed. Compare the number of data points to the number of unknown terms in the next order of Taylor series. If there are enough data to increase the order, then re-write the Taylor series. Of course, it is unwise to let the Taylor series grow to very high order. Some guidance in choosing the maximum order is provided by mechanical equilibrium, which requires that only the fourth derivatives be continuous within a domain. The method is challenged using GPS velocities in central California, where we are able to distinguish and describe the kinematics of different domains of deformation from the Pacific Ocean to the Sierra Nevada.
T41A-05
Detailed Shallow Structure of the Kunlun Fault Zone in Northern Tibetan Plateau, China: Implications from the 2001 Ms 8.1 Kunlun Earthquake
A detailed shallow structure of the Kunlun fault zone (KLFZ) was jointly deduced by the CDP stacking profiles, seismic refraction sounding profiles and the records of explosion-excited trapped waves which were conducted after the 2001 November 14, Ms 8.1 Kunlun earthquake in the northern Tibetan plateau. The shallow CDP stacking sections indicate that the rupture zone with high dip-angle (about 85 degrees) extends from the surface to a depth of 100 m. Seismic phases on the on-line record sections along the fault zone were analyzed, and 1-D P- and S- wave velocity models of shallow crust within the fault zone were determined by use of the seismic refraction method. The trapped waves on three-component seismic record sections along the profile perpendicular to the fault zone were simulated with the 3-D finite difference algorithm, where the primary 3-D model was constructed based on 1-D P- and S- velocity models within the fault zone, apparent velocities of P and S phases outside fault zone, dispersion of surface waves and Q-value estimation. With the trial and error method, the 3-D shallow structure of KLFZ was refined to a five-layer model with a low-velocity zone along the rupture trace on surface. Synthetic modeling indicates that the width of the low-velocity zone is approximately 300 m in the upper four layers (with thickness 1000 m), and 250 m in the fifth layer (semi-infinite). Due to the station deployment not being dense enough, the uncertainty of estimation of fault-zone width is large (probably up to 75 m). Shear wave velocities in the fault zone are reduced by 45-30% from surrounding rocks from the surface to a depth of 5 km. The low-velocity and low-Q zone in the KLFZ model is a result of major dynamic rupture in the 2001 Ms 8.1 Kunlun earthquake.
T41A-06
Seismotectonic results relative to the Isthmus of Tehuantepec: Mexico
The Isthmus of Tehuantepec (IT) is located at the southeastern of Mexico and at the North of the Zone of Fractures of Tehuantepec (ZFT), which subducts with the Cocos plate under the North American plate at the level of the Gulf of Tehuantepec. The seismotectonic study of the area of the IT showed that the current structure of the Isthmus results from the superposition of three distinct extensional types of tectonisms which took place since the upper Miocene (10 Ma). The oldest one took place between 10 Ma and 8 Ma, and had as a consequence a zone of trans-isthmian rupture along a major normal N-S fault system. This fault system is responsible for the tilting to the NW of the Chiapas batholith and the folded belt of the Sierra of Chiapas. After this first extensional phase, two distinct extensive episodes developed simultaneously and independently, respectively in the North and South areas of the IT. They are still active. One of then is linked to the multi-directional extension, and was highlighted by the study of the brittle structures in the North of the Isthmus, which is assigned to the evolution of the passive margin of the Gulf of Mexico. The other one is associated to the N-S and NNE-SSW extensions, related to the collapse of the coastal plain of Tehuantepec towards the southern part of the isthmus, which are associated with the subduction process of the ZFT. The field work made it possible to highlight the presence of recent N-S at NNE-SSW normal fault systems which mark the western limit of the Tehuantepec plain and that of the coastal Pacific plain of Central America.
T41A-07
The Cuenca de Oro, a Pull-Apart Basin Hosting Precious Metal Deposits Along the Re- Activated Seri-Tahue Terrane Boundary
At the intersection of Chihuahua, Sonora, and Sinaloa a boundary between the Seri and Tahue terranes has been hypothesized, and further refined as the Sinforosa Lineament. Near the western termination of the Sinforosa Lineament lies a topographic basin. Part of this study will be to better define this pull-apart basin, informally named the Cuenca de Oro due to its numerous precious metal deposits. The intention of this study is to test that the Seri-Tahue terrane boundary was re-energized during the beginning of extension related to the opening of the Sea of Cortez (~30ma). It is probable that the precious metal occurrences are related to the initiation of extension(alunite at El Sauzal has been dated at ~30ma). Five field excursions totaling sixty days of field work have been completed and a first draft of a regional geologic map has been made. Large shear zones support the hypothesis of a pull-apart basin. A study of the alteration and lineament intersections determine the location of many known precious metal deposits. By creating multiple cross-sections the basin can be modeled in three dimensions and a tectonic history can be interpreted. This study will present a structural analysis of the Cuenca de Oro and develop a tectonic history related temporally with the epithermal mineralization events.
T41A-08
Structural and morphological evolution of thrust wedges above a ductile layer with different viscous behavior
A series of scaled physical experiments illustrate the importance of differences in density and viscous behavior of décollement in the structural evolution of thrust wedges during shortening. In particular, we have analyzed the effect of changes in viscosity in the morphological evolution and strain of the brittle overburden surface. Ten models properly scaled in geometry and mechanical behavior of natural geological materials were deformed at the Modeling Laboratory (LAMMG) of UNAM. Mechanical stratification of the models included basal and upper brittle layers of 1 and 2 cm, respectively; separated by an intermediate viscous layer of 0.5 cm. Brittle layers were constructed with grains of quartz sand following a Mohr-Coulomb criterion of faulting and bulk density of ca. 1300 kg m-3. The viscous layer was composed of silicon-sand mixtures having differences in dynamic viscosity (Pa s) and density (kg m-3) as the following cases: (A) 2.0 e 4 and 978, (B) 3.3 e 4 and 1195, (C) 4.7 e 4 and 1270. The experiments were carried out in a Plexiglas box of 40x15x10 cm and deformed by moving a vertical wall at a constant velocity of 1.5 cm hr-1. Cross sections of the experiments were obtained for values of bulk shortening of ca. 20 and 40 percent. The modeling results suggest a close relation of structural style of the thrust wedge with the initial conditions of décollement viscosity. Low viscosity models have a structural development characterized by low angle napes and detachment folds with limb rotation indicating a predominant vergence towards foreland. High viscosity models have a greater mechanical coupling between décollement and overburden and develop preferentially detachment folds with higher elevation and undefined vergence. The evolution of the surface in two models with different initial dynamic viscosity, cases A and B, was analyzed at the optical interferometry laboratory of CIO with two full-field optical techniques: fringe projection and laser speckle photography. The use of these combined techniques permitted to obtain a full-field and high resolution map of the changes in elevation during deformation and the instantaneous displacement field of the particles in the surface. The detailed analysis of the instantaneous displacement field also suggests an intimate link of the surface strain with viscous flow of the ductile layer. The presence of displacement instabilities, such as small vortex-like structures and changes in displacement direction, suggests non-steady flow of granular material in the surface of models during the initial steps of deformation in both cases. However, quasi-steady displacement field is reached significantly earlier in the case B (ca. 6 mm of bulk shortening) than in the case A model (ca. 31 mm of bulk shortening). Finally, taking into account the limitations of modeling (e.g., no erosion and deposition, no thermal evolution) the comparative analysis of the models with natural examples can give insights into the structural evolution of thrust wedges developed above décollement layers. Some characteristics of the Laramide shortening and its kinematics reconstruction in southern Mexico can thus be constrained with these models.
T41A-09
Crustal thickness estimates of Northwestern South America and their relation to the presence of batholithic bodies of economy interest
A new model for the crustal evolution of northwestern South America (−8 S to 23.5 N, −90 W to −58.5 W) was developed from gravity derived MOHO depth estimates and tectonic features interpreted from correlative geopotential anomalies and seismic data. Crustal thickness estimates provide important constraints on the distribution of volcanic and seismic hazards, and mineral and energy deposits. Crustal thickness estimates were obtained by inversion of the compensated terrain gravity effects (CTGE) and compared against theoretical Airy MOHO and compiled seismic MOHO estimates. The thickness of the continental crust varies from 35 km to 55 km and shows that the mountain ranges are partially compensated by continental roots. The volumetric proportions of erupted rock types and geochemical characteristics likely are strongly correlated with the thickness and chemical characteristics of the crust through which the rising magmas travel. Crustal thicknesses from 45 to 55 km at the continent are related to the presence of andesitic batholiths of economic interest. Major deposits of base and precious metals of the Andean Mountains are related to intermediate to felsic intrusions. Porphyry copper-molybdenum deposits coincide with Mesozoic - Cenozoic orogenic belts and calc- alkaline volcanism. Major batholiths in the Central Andes, are related to gravity-inferred crustal thicknesses between 55 km to 60 km. Therefore, these results suggest that exploration of mineral deposits associated with batholithic intrusions in the Andes Mountains can be extended to crustal thicknesses from 45 km to 60 km.
T41A-10
Geophysical modeling of the Araguainha impact structure, Brazil
The Araguainha crater is the largest impact structure of South America, formed over horizontal sediments of the Paraná Basin. This structure of 40 km in diameter has a central uplift which presents a negative contrast of density. The depth analysis of Araguainha presented in this work is supported by gravimetric modeling in two profiles of E-W and N-S directions. The results of 2,5 D gravity models allow one to classify the Araguainha structure as a concentric, symmetrical structure, with an average basement depth of 1,0 km. However, at the southern rim, the structure shows a relatively smaller depth when compared with the others regions, with is coincident with a local low gravity anomaly; the eastern, western, and southern rims have depths between 0,5 and 0,9 km. The maximum basement depth is found in the annular basin, which reaches about 1,4 km, caused by sediment constriction at the transient cavity together with specular pairs of radial faults that appear in the modification stage of the crater. This thickening of the sedimentary layers is observed in numerical experiments involving other similar structures, what reinforces the results here obtained. The brittle structures are responsible for the greatest depths in the annular basin region, and they promote a sediment constriction with horizontal radial tension. These observations allow us to characterize Araguainha as a brittle-ductile domain impact structure
T41A-11
Revised Pacific-Antarctic rotations from anomaly 20 (~45 Ma) to present
T41A-12
Conflicting Stratigraphic and Geochronologic Data From the Acatlán Complex: "Ordovician" Granites Intrude Metamorphic and Sedimentary Rocks of Devonian-Permian age
Stratigraphic relationships in the Patlanoaya area of the Acatlán Complex, southern México, reveal conflicting data between absolute dating of minerals and field observations of the host rocks. The issue has central implications for the origin and evolution of the Acatlán Complex in the margins of the Iapetus (traditional view) or Rheic (innovative view) oceans. Three undeformed granites (Palo Liso, Los Hornos and Cuajilote) that show intrusive relationships with adjacent metamorphic rocks and with late Paleozoic fossiliferous sedimentary units, all yield apparent SHRIMP U-Pb Middle Ordovician zircon ages. Palo Liso K-feldspar-biotite megacrystic granite is a pluton intruding basement units that include Piaxtla high pressure and El Rodeo low pressure metamorphic suites, as well as sedimentary rocks of Late Devonian (?) Otate and latest Devonian-Permian Patlanoaya formations, and yet this granite yielded an age of 467.8 ± 4.2 Ma. Moreover, the Piaxtla Suite, which is nonconformably covered by the Devonian-late Paleozoic formations, rendered an U-Pb zircon age of 353 ± 1 Ma for a retrogressed eclogite, and 40Ar/39Ar ages of 341.57 ± 3.68 Ma (glaucophane) and 341.50 ± 2.82 Ma (phengite) for interlayered blueschists. Los Hornos granite, exposed 20 km SE from the former, and intruding greenstones of El Rodeo Suite of inferred Ordovician age, also yields similar zircon ages at 464.9 ± 2.3 Ma. Finally, the Cuajilote granite, although showing sheared contacts with the Pennsylvanian-Permian Tecomate Formation, it bears xenoliths from this unit indicating a younger stratigraphic age; nevertheless, its apparent zircon age is also Middle Ordovician at 463.5 ± 3.7 Ma. Possible explanations for this paradox include: (a) misinterpretation of field data (contacts and unit correlations), or (b) possible erroneous assumptions regarding equivalence of absolute mineral dates with rock-forming events. If the geochronologic data indeed correspond to crystallization ages of the granites, the deformation and regional metamorphism of El Rodeo Suite and the deposition and deformation of Otate Formation would predate the Middle Ordovician plutons. Consequently, a pre-Middle Ordovician orogenic history in the Acatlán Complex is evidenced, and the interpreted sedimentary contact between the high pressure Piaxtla Suite and the Otate Formation would be a major subhorizontal normal fault related to extensional exhumation of the Early Mississippian high-pressure rocks. If our stratigraphic observations hold instead, zircon inheritance and the petrologic (crystallization and dissolution) and U-Pb systematics behavior of this phase in contaminated granitoids should be understood better before conflicting isotopic ages are accepted.