General Tectonophysics Posters
Presiding: L D Brown, Cornell University; W Langin, Shell Exploration and Production
T43D-01 1330h
Has Northern Hemisphere Heat Flow Been Underestimated?
We present three lines of evidence to suggest the hypothesis that heat flow in the northern hemisphere may have been underestimated by 15 to 60 percent in shallow wells due to a large post-glacial warming signal. First, temperature vs. depth (T-z) measurements in parts of Europe and North America show a systematic increase in heat flow with depth. This phenomenon is best recognized in analyses of deep (greater than 2km) boreholes in non-tectonic regions with normal to low background heat flow. In Europe, the increase in heat flow with depth has been observed by analysis of more than 1500 deep boreholes located throughout the Fennoscandian Shield, East European Platform, Danish Basin, Germany, Czech Republic, and Poland. There are significantly fewer deep boreholes in North America, but the increase in heat flow with depth appears in a suite of 759 sites in the IHFC Global Heat Flow Database for the region east of the Rocky Mountains and north of latitude 40 N. Second, surface heat flow values in southern hemisphere shields average approximately 50 mWm-2, but surface heat flow values in northern hemisphere shields average 33 mWm-2. Unless crustal radioactivity or mantle heat flow or both factors are greater in southern hemisphere continents, there is no reason for the northern and southern shield areas having similar ages to have different heat flow values. Third, two recently published surface heat flow maps show anomalously low heat flow in the Canadian Shield in a pattern that is coincident with the Wisconsinan ice sheet. The coincidence of low heat flow and ice accumulation has no geophysical basis, thus the coincidence may suggest the existence of a transient signal caused by a warming event. Recent studies of heat flow in North America indicate that in several sites, the ice base temperature was close to the pressure melting point. We hypothesize that there may have been cold ice-free periods during the Pleistocene that would account for the apparent colder surface temperatures. If our first hypothesis is correct, a majority of northern hemisphere heat flow values require revision by as much as 60 percent because they were determined from boreholes too shallow for recognition of the gradient disturbance caused by a large post-glacial warming signal. Consequently, estimates of the total global heat flux may need revision by as much as 20 to 30 percent due to the underestimate of heat flow in the northern hemisphere. A critical aspect of such a revision is that in northern continental regions where a linear relation between heat flow and radioactive heat generation has been observed, revision of heat flow values would require a significant change in the estimate of mantle heat flow.
T43D-02 1330h
40Ar/39Ar dates from alkaline intrusions of the northern Crazy Mountains, south-central Montana
The Crazy Mountains basin of south-central Montana is a complex foreland basin that formed during the interaction of thin-skinned, decollement-style folds of the Montana thrust belt and the basement-involved folds and thrust faults of the Rocky Mountain foreland province. Near the depositional center of the basin, synorogenic strata of the Paleocene Fort Union Formation have been intruded and locally thermally metamorphosed by strongly alkaline to subalkaline Tertiary intrusive rocks. The subalkaline rocks are found mostly in the southern Crazy Mountains and form stocks (Big Timber stock, Loco Mountain stock), radiating dikes and sills. With the exception of the Ibex Mountain sill (?), the alkaline rocks are restricted to the northern Crazy Mountains. New 40Ar/39Ar dates are reported from the strongly alkaline rocks, including the Comb Creek stock and dike swarm, the Ibex Mountain sill(?), and sills from the Robinson anticline intrusive complex. The alkaline rocks of the Robinson anticline intrusive complex are exposed in the easternmost folds of the Cordilleran fold and thrust belt, but despite their arcuate and apparently folded map geometry they have been shown to post-date folding. Hornblende from a trachyte sill in the Robinson anticline intrusive complex yielded a relatively simple age spectrum with a weighted mean of 50.61 0.14 Ma (2σ), which probably records the age of sill emplacement. Nepheline syenite and mafic nepheline syenites of the Comb Creek stock and a dike from its radial dike swarm, two sills from the Robinson antlicline intrusive complex, and the Ibex Mountains sill(?) gave biotite plateau dates ranging from 50.03 to 50.22 Ma, with 2σ errors of 0.11 to 0.19 Ma. Because these dates are from fairly small, hypabyssal intrusions, they must have cooled quickly and thus these dates closely approximate the emplacement age of the intrusions. These data indicate that the strongly alkaline intrusions were emplaced during a fairly restricted interval of time at about 50.1 Ma. The dates from the alkaline rocks are somewhat older than dates from the subalkaline Big Timber stock in the southern Crazy Mountains, which gave biotite 40Ar/39Ar dates of about 49.3 Ma (du Bray and Harlan, 1996). However, because these dates represent cooling through closure temperatures of about 350 C, they are minimum estimates for the age of the stock. The limited span of 40Ar/39Ar dates between the alkaline and subalkaline rocks of the Crazy Mountains intrusions (i.e., 50.6 to 49.2 Ma) indicates that the magmas represented by these different geochemical groups were closely associated in both time and space, with emplacement occurring in as little as 1.5 Ma. On a regional scale, the 49-51 Ma age is similar to that of most of the igneous centers of the Central Montana alkalic province and is coeval with the peak of widespread volcanism in the Absaroka-Gallatin volcanic field immediately to the south of the Crazy Mountains Basin.
T43D-03 1330h
Seismicity, Tectonics, and Lithospheric Structure of the Tibetan Plateau
The Tibetan Plateau has an average elevation of nearly five kilometers above sea level and constitutes the highest and most conspicuous region of such significantly elevated topography on earth. Formation of the Tibetan Plateau commenced when, by convergence of the Indian and Eurasian lithosphere, the Indian continent began colliding with the Eurasian continent approximately 50 million years ago. This continent collision is continuing today and many aspects of the formation and evolution of the Tibetan Plateau remain unresolved, including the present-day accommodation of deformation within the plateau and the configuration of the Eurasian and Indian sub-crustal lithosphere. We present 267 local earthquake locations for central Tibet. These earthquakes exhibit both spatial and temporal clustering that may indicate swarm-like activity in the Tibetan crust. Calculated focal depths indicate that seismicity is confined to the upper crust, with only a few events occurring more than 25 kilometers below the surface of the plateau. We found no earthquakes in the lower crust or uppermost mantle. These results suggest that the middle and lower crust is aseismic and undergoing ductile deformation, thereby supporting models for elevated temperatures at and beneath mid-crustal levels. A newly-compiled catalog of nearly 900 earthquake focal mechanisms for the Himalayas and Tibetan Plateau clearly defines four discrete zones of deformation within the collision zone. These data confirm previous suggestions of thrusting along the Himalayan Arc, while indicating that deformation within southern Tibet occurs largely by normal faulting. An abrupt transition to strike-slip faulting takes place near 32 N. In the northeast margin of the plateau, thrust faulting is the primary mode of deformation. The transition from normal faulting to strike-slip faulting in the center of the plateau is correlated with a two-to-three fold decrease in the thickness of the lithosphere, suggesting that the change in surface tectonics reflects stress concentration by lithospheric thinning. A new seismically-constrained model of the gravity field within Tibet supports the underthrusting of Indian lithosphere beneath the plateau to approximately 32 N, where it descends nearly vertically into the mantle.
http://www.geo.cornell.edu/geology/indepth/indepth.html
T43D-04 1330h
Moho topography and lower crustal density in Southern Ontario from linearized gravity inversion
We attempt to map both crustal thickness variations and density variations in the lower crust of Southern Ontario. Since both cannot be determined independently we constrain our dataset, consisting of complete Bouguer gravity values from the GSC and PACES, by estimates of crustal thickness from semblance-weighted receiver function analysis and from seismic refraction studies. Our linearized gravity inversion parametrizes the Moho as discrete rectangular blocks sitting at a reference depth (mean of the seismic estimates). We assume that the mass of one block is concentrated at its base and thus obtain a linear relationship between the height - or the density contrast - of the block and the vertical component of gravity measured at the surface. In a first step we leave the density contrast constant and only allow the thickness to vary. Our results show the following tentative correlations with surface structural units: thin crust (~30 km) beneath the mid-continent rift is flanked by thick crust (~48 km) on either side, the Ottawa-Bonnechere graben has thick (~45 km) crust, and so has the Composite Arc Terrane of the Grenville Orogen. Some of these results contradict seismic estimates of crustal thickness. In a second step we grid all available seismic thickness estimates, calculate the resulting gravity field, subtract the latter from Bouguer gravity data, and invert the difference for density variations. The results show a clear distinction between dense lower crust of the Central Gneiss Belt and lighter lower crust of the Composite Arc Terrane in the NW and SE of the Grenville Orogen, respectively. The Abitibi belt of the Superior craton also seems to have lower density lower crust. Residual gravity maps display mainly short-wavelength anomalies which we believe to be caused by shallow anomalies. Crustal thickness variations are not reflected in the generally flat surface topography of the area. Our results may show that Moho topography of orogenic belts can be sustained long after tectonic activity has ceased because of an increase in the density of the lower crust.
T43D-05 1330h
Architecture of the Northwest Andean Microplates
Recently revised models on global plate boundary zones show that the North Andes microplate includes a wide distribution of seismicity, volcanic events, active faulting and extreme topography. The current description of the north Andean microplate boundaries is interpreted from a variety of geological and geophysical models including volcanism and seismicity with variable confidence levels. The poorly understood complex structure and geometry of plate boundaries limits the ability of current physical models to predict neotectonic and other effects including intra-plate lithospheric stresses and strain. Together with local surface gravity and topography data, a variety of available space geodetic sensors have substantially improved the modeling of the lithosphere for analyzing subsurface mass dynamics. They include the GPS-derived 3-D crustal velocities, high resolution (90-m) topography, seismic surveys and high resolution gravity models derived from integrated satellite (e.g., CHAMP and GRACE, 200-km resolution) and terrestrial observations (up to ~5 km resolution). Spherical wavelets is a modern tool for a multi-resolution representation of spatially heterogenously distributed gravity (consistent with the generalized boundary value problem) and for topography datasets with the distinct ability to enhance localized signals. Analysis of multi-resolution gravity and topography models combined with GPS velocities provide a unique opportunity to characterize the structure, isostatic conditions, mass dynamics, and intra-plate deformations of the North Andes microplate.
T43D-06 1330h
Magnetic Fabric of the K/T Breccia Sequence from the Yaxcopoil-1 Borehole, Chicxulub Impact Structure, Yucatan Peninsula
We report results of a magnetic fabric study of the malt-rich breccia sequence of the Chicxulub impact crater, Yucatan peninsula, Mexico. The breccia sequence has been sampled as part of the Chicxulub scientific drilling project (CSDP). The Yaxcopoil-1 borehole was drilled in the southern sector of the crater, south of Merida city. CSDP included continuous coring from about 404 m to 1511 m, and provided cores of the Tertiary carbonate sequence, the impact breccias and underlying carbonate megablocks. Ninety-two samples from 855.22 to 894.58 meters from Yaxcopoil-1 were selected for the magnetic fabric study. Measurements were performed using a Kappa-bridge KLY-2, which provided data on the magnetic anisotropy of low-field susceptibility (AMS). About 77 percent of the suevitic breccias had a prolate fabrics, 64 percent of the green monomitic breccia and 50 percent policmitic breccia had oblate fabrics. The values of AMS parameters such as L,F and P' of prolate fabric of the samples of monomitic breccia showed an increase with the depth, the opposite ocurre with the oblate specimens. As it is only possible to determine the upper side of the samples, the analysis of the directional results have to be careful, but tendencies of sedimentary-like fabrics can be determined. Results indicate a relative heterogeneity in the magnetic fabric of the breccia, which is expected from the high energy emplacement within the crater during crater formation; certain sectors provide some evidence for characteristic fabric patterns within the impact breccias.
T43D-07 1330h
Fracture propagation and fluid flow in fractured reservoirs: field studies and numerical models
In fractured reservoirs (e.g., for petroleum or geothermal water), fluid flow is largely controlled by the permeability of the fracture network. Together with shear fractures (faults), hydrofractures (extension fractures generated by internal fluid pressure, including mineral veins and joints) contribute considerably to the permeability in fractured reservoirs. The permeability of an individual fracture is proportional to the cube of its aperture. But for fluid flow to occur between two sites in a reservoir, there must be at least one interconnected cluster of fractures that links these sites, that is, the percolation threshold must be reached. Field observations show that in heterogeneous and anisotropic, e.g., layered, rocks many hydrofractures become arrested or offset at layer contacts (become stratabound) and do not form interconnected networks. Here I present results from field studies in layered sedimentary rocks from the Bristol Channel Basin, UK. The Lower Jurassic sections exposed near Kilve, Somerset Coast (Southwest England), and around Nash Point, Glamorgan Coast (South Wales) consist of limestone and shale layers dissected by normal faults (Kilve) or strike-slip faults (Nash Point). Whereas joints occur throughout the study areas, calcite veins occur almost exclusively in the cores and damage zones of the faults. These observations indicate that geothermal water was transported along the then-active faults into the host rocks. Furthermore, there is evidence that the veins were injected as hydrofractures from the fault planes into the limestone layers next to the faults. The most important factors that contribute to hydrofracture arrest or offset are discontinuities, stiffness (Young's modulus) changes between layers, and stress barriers - layers where the local stress field is unfavorable to the propagation of a hydrofracture. Using numerical models I explore the conditions for hydrofracture propagation and conclude that mechanical layering largely controls whether evolving hydrofractures become confined to single layers (stratabound) or not (non-stratabound) in which case a vertically interconnected fracture system may form. These results compare very well with the field results. Non-stratabound fractures often show great variations in fracture orientation and aperture. For vertical hydrofractures, field observations and numerical models indicate that the apertures tend to be greatest in the softest layers. However, the present field observations show that calcite veins and joints mostly follow inclined shear fractures in the soft shale layers but change into extension fractures in the stiff limestone layers. Normally, the inclined shear fractures have much smaller apertures than the vertical extension fractures. Variations of fracture aperture in layered fluid reservoirs are important because of possible channeling of fluid flow along the widest parts of a fracture. These aperture variations of fractures in layered rocks were studied using numerical models. The models indicate that in layered rocks, at the contacts between soft and stiff layers, the stress trajectories (the directions of the principal stresses) often become rotated. This conclusion fits with field observations indicating that not only do fractures change dip, but also strike, between layers, with important implications for reservoir permeability. This follows because fractures of unfavorable strike (with respect to the local stress field) tend to close and fluid flow is inhibited. By contrast, fractures with favorable strikes tend to open up, in which case fluid flow is enhanced.
T43D-08 1330h
Neotectonics and Geophysical Structure Under Armenia City (Colombia)
We have analyzed the structural and neotectonic behavior of the Armenian city (Colombia) with gravimetric, magnetometric, geoelectric and a seismic section. This information allow us to confirm the presence of an alluvial fan composed by several volcano-clastic events (Holocene), whose development were controlled by three structural tendencies: the El Danubio fault (N15W), the Hojas Anchas fault (E-W) and the Armenia fault (N20E). The Armenian fault shows evidences of paleo-liquefaction in volcanic deposits whose source is the Ruiz - Tolima volcanic complex (2.630 years B.P), and evidences of superficial rupture. This structure is about 18km at north of the epicentral area of the Armenian Earthquake (January 25, 1999; Mw=6.2) and it shares the area of aftershocks. Along the Armenia city, this fault shows structures that suggest possible events with superior magnitudes to Mw=5.8.