Distinct crustal structure of the North American Midcontinent Rift fromPwave receiver functions
Hao ZhangSuzan van der LeeEmily WolinT. A. BollmannJ. RevenaughDouglas A. WiensA. W. FrederiksenF. A. DarbyshireG. I. AleqabiM. E. WysessionSeth SteinDonna M. Jurdy
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Abstract Eighty‐two broadband seismic stations of the Superior Province Rifting Earthscope Experiment (SPREE) collected 2.5 years of continuous seismic data in the area of the high gravity anomaly associated with the Midcontinent Rift (MCR). Over 100 high‐quality teleseismic earthquakes were used for crustal P wave receiver function analysis. Our analysis reveals that the base of the sedimentary layer is shallow outside the MCR, thickens near the flanks where gravity anomalies are low, and shallows again in the MCR's center where the gravity anomalies peak. This pattern is similar to that found from local geophysical studies and is consistent with reverse faulting having accompanied the cessation of rifting at 1.1 Ga. Intermittent intracrustal boundaries imaged by our analysis might represent the bottom of the MCR's mostly buried dense volcanic layers. Outside the MCR, the Moho is strong, sharp, and relatively flat, both beneath the Archean Superior Province and the Proterozoic terranes to its south. Inside the MCR, two weaker candidate Mohos are found at depths up to 25 km apart in the rift's center. The intermediate layer between these discontinuities tapers toward the edges of the MCR. The presence of this transitional layer is remarkably consistent along the strike of the MCR, including beneath its jog in southern Minnesota, near the Belle Plaine Fault. We interpret these results as evidence for extensive underplating as a defining characteristic of the rift, which remains continuous along the Minnesota jog, where due to its orientation, it is minimally affected by the reverse faulting that characterizes the NNE striking parts of the rift.Keywords:
Receiver function
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Density contrast
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Free-air gravity anomaly
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The first degree polynomial order is fitted to the Bouguer gravity anomaly data to produce the first residual gravity anomaly map of Calabar Flank. The residual gravity data was computed by subtracting the regional trend from the Bouguer gravity field. The regional trend in the flank is the first degree surface fit and represents anomaly of long wavelength while the residual component have their origin from short wavelength sources (shallow sources). This quantitative approach is advantageous over the wavelength filtering methods. The result of this low order fitting shows that the residual gravity field is characterized by positive and negative gravity anomalies. This is consistent with the geologic setting and tectonics of the Calabar Flank. (Keywords: Bouguer gravity anomaly data, Calabar Flank, gravity field)
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This paper presents a number of new concepts concerning the gravity anomaly. First, it identifies a distinct difference between a surface (2-D) gravity anomaly (the difference between actual gravity on one surface and normal gravity on another surface) and a solid (3-D) gravity anomaly defined in the fundamental gravimetric equation. Second, it introduces the ‘no topography’ gravity anomaly (which turns out to be the complete spherical Bouguer anomaly) as a means to generate a quantity that is smooth, thus suitable for gridding, and harmonic, thus suitable for downward continuation. It is understood that the possibility of downward continuing a smooth gravity anomaly would simplify the task of computing an accurate geoid. It is also shown that the planar Bouguer anomaly is not harmonic, and thus cannot be downward continued.
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An isostatic residual gravity map of Brazil has been computed by removing from a 0.5° × 0.5° Bouguer anomaly grid a regional gravity field calculated for compensating masses of surface topography. The coherence function, a statistical measure of the correlation between Bouguer anomaly and topography, was first computed in order to constrain the compensation mechanism within Brazil. Similar to results for North America and Australia, the coherence function of South America has a broad transition between high and low coherence values, suggesting a combination of tectonic provinces with different flexural rigidities and/or loading processes. In view of this result, we have considered, as a first approximation, a model in which the surface topography is the only load acting on a nonrigid lithosphere. A regional gravity field has been computed assuming Airy‐Heiskanen isostasy with compensation at the crust‐mantle boundary. The residual gravity map, which was obtained by removing the computed regional gravity field from the observed Bouguer anomaly, shows a long‐wavelength N‐S trending negative anomaly over most of Brazil. This gravity feature of approximately 3000 km width is the southern continuation of the western North Atlantic negative geoid/gravity anomaly and reaches at least ∼15 mGal in the northern portion of Brazil. Using the upward continued isostatic residual gravity field at 300 km, this long‐wavelength component, which may be dynamically induced, has been removed to first approximation. The final isostatic residual gravity anomaly map depicts anomalies with wavelengths between 100 and 1000 km which correlate with major tectonic provinces. Negative anomalies occur mainly over Paleozoic intracratonic and Cretaceous rift‐type sedimentary basins, and granitic intrusions and along Proterozoic thrust belts. Positive residual anomalies are generally observed over regions affected by igneous activity and volcanism such as in the Amazon basin and the Paraná flood basalt province. Positive anomalies are also associated with overthrust crustal plates which define a suture zone in central Brazil and over sub‐Andean Tertiary foreland basins.
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One of the most difficult problems in gravity interpretation is the separation of regional and residual gravity anomalies from the Bouguer gravity anomaly. This study discusses the application of the minimum‐curvature method to determine the regional and residual gravity anomalies.
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Using the newly ultrahigh order gravity field model EIGEN-6C2,the global DEM model topo_15.1.img and the crust model CRUST1.0,we calculate three kinds of static gravity fields(Free-air gravity anomaly,Bouguer anomaly and residue gravity anomaly)of Sichuan-Yunnan region,and study the relationship between these gravity fields and regional earthquakes.As a whole,the Free-air gravity anomaly has a clear mirror-image relation with regional topography,which means that the regional topography has a big effect on Free-air gravity anomaly.By subtracting the topography gravity effect from Free-air gravity anomaly,we get the Bouguer anomaly of Sichuan-Yunnan region,which presents a characteristic of the lower northwest higher southeast;And the Bouguer anomaly basically reflect the crustal thickness changes of SichuanYunnan region,which be superposed of the intensity of various range of local anomalies.Based on the Parker method we calculate the gravity effect of regional Moho discontinuity,by subtracting it from regional Bouguer anomaly,we get the residue gravity anomaly, which can reflect the density difference of crust in Sichuan-Yunnan region.By comparing the patterns of these gravity fields with the earthquakes source mechanism which occurred after the year 2000,we found that,the thrust or normal type of earthquakes are likely to break on the high gradient zone of three kinds of static gravity fields,and the strike-slip type of earthquakes are likely to break on the smooth and steady area static of gravity fields,which may be concerned with the nature of the faults and the structural setting of the crust.
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The gravity anomalies of the Jurassic and deep structures were obtained by stripping the gravity effect of Cretaceous and Tertiary formations from the available Bouguer gravity map in central and south Iraq. The gravity effect of the stripped layers was determined depending on the density log or the density density obtained from the sonic log. The density relation with the seismic velocity of Gardner et al (1974) was used to obtain density from sonic logs in case of a lack of density log. The average density of the Cretaceous and Tertiary formation were determined then the density contrast of these formations was obtained. The density contrast and thickness of all stratigraphic formations in the area between the sea level to the top of Jurassic formations were used to determine the gravity effect of these layers. The gravity anomaly map of the stripped formation was determined. The gravity anomaly map of the stripped formation was subtracted from the Bouguer gravity map, and the gravity anomaly map of deep structures was obtained. The regional and residual maps (3rd order polynomial ) were determined for the gravity anomaly maps before and after stripping. The regional gravity map before stripping shows one positive anomaly located at the western part of the study area and west Abu-Jir and Euphrates faults. The regional gravity map after stripping shows a positive anomaly located along an axis extended from Kut toward Najaf. This positive anomaly map divided the sedimentary basin into two sub-basins. The positive gravity residual anomaly of the Bouguer map before stripping shows regionally three structural axes trending NW-SE. These axes are Baghdad-Kut axis, northwest Karbala axis and west Samawa- Nasiriyah axis. The positive residual anomaly map after stripping shows two important anomaly areas. The first area is located between Kut and Karbala-Najaf . and the second is located northwest Karbala by about 100-120 km. These two areas may be prospective areas for hydrocarbon. The stripping method application in the study area shows good result; therefor, it can be used to enhance the gravity data to investigate deep structures in other areas.
Density contrast
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Free-air gravity anomaly
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