William J. Hinze

Crustal structure of the Grenville front and adjacent terranes

Under the auspices of the Great Lakes International Multidisciplinary Program on Crustal Evolution, approximately 320 km of deep seismic reflection data were collected in Lake Huron along a profile that extends east from the Manitoulin terrane across the Grenville front to the interior of the Grenville orogen. The Manitoulin terrane is characterized by a series of gently east-dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer. Imaged by strong reflections at the western end of a spectacular band of moderately east-dipping reflections, the Grenville front clearly truncates Manitoulin terrane structures to the west. These data are interpreted in terms of a speculative two-stage model involving (1) creation of a major decollement during northward collision of an allochthonous terrane with the southern Superior cratonic margin (1.83-1.89 Ga; Penokean orogeny) and (2) northwest-directed stacking of microterranes at the southeastern margin of the craton, followed by crust-penetrating ductile imbrication under high-pressure-high-temperature conditions leading to the ramping of deeply buried rocks to the near surface (1.0-1.3 Ga; Grenvillian orogeny).

The Utility of Regional Gravity and Magnetic Anomaly Maps

Mapping of the Earths gravity and magnetic fields has a long and distinguished history as part of the investigations of the structure and petrologic variation within the Earths lithosphere. Prior to 1950, observations of anomalies in these planetary fields had gene rally been focused on limited areas for specific mineral-resource and geological objectives. The increasing availability of portable gravimeters and the development of aeromagnetic-survey technology shortiyafter World War II, however, led to efficient and precise gravity and magnetic mapping of extensive regions. These regional surveys were conducted for the most part by governmental organizations most of ten as a means of evaluating or stimulating the exploitation of earth resources. The data from these federal surveys are generally available in the public domain and have been augmented by large amounts of data collected by academic institutions for basic-research investigations. Vast amounts of gravity and magnetic-anomaly data also have been acquired by industrial firms in mineral-resource exploration, but with few exceptions these data are generally not available to the public. In the United States, the publicly available data have recently been composited into new or improved country-wide anomaly maps. In 1975, the Society of Exploration Geophysicists (SEG) and the U.S. Geological Survey, recognizing the value of regional gravity and magnetic-anomaly maps, jointly organized gravity-and magnetic-anomaly map committees to prepare anomaly maps of the United States. The immediate objective of the committe es was to compile and publish a revised gravity-anomaly map of the conterminous United States and the first magnetic-anomaly maps of the conterminous United States and Alaska. These objectives were met in late 1982 with the publication of the Gravity Anomaly Map of the United States by the SEG and the release of the Composite Magnetic Anomaly Map of the Conterminous United States and the Magnetic Anomaly Map of Alaska by the U.S. Geological Survey. Small-scale versions of these maps are reproduced in color in this volume together with the most recently published gravity-and magnetic-anomaly maps of Canada. In recognition of the publication of these national maps, and to illustrate the many uses of gravity-and magnetic-anomaly maps, aseries of special technical sessions was held at the 52nd Annual International Meeting of the SEG in the fall of 1982 in Dallas, Texas. A total of 33 of the 53 papers presented at the special sessions were accepted for publication in this volume, The Utility of Regional Gravity and Magnetic Anomaly Maps.

Bouguer reduction density, why 2.67?

One of the most widely recognized parameters in solid-earth geophysics is the assumed density of the surface rocks of the continental crust, 2.67 g/cm 3 , or 2670 kg/m 3 in today9s preferred SI units. In the calculation of Bouguer and isostatic gravity anomalies, which are widely used for geological studies, the gravitational effect of the earth mass between an observation site and a datum is calculated assuming an infinite, flat slab of a specified density that approximates the mass. The datum for most surveys is sea level or, in the case of surveys of limited areas, it may be the elevation of the lowest station in the survey. Although the choice of datum is arbitrary, commonly sea level is used in an attempt to standardize anomalies from different surveys. Similarly, a density of 2.67 g/cm 3 is generally assumed for the density of the included mass in regional gravity surveys.

Gravity inversion of an interface above which the density contrast varies exponentially with depth

Mapping of an interface above which the density contrast varies exponentially with depth, as is common at the basement surface of sedimentary basins, is efficiently achieved by a theoretically precise gravity method which can be applied to either profile data or twodimensional data. The contrast in mass above the interface is modeled by an array of vertical rectangular prisms with density contrasts varying exponentially with depth. Gravity anomalies due to the prisms are calculated in the wavenumber domain and then converted to the space domain. The precision of the inverse numerical Fourier transform in this procedure is significantly increased by a shift‐sampling technique based on the discrete Fourier deviation equation. Depth to the interface is determined by iterative adjustment of the vertical extent of the prisms in accordance with observed gravity anomaly data. The basement surface of the Los Angeles basin, California, calculated by this method, closely duplicates the published configuration based...

Transcontinental Proterozoic provinces

Research on the Precambrian basement of North America over the past two decades has shown that Archean and earliest Proterozoic evolution culminated in suturing of Archean cratonic elements and pre-1.80-Ga Proterozoic terranes to form the Canadian Shield at about 1.80 Ga (Hoffman, 1988,1989a, b). We will refer to this part of Laurentia as the Hudsonian craton (Fig. 1) because it was fused together about 1.80 to 1.85 Ga during the Trans-Hudson and Penokean orogenies (Hoffman, 1988). The Hudsonian craton, including its extensions into the United States (Chapters 2 and 3, this volume), formed the foreland against which 1.8- to 1.6-Ga continental growth occurred, forming the larger Laurentia (Hoffman, 1989a, b). Geologic and geochronologic studies over the past three decades have shown that most of the Precambrian in the United States south of the Hudsonian craton and west of the Grenville province (Chapter 5) consists of a broad northeast to east-northeast-trending zone of orogenic provinces that formed between 1.8 and 1.6 Ga. This zone, including extensions into eastern Canada, comprises or hosts most rock units of this age in North America as well as extensive suites of 1.35- to 1.50-Ga granite and rhyolite. This addition to the Hudsonian Craton is referred to in this chapter as the Transcontinental Proterozoic provinces (Fig. 1); the plural form is used to denote the composite nature of this broad region. The Transcontinental Proterozoic provinces consist of many distinct lithotectonic entities that can be defined on the basis of regional lithology, regional structure, U-Pb ages from zircons, Sr-Nd-Pb isotopic signatures, and regional geophysical anomalies.

Tectonic development of the New Madrid rift complex, Mississippi embayment, North America

Abstract Geological and geophysical studies of the New Madrid Seismic Zone have revealed a buried late Precambrian rift beneath the upper Mississippi Embayment area. The rift has influenced the tectonics and geologic history of the area since late Precambrian time and is presently associated with the contemporary earthquake activity of the New Madrid Seismic Zone. The rift formed during late Precambrian to earliest Cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions. The configuration of the buried rift is interpreted from gravity, magnetic, seismic refraction, seismic reflection and stratigraphic studies. The increased mass of the crust in the rift zone, which is reflected by regional positive gravity anomalies over the upper Mississippi Embayment area, has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex. The correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rift structures. The slippage is due to reactivation of the structure by the contemporary, nearly E-W regional compressive stress which is the result of plate motions.

Spherical earth gravity and magnetic anomaly analysis by equivalent point source inversion

Abstract To facilitate geologic interpretation of satellite elevation potential field data, analysis techniques are developed and verified in the spherical domain that are commensurate with conventional flat earth methods of potential field interpretation. A powerful approach to the spherical earth problem relates potential field anomalies to a distribution of equivalent point sources by least squares matrix inversion. Linear transformations of the equivalent source field lead to corresponding geoidal anomalies, pseudo-anomalies, vector anomaly components, spatial derivatives, continuations, and differential magnetic pole reductions. A number of examples using 1°-averaged surface free-air gravity anomalies and POGO satellite magnetometer data for the United States, Mexico and Central America illustrate the capabilities of the method.

Major Proterozoic basement features of the eastern midcontinent of North America revealed by recent COCORP profiling

COCORP profiling in the eastern midcontinent of North America has (1) traced an extensive sequence of Precambrian layered rocks beneath southern Illinois, Indiana, and western Ohio; (2) detected a broad zone of east-dipping basement reflectors associated with the Grenville front beneath western Ohio; and (3) discovered a wide region of west -dipping reflectors penetrating most of the crust beneath eastern Ohio. The Precambrian layered assemblage may be as much as 11 km thick beneath southern Illinois, extends at least 170 km in an east-west direction, and contains several strong reflectors that have a lateral continuity of 80 km or more. Industry seismic data indicate that the layering is extensive in a north-south direction as well. Possible explanations for the layering include the silicic igneous rocks of the ca. 1.48 Ga eastern granite-rhyolite province, which are penetrated by basement drill holes throughout the region, perhaps intermixed or underlain by mafic igneous or sedimentary rocks. The 40-50-km-wide zone of strong, east-dipping (25°-30°) reflectors beneath west-central Ohio corresponds to the position of the Grenville front as determined from potential field and drill-hole data. These dipping reflectors in the upper and middle crust are interpreted to result from ductile deformation zones (mylonites) like those exposed at the Grenville front in Canada and imaged on the GLIMPCE seismic reflection lines in Lake Huron. Both the COCORP and GLIMPCE lines show a remarkably similar reflection geometry, despite the more than 500 km separating the two profiles. Easternmost Ohio appears to be underlain by pronounced west-dipping (<40°) reflectors in the middle and lower crust, which are also interpreted as marking a region of pervasive ductile deformation 80 km or more in width. Analogy with similar reflection packages elsewhere suggests that these reflections may mark a major collision zone. The west-dipping reflectors may be correlative with similar reflectors imaged on another COCORP survey in northern Alabama. The correlations suggested by these new results, though tentative, imply that the eastern midcontinent is composed of a relatively simple assemblage of crustal blocks.

The Role of Rifting in the Tectonic Development of the Midcontinent, U.S.A.

Abstract Recent studies have proposed the existence of several major ancient rift zones in the midcontinent region of North America. Although the dating of some of these rifts (and even the rift interpretations) are subject to question, an analysis of these “paleo-rifts” reveals three major episodes of rifting: Keweenawan (~ 1.1 b.y. B.P.), Eocambrian (~ 600 m.y. B.P.), and early Mesozoic (~ 200 m.y. B.P.). The extent of these events documents that rifting has played a major role in the tectonic development of the midcontinent region. This role goes well beyond the initial rifting event because these features display a strong correlation with Paleozoic basins and a strong propensity for reactivation. For example, the Eocambrian Reelfoot rift was reactivated in the Mesozoic to form the Mississippi embayment and is the site of modern seismicity which suggests reactivation in a contemporary stress field of ENE compression. Even though the importance of rifting can be established, recognition of rifts and delineation of their complexities remain a major problem which requires more study.

Geophysical Studies of Basement Geology of Southern Peninsula of Michigan

Only fragmentary direct information is available on the basement complex of the Southern Peninsula of Michigan because of limited and poorly distributed basement drill holes. This has encouraged the use of geophysical methods, primarily gravity and magnetic, to study the Precambrian formations. A basement configuration map prepared from magnetic depth estimates and basement drill tests confirms that the basement surface under the Southern Peninsula has the form of an oval depression reaching a maximum depth of more than 15,000 ft (4.5 km) below sea level on the western shore of Saginaw Bay. A basement topographic high is associated with the Howell anticline and a roughly north-south-striking basement trough plunges into the basin from the common boundary point of Indiana, Ohio, and Michigan. Aeromagnetic and Bouguer gravity anomaly maps, together with isotopic dates of samples obtained from basement drill holes and extrapolation of known Precambrian geologic trends, indicate that four basement provinces underlie the Southern Peninsula. The Penokean province can be traced geophysically from Lake Michigan into the Southern Peninsula, where it is characterized by east-southeast-striking geophysical anomalies. Central and southwestern Michigan is underlain primarily by felsic rocks correlating with the Central province. Basement rocks in southeastern Michigan strike north-northeast and are interpreted to be metamorphosed intrusive and extrusive rocks and mafic and felsic gneisses of the Grenville province. The Grenville front strikes south-southwest from Sagi aw Bay to west of the Howell anticline and from there due south to the Michigan Ohio boundary. A Keweenawan rift zone characterized by mafic intrusive and extrusive rocks and by uplifted gneisses transects all of the provinces and extends from Grand Traverse Bay to southeastern Michigan. Another subparallel ancient rift zone may be present in southwestern Michigan. These zones were formed during an episode of crustal extension in Keweenawan time. Subsequent deformation of the sedimentary rocks within the basin generally has been associated with movement along lines of basement weakness apparently related to the rift zone and Penokean structural trends.