Edward G. Lidiak

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.

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.

Basement Rocks in Continental Interior of United States

This paper outlines the Precambrian geologic history of the continental interior of the United States by describing several selected areas from north to south. In this way the history is developed chronologically and the various areas illustrate the methods used in preparing a map of the buried basement. Lateral continental accretion is of lesser importance than is usually postulated; at least 50 percent of North America was in existence 2,500 m.y. ago. Crustal stabilization occurred at different times (about 2,500, 1,700, 1,350, and 1,000 m.y. ago) in the North American continental interior. Buried basement rocks from more than 3,000 wells and scattered outcrops in the continental interior of the United States were studied petrographically. This information, in conjunction with geophysical data and isotopic ages, was used to outline geological units and to work out the geological history. Except for the Cambrian igneous complex of the Wichita Mountains, southern Oklahoma, all basement rocks between the Rocky and Appalachian Mountains are Precambrian. Included here as basement are the widespread Precambrian sedimentary and volcanic rock sequences as well as the crystalline continental crust usually defined as basement. Rocks 2,500 m.y. old or older exposed in northern Minnesota occur as belts of west-southwest-trending granite and gneiss alternating with greenstone and schist. They extend without interruption into eastern North Dakota and northeastern South Dakota. A large part of Wyoming also is underlain by rocks at least 2,500 m.y. old. Well data for the Williston basin in eastern Montana and western North and South Dakota are too sparse to permit construction of a geologic map of the basement but isotopic ages and gravity anomalies suggest that this region is underlain by rocks deformed and consolidated about 1,700 m.y. ago during the Black Hills orogeny. The sedimentary parent rocks of the Precambrian rocks exposed in the Black Hills were deposited on an older basement that contains zircon dated at 2,550 m.y. The Black Hills are the culmination of the long north-northwest-trending basement high that includes the Chadron and Cambridge arches of Nebraska and the Central Kansas uplift. Most wells to basement have penetrated metamorphic rocks along this uplift. A granite batholith underlies the End_Page 2351------------------------------ area along the Nebraska-Kansas boundary. North of this batholith, along the uplift, isotopic ages of granite and gneiss are similar to those from the Black Hills (ca. 1,700 m.y.). The batholith intrudes mafic schist as well as the granite and gneiss; Rb-Sr ages are 1,490 m.y.; K-Ar ages of biotite from the granite batholith and schist as well as whole-rock ages of the schist are between 1,360 and 1,300 m.y. These 1,360-1,300-m.y. ages reflect a period of metamorphism or thermal activity subsequent to the original formation of the rocks but are not known to be associated with a granite-forming event here. K-Ar ages of biotite from cataclastic zones in southwestern Nebraska are 1,170 m.y. A body of anorthosite-gabbro in southwestern Nebraska has not been dated, but its cataclastic textu e and a zone of cataclasis that trends toward the body suggest that its intrusion is either contemporaneous with, or older than, the time of cataclasis. Isotopic ages in central Kansas suggest two thermal events: 1,450-1,350 m.y. and 1,250-1,100 m.y. The latter is defined on the basis of Rb-Sr and K-Ar mica ages, the former on Rb-Sr feldspar or whole-rock analyses. Granite in southeastern Nebraska, northeastern Kansas, and central Missouri is at least 1,450 m.y. old. Most of the Nemaha uplift as well as many granites at least as far west as south-central New Mexico give isotopic ages in the 1,450 to 1,350-m.y. range. During this period (the Nemaha igneous activity), rocks from New Mexico eastward to Ohio were consolidated to form the basement for younger extensive volcanic-intrusive complexes. Keweenawan basalt and associated sedimentary rock units are nearly continuous from northeastern Kansas to the outcrop belt around Lake Superior. An offset in trend and a break in continuity near the Nebraska-Kansas border have been postulated to be the result of a post-Precambrian transcontinental wrench fault. Cataclasis, recognizable in some basement samples within this discontinuity, could be associated with either rifting of the Keweenawan trough or Paleozoic movement along the adjacent north-trending Nemaha uplift. Extensive areas in southeastern Missouri, northeastern Oklahoma and vicinity, and in and near the Texas Panhandle are underlain by rhyolite and related granite. Rhyolite is found in isolated localities in Wisconsin and in some of the few basement tests in Illinois, Indiana, and Ohio. Three distinct volcanic-intrusive events can be defined: St. Francois igneous activity of southeastern Missouri (to Ohio?), 1,350-1,200 m.y. ago; Spavinaw igneous activity of northeastern Oklahoma, 1,300-1,150 m.y. ago (with a 1,200 ± 30 m.y. isochron); and Panhandle igneous activity of Texas and eastern New Mexico, 1,200-1,100 m.y. ago (1,140 ± 50 m.y. average). Each igneous event produced micrographic granite as well as rhyolite (many samples show well-preserved welded tuff textures). Rhyolite from the Spavinaw and Panhandle igneous activities covers at least 24,000 and 21,500 sq mi, respectively; the original extent of the volcanic fields is unknown. Exposed in the Llano uplift of central Texas is a thick sequence of sedimentary rocks that were deposited, folded, metamorphosed, and intruded 1,150 to 1,000 m.y. ago during the Llano orogeny. Rocks associated in time with this event are found in the subsurface of north-central Texas and Trans-Pecos Texas. Extensive areas are covered by sedimentary rocks of probable or known Precambrian age. From north to south these include: Sioux Quartzite, minimum age 1,200 m.y.; upper Keweenawan sediments in the Keweenawan trough; sediments of southwestern Missouri-southeastern Kansas; Rice Formation of Scott, central Kansas; Tillman metasedimentary group of Ham et al., southern Oklahoma and adjacent parts of Texas; De Baca terrane of this report, south-central New Mexico (possibly correlative with the Swisher diabasic terrane = Swisher gabbroic terrane of Flawn, of the Texas Panhandle); and Fisher metasedimentary terrane of Flawn, north-central Texas (probably correlative with the Llano uplift metasedimentary rocks). The last major igneous event of the central United States before the Paleozoic marine inundation was the intrusion and extrusion of the early to mid-Cambrian basalt-gabbro-rhyolite-granite igneous complex of the Wichita Mountains of southern Oklahoma. Known Precambrian rocks have yet to be penetrated by wells in the Gulf coastal plain. Crystalline basement rocks in the inner coastal plain appear to be metamorphosed Paleozoic rocks of the Ouachita system. Southward the basement is covered by thick sedimentary rocks of the Gulf Coast geosyncline.

Cretaceous to Mid-Eocene pelagic sediment budget in Puerto Rico and the Virgin Islands (northeast Antilles Island arc)

Island arc basalts (IAB) in the Greater Antilles, dating between Albian and mid-Eocene time (~112 to 45 Ma), consist of an early low-K, primitive island arc (PIA) basalt series and a later, predominantly intermediate calcalkaline (CA) series. The rocks resemble modern sediment-poor, low-light rare earth element (LREE)/heavy rare earth element (HREE) arc basalts from intra-oceanic tectonic settings and sediment-rich, high-LREE/HREE types from continental margin arcs, respectively. Isotope and incompatible trace element distribution along a 450 km segment of the arc in the northeast Antilles demonstrates that low-LREE/HREE basalts predominate in Albian to Santonian (~85 Ma) stratigraphic sequences in the Virgin Islands (VI) and northeast Puerto Rico (NEPR), while there is a gradual but spectacular increase in both LREE/HREE and absolute abundances of incompatible elements in central Puerto Rico (CPR). Northeastern Antilles basalts have consistently elevated La/Nb and relatively low Nb/Zr, both inconsistent with the presence of a significant ocean island basalt component. Hence, observed differences are interpreted to reflect variation in proportions of pelagic sediment subducted by the south-dipping Antilles arc system as it swept north-eastward across the Caribbean region and eventually approached the Bahama Banks along the south-eastern fringes of the North American Plate. Trace element mixing models indicate sediment proportions in VI and NEPR were limited, averaging considerably below 1.0%. In comparison sediment content in CPR increased from an average slightly above 1.0% in Albian (~112 Ma) basalts to as high as 8% in Cenomanian (100-94 Ma) types. Hypothetical pre-arc pelagic sedimentary facies in the subducted proto-Atlantic (or proto-Caribbean) basin, included 1) a young, centrally located longitudinal ridge-crest facies, with a thin sediment cover, eventually subducted by VI and NEPR, 2) a slightly older basin-margin facies of variable width and moderate sediment thickness, subducted by CPR during Albian time, and 3) a thick, pre-arc continental margin facies in the vicinity of Central America, subducted by CPR during Cenomanian time. Following collision of neighboring Hispaniola with the Bahamas sediment budgets in the northeast Antilles stabilized at moderate levels from 2 to 3%, reflecting widespread subduction of North Atlantic Cretaceous pelagic sediment (AKPS).

Crustal Study of a Continental Strip from the Atlantic Ocean to the Rocky Mountains

Twenty aeromagnetic profiles over a 100-mile-wide strip along the arc of a great circle passing through Denver, Colorado, and Washington, D. C, reveal large anomalies of major crustal significance. Contoured data disclose several areas of distinct magnetic patterns reflecting basement lithology and structure. The mafic rocks of the Blue Ridge and Piedmont and the Keweenawan mafic belt in Iowa and Nebraska give rise to strong linear trends. Areas with a more random pattern of closely spaced magnetic anomalies appear in central Ohio, eastern Iowa, and central and western Nebraska. Except in the Blue Ridge and Piedmont areas, crystalline basement rocks are covered by a thick blanket of virtually nonmagnetic sedimentary rocks, and lithology must be inferred from correlations of the magnetic data with scattered drill-hole data and regional gravity data. The area of highly magnetic rocks in central Ohio has a sharp western boundary that coincides with the western limit of metamorphism associated with the probable extension into Ohio of the Grenville province of Canada. This area and a similar one in eastern Iowa are linked by an arcuate, nearly continuous belt of positive gravity anomalies that extends north into Wisconsin and northern Michigan and then swings southeast across central Michigan. This horseshoe-shaped feature is associated with lithologically diverse but highly magnetic basement rocks. A group of linear magnetic anomalies in western Iowa and eastern Nebraska correlates with the well-known midcontinent gravity high. In Nebraska the magnetic data provide a basis for grouping the extremely complex drill-hole data into three over all lithologic terranes. An analysis of the long-wavelength (>40 miles) variations of the profiles shows that they form a number of large coherent anomalies, many of which show little relation to the major tectonic trends and lithologic patterns of the basement surface. The very broad and less numerous anomalies in the east, which have more or less north-south trends, are significantly different from the more numerous anomalies in the western part of the strip, which tend to trend east-west. One linear anomaly extends for nearly 500 miles across Nebraska and Iowa and may mark a zone of rifting. Heat-flow data show that rocks at the Curie point, which determines the depth below which rock magnetization cannot occur, may be deep enough, at least in shield and other stable parts of the continent, to include a part of the upper mantle. The concentration of the large magnetic features in Iowa and Nebraska may indicate that the thickness of magnetized rock is greater in this area and that perhaps some of these features originate in the upper mantle.

Role of crustal melting in petrogenesis of the Cretaceous Water Island Formation (Virgin Islands, northeast Antilles Island arc)

The latest Aptian to earliest Albian (~115 Ma) Water Island Fm in the Virgin Islands contains some of the oldest known arc-related strata in the Greater Antilles Island Arc. Hence, the unit is of considerable significance in tectonic reconstructions of initial subduction parameters along the long-lived destructive plate margin separating the North American and Caribbean Plates. Exposed Water Island strata are bimodal, consisting predominantly of altered dacite and rhyolite (originally called keratophyre; 65-85% SiO2) and subordinate degraded (spilite; 46-57% SiO2). TiO2 content of Water Island basalt averages approximately 0.5%, resembling borderline intermediate-Ti boninite basalts, consistent with low incompatible element abundances and low normalized light rare earth elements (LREE) with respect to Sm. Trace element patterns of the felsic suite, characterized by pronounced negative normalized anomalies for high field-strength elements (HFSE), low Sr/Y, and low absolute rare earth element (REE) abundances, and relatively flat normalized REE patterns, have analogues in plagiorhyolite suites from bimodal Cenozoic arcs, including the western Aleutians, Izu-Bonin, the Kermadecs, and South Sandwich. Relatively low incompatible element concentrations in plagiorhyolites and contrasting normalized incompatible trace element patterns in basalts preclude an origin of Water Island plagiorhyolite through MORB-type fractional crystallization. Compositions are consistent instead with melting models involving partial fusion of amphibole-bearing gabbro at low pressures (within the stability range of plagioclase) in response to introduction of heat and aqueous flux by arc-related basalt melts and associated hydrothermal fluids during transmission to the surface. Truncation of the basalt fractional crystallization trend at SiO2 = 57% indicates evolved island arc basalt (IAB) crystal fractionates were gradually displaced from crustal magma conduits by more buoyant plagiorhyolite melt, and trapped in underplated, sub-crustal magma chambers. Basalts have low (Ce/Ce*)N (average ˜ 0.78), indicating the presence of significant pelagic sediment (0.5 to 1.5% Atlantic Cretaceous pelagic sediment, AKPS). One subunit of relatively high-HFSE plagiorhyolite has (Ce/Ce*)N near-expected values, but another with low-HFSE has slightly lower than expected (Ce/Ce*)N, consistent with a small sediment component. Absence of intermediate andesite from the Water Island Fm is inconsistent, however, with basaltrhyolite magma mixing processes. Consequently, incorporation of sediment by low-HFSE plagiorhyolite is inferred to have resulted from re-melting of arc-related gabbro.

Petrogenesis of fertile mantle peridotites from the Monte del Estado massif (southwest Puerto Rico): a preserved section of Proto-Caribbean lithospheric mantle?

The Monte del Estado massif is the largest and northernmost serpentinized peridotite belt in southwest Puerto Rico. It is mainly composed of spinel lherzolite and minor harzburgite with variable clinopyroxene modal abundances. Mineral and whole rock major and trace element compositions of peridotites coincide with those of fertile abyssal mantle rocks from mid ocean ridges. Peridotites lost 2-14 wt% of relative MgO and variable amounts of CaO by serpentinization and seafloor weathering. HREE contents in whole rock indicate that the Monte del Estado peridotites are residues after low to moderate degrees (2-15%) of fractional partial melting in the spinel stability field. However, very low LREE/HREE and MREE/HREE in clinopyroxene cannot be explained by melting models of a spinel lherzolite source and support that the Monte del Estado peridotites experienced initial low fractional melting degrees (~ 4%) in the garnet stability field. The relative enrichment of LREE in whole rock is not due to alteration processes but probably reflects the capture of percolating fluid/melt fractions or the crystallization of sub-percent amounts of hydrous minerals (e.g., amphibole, phlogopite) along grain boundaries or as microinclusions in minerals. We propose that the Monte del Estado peridotite belt represents a section of ancient Proto-Caribbean (Atlantic) lithospheric mantle originated by seafloor spreading between North and South America in the Late Jurassic- Early Cretaceous. This portion of oceanic lithospheric mantle was subsequently trapped in the forearc region of the Greater Antilles paleo-island arc generated by the northward subduction of the Caribbean plate beneath the Proto-Caribbean ocean. Finally, the Monte del Estado peridotites belt was emplaced in the Early Cretaceous probably as result of the change in subduction polarity of the Greater Antilles paleo-island arc without having been significantly modified by subduction processes

Buried Precambrian Rocks of South Dakota

Early Precambrian granitic gneiss, mafic schist, and granite, outlined by prominent east-northeast-trending magnetic and gravity anomalies, are extensive in the buried basement of northeastern South Dakota, the southwestern-most extension of the Superior geological province. These rocks were involved in widespread orogenesis about 2500 m.y. ago. In central and western South Dakota equally prominent geophysical anomalies define the trend of middle Precambrian rocks. A middle Precambrian sedimentary succession in the Black Hills was deformed and intruded by granite approximately 1700 m.y. ago. Four distinct metamorphic belts which possibly date mainly from middle Precambrian time are recognized. A large granite terrane in southern South Dakota is tentatively interpreted as having formed 1430 to 1460 m.y. ago. The basement complex in southeastern and central South Dakota is overlain by a sheet of Sioux Quartzite. A new Rb-Sr age determination on a rhyolite suggests that the Sioux Quartzite is at least 1470 m.y. old.

The case for persistent southwest-dipping Cretaceous convergence in the northeast Antilles: Geochemistry, melting models, and tectonic implications

Constraints on the polarity of Cretaceous subduction in the Greater Antilles are provided through geochemical comparison between the erupted island arc lavas in central Puerto Rico and potential pelagic sediment reservoirs in the fl anking ocean basins. Early Jurassic to mid-Cretaceous (185- to 65-Ma) sediment from the open Pacifi c on the southwest is dominated by pelagic chert, which is highly refractory and depleted with respect to incompatible elements. In comparison, mid- to Late Cretaceous (ca. 112- to 65-Ma) sediment from the younger Atlantic basin on the northeast was dominated by mixtures of two end members. These include (1) biogenic clay and carbonates with elevated light rareearth element (LREE) abundances, negative MORB-normalized, high fi eld-strength element (HFSE) anomalies, and low Zr/Sm; and (2) turbiditic detritus of upper continental crust composition with high LREE, comparatively shallow HFSE anomalies, and high Zr/Sm. Compositions of Puerto Rican arc basalts are inconsistent with incorporation of Pacifi c pelagic chert. Instead, patterns characteristic of high-Fe island arc tholeiites are reproduced by incorporation of up to 4% of a low-Zr/Sm biogenic sediment component of Atlantic origin, whereas patterns of low-Fe lavas require, in addition to biogenic sediment, introduction of up to 2% of a high-Zr/ Sm crustal turbidite component. The Atlantic origin of all the subducted sediments indicates the polarity of subduction throughout the Cretaceous in the northeast Antilles was persistently southwest dipping. This conclusion is supported by the presence of a lowZr/Sm suprasubduction zone component of Atlantic origin in Caribbean plateau basalts (91‐88 Ma) from southwest Puerto Rico, which were erupted within the broad backarc region of the Greater Antilles during intermediate stages of arc development.