North American Continental Margin Research Articles

Secondary magnetization of Triassic‐Jurassic volcaniclastic rocks of the Quesnel Terrane, Quesnel Lake, B.C.

Upper Triassic‐Lower Jurassic rocks at Quesnel Lake form part of the Quesnel terrane, which was emplaced against the Omineca Belt, part of the North American (paleo‐)continental margin, in the late Early Jurassic. Hard magnetizations, which may or may not be primary, were identified in volcanic wackes, but they are poorly grouped. However, removed magnetizations of intermediate unblocking temperatures (200‐500°C) and coercivities (30‐50 mT) are systematically directed northeast and downward (D=35°, I=57°, a95=9° 5 sites, 22 specimens; paleopole 62°N, 15°W, dm=13°, dp=10°) with respect to present horizontal. We interpret this to be an overprint, acquired after Middle to Late Jurassic folding, as a result of mid‐Cretaceous plutonism during the Cretaceous Normal Superchron. The paleopole is compatible with Early to mid‐Cretaceous paleopoles from other terranes in the southern Canadian Cordillera, but does not agree with observations from post‐Jurassic cratonic rocks. This disagreement indicates that in the mid‐Cretaceous these terranes, along with at least part of the Omineca Belt to which they were attached, formed a quasi‐rigid super‐terrane which was situated about 2000 km farther south than its present position relative to the craton.

Paleoceanography of the Deep Western Boundary Undercurrent on the North American continental margin for the past 25 000 yr

Temporal fluctuations of silt mean particle size and foraminiferal indices were used to examine fluctuations in velocity and position of the axis of the Deep Western Boundary Undercurrent (DWBUC) on the North American continental margin. From 25 to 17 ka the DWBUC axis was 500 m shallower and farther inshore than its present position on the lower rise; current speed was high from 25 to 20 ka and then waned until 17 ka. Between 17 and 7 ka the major axis of the DWBUC shallowed to 4000 m; current speed increased from 17 to 11 ka but slowed from H to 7 ka. Since 7 ka the DWBUC axis has deepened to its modern position at 4900 m. The zone of maximum carbonate dissolution remained below the DWBUC; the most intense dissolution was from 22 to 9 ka and 5 to 2 ka. Highest abundance of Uvigerina and highest percent benthic foraminifera were immediately above the DWBUC from 22 to 7 ka, after which values of both parameters fell to near zero.

Evolution of the Yukon-Tanana terrane: Evidence from southeastern Yukon Territory

The Yukon-Tanana terrane (YTT) in southeastern Yukon consists predominantly of a mid-Paleozoic volcanic-plutonic (arc?) assemblage built on continental crust. The YTT had experienced strong deformation and metamorphism by Late Triassic time. By mid-Cretaceous the metamorphic complex and Late Triassic clastic rocks derived from it were imbricated with middle and upper Paleozoic “ophiolitic” sheets. The YTT differs markedly from adjacent parts of the North American continental margin in both its depositional and tectonic histories. The composition and tectonic history of the YTT is comparable to that of the Kootenay and Barkerville terranes of southern and central British Columbia from which the YTT may have been displaced.

Tectonic synthesis of the Taconian orogeny in western New England

A tectonic synthesis based on stratigraphic and structural analysis of western New England is proposed for the Ordovician Taconian orogeny. It emphasizes arc-continental collision in which ocean-floor, continental-margin, and ensialic-rift rocks were imbricated westward in a repeatedly deformed accretionary wedge. Continued compression displaced segments of the North American sialic crust to the west and deformed the earlier emplaced slices of the Taconic allochthons which were derived from the continental margin. Critical arguments for this synthesis are (1) the west-to-east stratigraphic relations among the basal rift clastic rocks of the Dalton, Pinnacle, and Hoosac Formations of late Precambrian to Early Cambrian age; (2) the stratigraphic and sedimentological similarities between the rocks of the lower Taconic sequence and rocks in the Pinney Hollow and Underhill slices to the east and north of the Green Mountain massif; (3) the environmental similarities between the Cambrian and Lower Ordovician section of the Giddings Brook slice and the age-equivalent section in the St. Albans synclinorium; (4) the presence of carbonate platform rocks as slivers between each of the successively higher and younger premetamorphic slices (groups 1 and 2) of the Taconic allochthons; (5) the presence of synmetamorphic, fault-related structures in the youngest and highest slices (group 3) of the Taconic allochthons; (6) the recognition of extensive thrust zones in the pre-Silurian eugeoclinal sequence east of the middle Proterozoic basement of the Housatonic, Berkshire, Green Mountain, and Lincoln massifs; (7) the location of the Taconic root zone within the pre-Silurian eugeoclinal sequence; (8) the recognition of numerous faults in the serpentinite belt; (9) the similarity between the rocks of the Moretown Formation and modern fore-arc basin sequences; (10) the recognition that the volcanic arc-continental complexes of the Ascot-Weedon and Bronson Hill have been displaced westward over the Moretown and/or Hawley Formations along such faults as the Bristol and Coburn Hill thrusts; (11) the allochthonous and internally imbricated nature of the North American basement in the Berkshire massif; (12) the proposition that the Housatonic, Green Mountain, and Lincoln massifs, as well as the middle Proterozoic cored domes of southeastern Vermont, are also thick sialic slices of North American basement; (13) the recognition of medium-high- to high-pressure metamorphic mineral assemblages in the pre-Silurian eugeoclinal rocks of Vermont; and (14) the recent synthesis of isotopic age data by Sutter and others (1985). On the basis of an analysis of the foregoing arguments and relationships, a chronological sequence of seven structural sections between Albany, New York, and the Bronson Hill anticlinorium in central Massachusetts is used to depict the evolution of the Taconian orogeny. Retrodeformed distances are based on structural overlap and restoration of the Taconic slices to their depositional setting along the ancient North American continental margin. These easterly younging, diverticulated slices formed as a result of horizontal compression rather than gravity sliding. This palinspastic analysis implies the following. (1) Approximately 1,000 km of shortening has occurred during the emplacement of the Taconic allochthons and the subsequent imbrication of North American basement as thick sialic slices. Approximately 330 km of this shortening is attributed to multiple cleavage generations. (2) Repeated movement along such major surfaces as the Cameron9s Line-Whitcomb Summit-Belvidere Mountain thrust zone has buried the Taconic root zone. We suggest that the northern extension of this root zone is exposed to the east of the Lincoln massif in Vermont where the Underhill, Pinney Hollow, and Hazens Notch Formations are exposed. These formations, here considered thrust slices, disappear along the Belvidere Mountain-Whitcomb Summit thrust zone as it is traced southward into western Massachusetts and western Connecticut. (3) Taconian metamorphic rocks, particularly the older medium-high-pressure rocks in northern Vermont, have been transported westward on such reactivated surfaces as the Belvidere Mountain thrust. (4) The anticlinorial form of the middle Proterozoic basement in the Green Mountain and Lincoln massifs may have resulted from fault-bend folding on deep mantle-involved thrusts that developed late in the Taconian orogeny.

Paleoceanography of the Deep Western Boundary Under-current on the North American continental margin for the past 25,000 yr. : Ledbetter, M.T. and W.L. Balsam, 1985 Geology, geol. Soc. Am., 13(3):181–184

Paleoceanography of the Deep Western Boundary Under-current on the North American continental margin for the past 25,000 yr. : Ledbetter, M.T. and W.L. Balsam, 1985 Geology, geol. Soc. Am., 13(3):181–184

High-Resolution Seismic Stratigraphy of North Carolina Continental Margin, Cape Fear Terrace: Sea Level Cyclicity, Paleobathymetry, and Gulf Stream Dynamics: ABSTRACT

A high-resolution seismic stratigraphic study of the Cape Fear Terrace (outer continental shelf off North Carolina) combined with biolithostratigraphic data has yielded a chronostratigraphic framework of the Quaternary sequences that comprise this portion of the North American continental margin. The Cape Fear Terrace is an anomalous, point-source, prograding, shelf-margin feature that has experienced positive relief through much of the Quaternary. This upbuilding or outbuilding followed a period of active, early Pliocene, submarine erosion in which the ancestral Gulf Stream cut an erosional path beneath the present shelf margin. The terrace was originally built up during a relative lowstand of sea level with the construction of a shelf-edge deltaic feature. Severe modification of this delta front occurred during a relative highstand of sea level as the Gulf Stream began to impinge upon the margin. The anomalously thick accumulation of shelf-edge sediments acted as a barrier to flow, inducing complex flow patterns of the Gulf Stream. Excavation of these sediments yielded a ter ace feature with preferential erosion on the upstream side. Subsequent deposition in the terrace region may have resulted during fairly highstands of sea level, as evidenced by the presence of active seaward-prograding sand waves in the terrace region today. Once this shelf-edge bathymetric irregularity (the terrace) had been established, the Gulf Stream acted as a dynamic force inducing cellular flow structures within the shelf environment, which enabled sediments to be transported seaward along the paleo-shoals complex. End_of_Article - Last_Page 284------------

Provenance of Upper Ordovician (Richmondian) Bald Eagle and Juniata Formations, Central Pennsylvania: Implications for Nature of Taconian Orogeny in Central Appalachians: ABSTRACT

Medium to coarse-grained lithic sandstones and lithic-pebble conglomerates of the Bald Eagle and Juniata Formations contain recycled sedimentary, End_Page 1922------------------------------ low-grade metamorphic, and volcaniclastic detritus that was shed northwestward in an alluvial and/or fluvial system from an uplift produced during the Taconian orogeny. Intermediate to felsic volcaniclastic rock fragments comprise approximately 3% of the framework grains. They consist of devitrified shards, partially kaolinized plagioclase laths, and biotite in a cryptocrystalline groundmass. Siltstone, mudstone, chert, radiolarian chert, and recycled grains are indicative of a sedimentary source. Chlorite-quartz, epidote-quartz, and muscovite-chlorite phyllite clasts indicate a low-grade metamorphic provenance. Quartz varieties plotted on a diagram are consistent with a low-grade metamorphic provenance. Metaquartzite, slate, and cataclastic lithic fragments indicate tectonic deformation in the source area. Provenance criteria support a plate tectonic model involving an eastward-dipping subduction zone in the central Appalachians during the Middle to Late Ordovician. The climax of the Taconian orogeny has been described as a collision of an island arc with the North American continental margin. Petrologic data from the Bald Eagle and Juniata Formations suggest that this collision was accompanied by uplift of accretionary prism and continental margin or forearc basin sediments prior to the Richmondian. The presence of sedimentary, low-grade metamorphic, cataclastic, and volcaniclastic lithic fragments, and the notable absence of higher grade metamorphic and plutonic grain-types, indicate unroofing at relatively shallow levels of the orogene. End_of_Article - Last_Page 1923------------

Basement massifs in the Appalachians: their role in deformation during the Appalachian Orogenies

AbstractBasement is constituted of rocks which belong to a previous orogenic cycle which have been reactivated and incorporated into a younger cycle. Basement massifs may be classified according to their relative position in an orogen as external or internal massifs. They may also be categorized according to their role in deformation, as thrust‐related, fold‐related and composite massifs.All Appalachian external massifs were transported following their removal from the overridden edge of the ancient North American continental margin. Most of the internal massifs are also probably transported, but several (Pine Mountain and Sauratown Mountains) may be present as windows exposing parautochthonous basement beneath the main thrust sheet. The latter reside immediately west of the low (west) to high (east) gravity gradient which probably outlines the old edge of Grenvillian crust. Reactivated crustal material generated during early Palaeozoic orogeny plays the same mechanical role in reactivation as basement from the previous Grenville cycle. The domes of the Bronson Hill anticlinorium cored with Ordovician or older gneisses illustrate this behaviour.Basement (Grenville) massifs are distributed throughout the Appalachians as a belt of external massifs (Blue Ridge, Reading Prong, Hudson and Berkshire Highlands, Green Mountains, and Long Range Mountains) along the western edge of the crystalline metamorphic core. Additionally, internal massifs are also present (Pine Mountain belt, Tallulah Falls and Toxaway domes, Sauratown Mountains anticlinorium, State Farm gneiss dome, Baltimore Gneiss domes, Mine Ridge anticline, and Chain Lakes massif).Basement internal massifs probably served to localize thrusts by causing them to ramp over and around the massifs. Their antiformal shape may in part be as much related to thrust mechanics as to folding.

Phenomena of pulsation tectonics related to the breakup of the eastern north American continental margin

New data from the recent IPOD drilling of DSDP Site 534 in the Blake-Bahama Basin give a definite age for the spreading-center shift involved in the breakup of the North American Atlantic margin. A basal Callovian age (~ 155 m.y.) is determined for the Blake Spur magnetic anomaly marking this spreadingcenter shift that signals the birth of the modern North Atlantic Ocean. This is some 20 m.y. younger than previously thought. An implication of this younger age for the Blake Spur event is that very high spreading rates are now required for the Jurassic outer magnetic quiet zone along the North American margin. This association of a relatively high spreading rate with a magnetic quiet zone is similar to that for the mid-Cretaceous and implies a link between the processes controlling plate spreading, which are in the upper mantle, and the processes controlling the magnetic field, which are in the outer core. A theory of pulsation tectonics involving the cyclic eruption of plumes of hot mantle material from the lowermost mantle could explain the correlation. Plumes carry heat away from the core-mantle boundary and later reach the asthenosphere and lithosphere to induce faster spreading. The pulse of fast spreading in the Jurassic apparently followed the breakup of continents bordering the North Atlantic Ocean. Other pulses of fast spreading appear to correlate with major ocean openings on various parts of the globe, implying that this might be a prevalent process. Rifting of passive margins may be controlled by the more fundamental global processes described by the theory of pulsation tectonics.

Rb–Sr whole-rock age of the Klondike Schist, Yukon Territory

Seven samples of metarhyolite from a measured section of the Klondike Schist, in the Yukon Crystalline Terrane, 5 km southwest of Dawson City, give a whole-rock Rb–Sr isochron age of 202 ± 11 Ma with an initial 87Sr/86Sr ratio of 0.7140 ± 0.0001. The high initial ratio suggests that this is a metamorphic age, recording the time of shearing of the metarhyolites. An estimated minimum age of formation of the protolith of approximately 260 Ma was obtained by rotating the isochron to an assumed primary initial 87Sr/86Sr ratio of 0.707.The siliceous cataclasites of the Klondike Schist may have formed as a forearc wedge of arkose with intercalated volcanics. The unit was intensely sheared in a subduction zone, prior to obduction onto the ancient North American continental margin. The results of this isotopic study suggest an age of formation of 260 Ma and an age of cataclasis of 202 Ma. The metarhyolites remained closed to Rb–Sr isotopic equilibration during obduction, though the thermal event is recorded in K–Ar isotopic data reported by other authors. The results of this study are consistent with theories of formation of this assemblage.

Late Quaternary sedimentation in the western north Atlantic: stratigraphy and paleoceanography

A detailed analysis of stratigraphic indices was undertaken to study paleoclimatic change in the western North Atlantic. Three stratigraphic indicators — carbonate content, Globorotalia menardii, and Uvigerina peregrina — were tested in ten radiocarbon dated cores from the North American continental margin. Of these, carbonate content appears to be most accurate. Carbonate content exhibits two major increases from late glacial time to the present; estimated ages of the average of the mid-point of these increase are 13 200 B.P. and 8600 B.P. An absolute chronology was established down fifteen cores using these stratigraphic indicators and the radiocarbon dates. Sea-surface temperature (SST) estimates derived from planktonic foraminifera were mapped at 2000-yr intervals from 2000 B.P. to 14 000 B.P. SST warms rapidly from 12 000 to 8000 B.P., reaches a maximum between 8000 and 6000 B.P., and gradually cools after 6000 B.P. Bottom waters were monitored with Uvigerina peregrina and dissolution indices. The disappearance of Uvigerina suggests that the water type it lives in (low O 2?) started to thin from both the top and bottom by 13 000 B.P. and by 7300 B.P. had disappeared. Fragmentation of planktonic foraminifera was used as an index of dissolution. In most cores dissolution was intense during late glacial time, lessened during early postglacial time and resumed in the late postglacial.

Phenomena of Pulsation Tectonics Related to Early Rifting of Eastern North American Continental Margin: ABSTRACT

New data from the recent IPOD drilling of DSDP Site 534 in the Blake-Bahama Basin give a definitive age for the spreading-center shift involved in the early rifting of the North American Atlantic margin. A basal Callovian age (~155 m.y.) is determined for the Blake Spur magnetic anomaly marking this spreading-center shift that signals the birth of the modern North Atlantic Ocean. This is some 20 m.y. younger than previously thought. One implication of this surprising result is that this spreading-center shift starting North Atlantic rifting is now of an age which could be assigned to the spreading-center shift needed to end the rifting in the Gulf of Mexico. It is suggested that this might be one and the same event. Another implication of this surprisingly young age for t e Blake Spur event is that very high spreading rates are now required for the Jurassic outer magnetic quiet zone along the North American margin. This association of a high spreading rate with a magnetic quiet zone is similar to that for the middle Cretaceous and implies a link between the processes controlling plate spreading, which are in the upper mantle, and the processes controlling the magnetic field, which are in the outer core. A theory of pulsation tectonics involving the cyclic eruption of plumes of hot mantle material from the lowermost mantle could explain the correlation. Plumes carrying the heat away from the core/mantle boundary later reach the asthenosphere and lithosphere to induce faster spreading. The pulse of fast spreading in the Jurassic apparently caused the riftin of the North Atlantic. Other pulses of fast spreading appear to correlate with major ocean openings on various parts of the globe, implying that this may be a consistent process. Rifting of passive margins may be controlled by the more fundamental global processes described by the theory of pulsation tectonics. End_of_Article - Last_Page 1670------------

The distribution of 3He anomalies in the deep Atlantic

The 3He/ 4He results of the Atlantic GEOSECS program are interpreted in terms of a simple mixing model which allows the contribution of the 3He-rich southern component of deep water to be separated from that of the 3He-poor northern component. The anomalies from the mixing line connecting these components reveal a 3He-rich zone over the Mid-Atlantic Ridge 16° south of the equator and one against the North American continental margin off New Jersey at about the same elevation. The origin of the latter feature remains a mystery.

Pacific-North American plate interaction and Neogene volcanism in coastal California

Palinspastic and global plate tectonic reconstructions of the southwestern United States, combined with analysis of the distribution of Cenozoic igneous activity suggest that Neogene and younger volcanic rocks in the Continental Borderland and Coast, Transverse and Peninsular Ranges appear to reflect subduction of the Farallon plate south of the Mendocino Fracture Zone and overriding of the Pacific—Farallon ridge. The igneous activity of Neogene age in coastal California appears to be a continuation of fields of magmatism in adjacent areas of southeastern California, Arizona and Baja California. With the close approach of the Pacific—Farallon ridge to the North American continental margin, progressively younger, hotter and thinner Farallon plate underwent subduction beneath California, resulting in more elevated temperatures at shallower depths and closer to the trench. Consequently, the associated volcanic arc apparently shifted to the west. Volcanic activity in coastal California appears to have been most widespread shortly after overriding of the Pacific—Farallon ridge occurred— that is, at that time at which the oceanic lithosphere was thinnest, and hot, partially molten asthenosphere was closest to the subduction zone. The zone of near-coast igneous activity in California was probably limited on the north by the extension of the Mendocino Fracture Zone in the descending Farallon plate. South of the fracture zone the lithosphere was young and hot; to the north it was older and, therefore, colder. Thus, Neogene igneous activity in the Basin-Range Province was associated with more typical subduction of older lithosphere involving a deeper, down-dip locus of melting, resulting in a wider arc-trench gap. The abrupt narrowing of the Neogene arc-trench gap from north to south occurred near the present-day Garlock Fault. The role of oblique and en echelon strike-slip faulting in permitting and localizing middle Cenozoic magmatism in the areas west of the San Andreas Fault is uncertain. Discrepancies in timing of movement on the northern and southern San Andreas Fault seem to require oblique strike-slip faulting, and by association, igneous activity, in the Borderland and Transverse Ranges prior to 10 m.y. B.P. Oblique, divergent strike-slip faulting in the San Francisco Bay area does seem to be responsible for significant igneous activity between 14 m.y. B.P. and present. The Page Mill, Grizzly peak , Sonoma and Clear Lake volcanic fields occur in zones of divergence between faults of the San Andreas, Calaveras and Hayward systems. Further, the age of these volcanic rocks corresponds with the documented displacement history of the San Andreas Fault.

Evidence for Post-Jurassic Tectonism in Eastern North America: ABSTRACT

The Gulf Coastal Plain province extends eastward along the North American continental margin, and makes up a large part of the terrain of eastern North America. Both the Gulf Coast and eastern North America have been regarded as regions quiescent since the Jurassic Period because there appears to have been no obvious tectonic activity there, or any reason for it to occur. However, recent investigations, especially in eastern North America, suggest that this view may be too conservative. Post-Jurassic tectonic activity in eastern North America is indicated by widespread faulting, extensive subsidence and uplift which continue to the present, igneous activity, and a regional horizontal compressive stress. Much of this activity seems to be associated with compressional deformation, and vertical uplift and subsidence. The regional extent of these events is very large, though the magnitude of the diastrophism is less spectacular than in other regions generally associated with orogenic activity. Any model that attempts to explain post-Jurassic tectonism in eastern North America must account for these types of activity. End_of_Article - Last_Page 1610------------

Sedimentation Processes and Hydrocarbon Potential of Continental Rise off North America: ABSTRACT

The continental rise, within the 200-mi (320 km) economic zone, represents both the largest accumulation of terrigenous sediment on this planet and perhaps the largest unexploited hydrocarbon repository. Several models have been presented for the origin of the rise; in the 1950s and 1960s, R. Dietz, M. Kay, C. Drake, M. Ewing, and others suggested a model of pyroclastics and turbidity-current deposition which produced a thick accumulation (eugeosyncline) of alternating coarse- and fine-grained sediment. This view of continental-rise sedimentation was proposed prior to the discovery that deep thermohaline circulation could play a significant geologic role at abyssal depths. This latter concept was first hypothesized by the writer, who presented a model for along-slope fine-sediment dispersal driven by thermohaline circulation. This view held that turbidity currents inject sediment into contour-following, near-bottom currents; only fine-grained material is wafted parallel with contours, whereas the coarser turbidite material largely bypasses the continental rise and is deposited on the abyssal plains. This concept is supported by the results of Leg 11 of the DSDP. The present sedimentary environment of the lower continental rise (3.5 to 5 km deep) in the western North Atlantic is now known to be dominated by vigorous near-bottom, contour-following currents (i.e., contour currents). Dense water originating in the Labrador, Norwegian, and Irminger Seas flows south and west through various fracture zones until it reaches the North American continental margin, where it is constrained to flow against the massive sedimentary apron of the continental rise. The obvious implication of the contour-current concept for petroleum interest is that there would be little or no deposition on the continental rise of sediments End_Page 467------------------------------ suitable for forming reservoir rock during the time that deep circulation has been an important process in the western North Atlantic. This process is thought to have begun about 50 m.y.B.P. There would be deposition only of fine-grained, relatively impermeable, potential source beds. It is concluded that Cenozoic sediments of the continental rise, at least off the east coast of North America, may not be a likely source for future hydrocarbon recovery. A few deep holes into the continental rise (preceded by complete seismic surveys) are needed to assess the potential of the underlying, deeply buried Paleozoic section. End_of_Article - Last_Page 468------------

Tectonically induced deformation of deep-sea sediments off Washington and northern Oregon: Mechanical consolidation

Convergent motion of the North American and Juan de Fuca plates has resulted in deformation of Cascadia Basin sediments and accretion of these deposits to the North American continental margin. The accreted deposits, which occur as anticlinal ridges and thrust blocks, constitute the lower continental slope or borderland off Washington and northern Oregon. Over the past 2.0 m.y., approximately 30 km of this deformed material has been added to the lower slope, removing undeformed deposits from Cascadia Basin at a rate of 2.3–2.9 cm/yr. Near-surface sediments involved in this accretionary process are mechanically consolidated: muds̀tones dredged from the lower slope exhibit physical properties (water contents, 20–47%; void ratios, 0.4–1.2; preconsolidation pressures, 0.8–8.2 MPa) which differ significantly from properties of similar, but undeformed sediments (water contents, 50–250%; void ratios, 1.1–1.9). While some consolidation may be attributable to prior burial ( < 2.0 MPa ) or carbonate precipitation, neither mechanism can wholly account for the values observed. It appears that most of the consolidation has occurred in response to tectonically induced overpressures. Initial consolidation occurs rapidly across a narrow ( < 3 km ) front, defined by the base of the continental slope. Further consolidation and dewatering appears to take place, at a much reduced rate, over the entire width of the lower slope. Development of foliation is nearly ubiquitous in the deformed mudstones. This property limits the strength of the deposits (shear strengths, 90–416 kPa) and movement along these planes probably accommodates much of the strain after initial consolidation. The physical properties characteristic of Washington—Oregon deformed sediments may represent limiting values for mechanical consolidation of near-surface terrigenous sediments under horizontal stress.

Possible Role of Transform Faults in the Development of Apparent Offsets in the Ouachita-Southern Appalachian Tectonic Belt

An apparent offset between the Marathon Mountains and the buried western extension of the Ouachita Mountains, and another between the Ouachita and southern Appalachian Mountains have been the subject of numerous and variable interpretations. Here, they are explained in terms of plate-tectonic theory, as previously applied to the Ouachita and southern Appalachian systems. The presently exposed tectonic belt lies somewhat north of the boundary between continental crust and upper mantle to the north and oceanic crust and upper mantle to the south. This boundary is interpreted as a Paleozoic plate margin. Apparent offsets in the boundary and in the tectonic belt are thought to represent the location of former trench-trench (or arc-arc) transform faults which allowed subduction and subsequent orogeny to take place in en echelon fashion along an irregular Precambrian-Paleozoic North American continental margin. The irregularity of continental-margin shape may be a result of supercontinent (Pangaea I?) fragmenta...

Turbidites of the Aleutian abyssal plain: Mineralogy, provenance, and constraints for Cenozoic motion of the Pacific plate

The Aleutian abyssal plain is a fossil abyssal plain of Paleogene age in the western Gulf of Alaska. The plain is a large, southward-thinning turbidite apron now cut off from sediment sources by the Aleutian Trench. Turbidite sedimentation ceased about 30 m.y. ago, and the apron is now buried under a thick blanket of pelagic deposits. Turbidites of the plain were recovered at site 183 of the Deep Sea Drilling Project on the northern edge of the apron. The heavy-mineral fraction of sand-sized samples is mostly amphibole and epidote with minor pyroxene, garnet, and sphene. The light-mineral fraction is mostly quartzose debris and feldspars. Subordinate lithic fragments consist of roughly equal amounts of metamorphic, plutonic, sedimentary, and volcanic grains. The sand compositions are arkoses in many sandstone classifications, although if fine silt is included with clay as matrix, the sand deposits are feldspathic or lithofeldspathic graywacke. The sands are apparently first-cycle products of deep dissection into a plutonic terrane, and they contrast sharply with arc-derived volcanic sandstones of similar age common on the adjacent North American continental margin. The turbidite sands are stratigraphically remarkably constant in composition, which indicates derivation from virtually the same terrane through a time span approaching 20 m.y. Comparison of Aleutian plain data with the compositions of coeval sedimentary rocks from the northeast Pacific margin shows that the Kodiak shelf area includes possible proximal equivalents of the more distal turbidites. Derivation from the volcaniclastic Mesozoic flysch of the Shumagin-Kodiak shelf is unlikely; more probably the sediments were derived from primary plutonic sources. The turbidites also resemble deposits in the Chugach Mountains and the younger turbidites of the Alaskan abyssal plain and could conceivably have been derived from the coast ranges of southeastern Alaska or western British Columbia. The Aleutian plain sediment most likely was not derived from as far south as the Oregon-Washington continental margin, where coeval sedimentary deposits are dominantly volcaniclastic. This work lends some support to earlier suggestions that the fan-shaped turbidite body originated on the continental margin in the Gulf of Alaska, and it supports models of little or modest motion of the Pacific plate relative to North America. The mineralogy alone cannot refute more ambitious motion models, but when combined with previously published evidence on size of the plain, sediment thicknesses, and nan-nofossil species diversity, the data seriously constrain models requiring large-scale northwestward motion of the Pacific plate in post-Eocene time.

Evidence for the Emergence of the Hercynian Front on the North American Continent

Recent marine seismic reflection, bathymetric and magnetic measurements made across the Charlie Gibbs Fracture Zone have clearly shown that the fracture zone extends to the east of 17 °west longitude. Projections to the west of the Mid-Atlantic Ridge extend the fracture zone to the North American Continental margin northeast of Newfoundland. Projection of the structural trends of the Hercynian Front from the European continental margin offer a remarkable linearity with the Charlie Gibbs Fracture Zone on the west. Remnants of the Hercynian Front have been identified in New Brunswick. Flemish Cap, which foundered during the initial phase of the latest opening of the Atlantic is believed to have been located on the fracture zone's southern edge, before drifting southeasterly to its present position. The alignment of the Hercynian Front with the Charlie Gibbs Fracture Zone is linked up on the North American continent and is offered as a possible clue to the pre-drift configuration of the Laurasian continent.