Undeformed Sediments Research Articles

Magnetic fabric analysis of the Plio-Pleistocene sedimentary formations of the Coastal Range of Taiwan

Over 2100 samples were collected from 130 sedimentary sites of early Pliocene to Pleistocene age in the Coastal Range of Eastern Taiwan. Their lithology is mainly unmetamorphosed marls, siltstones and sandstones and the formations are folded and faulted at different scales. Different shapes of the susceptibility ellipsoid were observed from an oblate form, with K min closely perpendicular to the bedding plane, to a “pencil” structure, with K min ≈ K int forming a girdle around K max, reflecting an increasing tectonic contribution to the magnetic fabric. At sites where fault tectonic analysis is available, the magnetic lineation is perpendicular to the main compression. These results have a twofold significance: first, confirming previous preliminary results, they show that in such visually undeformed sediments the magnetic fabric may be, at least in part, of tectonic origin. Second, when considered together with recent paleomagnetic and tectonic data, they suggest that in the Plio-Pleistocene sediments of the Coastal Range of Taiwan the magnetic lineation arises from the compression due to the collision of the Philippine Sea and the Eurasian plates. In our interpretation, the stress pattern has rotated clockwise along with the structures during this collision.

The Thermal History of the Arkoma Basin and the Eastern Ouachita Mountains from AFTA (Apatite Fission Track Analysis): ABSTRACT

Results of an Apatite Fission Track Analysis (AFTA) study indicate that regional cooling commencing around the middle Cretaceous ({approximately} 100 Ma) has affected both the relatively undeformed Arkoma basin sediments and the folded rocks of the eastern Ouachita Mountains (Benton uplift). Outcropping Pennsylvanian rocks of the Arkoma basin sequence in a section along the Arkansas River between Little Rock and Ozark, Arkansas, as well as Mississippian and older rocks in a section across the Ouachitas between Russellville and Hot Springs, Arkansas, experienced paleotemperatures between about 90 and 110{degree}C immediately prior to middle Cretaceous cooling. Cretaceous intrusive rocks have apatite fission track ages indistinguishable from K-Ar and Rb-Sr ages at about 95 Ma with mean track lengths greater than 14 {mu}m indicating rapid cooling from temperatures greater than approximately 110-50{degree}C or less at this time. AFTA data from Missouri also show evidence of cooling from elevated paleotemperatures during the middle Cretaceous and suggest the possibility of raised thermal gradients ({approximately}50{degree}C/km). These data emphasize the regional extent of the observed cooling episode, which appears to have involved uplift and erosion of at least 1-2 km of section from across the region since the middle Cretaceous. Comparison of these results with published vitrinitemore » reflectance data suggests that the Arkoma basin sequence achieved maximum paleotemperatures during late Paleozoic burial, and that paleotemperatures reached in the Mesozoic, during rifting in the Mississippi Embayment, were of lower magnitude. These observations have important implications for hydrocarbon generation and exploration in the Arkoma basin.« less

Structural styles of growth faults in the U.S. Gulf Coast Basin

Abstract Structure and sediments related to shelf margin growth faults are an important exploration objective in the Cenozoic in the U.S. Gulf Coast basin. The syndepositional nature of these fault systems has been recognized for decades with well control and seismic. Recent seismic of higher quality and deeper penetration has begun to reveal more clearly the variety of structural styles of the ‘typical’ Gulf Coast shelf margin growth fault. Generally five different shelf margin growth fault types are recognized: (1) listric faults which detach above undeformed older section, (2) listric faults which detach on mobile, overpressured shales above older undeformed sediments, (3) faults which sole into autochthonous salt withdrawal basins, (4) faults which detach along allochthonous salt sill intrusions, and (5) non-detached faults caused by differential compaction of thick shale intervals or basement faulting. The first four are the dominant structural types and their different structural styles reflect various modes of accommodation of fault offset within the sedimentary section. The fifth structural type, although not as commonly recognized, may be obscured by other structural complexities. These five fault types have different structural and stratigraphic characteristics which create different trap-source-reservoir relationships for each type. By understanding and identifying the distribution of these structural types one may better understand the complex production trends established in the Gulf Coast basin and more accurately assess potential production on prospective traps.

Thickening of fault zones: A mechanism of melange formation in accreting sediments

Sediments accreted at subduction zones undergo stratal disruption and form a type of melange. The thickness of the disrupted zones grows with progressive deformation. This suggests that initial fault surfaces are abandoned and deformation propagates into adjacent undeformed sediment. Factors causing the abandonment of fault surfaces during continuing deformation include (1) strengthening owing to porosity loss during consolidation, (2) localized drops in fluid pressure on fault surfaces that act as dewaterinig conduits, and (3) reorientation of fault surfaces. The disruptive processes occurring in accretionary prisms result principally from the deformation of a consolidating sediment mass.

Origine des chlorites de l'épizone. Héritage et cristallisation synschisteuse. Exemple des grauwackes cambriennes du Maroc occidental

Several representative samples of greywacke have been studied from a regional section in NW Morocco. The metamorphic and structural grade is shown to increase along this section. It leads from undeformed sediments to schists affected by a conspicuous schistosity. The undeformed greywackes contain two types of 14 Å minerals : the coarser types are detrital biotites, more or less weathered, lying flat in the bedding plane. The finer types are clay minerals : chlorite and 14C-14V mixed-layer. In the course of tectono-metamorphic evolution, both 14 Å phases are transformed : the clay minerals evolve to true chlorites by expulsion of interlayer alkaline cations, fixation of (Fe + Mg) and increased tetrahedral substitution ; meanwhile, blast sizes increase during growth parallel to the cleavage plane. Detrital biotites undergo a similar chemical evolution, contemporaneous with structural changes brought about mainly by intracrystalline shear and slide. At the highest grade attained, the low-grade metamorphic chlorites constitute an homogeneous set where minerals from various origins are no longer distinguishable on chemical or structural criteria.

Long decollements and mud volcanoes: Evidence from the Barbados Ridge Complex for the role of high pore-fluid pressure in the development of an accretionary complex

Research Article| May 01, 1983 Long decollements and mud volcanoes: Evidence from the Barbados Ridge Complex for the role of high pore-fluid pressure in the development of an accretionary complex G. K. Westbrook; G. K. Westbrook 1Department of Geological Sciences, University of Durham, Durham DH1 3LE, England Search for other works by this author on: GSW Google Scholar M. J. Smith M. J. Smith 1Department of Geological Sciences, University of Durham, Durham DH1 3LE, England Search for other works by this author on: GSW Google Scholar Geology (1983) 11 (5): 279–283. https://doi.org/10.1130/0091-7613(1983)11<279:LDAMVE>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation G. K. Westbrook, M. J. Smith; Long decollements and mud volcanoes: Evidence from the Barbados Ridge Complex for the role of high pore-fluid pressure in the development of an accretionary complex. Geology 1983;; 11 (5): 279–283. doi: https://doi.org/10.1130/0091-7613(1983)11<279:LDAMVE>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Mud volcanoes in front of the Barbados Ridge accretionary complex, discovered in multichannel seismic reflection profiles, provide evidence of very high pore-fluid pressures in the sedimentary cover on the oceanic lithosphere that is being subducted beneath the Lesser Antilles island arc. These high pore-fluid pressures can be attributed to the load imposed on the sedimentary cover by the weight of the advancing accretionary wedge. They offer a mechanical explanation for the very wide decollement that separates the Barbados Ridge Complex accretionary wedge from undeformed sediments on the oceanic lithosphere beneath it. Changes in level of this decollement may be induced by local variations in pore-fluid pressure. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Laramide Sedimentation, Folding, and Faulting in Southern Wind River Range, Wyoming: ABSTRACT

Surface observations along the southern margin of the Wind River Range in Fremont County, Wyoming, indicate that early motion along the Wind River and Continental faults controlled depositional patterns and lithologic characteristics of the local syntectonic sediments, and that the latest motion on a segment of the Wind River fault between Oregon and Pacific buttes folded some of these same sediments into a monocline. The stratigraphic sequence exposed in the monocline consists (in ascending order) of a lower distal fan or alluvial plain unit (main body of the Wasatch Formation), a lake margin unit (Tipton Tongue of the Green River Formation), a fluvial and deltaic sandbody (Tipton Sandstone), and an alluvial fan unit (Cathedral Bluffs Tongue of the Wasatch Formation). Cu rent direction, clast composition, and clast-size data indicate that a granitic and mafic distal source to the east and a proximal granitic source to the north supplied sediment. Subsequent movement on the Wind River fault warped this sequence into a monocline 2 mi long. This structure dies out in both a northwest and southeast direction along the inferred trace of the Wind River fault and is overlain by undeformed middle Eocene sediments. Other syntectonic units (e.g., Fort Union, Ice Point, White River, Arikaree, and South Pass conglomerates) occur in patches along the Wind River and Continental faults in this area. Each deposit is of local extent, exhibits rapid thickness and petrofacies changes, and probably represents proximal alluvial fan deposition. These characteristics are typical of syntectonic sediments in transcurrent-faulted terrains, and we are investigating the possibility of such faulting in this area. Tectonic implications of these interpretations are: (1) early motion on the Wind River fault controlled the margin of Eocene Lake Gosiute and generated a distal sediment source to the east; (2) late early Eocene uplift of the north side of the Continental fault provided a proximal source for pegmatitic and granitic boulders to the north; (3) last motion on the Wind River fault was latest early Eocene or earliest middle Eocene between Oregon and Pacific buttes; (4) the Wind River fault consists of several segments which moved separately rather than as one, long continuous zone of concurrent faulting; (5) while the Wind River Range was being thrust to the southwest it may have been uncoupled from the basins to the south by a zone of transcurrent faulting; (6) Pliocene or younger recurre t motion along the Continental fault was opposite to that in the Eocene. End_of_Article - Last_Page 1357------------

Extensive underthrusting of undeformed sediment beneath the accretionary complex of the lesser antilles subduction zone

Extensive underthrusting of undeformed sediment beneath the accretionary complex of the lesser antilles subduction zone

Extensive underthrusting of undeformed sediment beneath the accretionary complex of the Lesser Antilles subduction zone

Along the eastern margin of the Caribbean Plate, which overrides the Atlantic oceanic lithosphere at ∼20 km Myr−1 in an easterly direction1, east of the Lesser Antilles island arc, lies a wide accretionary complex (Barbados Ridge Complex); 260-km wide at the latitude of Barbados and >20-km thick beneath Barbados3,4, it increases in width and thickness from north to south2 (Fig. 1). The style of deformation varies considerably along the eastern margin of the complex2,3,5–11, and is related to the thickness and type of sediment on the Atlantic Ocean floor. A common feature is that a proportion of undeformed sediment from the ocean floor is brought beneath the deformed sediments of the accretionary complex, and in some areas a clearly defined seismic reflector separates the deformed and undeformed sediments, which has been interpreted as a decollement surface5,12,13. A seismic section from an investigation of the accretionary complex, conducted from RSS Discovery in 1980 using 12-channel seismic reflection and long range side-scan sonar (GLORIA), shows undeformed sedimentary rocks carried horizontally 74 km beneath deformed rocks forming the accretionary wedge. We report here that the low shear stresses required on the decollement between the deformed and undeformed rocks are made possible by porewater at pressures close to that of the lithostatic load.

Plate margin transition from oceanic arc-trench to continental system: The Kermadec-New Zealand example

The East Coast Fold Belt (ECFB) of the North Island, New Zealand, is the continuation of the Tonga-Kermadec arc-trench system. Structurally its tectonic front to the east defines the Indian-Pacific plate boundary. This, however, is not continuous with the Kermadec Trench. Large-scale fragmentation of the ECFB into segments of greatly varying width, strike and structure may be caused by a strongly segmented subducting plate, individual segments of which strike in different directions and have different dips and rates of subduction. Towards the southwest, regional change of strike with respect to plate motion has resulted in the formation of a broad shear zone marked by a strongly subsiding trough filled with rapidly deposited, largely undeformed sediments in front of the ECFB. This foredeep (Hikurangi Trough), which thus occupies the gap between ECFB (Indian plate) and the continental Chatham Rise (Pacific plate) is gradually being involved in the overall deformation, due to continuing motion of the Pacific plate to the southwest, in a slightly oblique sense along the shear zone. As a result, the Hikurangi Trough is shifting with time to the east-northeast. From a tectonic, structural and morphological point of view, it is unrelated to the Kermadec Trench which terminates in the region of East Cape. The structure of the ECFB is characterized mainly by extension normal to the plate boundary, with regional tilting and down-faulting of the continental margin. Effects of compression are observed only locally, and are often due to diapiric uplifts caused by widespread, undercompacted shale. Such diapirs form elongate structural highs which in many cases have supplied sediments into adjacent basins on their landward side. Overall the continental slope and margin are underlain by land-derived sediments which exhibit in-place deformation. Locally they extend as undeformed sediment aprons beyond the tectonicfront. There is no compelling evidence of a subduction complex of imbricate thrust slices. It is concluded that the tectonic evolution is not controlled by accretion but rather by subsidence and tectonic erosion along the continental margin. The conditions are complicated, however, because of the discrete change from an oceanic arc-trench subduction system to an intercontinental shear zone.

RELATIONSHIP BETWEEN AGES OF SHIELDS, DIPS OF ADJACENT SEDIMENTARY BASEMENTS, AND OCCURRENCE OF OIL AND GAS

Crystalline shields can become stable at different times. This stability is commonly marked by the end of magmatism and compressive deformation and the beginning of deposition of platformal sedimentary sequences. The margins of the shields are then the basements for undeformed sediments that dip away from the exposed crystalline rocks.Dips of basements on shield margins directly underlying Phanerozoic sediments have been determined in seven areas in which the margin has not been altered by rifting or other tectonic activity. The adjacent shields have stabilization ages ranging from more than 3,000 m.y. to 600 m.y. The basement dips (tilts) increase progressively from about 3 m/km for older shields to 16 m/km adjacent to the youngest shield (the Nubian‐Arabian shield). These tilts can only approximately be recalculated into uplift rates of the shields or subsidence rates of their margins. The dips are, however, consistent with Phanerozoic uplift rates of Archean cratons of less than 5 m/m.y., or comparable subsidence rates of the margins; such low uplift rates could be caused almost wholly by erosion and isostatic adjustment. Younger shields have undergone some other, as yet unidentified, process that causes more rapid uplift.The higher dips of basements around younger shields apparently promote the development of thick sections of mildly‐deformed Phanerozoic sediments. Sequences of this type are commonly favorable for accumulation of commercial concentrations of oil and gas. A correlation has been found between age of shield, basement dip, and hydrocarbon occurrence around the western edge of the Canadian shield and the northern edge of Africa. The data show that oil and gas are more abundant in Phanerozoic sediments adjacent to younger shields, which supports the common observation that the world's largest hydrocarbon accumulation (the Arabian Gulf) is adjacent to the world's youngest shield (the Nubian‐Arabian shield).This criterion for hydrocarbon accumulation may be of some value in exploration of little‐known areas. For example, basements of different ages occur adjacent to the overthrust belt of the northern Rockies, and the generally‐young age of Precambrian basement in the eastern US may indicate considerable thickness of sediment under crystalline thrusts in the Appalachians. Different ages of shields in different parts of China may also have broadly affected sedimentary patterns in that country.

Late Quaternary sedimentation in a zone of continental plate convergence — The central Hellenic Trench system

The central Hellenic Trench system, south of Crete, is a tectonically active region forming part of a complex geotectonic regime involving continental plate collision and convergence. Gravity and piston cores, together with seismic reflection profiles, provide evidence of significant variations in Plio-Quaternary lithology and sediment geometry that broadly reflect the morphologic diversity of this area and yield data important in the elucidation of analogous ancient sequences. Gravitite sediments are typically found in the trenches and deep basins, while suspensite sediments predominate on highs and in some perched basins. In detail, the trench floors are characterized by cyclic repetitions of gravitite sediments (in high proportions, comparable only to submarine fans elsewhere in the Mediterranean), thick diluted sapropels and hemipelagic calcareous oozes. The substantial core-to-core variations in this and other facies assemblages are attributed to the variety of sub-environments present within each depositional province. The Late Quaternary sediment sequences present in cores are well organized and display a cyclic stratigraphy that can be correlated with other eastern Mediterranean areas. Sedimentation rates deduced on the basis of this correlation are noticeably lower than in the western Hellenic Trench or the Nile Cone, probably because of the reduced supply of fine-grained terrigenous material to this area. Although there is a marked correlation between the Late Quaternary climatic-eustatic oscillations and the sediment types and rates, some of the overall variations in lithofacies patterns are best attributed to the tectonic mobility of the area. Seismic profiles reveal substantial regional variations in the geometry and organization of the Plio-Quaternary sedimentary fill in adjacent sectors of the trenches. The thickness of unconsolidated, undeformed sediment below the trench floor is also much smaller than that extrapolated from the Late Quaternary sedimentation rates computed from the cores. It is concluded that these variations in geometry and thickness of the sedimentary infill reflect different stages in the process of convergence and plate consumption. The differential removal of sediments from adjacent sectors of the trench floor is further interpreted as a discontinuous and diachronous process. The geometry, thickness and lithofacies represented in the central Hellenic Trench sequences appear more varied and complex than those reported from modern Pacific trenches and it is suggested that the Quaternary sediments of this region may furnish more appropriate examples of the deposition associated with alpine-type compressional settings than the sedimentary fill of Pacific-type trenches.

Submarine topography and shallow structure of the Madagascar Ridge, western Indian Ocean

Bathymetric, seismic-reflection, seismic-refraction, and magnetic-anomaly data suggest that the Madagascar Ridge consists of two different domains. North of 31°S, both the sea floor and the basement topography appear to be locally and regionally complex. Small sediment-filled pockets are present between numerous basaltic basement highs. Large-scale normal faulting shapes the western flank; on the east, the Late Cretaceous fracture zones of the Madagascar Basin penetrate deeply into the ridge. South of 32°S, an extensive area of thick undeformed sediments is found over the central part of the ridge. Two prominent seismic horizons — the upper one being attributed to an unconformity between lower Eocene–upper Oligocene and lower Miocene sediments — can be mapped throughout this area. The steep western slope reflects the Mozambique Basin fracture-zone trends. Both the topography and sediment cover at the southeastern limit are closely related to the north-south–trending fracture zones of the Southwest Indian Ridge. The character of magnetic anomalies is in accord with this regional subdivision. An east-west–trending rise situated near 32°S and marked by the 4,000-m isobath marks the former boundary of two spreading systems that are related to the Central Indian Ridge and the Southwest Indian Ridge. The shallow structural data and our interpretation of those data favor an oceanic nature for the entire Madagascar Ridge.

Deformation of Continental Slope Sediments Resulting from Large-Scale Downslope Creep: ABSTRACT

Recently acquired high-resolution seismic data suggest that a previously undescribed style of deformation may be common in sediments on the continental slope. This deformation is characterized by folds whose axes are oriented roughly parallel with the slope of the seafloor (perpendicular to isobaths). In cross section, the amplitude and wavelength of folds range from 5 to 50 ft (1.5 to 15 m) and from 300 to 2,000 ft (100 to 600 m) respectively. The amplitude generally decreases with depth, and undeformed sediments form the base of the folded sequence. Folded sequences up to 400 ft (120 m) thick have been observed on the continental slope offshore Louisiana. Where not buried beneath younger, undeformed sediments, these folds are expressed at the seafloor as a series of rid es and furrows which are mappable using side-scan sonar. Detailed mapping has shown that individual folds are several hundred to several thousand feet long and the areal extent of folding is tens of square miles. Where the continental slope is irregular, fold axes converge downslope into bathymetric lows forming a fanlike pattern. Folds are rare or absent where the continental slope is planar and on the crest of bathymetric highs. This style of deformation has been detected in sediments consisting primarily of cohesive clays on the upper continental slope off Louisiana, Texas, and California where seafloor slopes range from 2 to 15%. It is End_Page 549------------------------------ proposed that these cohesive sediments have moved downslope, and that folds developed in response to increasing lateral confinement as sediments converged within bathymetric lows. The general style of deformation, preservation of bedding, and absence of faults suggest that movement occurred at a slow rate, probably as creep. End_of_Article - Last_Page 550------------

Pliocene-Pleistocene Diastrophism of Santa Monica and San Pedro Shelves, California Continental Borderland

Data from geophysical profiling and from jet and dart coring have been used to study the late Cenozoic history of the Santa Monica and San Pedro Shelves. Deformed strata commonly crop out on the outer shelf and consist chiefly of shale belonging to the Monterey Shale and the Repetto Formation (middle Miocene through lower upper Pliocene). In contrast, undeformed sands and silts, primarily beneath the inner shelf, represent the Pico and San Pedro Formations (uppermost Pliocene through middle Pleistocene) as well as unnamed upper Pleistocene and Holocene deposits. Together with the Palos Verdes Hills and the Lasuen Knoll, outer-shelf strata have been folded into five northwest-trending en echelon anticlinoria. One anticlinorium is in a structural depression directly north of Canyon and has been buried beneath undeformed sediments. This depression facilitated initial development of the canyon during late Pleistocene time. An east- ortheast-striking termed the Redondo Canyon fault, separates the platform anticlinorium from the Palos Verdes Hills anticlinorium. Formation of the anticlinoria and the Canyon fault is attributed to convergent dextral shear along the northwest-striking Palos Verdes fault after early Pliocene time. Although displacement along the Palos Verdes fault has continued in San Pedro Bay through late Pleistocene time, the absence of large vertical separation in the upper 300 m of strata in Santa Monica Bay north of the platform anticlinorium indicates that the fault has been relatively inactive there at least since the beginning of late Pleistocene time. These observations infer that the late Quaternary uplift of the Palos Verdes Hills was decoupled from the Santa onica shelf along the Canyon and may have occurred because the Santa Monica Shelf restrained the relative northwestward movement of the Palos Verdes Hills block. It is suggested that late Pliocene and early Pleistocene diastrophism in the California continental borderland was characterized by convergent dextral shear accompanied by en echelon folding. This contrasts with the middle and early late Miocene history of widespread faulting and large lateral displacements proposed by other investigators. The transition in the style and locus of deformation between 12 and 3 m.y.B.P. may be related to a change in the orientation of the transform boundary between the North American and Pacific plates, changing slip rates along the San Andreas and, possibly, differences in slip rates along the San Andreas north and south of the Transverse Ranges.

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.

The changes in shape of the magnetic susceptibility ellipsoid during progressive metamorphism and deformation

The magnetic anisotropy of a sequence of rocks ranging from shales and greywackes to gneisses was investigated. The oblate and slightly eccentric magnetic susceptibility ellipsoids in sediments are progressively changed when sediments are subject to metamorphism and ductile deformation. In the initial stages of low-grade metamorphism and ductile deformation, the ellipsoids become more eccentric and triaxial, later they are even more eccentric, but again predominantly oblate. The directions of principal susceptibilities are closely related to the primary-fabric elements in undeformed sediments; during deformation they come to be related to the deformational-fabric elements.

The geology of part of the Southern foreland of the Damara orogenic belt in South West Africa and Botswana

A characteristic feature of the Damara geosyncline in northern South West Africa is the gradual transgression from the relatively undeformed sediments of the miogeosynclinal facies to the metamorphosed eugeosynclinical facies. A crude mirror-image of the arc separating the two facies is present along the southern foreland of the Damara geosyncline and conforms quite clearly with the northern limit of the Kalahari craton. However, owing to the presence of major thrust-faulting which eliminates the transition-beds between the two facies a direct correlation is not possible. The pre-Damara rocks on the southern side of the Damara geosyncline have been divided into the Marienhof Formation and the Dordabis Group in central South West Africa, and the Kgwebe Group in Botswana. These rocks form the base on which the strata constituting the Damara geosyncline have been deposited, and have been subjected to metamorphism both before and during the Damara orogenesis. The term Southern Marginal Facies is proposed for the unmetamorphosed strata deposited unconformably on the pre-Damara rocks along the southern flanks of the Damara geosyncline. These unmetamorphosed strata are composed of the Klein Aub, Witvlei, Ghanzi and Tsumis Formations which are regarded as the equivalent of the Nosib Group. They are unconformably overlain by the Nama Group which is considered to be, at least in part, the equivalent of the Hakos and Khomas Formations. Copper mineralization associated with calcareous shale, limestone and argillite occurs within the Klein Aub, Witvlei and Lower Ghanzi Formations. The mineralized strata closely resemble that of the Zambian Copper-belt with which they may well be correlated.

The development of stress systems and fracture patterns in undeformed sediments : Report. In Advances in Rock Mechanics V1, Part A. In English, French and German. 19F, 11R. Proc. 3rd Congress Int. Soc. Rock Mech. Sept. 1974, Denver, USA, Acad. Sci. V1, Part A, 1974, P487–519 Price N.J.

The development of stress systems and fracture patterns in undeformed sediments : Report. In Advances in Rock Mechanics V1, Part A. In English, French and German. 19F, 11R. Proc. 3rd Congress Int. Soc. Rock Mech. Sept. 1974, Denver, USA, Acad. Sci. V1, Part A, 1974, P487–519 Price N.J.

Steady–State Trenches?

Values obtained for both sedimentation and convergence rates in certain trenches are inconsistent with the presence of thick, undeformed sediments. We propose a steady-state model in which the geometry of the sedimentary prism in a trench could remain constant by having inward-advancing deformation of sediments along the inner wall exactly balance outward-advancing overlap of new sediments on the outer wall. However, if the advance of deformation into a trench is considered to be equal to or greater than the convergence rate across the trench, it is clear that significant thicknesses of undisturbed trench sediments should not exist and that a steady state cannot exist. Therefore, for trenches containing thick, undeformed sediments, the convergence of crustal plates is being accommodated by erogenic deformation away from the trench, and the oceanic lithosphere is, in fact, strongly coupled to the continental lithosphere. There may be no discrete structural expression of a subduction zone at shallow crustal levels. All plate convergence during the time of trench filling can be reasonably converted into an episode of deformation and uplift of the adjacent erogenic belt. The convergent motions of lithosphere plates through geologic time are too great to have all convergence accounted for by orogenic deformation and uplift. A bimodal behavior of subduction zones seems necessary, having lithosphere coupling and orogenesis alternate with lithosphere decoupling and simple underthrusting in trenches. The two modes may not be exclusive nor may they be the only modes. But perhaps such modes explain the paradox of continuous sea-floor spreading versus episodic orogenesis.