Margins A Research Initiative for Interdisciplinary Studies of the Processes Attending Lithospheric Extension and Convergence (1989) / Chapter Skim
Currently Skimming:
Part III: Background Papers
Part III: Background Papers
Pages 113-276
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 113... ...
PART III BACKGROUND PAPERS These papers express the views of the authors, not necessarily those of the National Research Council. As is customary with reports of this kind, the background papers are reproduced here, for the reader's convenience, as they were received from the authors without the NRC review and editorial attention given to the preceding sections of this report.
|
From page 115... ...
. e Section 2: Passive Continental Margins I
|
From page 116... ...
Dative ~i~ttal Mars ME=P,2~CI; OF HE Gallon 116
|
From page 117... ...
Davis INTRODUCTI ON Much of the deformation that affects the crustal portion of the lithosphere occurs at convergent plate boundaries, which in the broadest sense encompass oceanic subduction zones, oniand thrust belts, and everything in between. Horizontal convergence at these boundaries is accommodated by a combination of two processes.
|
From page 118... ...
We contend that modern subduction zones (Figure ~ ) , where the downgoing plate is oceanic, provide a unique natural laboratory for the study of erogenic deformation at a variety of scales.
|
From page 119... ...
Convergence rates range from 10 to 1 00 mm/yr, and sedimentary cover on the downgoing plate from 0.2 to 10 km thick. Subduction complexes vary widely in their accretionary behavior: some grow by accretion of sediment and possibly oceanic crustal rocks, whereas others show evidence for long periods of non-accreting, possibly accompanied by loss of material (subduction erosion)
|
From page 120... ...
However, the factors controlling the trenchward limit of such earthquakes are less well understood. Wedges at seismogenic subduction zones may show a mix of Reformational mechanisms, 320
|
From page 121... ...
The geometry and position of this master thrust probably exerts an important control on accretionary processes and ultimately determine the flux of crustal and sedimentary materials in and out of the wedge. (5 ~ What is the nature of heat and fluid flow within subduction complexes?
|
From page 122... ...
? How much "aseismic" slip occurs at seismogenic subduction zones?
|
From page 123... ...
~ ~ ~ How is the seismogenic character of a subduction zone related to geodetically measured strain? To the first order, the elastic dislocation model can account for the temporal changes in geodetically measured strain at seismogenic convergent margins (e.g., Savage, 1983; Thatcher and Rundle, 1984~.
|
From page 124... ...
Unfortunately, MCS surveys have had only limited success at imaging the deep structure of convergent margins. Part of the problem is due to the rough seafloor and relatively steep slopes that characterize this tectonic setting.
|
From page 125... ...
(4) Long-term deployment of onshore-offshore arrays of geodetic instruments and seismometers across some well characterized subduction zones.
|
From page 126... ...
Sykes, Mechanics of the Shallow Regions of Subduction Zones and the Loci and Maximum Size of Thrust Earthquakes, Tectonics, 7, 833857, 1987. Chapple, W.M., Mechanics of Thin-Skinned Fold-and-Thrust Belts, Geol.
|
From page 127... ...
Tectonic Processes Along the Front of Modern Convergent Margins Research of the Past Decade, Ann.
|
From page 128... ...
convergent margin is controlled to varying degrees by the Ethology, strain, seismicity, and rheology of the subduction zone. These features vary widely between various convergent margins, offering a variety of well-controlled natural laboratories.
|
From page 130... ...
PI GORE 2 1 ~ c:) C, fir 141 C=~ 1 trap ~= o fir lo: lo I ~ _ ~ a ~ - ~# o ~ ~ Z Vat Let fir IL fir ~ Q A Z ~.
|
From page 131... ...
For example, plates with subducting slabs attached move faster Tan plates without sub~uc~ang slabs, consistent wad Me negative buoyancy associated wad slabs being a dominant Hiving force (Forsythe and Uyeda, 1975~. The maximum size of earthquakes at subduction zones is dimly proportional to convergence velocity and inversely proportional to We age, and hence negative buoyancy, of He subducting plate (Ruff and Kan~nor~, 198O)
|
From page 132... ...
This subhonzontal subduction has been proposed for North America clueing He I~ramide orogeny (e.g., Bird, 1988) The state of stress In subduction zones is minutely relay to He dynamic processes occulting.
|
From page 133... ...
Such ~ model is also consistent wad He state of Stress In subducted slabs and He infers advec~ve thickening of slabs at the base of He upper mande. Simple v~scoelasac models of subduction zones have been proposed to address the Observer!
|
From page 134... ...
DeteImining the shape of subducting slabs places strong constraints on dynamic models. InvesugaDon of locations in He deep n~nrltle beneath fossil subduction zones are important to ~nacase He temporal coverage of the subduction process.
|
From page 135... ...
These measurements should ~ made frequently enough ~ Be coupling of forces Cam convective tunescales to the omescales of Be seismic cycle can be addressed Since tile basic mmpmal spectrum of regiorm1 crustal defoImaaon is as yet unconstrained by obsen~a~ons, permanent, congruously mo'iitared regional swain networks should be installed in a few active regiOlISe Transfer of stress and swain after large subduction zone events should be monitored to address Be questions of stress level anti coupling of subduction zones. This requites Anal epoch surreys.
|
From page 136... ...
For convection modeling, important problems to address include He effects of phase changes and vanadons in composition and rheology on flow. Specifically, He mteraciion of subducted slabs wad He 670 km discontinue must be addressed for a wide range of models of upper and lower mande composition.
|
From page 137... ...
Stress transfer and seismic phenomena in coupled subduction zones during the earthquake cycle, J Geophys.
|
From page 138... ...
and Barazangi, M., 1977. Geometry of Benioff zones: Lateral segmentation and downwards bending of He subducted lithosphere, in Island Arcs, Deep Sea Trenches, and Back-Arc Basins, ed by M
|
From page 139... ...
Seismicity and the subduction process, Phys. Earth Planet.
|
From page 140... ...
Figure Ib) The observed geoid, filtered to include spherical harmonic degrees 4-9 to emphasize We association win subduction zones.
|
From page 141... ...
~ 1 ma so o c: v: o 141 Lo Ct c Or L
|
From page 142... ...
9 in l Don an; o c-' I= o 142 i a, CO A C~ UD teD Lr)
|
From page 143... ...
l to o C:5 = ~~ :: Hi: !
|
From page 145... ...
@ @ ma -ma 145
|
From page 146... ...
Because convergent Pilate boundaries lie both along continental margins, as marginal arc systems, and within ocean basins, as island arc systems, there is still a question as to which are preferred settings and why. There is a general sense that most convergent boundaries form near a pre-existing interface, but largely this ~ s an exercise in inductive logic .
|
From page 147... ...
Ironically there is no example in which a convergent margin is observed to be initiating or to have recently initiated along a passive continental margin. There are, however, examples in which arcs are or have recently formed in back arc basins, along the rear margins of island arcs (e.g.
|
From page 148... ...
Of emergent forearc basins, the Great Valley of California has probably influenced our thinking the most, as it is located near a concentration of geologists who shaped early ideas about convergent margins. Subsequent information from this area suggests' however, that the Coast Range ophiolite, which underlies the western flank of this basin, is not autochthonous with respect to the Sierran arc.
|
From page 149... ...
, but the oceanographic data even- in their treatment are scant, biased, and in some cases erroneous. The problem of lack of integration of land and oceanographic approaches to the origin of ophiolites, and indirectly to arc initiation, was illustrated by the near absence of an oceanographic input at a major conference on "Ophiolites and Oceanic Lithosphere" (lamentation of Gass, et al ., 1984, p .
|
From page 150... ...
. Stress concentrations at these tips could alter calculations of stressstrength relations along continental margins, such as those of Cloetingh et al., 1983 e Perhaps because of the lack Of concensus concerning the formative processes for convergent margins, very little effort has been expended on the characterization of these zones.
|
From page 151... ...
-What implications does this geometry have for the basement of forearms and the development of-ophiolites? If convergent margins originate at different settings, how are these identified and discriminated in the geologic record, and what is the distribution of the various initial settings ?
|
From page 152... ...
1. Conventional Oceanographic Studies The most pressing task that would employ conventional oceanographic techniques in the characterization of very young or incipient convergent margins.
|
From page 153... ...
The danger in misinterpreting or missing the earliest phases of arc activity in an emergent convergent margin are offset by the exposure and ability to conduct studies of details and of deeper structures. Regional relationships are often better expressed, with the constraints of stratigraphic control.
|
From page 154... ...
tip of a new trench could be used to determine the stress in that area. Seismologic studies, perhaps with arrays of OBS's could be used to evaluate the geometry and Precocity structure of the descending oceanic lithosphere .
|
From page 155... ...
269-291. Cloetingh, S.A.P.L., Uortel, M.J.R., and Vlaar, N.J., 1983, State of stress at passive margins and initiation of subduction zones-AAPG Mem.
|
From page 156... ...
showing common interpretation of oceanic crust ~ in black) under the forearc (outer-arc)
|
From page 157... ...
showing zones of trapped oceanic crust in reentrance of the continental margin.
|
From page 158... ...
.~ ~_ 6 8 1 0 0 50k'~i ~ _ . NW SE | W E VOLCANIC ~ BARBADOs BARBADOS CENTRAL RECENT ATLANTIC ARC TOBAGO TROUGH - Ct.E-ST ' TROUGH BARBADOS RIDGE ACCRETION t- ARYSSAL Fig 18 Fig t7 ~ 1 Fig 9 figil F;~7Fig 13 ~ ·~_ a_ _~ r~.
|
From page 159... ...
~ Several examples of plate geometries along convergent margins in which transverse upper plate spreading zones could lead to creation of ophiolite sheets in a forearc setting.
|
From page 160... ...
The volcanic front of an island arc is typically segmented, and the distance between volcanoes varies from 10 to 100's of kilometers. Not all subduction zones have volcanoes.
|
From page 161... ...
A new volcanic chain commonly forms following arc rifting also. The active volcanoes of the Montana, Ton ,a and Scotia arcs have been erupted through, and built on, oceanic crust formed at their backarc spreading centers.
|
From page 162... ...
a) Using backslapping techniques on cored/logged driliholes and seismic straugraphic analysis of interconnecting seis~c-profiles, we need to determine, for example, whether the frontal arc and outer-arc high develop by igneous intrusion or differential uplift, whether the upper-sIope basin between them is due to forearc spreading or differential subsidence, and whether flexural loading by either arc volcanoes or by coupling ~~ He subducting plate is an important process.
|
From page 163... ...
However He thermodynamics of intraocean~c convergent margins is almost totally unknown. All quanuta~ave models of the forearc temperature field relate to the effect of the subducting slab and do not consider the influence of the the volcanic arc, the age of and fluid flow through the fore arcs and the thermal blanketting effect of sediments.
|
From page 164... ...
For shallow Selected References Arculus, R.J., 1985, Arc magmatism—an unresolved problem of sources, material fluxes, tectonic evolution and thermochemical regions of subduction zones, in Nasu, N et al., eds., Formation of Active Ocean Margins: Tokyo, Terra Scientific Publishing Co., p.
|
From page 165... ...
and Uyeda, S., 1981, Tectonic processes and the history of the Mariana Arca synthesis of the results of Deep Sea Drilling Project Leg 60, in Hussong' D.M., Uyeda, S., et al., Initial Reports of the Dep Sea Dnlling Project, v.
|
From page 166... ...
Relative motion of the subducted plate is shown by arrows.
|
From page 168... ...
A sprawling h;~ck-arc basin separates a remn.`nt arc from an active volcanic arc which has been superposed on part of the back-arc crust. Tectonic erosion has removed fore-arc m;~tcri;`l exposing Ants of alder arc c`,mp<,nents on the trench slope.
|
From page 169... ...
Vertical exaggeration makes it appear impossible for the slope to accommodate colliding seamounts, and lends support to geologic interpretations Hat seamounts are sheared from the subducting ocean crust and were accreted in ancient subduction zones now exposed on land. However, surveys of He intersection of edges with trenches using conventional echo-sounding profiles showed little chaotic local topography (Nazca Ridge in the Peru Trench, for instance)
|
From page 170... ...
Such collision commonly involves large scale strik~slip movement of crustal slices parallel to the margin. SOME TECTONIC PROCESSES ACCOMPANYING COLLISION Examining the consequences of collision along convergent margins Is somewhat analogous to testing the physical state of materials in He laboratory.
|
From page 171... ...
The scar where the seamount was suWuctecI is healed and it-contains a hiatus between a young accretiona~y complex and~an older failed slope, as is observed along convergent margins, where a accre~donary complex is stacked against a "'backstop." This example of a mesmscale collision shows how a scar is former! in a convergent margin without "chain-saw" abrasion, the forcible shouldering asicle, or long distance displacement of large volumes of matenal.
|
From page 172... ...
The effect of oblique subduction causes larg~scale lateral transport of continental margin or island-arc fragments. For instance, where the Indian Ocean is subducting obliquely under Southeast Asia at the Suda subduction zone, the entire Sumatra forearc region is being translated northward along the Sumantran Fault zone (Moore et al., 1980~.
|
From page 173... ...
What tectonic processes shape the convergent continental margins?
|
From page 174... ...
REFERENCES Bourgois, J., Pautot, G., Bandy, W., Boinet, T., Chotin, P., Huchon, P., Mercier de Lepinay, B., Mange, F., Monlau, J., Pelletier, B., Sosson, M., and van Huene, R., 1988, SEABEAM and seismic-reflection imaging of the tectonic regime of the Andean continental margin off Peru (4°S to 10°S) : Earth and Planetary Science Letters, v.
|
From page 175... ...
Oshirna, S., Ogimo' T., Katsura, K., Ikada, M., Uchida, M et al.' 1985, Subduction of the Daiichi-Kashima Seamount into the landward slope of the Japan Trench: Rep.
|
From page 176... ...
Seamount has entered trench axis and begins to break along normal faults that develop as the ocean crust is fixed downward into the subduction zone.
|
From page 178... ...
`;~Hi `co' B Figure 2 Ha.
|
From page 179... ...
Figure 2 :~ Hi,..
|
From page 180... ...
Active Conti~t=1 M—ins MASS AND C~CAL ~RP115E~ 180
|
From page 181... ...
illustrates that the formation of relatively durable continental crust accompanies subduction of relatively ephemeral oceanic crust at convergent plate margins. Fundamental to understanding the significance of subduction for both crust and mantle is the use of compositional tracers to calculate mass flux in the system.
|
From page 182... ...
o to = e o o = : ~ / .........
|
From page 183... ...
There are benefits from looking at indivdual arcs with a this systems-analysis type of approach. Past experience has shown that identification of discrepancies in input - output budgets, and of inconsistencies in perturbation response times are some of the best ways to discover unanticipated processes: an example is the failure of budgets for Mg in sea water to balance until the hydrothermal uptake of Mg by the oceanic crust was discovered.
|
From page 184... ...
Global Overview Igneous crust that forms the top of oceanic lithospheric plates is very efficiently returned to the asthenospheric mantle at subduction zones. This conclusion follows directly from the exponentially decreasing total - mass of preserved oceanic crust (including ophiolites)
|
From page 185... ...
Appealing as it is, simple melting of the subducted oceanic crust is not an adequate source for arc magmas. However, sedimentary part of the subducted oceanic crust is an important source for many trace elements -- especially those that are enriched in arc basalts compared to altered Mid-Oceanic Ridge Basalt (MORB)
|
From page 186... ...
Segregate on of mafic minerals from ponded oliv~ne tholeiite forms of cu~n~nulate ultramafic rocks that occur at the crust-mantle boundary. A basic lower crust consists of unassimilated residues of crystalline oceanic crust and intrusive hi-A1 basalt and its mafic crystalline fractionates.
|
From page 187... ...
element basalts laventory H2O - 21 - 8t K 11 65\ Bb 20 ppm ~ 100% C 1t >> 100' Othcr ~Excess~clea~nts in arc besalts are not in sea water, and are not ·dded to oceenic crust during, hydrother~1 clteration: Th, Pb, 1OB. TABLE 2 Sourees of ~Excess ~ Element~ in Volcanic Arc l5~-lts Top Scd~nt - Carbonate X C H2O K U Th Pb 1OBe C1~y -- X X X X X Altered Oceanic Crust (low T)
|
From page 188... ...
paper. More than 500 km3 of igneous oceanic crust and perhaps 50 km3 of sediment is being transported to the Mariana arc per million years.
|
From page 189... ...
, Flow diagram of the Hariana arc, illustrating the mass and of K2O involved during processes of plate convergence and arc magmatism. Use of realistic element ratios in possible sources res~slts i~ teh conclusion that no more than a few tens of meters of the basal pelagic sediments are in`,o1`red in magma production.
|
From page 190... ...
The contrasting compositions of input and output to the arc implies that some elements, such as potassium, are in much shorter supply than others. Even though K2O content of the outputs is higher than that of the hydrothermally altered igneous oceanic crust, that source as well as the sediment or mantle wedge above the descending plate could supply sufficient K2O to account for the K2O in Mariana arc lavas (e.g., the ER for K is much less than 1~.
|
From page 191... ...
. ~ Sr/" Sr values assun~ed to bc .709 in sculiment .702 5 in deoleted mantic and oceanic crust.
|
From page 192... ...
. About 50 m of sediment and 700 m of altered oceanic crust are sufficient to furnish all the required K
|
From page 193... ...
In general, the currently popular source models (of the past decade) call for linear mixtures of meltdepleted peridotite' undepleted peridotite' and subducted oceanic crust (including sediment and hydrothermal alteration components)
|
From page 194... ...
A decreasing growth rate is implied -- perhaps an exponentially decreasing rate, but this is not required by the data. Given the probable long-term return of crust to mantle, some types of mantle heterogeneity must be closely related to the Subduction Process (S-Process)
|
From page 195... ...
The compositional relationship between the subducted crustal components and those returned to the mantle is not obvious, although it seems probable that intraelement fractionation that occurs would return K, Rb, Sr, and Ba to the crust in higher concentrations than those of the subducted material. In arcs with associated back-arc basins, some of the crustal elements appear to be extracted from the mantle in the back-arc basin basalts.
|
From page 196... ...
formed' Second, basaltic continental crust, once formed, is transitory, and returns to the mantle by crustal delamination together with some mantle lithosphere at continental convergent margins or at continent-continent collision zones, where crust is unusually thick. At the base of these thick crustal sections, crust recrystallizes to a dense garnet-bearing mineral assemblage (garnet granulite, then eclogite)
|
From page 197... ...
, Lithologic cross section of Coast Plueonic Complex, British Columbia (after Longer et al., 198S) showing granodiorite-rich upper crust and lower crust comprised of slices of oceanic crust in a matrix of accretionary prism material (greywacke)
|
From page 199... ...
Often, models of deformation at convergent margins are dominated by the thermal structure alone, but it is increasingly apparent that tectonics may be more fundamentally controlled by chemical heterogeneities that control the rheology -- e."., Hollister and Crawford's (1986) concept of tectonic surges that correlate with plutonic episodes, and Kay et al's (1986)
|
From page 200... ...
b) Mass Flux and the Geological Record The geological record reveals that the rate of mass transfer at convergent margins is nonlinear on a wide range of timescales.
|
From page 201... ...
Formation of these mantle heterogeneities may be reasonably linked to the subduction process -- where inefficiency of crustal recycling leaves elements from the upper continental crust in the mantle, or continental collisional processes, where lower continental crustal rocks may be mixed into the mantle. At present, the subduction process is specially concentrated.
|
From page 202... ...
A convincing compositional match between observed heterogeneities in the subducting plate and those in the oceanic mantle could put a modern twist into Barth's (1968) prescient observation that "The diversification of igneous rocks is caused by sedimentary processes." 202
|
From page 203... ...
67, 405. A, Armstrong, R., 1971 , Isotopic and chemical constraint on models of Vagina genes is in volcanic arcs, Earth Planet.
|
From page 204... ...
and Hawkesworth, C.J ., ~ 988b, Is average continental crust generated at subduction zones? Geology, v.
|
From page 205... ...
Kay, S Mahlburg, and Kay, Rig., 1985, Role of crystal cumulates and the oceanic crust in the formation of the lower crust of the Aleutian arc, Geology v.
|
From page 206... ...
Tera, F., Brown, L., Morris, J., Sacks, I.S., Klein, J., and Middle ton, R., 1986. Sediment incorporation in island-arc Magyar: Inferences from 1OBe Geochim Cosmochim.
|
From page 207... ...
Passive Cont~n~1 trains ME~NIC~; OF RIFTrNG OF WE LrI~EKE: 207
|
From page 208... ...
The pre-plate tectonic literature is replete with examples of analyses of continental rift systems and sedimentary basins from which deductions were made that hold very nearly true today; even deductions made about passive margin evolution that were little more than inferences and speculations based upon a tiny amount of marine data, are astonishingly close to the mark when viewed today (see Bond, 1988 for an extensive review)
|
From page 209... ...
Sleep (1971) first addressed the problem of passive margins forming by thermal contraction of a thin lithosphere, but was puzzled by the mechanism of both crustal and lithospher~c thinning.
|
From page 210... ...
Subsidence information was derived from the stratigraphic record in exploration wells after suitable corrections were made for sediment loading and compaction, and suitable account taken of the mechanism of isostatic compensation (Watts and Ryan, 1976; Steckler and Watts, 1978)
|
From page 211... ...
Closer attention was paid to the actual mechanism by which the crust reacted to extensional stresses, and the importance of listric and low-angle, normal faulting, including subhorizontal detachment faulting, emerged. Studies on continental extensional systems contributed in a fundamental way.
|
From page 212... ...
If so one would expect to see some evidence for detachment faulting on the outer part of virtually all passive margins and planar normal faults on the inner margins. Do we have the data base to assess the problem?
|
From page 213... ...
Figure 2 cartoons a comparison that is now commonly made between simple and pure shear extensional deformation of the lithosphere. In particular, simple shear deformation contrasts in generating strong asymmetry in the extensional system?
|
From page 214... ...
. Simple shear tectonics is in the process of rapidly gaining ground as a principal component of passive margin models.
|
From page 215... ...
While these studies represent a clear departure point in research into extensional tectonics, it has by no means been demonstrated that simple shear is indeed the dominant mechanism for -the extensional deformation that culminates in seafloor spreading. On some passive margins mid-crustal seismic reflecting horizons have been recognized that may be detachment surfaces (Boillot et al., 1987; Wernicke and Burchfiel, 1981; Lister et al, 1986, in press; LePichon and Barbier, 1987; Mutter et al., in press)
|
From page 216... ...
While the majority of seismic reflection interpretation of passive margins focuses on large half-grabens developed under thick post-rift cover, work in the Basin and Range is now showing that the structural expression of large-scale extension may be only subtly expressed in 216
|
From page 217... ...
A national program of research that strongly interfaces onshore and offshore research is needed. This problem is particularly acute for extended continental margins relative to compressional ones because the end product of rifting is so difficult to access and the active examples so very few.
|
From page 218... ...
The Continental Margins Vorkshop creates a timely opportunity to define the key questions that we must answer in order to advance our understanding of the processes underlying the formation of these fundamental Earth structures, and set forth plans for long-term global investigations that can answer these questions.
|
From page 219... ...
Boutillier, 1982. On the Evolution of Rifted Continental Margins: Comparison of Models and Observations for the Nova Scotia Margin, Geophys.
|
From page 220... ...
Dewey and P.L. Hancock, eds., Continental Extensional Tectonics, Geol.
|
From page 221... ...
Symonds, 1986. Application of the Detachment Fault Model to the Formation of Passive Continental Margins; Geology, Vol.
|
From page 222... ...
Vail, 1982. A Thermal-Hechanical Model of Rifting With Implications for Outer Highs on Passive Continental Margins, In: J.S.
|
From page 223... ...
F., 1986. FTexure of the Lo thosphere and Continental Margin Basins, Tectonophysics, 36, p.
|
From page 225... ...
or '/~-~/]
|
From page 226... ...
lo. FIGURE 3 me zone of permanent subvertical simple shear strain 226
|
From page 227... ...
1~ ~ \ \ \ o \ \ ' \ - 1 a At onset of debn~inatior, ~ $,N, b Low-angb normal faults "fire'' from disseminating Bayer MuNipb d~eh~ tam ,9 Mylon~t¢tront C Lower pi - e bow upward ~\ actn,. insctrve mylonites _~_ mvionites __ d l~letemorphic core complex in lower pl - e culmination FIGURE 4 227 Mylonitised granite Jn~cinre ~r~==
|
From page 228... ...
R3 'iai~iii~ = ~ ~ ~ ~ ~ 5_.
|
From page 230... ...
Magmatism is therefore intimately tied to extensional tectonics so that an understanding of extension is incomplete without a full description of the magmanc processes. Geophysical surveys and deep sea drilling of many passive continental margins have confined Cat volcanic and intrusive bodies are generally to be included as components of the rifted margin architecture, but their relative volumes and distributions are highly variable.
|
From page 231... ...
Comparison of the typical structure elements of volcanic and nonvolcaruc margins. The n~nbers refer to the following structural elemenm: 1, the nonnal thickness oceanic crust; 2, the Wick volcanic succession associated with the initiation of seafloor spreading of volcanic margins of which the sowed dipping Its form the upper sequence; 3, a s~ctura1 high in continental crust that often occurs adjacent to the thick volcanic succession; 4, Tinned, subsided continental crust; 5, unstretched con~ntal crush The dot dash line mark Me s=ugraphic level Of blowup.
|
From page 232... ...
. Magmatic activity plays an important role in the structural development of the crust, the thermal and mechanical evolution of the lithosphere, and subsidence history of rifts and continental margins.
|
From page 233... ...
Seafloor ~preading =dally develops in equidistant axial "hot pO~ltS", which are probably reLited to upwelling a~henospheric diapirs ~igg~d by a R~leighl Taylor-~e ins~bility in the upper mantle. These "hot points" act as nuclei f~ a~cial propagation of the oceanic rift, evenmaBy resu~ng in a more or less continuous ems of pp~ing.
|
From page 234... ...
e: lbe different histories of nfung can result in the juxtaposition of margins Mat are largely arnagmaiic, producing nonnal Sickness oceanic crust at the onset of seafloor spreading, and margins Hat indicate a prolific magmaiic episode that accompanied the onset of seafloor spreading. This cartoon depicts ~ juxtaposition of the nonvolcanic Exmouth margin (background)
|
From page 236... ...
, , . Clearly the formation of continental margins by rifling and magmausm is one of die primary processes that has shaped the Earth, and we need to~develop a strategy: to study in detail a process that operated on such a large scale and Hat is often not directly accessible to us.
|
From page 237... ...
for structural and magmatic segmentation of m~ocean ridge spreading centers (e.g. Langmuir et al., 198~6~.
|
From page 239... ...
3. What defines the Moho; how do we measure crustal thicknesses in presently extending rifts and quiescent passive margins, and how can we distinguish deformed continental crust from uplifted highly defonnec} pre-r~ft mantle, residual lower crystal rocks depleted by partial melting, layered mafic and ul~amaf~c cumulates, or some comb~nai~on of these (Af~mendinger et al., 1987; Cans, 1987)
|
From page 240... ...
What dis~anct geocheTn~smes am evident In ma~=,rnai~sm due to back arc extension, and what infom~anon does this reveal for He upper mantle in these regions? Since many immature ocean basins are of the back-arc ~e, an assessment of the similani~es and differences between Weir development and the development of passive margins In general would be a useful constraint in process evaluation for both systems.
|
From page 241... ...
How are highly fractionated ~,oicanics formed, and what volume of residual cumulate matenal is required In the crust beneath rifts? SOME SUGGESTIONS FOR FUTURE RESEARCH Future research that could provide important insights into these problems would involve multidisciplinary studies carried out both along and across presently extending rifts as well as.passive continental margins In apparently different Pragmatic and tectonic settings.
|
From page 242... ...
In parallel with the above studies, drilling Reverses along a flow line of oceanic crust immediately adjacent to and seaward of contend margins should be castled out proximal to magmanc, amagmaiic, and back-arc margins to determine the mantle evolution associated with the development of an ocean basin in different tectonic settings. These should be accompanied by multichannel refiecuon and refraction, geopotennal, and heat flow surveys that extend landward into Me rifted margins.
|
From page 243... ...
G., 1986. Tectonic configuration of the western Arabian continental margin, southern Red Sea.
|
From page 244... ...
A., 1986. Application of the detachment model to the fo~Tnai~on of passive continental margins.
|
From page 245... ...
N., 1987. Magmatism at rifted continental margins.
|
From page 246... ...
Passive Continental Groins Rip AND PASSIVE MARGIN USE THE SEOI~ WARD 246
|
From page 247... ...
sequences tend to be thicker and contain more information about nondepositional environmental changes associated with sea level changes. Other marine libraries, particularly the creep sea basin sediments, also contain important sea level related data, as, for example, those associated with chemical and temperature changes, which the oxidizing environment and stronger currents on the outer shelf and upper slope tend to destroy.
|
From page 248... ...
sea level change. Sequence stratigraphy records a multidimensional matrix with dimensions of Ethology, climate, tectonic regime, sediment flux, and other parameters which are useful in the study of global sea level changes and associated phenomena.
|
From page 249... ...
Legitimate differences in interpretation of systems tracts may result. The problem is most-acute in data from continental margins.
|
From page 250... ...
Sequences and systems tracts are likely to differ significantly from one of the above depositions environments to the other Sequences developed in basins underpinned by subsiding continental crust, e.g., the North Sea and East Texas Salt Basin, appear to differ from those underpinned by subsiding oceanic or greatly thinned continental crust. Stratigraphic signatures in these environments are poorly investigated.
|
From page 251... ...
Fin any, comple~nentary outcrop studies are valuable in the investigation of offshore sequence stratigraphy. Global Sea Level Changes The concept of global or eustatic sea level changes is a powerful tool, central to sequence stratigraphy.
|
From page 252... ...
examined this issue in his study of data from the Santos Basin in the western South Atlantic, the Grand Banks of southeastern Canada, and the Beaulort Sea in the Arctic Ocean. His results are shown in Figure 6.
|
From page 253... ...
comm., 1988) reports that coring in continental margin carbonates has produced results similar to those from atolls.
|
From page 254... ...
have presented a mode} suggesting that intraplate stresses during plate collison, fragmentation or reorganization at convergent plate boundaries are sufficient to cause sea level changes of 100 m at rates of ~ cm/k.y. There is some evidence to support this model.
|
From page 255... ...
correlated vanations in the sediment carbon budget with sea level changes, and Prakes and Bolton (1984) correlated sea level regression on the Australian margin with giant manganese deposits.
|
From page 256... ...
Objective testing of mechanistic models responsible for 28- and 36-order sea level changes are not possible until these matters are resolved.
|
From page 257... ...
E.? 1986, Cretaceouspaleoceanographyof the western North Atlantic Ocean; in Vogt, P
|
From page 258... ...
J., 1988, Age and significance of sequence boundaries on Jurassic and Early Cretaceous rifted continental margins: Am. Assoc.
|
From page 259... ...
C., III, and Golovchenko, X., 1983, The effect of sea level change on the shelledge and slope of passive margins: SEPM Sp.
|
From page 260... ...
W., and Ward, L W., 1987, Cenozoic unconformities and depositional supersequences of North Atlantic continental margins: Testing the Vail model: Geology, v.
|
From page 261... ...
Cretaceous global sea level changes.
|
From page 262... ...
1- Hlasa- ~ 1 I'm - ~ 11 ~ it At ARC a_ LL — In _ z r r \ _ In X a)
|
From page 263... ...
ir o ~ al 1:5 z ~ ° m 0 ~ { LL a)
|
From page 264... ...
DSDP 612 _ USGS ISEAcH COST COST MA AGE No.1 S2H2-tlL B-2 B O PLEIST ; 10~ 1 1 I?
|
From page 265... ...
~ 1 In ` A o, 2 _ at: ~ [Ui_ 0 #,C Carl IIJ t- Z Z ~ Us C]
|
From page 266... ...
70 75 80 85 90 95 100 105 1 1 0 30 33 ~4 C30 C31 C32 a: LL AL Q en o LL 111 66.5 MAASTRICHTIAN A: z o z 111 An 74 CAMPANIAN 84 89 SANTONIAN 88 CONIACIAN - 92 TU8ONIAN CENOMANIAN ALBIAN 106 APTIAN _ _ LONG TERM ~—SHORT TERM 70 75 80 85 90 95 100 105 110 Fig.
|
From page 268... ...
~ cn ~ - ~ At: a)
|
From page 269... ...
Crustal processes such as subsidence rate and isostatic response to sediment loading also affect the conditions in passive margin sediments. The proposed Continental Margin Workshop is an opportunity to examine the interactions among these diverse phenomena and devise strategies for studying ~em.
|
From page 270... ...
Compaction produces an upward flow of water when permeabilities permit, but leads to the formation of ove~pressure when water camlot escape. The thickness and physical properties of sediments deposited in passive margin basins are often laterally heterogeneous leading to significant lateral fluid flow.
|
From page 271... ...
Tectonic subsidence history may be used to constrain me mechanisms Mat formed the margin. Two end-member models of continental rifting and subsequent passive continental margin formation are currently popular.
|
From page 272... ...
This or other mechanisms are required in many basins to move energy upward faster than conduction should allow. The physical properties of margin basin sediments and the lithosphere below influence literally all of the geophysical observations: gravity, geoid height, seismology, magnetics, well logs, and physical processes we seek to understand: fluid flow, heat flow, deformation, fracture, sediment compaction, and etc.
|
From page 273... ...
It win allow us to build quantitative models of diagenes~s. The magnitude of inorganic chemical fluxes into the ocean through passive margin basins are largely unknown.
|
From page 274... ...
What are the chemical fluxes into the global seawater system at passive continental margin basins? How, when, and where do hydrocarbons mature in passive continental margin basins?
|
From page 275... ...
Drilling will eventually be required but is not useful until more survey work is complete. The principal means of study of the isostatic response of the lithosphere under passive continental margin basins involve comparing observations of gravity, geoid height, topography, sediment density and distribution, crust thickness, density and li~ology' and subsidence history' using geodynamic models.
|
From page 276... ...
90-92* Wernicke, B., 1985, Uniform-sense normal simple shear of the continental lithosphere, Canadian Journal of Earth Sciences, v.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.