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Compressional Styles - Basement Involved

The two principal elements of basement-related structural styles are compressional fault blocks and their

bounding basement thrusts which may range in inclination from near-vertical to less than 30ordm Both high

and low angle faults occur along a single thrust front and are now thought to represent the end members of

a continuous spectrum of basement-involved deformation

Reverse faults involving basement generally occur along convergent plate margins primarily in foreland

regions characterized by A-type subduction and in the inner trench slope and outer high portions of B-type

subduction zones Of these the foreland regions are by far the most important to petroleum exploration

Basement-involved forelands primarily develop in two major settings between a volcanic arc and a craton

(ie in a backarc setting) and in front of the arc during continental collision The first of these is

particularly characteristic of the North and South American cordillera while the latter typifies the great

Alpine-Himalayan orogenic zone

Compressional basement block faulting is particularly well known and well studied in the Rocky Mountain

foreland region of the western United States

( Figure 1 Principal basement uplifts and associated basins of the Rocky Mountain foreland region Cross

section lines refer to Fig 3) In this area large rigid masses of Precambrian crystalline rock have been

forced up and to some degree laterally outward many thousands of feet The larger uplifted blocks appear

to be intimately associated with deep foreland basins in general each block has its related basin(s) whose

timing of subsidence seems directly related to uplift history Most often these basins are strongly

asymmetric with their structural axes running close to the thrust front and their back flanks forming

relatively gentle basement slopes away from the uplift

These basins have served as the sites for considerable sediment accumulation and subsequent petroleum

generation and entrapment Structural relief between the present crest of basement uplifts and their

associated basins is frequently on the order of 10 km or more Thus the vertical component to faulting is

undoubtedly considerable The overlying sedimentary cover on these blocks has usually acted in a passive

manner accommodating basement uplift by drape folding and brittle fracture Blocks are typically bounded

by faults on both sides and are tilted at various angles

Figure 2 ( Basement faulting in the Rocky Mountain foreland region) shows four styles of faulting that have

been proposed to explain the geometries observed in the field and in seismic profiles To some degree the

differences between these reflect the fact that various investigators have concentrated their efforts in

different parts of the region along different major fault zones Such zones display variable geometries

which range from reverse listric planes (steepening with depth) to low-dipping thrusts of 30ordm or less (



Figure 3 Two interpreted seismic profiles from Wyoming showing expression of rigid basement uplifts (a)

Southwest Wind river fault (b) Casper Arch thrust Note how these end member styles of faulting

correspond to (b) and (d) of Fig2) Single faults apparently show both these geometries along their strike

At present the proposed styles of faulting reflect two major schools of thought These agree that

compression is involved to some degree in deformation but part company on the question of whether

displacement is dominantly vertical upthrusting of Figure 2 parts (a) and (b) or horizontal

overthrusting of parts (c) and (d) The seismic profiles given in Figure 3 illustrate how apparently

irreconcilable these two schools are

Many of the principal faults have also apparently suffered a degree of ancillary strike slip displacement For

these reasons some geologists now think that at least several of the major blocks such as the Wind River

have been uplifted in a type of scissorlike rotational pattern with the inclination of the fault plane

decreasing and the amount of lateral movement increasing with distance from a nodal zone of almost total

vertical uplift This has in turn been related to the clockwise rotation of the entire Colorado Plateau duringLaramide time (see for example Gries 1983)

With respect to those blocks bounded by near-vertical faults Harding and Lowell (1979) describe three

basic structural levels from basement up into the sedimentary cover

bull a tilted fault block of basement and immediately overlying units

bull an intermediate level where the principal fault becomes a zone of steep drag folding

bull an uppermost level of gentle drape or monoclinal folding

The structure of fault segments can be complex involving normal faulting along the crest of the block and

drape fold tear faulting of the cover and basement and more minor reverse faulting in the foot-wall of the

main fault

Some geologists have pointed to older zones of Precambrian shearing within the basement itself as a

probable control on the location and possibly geometry of faults Certainly in any region where basement

faulting is a dominant structural style the geologist should understand the preexisting structural character

of the rocks involved Metamorphic rocks are in general the products of orogenesis and very often contain

important planes of weakness such as shear zones and major lithologic boundaries that will have some

amount of influence on all later deformation



Figure 4 ( Regional east-west cross section through the Rocky Mountain foreland from northeastern

Wyoming to southeastern Idaho The deep crustal faulting shown for the Wind River Mountains is based on

recent COCORP seismic reflection profiles Note that faults are interpreted to reach to the crust - mantle

boundary Western end of section shows decollement thrusting of the western overthrust belt ) shows

basement faults as both relatively planar and as listric to the crust-mantle boundary Compressive

decoupling is therefore proposed to occur at or near the Moho At present this interpretation is highly

speculative It does however take account of the prevailing hypothesis of intracontinental underthrusting

(a form of A-type subduction)

The most generally accepted idea about the genesis of these basement foreland uplifts is that the principal

compressional stresses are in some way directly related to B-subduction as appears to be the case in the

South American cordillera ( Figure 5 Basic tectonic setting and regional cross section through northwest

Columbia showing three principal tectonic divisions Note the extensive involvement of continental

basement in faulting of the eastern cordillera) Here an A-B subduction couple seems responsible for theopposing directions of tectonic transport

Broadly speaking the vertical rise of large rigid tectonic blocks has generated a good deal of local

variability and thus a considerable diversity of structural traps should be expected ( Figure 6 General

hydrocarbon trapping possibilities associated with basement block uplifts) The Elk basin oil field in the

Big horn basin of northern Wyoming is an example of one of the larger traps discovered thus far To date it

has produced over 500 million bbl out of fault-controlled closures such as those shown in Figure 7

(Uninterpreted and interpreted seismic sections through South Elk basin producing area (northeast Big

Horn basin Wyoming ) showing anticlinal folding over basement thrusts Note how deformation in thissection appears to correspond with style (c) in Fig 2) and Figure 8 (Cross section of Elk Basin field

approximately 10 miles north of the seismic profile shown in Fig 7 )

In more recent years some companies have attempted to penetrate several of the lower-dipping

Precambrian thrusts in order to explore the sedimentary section beneath certain formations of which are

known to be productive in nearby domes and folds To date only a few of these expensive wells have been

successful Oil and gas leaks within the thrust zones however have been reported

More generally in addition to providing drape and fault-related closures basement uplift can also influence petroleum generation and trapping in more subtle ways For example in the Hardeman basin of northeast

Texas high-angle splinter faults related to late Paleozoic uplift of the Red River-Matador Arch and the

Wichita Mountains created local avenues for the invasion of dolomitizing solutions into the cores and

flanks of Mississippian bioherms As a result these became excellent though highly localized reservoirs



Another possibility involves the inversion of rift-related block faults during later compressional tectonism

Convergence of older passive plate margins can lead to the rejuvenation of originally normal faults into

high-angle reverse faults or thrusts Such reactivation since it reverses the sense of displacement is said to

create inverted basins This can be crucial to understand in certain regions since the sediments normally

assumed to characterize rift provinces will become involved in anticlinal thrust and compressional-wrench

tectonism In such cases apparent horst-and-graben morphology may be retained however the more recent

uplift will have substantially altered the original structural configuration

Compressional Styles - Basement Not Involved

The primary structural style in this category is the decollement foreland thrust-fold belt Due to the

thorough work of such authors as Price and Mountjoy (1970) Dahlstrom (1970) and Bally Gordy and

Steward (1966) the Canadian cordillera has come to be generally treated as a type locality for such

deformation Figure 1 ( shows basal decollement and telescoped nature of regional thrust deformation

Note the interpreted involvement of matamorphism in thrusting toward the west ie closest to the hot

mobile core of the orogen ) is the classic cross section by Price and Mountjoy (1970) through the southern

portion of this structural belt and shows most of the principal features we have come to expect in this style

The majority of hydrocarbon production exists in the foothills region where faulting is especially complex

Figure 2 is a cross section through the Jumpingpound gas field which amply shows this complexity Note

the degree of imbrication above the ramp anticline where closure is best this is the primary trap in this and

many other fields of the foothills area

Thrusted anticlines appear to be the most common traps in productive foreland regions throughout the

world but with regard to the size and specific geometry of traps it should be emphasized that considerable

variety exists within and between these regions Over the span of Proterozoic time differences in the size

and morphology of plates in their marginal sedimentary character inherited structural features and

specific motions have all ensured a high degree of structural variation in foreland orogenic zones

Like compressive basement faulting decollement thrusting is most often related to processes occurring

along convergent plate boundaries both in the mentioned foreland belts and along the leading edges of

subduction zones We have already discussed some of the details of B-type subduction zones Figure 3

displays the interpreted structure on a seismic section across the active subduction complex offshore of

northern California This section shows the decollement faulting in the accretionary prism quite well



Note the change from the chaotically deformed accretionary prism sediments into the more conformable

forearc basin at the far right The deformed complex is broken into thrust slices about 400-500 m (1300-

1600 ft) thick whose bounding faults display variable curvature This is probably due to a degree of

stratigraphic control and ramping of fault development We can also see that where compressional features

dominate the accretionary prism extensional faulting characterizes the deep sea sedimentary section west

of the trench This has been observed in a number of active subduction zones Often these faults have

developed in the underlying ocean crust and have been interpreted as being the result of lithospheric

bending and consequent tensional rupture

Collision-related thrust belts verge toward the subducting (on-coming) plate and represent the succession

of B-subduction by A-subduction ( Figure 4 a map showing distribution of the major collision-related

fold and thrust belts and related foredeeps (regional foreland basins) Many of the worlds foredeeps

(Appalachian Canadian Rocky Mountain Arabian Uralian) are highly productive of oil and gas Cross

section lines refer to Figure 5) A transition to basement-involved thrusting often occurs in these settings behind the thrust front Figure 5 and Figure 6 (Generalized cross section through four of the worlds major

collision-generated mountain systems at comparable scales) show how this involvement is usually

interpreted to increase into the mobilized metamorphic core of an orogen such as the Canadian

cordillera In the case of trench deformation the situation is less well understood Two forms of basement-

involved faulting are presumed that which incorporates slivers of ocean crust (known as ophiolites) into

the accretionary prism and that which cuts continental crust For both trench and foreland thrust zones

then overlap occurs between detached and connected structural styles

Most thrust belts presently exposed at the earths surface occur as sinuous belts up to thousands of kilometers long Their width is not uniform but is instead characterized by sharply recessed and more

gently extended portions known respectively as reentrants and salients (see Figure 4 ) The cause for

this type of variation along tectonic strike is not well understood but often thought to be related to

preexisting basement features

Figure 5 and Figure 6 compare the overall features of several major collision-generated foreland regions

The Alps appear to have resulted from a series of collisional episodes between the Eurasian continent and

various arclike continental fragments that once bordered it to the south The Zagros orogen as we have

noted (see Figure 7 Generalized map and cross section showing continental breakup along the Red Searift and collision in the Zagros region of southeastern Iran Numbers indicate total estimated separation

(in km) between Africa and Arabia) is the continuing result of Eurasia colliding with the much smaller

Arabian continent Both the extensive Himalayan Mountain belt and the Appalachian-Ouchita-Marathon

system are interpreted as the result of the impact between larger continental masses-in the first case India

and Eurasia in the second those Paleozoic continents (proto-North America South America Africa and

Europe) whose collision marked the early formation of Pangaea In both the Alps and the Himalayas great



thicknesses of crystalline basement rocks are involved in the deformation and have exerted considerable

control on resulting structures Though decollement thrusting is evident in the Jura Mountains of western

Switzerland and eastern France it does not dominate the regional structural style of the Alps as it does the

Appalachian Zagros or Canadian forelands

At the same time however we can also think of the Alps and the Himalayas as two opposite end-members

with respect to the general style of deformation The the sedimentary cover The central and southern

European Alps with their spectacular development of thrust and fold nappe structures represent the most

highly contorted foreland region in the world Broadly speaking they show the effects of very rapid

collision-related diastrophism on a thick only semicompacted sedimentary pile Folding of a highly ductile

nature is common recumbent and overturned nappes are piled up on top of each other like rumpled carpets

to form the higher ranges (see Figure 8 Alpine nappe structures) Sediments appear to have been squeezed

up and out of forearc and backarc basins as the various island arc fragments were progressively welded to

the Eurasian continent

The Himalayas present a very different case Here great thicknesses of well-indurated Paleozoic and Early

Mesozoic sandstone and carbonate rocks were involved in the deformation This generally created less

contorted more widely spaced and far more massive structures

As in the Canadian Rocky Mountains and the western overthrust belt of the United States ramping is well

developed in the Appalachian foreland This has been interpreted as being the result of both ductility

contrast related to stratigraphic variation and preexisting block faulting in the basement

The well-exposed dramatic structures of the Zagros foreland appear to resemble those of the Appalachians

more than those of the Alps or Himalayas Some of the larger folds stretch for as much as 160 km (100

miles) along strike before plunging beneath the surface The existence of salt layers in the lithologic section

has resulted in a high degree of complex local decollement In places for example slip has occurred along

and within salt intervals This has apparently created shallow anticlines that have subsequently been pierced

by the tectonically mobilized salt

In discussing foreland thrust belts there are a number of general aspects that are of direct importance to

hydrocarbon exploration

1 Anticlines occur primarily in the hangingwall and are asymmetric toward the direction of

tectonic transport



2 The geometry of thrusting is much dependent on the competence of the sedimentary units

involved The number of thrusts as well as the intensity of folding decreases in distinct

proportion to increases in the amount of thick competent units (eg sandstone carbonates)

3 Sedimentary thicknesses decrease and deposits change character to more shallow-water clastics

in the direction of tectonic transport ie away from the metamorphic core or in stratigraphic

terms toward the basin margin This usually results in an increase in the overall density of faulting

in the same direction

4 Structures become progressively younger in the direction of tectonic transport

5 Rocks involved in thrusting also become younger in this direction

6 Advancing thrust sheets act to load and depress the crust into local fore-land basins (sometimes

called molasse basins) These fill with coarse marine and nonmarine detritus which then also

becomes involved in deformation Such basins are often rich in plant-derived organic matter

7 In many cases a regional foreland basin relatively rich in petroleum will exist immediately out

in front of the thrust belt presumably created by crustal loading on a more massive scale

Thus deformation appears to begin in the deeper thicker portions of the sedimentary wedge and progress

upward and on-to the craton In a broad sense this has meant that hydrocarbons have had the best chance to

accumulate and remain undisturbed near the youngest leading portions of thrust belts and in the regional

foreland basins out in front of them

As mentioned traps in thrust belts are mainly associated with asymmetric anticlines ( Figure 9 Seismic

profile through the eastern Po plain in northeastern Italy (approximately 50 km southwest of Venice)

showing thrust structure of the Apennines This profile reveals the thrust belt at its widest point Gas pools

exist in Pliocene sandstones that wedge out against the rising structures Oil is produced from complex

structural traps in underlying Mesozoic carbonates) and to a lesser extent fault truncations Closures are

most often at less than about 3000 m depth (Harding and Lowell 1979) The actual size of individual traps

and their height of closure can vary a great deal depending on the spacing of thrusts and the degree of

asymmetry in folds

Substantial-even giant-accumulations have been discovered in a number of the worlds foreland regions

Perhaps the most impressive example of production from thrust-fold structures is offered by the oil fields in

the Zagros Mountains Here reservoir quality is due to an extensive interconnected fracture system

generated by the folding Production is from the Asmari Limestone which by itself yields more than 75



of all petroleum currently being recovered from traps in foreland belts In addition several recent major

discoveries along the Idaho-Wyoming thrust belt have encouraged continued exploration and have caused

geologists to take another more detailed look at the petroleum potential of other foreland regions such as

the Appalachian-Ouchita system

With regard to active subduction zones several general statements can be made concerning overall

hydrocarbon potential In the forearc for example both source and seal can be more than adequate but the

large amount of volcanogenic material generally makes for rather poor reservoir quality Not only is the

sediment matrix often fine-grained but both primary and secondary pore-plugging and swelling clays-most

notably illite and montmorillonite-are abundant

Moreover low heat flow is characteristic of the forearc this is even more the case in the forearc basins that

develop over the subduction complex itself Structural traps however abound and consist of anticlinal and

thrust closures and relatively shallow drape folds above thrusts

The problems mentioned for forearc regions appear to characterize the Makran subduction complex which

because of its unique setting may otherwise appear to offer relatively strong hydrocarbon potential ( Figure

10 Interpreted structure of the Makran accretionary prism Gulf of Omansee Figure 4 for approximate

location) This very large accretionary prism is basically the continuation of the Zagros collision zone into a

B-type subduction zone that stretches nearly 900 km eastward from the southwestern coast of Iran to

Pakistan It shows the development of many thick coherent thrust-fold structures whose amplitudes are

unusually large and whose petroleum potential might therefore seem to be substantially greater than that in

most other forearc arc systems As much as 7 km of mostly late Tertiary abyssal plain sediments (most

likely resulting from high erosion rates that began with the India-Eurasia collisional event to the north and

west) have been deformed into an imbricated stack

Despite this structurally attractive setting potential appears moderate at best due to relatively low

geothermal gradients which average 1 ordm per 30 m (Harms et al 1983) Gas seeps and traces of heavy

hydrocarbons however have been found At the same time wells drilled in coastal Makran have

encountered very high pressures Because of the young age of the sediments penetrated (Pliocene at total

depth) their rapid accumulation (about 300 m per my) and their subsequent deformation we can expect

pore pressures to be quite high As a whole then the combination of remote access potential drilling problems and low-to-moderate maturation potential makes most active forearc systems relatively high

exploration risks at present



Figure 3 Two interpreted seismic profiles from Wyoming showing expression of rigid basement uplifts (a)

Southwest Wind river fault (b) Casper Arch thrust Note how these end member styles of faulting

correspond to (b) and (d) of Fig2) Single faults apparently show both these geometries along their strike

At present the proposed styles of faulting reflect two major schools of thought These agree that

compression is involved to some degree in deformation but part company on the question of whether

displacement is dominantly vertical upthrusting of Figure 2 parts (a) and (b) or horizontal

overthrusting of parts (c) and (d) The seismic profiles given in Figure 3 illustrate how apparently

irreconcilable these two schools are

Many of the principal faults have also apparently suffered a degree of ancillary strike slip displacement For

these reasons some geologists now think that at least several of the major blocks such as the Wind River

have been uplifted in a type of scissorlike rotational pattern with the inclination of the fault plane

decreasing and the amount of lateral movement increasing with distance from a nodal zone of almost total

vertical uplift This has in turn been related to the clockwise rotation of the entire Colorado Plateau duringLaramide time (see for example Gries 1983)

With respect to those blocks bounded by near-vertical faults Harding and Lowell (1979) describe three

basic structural levels from basement up into the sedimentary cover

bull a tilted fault block of basement and immediately overlying units

bull an intermediate level where the principal fault becomes a zone of steep drag folding

bull an uppermost level of gentle drape or monoclinal folding

The structure of fault segments can be complex involving normal faulting along the crest of the block and

drape fold tear faulting of the cover and basement and more minor reverse faulting in the foot-wall of the

main fault

Some geologists have pointed to older zones of Precambrian shearing within the basement itself as a

probable control on the location and possibly geometry of faults Certainly in any region where basement

faulting is a dominant structural style the geologist should understand the preexisting structural character

of the rocks involved Metamorphic rocks are in general the products of orogenesis and very often contain

important planes of weakness such as shear zones and major lithologic boundaries that will have some

amount of influence on all later deformation































































































































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tectonic transport




























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tectonic transport




























asymmetry in folds




































































































tectonic transport




























asymmetry in folds


































































tectonic transport




























asymmetry in folds































































asymmetry in folds



































































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Compressional Styles.doc