DEEP SEISMIC DATA OFFER NEW VIEW OF BASIN

Tim Stern
Geophysics Division Department of Scientific and Industrial Research
Wellington, New Zealand

Deep seismic reflection is the application of standard oil industry seismic methods to the study of the whole of the earth's crust. In continental areas the crustal thickness generally varies from about 20 to 50 km. Therefore, the principal difference from oil company seismic surveys is that larger energy sources are used, if possible, and seismographs record longer for reflected energy; i.e., typically 15-20 sec two-way travel time (TWT) rather than 5-6 sec. Deep seismic profiles at 100-300 km long are also typically much longer than industry lines. This has two principal advantages: first, geological resolution for structures deep in the crust; second, surface features, such as overthrust belts, can be followed to great depths into the crust and even the upper mantle.1

The main goal of deep seismic has been to further our understanding of the makeup and evolution of the continental crust. The questions addressed have largely been global and at times academic. Recently, there has been a more pragmatic move to applying crustal seismic studies to specific questions associated with the deep crustal structure beneath oil-producing sedimentary basins. This paper is, therefore, directed to a brief description and interpretation of a deep seismic reflection profile from New Zealand's principal oil-producing basin-the Taranaki basin. In particular, one of the main intents of the paper is to demonstrate how deep seismic data have led to an interpretation of the South Taranaki basin in terms of a compressionally driven, foreland basin structure.

EXTENSIONAL, FORELAND BASINS

Early seismic refraction studies showed the earth's crust to consist of two or three plane layers of velocity 5-7 km/sec overlying an upper mantle of velocity around 8 km/sec. The boundary between the crust and the upper mantle is referred to as the "Moho" after the Yugoslavian seismologist Mohorovicic who discovered it.2

More recent deep seismic reflection studies show that the crust is much more complex than the early refraction results suggest. In particular, the Moho is often marked by a broad transition zone, up to 10 km thick, of high reflectivity, and the crust itself displays lateral heterogeneity over distances of only 10-20 km.3 In reflection seismology the "reflection Moho" is taken as the depth where reflectivity at the base of the crust ceases,4 as this generally is found to coincide with other geophysical determinations of depth to Moho.

Because the Moho represents a first-order change in seismic velocity, it is also inferred to represent a similarly large increase in density. Therefore, if the Moho is arched upwards, say by stretching the crust, isostasy will require that the earth's surface bend downwards to compensate. This process results in an extensionally driven basin5 and is one of the two principal processes by which sedimentary basins are formed.

If, on the other hand, thrust sheets are loaded onto a portion of the lithosphere, the earth's crust will respond isostatically by flexing downward ahead of the thrusts. This is the other end member of sedimentary basin types, variously labeled exogeosyncline,6 fore-deep or foreland basin;7 8 the term foreland basin will be used here.

For basins formed by crustal extension, or stretching, the Moho appears flat on a time section because, as Warner9 argues, lithospheric strength is low, local isostasy prevails, and therefore both the density and travel time integral down to the Moho remain constant. In contrast, a foreland basin shows a dipping Moho, with the thickest section of the crust immediately adjacent to, or beneath, the overthrust belt. Hence, if deep seismic reflection data can track the Moho beneath a sedimentary basin, then they may provide an insight to the driving mechanism for the basin in question.

In this study we apply this simple principle to the South Taranaki basin. Although the prevalent thinking is that the South Taranaki basin is a rift structure,10 11 the deep seismic data from this area show at least 10 km of crustal thickening beneath, and a broad crustal flexure down towards, its eastern margin.

A compressional, foreland-basin origin for the South Taranaki basin is thus proposed.

STRUCTURAL BACKGROUND

A location map for both the Taranaki basin and the seismic line discussed here are shown in Fig. 1. Over 25,000 km of oil industry shallow seismic (5-6 sec TWT) and more than 20 drill holes have provided the principal geological and structural information about the basin.11 12 13 Existing seismic reflection data indicate that the North Taranaki basin is structurally different from the South Taranaki basin.13 The results from this study, therefore, pertain to the South Taranaki basin only.

For this study the eastern boundary of the Taranaki basin is taken at the Taranaki Boundary fault as shown in Fig. 1. The Taranaki fault zone has been regarded as a relatively narrow zone taking in the Taranaki Boundary fault and fault offshoots like the Manaia fault.12 In this study "Taranaki fault zone" refers to the zone of inferred stacked thrusts that takes in the Manaia and Taranaki Boundary faults and also extends well to the east of these two faults, as will be discussed shortly. The southern boundary of the South Taranaki basin is taken as the Northwest Nelson coastline, the northern boundary as the Taranaki coastline.

These are arbitrary choices for the northern and southern boundaries but will suffice for the present study. To the west the South Taranaki basin merges into the Western platform-an area of relatively undisturbed continental shelf dominated by a large thickness of westward prograding Plio-Pleistocene sediments called the Giant Foreset beds.12

A cross section of the South Taranaki basin is shown at the bottom of Fig. 1 and demonstrates its asymmetry.

Implicit in this cross section are two distinct phases to the geological history of the Taranaki basin: a Cretaceous-Oligocene rifting phase, linked to the breakup of Gondwanaland, and a late Oligocene-Pliocene phase of tectonic activity associated with the propagation of the present Australian-Pacific plate boundary into the eastern North Island.

The Taranaki basin lies behind this plate boundary, and the subducted Pacific plate is present beneath the basin to depths of up to 300 km.14

The seismic line crossed the southern portion of the Wanganui basin and then the South Taranaki basin. Anderton has analyzed industry seismic from the Wanganui basin and describes the basin as a quasicircular, 4-5 km deep Plio-Pleistocene basin filled with shallow marine sediments. Therefore, the Wanganui basin is a younger feature than the adjacent South Taranaki basin. Up to 2 km of Plio-Pleistocene of onlapping sediments cross the eastern margin of the South Taranaki basin.15

The initial subsidence responsible for these sediments can be attributed to distributed flexure linked to Plio-Pleistocene crustal down-warping centered within the Wanganui basin.16 17

DEEP SEISMIC DATA

The deep seismic line was 220 km long, shot by Western Geophysical with processing carried out by GECO (NZ). Initial interpretations of these data have been presented by Davey18 and Stern and Davey.16

Fig. 2 shows sections of the seismic data from each side of the Cape Egmont fault zone. To the west of the Cape Egmont fault zone the crustal section resembles a classic deep crustal section from an area that has undergone extensions;19 20 that is, a strongly reflective sedimentary section down to about 2.5 sec TWT and an almost equally reflective crustal section down to 10 sec TWT (approximately 30 km deep). At about 10 sec TWT the reflectivity ceases. This has been noted worldwide to correspond to the base of the crust, or Moho, where the Moho has been detected by other geophysical means.4 East of the Cape Egmont fault zone the crustal reflectivity is not as strong, and the Moho is marked by a distinct band of reflectors that form a ramp dipping to the east.

Fig. 3 shows a line drawing presentation of the full 220 km long seismic line. Salient points to note on this line drawing are:

• The long-wavelength apparent flexure of the crust with the maximum deflection occurring adjacent to the Taranaki Boundary fault.

• Crustal thickening adjacent to the Taranaki Boundary fault of about 3 sec TWT, which corresponds to about 10 km in depth.

• Beneath the center of the Wanganui basin there is a wide "bow tie" of criss-crossing reflection events at about 10 14 sec TWT. Previous work16 18 shows this bow tie could be resolved by line-segment migration into the westward dipping reflections from subducted Pacific plate and eastward dipping Moho reflections associated with the Australian plate.

• A zone about 50 km wide between the center of the Wanganui basin and the Taranaki Boundary fault where few, if any, coherent deep reflections could be obtained. This is the area that will be referred to here as the Taranaki fault zone. In this zone it is proposed the crust is broken by thrust faults that dip steeply eastward into the crust as will be discussed shortly.

STRUCTURAL INTERPRETATION

Fig. 4 summarizes an interpretation of the deep seismic in terms of a classic foreland basin structure as, for example, outlined by Coney21 and Beaumont.8 It is proposed that the long-wavelength bending of the lithosphere is due to lithospheric flexure induced by the loading of thrust sheets at and east of the Taranaki Boundary fault. Between the Taranaki Boundary fault and the Wanganui basin there are few coherent reflections apart from some strong amplitude reflections, about 40 km from the eastern end of the seismic line, with apparent easterly dips of about 40. Similar steeply dipping and strong reflections are associated with the Wind River thrust, western U.S.,22 and the overthrust Arunta block within central Australia.23 These reflections have been interpreted to result from shear or mylonite zones associated with "thick-skinned" crustal thrusting. Thus, on the interpretation of Fig. 4 a zone of crustal thrusts is interpreted as extending from the middle of the Wanganui basin westward to the eastern margin of the South Taranaki basin. It is not clear why few crustal reflections are evident just east of the Taranaki Boundary fault, but if the thrust zones become greater than about 45 in dip, reflections from them become increasingly difficult to image.24

As shown in Fig. 4 the principal geological characteristic of a foreland basin is that at its base is a sequence of rift margin, or miogeosynclinal, sediments, generally the source rocks for the hydrocarbons, that have been buried by younger flysch-type sediments. Many of the major oil-bearing basins of the world are foreland basin structures. Examples include the Persian Gulf, the Alberta basin of western Canada, the Ouachita basin of the southern U.S., and the Appalachian basin of the eastern U.S.7 25

An interesting departure of South Taranaki basin from the generic foreland basin model shown in Fig. 4, and from the above mentioned examples, is that the polarity of the miogeosyncline is reversed; i.e., the late Cretaceous rift margin on the western side of New Zealand was open to the west, rather than to the east as suggested by the model. In this regard a possible analogy to the South Taranaki basin is the Alaskan North Slope, where the miogeosyncline thickens to the north but present-day thrusting is directed from the south.26

TIMING

Four subsidence curves from just west of the Taranaki Boundary fault are shown stacked together in Fig. 5. From 80 million years before present (Ma) until about 32 Ma the curves show an exponential decay that is characteristic of passive margins where rifting is followed by thermal contraction.27 At approximately 32 Ma another subsidence phase began slowly and then rapidly accelerated between 25 and 22 Ma. This is interpreted to reflect subsidence due to loading from a thrust front that initially formed well to the east of the Taranaki Boundary fault and progressively advanced westwards, pushing a fore-deep trough in front of it, The overall S-shape pattern of subsidence is also recognized in North America.28 Fig. 5 shows representative forms of subsidence curves from North America that contrast the exponential-decay shape of rift subsidence with the exponential growth shape of compression induced subsidence. An S-shape curve therefore results from the full cycle of rifting followed by at least one lithospheric thermal-time constant (65 million years) later by overthrusting.

In the last 15 million years all of the curves show a strong uplift. This is explicable in terms of an uplift on a moving thrust front that catches up with drill holes that are effectively fixed in the lithosphere. Furthermore, most of the drill holes are aimed at structural highs for exploration purposes, so that parts of the lithosphere that are not uplifted by the advancing thrust front tend not to be sampled.

COMPARISON WITH EUROPEAN BASINS

In Fig. 6 a comparison is shown between deep seismic sections from the South Taranaki basin and two basins from within Europe-the Celtic basin between the U.K. and Ireland and the Ebro (Spain) and the Aquitaine (France) basins found each side of the Pyrenees collision front. This comparison underscores the difference in deep structure between foreland basin and extensionally driven sedimentary basins. The Celtic basin, which has an extensional origin, displays a flat Moho on the time section. As previously explained, this is a consequence of the respective travel-time anomalies of the sediments and the subjacent thinned crust canceling each other. In comparison, Moho beneath both Taranaki and the foreland basins each side of the Pyrenees is flexed strongly downwards.

The Pyrenees collision zone is different from the Taranaki fault zone in many regards. For example, there are several kilometers of topography created at the Pyrenees collision zone, whereas most of the surface expression of the Taranaki Boundary fault is below sea level and in some places covered by up to 2 km of Plio-Pleistocene sediments. Also, two foreland basin structures, the Ebro and the Aquitaine basins, have developed each side of the Pyrenees collision zone as thrusting emanated both north and south from the collision zone.

Nevertheless, given these differences, there is still a broad similarity in the deep structure of the South Taranaki basin and the Ebro basin on the south side of the Pyrenees collision zone.

IMPLICATIONS

Recognition that the main subsidence phase of the South Taranaki basin was compressionally, rather than extensionally, driven has important thermal and hydrological implications. For example, the variation of heat flux with time due to the formation of extensional basins5 is straightforward and predicts an initial heat flux peak followed by an exponential decay with a time constant of about 65 Ma.27 Heat flux per turbations associated with overthrusting are, however, more subtle. Brewer29 shows that for regions overridden by thrust sheets of the order of 5-15 km thick, frictional heating will induce a positive heat flux anomaly that will persist for 5 Ma after thrusting has ceased. Thereafter, a negative heat flux anomaly will be present, and this will persist for 200-300 million years.

Perhaps of even more import is the role of predicted fluid and hydrocarbon migration within foreland basins. Dickinson,7 Oliver,25 and Ge and Garven30 all argue that large scale compression and overthrusting can initiate transient fluid flow hundreds of kilometers into the foreland with consequences for hydrocarbon migration, sediment-hosted ore deposits, and heat flux. For example, Oliver examines the spatial occurrence of oil and gas fields in the U.S. and notes that all major discoveries, with the exception of the Gulf Coast province, are found in foreland basin structures behind orogenic belts. He argues that hydrocarbons have been pushed along porous paths within the foreland basins by advancing thrust fronts.

The analogy Oliver uses is one of a tectonic "squeegee" driving fluids ahead of it. He also notes that meteoric water being driven through a basin will perturb geothermal gradients with the net effect of lowering the geothermal gradient near the thrust belt and raising the gradient far from the belt. Thus, some forms of metamorphism and thermal maturation could be achieved far from the thrust front without the depth of burial that would otherwise be needed.

Within the Taranaki basin all the present producing oil fields are found immediately adjacent to or just west of the Taranaki Boundary fault (Fig. 1). Source materials drilled in the McKee, Kapuni, and Maui fields are immature, suggesting that these accumulations have resulted from a migration of oil from a distant and deeper source.31 If there has been widespread up-dip migration of hydrocarbons, driven by westward moving thrust sheets in a manner similar to that described by Dickinson and Oliver, then promising prospects would be predicted within the vicinity of the Cape Egmont fault zone and even farther westward on the western platform.

CONCLUSIONS

This study demonstrates how deep seismic exploration has led to an interpretation of an oil producing basin that was not immediately apparent from the approximately 25,000 km of shallow seismic reflection data shot in the 20 years prior to the deep survey. Pronounced crustal thickening and a broad crustal flexure are the principal features seen in the deep seismic data. These features are most easily reconciled with crustal shortening and loading of thrust sheets within a 50 km wide Taranaki fault zone. Although the seismic resolution is at its poorest within the zone of the overthrusts, most of the crust is inferred to be involved in the overthrusting. Thus, evidence from deep seismic data points to a foreland basin style of late-Tertiary development for the South Taranaki basin, rather than a rift graben as previously assumed.

ACKNOWLEDGMENT

The seismic data described here were collected by Western Geophysical and processed by GECO (NZ). The cooperation and assistance of these two companies is gratefully acknowledged.

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