Sandbox model studies of inversion tectonics

Tectonophwics,

379

137 (1987) 379-388

Elsevier Science Publishers

B.V., Amsterdam

Sandbox

- Printed

model studies of inversion

A. KOOPMAN, Koninklijke

in The Netherlands

Shell Exploratie

A. SPEKSNIJDER

en Produktie Laboratorium,

(Received

February

and W.T. HORSFIELD

LREP/?,

17.1986:

tectonics

Volmerlaan 6, Rijswijk

accepted

March

1987. Sandbox

model

(The Netherlands)

26, 1986)

Abstract Koopman,

A., Speksnijder,

Ziegler (Editor), Scaled

plane-strain

sedimentary

A. and

Compressional sandbox

Horsfield,

W.T.,

Intra-Plate models

Deformations

have

been

used

in the Alpine to simulate

studies

of inversion

Foreland.

tectonics.

Tectonophysics,

basement-controlled

In: P.A.

137: 379-388.

structural

inversion

in a

overburden.

Two basement that controlled The inversion

configurations

a graben produced

were tested.

relief induced

In the second

configuration

block

generates

an inverted

Both experiments

structure

bounded

show that structural

fault blocks,

between

to its bounding

inversion fault

overburden

and tilted fault blocks

a compression

is induced

inversion

not only reactivates

are the result of horizontal

uplift of basement

pre-existing compression,

The dynamic and kinematic processes associated with structural inversion are poorly known. Generally two controversial models are proposed in literature: (a) Structural inversion as a two-dimensional

Equipment

normal

structure.

was tested.

When a tilting

in the overburden,

which

either associated

new faults

with rotation

of

and configurations

without movement in the third dimension, the plane of section. Testing a possible

three-dimen-

0 1987 Elsevier Science Publishers

faults but also generates

of the faults and the sequence of their formation (Fig. 1). By its nature, it enables investigation of a cross-sectional, plane-strain situation only, i.e. out of strike-

slip/oblique-slip origin of structural inversion is therefore beyond the scope of these experiments. The rigid blocks at the base of the box represent a structural basement, whilst the sand represents overlying moderately cohesive sediments. Sand was fed into the box from a hopper driven by an electric motor, to give a reproducible, ho-

sional strain (see, for example, Glennie and Boegner, 1981; Ziegler, 1981, 1983). Part of the controversy stems from the poor knowledge of the geometries of fault patterns delimiting inverted structures. The sandbox models described in this paper were designed to study inversion-related fault geometries and kinematics. 0040-1951/87/$03.50

faults.

An elongate sandbox was used (Horsfield, 1977), which allows a visual cross-sectional study

(i.e. plane-strain) process, related to the generation of contractional fault-blocks (Harding and Lowell, 1979; Harding, 1985). involving

faults

overburden

blocks.

The experiments

inversion

and normal

faults. The reactivated

at the edges of the inverted

Introduction

(b) Structural

along vertical

along the pre-existing

by new reverse faults.

Faults related to inversion

one of the fault blocks or with vertical

of movement

of displacement

of the pre-existing

wedge-shaped

the relationship

does not move parallel

in the overburden.

recovery

new reverse faults in the footwall

faults and the new reverse faults defined basement

In the first one. reversal

only a partial

B.V.

LASS

MARKER

FRONT

AND REAR

WALL

LAYERS

BLOCKS

Fig. 1. Simplified

sketch of the plane-strain

(Teflon

coa,ed )

sandbox.

mogeneous packing density. By adding small amounts of ink-stained sand at regular intervals, a series of coloured marker horizons was obtained, which have no mechanical significance. Laboratory models provide analogues of geological structures only if they are properly scaled (Hubbert, 1937). A reasonable mechanical scaling is fulfilled by the use of dry sand (grain size 0.15-0.30 mm), which has a very low cohesion and a frictional plastic behaviour. It will fail according to the Mohr-Coulomb slip concept (see, for example, Jaeger and Cook, 1979), like most low to moderately cohesive materials that occur in natural sedimentary overburdens. The low cohesion of the sand complies with the very low confining stresses in the model. Faults in the sand develop by shear dilatancy (i.e. volume increase produced by relative movement between the grains), accompanied by strain softening in narrow planar zones. The width of the shear zone is not to scale, because the dilatancy is controlled by the size, distribution and packing of the grains (Mandl et al., 1977; Horsfield, 1977). Two experimental configurations were tested. The first one (Figs. 1, 2A) comprised a central basement block (a) which was allowed to move upward or downward with respect to the confining blocks (b), thus inducing faulting in the overlying sand. The dip angles of the basement faults (c) could be changed (60” and 90” dip angles were tried). In the case of a 60” basement fault the vertical movement was induced by outward (first stage) and inward (second stage) movement of the confining blocks, which were fixed to shafts driven by electric motors (d). The dip angles of the basement faults controlled the amount of horizon-

tal extension during the development of the graben (first stage), and the amount of shortening during the process of inversion (second stage). In the case of vertical basement faults no horizontal extension/shortening occurred. Here the central block was simply lowered and subsequently uplifted by a motor-driven vertical piston (e ). A second configuration was designed to test a possible plane-strain origin for inversion in the overburden associated with tilted basement blocks, as is suggested by Harding (1985; see Fig. 2C). In the model horizontal compression in the overburden is caused by a shift of rotation axis of one of the basement blocks, thus inducing a progressive misfit of the blocks along the basement fault plane. For this purpose two basement blocks were fitted into the sandbox (Fig. 2B). One block (a) remained fixed, this being the footwall of a 60” basement fault (b). In the first stage of the experiment, the other block (c) was allowed to rotate around a rotation axis (d) by moving a motordriven piston (e) down, which induced dip-slip movement along the basement fault. In the second

Fig.

2. Models

inversion tion

2: inversion

pre-existing

tested

in the sandbox.

by uplift of a central by

rotation

A. Configuration

1:

b asement

block. B. Configuraof a fault block towards a

fault, based on a model (C) after Harding

(1985).

3x1

stage of the experiment

the piston

new rotation

axis for a further

block towards

the basement

tilt was accomplished (f)

horizontally

indicated tion

experiment the footwall

(d)

block,

leakage

upward. block

greased

During

faults

sticking

and

were run for

each configuration, in order to establish tability of the results.

the repea-

results

Configuration I: Inversion basement block The experiment faults is considered the central

the

In both

the basement

to prevent

of sand. Several experiments

Experimental

as

was held against

by a lead weight (8).

configurations

carefully

device

block,

i.e. the first stage rota-

moved

the rotational

experimental were

was

a tapered

the basement

with the arrow;

axis

iOcm L-i

fault. This additional

by wedging

under

(e) acted as a tilt of the fault

by uplift of a central C

with 60” dipping basement first. Downward movement of

basement

block created

a graben

j?

marginal

wedge-shopd

Expt.i76-14

marginal wedge-shaped

block

block

in the

overburden. In the course of the experiment sand was added uniformly across the model, simulating a syn-sedimentary

development

After termination

of the downward

graben

edges had become

drape and the remaining

of the

smoothed

depression

with extra sand (Fig. 3A). Positive inversion of the graben the

upward

movement

graben.

movement

the

by sediment was then filled resulted

of the central

from

basement

block driven by the inward movement of the confining blocks. Syn-sedimentary deformation was maintained

by adding

sand layers at regular

vals during the inversion (Fig. 3B-D). The original normal faults bounding

were subsequently important during

3. Sequential

development

by uplift

basement

faults.

movement

of central

faulting

of a central A. Final

basement

structure

basement

sand layer. B-D.

of sandbox basement

configuration

block.

of stage

block:

Stage 2: upward

1:

with dipping 1: downward

S indicates movement

first synof central

block.

inter-

the graben

were reactivated as reverse faults. The faults propagated upward to the surface and tended to curve outwards in the previously unfaulted, sand (Fig. 3B-D). The new reverse

Fig.

inversion

newly added faults which

developed became increasingly the progressive uplift of the

central basement block. These new faults developed at the footwall sides of the pre-existing normal faults, i.e. outside the inverted graben, and they tended to flatten outwards. In some experiments reverse faults were also generated from the top of the basement upwards (Fig. 4A), notably

after

the normal

dip-slip

component

of displace-

ment at basement level was recovered. In the upper part of the sandpack, boundaries of the inverted structure.

at the the re-

activated originally normal faults and the new reverse faults defined narrow, elevated, wedgeshaped blocks (Fig. 3D). With further movement of basement blocks, wedge-shaped blocks may also appear at a deeper level (Fig. 4A). Similar elevated wedge-shaped blocks are known from seismic sections across the southern North Sea (Fig. 5). At the

end

of

the

experiment

the

internal

c

Ikm

,

Fig. 4. Comparison between the geometry of an inverted structure in the sandbox (A), and in nature fB; example from the Middle East).

stratification of the inverted structure diwd towards its centre, resulting in a saucer-shaped geometry. This geometry was inherited from the sedimentary drape over the orig@l graben edges. Moreover, one can imagine that along the edges of natural grabens such inclinations are increased by drag along the original normal faults. Similar saucer-shaped geometries are also known from seismic sections across inverted structures (Fig. 4B). Experiments on inversion carried out with

vertical faults in the basement gave similar results (Fig. 6). However, downward movement of the central basement block fed inlay to the generation of one or more convex-upward reverse faults facing towards the central, downtbrown block (precursor faults, Horsfield, 1977). Only after continued downward movement did the displacements in the overburden become concentrated along m&r-vertical normal faults. In the course of the experiment the graben was filled by adding sand at regular intervals.

I

Fig. 5. Example

of structural

inversion

in the southern

North

Sea; note wedge-shaped

marginal

1 km

highs.

Upward movement of the central basement block led to reactivation of the near-vertical normal faults in the overburden, whereas the early

same level, were overlain by a 12 cm thick sandpack with horizontal bedding markers (Fig. 7A). Downward movement of the piston below the

precursor

left-hand block allowed this block to rotate in the first stage of the experiment. Movement along the basement fault (60” dip angle) initially induced a convex-upward precursor fault (Fig. 7B), which

faults remained basically inactive (Fig. was simulated by levelling the top

6B). Erosion surface. When

the reversal

of vertical

block

movement

was implemented in an early stage of the experiment, before the near-vertical faults developed. new precursor faults were generated, facing away from the inverted graben. The end result of this experiment again shows the elevated wedge-shaped blocks. defined on both sides by convex-upward reverse faults. Configuration 2: Inversion by rotation of a fault block towards a pre-existing fault The basement blocks. which are initially at the

was followed only shortly afterwards by a straight normal fault, slightly steeper than the basement fault (Fig. 7C). Subsequently an antithetic normal fault was generated, which, together with the synthetic normal fault, formed the boundaries of a small graben (Fig. 7D). This small graben structure is superimposed on the deepest part of the larger asymmetric half-graben, which develops at the downthrown side of the synthetic fault. In the second stage the piston was kept in place to act as a hinge for an addition tilt of the

precursor

faults;

b-normal faults. B. Stage 2: upward movement of central basement block

Y/A

_lOcm

,

” /

”

,/

F

G

Fig. 7. Sequential development of sandbox configuration 2: inversion by rotation of a fault block towards a pre-existing fault. A-D. Stage 1. E-H. Stage 2. See text for explanation.

385

I

I

Fig. 8. Photograph

(A) and sketch with extrapolation

basement block towards the basement fault (Fig. 7E-H). After a small additional tilt of only 6” a reverse fault was generated, propagating upward from the base of the sandpack (Fig. 7E). In the

of faults in an imaginary

syn-tectonic

overburden

(B).

lower part of the sandpack this fault was strongly convex upward. resembling the precursor fauit of the first stage. Nearer the surface the fault was straight with a moderate inclination of 4o” to-

6

D O-.

4000-

: :

Zechstein

Fig.

9. Four

configuration after Michelsen

exampies

of inverted

structures

2. A and B from the Central and Andersen,

with

Graben,

similarities North

between

cross-sectional

Sea: C and D from Northwest

1983; C and D after Baldschuhn

et al., 1985).

geometries Germany

and

the sandbox

(A after Skjewen

model

in

t’t al.. 19x3: n

wards the basement fault. With further tilt a second reverse fault developed (Fig. 7F-H) with a dip of 40’ opposite to the earlier one. The reverse faults defined a pop-up structure, more or less centred above the hinge of the tilted block (Fig. 7H). One can envisage that, if sedimentation had continued during the first stage of block tilting, a sedimentary wedge would have formed with the thickest section on the downthrown side of the main normal fault. The subsequent development of the pop-up structure during the additional tilting would cause positive inversion of the thickest section of this sedimentary wedge (Fig. 8). Similar cross-sectional geometries of inverted structures have been encountered in nature (Fig. 9). Discussion Origin and signjficance of precursor faults

Convex-upward precursor faults are due to the effect of shear dilatancy on the orientation of the fault-~ontrolhng stress state in the overburden (Horsfield, 1977; Walters and Thomas, 1982). They are non-scaled features, inherent in the usage of unconsolidated sand and very low confining stresses. The generation of precursor faults is increased by compression in the overburden across the fault (Stearns et al., 1978, 1981). In nature, precursor faults are not likely to develop in purely extensional settings, because the volumetric increase associated with shear dilatancy is generally largely suppressed by high confining stresses (e.g. a large effective overburden weight). Only at very shallow levels, where the horizontal stress may be significantly higher than the vertical stress, might precursor faults be expected under plane-strain conditions. This may explain the occurrence of precursor faults at very shallow depths, documented in literature (e.g. Mercier, 1976; Angelier, 1979). The occurrence of large convex-upward reverse faults associated with normal dip-slip faults in an overburden is related to compression at large angles to the faults. A way in which such lateral compressive stresses may be generated is the involvement of a basement-induced strike-slip com-

ponent of movement as well as the dip-slip displacement. Indeed, composite convex-upward fault patterns (palm-tree structures), similar to those in the two-dimensional sandbox experiments, are known from areas of wrench faulting (see. for example, Wilcox et al., 1973; Sylvester and Smith, 1975; Harding and Lowell, 1979). They result primarily from oblique-slip displacement along a steeply dipping basement fault (Naylor et al., 1986). Reactivation of existing faults versus generation of new (reverse) faults

The common feature of the experiments reported here is that the inversion is partly or completely accommodated along newly developed reverse faults. In configuration 1 the (partial) recovery of normal displacement along the pre-existing faults is a direct result of the upward motion of the central basement block, thus excluding finite rotation of the blocks with respect to each other. As this upward movement proceeds, lateral compression is gradually built up in the overburden as a result of the inward movement of the confining blocks. This compression creates conditions of increased normal stresses across the steep faults, which cause a substantial increase in the sliding friction (Handin et al., 1963). Therefore the reverse movement along the pre-existing overburden faults is gradually inhibited, and above a certain limit value it becomes mechanically easier for the shear stress to be released along new reverse faults. The positions and shapes of these new faults are determined by the orientations and magnitudes of the local principal stresses, according to the Mohr-Coulomb slip concept. The strong curvature of the reverse faults near the free surface may be ascribed to the surface-parallel compressive stresses resulting from the gravitational load of the developing surface slopes (Sanderson and Marchini, 1984). Inversion in the experiments of configuration 2 is not associated with reverse movement along the basement fault. Instead, along this basement fault minor normal movement has continued. The additional tilt of the downthrown fault block in the second stage of the experiment induced lateral

387

compression, part

which

is felt strongest

of the overburden.

stresses faults

have increased develop faults,

which

oriented

new conjugate

faults

now remain

faults

of the pre-existing

newly

applied

easier

to reactivate

basically

slip planes.

(overcoming

tion of the fault) or to generate friction

the inac-

The oriento the

whether

it is

the sliding

fric-

a fault (overcom-

of the surrounding

rock).

These results are in accordance with theories on fault reactivation (Jaeger, 1960; Adler, 1963; Stearns et al., 1981; Sibson, 1985), and experiments on natural Lajtai (1969).

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by Donath

(1961)

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In contrast to our sandbox experiments. comburden sequences in nature are generally posed of units having different material properties. Ductile horizons within such multilayered

of

Tectonophysics. strukturen

favourably

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stress field determines

ing the internal

transect

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