Strain partitioning and deformation mode analysis of the normal faults at Red Mountain, Birmingham, Alabama

Tectonophysics,

171

170 (1989) 171-182

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Strain partitioning and deformation mode analysis of the normal faults at Red Mountain, Birmingham, Alabama SCHUMAN WU Department

of Geology, The University of Alabamn, (Received

April 20.1989;

accepted

Tuscaloosa, AL 35487 (U.S.A.) May 23,1989)

Abstract Wu, S., 1989. Strain partitioning Alabama.

Tectonophysics,

In a low-temperature confining

pressure

published

experimental

Four deformation uniform

and deformation

environment,

and

mode analysis

total

strain

the thin-section at geological

data by plotting

strain

the deformation

modes are shown on the diagram:

flow. By determining

scale

rock-deformation A deformation

mode on a graph

extensional

and hence the depth at which the rock was deformed on the gently dipping Alabama.

structures,

intergranular

southeast

Samples

including

of the Ordovician pressure

about 45 to 80% in shale is indicated by stylolites.

associated

veins truncate

was caused

The normal faults

solution,

faults formed burrows

during

(about diagram,

22”)

to bedding.

The strain

and deformed

magnitude

the normal

that

pressure

(< 18 MPa)

at shallow

uniform

flow occurred

under higher confining

depth

burrows

faulting

deformed

sample,

burrows.

after the Ordovician

is about

Rock ductility, which is defined as the total permanent strain before fracturing (Handin and Hager, 1957), is a function of many variables, among which confining pressure, temperature, pore fluid pressure, strain rate, and lithology are the most important. On the basis of experimental data, Handin et al. (1963) constructed diagrams of rock ductility versus the depth of burial. The diagrams of both dry and water-saturated tests B.V.

is applied

and adjacent features

compaction

shortening

by brittle

the homogenous

and

the confining

to normal expressway

faults cut in

to the faults contain calcite

shortening

of shale to burrows, because

the calcite cement. deformation,

pressure.

faulting,

a vertical

rocks were consolidated

which truncate

of from

including

compaction,

about

twins, of

6% in

the faults and

A total strain of 2.0%

indicated

by the calcite

strain in the dip direction

at a low angle

the observed

data on the deformation

deformation

occurred

horizontal

(at least 60 MPa) at greater

Introduction

0 1989 Elsevier Science Publishers

During

2.6%. By locating

pressure

within

later homogenous

characterized

(< 800 m), and

The method

in the Red Mountain

deformation

and relative

faulting..A

a function

mode of a naturally

ductile

has its major principal

it is found

0040-1951/89/$03.50

Birmingham,

is constructed incipient

Limestone

and

and stylolites,

normal

twins in veins, cement and fossil fragments,

veins,

is primarily diagram

faulting,

anticlinorium

Chickamauga

and

by deformed

the deformed

by mesoscopic

faults

mode mode

mesoscopic

can be evaluated.

limb of the Birmingham

mesoscopic

and transgranular

limestone

at Red Mountain,

of total strain versus the confining

fracturing,

the total strain and the deformation

exposed brittle

faults

rates.

pressure

Birmingham,

of the normal

170: 171-182.

under

compression

depth

(>

mode

low confining

characterized

by

2.5 km).

show that the ductility of any particular rock type increases with depth because of the increase of confining pressure and temperature. These diagrams have important implications because they can be used for evaluating the natural deformational environment. For example, if a deformed limestone is found to have a prefailure strain of 30%, the diagrams of Handin et al. (1963) suggest a 5 to 7 km depth for dry deformation and 8 to 9 km for water-saturated deformation. It maybe difficult, however, to determine the prefailure strain

172

s. wu

for naturally

deformed

ing deformation

rocks, because

post-fault-

may also contribute

strain.

Deformation

portant

additional

mechanisms information

to the total bear

about

very

im-

the environ-

mental conditions and should be taken into consideration. A significant move towards the quantitative ment

using

sideration

of deformational

total strain

data combined

of deformation

by Donath (1971).

evaluation

mechanisms

et al. (1971) and Tobin

On the basis

environwith conwas made

and Donath

of their experimental

data,

Donath et al. (1971) distinguished four deformation modes in the total strain versus confining pressure diagram, which is called a deformation mode field (DMF) diagram: homogenous deformation (field A), microscopic ductile faulting (field B), macroscopic ductile faulting (field C), and brittle faulting (field D). If the total strain and the deformation mode are determined by using Donath’s DMF diagram, the range of confining pressure of the deformation can be evaluated. Donath et al. (1971) used cohesion as a basic criteria to distinguish between a brittle fault and a ductile fault that is, a brittle fault results in total loss of cohesion, and a ductile fault develops without total loss of cohesion. This distinction may be good for laboratory deformed rock samples, however, the primary cohesion of natural

Fig. 1. Total

strain

versus

confining

mentally

deformed

different

modes are distinguished

tensional faulting,

limestone

fracturing, o-uniform

samples.

+ -mesoscopic

before

failure.

Depth

faulting,

O-ex-

A-incipient

1966, Donath

the ductility

scale assumes

adapted

failed by

symbols:

flow (data from Handin

(pressure-depth

plot of experi-

Rock samples

by different

al., 1971). The solid line represents strain

pressure

et

as percent

zero pore pressure

from Hyndman,

1985).

faults is not always determinable because of syndeformational and post-deformational alteration.

The second group includes all the rocks which reached or passed the ultimate rock strength. When all the data are plotted on a total-strain versus confining-pressure diagram, the boundary between these two groups is the ductility curve (Fig. 1). The second group of data can be subdivided into three categories: (1) Those that reached or passed the ultimate rock strength, for which the stress-strain curve fell but no fault appears meso-

Furthermore, the difference between the fault types of macroscopic ductile and microscopic ductile

scopically, although An incipient-faulting

faulting

for this category because transgranular structures appear microscopically (Tobin and Donath, 1971). (2) Those that passed the ultimate rock strength, for which the stress-strain curve fell and faults appear mesoscopically. A mesoscopic-faulting deformation mode is used for the data of this category because the faults can be seen in thin sections and hand samples without a microscope (the scale terms used in this paper such as microscopic. mesoscopic, and macroscopic follow the definitions of Turner and Weiss, 1963, pp. 15-16). (3) Rocks that deformed at extremely low confining pressure which failed by extensional fractures. Extensional-fracturing deformation mode is used for the data of this category. The data plotted in Fig. 1 are taken from Handin (1966) and Donath et al.

is subtle.

To be able

to determine

the

deformation mode more precisely, Tobin and Donath (1971) used microscopic criteria for some of their laboratory-deformed limestone samples. Examination of some of the published laboratory experimental data by Handin (1966) Handin and Hager (1957) Handin et al (1963) and Donath et al. (1971) reveals that the data fall into two major groups. The first group includes the data for the rocks which did not fail, that is, the experiments were terminated before the stressstrain curve was falling. Following Donath et al. (1971) the uniform-flow deformation mode is used for this group. The deformation mechanisms of the uniform-flow mode are dominated by intracrystalline plasticity (Tobin and Donath, 1971).

the samples are greatly bulged. deformation mode is used

STRAIN

PARTITIONING

AND

(1971) for limestone dry,

strain

confining

180 MPa, and

at room

Silurian

temperature,

the ductility

For confining because

the

pressures

curve

and

ductility

of the larger

pressures

greater

less than

is well defined

pressures

between

as

180 MPa

curve

is less well

overlap

of the uni-

than

modes.

For

200 MPa,

the

ductility curve is poorly defined because of a lack of data. At 200 MPa confining pressure and total strains of 25 to 30’%, some rocks failed by incipient faulting, and others deformed by uniform flow (Fig. 1). This could be caused by different strain rates. Above the ductility curve, the boundaries between incipient faulting, mesoscopic faulting, and the extensional fracturing modes are approximately located because of a data overlapping, which strain rates.

also The

could scale

173

ALABAMA

from 0 to 295 MPa. Although

form flow and the incipient-faulting confining

ANALYSIS.

10m3 to 10W5 s-l,

1). For confining

200 MPa,

defined

MODE

exist, the four deformation modes are by the distribution of different data

(Fig.

shown.

tested

from

pressures

overlaps indicated types

rates

DEFORMATION

be caused by different on the right of Fig. 1

strata

are offset. The total strain

stone samples at

Red

from adjacent

Mountain

is analysed

among deformation strain

episodes.

and the mesoscopic

plotted

on

evaluate

the confining

deformation. consistent Thomas

a

and

with

pressure

the

partitioned

deformation

results

total

modes

are

diagram

to

and the depth

will

be

shown

sedimentologic

(1968) and Thomas

faults

The partitioned

deformation-mode

The

of lime-

to the normal

of

to be

evidence

of

and Bearce (1986).

Rock fabrics of the normal faults at Red Mountain The cut for the Red Mountain Expressway in Birmingham (Fig. 2) exposes Paleozoic strata from Cambrian to Mississippian age on the southeast limb of the Birmingham anticlinorium. The oldest strata (Fig.

exposed in the cut are in the Knox Group 3), dominated by dolomite; the Chic-

kamauga Group is Middle to Late Ordovician in age and dominated by limestone; the Lower and Middle Silurian Red Mountain Formation is a

indicates the depth of lithostatic confining pressure based on a pressure-depth relationship from Hyndman (1985, p. 15). Several predications can be made from Fig. 1. First, for a given amount of strain, the deformation mode can vary from extensional fracturing to mesoscopic faulting, incipient faulting, and uniform flow with increase of confining pressure. Second, at a constant confining pressure, increasing strain will cause the deformation mode to change

from uniform

flow to incipient

faulting

to

mesoscopic faulting. The idea that total strain and deformation mode can be used as indications of the deformational environment was proposed years ago (Donath, 1970; Donath et al., 1971), but as yet little effort has been made to use this concept in the quantitative analysis of naturally deformed rocks. In this paper, the normal faults exposed at the Red Mountain expressway cut in Birmingham, Alabama, were chosen for a field test. The faults are on the gently dipping southeast limb of the Birmingham anticlinorium which is a major Alleghanian structure in the southenmost Appalachian fold and thrust belt. Ordovician through

EXPLANATION

-. --p 0

\---‘ by

structural

arrow), geology

Antkline Thrust fault Metamorphic

50

( \ boundary

i

100Km

i

_-____.-__-__-__-__-_.______

Fig. 2. Index map showing (indicated

-

the Red Mountain

Birmingham,

expressway

Alabama.

map is from Thomas

The

cut

outline

et al. (1982).

174

s wu

elastic sequence of iron-bearing sandstone, siltstone, shale, and sedimentary hematite; the Fort Payne Chert is of Mississippian faults are exposed The

in the expressway

Chickamauga

Mountain meters

Group

Formation (Thomas

Chickamauga

bedded

with

up

cut (Fig. 4).

through

the

Red

are offset as much as several

et al..

Group

1982).

Samples

of the

are from F2 and a conjugate

fault F3 (Fig. 4). Sample are fossiliferous,

age. Three normal

SW-A, SW-B, and SW-D

coarse-grained

shale.

Sample

limestones SW-C

inter-

is mudstone.

The observed mesoscopic and microscopic structures include normal faults, stylolites, veins, deformed burrows and wrapped shale lamination around burrows, and calcite twins. Mesoscopic faults appearing in the samples are parallel to either the macroscopic fault F2 (Fig. Sa) or F3 (Fig. 5b). They are at high angle (60 “-75 about 1 stylolites burrows

o ) to bedding and have normal offsets of to 1.5 mm. Mesoscopic faults offset the in limestone (Fig. 5a) and the deformed in shale (Fig. 5b).

Veins are at high angle to or nearly normal to bedding and usually are associated with a fault. The width of veins sampled ranges from 0.5 mm to 6.5 mm. More than half of the calcite fillings in veins are twinned (Fig. 5~). Stylolites are mainly parallel to bedding (Figs. 5a, d, 6a, b). Stylolites in coarse-grained limestone are more abundant than in shale and fine-grained limestone. In coarse-grained limestone, calcite cement is truncated by stylolites (Figs. 5d, 6a). Heights

of stylolite

columns

range

from 0.32 to

0.94 mm. Except for a few local high-angle-tobedding stylolites, most stylolites indicate layerperpendicular compression. Elliptically shaped burrows appear in the shale of sample SW-B. If the burrow fillings were originally cylindrical, the elliptical shape in the thin section could result from either the oblique cut, or deformation. Perhaps some of the extremely elongated burrows are the result of both effects, but most of the moderately elliptical burrows re-

Fig. 3. Stratigraphic

section

in the Red Mountain

cut (from measured

section

by Thomas,

is shown on Fig. 2).

expressway

et al., 1982. Location

STRAIN

PARTITIONING

AND

DEFORMATION

MODE

ANALYSIS.

175

ALABAMA

Northwest

Southeast

Road

Fl

Fig. 4. Cross section of west wall of the Red Mountain

sulted from fine quart petent than compaction

deformation. The burrow grains and appear to be the surrounding shale. The of shale relative burrows

by the wrapping

of shale lamination

Fig. 5. a. SW-A, limestone normal

limestone,

expressway

cut (location

is on Fig. 2) and the sample locations.

fills contain more comdifferential is indicated

tial compaction indicate vertical shortening. In the coarse-grained limestone, about 60% of the calcite grains are twinned (Fig. 6d), the aver-

around

age number

and shale cut by normal

faults. c. SW-C, fine-grained

F3

F2

level

faults,

veins formed

the

burrows.

Fitted

note the offset of stylolites.

boundaries

of twin

stylolites.

sets per grain

b. SW-B, deformed

near fault. d. SW-A, syntaxial

layer-parallel

(Fig. 6c) and differen-

cement

burrows

of a fossil fragment

is 1.7. Most

in shale cut by truncated

by

176

Fig. 6. a. SW-A, a fossil and its syntaxial can be estimated fragment,

by restoring

the height

cement

the original

of the stylolite deformed

column

are partially

elliptical

shape

dissolved

is used to estimate

burrows,

by a layer-parallel

stylolite,

of the fossil. b. SW-B, a layer-parallel the minimum

disolved

material.

the minimum stylolite

material

that dissolved

c. SW-B, the elliptical

dissolved a fossil shape

of

note the fitted shape. d. SW-B, calcite twins in cement.

twins are thin twins (Groshong, 1988). Twin thickness ranges from 1 to 4.5 microns. A few grains show thick twins. Twins are developed rather homogeneously in calcite cements, fossil fragments, and calcite fillings in veins.

ing of the microfaults, is perhaps more appropriate. Considering the variety of the rock fabrics, different techniques are used in calculating the strain

associated

with the each individual

type of strain fabric to reveal magnitude and orientation.

the bulk

strain

Bulk strain estimation

Strain of stylolites In a standard definition, strain is a continuum measurement which describes the changes in volume and shape of a deformed object. In structural geology, strain is also often used to describe discontinuous deformation such as faults (Jamison and Sterns, 1982) veins (Ramsay and Huber, 1983), and strain fabrics caused by pressure solution (Mitra, 1976). In this case, the term “bulk strain”, which Jamison and Stearns (1982) used to refer to the net distortion of the rock occurring over a region much larger than the average spac-

Stylolites are mostly parallel to the bedding and indicate layer-perpendicular shortening. If a grain or fossil is partially dissolved by a stylolite, and if the shape of the grain or fossil can be restored, the minimum amount of dissolved material can be estimated by adding the dissolved part to restore the original shape (Engelder, 1982, p. 74) (Fig. 7a). Another method to estimate the minimum amount of dissolved material is to use the maximum column height of a stylolite as the measure of the

STRAIN

PARTITIONING

AND

DEFORMATION

MODE

ANALYSIS.

177

ALABAMA

TABLE Strain

1 of stylolites

Sample

a a

Tk T

i--I 1

0

dissolved

materials

of estimating

on a stylolite.

to restore the original

Average spacing

measure-

of stylolites

ments

(mm)

- 5.75

*0.45

44

4.5

- 5.15

*1.77

40

5.0 (dip section)

SW-B

- 5.94

kO.98

38

4.7 (strike section)

SW-D

-6.15

+0.86

76

Average

-5.9+1.0

Strains

amount

the dissolved

of

SW-B

2.6 4.2

2 of faults

Sample a. Adding

Number

SW-A

TABLE

I

the minimum

Standard deviation

* - = shortening.

b Fig. 7. Two methods

Strain,

of

Strain + standard

Number

deviation

measurements

*

part

of

Fault spacing

(md

e3

shape. b. Using the height of the stylolite

(W

6)

SW-A

-1.54+0.25

1.52*0.28

4

SW-B

-2.00+0.30

1.96kO.29

5

29.7

SW-D

-2.53+0.89

2.46+0.85

4

69.0

Average

-2.0

2.0 *0.5%?

column.

amount of dissolved material (Stockdale, 1922) (Fig. 7b). Both methods have been used to determine the minimum amount of dissolved material of stylolites along four transect lines in sample

+0.5%

13.5

51.4

* - = shortening.

SW-A, SW-B, and SW-D (Table 1). The average vertical shortening indicated by stylolites is about

Strain of faults

ratio fault Fig. scale

A fault is usually described in terms of displacement. If the fault plane is taken as a simple shear surface, the shear strain expressed by the

from a distance the sheared card deck appears to show homogeneous simple shear, but a close look reveals that the shear takes place along individual

5.9%.

of to 8a) (y

displacement (dx) along an individual the average fault spacing (L) (y = dx/L, is the same as the shear strain at the large = lOOdx/lOOL, Fig. 8~). This is similar to

the shearing

B

faults,

the ratio dx/L

b

from fault dispalcement.

can be used to estimate

looked

at

C

a. dx is the displacement

the bulk shear

increasingly

When

b

a Fig. 8. Shear train estimated

‘of a deck of cards.

strain.

resembles

along the fault and L is the average

b and c. As the scale of observation

homogeneously

strain.

spacing

increases,

between

the fault slip

178

s wu

faults. After the shear strain ated principal

strains

be calculated e,, e3 =

is found,

and strain

the associ-

orientations

l/2(

y2 + 2 + y/G)

- 1

(la) (lb)

and Huber,

the faults in samples given Table sional direction

1983). Estimated

1

strains

for

are about

shortening

2%, and

2t

] Fig. 9. Diagram a burrow.

showing

The relative

the shale lamination compaction

be estimated

SW-A, SW-B, and SW-D are

2. The average

strains

fi +

tan 28 = -2/y (Ramsay

can

by:

wrapped

around

of shale to the burrow

and exten-

the shortening

is at a high angle to bedding.

the long half axis (a) and short half axis (b), the principal

strains

eFr = Jb7a

Strain of veins

are given by:

- 1

(2a)

ey=m-1 Veins are observed in sample nearly perpendicular to bedding.

SW-C and are Strain of veins

contributes only to bedding-parallel extension of the sample. The method for estimating extension caused by veins is discussed by Ramsay and Huber (1983). Four lines are measured in sample SW-C; the average extension by veins in that sample is

(2b)

(Cloos 1945) assumed. the

in which

The differential burrows can

compaction be found

The elliptical shape of burrows results from compaction but may be exaggerated by an oblique cut. Assuming that those burrows with minimum to moderate ellipticity are nearly perpendicular to

The average

35 L- 5% perpendicular

TABLE

3 and relative compaction

between shale and by comparing the

to be relatively undegives the minimum

estimation of the compaction of shale. If the burrows are assumed deformed from their original cylindrical shape, then the shortening strain eih + eFr gives the maximum estimation of the compaction of shale. The measured elliptical burrows and the wrapped shale thickness are listed in Table 3.

similar

burrows

shortening

strain

of burrows

to bedding.

in shale *

2a

2b

21

e,”

et

m-0

(nm

6)

6)

2

2.21

0.86

0.63

- 37.78

60.72

- 26.67

- 64.45

3

2.92

1.43

0.86

- 30.16

43.18

- 40.00

- 70.16

4

1.71

0.80

0.51

- 31.68

46.38

- 35.71

- 67.39

5

2.39

1.07

0.49

- 32.98

49.22

- 54.66

- 87.64

6

1.86

0.86

0.34

- 32.06

47.19

- 60.00

- 92.06

7

4.51

1.36

0.89

- 45.16

82.38

- 34.74

- 79.90

8

3.79

1.29

0.57

- 41.17

71.59

- 55.55

- 96.72

-35*5

55 f 14

-45

Average * - = shortening.

ep’ + esh

* 11

is about

The relatitive

(mm)

No.

is

(3)

likely

By measuring

change

t)/b

the thin section and deformed by a plane-strain deformation, then these elliptical burrows are very ellipse.

volume

(2t) to the short axis of

If the burrows are assumed formed, then this equation

Strain of deformed burrows

Strain of deformed

zero

wrapped shale thickness the burrows (Fig. 9): efh= (b-

about 4.8 + 2.5%.

to the strain

can

by (b - t )/h.

80+11

STRAIN

PARTITIONING

AND

DEFORMATION

MODE

ANALYSIS,

ALABAMA

179

procedure of Groshong et al. (1984), the principal strains and the nominal errors are computed. The shortening strain is about 2.6 + 0.5% in the dip direction at a low angle (about 22”) to bedding (Fig. 10).

S60’E

Deformation

Fig. 10. Stereonet SW-A, looking tion

of Fig.

compression,

projection along

of strain of calcite twin in sample

strike toward

4). The

symbols

intermidate

strains respectively

o,

extension, as indicated

the southwest

(at the sec-

n are

maximum

and maximum

extension

A,

and

by the strain magnitute.

compaction of shale is 45 * 11%. The compaction of shale is estimated between 45 and 80%. Calcite twin strain

Calcite twin strain can be calculated by the calcite strain-gauge technique (Groshong, 1972). Two perpendicular thin sections are cut from SWA. Thirty-two twins sets are measured from the dip section, and 25 twin sets are measured from the strike section. The small percentage of negative expected values (7%) indicates deformation in one episode of homogeneous strain. Following the

The rock fabrics and strain analysis reveal that at least three deformation episodes happened during the history of the rocks adjacent to the normal faults at Red Mountain (Fig. 11). The first deformation occurred during compaction in which a vertical shortening strain of about 45 to 80% in shale is indicated by the deformed burrows and the differential compaction between shale and burrows, and about 6% in limestone is indicated by stylolites. The normal faults formed after the Ordovician rocks were consolidated because faults truncate the burrows, stylolites, and calcite cement. During the normal faulting the shortening strain caused by mesoscopic faults is about 2% at a high angle to bedding. Calcite twins post-date the veins which formed during normal faulting, thus the twins represent the last episode of deformation. Strain analysis reveals 2.6% shortening strain in the dip direction at a low angle to bedding. The last deformation episode as indicated by Mesoscopic. microscopic structures

Approximate shortening direction

Deformation episode

episodes

Amount of shortening

c,

45-80%

1 Compaction z

5.9+1.0%

4b

K7 2 Normal

faulting

2.0f0.5% 4L

3 Alleghanian orogeny

Fig. 11. Schematic

diagrams

2.6f0.6% v

showing

the deformation

episodes

in rocks adjacent

to the normal

faults at Red Mountain.

180

s. wu

calcite twins in this vicinity during

the

thrusting

regional

that

most likely happened

Alleghanian

formed

the

folding

and

Birmingham

anti-

During tening

the Alleghanian

strain

formed

tion. No related

orogeny,

a 2.6% shor-

by crystal-plastic

deforma-

faults are seen in the vicinity.

clinorium.

sampled

Interpretation of deformation environment

pressure

of uniform

is about

50 MPa (Fig. 12). Considering

It is well known that brittle deformation occurs at shallow depth with low confining pressure and

error in the strain magnitude related

ductile

perhaps

rocks

this deformation.

deformation

confining shallow

pressure. is shallow

at greater

depth

But the question

with higher remains,

how

and how deep is deep? Using

the estimated total strain and the associated deformation mode, a specific area can be located in Fig. 1 and the range of confining pressure can be quantitatively

evaluated.

During the normal faulting deformation in the Red Mountain Expressway, mesoscopic faults de-

experienced

The minimum

The

flow during

limit of confining

flow for a total strain of 2.6%

faults developed a safe estimate

ing pressure

uniform

the 0.5%

and that there are no

in the vicinity,

60 MPa is

for the minimum

confin-

for this event.

The confining pressure cannot be directly converted into depth unless the pore-fluid pressure is known. Unfortunately the pore-fluid pressure is not easy to estimate in most cases. From the fabric observations, it is apparent that the rocks were consolidated

and well cemented,

no apparent

veloped in rocks adjacent to the outcrop-scale normal faults and the total strain for this event is about 2%. The upper and lower boundaries of the mesoscopic faulting deformation mode for a total strain of 2% indicate a confining pressure range of 6 to 30 MPa (Fig. 12). The middle of this range (18 MPa) is used for the estimation of maximum confining pressure because the faults are distinct and conditions should depart from those in prox-

porosity is seen under the microscope except along the faults and the secondary fractures. So the primary porosity is estimated to be no more than 1 to 2%. It is most likely that the pore-fluid pressure was not important during the normal faulting or the crystal-plastic deformation. Using this assumption to neglect the pore-fluid pressure, confining pressure of 18 MPa and 60 MPa are converted to depths of about 0.8 km and 2.6 km, respectively.

imity to the ductility just about to occur).

Stratigraphic and metamorphic evidence is consistent with the burial depth inferred from the

curve

Total 5

strain,

15

18

(on which

failure

is

deformation mechanisms. On the opposite wall of the same road cut, Fl has been interpreted as a synsedimentary structure formed during the de-

% 25

28

38

250 I

I

1

t Fig. 12. Total agram.

strains

plotted

It shows that the normal

ing pressure uniform

10

less than

shortening

on the deformation faulting

18 MPa (point

formed

formed

2).

di-

with confin-

I ) and the horizontal

with confining

MPa (point

mode

pressure

at least 60

position of the top sandstone unit of the Red Mountain Formation (Thomas et al., 1982; Thomas, 1986; Thomas and Bearce, 1986). The evidence is that this unit exhibits soft-sediment deformation, slump structures, and local thickening on the downthrown fault block (Thomas, 1986, his fig. 4). Though the upper part of F2 has been eroded away, it is most likely that F2 shares the same characteristics with Fl, which means that F2 also formed at a very shallow depth, perhaps no more than a few hundred meters (the total thickness from above the Chickamauga Group to the top of Red Mountain Formation is no more 100 m in this vicinity (Fig. 3). The stage of organic metamorphism in this area based on coal rank, vitrinite reflectance. and the

STRAIN

PARTITIONING

conodont vitrinite

AND

alteration reflectance

sequence

1.5 and

4 km,

of the base

Alleghanian Group

is

Ordovician

equivalent

800 meters during

to

a

pers.

depth depth

of the Pennsylvanian

(Thomas, rocks

a burial

The

181

ALABAMA

Acknowledgments

at the base of

the probable

deformation.

is about

Pennsylvanian

ANALYSIS,

(R.H. Groshong,

1989). This represents

between burial

index

MODE

of about 1.5-2.0

the Pennsylvanian commun.,

DEFORMATION

of

Richard

of

academic

during

Chickamauga

below

1972), giving

The work

was done

H. Groshong,

valuable

discussions

David A. Ferrill.

It also has benefited

with William

of

improved

of

The work was partially

and

has been greatly

by reviews from Groshong

EAR-8402915

of from

A. Thomas

The manuscript

a depth

of about

the supervision

Jr. and the help is not only

but linguistic.

the base

deformation

under

and Thomas.

supported

by NSF-grant

to R.H. Groshong,

Jr.

2-5 km. References Conclusions

Cloos,

E., 1947. Oolite

fold, Maryland.

Strain partitioning

and deformation-mode

anal-

ysis allows a sample to be located on the deformation mode diagram. This allows the conditions of deformation to be estimated directly from the deformation mechanisms. The results obtained here are consistent with and, in part, improve upon the depths estimated from the stratigraphic and organic metamorphism data. Three deformation episodes are indicated by mesoscopic and microscopic structures in rocks adjacent to the normal faults at the Red Mouncompaction, shallow-burial tain : soft-sediment normal faulting, and deeper-burial homogeneous subhorizontal compression during the Alleghanian orogeny. The minimum vertical shortening during compaction is between 45 and 80% in shale and 6% in limestone; the normal faulting caused about 2% layer-perpendicular shortening strain; and the homogeneous subhorizontal shortening strain during the Alleghanian orogeny is about 2.6%. The normal faulting formed at a depth less than 800 m with confining pressure less than 18 MPa; the subhorizontal compression during the Alleghanian orogeny occurred at a depth greater than 2.6 km with confining pressure greater than 60 MPa. This approach could be refined by including the effects on rock ductility of other parameters such as strain rate and temperature (even under a low temperature range). Better deformational mode diagrams (even multi-dimensional) could be prepared with more detailed information.

Donath,

F.A.,

deformation

in the South

Mountain

Geol. Sot. Am. Bull., 58: 843-918.

1970. Some information

squeezed

out of rock.

Am. Sci., 58: 54-72. Donath,

F.A., Fall, R.T. and Tobin,

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D.G.,

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deformed

rocks.

Geol.

Sot.

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T., 1982. A natural

example

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operation

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dissolution

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Acta, 46: 69-74.

Groshong,

R.H.,

calcite.

Jr., 1972. Strain

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pressure

calculated

solution.

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in

Geol. Sot. Am. Bull., 82: 2025-2038.

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R.H.,

mechanisms

Jr.,

1988.

Low-temperature

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100: 1329-1360. Groshong,

R.H.,

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L.W.

and

Gasteiger,

C., 1984.

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technique.

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J.,

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Am.,

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R.V., 1957. Experimental

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temperature

and

on dry

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pressure:

test at room

Am. Assoc.

Pet. Geol.

Bull., 41: l-50. Handin,

J., Hager,

R.V.,

1963. Experimental der confining

Friedman,

M. and

deformation

pressure:

Feather,

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pore pressure

J.N.,

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D.W., 1985. Petrology

Rocks.

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and Metamorphic

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D.W.,

1982. Tectonic

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