Tectonophysics, 92 (1983) 253-274 253

Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netheriands

MECHANISMS OF NAPPE EMPLACEMENT AT THE SOUTHERN MARGIN

OF THE DAMABA OROGEN (NAMIBIA) *

KLAUS WEBER and HANS AHRENDT

Geologisch - Paltiontologisches Institut, Golakchmidtstr. 3, D - 3400 Gtittingen (Federal Republic of Germany)

(Received September 1, 1983)

ABSTRACT

Weber, K. and Ahrendt, H., 1983. Mechanisms of nappe emplacement at the southern margin of the

Damara Orogen (Namibia). In: M. Etheridge and S. Cox (Editors), Deformation Processes in

Tectonics. Tectonophysics, 92: 253-214.

The Naukluft nappe complex is exposed approximately 50 km south of the present southern margin of

the late Precambrian to early Palaeozoic Damara Orogen. This nappe complex overlies the aut~hthono~s

Nama beds which belong to a platform area adjacent to the Damara mobile belt. The total displacement

from the NW to the SE amounts to 50-80 km. The base of Naukluft Nappes is formed by a dolomite

horizon ranging in thickness between zero and about 30 m.

Recent inv~tigations of the mineral content, fluid inclusions, grain fabric, and deformation lead to the

conclusion that continental playa-lake evaporites must be assumed as source rocks of this dolomite. This

unit, named the “Sole Dolomite” contains 35 different minerals with sparitic dolomite, albite, quartz,

tourmaline, Mg-riebeckite, talc, and sericite being the main components.

The Sole Dolomite contains numerous rock fragments of granites, gr~~io~tes, gneisses and mica

schists with tourmahne and Mg-riebeckite as metasomatic minerals. Fragments of quartz-albitolites,

which contain more than 50% vol. of albite, and various amounts of tourmaline and dolomite are very

frequent. The Sole Dolomite is extremely rich in fluid inclusions of different, but mostly high salinity. The

minerals of the Sole Dolomite display no primary r~~stallisation and no crystal plastic deformation. No

preferred lattice orientation is developed. All deformation found in the Sole Dolomite is of the brittle

type. Catadastic mylonitisation without any preferred lattice orientation has been encountered only in the

lowermost few centimetres of the Sole Dolomite. In contrast to the Sole Dolomite the underlying

autochthonous Nama limestones have been transformed over a vertical distance of several metres into

fine-grained mylonites which reveal a well-developed preferred lattice orientation. The formation of these

two types of mylonites will be discussed in more detail.

The Sole Dolomite is interpreted as a discordant intrusion under higb pore fluid pressure into the base

of a nappe sequence. It may be assumed that before its tit~fication the water-rich carbonate mush has

acted as a lubricant. The final displacement of the nappes with the Sole Dolomite at its base must have

taken place after lithification of the intrusion. Otherwise, the intense low temperature mylonitisation of

the autochthonous and parautochthonous Nama limestones cannot be explained adequately.

* This work forms part of the research program of the Sonderforschungsbereich 48 “Entwicklung,

Bestand und Eigenschaften der Erdkruste, insbesondere der Geosynklinalraume”, University of Gbttin-

gen, Federal Republic of Germany. Funds for this work were provided by the German Research

Association.

0040-1951/83/0000-0ooo/%03.00 Q 1983 Elsevier Scientific Publishing Company

254

INTRODUCTION

The Damara Orogen of Namibia (Southwest Africa) forms part of the late

Precambrian to early Palaeozoic Pan-African mobile belt system (Martin, 1965;

Martin and Porada, 1977; Kroner, 1977, in press; Porada, 1979). It consists of a

N-S trending coastal branch and a NE-SW trending intracontinental branch. These

branches are connected stratigraphi~ally and structurally (Guj, 1970; Weber et al., in

press; Porada et al., in press). Sedimentation began about 900 Ma ago with a stage

of rifting which produced three widely spaced grabens in the 2000-1000 Ma old

sialic crust (Martin and Porada, 1977). These grabens were filled with 6000 m of

feldspathic to arkosic, locally conglomeratic sands of the Nosib Group. In the

northern graben, rifting was associated with rhyolitic volcanism.

The rhyolites and ignimbrites of the upper Nosib Naauwp~rt Formation reach

thicknesses of more than 6000 m. They are associated with alkaline and peralkaline

plutonic and volcanic rocks (Frets, 1969; Jacob, 1974; Miller, 1974, 1979; Martin

and Porada, 1977).

The upper Nosib Group along the southern margin of the Damara Orogen is

characterized by evaporitic playa-lake deposits of the Duruchaus Formation (Behr et

al., in press). Similar environments may have also existed in parts of the northern

graben (Weber et al., in press). These evaporitic sequences have important implica-

tions for the structural development of the Damara Orogen, particularly on the

Naukluft Nappe Complex which will be discussed in more detail in the following.

GEOLOGICAL SETTING

The Naukluft nappe complex (NNC) is exposed approximately 50 km south of

the present southern margin of the Damara Orogen (Fig. 1). This nappe complex

overlies the autochthonous and p~aut~hthonous Nama beds (Korn and Martin,

1959) which belong to a platform area adjacent to the Damara mobile belt (Germs,

1974). The total displacement from the NW to the SE amounts to 50-80 km

(Ahrendt et al., 1977; Hartnady, 1978). Detailed investigations on the NNC were

done by Korn and Martin (1959), Ahrendt et al. (1977), Munch (1978), Hartnady

(1978) and Weber et al. (in press).

The age and degree of metamo~~sm of the nappes and the Nama basin were

investigated by Ahrendt et al. (1977). According to these investigations, inside the

Nama basin the transition from diagenesis to anchimetamorphism can be observed.

Within the diagenetic area the Nama beds are unfolded, whereas within the

an&metamorphic area the Nama beds are folded, forming SE-facing asymmetric

folds. Deformation increases from SE to NW. Folding and metamo~~sm of the

Nama beds predate the final emplacement of the NNC.

Inside the NNC metamorphism increases from S to N (Ahrendt et al., 1977; Behr

et al., 1981; Weber et al., in press). The highest temperatures have been found in the

Fig. 1. Sketch map of the area of the Naukluft nappe complex. Temperatures of formation of the Sole Dolomite aft’er Behr et al. (1981). Isolines and values of illite crystallinity after Ahrendt et al. (1977).

northernmost part of the NNC where biotite locally occurs in pelitic sediments of

the Cudu Nappe.

K/Ar age determinations on white micas (Ahrendt et al., 1977) reveal two groups

of ages. The syntectonically recrystallized slates of the folded Nama beds give an age

of 532-537 Ma. The temperture during metamorphism did not exceed the blocking

temperature of about 350°C for the K/Ar system for white mica. Therefore, the age

of about 535 Ma is interpreted as representing the peak of syntectonic metamor-

phism in the folded Nama sediments.

In contrast to the Nama rocks, the ages found in the nappes give two different

groups of ages. Ages between 532 and 547 Ma have been found in the weakest

metamorphic parts of the NNC. The ages are interpreted-like in the Nama

beds-as representing the peak of metamorphism. Ages of 492-494 Ma are found in

the highest metamorphic parts of the NNC where locally biotite has grown. These

ages are interpreted as cooling ages which represent the time of imbrication and

accompanied uplift of the nappe complex. The final emplacement of the NNC

possibly took place at about 480 Ma ago (Weber et al., in press).

SOLE DOLOMITE

The base of the Naukluft Nappes is formed by a dolomite horizon ranging in

thickness between zero and about 30 m. Due to its discordant relation with the over-

and underlying rocks, this strata-like dolomite layer was named “Unconformity

Dolomite” by Korn and Martin (1959). They ascribed a sedimentary origin to this

dolomite, while Munch (1978) regarded it as a blastomylonite representing the

lubrication layer on the sole of the Naukluft nappes. Hartnady (1978) has tentatively

postulated that the Unconformity Dolomite has formed in situ by very low-grade

hydrothermal metasomatic reactions between the dolomite of the nappes and its

cooler pelitic substratum.

Recent investigations (Behr et al., 1981, in press) of the mineralogy, fluid

inclusions, grain fabric and deformation lead to the conclusion that playa-lake

evaporites are the most likely source rocks of this dolomite, referred to in the

following as “Sole Dolomite”. Searching for the primary depository of such rocks

Behr et al. (1982) identified the Duruchaus Formation at the southern margin of the

Damara Orogen as a likely source. The present paper gives a more detailed

description of the microstructures and deformation processes inside the Sole Dolomite

and the underlying autochthonous Nama limestones.

MINERALOGICAL COMPOSITION

Thirty-five different minerals have been found to date in the Sole Dolomite

(Table I). The main component is dolomite which makes up about 60 to 75% of the

Sole Dolomite. Twenty-five to 40% of the material is silicates, predominantly albite,

quartz and tourmaline.

TABLE I

Minerals found to date in the Sole Dolomite (the main components are itahzed)

Dolomite

Calcrte

Albite

Microcline

Tourmaline

Quartz

Quartzine

Chalcedony

Talc

Brucite

Magnesite

Paragonite

Sericite

Phlogopite

Kaolinite

Apatite

Zircon

Zeolite

Mg - riebeckite

Titanomagnetite

Aegirite

Goethite

Saponite

Searlesite

Danburite

Borazite

Rutile

Broohite

Pyrite

Lepidocrocite

Chlorite

Coelestine

Analcite

Hibschite

Four generations of dolomite may be distinguished: D&mite I is a micritic dolomite mostly found inside quartz-albite cherts. Dolomite II is a sparitic dolomite of stoichiometric composition, up to 1 cm in

grain size. It contains inclusions of several silicate minerals, mostly albite, quartz, white mica, talc, and tourmaline. These minerals often display multiple zoning indicating several stages of crystal growth (Fig. 2A, B). Sometimes the mica and talc flakes are deformed. Multiple zoning is displayed more frequently by trails of fluid inclusions. Particularly at the base of the Sole Dolomite and in more strongly deformed layers arranged parallel to the macroscopic lamination, dolomite II grains contain concentric trails of fluid inclusions. These may be interpreted as being the result of grain rotation during crystal growth. Some parts of the Sole Dolomite, particularly at its base are rich in more or less completely rounded grains (Fig. ?‘A). Other parts display fractured and strongly corroded grains of dolomite II (Fig. 2C, D). These grains have oriented overgrowths of zoned dolomite III.

Dolomite III forms oriented overgrowths on the sparitic dolomite II grains and has the same composition as dolomite II. It fills up pore spaces between grains of dolomite II (Fig, 2C, D). The dolomite III generation is extremely rich in fluid inclusions of high salinity (Behr et al., in press) and appears like a sponge under the SEM (Fig. 3).

Dolomite 0’ forms small, clear crystals on the walls of late tectonic fracture and residual pore spaces.

Albite is the most frequent silicate mineral, making up in some places about 35% of the total rock volume. Zoned inclusions are frequent in single crystals. Sometimes euhedral crystals contain a rounded core of an older albite grain (Fig. 4A). Occasionally albite grains are broken and sometimes have healed again (Fig. 4B). Twinned and untwinned albites together with quartz form the main constituents of the frequent albitolite fragments.

Quartz forms euhedral short prismatic crystals and also rounded grains, usually 200-500 pm in size. Like dolomite and albite, quartz is often multiply zoned as indicated by inclusions of several silicate minerals and fluids (Fig. 4C).

The rounded quartz grains display oriented overgrowth of quartz which is extremely rich in inclusions of other silicate minerals (albite, phyllosili~ates, tourma- line) and fluids. This secondary generation of quartz fills up pore spaces like dolomite III (Fig. 2D) and forms together with dolomite III the main cement mineral.

Quartzine or pseudoquartzine together with dolomite IV and sometimes kaolinite fills pore spaces and late tectonic fractures. It is extremely rich in fluid inclusions. The fluid inclusions show boiling phenomena. The boiling-point varies from sample to sample between 180°C and 300°C. Inside the central parts of the Sole Dolomite the boiling-temperature was higher than at its base. The boiling possibly was caused by an opening of fractures and thus in a sudden change from lithostatic to hydrostatic pressure (Behr et al., in press).

2%

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261

Tourmaline is represented up to l-2 vol.% in the matrix of the Sole Dolomite. In some rock fragments the content in tourmaline is much higher, sometimes more than 50 vol.%. Larger tourmaline crystals are often zoned.

Mg-riebeckite occurs as a metasomatic mineral, particularly in granitic rock fragments.

Talc forms several generations of inclusions in dolomite and silicate minerals. As a rock forming mineral it occurs in the so called “white sand” in the NE part of the NNC (Munch, 1978). Here strongly deformed talc grains are concentrated along grain boundaries and particularly in pore spaces between sparitic dolomite grains. Due to the high content of interstitial talc, the “white sand” is less strongly lithified than other parts of the Sole Dolomite. Deformed talc grains also occur inside sparitic dolomite and inside euhedral albite grains.

Most of the Sole Dolomite minerals are rich in fluid inclusions of different, but mostly high salinity (up to 50%) (Behr et al., in press). Three generations of decreasing salinity may be distinguished. The oldest generation is related to dolomite II, while generations 2 and 3 are found in albite and dolomite III. The maximum trapping conditions were 3-4 km depth and a temperature of 450-500°C in the northern part and 2-3 km depth and about 400°C in the southern part of the NNC (Behr et al., 1981).

ROCK FRAGMENTS

The Sole Dolomite contains numerous rock fragments of different size and composition. The largest but very rare may have volumes of up to 2000 m3 (Munch, 1978) while the smallest are less than 100 pm across. Two groups of rock fragments may be distinguished:

More or less rounded xenolithic rock fragment of granites, granodiorites, gneisses, quartzites, and mica schists. Mg-riebeckite and tourmaline are the characteristic metasomatic minerals in these fragments. Biotite is largely replaced by tourmaline.

The second group comprises angular to completely rounded cognate or “autho- lithic” rock fragments of quartz-albitolites with various amounts of tourmaline (up to 75 vol.%) and dolomite. Macroscopically the fine-grained “autholithic” rock fragments reveal a chert-like fabric (quartz-albite cherts), whereas coarser grained varieties resemble aplites. Most of the albites are untwinned, particularly in the fine-grained albitolites. Concerning the grain fabric and mineralogical composition,

Fig. 4. A. Zoned albite with rounded core; nicols crossed, Sole Dolomite.

B. Broken grain of zoned albite; nicols crossed, Sole Dolomite.

C. Euhedral crystal of zoned quartz. The inclusions consist of phyllosilicates, tourmaline, rutile, dolomite

and ore, nicols crossed, Sole Dolomite.

D. Dolomite breccia from the Sole Dolomite. Farm Spitskop Suidwes; negative print. The (dark)

fragments consist of dolomitic wall rocks; the (light) matrix of Sole Dolomite material. The fracture fabric

of the fragments might be interpreted as indicative of hydraulic fracturing.

Fig. 5. A. Sole Dolomite (S.D.) on Farm Naukluft 9. Notice the very sharp contact with

authochthonous Nama limestone mylonite (N.1.m.).

B. SE-facing flow folds in the Sole Dolomite. Farm Naukluft 9.

the unde

263

265

these albitolites are very similar to those of the Duruchaus Formation, which must

be regarded as a source rock of the Sole Dolomite (Behr et al., in press).

DEFORMATION FABRIC OF THE SOLE DOLOMITE

The plate of Sole Dolomite at the base of the Naukluft nappes may be developed as a brown coloured, compact (Fig. SA) sometimes well-laminated, sparitic dolomite (Fig. SB). Both layer-parallel as well as discordant intrusions of the Sole Dolomite material occur up to 60 m in the hanging wall rocks (Munch, 1978; Behr et al., 1981). Ductile deformation of mineral grains were not observed, except for very rare twins in more strongly deformed parts of the Sole Dolomite. In such layers some sparitic grains are undulous and show subgrains. With the exception of this case, all mineral deformation found in the Sole Dolomite is cataclastic. Folds, as shown in Fig. SB, are very rare and represent no ductile deformation. They must be interpre- ted as isoclinal flow folds formed inside a water-rich crystal mush. No slaty cleavage fabrics are developed.

Breccia horizons up to 30 m in thickness are very frequent, particularly in the hanging wall of the Sole Dolomite. However, they also occur as 15-50 cm thick layers and lenses at the base of the Sole Dolomite (Munch, 1978). The breccia consist of angular, sometimes rounded fragments of fine-grained dolomites, dolomitic sandstones, quartzites, phyllitic quartz&es and phyllites, which can be derived from the hanging wall Naukluft nappes. Some of the xenolithic fragments have a fine-grained blastomylonitic fabric which predates the brecciation. The matrix of the

breccia consists of fine-grained siliceous dolomite and/or Sole Dolomite material which intrudes the fractures inside rock fragments (Fig. 4D).

The base of the Sole Dolomite is formed by a few centimetres thick layer of mylonitized Sole Dolomite (Fig. 6) which grades into ultramylonitic Nama limes- tones. Along this boundary the sparitic dolomite grains are well rounded and show particularly well-developed trails of fluid inclusions (Fig. 7A). Dolomite III which is

extremely rich in fluid inclusions is transformed into a very fine-grained dolomite which grades into the Sole Dolo~te-mylonite. The boundary between the Sole Dolomite and its mylonitized base is frequently extremely sharp (Fig. 6, 7A, B).

Neither the Sole Dolomite nor its mylonitized base show any preferred lattice

Fig. 7. A. Sole Dolomite showing rounded grains of sparitic dolomite along the contact to the Sole

Dolomite-mylonite. (Contact perpendicular to scale.) Most of the sparitic dolomite grains are rich in more

or less concentrically arranged trails of fluid inclusion. Nicols crossed; Farm Naukluft 9.

B. Relic grain of sparitic dolomite II grades into Sole Dolomite-mylonite. Nicols crossed. Same sample as

in Fig. 7A. (Contact parallel to scale,)

C. Grain fabric of the Sole Dolomite-mylonite. Notice the very fine-grained undulose grains and serrated

grain boundaries. Nicols crossed; Farm Naukluft 9. (Same orientation as in B.)

D. Dolomitic Nama limestone mylonite showing recrystallized calcite in a very fine-grained non

recrystallized matrix of dolomite. Macroscopic foliation perpendicular to scale.

266

orientation (Fig. 6). Under the optical microscope the Sole Dolomite-mylonite shows

angular fragments of formerly sparitic dolomite grains which are surrounded by an

extremely fine-grained matrix of dolomite (Fig. 7C). The Sole Dolomite-mylonite is

strongly depleted in silicate minerals like quartz and albite. Late tectonic cracks and

pores are filled with kaolinite and quartzine. The quartzine is rich in fluid inclusions

which show boiling phenomena.

The fine-grained matrix of the Sole Dolomite-mylonite is strongly undulose but

free of twin-lamellae. The grain boundaries are unstable (Fig. 7C). The instability of

the grain boundaries increases with decreasing grain size. The smallest grain sizes are

less than 0.5 pm. Recognizable larger fragments of sparitic dolomite grains are

composed of grains between 5 and 10 pm in size. The grain boundaries between

these grains are generally unstable but occasionally high angle boundaries are

developed.

LIMESTONE MYLONITE

In contrast to the Sole Dolomite the underlying autochthonous Nama limestones

have been transformed over a vertical distance of several metres into fine-grained

mylonites (Fig. 9A) which reveal a well-developed preferred lattice orientation (Fig.

8A). The patterns of preferred orientation of the e-lamellae are similar to those of

the Morcles nappe described by Schmid et al. (1981). The patterns show a strong

tendency towards axial symmetry with the axis of symmetry oblique to the symmetry

of the macroscopic fabric (Fig. 8). The inferred direction of shear is identical to the

direction of tectonic transport of the NNC.

Fig. 8. Pole figures for e-lamellae from mylonitic Nama limestone

A. Same sample as in Figs. IO and Il.

B. Dolomitic Nama limestone mylonite (Fig. 7D). The dolomite component has no preferred lattice

orientation.

Fig.

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269

The grain sizes are between 0.5 and 30 pm. All grains are undulose. Larger

intraclasts are strongly twinned and contain subgrains. Near the grain boundaries of

the intraclasts the subgrains grade into recrystallized grains by subgrain rotation

(Fig. 9B, C). These have the same grain size as the recrystallized matrix (5-10 pm).

Besides recrystallized matrix grains the limestone mylonite contains matrix grains

with strongly serrated grain boundaries which are indicative of grain boundary

migration (Fig. 9D). The serrated grains exhibit subgrains of less than 1 pm in grain

size (Fig. 10B). At the northwestern rim of the NNC the mylonitized Nama

limestones are somewhat coarser grained than at the southeastern border where the

metamorphic temperatures were lower.

The boundary between the mylonitized Nama limestones and the Sole Dolomite-

mylonite is not sharp. The transition zone may be more or less dolomitic and shows

irregular layers of dolomitic and of calcitic material arranged parallel to the plane of

the mylonitic lamination. The dolomite may be derived from primary dolomitic

parts of the Nama limestones or from metasomatic dolomitization related to the Sole

Dolomite. The dolomite component of limestone mylonites reveal no preferred

lattice orientation. The grain sizes and grain shapes are similar to that of the Sole

Dolomite-mylonite and may be interpreted as a result of cataclastic mylonitization.

The calcite component is crystallographically oriented (Fig. 8B), but the texture is

weaker when compared with the pure limestone mylonite. Furthermore, the grains

are coarser (about 10 pm) and more equigranular. High angle boundaries are

developed and twins are less frequent (Fig. 7D).

INTERPRETATION

Polyphase crystal growth as well as corrosion of crystals and the wealth of fluid

inclusions can be interpreted as an indication of the former presence of abundant

fluid phases. According to fluid inclusion measurements the maximum trapping

temperature in the Sole Dolomite was about 100°C higher than the metamorphic

temperature in the overlying Naukluft Nappes and about 100°C to 200°C higher

than in the underlying Nama rocks. Cataclastic mineral deformation, brecciation

and hydraulic fracturing of rocks, the lack of crystal plastic deformation and

primary recrystallisation point to low effective stresses which can be best explained

in the present case by abnormally high pore fluid pressures.

On the basis of the mineralogical and fluid inclusion composition Behr et al.

(1981, in press) have interpreted the Sole Dolomite as being derived from meta-

morphosed, hypersaline, evaporitic sediments. Autochthonous equivalents of these

evaporitic sediments have been found by Behr et al. (in press) in the Duruchaus

Formation at the southern margin of the Damara orogen about 80 km to the NE of

the NNC. Here, several more or less circular pipes of siliceous dolomite indicate the

intrusion of siliceous dolomite into higher structural levels. The deformational

fabrics are similar to that of the Sole Dolomite. Similar intrusive bodies of siliceous

270

dolomite have been found by Weber et al. (in press) in the northern part of the

Damara orogen.

The intrusive character of the Sole Dolomite is supported by its discordant

relation with the over- and underlying rocks and by layer-parallel as well as

discordant intrusions of Sole Dolomite material into the hanging wall rocks. There-

fore, the Sole Dolomite can be interpreted as an intrusion of a metamorphosed,

water-rich evaporitic sequence into the base of an earlier formed nappe sequence.

During intrusion grains of sparitic dolomite II, earlier formed silicate grains,

xenohthic and autolithic rock fragments were rounded and broken. Breccia horizons

were formed by hydraulic fracturing of the wall rocks. Multiple zoned crystals of

dolomite II and silicate minerals and the corrosion of dolomite II crystals document

changes in the PTX-conditions during the subsidence of the sedimentary sequence

and its later ascent. The intrusion bears analogies to the mechanical behaviour of

crystal magmas in layered igneous complexes.

After intrusion the mush of siliceous dolomite was lithified mainly by dolomite

III and secondary quartz. The high content of fluid inclusions of high salinity is

indicative of the former presence of interstitial brines.

Deformational fabrics which post-date the lithification of the Sole Dolomite are

scarce, except in the Sole Dolomite-mylonite, which is at the base of the Sole

Dolomite, and in rare, thin mylonitic layers inside the Sole Dolomite. The latter

consist of rounded grains of dolomite II. These grains are surrounded by an

extremely fine-grained matrix of dolomite, which grades into dolomite III. The same

transformation can be observed inside the transition zone from the Sole Dolomite to

the Sole Dolo~te-mylonite (Fig. 7B).

Intracrystalline slip mechanisms as well as grain boundary sliding may be

excluded as dominant deformation mechanisms due to the lack of a preferred lattice

orientation and the strongly serrated grain boundaries. The absence of twins agrees

well with the fact that the temperature during mylonitization was relatively low, and

the thickness of the overlying Naukluft nappes was not greater than 3~-4~0 m

(Korn and Martin, 1959; Hartnady, 1978). Confining pressure of approx. 1 kbar and

possible shear stresses in the range of 500 bars at temperatures between 2OO’C and

300°C are not high enough for dislocation glide and the formation of twins in

dolomite (Handin and Fairbairn, 1956; Higgs and Handin, 1959; Griggs et al., 1960;

Tullis, 1980). Barber et al. (1981) have shown that under expe~mental conditions

(strain rate of 1.3. lo-’ set-’ and confining pressure of 700 MPa) at temperatures

below 300°C there is considerable cataclasis, and shear-fracturing must be counted

as a significant deformation mechanism. Due to the low temperature and low

confining pressure, cataclastic mylonitization might have been the dominant defor- mation mechanism in the Sole Dolo~te-mylonite.

After Ashby and Verral(1977) cataclastic flow can be achieved by “cleavage and

rolling-plus-sliding”. Rolling-plus-sliding is more pressure sensitive then fracturing.

Therefore, it can be assumed that the first mentioned deformation mechanism was

271

dominant during the intrusion of the Sole Dolomite, because grains carry only a

small portion of lithostatic load due to the elevated pore fluid pressure.

Inside the Sole Dolomite-mylonite microfracturing must be assumed as the

dominant deformation mechanism. Fracturing started inside the highly porous and

therefore mechanically most unstable dolomite III cement. Continued shearing may

break the fragments into smaller and smaller pieces. Rounded cores of dolomite II

crystals can be preserved, but the relics of sparitic dolomite II grains mostly form

angular fragments.

The Nama limestone which underlies the Sole Dolomite-mylonite is transformed

over a distance of several metres into a ductile ultramylonite which is similar to that

of the Morcles nappe described by Schmid et al. (1981). Undulose extinction,

subgrains and bent twins (Fig. 10A) indicate that dislocation glide systems were

active. Based on the present investigations and in contrast to former interpretations

it must be assumed that the Nama limestone mylonite represents the lubricating

layer for the displacement of the NNC.

CONCLUSIONS

Microstructures and mineralogical composition of the Sole Dolomite reflect a

complex and polyphase geological history.

The development of the Sole Dolomite may be characterized by the following

steps (Fig. 11):

(1) A sequence of hypersaline playa-lake sediments, which were deposited at the

southern margin of the later Damara Orogen, has been transformed during in-

creasing burial into an albite-tourmaline bearing siliceous dolomite which was rich in

fluid inclusions and interstitial brines (Fig. 1 lA, B). Compactional disequilibria,

dewatering of hydrous carbonates and silicates, osmotic processes and crystallization

as well as grain growth may have produced abnormally high fluid pressures (Weber,

1980).

(2) During the Damara orogeny a nappe sequence was formed and thrust to the

SE. The evaporitic sequences together with the Damara foreland were progressively

subsided due to the southward migration of the Damara front (Fig. 11C). The

maintenance of and the further increase in pore fluid pressure may have been

induced by aquathermal pressuring and deformation. The arrival of the nappes

above the buried evaporitic sequences must have led to a sudden increase in the

overburden pressure and to further reduction of the mechanical stability of the

evaporitic sequence through an increase in pore fluid pressure.

(3) The now metamorphosed, but highly mobile evaporitic material intruded into

higher structural levels and spread along the structural discontinuity at the base of

the previously formed nappe sequence (Fig. 11D). In some places the Sole Dolomite

mush has intruded the overlying Naukluft Nappes. It may be assumed that during

the time of spreading along the base of the nappe sequence and before its lithifica-

212

273

tion, the water-rich carbonate mush has acted as a lubricant. However, the distance

of transport during this stage is not known. Since the lithification of carbonate rocks

is a relatively short-termed process, and after intrusion a relatively fast expel1 of

water must be assumed, probably no far-reaching transport has taken place after

intrusion and prior to lithification of the Sole Dolomite.

(4) The final emplacement of the NNC (the Naukluft nappes with the Sole

Dolomite at its base) must have taken place after lithification of the intrusion.

Otherwise, the intense low temperature ductile mylonitization of the underlying

Nama limestones cannot be adequately explained. Due to the high rigidity of

dolomite under confining pressure, only the lowermost few centimetres of the Sole

Dolomite sheet were cataclastically mylonitized without any preferred lattice orien-

tation. The main glide horizon of the Naukluft nappe complex was not the Sole

Dolomite, but the underlying Nama limestone-mylonite.

ACKNOWLEDGEMENTS

I owe my thanks to my colleagues of the Sonderforschungsbereich 48, especially

to Dr. H.J. Behr, Dr. H. Martin and Dr. H. Porada for helpful discussions and their

permission to use unpublished data.

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