an early cretaceous phase of accelerated erosion on the south-western margin of africa: evidence...

Nucl. Tracks Radiat. Meas., Vol. 17, No. 3, pp. 339-350, 1990 Int. J. Radiat. Appl. Instrum., Part D Printed in Great Britain

0735-245X/90 $3.00 + .00 1990 Pergamon Press plc

AN EARLY CRETACEOUS PHASE OF ACCELERATED EROSION ON THE SOUTH-WESTERN MARGIN OF

AFRICA: EVIDENCE FROM APATITE FISSION TRACK ANALYSIS A N D THE OFFSHORE

SEDIMENTARY RECORD

RODERICK W. BROWN,* DEREK J. RUST,t MICHAEL A. SUMMEgFIELD,~" ANDREW J. W. GLEADOW* and MICHEIL C. J. DE WlT~

*Department of Geology, La Trobe University, Bundoora, Victoria 3083, Australia; tDepartment of Geography, School of Earth Sciences, University of Edinburgh, Edinburgh EH8 9XP, U.K.; and

:~De Beers Consolidated Mines Ltd, PO Box 47, Kimberley 8300, South Africa

(Received 5 September 1988; /n revised form 17 May 1989)

Abstraet--Apatite fission track ages and confined track length distributions have been determined for rock samples from the south-western continental margin of Africa. The apatite ages fall into two groups, one having early Cretaceous ages and mean confined track lengths of ~ 14/~m with very few short tracks, and the other having older ages with confined track length distributions containing a significant proportion of strongly annealed tracks (< 10/am). In any particular area the older apatite ages only occur above a critical threshold elevation, forming a regional pattern in the data and indicating cooling of the upper few kilometres of the crust during the early Cretaceous. This episode of cooling is shown to have been the consequence of an accelerated phase of erosion associated with the early stages of rifting and break-up of Gondwana, and correlates with sedimentation patterns derived from borehole data for the adjacent offshore basin.

1. INTRODUCTION

THE ¢tmONOLOGY and nature of landscape development in southern Africa has stimulated debate since the beginning of the present century, and yet no consensus has been reached (Partridge and Maud, 1988). Major differences in viewpoint result largely from a lack of quantitative information concerning the timing and magnitude of denudation episodes affecting the sub-continent, particularly since the break up of Gondwana. The major discrepancies between the various proposed landscape chronologies (Partridge and Maud, 1987) imply that there are fundamental problems associated with dating ap- proaches used up until now, for example, in the correlation between landsurfaces and unconformities in the offshore sedimentary record (Summerfield, 1985).

This situation suggests that the use of a more direct dating technique may help to resolve these discrepancies, and with this objective a comprehensive study of the morphotectonic development of southern Africa has been initiated. A mejor component of this study was the application of apatite fission track analysis (AFTA) to rocks collected at a range of elevations across the region. This technique provides a valuable tool for evaluating the thermal history of rocks at temperatures below -.. 125°C and consequently for examining the thermo-tectonic development of the upper few kilometres of the Earth's crust (Gleadow

et al., 1986). As a result A F T A can be used to date major periods of denudation directly (Gleadow and Fitzgerald, 1987; Green, 1986; Miller and Duddy, 1989).

The aim of the study now underway is to provide quantitative data on the timing and extent of major denudational episodes affecting the southern African sub-continent since the initiation of rifting and the break up of Gondwana. This paper presents the AFTA results obtained from an initial suite of outcrop and borehole samples from the south-western margin, together with previously unpublished borehole data from the adjacent offshore basin.

2. RESULTS AND INTERPRETATION

Thirty-five outcrop and borehole samples were analysed from the western margin of southern Africa between Cape Town in South Africa and Luderitz in Namibia, a distance of approximately 1000 kin. Nine- teen of the samples were of igneous or metamorphic crystalline basement rocks of Palaeozoic age or older, and six were sedimentary rocks. The outcrop samples were collected, where possible, along transects roughly perpendicular to the present coastline and over a range in elevation from sea level to above the prominent erosional escarpment known as the Great Escarpment (Oilier and Marker, 1985). This feature trends roughly parallel to and at distances of

339

340 RODERICK W. BROWN et al.

- - 3 0 S

- - 3 5 S

1 5 E 2 0 E I I

FIG. i. Location map for south-western Africa showing outcrop sample sites (small filled circles), boreholes (large open circles) and the position of the "Great Escarpment". Bathymetrie contours are shown at

I000 m intervals.

50-300 km inland from the coastline, and the highest elevation sampled was 1550 m (8732-41), In addition to the outcrop samples, three samples were analysed from a deep borehole (QU 1/65) through the Karoo sequence (Dingle et al., 1983). Sample elevations were estimated from 1:250,000 topographic map sheets with an estimated uncertainty of + 50 m. Locations

2 Fish River

0

0 . sp. , 1.00 Luderitz-Keetmanshoop km

2 Koa valley Orange river

0

o . ~ . 1.oo Kleinsee-Uplngton km

k m l -

0-

9 ,~, . 1.oo Groenrivler-Kamieskroon km

2 i T a b l e Mt 34 ~ 41

o . sp . 1.oo Cape Town-Sutherland

FIG. 2. Topographic profi les showing outcrop sample locations with respect to elevation and distance f rom the

present day coastline.

of outcrop samples and boreholes are indicated on Fig. 1, and together with sample elevation information on Fig. 2. Apatite separates were obtained from the rocks using conventional heavy liquid and magnetic techniques and analysed using the external detector method as described by Green (1986). Apatite ages were calculated using a Zeta calibration factor of 347 for NBS glass dosimeter SRM 612 (Hurford and Green, 1983; Green, 1985) obtained by 14 independent calibration measurements against three apatite age standards; 4 × Fish Canyon Tuff, 5 × Mount Dromedary and 5 x Durango (Mexico) (Miller et al., 1985). The apatite fission track dating results and the confined track length results for the 25 analysed samples plus the results from De Wit (1988) (measured by A.G.) are shown in Table 1 and Table 2, respectively.

The most significant feature of the data is that all the apatite ages are younger than the stratigraphic age of the rocks sampled, indicating that all the samples have experienced some degree of thermal annealing. Another important feature is that there is no systematic variation of apatite age with distance from the coastline unlike the pattern observed else- where in Gondwana (Moore et al., 1986). There is also a striking lack of variation in apatite age with topographic elevation up to some critical threshold elevation, for any particular area. Above these threshold elevations the apatite ages increase significantly. Four representative apatite age--elevation

EARLY CRETACEOUS PHASE OF ACCELERATED EROSION 341

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342 R O D E R I C K W . B R O W N et al.

Table 2. Apatite confined track length results

Mean confined track N u m b e r o f Standard deviation Fission track age Sample length ~ m ) ( + ltr) tracks of distribution (Ma) ( + l(r)

8732-34 12.93 -t- 0.22 125 2.43 155 + 4 8732-35 12.82 4- 0.20 100 2.01 129 4- 5 8732-36 14.02 4- 0.20 100 0.96 124 4-4 8732-37 13.38 _+ 0.23 75 2.02 143 4- 7 8732-39 12.07 4- 0.23 102 2.36 142 -I- 12 8732-40 1 i.83 ___ 0.28 126 3.12 160 -I- 11 8732-41 12.71 + 0.28 120 3.12 178 4- 12 8732-42 13.36 _ 0.25 100 2.51 100 _+ 5 8732-43 13.44 4- 0.22 37 1.36 120 + 5 8732-44 13.34 +_ 0.13 115 1.36 108 _ 3 8732-45 12.75 + 0.25 100 2.47 110 4- 5 8732-46 13.30 -t- 0.15 116 1.6 153 -I- 7 8732-51 13.70 + 0.18 63 1.45 116 + 6 8732-52 12.93 + 0.50 14 1.86 107 +_ 7 8732-55 13.64 4- 0.12 102 1.21 72 _+ 3 8732-56 13.80 + 0.27 26 1.38 127 _ 10 8732-86 13.47 __+ 0.15 85 1.42 113 4- 7 8732-81 14.10 4- 0.20 31 1.10 100 +_. 5 8732-82 13.54 _ 0.14 94 1.37 95 4- 3 8732-83 13.72 4- 0.11 115 1.15 102 + 4 8732-84 12.09 + 0.35 85 3.25 138 +__ 8 8732-85 13.40 __+ 0.19 68 1.60 83 + 3 8732-96 13.18 + 0.30 50 2.12 127 +_. 7 8732-98 13.03 _ 0.29 52 2.09 114 4- 6 8732-105 9.83 +0 .42 41 2.71 11 4- 1 MDW-01* 14.03 + 0.12 81 0.88 119 +__ 9 MDW-02 13.68 + 0.12 100 1.17 108 _+ 4 MDW-03 13.69 4- 0.11 I00 1,11 123 ___ 7 MDW-04 13.33 +0 .68 9 2,04 !11 + 12 MDW-05 14.01 -t- 0.12 100 1,18 129 + 6 MDW-06 13.16 4- 0.79 10 2.51 125 _+ 26 MDW-07 14.05 -t- 0.12 100 I. 19 125 +__ 4 MDW-08 13.93 __+ 0.11 100 1.13 126 + 6 MDW-09 13.86 _ 0.11 I00 1.10 111 + 5 MDW-10 13.81 4- 0.14 100 1.37 107 + 6 MDW-11 13.98 + 0.13 76 1.12 70 4- 5 MDW-12 14.23 4- 0.09 100 0.88 73 ___ 4

* M D W indicates the samples from De Wit (1988).

2500

2000 E 1500

1000 u1, tu 500

;'500

A 2000 E z 1500 0

1000

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0

L u d e r i t z - K e e t m a n s h o o p

100 200

Cape Town-Sutherland

Q

15oo.

lOOO,

5oo,

o,

15oo

1~o

5 ~

o

G r o e n r i v i e r - K a m i e s k r o o n

100

Kleinsee-Upington

200

o 16o 200 o 16o 200 APATITE AGE (Ma) APATITE AGE (Ma)

FIG. 3. Apatite fission track age-elevation profiles for the four transects shown in Fig. 2.


profiles from the study area are shown in Fig. 3, and these indicate that the pattern observed in the apatite age-elevation profiles is a regional feature.

The mean apatite age for samples below this threshold elevation ranges from ~ 1 2 0 M a in the Cape Town area to ~ 100 Ma in the Luderitz area. The mean confined track length distributions for these samples are also similar, having strong peaks at ~14/am with very few short tracks (<10/am) present. A more detailed analysis of the relationship between apatite age and mean track length reveals a distinct pattern: there is a continuous trend between the youngest ages, which have the longest mean track length with the lowest standard deviations, and the older ages which have progressively shorter mean track lengths with increasing standard deviations (Fig. 4). This type of pattern between apatite age and mean track length is indicative of a period of thermal annealing, affecting the suite of samples to varying degrees, followed by an episode of rapid cooling (Gleadow and Fitzgerald, 1987; Fitzgerald and Gleadow, 1988). The samples with the youngest

~ 15 '

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16o 260 APATITE AGE (Ma)

16o 26o APATITE AGE (Ma)

i

• 1

i I i

i •

. , - , . ,

12 13 14 15 MEAN TRACK LENGTH (um)

FiG. 4. Details of the relationship between apatite fission track age and mean confined track length for the samples from the Cape Town-Sutherland and the Luderitz-

Keetmanshoop transects.

apatite ages and longest mean track length having experienced the highest temperatures of annealing (sufficient to completely anneal all previously accumulated tracks), and the samples with progressively older ages and shorter mean track lengths represent successively lower temperatures of annealing at higher structural levels in the pre-uplift crust.

The total number of fission tracks in each sample can be regarded as comprising two components. One is the contribution of earlier tracks which were shortened during the period of annealing, and the other is the contribution of younger tracks that accumulated after the episode of rapid cooling, and which have experienced only a minor amount of annealing and thus have longer lengths (> 13/am). The maximum etchable length of the tracks, and hence the proportion of tracks from the earlier component that contributes to the measured fission track age of each sample, is inversely proportional to the maximum temperature of annealing experienced by that sample. Obviously, for samples that have experienced temperatures high enough to completely anneal all previously accumulated tracks, the proportion of earlier tracks that contribute to the measured fission track age will have been reduced to zero.

It is the mixture of these two fission track components, in varying ratios, that produces the continuous trend of decreasing mean track length with increasing apatite age (Fig. 4). This type of relationship between apatite age and mean track length has been observed and well documented in several other studies. Moore et al. (1986) were the first to demonstrate this trend i~ their investigation of the thermal evolution of the south-eastern Australian continental margin. Green (1986) has shown and explained in more detail the same pattern in his documentation of the early Tertiary uplift history of northern England. More recently, Miller and Duddy (1989) have also observed this relationship for the early Cretaceous uplift and cooling of the Appalachians in the north-eastern United States.

Application of the principles of interpretation explained by Gleadow et aL (1983, 1986) and Green et al. (1989) allows more quantitative constraints to be placed on the nature of the thermal history that produced the observed pattern in the present fission track data set. The confined track length distributions and single grain age distributions for the five samples along the Luderitz-Keetmanshoop transect are shown in Fig. 5 together with a topographic section. Samples 81, 82, 83 and 85 all have narrow, symmet- rical single grain age distributions with a mean age of ~ 100 Ma, and which pass the ~2 test of Gaibraith (1983) indicating that the distributions are consistent with a single age population. The confined track length distributions for these four samples are also similar in that they all contain very few short tracks (< 10/am) and have well defined peaks at ,,, 14/am with mean confined track lengths between 13.54 and 14.10/a m. The details of the apatite ages and confined


0 2OO 40O Apatb ~ (Me)

o ~o 2o / ~ ~ / (mleronl)

km 1

0

0 50 100 I I ! I I

km FIG. 5. Single-grain age distributions and confined track length distributions for samples from the

Luderitz-Kcctmanshoop transect.

track lengths for these four samples show that they all must have cooled quite rapidly from temperatures above ~ 125°C to temperatures below ~60°C about 100 Ma ago and have remained below ~60°C until the present. The exact duration of this cooling episode is not well constrained at this stage but would be of the order of 10 Ma.

In contrast, sample 84 has an older age of 138 + 8 Ma and is the only sample containing a larg¢ proportion of strongly annealed tracks ( < 10pm). The single grain age distribution for this sample has a peak at ~ 100 Ma, similar to the other samples on this transect, as well as a large proportion of older grains (up to ~ 180 Ma). This spread in single grain ages is reflected by the X 2 value (P < 1%), which is not consistent with a single age population. These details indicate that sample 84 must have spent an appreciable time at temperatures in the range 125-80°C before it too cooled to below ~60°C about 100Ma ago. The observed age of ~ 1 4 0 M a for sample 84 therefore represents a "mixed age" lying between the time when tracks first began to accumu- late and the time of cooling ~ I00 Ma ago.

The apatite fission track data for the eight samples from the region west of Upington (46, 45, 44, 43, 42, 51 and 86) have been combined with the AFTA results published by De Wit (1988) for this area. The apatite ages range from 100 + 6 Ma to 129 + 6 Ma with a mean age of ~120Ma . Only one sample (8732-46) has a significantly older age of 153 + 7 Ma. The confined track length distributions for these samples are strikingly similar; all have well developed peaks at ~ 1 4 p m with very few short (<10/~m) tracks present. The mean confined track lengths range between 13.16 and 14.23pm. The similarity of the apatite ages and confined track length

distributions for this set of samples implies that they have all experienced a similar cooling history, that is, an episode of accelerated cooling from temperatures in excess of 125°C to temperatures less than ~ 60°C about 120 Ma ago. Sample 8732-46 (with an apatite age of 153 + 7 Ma) is believed to represent a mixed age similar to sample 8732-84, and thus reflects a lower maximum temperature of annealing than the other samples in this set.

It seems likely that this regional pattern in the AFTA data for the area west of Upington is a continuation of the pattern observed for the Luderitz-Keetmanshoop transect. However, the present regional level of erosion in this area is deeper than that for the Luderitz area, hence the older apatite ages occurring at higher stratigraphic levels have been removed. Consequently, most outcropping rocks in the region have similar apatite ages and track length distributions, indicating accelerated cooling from high temperatures (>125°C) in the early Cretaceous.

Most of the samples from the Cape Town- Sutherland transect have single-grain age distributions and accompanying confined track length distributions which represent maximum annealing temperatures of less than 125°C (Fig. 6). As a result, the proportion of tracks in these samples which accumulated prior to the period of annealing is greater than zero. Evidence for this is provided by samples 34, 35, 39, 40 and 41 all of which have a significant number of short tracks ( < 10 ~m) represented in their confined track length distributions, as well as a well developed peak at ~ 1 4 p m . This pattern in the confined track length distributions demonstrates that there are at least two fission track components in these samples, an early component of


| , ,

0 2OO 4OO AOama X~ (ua)

0 10 20 \ / \ // J Track lamOtn

( m i c r o ~ )

0 so 100 I I = i

km

FIG. 6. Single-grain age distributions and confined track length distributions for samples from the Cape Town-Sutherland transect.

strongly annealed tracks and a later component of tracks that have never experienced temperatures above ,,,60°C.

The single-grain age distributions for these five samples support this interpretation. The apatites separated from samples 39, 40 and 41 arc likely to have been derived from more than one sourec be- cause these three rocks are sandstones from the Karoo sequence (Dingle et al. , 1983). The apatites from these three samples should thus exhibit a range of compositions, specifically in the F/CI ratio of the individual apatitc grains. Green et al. (1986) have shown that the F/CI ratio of the host apatitc crystal has a significant effect on the rate at which fission tracks will be annealed at any given temperature. Hence, the spread in single-grain ages for samples 39, 40 and 41 should reflect this compositional range of apatite in these rocks. All three samples (see Fig. 6) have well developed peaks in their single-grain age distributions at ~120 Ma as well as containing a significant proportion of older grains. This pattern, in conjunction with the confined track length data, indicates that these samples must have experienced temperatures high enough to completely anneal the fission tracks in the apatites with the highest F/C1 ratios but not high enough to erase the tracks in apatites with lower F/CI ratios. The temperatures that produced this effect are likely to have been in the range of ~90°C to 125°C (Green et al. , 1986; Green, 1986).

Samples 35 and 34, on the other hand, are both granites and so would be expected to show a narrow range in F/Ct ratio, as is the case for igneous rocks in general (Sieber, 1986). This means that all the apatit¢ crystals in each sample will have similar annealing properties and, unlike apatites from the sedimentary samples, all have ages that have been

partially reduced by the same extent. As a consequence single-grain age distributions are produced which are consistent with a single age distribution. This does not imply, however, that the apatite ages for these two samples represent discrete geological events. On the contrary, the ages arc mixed ages produced by the addition of a younger set of tracks to an older set that have been strongly annealed but not completely erased. These "mixed ages" or "partially over-printed ages" arc therefore diagnostic of the degree of thermal annealing with the extent of age reduction (from some age prior to the time of annealing) being proportional to the maximum palaeotemperature experienced.

Sample 36 has a confined track length distribution with a narrow, well developed peak at ~ 14 #m with no short tracks represented (Fig. 6). In addition, the single-grain age distribution has a narrow, sym- metrical peak at ,-, 120 Ma and an absence of older and younger grains. These details indicate that this sample has experienced a discrete episode of accelerated cooling from temperatures in excess of 125°C to below ~60°C during the early Cretaceous. This episode of accelerated cooling of the upper few kiiometres of the earth's crust was also recorded by the other samples, as evidenced by well developed peaks in the confined track length distributions (at ~ 14 #m) and a peak at ~ 120 Ma in the single grain age distributions for samples 39, 40 and 41.

Samples 96, 98 and 105 are the three borehole samples from QU 1/65 and are from depths of 470 m, 1460 m, and 2750 m, respectively. Both samples 96 and 98 are fine-grained feldspathic sandstones and sample 105 is a porphyritic biotite granite. Like sample 36, samples 96 and 98 have confined track length distributions and single-grain age distributions indicative of accelerated cooling from elevated temperatures

346 R O D E R I C K W. BROWN et al.

(> 125°C) during the early Cretaceous. Sample 105, on the other hand, has a apatite age of ~ 10 Ma and a mean confined track length of 9.83 #m, reflecting the present day down-hole temperature of ~ 110°C.

3. DISCUSSION

On the regional scale the apatite ages can be placed in two groups. One group represents samples that have cooled rapidly from temperatures above ~ 125°C to temperatures below ~60°C in the early Cretaceous and have subsequently remained cool to the present day. The second group represents samples that have spent a significant amount of time (of the order of 1 0 7 years) at temperatures between ~ 125°C and ~ 80°C before they too underwent rapid cooling at the same time as the first group of samples. In addition, the samples in the second group are invari- ably those from the highest elevations in any of the areas sampled. The consistency of this pattern in the apatite fission track data, as seen particularly in the region between Kleinsee and Upington (Fig. 1), is a feature that must be explained by any interpretation.

Samples 41, 40 and 39 together with the three borehole samples from QU 1/65 (96, 98 and 105) have been used to construct a conceptual diagram (Fig. 6) which depicts the interpretation of the vertical structure of the observed AFTA results over the study region. The critical feature of this interpretation is the recognition of a "break in slope" on the apatite age-elevation profile at ~ 7 5 0 m elevation and ~ 125 Ma. This break is complemented by a sharp decrease in the mean confined track length at the same elevation. The break in slope is interpreted as representing the base of the "Partial Annealing Zone" (PAZ), in the sense of Gieadow and Fitzgerald (1987), that existed in the crust prior to the rapid cooling episode. The present day PAZ is shown as the shaded area in Fig. 7.

Cooling of the upper crust can be produced by either reducing the regional geothermal gradient or by removal of material from the surface of the crust (or a combination of these mechanisms). The apatite fission track data alone do not provide direct information about the mechanism involved in producing the cooling episode observed. However, the data do allow constraints to be placed on mechanisms that are geologically reasonable. For example, the observed pattern in the apatite fission track data could conceivably be produced by a rapid decrease in the regional geothermal gradient from ~ 180°C km -~ to ~ 30~C km-~ about 140 Ma ago with no accompanying denudation, although this is clearly an unrealistic interpretation.

The present day geothermal gradient in the vicinity of QU 1/65 is approximately 30°C km-l . This estimate was made from three down-hole temperature measurements for another deep borehole (WE 1/66) located about 200 km to the east. The uniformity of the geology across the Karoo Basin (Dingle et al. ,

0

2.0

1.5

1.0

o~

0.5

1.0

1.5

2.0

APATITE AGE (Me) 100 200

i , ~ , I I I , I I I • APATITE AGE

- ! __[_

5 10 MEAN TRACK LENGTH (pro)

0.5

1.0 ~ o~"

2 . 5 ~

"° M:',o

FIG. 7. Apatite fission track age and mean confined track length plotted against sample elevation for outcrop samples 41, 40 and 39 (filled symbols) and borehole core samples 96, 98 and 105 (open symbols). Temperature scale refers to the estimated present-day down-hole temperatures for the QU 1/65 borehole. The shaded area marks the position of the

present-day Partial Annealing Zone for QU 1/65.

1983) suggests that the geothermal gradient obtained for WE 1/66 is likely to be a reasonable approxima- tion of the geothermal gradient for QU 1/65. It should be noted, however, that heat flow measurements from the region between Kleinsee and Uping- ton, based on numerous down-hole temperature measurements, indicate a present day geothermal gradient of ~20°C km -] for that area (Jones, 1988).

In order to estimate the amount of denudation that must have occurred it is necessary to know what the geothermal gradient was at the time of this event. Any estimate of this palaeo-geothermal gradient would simply be conjecture at this stage. However, the effects of a range of higher geothermal gradients, which may have existed at the time of the denudation and have now declined to the present day gradient, can be calculated. The effect of a higher geothermal gradient at the time of accelerated cooling would be

1 8 0 . . . . . . . . " ' E ,.~ 160

o~ 140

100 Present geothermal .¢ so gradient

i,~ . , . i . [ , J , i . . . . . . . .

~ 0 1 2 3 4 5 6 7 8 g 1 0

Required denudation (km) FIG. 8. Diagram illustrating the relationship between the geothermal gradient at the time of rapid cooling and the amount of required denudation indicated by the apatite

fission track data.


an overall decrease in the amount of denudation required to produce the observed pattern in the AFTA data. Using the break in slope, shown in Fig. 7, as a marker for the 125°C isothermal surface in the crust prior to cooling it is possible to calculate the amount of denudation required to elevate the break in slope to its present level of ~ 500 m below the surface. For reasonable values of the initial geothermal gradient this calculation indicates that some kilometers of denudation must have taken place. The resulting curve for this calculation is shown in Fig. 8.

In addition, the fission track data imply that significant denudation has occurred on the landward as well as the seaward side of the present position of the "Great Escarpment" (Ollier and Marker, 1985) along the western continental margin. This clearly has important implications for understanding the post-Gondwana landscape history of southern Africa, since many previous interpretations have invoked relatively modest rates of denudation inland of this feature. It is interesting to note that the fission track data presented by Miller and Duddy (1989) for the northern Appalachian Basin also demonstrates that several kilometers of early Cretaceous erosion occurred both seaward and landward of the present escarpment in the Catskills region. The timing and rate of cooling associated with this phase of erosion is remarkably similar to the episode of erosion documented in this paper for the south-western African margin.

Regional denudation of the order of some kilometers would have induced compensatory isostatic uplift of the rock column but an overall lowering of the pre-existing land surface. The present day mean elevation inland of the Great Escarpment is approximately 1000 m, and using a figure of 3 km of denudation (Fig. 8) for purposes of illustration, the ultimate decrease in mean elevation would be about 600 m (using local Airy isostasy, as a first approxi- mation, and a mean density of eroded material of ~ 2700 kg m-3).

If an initial mean elevation of ~ 1600 m is assumed, this denudation episode could conceivably have been accomplished as a result of a new drainage base level being established along the coast of the developing continental margin. Uplift of the rock column sufficient to expose apatites at the surface with the apparent ages and confined track length distributions observed could then have been accomplished solely by isostatic uplift in response to denudational unloading. However, current knowledge of the tectono-thermal effects of the rifting process (Sclater and Christie, 1980; Keen, 1985; Issler et al., 1989) indicates that some tectonic uplift can be expected to have occurred prior to and/or during the denudational episode.

The underlying mechanism that produced this denudational episode would thus include a contribution from new drainage base levels established as a

consequence of rifting, as well as a contribution from tectonic uplift. The apatite fission track data suggest that the denudation was of regional extent, however the data available so far are insufficient to indicate if the magnitude of the denudation varies in a systematic way; for example with increasing distance from the continent-ocean boundary. The proposal for an early Cretaceous phase of accelerated denudation is consistent with the sedimentary record from the Orange basin adjacent to this part of the continental margin (Fig. 9). The timing of this phase of denudation corresponds broadly with the syn-rift and early drift stage (Falvey, 1974) of margin development in southern Africa.

Initial igneous activity believed to be related to rifting along this margin is represented by dolerite sills dated at 178-199 Ma in the Keetmanshoop area of southern Namibia, by lavas in the Hoachanas area of central Namibia dated at 161-173Ma, and by dolerites and basalts occurring along the Orange River and dated at 166-191 Ma (Siedner and Miller, 1968; Gidskehaug et al., 1975; Siedner and Mitchell, 1976). Gerrard and Smith (1982), using seismic reflec- tion and borehole data offshore from this margin, identify an angular unconformity at the base of what they consider to be the rift valley lithotectonic unit, assigning it a Kimmeridgian (~ 155 Ma) age. Investi- gations of sea-floor magnetic anomaly patterns in this area have recognised lineated anomalies as old as M9 (Austin and Uchupi, 1982) and possibly MI0 (Larson and Ladd, 1973), indicating that oceanic crust began to be emplaced in the late Valanginian about 130-135 Ma ago.

This chronology of margin evolution provides a framework for considering the AFTA results now available, particularly the suggested episode of accelerated denudation. The time of the change from low rates of denudation to significantly increased rates can be estimated using the break in slope. This break occurs about 125 Ma (Fig. 7), and after a minor correction for the small degree of annealing that has occurred during the time spent passing through the PAZ (Green, 1988), provides an age of about 140 + 10 Ma for the onset of the cooling of the rock column in response to the proposed denudation. Such timing clearly suggests a causal relationship between the rifting process and the phase of denudation, and may be explained by initial thermal uplift with associated extensional faulting and the development of rapidly eroding fault related topography with high local relief.

The borehole records of the Orange basin shown in Fig. 9 allow sedimentation rates to be calculated for post-earliest Cretaceous time but not for the older interval extending to the beginning of the Cretaceous. As a result the high rates which might be expected during what may have been the most tectonically active phase of rifting during the earliest Cretaceous cannot be quantified. Furthermore the beginning of the quantifiable record for many of the boreholes


100

5O

UJ~ EARLY

KUDU 9A-1 K-A1 K-A2 K-A3

L 15 58 t

0 >28

U 18 , , 3

'33

I 1271

176

• >576

50

100

MIOCENE

OLIGOCENE

EOCENE

PALEOCENE

LATE

~ EARLY

A-D1 A-HI A-K1 A-A1 A-C1 A-C2 A-C3

285 ? t83 128 U 232 269 7197 ?184 ?214

c3 . . . . ~oo mMl-1

FIG. 9. Histograms illustrating sedimentation rates (corrected for compaction) obtained from SOEKOR borehole records. See Fig. l(a) for borehol¢ locations.

occurs at about 115 Ma ago, coinciding approximately with the widespread All angular unconformity which generally defines the base of the drift sequence (Gerrard and Smith, 1982; Emery and Uchupi, 1984), and this interval in southern Africa is likely to exhibit lower sedimentation rates than the preceding rift sequence deposits (Dingle et al., 1983). In view of these considerations the documented pattern of sedimentation assumes greater significance, showing as it does high rates at the beginning of the record and a significant reduction in sedimentation up to the end of the Cretaceous (Fig. 9). This pattern is most pronounced in the records for boreholes K-AI, K-A2 and K-A3 in the centre of the Orange basin (Fig. 1). The only notable exception is the Kudu 9A-I record from the north of the basin which, in addition to displaying high sedimentation rates in the early Cretaceous, also documents high rates in the late Cretaceous. However, this later period of deposi- tion may be related to the establishment of the Orange River outlet at this part of the margin (Dingle and Hendey, 1984). In all of the boreholes sedimentation rates become significantly reduced in the post- Cretaceous record, and recently published isopach maps for this area suggest that these younger deposits constitute a relatively small proportion of the total

offshore sediments (Gerrard and Smith, 1982; Dingle et al., 1983; Emery and Uchupi, 1984).

Geomorphological evidence (De Wit, 1988), in the form of calcrete duricrusts of early-middle Tertiary age affecting the crater facies of ultrabasic intrusive pipes of late Cretaceous age occurring inland of the Great Escarpment in the region west of Upington, supports the idea that relatively little denudation has occurred inland of the present day escarpment since the end of the Cretaceous.

4. CONCLUSIONS

The application of apatite fission track analysis to rocks collected from a range of elevations along the western continental margin of southern Africa indicates that the present land surface exposes rocks that have cooled rapidly from temperatures above ~125°C to temperatures below ~60°C during the early Cretaceous. This rapid cooling of the upper crust is interpreted as the result of accelerated denudation and consequent uplift of the rock column associated with the early development of the continental margin. The timing of this episode of denudation is broadly synchronous with the break up of West Gondwana and correlates with the pattern of

EARLY C R E T A C E O U S P H A S E O F A C C E L E R A T E D E R O S I O N 349

sedimentation derived from borehole data in the adjacent offshore basin.

Furthermore, the ,,, 125°C palaco-isothermal surface for the pre-uplift crust is recorded by the apatite age--elevation profiles as a distinct break in slope. The present-day elevation at which this break occurs allows an estimate of the amount of denudat ion to be made. For reasonable estimates of the palaeo- geothermal gradient some kilometers o f denudat ion must have taken place. For a palaeo-geothcrmal gradient of 30°C km -~ the calculated amount o f denudation is 3 km. In addition, the presently available A F T A data indicate that the denudation was regional in extent.

Acknowledgements--RWB would like to thank De Beers Consolidated Mining Company for their generous financial and logistical support, as well as John Bristow for his enthusiastic help during this study. DJR and MAS grate- fully acknowledge financial support from the Natural Environment Research Council (Grant No GR3/6693) and from Texaco Inc. We are also grateful to SOEKOR (Southern Oil Exploration Corporation) for allowing access to proprietary offshore well-log records and to Dr N. van Vuuren of the Geological Survey of South Africa for his help in organising sampling of the Karoo borehole cores. Thanks to Susan Scott for kindly typing the manuscript.

REFERENCF~

Austin J. A. and Uchupi E. (1982) Continental-oceanic transition off south-west Africa. Am. Ass. Petrol. Geol. Bull. 66, 1328-1347.

De Wit M. C. J. (1988) Aspects of the geomorphology of the north-western Cape, South Africa. In Geomorphologi- cal Studies in Southern Africa (Edited by Dardis G. G and Moon B. P.), pp. 57-69. Balkema, Rotterdam.

Dingle R. V. and Hendey Q. B. (1984) Late Mesozoic and Tertiary sediment supply to the eastern Cape basin (SE Atlantic) and Palaeo-drainage systems in southwestern Africa. Mar. Geol. 56, 13-26.

Dingle R. V., Siesser W. G. and Newton A. R. (1983) Mesozoic and Tertiary Geology of Southern Africa, p. 375. Balkema, Rotterdam.

Emery K. O. and Uchupi E. (1984) The Geology of the Atlantic Ocean, p. 1050. Springer, Berlin.

Falvey D. A. (1974) The development of continental margins in plate tectonic theory. J. Austr. Petrol. Explor. Ass. 14, 95-105.

Fitzgerald P. G. and Gleadow A. J. W. (1988) Fission-track geochronology, tectonics and structure of the Tram- Antarctic Mountains in northern Victoria Land, Antarctica. Isotope Geosci. 73, 169-198.

Galbraith R. F. (1983) Goodness-of-fit tests in fission track dating. J. Math. Geol. 15, 567-569.

Gerrard I. and Smith G. C. (1982) Post-Palaeozoic succes- sion and structure of the southwestern African continental margin. Am. Ass. Petrol. Geol. Mere. 34, 49-74.

Gidskehaug A., Crcer K. M. and Mitchell J. G. (1975) Palaeomagnetism and K-Ar ages of the south-west African basalts and their bearing on the time of initial rifting of the South Atlantic. R. astr. Soc. geophys. J. 42, 1-20.

Gleadow A. J. W. and Fitzgerald P. G. (1987) Uplift history and structure of the Trans Antarctic Mountains: new evidence from fission track dating of basement apatites in the Dry Valleys area, Southern Victoria Land. Earth Planet. Sci. Lett. 82, 1-14.

Gleadow A. J. W., Duddy I. R., Green P. F. and Lovering J. F. (1986) Confined fission track lengths in apatite: a diagnostic tool for thermal history analysis. Contr. miner. Petrol. 94, 405-415.

Gleadow A. J. W., Duddy I. R. and Lovering J. F. (1983) Fission track analysis: a new tool for the evaluation of thermal histories and hydrocarbon potential. J. Austr. Petrol. Explor. Ass. 23, 93-102.

Green P. F. (I 985) Comparison of zeta calibration baselines for fission-track dating of apatite, zircon and sphene. Isotope Geasci. 58, 1-22.

Green P. F. (1986) On the thermo-tectonic evolution of Northern England: evidence from fission track analysis. Geol. Mag. 123, 493-506.

Green P. F. (1988) The relationship between track shorten- ing and fission track age reduction in apatite: combined influences of inherent instability, annealing anisotropy, length bias and system calibration. Earth Planet. Sci. Lett. 89, 335-352.

Green P. F., Duddy I. R., Gleadow A. J. W. and Lovering J. F. (1989) Apatite fission track analysis as a palaeotemperature indicator for hydrocarbon exploration. In Thermal Histories of Sedimentary Basins: Methods and Case Histories (Edited by Naeser N. D. and McCulloh), pp. 181-195. Springer, New York.

Green P. F., Duddy I. R., Gleadow A. J. W., Tingate P. R. and Laslett G. M. (1986) Thermal annealing of fission tracks in apatite I: a qualitative description. Isotope Geosci. 59, 237-253.

Hurford A. J. and Green P. F. (1983) The zeta age calibration of fission-track dating. Isotope Geasci. I, 285-317.

Issler D., McQueen H. and Beaumont C. (1989) Thermal and isostatic consequences of simple shear extension of the continental lithosphere. Earth Planet. Sci. Lett. 91, 341-358.

Jones M. Q. W. (1988) Heat flow and heat production in the Namaqua Mobile Belt, South Africa. J. geophys. Res. 92, 6273--6289.

Keen C. E. (1985) The dynamics of rifting: deformation of the lithosphere by active and passive driving forces. Geophys. J, R. astr. Soc. 80, 95-120.

Larson R. L. and Ladd J. W. (1973) Evidence for the opening of the South Atlantic in the early Cretaceous. Nature 246, 209-212.

Miller D. and Duddy I. R. (1989) Early Cretaceous uplift and erosion of the northern Appalachian basin, New York, based on apatite fission track analysis. Earth Planet. Sci. Lett. 93, 35-49.

Miller D., Duddy I. R., Green P. F., Hurford A. J. and Naeser C. W. (1985) Results of inter-laboratory comparison of fission track age standards: Fission-track Workshop, 1984. Nucl. Tracks Radiat. Meas. I0, 383-391.

Moore M. E., Gleadow A. J. W. and Lovering J. F. (1986) Thermal evolution of rifted continental margins: new evidence from fission tracks in basement apatites from south-eastern Australia. Earth Planet. Sci. Lett. 78, 255-270.

Oilier C. D. and Marker M. E. (1985) The Great Escarp- ment of southern Africa. Z. Geomorphologie, Suppl. 54, 37-56.

Partridge T. C. and Maud R. R. (1987) Geomorphic evolution of southern Africa since the Mesozoic, S. Afr. J. Geol. 90, 174-208.

Partridge T. C. and Maud R. R. (1988) Geomorphic evolution of southern Africa: a comparative review. In Geomorphological Studies in Southern Africa (Edited by Dardis G. F. and Moon B. P.), pp. 5-15. Balkema, Rotterdam.

Sclater J. G. and Christie P. A. F. (1980) Continental stretching: an explanation of the post mid-Cretaceous

350 R O D E R I C K W. B R O W N et al.

subsidence of the Central North Sea Basin. J. geophya. Res. B85, 3711-3739.

Sieber K. G. (1986) Compositional variation in apatite. B.Se. Honours thesis, University of Melbourne.

Siedner G. and Miller J. A. (1968) K-At age determinations on basaltic rocks from southwest Africa and their beating on continental drift. Earth Planet. Sci. Lett. 4, 451-458.

Siedner G. and Mitchell J. G. (1976) Episodic Mesozoic volcanism in Namibia and Brazil: a K-Ar isochron study bearing on continental drift. Earth Planet. Sci. Lett. 30, 292-302.

Summerfield M. A. (1985) Plate tectonics and landscape development on the African continent. In Tectonic Geomorphology (Edited by Morisawa M. and Hack T. J.), pp. 27-51. Allen and Unwin, London.

an early cretaceous phase of accelerated erosion on the south-western margin of africa: evidence...

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