- Email: [email protected]
The evolution of the Rio Grande Rise in the southwest Atlantic Ocean
Marine Geology, 58 (1984) 35--58
35
Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands
T H E E V O L U T I O N O F T H E RIO G R A N D E ATLANTIC OCEAN
LUIZ ANTONIO
PIERANTONI
CAMBOA
RISE IN T H E S O U T H W E S T
I'2 and PHILIP D. R A B I N O W I T Z ' . 2
ILamont-Doherty Geological Observatory of Columbia University,Palisades,N Y 10964 (U.S.A.) 2Department of Oceanography, Texas A&M University, College Station, TX 77843 (U.S.A.) (Accepted for publication November 7, 1983)
ABSTRACT Gamboa, L.A.P. and Rabinowitz, P.D., 1984. The evolution of the Rio Grande Rise in the southwest Atlantic Ocean. Mar. Geol., 58: 35--58. The Rio Grande Rise, a major aseismic rise in the western South Atlantic Ocean, is composed of two portions with distinct morphologies and geological histories. The western portion of the Rise (WRGR) is a large elliptical bulge with its crest at a mean depth of about 2000 m. Numerous guyots protrude from its main platform. The crust underlying this portion of the Rise is older than 80 m.y.B.P, and was formed at a spreading center probably situated close to the sea level. A widespread volcanic event affected this region during the Eocene and generated several volcanic islands located at the center of the present WRGR. This Eocene volcanism is responsible for the unusual high elevation of this region and for the generation of the observed guyots. Much leas is known about the eastern portion of the Rio Grande Rise (ERGR). It has a north--south trend, parallel to the present South Atlantic spreading center, is bounded by fracture zones, is in a conjugate position to part of the Walvis Ridge in the eastern South Atlantic and may represent an abandoned spreading center. INTRODUCTION
The broad structural highs observed within the oceanic basins and not associated with the occurrence of earthquakes are called aseismic rises. The anomalous topography of these rises and ridges (term used for linear features) suggests that they have a different geological history from the crust adjacent to them. Some of these features are pieces of continental crust (microcontinents) separated from the main continental masses during rifting and subsequent generation of oceanic basins, and thus have a fairly distinct structure from the adjacent oceanic crust (as for example the Orphan Knoll and Rockall Plateau; Laughton, Berggren et al., 1972a, b). Most of the aseismic rises and ridges, however, are composed primarily of basaltic rocks formed
0025-3227/84/$03.00
© 1984 Elsevier Science Publishers B.V.
36 within the oceanic realm as, for example, the Rio Grande Rise. In such cases the knowledge of their genesis is still not clear and several different theories about their origin have been proposed. The Rio Grande Rise, one of the major aseismic rises in the South Atlantic Ocean, extends between 28 ° and 34°S lat. and 28 ° and 40°W long. and separates the Brazil Basin to the north from the Argentine Basin to the south (Figs.1 and 2). Another major aseismic ridge, the Walvis Ridge, is located in the eastern South Atlantic at an approximate conjugate position at the opposite side of the Mid-Atlantic Ridge in relation to the Rio Grande Rise in the western South Atlantic (Fig.l). The early ideas on the origin of the Rio Grande Rise were proposed by Wilson (1963, 1965), Dietz and Holden (1970) and Morgan (1971), who suggested that the Rio Grande Rise and Walvis Ridge were formed as the South American and African plates moved away from a stationary mantle " h o t s p o t " located under the South Atlantic spreading center. Le Pichon and Hayes (1971) suggested that the east--west part of the Rise represents a transverse ridge formed along a fracture zone trend and that the north--south part is related to a rearrangement in the pattern of opening of the South Atlantic. Kumar (1979) suggested a common origin for the Rio Grande Rise and Walvis Ridge as products of an excess of volcanism between 100 and 80 m,y. ago in a segment of the Mid-Atlantic Ridge bounded by fracture zones. A synthesis of the tectonic evolution of the Rio Grande Rise is also presented by Barker (1983). We show in Fig.2 a generalized tectonic map for the western central South Atlantic Ocean. Note that the western portion of the Rio Grande Rise has an approximate elliptical shape; the eastern portion is approximately north-south trending. This later segment may be bounded by fracture zones. A northern fracture zone is well defined as a continuous series of east--west trending basement highs, that can be traced westward to form the southern boundary of the prominent Silo Paulo salt plateau before continuing to the coastline as the Florianopolis High (Gamboa and Rabinowitz, 1981). This northern fracture zone lies close to the same synthetic flow line as does the northern portion of the Walvis Ridge (Fig.l). The possible boundary fracture zone to the south is n o t well defined; its position is based primarily on offsets in magnetic lineations (Ladd, 1974;Cande and Rabinowitz, 1978; Rabinowitz and LaBrecque, 1979) and changes in basement elevation as observed on a few seismic profiles. In this paper we will discuss the geologic history of the Rio Grande Rise based primarily on interpretation of lines of multi- and single-channel seismic reflection data, and deep-sea drilling sites (Fig.3). Our studies suggest that the Rio Grande Rise is a complex feature composed of two major morphological units of different geologic history. The western portion of the Rise (WRGR) has resulted from widespread Eocene volcanism which uplifted the oceanic crust and created numerous oceanic islands. The eastern segment of the Rise (ERGR) may be the manifestation of a ridge-crest migration.
60=w.
5o °
40 = I .....
15%-~-
RIO
":
35=
._.
GRANDE
30=
2o =
I
I
RISE
~ . , . . . . ~
~III~
..----
4 5"S' -~r--
,,o'w.
/
4"
-
4"
~r
I 2O°
Fig. 1. General bathymetry of the central South Atlantic showing the relative position betw thetic flow lines from Ladd (1974) and Rabinowitz and LaBrecque (1979). Note that the es~ part of the Walvis Ridge and approximately parallel to the mid-ocean ridge axis, whereas the flow lines. The inset shows all geophysical track lines that were available for this study. They ( tory, R/V "Fred Moore" of the University of Texas Marine Science Institute and D/V " G l o text are shown in Fig.3. Note paucity of data across the ERGR. Solid triangles denote location
pp. 3 7 - - 3 8
I0°
,~
I
__
0"
EO°
I
t
..,"
I ~
,++
2()°E.
~ if:
"
.15os.
+++i
++"
~'''" J
A&
I
"•
i~)o
(~o
I~
~5°S. 2< 'E.
~n the Rio Grande Rise and the Walvis Ridge (after Heezen and Tharp, 1978). Synern part of the Rio Grande Rise (ERGR) is in an approximate conjugate position to ,estern part of the Rio Grande Rise ( W R G R ) is more closely parallel to the synthetic ,nsistof R / V "Vema"; "Robert D. Conrad" of Lamont-Doherty Geological ObservaLar Challenger" of the Deep Sea Drilling Project. Selected track lines discussed in the of earthquake epicenters at the mid-ocean ridge.
pp. 39--40
_60 °
-55 °
_ 50 °
_45 °
BASALTS (
]
PRECAMBRIAN
*"
ROCKS
[~
i
,,
•
,{.
.
-¢-=~'~~'~@.,,o ~-~ t~" . #
,
~
, ~,,
~ ~ ~--
~"0 ' ' "
--
~t'?~
- -
SAO PAUl PLATEAI
/'~
Oiapir
._,~
\~..~~~
Argentine
Basin
-,,o-
-60 o
~
'~
F]or i a n o p o l i ~ ~ = ~
•
-3°°
* ''''''"*%**'*'"*"*
-55 o
.50 o
_45o
Fig. 2. Structural map of the Southern Brazilian Continental Margin and adjacent ocm geology from the geologic map of South America (1971) and Folha Asuncion and Folh to the west (WRGR) and a north--south trending unit to the east (ERGR). Several gm the northern limit of the eastern segment of the Rise. The Rio Grande Fracture Zone, t~
_ 55
40 °
~:/
°
-30
°
_ 25 °
_20
°
Brazil Basin ~ '
:~
515
G~. •
~°
RIO GRANDE ;
.-
_30 °
o
~g _mE
•
. . . . . ,,H,,.s,,
g /
\
.
.
.
.
.
.
.
.
.
""~'T'°°"°Y°" I
~,.../.
• •
,
,o.o..,.~..,ooo..,.,..oo.o..,ool i 40
I °
I
I
I
~ -35
I °
I
I
I
J _30
I o
I
I
I
] -25
I o
I
I
I -20
o
dc basin showing the basement expressions o f the major tectonic features in this area. Land Curitiba (1974). Note that the Rio Grande Rise comprises two main units, an elliptical one ~ts and seamounts are present at the western end o f the Rise. An axial trough is observed at the north and northwest o f the ERGR, consists o f linear ridges and troughs.
41
59 °
272°8o
"I
'
'
'
'
'
'
I
22
-~c~Wsi6 _ ~ 2 ~ •
r'ws~-''
\
"< ~
/
E'
B
guyot
0
seamount
\
\
"
/
~
vv
•
/ ~,
t
S
"
J ....
.....
56~
~
_
~ .....
_
36 °
27 °
39 °
Fig.3. Structural map of the Rio Grande Rise showing the location of the seismic lines discussed in the text. Small crosses mark the location of DSDP Sites 21, 22, 357 and 516F. Seismic lines WSA-10, -11 and -13 are multi-channel lines, and are shown in Figs.5 and 6. All other seismic lines shown consist of single channel analog seismic data. Lines A - - A ' to O-'G' are given in Fig.4; Lines 1--1', 11--11' and II1--111' are given in Fig.8.
MORPHOLOGY RISE (WRGR)
AND BASEMENT
STRUCTURE
OF THE WESTERN
RIO G R A N D E
T h e w e s t e r n p o r t i o n o f t h e R i o G r a n d e Rise ( W R G R ) is an e l e v a t e d a r e a o f elliptical s h a p e (Fig.2). T h e c r e s t o f this p a r t o f t h e Rise has a m e a n d e p t h o f a b o u t 2 0 0 0 m. G u y o t s a n d s e a m o u n t s rise f r o m t h e c r e s t o f this f e a t u r e w i t h t h e i r t o p s e x t e n d i n g t o d e p t h s less t h a n 700 m b e l o w sea level. T h e s e g u y o t s are p a r t i c u l a r l y c o m m o n in t h e c e n t r a l region of the W R G R (Fig.2) w i t h a large isolated g u y o t o b s e r v e d a t t h e s o u t h e a s t b o u n d a r y o f t h e Rise. T h e W R G R is also incised b y several c a n y o n s t h a t s t a r t in t h e c e n t r a l high a r e a a n d t r e n d n o r t h t o w a r d s t h e Brazil Basin (Fig.2). We do n o t h a v e t h e d a t a c o n t r o l t o d e t e r m i n e if similar c a n y o n s t r e n d t o the s o u t h t o w a r d s t h e A r g e n t i n e Basin. T h e o b s e r v e d c a n y o n s c u t several h u n d r e d m e t e r s i n t o t h e s e d i m e n t s a n d m a y a t t a i n 5 - - 2 0 k m in w i d t h (McDoweU et al., 1977). T h e origin o f such c a n y o n s was discussed b y J o h n s o n a n d Peters ( 1 9 7 9 ) , w h o suggested t h a t t h e s e f e a t u r e s w e r e first f o r m e d d u r i n g t h e late C r e t a c e o u s or early T e r t i a r y w h e n p o r t i o n s o f t h e Rise w e r e a t or a b o v e sea level.
42 Basement in the WRGR shows two basic characteristics: it is broken by guyots and seamounts protruding above the main platform and by seamountlike structures buried unde r the sedimentary sequence above the Rise; and is fairly s mo o th in areas away f r om the volcanic structures, being tilted away only from the higher central parts of the Rise (Figs.3 and 4). Volcanic structures, such as seamounts and guyots, are the most c o m m o n features defining the b o u n d a r y of the western Rio Grande Rise (see profiles E--E', B--B' and C--C' in Fig.4). At other places, a major scarp defines the b o u n d a r y o f the Rise (see profiles G--G' and F--F', Fig.4). SEISMIC STRATIGRAPHY Three lines of multi-channel seismic (MCS) data were obtained over the central high area of the western part of the Rise (WSA-10, -11 and -13, Fig.3 and Fig.5, inset). These seismic measurements were collected aboard the University of Texas Marine Science Institute R/V " F r e d Moore". The MCS lines were recorded 12-fold and the interval velocities discussed below were obtained f r o m the velocity analyses using the semblance technique of Taner and Koehler (1969). Two of the lines (WSA-11 and WSA-13) were shot over the areas o f relatively s m o o t h basement; line WSA-10 was n o t only shot over an area where basement is highly faulted but also crossed one of the largest guyots o f the Rise. We have identified several seismic sequences on these lines which demonstrate that the guyots are younger features than the basem e n t th at forms the WRGR. In the areas o f s m o o t h basement over the main platform of the Rise, five seismic sequences can be defined (Figs.5 and 6). Sequence I corresponds to acoustic basement and is characterized by a high-amplitude reflector at its top. This sequence has a strong impedance contrast between it and the sequence above and will be shown to be representative of basaltic basement rock. The few interval-velocity values calculated for the basement sequence yield an average value of 4.3 km s-1. Barker et al. {1983} pointed out the resemblance between the reflectors within this sequence and the "dipping reflectors" believed to be intercalations of basaltic lavas with shallow marine of subaerial sediments and considered to represent an initial phase of volcanism associated with a subaerial spreading center (Talwani and Udintsev, 1976; Hinz, 1981; Mutter et al., 1982; Roberts, Schnitker et al., 1982). Sequence II is, in general, weakly stratified in the frequency range of recording (acoustically transparent) and is t runcat ed at its top by a band of high-amplitude reflectors. This sequence has an average interval velocity of 3.2 km s -~.
Fig.4. Seismic reflection profiles across the western portion of the Rio Grande Rise. Arrows locate the structural boundaries of the western portion of the Rise as seen in Figs.2 and 3. Note that steep scarps and seamounts are common features defining this boundary.
43 A (s)
c~c
_
c./c _
3
i ,-,30 Km
B(NW)
(SE)B °
A' (N) -
C (w)
C'(E) ii
I
I
=3
V.E.= 22:1
.
.
.
•
i . i , , , i 5~o - R e ,6 i o . , . . . . . . . . . •
.
.,3_
i
'"t
,
D (s)
:,
D'(w~
I-
I , i '
3-
~
i
I E (N) --Iv31-oq
E ~(s) ~
~
P05
~
'~
-r-
F (w)
3 5
FZ(E)
G
(sw)
3 _ ~ ~
G~
(NE)
44 Sequence III has many m o u n d e d reflection patterns with a more weakly stratified material in the upper portions of the sequence. Sequence III thins away from the guyots, where the mounds are most pronounced, towards the deeper parts of the Rise where no mounds are observed. This sequence has an average interval velocity of 2.35 km s -~. The upper boundary of this sequence is a high-amplitude reflector which seems to truncate the underlying reflectors, thus suggesting a n unconformity. Sequence IV has continuous, high-amplitude reflectors. Sequences IV and V cannot be separated on the basis of measured interval velocities that vary from 2.08 to 1.59 km s -~. Line WSA-10 joins line WSA-11 at the base of the slope of one of the largest guyots on the Rio Grande Rise (Figs.3, 5 and 6). The basal sequences observed over the main platform of the Rise (on lines WSA-11 and -13) exist at line WSA-10 over the guyot. The acoustically transparent sequence observed above basement on Lines WSA-11 and WSA-13 (sequence II) can be continuously followed upslope of the guyot, based on seismic characteristics and interval velocity. Above sequence II, a thick sequence of steeply dipping reflectors is observed at the northeast side of the guyot that merges with the mounded sequence present over the platform of the Rise. A pronounced surface of unconformity truncates the dipping reflectors. LITHOLOGIES Four DSDP sites were drilled on the Rio Grande Rise (Maxwell, Von Herzen et al., 1970; Supko, Perch-Nielsen et ah, 1977; Barker, Carlson et al., 1981; Fig. 3). Only Site 516F was continuously cored. We have correlated the results of Site 516F with the seismic reflection data (Figs.5 and 6). The sedimentary columns indicate that after the formation of the crust of the western part of the Rio Grande Rise, pelagic sedimentation prevailed. Limestones and marly limestones of pelagic nature were deposited above basement from Coniacian/Santonian up to the Middle Miocene. By the beginning of the Middle Eocene a drastic change in the sedimentation pattern occurred and large amounts of terrigenous material deposited. Ash layers, turbidites with volcanic fragments and volcanic breccia were deposited above the previously deposited sediments. By the Late Eocene the terrigenous influx ceased and pelagic sedimentation prevailed again in the region. Basement rocks drilled at Site 516F over the main platform of the western Rio Grande Rise consist of tholeiitic basalts, mildly LREE-enriched, presenting a trace-element chemistry characteristic of transitional mid-ocean ridge basalts {Thompson et al., 1983; Weaver et al., 1983). However, rocks which were dredged earlier from the escarpments of the guyots, towering over the platform of the Rise, yielded an alkaline basalt suite characteristic of oceanic islands and seamounts and distinct from ocean-ridge tholeiites (Fodor et al., 1977).
1 . 0 ~
A
-
~ ' ~ ' ~ - ~ -
wsa
~.-.~~
guyot
go
~
WSA 13 : wsa
AI
Main platform of the WRGR
~./c II
,
,
,
,
- 1.0
/
/
"*--
30 00
-
~
13
~
'
.
,
~
~
1.0_
~
_
,
_
t~
~
~
~
~
~
~,
~
' '
,
Mltt~|
,,oo ~
~
...................
" ~ " ° ~ " ' ~ = " " *"~
u.,,
',,,,, E ....
o, L.,. o,,..o..
¢hQlk Qnd
11
....
,',',', , , ,
,~o . . .
400
~oo/, . : . j . , ~ : , , ~ , : : . . . . . I ] ....... ~. . . . . . . .
' ~oo
',
,,
, ,
'-~ ,
.
~ ........... ~" ~ • :~.t •
~)~'~ .. ~
~'~'-"~
~
~
c~.~:~
516
" ~'::,~. ~
SITE
~
~2~--~.~
P
I
11
'
WSA
cglcg,.v, o., of Holoceal to
Fig. 5. Multi-channel lines W S A - 1 0 , W S A - 1 1 and W S A - 1 3 shot across the s u m m i t and slope o f o n e o f the largest g u y o t s o f the R i o Grande Rise. L o c a t i o n s o f lines in Fig. 3. N o t e that the buried m o u n d s observed along the platform o f the Rio Grande Rise (line WSA11) can be traced c o n t i n u o u s l y t o w a r d s the g u y o t where t h e y merge w i t h the layers dipping a w a y f r o m the volcanic core. The m o u n d s f o r m a w e d g e that is thicker close to t h e g u y o t and thins t o w a r d s the deeper parts o f the Rise. This o c c u r r e n c e and distribution o f the m o u n d s suggest that these features represent s l u m p e d material derived f r o m the v o l c a n i c structures. Line A--A' is a single-channel ref l e c t i o n profile. T h e m o u n d s o n line A--A' were t h o u g h t to be volcanic b a s e m e n t prior t o the a c q u i s i t i o n o f t h e m u l t i c h a n n e l seismic data. N o t e that the unit n u m b e r s in D S D P Core 5 1 6 F are n o t the same as the seismic s e q u e n c e numbers given in Fig.6. The d o t s o n W S A - 1 3 d e l i m i t t h e insert to the right.
--
w s a
$1te516
0 f313m
I 0o
ol
49 WSA, 13 -10
W S A
guyot
~, -10 k m
IV& V
:~.~-
_. ~ -
~
mounds
" --~:'--
--~--~~
WSA-11 Main platform of the WRGR
~
WSA-10
'*
-
--3.0
¢~
WSA-11
_ _~-~_~, ~--
~, ;,,---~,_ -~-~:-
~=
~
~ -
-'~ '
velocities
~
s
_
.~ :
_
~
20
c
s -~
l
-13
~
=
_
,~
SITE 516
S
_
'
in km
WSA 11 i '
mounds
- ~._~.~.~....
-
~
N
-~.~
~
.
,
- 1 0 k m
.
-2.o
.
.
~ - - - - - - ~
-30
¢
-15 196
-
-
2-13
t59 '~
- ~ _4 ~ - 4 ~ -~., ze2 ~---~.~-
4.00 -
k69
~
--JJ~--~eF.--;---'~5
:-
2~50
2.as 3.30
t93 ___
-
205
-;--:__:_
187 204
1.67 ~----l~-~----..~t=o
I
~ - - ~ - - - ~ ~ - : ~ - : ~ _ ~ t " o 6 q_~w
;
velocities
~§8 ¢.41
in km s-1
3SS -
--~9~ 42~
2S4 -26
262 -
-~3 ;
~?e~
: ,,2#
ZOO
45S
-
"
--30 "
~)
~:
Fig.6. Tracings s h o w i n g the interp]:etation and v e l o c i t y analyses o f lines W S A - 1 0 , WSA-11 a n d W S A - 1 3 . L o c a t i o n o f lines in :~Fig.3. N o t e t h a t the same sequences observed over the m a i n p l a t f o r m o f the Rise can be c e n t i n u o u s l y f o l l o w e d ups]ope o f the g u y o t . T h e s t e e p l y d i p p i n g layers observed at line W S A - 1 0 merge w i t h the m o u n d s observed over the platf o r m of the Rise, indicating t h a t ! t h e mounds are probably slump deposits of material
originated at the volcanic structure. Note also that the mounds at the base of sequence III disappear towards the north away ~ o m the guyots on line WSA-13. The velocity structure derived from the multi-channel seismic semblance plots agrees, in general, with the sequences defined by the seismic reflection character.
50 EVIDENCE FOR EOCENE VOLCANISM The cores from Site 516F (Barker, Carlson et al., 1981) and 357 (Supko, Perch-Nielson et al., 1977; Zimmerman, 1977) show that during the Middle Eocene m a n y turbidite layers containing volcanic debris were deposited over the western part of the Rio Grande Rise. No such turbidites were observed elsewhere in the sedimentary sequence at Sites 516F and 357. Ash layers indicating subaerial volcanism are common throughout the Middle Eocene section at Site 516F. These ash layers contain fresh euhedrat biotites which yielded a K-Ar date of 47 m.y.B.P. (Bryan and Duncan, 1983). At Site 516F a large slump deposit (~ 15 m thick) of Cretaceous sediments occurs at the base of the Middle Eocene sequence. Below this slump layer, no turbidite or ash layers occur; however, they are abundant above this allocthonous block. This slump deposit was probably displaced from its original position by the uplift of the crust related to the formation of the volcanic structures that yielded the turbidite and ash. At Site 357, located 36 km from Site 516F, a layer of fresh volcanic breccia occurs within the Eocene calcareous sequence. This layer of volcanic breccia shows coarse fragments at the b o t t o m fining upwards gradationally. The layer also contains red algae, gastropod shells and large foraminifera. The granulometric evidence found in this deposit suggests that it was deposited as a single event, probably a slump (Supko, Perch-Nielsen et al., 1977). The organic fragments indicate shallow water components coming from the adjacent high peaks (Fodor and Thiede, 1977; Supko, Perch-Nielsen et al., 1977). Thin turbidite layers containing volcanic material occur above and below the breccia layer. This breccia layer probably represents one of the major volcanic events during the Middle Eocene. Additional evidence for the Eocene volcanism at the WRGR comes from the study of the seismic lines and their correlation with the sequence drilled at Sites 516F and 357. The multi-channel lines on the western part of the Rio Grande Rise show mound-like features which indicate a change in the depositional processes over the WRGR (sequence III, Fig.5). The velocity analyses of MCS data have shown that the mound-like sequence III correlates well with the Eocene turbidites and associated ash layers drilled at Site 516F (DSDP Unit IV, Fig.5). This sequence, as noted earlier, is quite pronounced near the volcanic peaks and can be continuously followed from the location of Site 516F towards the slopes of the volcanic structures. The mound-like features gradually disappear towards the deeper water. These mounds are interpreted to be slump and turbidity current accumulations generated at the slopes of the volcanic islands. In the original interpretation of the single channel seismic data, the top of this sequence in areas close to the guyots was considered to be the top of the oceanic basement. The new MCS data show, however, that a thick sedimentary sequence exists below these mounds. The observations above indicate that the guyots are the source for the volcanic debris mixed with the turbidites. The occurrence of these layers with the turbidites and slump deposits suggest that volcanism triggered the
51 gravity flows down the slopes of the volcanoes. Layers containing reefal debris with calcareous algae, bryozoans and large foraminifera also occur within the Middle Eocene sequence containing the turbidites and ash layers and thus indicating a nearby shallow-water environment. ORIGIN AND SUBSIDENCE HISTORY OF THE WESTERN PORTION OF THE RIO GRANDE RISE Site 516F is situated at about 22 km to the northeast of a large g u y o t (Fig.3), This site is in water depths of 1328 m and basement was drilled at 1250 m below the seafloor. Thiede (1977), under the assumption that aseismic ridges similar to the Rio Grande Rise subsided at the same rate as the surrounding normal oceanic basement, used DSDP sites to examine the subsidence history of the Rio Grande Rise. He concluded by backtracking paleodepth information from Sites 21 and 357 and from dredge samples, that the Rise was a large island towering 2 km above sea level during Santonian--Campanian times and t h a t it has slowly submerged since that time. The basement at Site 516F is of Santonian/Coniacian age (~85 m.y.B.P.), as determined from the sediments overlying it, and by radiometric dates (Musset and Barker, 1983). These dates are in agreement with the age predicted for the oceanic crust of this region based on the identified magnetic anomalies and rates of opening of the South Atlantic (Ladd, 1974; Cande and Rabinowitz, 1978; Rabinowitz and LaBrecque, 1979). Basement is now at 2578 m below sea level at Site 516F. If we assume that these rocks were formed at ~ 8 5 m.y. ago and use the procedure for backtracking from present basement elevations, as described by Parsons and Sclater (1977), and correct for sediment loading, we find the basement should have been formed at ~ 2 km above sea level, in agreem e n t with Thiede's (1977) results. However, the seismic data and drilling results strongly suggest t h a t the observed guyots and seamounts are younger features (Eocene) in relation to the basement (Santonian/Coniacian). This Eocene volcanism must have influenced the subsidence of the Rise and thus prevented its subsidence history from following the normal subsidence curve expected for oceanic crust since the time of the crust's formation. We suggest here that although the original ~ 8 5 m.y. ocean crust was formed at a high elevation (close to sea level), it was n o t formed at the ~ 2 km above sea level, as suggested by Thiede (1977), but was formed below sea level and was rejuvenated in the Eocene. Milliman (1983) reported that the sediments directly overlying the basalts at Site 516F indicate shallow-water environmerits, close to sea level (possibly less than 20 m water depth). Further, there i s n o evidence at either of the deep sites (357 and 516F) for the basem e n t having been exposed subaerially. Based on the seismic and drilling data, an evolutionary model for the central portions of the WRGR can be developed as shown in Fig.7. The western portion of the Rio Grande Rise was formed at a spreading center at unusually high elevations (close to sea level) during Santonian/Coniacian
52
5 - Present
S.I. mentation
Evolutionary model for
~
the guyots and adjacent
~
a r e a s of the central platform of the WRGR
4-
J
<
Late Eocene/Early Oligocene wave erosion leveledoff the island
3-
Middle Eocene
2 -
¢
S.J.
osh ~ wave erosion ,~ " ~ _ _ S I .// "-.~.~..~,,~ I ur bi d i t y currents P/ ~ " ~ ' X ~ I Um~P-s turbidit e5
r - ~ "k--/'~
Western part of the Ri0(WRGR)Gronde Rise
~
•
/
~
__
J
"/
7/
-._dumps - -
~-
~ " ' - , ~
~1, I
I - Early Middle Eocene --~ ~ ~-
S. I
depositionof
~
-
==
s,omped material
S.I.
-]
T pelagic sedimentation
Upp--~r Cretaceous to Lower Middle'Eocenepelogi~sed iments. • . ,, , ,: , , , = Upper Cretaceous crust begins to be heated end to bulge
Fig.7. Evo|utionazy mode] for the guyots and adjacent areas of the centra] platform of the WRGR based on seismic and drilling data. Not drawn to scale. times (Barker, Carlson et El., 1981). Pelagic sedimentation prevailed over this cooiLng and subsiding crust. During the Eazly Midd]e Eocene, a thermal anomaly affected the crest of the western Rio Grande Rise creating a bulge and the commencement of the present morphology of the western Rio Grande Rise. SeverEl volcanic islands were formed mainly at the center of thLs thermal anomaly and numerous smaller volcanic structures (seamounts) pierced through the pre-existing crest in many places (stages 1 and 2 Ln Fig.?). A t stage 3 the crest at the summit of this broad rise was further tilted and faulted during the formation of large volcanic structuxes. These crustal movements also affected the pelvic sediments previously deposited. In the areas between major volcanic structures, basement was affected by intense faulting and the pre-exist£ng sediments were tilted as a result of the differentia] movements of the basement blocks. The pre-existing sediments were cfisplaced by slumps and deposited over the Rise's main platform (stages 2 and
53 3 in Fig.7). After the volcanic structures reached subaerial conditions and formed volcanic islands, they were eroded by the sea. Active volcanism in these islands yielded large amounts of pyroclastic material. The finer ash expelled by the eruptions settled over the main platform of the Rise and the coarser material was deposited closer to the volcanic pillar, prograding outward and forming a platform under the existing erosional level (wave base?), as seen at stage 3 (Fig.7). Earthquakes associated with the volcanic activity generated turbidity flows whose sediments were deposited over the main platform of the Rise in close association with the ash deposits. The volcanic activity eventually ceased, erosion by wave action proceeded to level off the island, and the area began to subside (stage 4, Fig.7). After the subsidence of the volcanoes, pelagic sedimentation prevailed again over the entire region (stage 5, Fig.7). Similar evolution is proposed by Barker (1983) who presents a detailed subsidence curve for this portion of the Rio Grande Rise, based on the results of DSDP Leg 72. THE EASTERN
SEGMENT
O F T H E RIO G R A N D E
RISE (ERGR)
The E R G R is a b o u t 600 km long and has a north--south trend, parallel to the trend of the Mid-Atlantic Ridge axis. It is b o u n d e d to the north by a fracture zone; it may also be b o u n d e d by a fracture zone to the south. Transverse profiles across this segment of the Rio Grande Rise show a triangular cross-section (Fig.8). At the northern portion of this segment a 10 km wide axial trough is observed. Although only a few seismic profiles have been obtained across this part of the Rise, this axial trough can be mapped for at least 30 kin. The trough is filled by at least 800 m of sediments and the basement relief from trough to crest is on the order of 1 km (profile I--I', Fig.8). The trough is n o t observed in the southern extent of this segment (profile II--II', Fig.8). The E R G R is separated from the WRGR by a narrow and constrained abyssal plain in which depths exceed 4400 m. A few seamounts protrude from the plain. The seismic records (Fig.8) across the plain show flat-lying well-stratified ponded sediments suggesting that gravity-displaced deposition floors it. The E R G R and W R G R may, perhaps, be morphologically continuous at their northern extremities following the strike of the Rio Grande Fracture Zone (Figs.1 and 2). The E R G R is bounded by the same synthetic flow lines and is of similar distance from the present mid-ocean ridge axis as a major part of the Walvis Ridge in the eastern South Atlantic (Fig.l). This strongly suggests that the eastern Rio Grande Rise and its conjugate portion on the Walvis Ridge were formed at the same time by the same processes. There is no feature conjugate to the W R G R in the eastern South Atlantic. We believe that these observations, together with the major morphological differences between the W R G R and the E R G R , clearly demonstrate that these two segments are distinct features with independent origins.
> o
0
0
¢-
~
o--
•
5~'~:i
~ .... '
II , - wRGR
e
WRGR
III
"'
~.iil
F
,,r,~ ~~--"~IL __
IIr
I1'
on
55 DISCUSSION The Rio Grande Rise is a complex feature, composed basically of two distinct units of different geological histories. The WRGR is an area of elevated basement, elliptical in shape, from which numerous guyots and seamounts protrude. The exact depth at which the original Cretaceous crust of this area was formed is still n o t known. However, there is no conclusive geological evidence to indicate that it was above sea level during its early formation. The present morphology of the WRGR was greatly influenced by widespread volcanism during the Eocene. This is well d o c u m e n t e d by seismic and deepsea drilling data. It is suggested here that during the Eocene a thermal anomaly developed underneath the crustal area corresponding to the WRGR and that this thermal anomaly was responsible for the generation of several oceanic islands and seamounts and for the increased elevation of this part of the Rio Grande Rise, We also propose that the canyons observed on the main platform of the WRGR were formed during the Eocene uplift of the Rise. Thus far, the origin of these canyons has been enigmatic (McDoweU et al., 1977; Johnson and Peters, 1979). In contrast to the WRGR, the ERGR has a north--south trend, parallel to the trend of the present Mid-Atlantic Ridge. The E R G R is believed to be bounded to the north and south by fracture zones, and is in an approximate conjugate position to part of the Walvis Ridge in the eastern South Atlantic Ocean. The two structural elements of the Rio Grande Rise are separated by a deep abyssal plain. The ERGR ascends gradually from this plain, whereas the contact between the plain and the WRGR is marked by steep fault scarps. Intraplate Eocene volcanism, as postulated here for the western portion of the Rio Grande Rise, also occurred extensively in the eastern South Atlantic. During the Eocene, numerous volcanic islands were built at the southwestern portion of the Walvis Ridge. These islands were later eroded, then subsided and now form the G u y o t Province of the Walvis Ridge (Connary, 1972). DSDP Site 359 was drilled over one of these guyots and confirmed its Middle Eocene age (Supko, Perch-Nielsen et al, 1977). However, the Eocene volcanism at the Rio Grande Rise and Walvis Ridge occurred at independent crust areas. The WRGR and the G u y o t Province of the Walvis Ridge are n o t contiguous features in paleoreconstructions. The G u y o t Province of the Walvis Ridge, as mapped by Connary (1972), is centered on crust varying in age from approximately 40 to 68 m.y.B.P., whereas the oceanic islands and seamounts formed by intraplate volcanism during the Middle Eocene at the WRGR is on crust older than 80 m . y . B . P . Uplift of pieces of oceanic crust related to off-axis volcanism has also been documented in other regions. For example, the Bermuda Rise in the Fig.8. Seismic profiles across the abyssal plain that separates the two portions of the Rio Grande Rise. Arrows locate the boundaries of the Rise (see locations in Fig.3). Note that in profiles I and H the sediments in the western portion of the Rise are ponded against a seamount in the abyssal plain. Note also the triangular cross-section and axial valley observed in ERGR in profile I--I'.
56 w e s t e r n central A t l a n t i c O c e a n was similarly f o r m e d during the E o c e n e b y i n t r a p l a t e v o l c a n i s m w i t h i n C r e t a c e o u s c r u s t ( T u c h o l k e a n d V o g t , 1 9 7 9 ) ; the Cape Verde Rise has b e e n s h o w n b y L a n c e l o t a n d S e i b o l d ( 1 9 7 7 ) t o consist o f Jurassic o c e a n i c c r u s t u p l i f t e d b y t h e events associated w i t h t h e f o r m a t i o n o f the Cape V e r d e Islands during the Miocene. T h e s e o c c u r r e n c e s suggest t h a t i n t r a p l a t e v o l c a n i s m m a y be a c o m m o n p h e n o m e n a t h r o u g h geologic time. ACKNOWLEDGEMENTS T h e d a t a s h o w n in this p a p e r w e r e c o l l e c t e d on cruises of L a m o n t - D o h e r t y Geological O b s e r v a t o r y research vessels " V e m a " and " R o b e r t D. C o n r a d " , t h e U n i v e r s i t y o f T e x a s Marine Science I n s t i t u t e ( U T M S I ) , research vessel " F r e d M o o r e " a n d the D e e p Sea Drilling P r o j e c t D / V " G l o m a r C h a l l e n g e r " . We wish t o t h a n k the officers, c r e w a n d scientists a b o a r d t h e s e vessels f o r their assistance in g a t h e r i n g t h e data. We are p a r t i c u l a r l y g r a t e f u l t o Drs. J o h n L a d d , R i c h a r d Buffler a n d T o m Shipley, the c h i e f scientists a b o a r d U T M S I ' S R / V " F r e d M o o r e " . A r v o B u c k , J o h n Diebold and R o b e r t H o u t z r e v i e w e d t h e m a n u s c r i p t and m a d e valuable suggestions f o r its i m p r o v e m e n t . Financial s u p p o r t f o r this s t u d y was p r o v i d e d b y N a t i o n a l Science F o u n d a tion G r a n t s OCE 7 2 - 6 0 4 2 6 A 0 4 (Office of I D O E ) , D E S 7 1 - 0 0 2 1 4 , OCE 772 5 9 9 2 a n d OCE 7 9 - 1 9 3 8 9 (Office of O c e a n o g r a p h y ) , a n d b y Office o f N a v a l R e s e a r c h C o n t r a c t N 0 0 0 1 4 - 7 5 - C - 0 2 1 0 . Financial s u p p o r t f o r L.A.P. G a m b o a was p r o v i d e d in p a r t b y C o o r d e n a n c ~ o do A p e r f e i c o a m e n t o de Pessoal de Nivel S u p e r i o r (CAPES). This is L a m o n t - D o h e r t y G e o l o g i c a l O b s e r v a t o r y C o n t r i b u t i o n No. 3 6 0 2 . REFERENCES Barker, P.F., 1983. Tectonic evolution and subsidence history of the Rio Grande Rise. In: P.F. Barker, R.L. Carlson et al., Initial Reports of the Deep Sea Drilling Project, v. 72. U.S. Govt. Printing Office, Washington, D.C., pp. 953--976. Barker, P.F., Carlson, R.L. et al., 1981. Deep Sea Drilling Project Leg 72: Southwest Atlantic paleocirculation and Rio Grande Rise tectonics. Geol. Soc. Am. Bull., 92: 294--309. Barker, P.F., Buffer, R.T. and Gambos, L.A.P., 1983. A seismic reflection study of the Rio Grande Rise. In: P.F. Barker, R.L. Carlson et al., Initial Reports of the Deep Sea Drilling Project, v. 72. U.S. Govt. Printing Office, Washington, D.C., pp. 499--517. Bryan, W.B. and Duncan, R.A., 1983. Age and provenance of clastic horizons from Hole 516F. In: P.F. Barker, R.L. Carlson et al., Initial Reports of the Deep Sea Drilling Project, v. 72. U.S. Govt. Printing Office, Washington, D.C., pp. 475--477. Cande, S.C. and Rabinowitz, P.D., 1978. Magnetic anomalies of the continental margin of Brazil. Am. Assoc. Pet. Geol., Offshore Brazil Map Set. Connary, S.D., 1972. Investigations of the Walvis Ridge and environs, Ph.D. thesis, Columbia University, Palisades, N.Y., 228 pp. Dietz, R.S. and Holden, J.C., 1970. Reconstruction of Pangea: breakup and dispersion of the continents, Permian to present. J. Geophys. Res., 75: 4939--4956. Fodor, R.V. and Thiede, J., 1977. Volcanic breccia from DSDP Site 357: Implications for the composition and origin of the Rio Grande Rise. In: P.R. Supko, K. Perch-Nielsen et al., Initial Reports of the Deep Sea Drilling Project, v. 39, U.S. Govt. Printing Office, Washington, D.C., pp.537--543.
57 Fodor, R.V., Husler, J.W. and Keil, K., 1977. Petrology of basalt recovered during D S D P Leg 39B. In: P.R. Supko, K. Perch-Nielsen et al.,InitialReports of the Deep Sea Drilling Project, v. 39. U.S. Govt. Printing Office, Washington, D.C., pp.513--523. Folha Asuncion (SG-21) and Folha Curitiba (SG-22), 1974. Ministry of Mines and Energy, Brazil. Gamboa, L.A.P. and Rabinowitz, P.D., 1981. The Rio Grande Fracture Zone in the western South Atlantic and its tectonic implications. Earth Planet. Sci. Lett., 52: 410--418. Heezen, B.C. and Tharp, M., 1978. The b a t h y m e t r y of the South Atlantic Ocean. In: General Bathymetric Charts of the Oceans (GEBCO) (5th ed.), Can. Hydrogr. Service, Ottawa, Ont. Hinz, K., 1981. A hypothesis on terrestrial catastrophes: wedges of very thick oceanward dipping layers beneath passive continental margins. Geol. Jahrb., Reihe E, 22: 3--28. Johnson, D.A. and Peters, C.S., 1979. Late Cenozoic sedimentation and erosion of the Rio Grande Rise. J. Geol., 87: 371--392. Kumar, N., 1979. Origin of "paired" aseismic rises, Ceara and Sierra Leone Rises in the Equatorial, and the Rio Grande Rise and Walvis Ridge in the South Atlantic. Mar. Geol., 30: 175--191. Ladd, J.W., 1974. South Atlantic sea floor spreading and Caribbean tectonics. Ph.D. thesis, Columbia University, Palisades, N.Y., 200 pp. Lancelot, Y. and Seibold, E., 1977. The evolution of the Central Northeastern Atlantic - Summary of results of DSDP Leg 41. In: Y. Lancelot, E. Seibold et al., Initial Reports of the Deep Sea Drilling Project, v. 41. U.S. Govt. Printing Office, Washington, D.C., pp.1215--1245. Laughton, A.S., Berggren, W.A. et al., 1972a. Site 111. In: A.S. Laughton, W.A. Berggren et al., Initial Reports of the Deep Sea Drilling Project, v. 12. U.S. Govt. Printing Office, Washington, D.C., pp.33--159. Laughton, A.S., Berggren, W.A. et al., 1972b. Sites 116 and 117. In: A.S. Laughton, W.A. Berggren et al., Initial Reports of the Deep Sea Drilling Project, v. 12. U.S. Govt. Printing Office, Washington, D.C., pp.395----671. LePichon, X. and Hayes, D.E., 1971. Marginal offsets, fracture zones, and the early opening of the South Atlantic. J. Geophys. Res., 76: 6283--6293. Mapa Geologico da America do Sul, 1964. Ministry of Mines and Energy, Brasilia, Brazil. Maxwell, A.E., Von Herzen, R. et al., 1970. Initial reports of the Deep Sea Drilling Project, v. 3. U.S. Govt. Printing Office, Washington, D.C. McDowell, S., Kumar, N., Jacobi, R.D., Johnson, D.A. and Bunce, E.T., 1977. Regional setting of Site 357, North flank of Rio Grande Rise. In: P.R. Supko, K. Perch-Nielsen et al., Initial Reports of the Deep Sea Drilling Project, v. 39. U.S. Govt. Printing Office, Washington, D.C., Pp.955--969. Milliman, J.D., 1983. Coniacian/Santonian depositional environments on the Rio Grande Rise as evidenced from carbonate sediments at Hole 516F. In: P.F. Barker, R.L. Carlson et al., Initial Reports of the Deep Sea Drilling Project, v. 72. U.S. Govt. Printing Office, Washington, D.C., pp. 395--397. Moody, R., Hayes, D.E. and Connary, S., 1979. Bathymetry of the continental margin of Brazil. Am. Assoc. Pet. Geol., Offshore Brazil Map Ser., Catalog No. 832, Tulsa, Okla. Morgan, W.J., 1971. Convection plumes in the lower mantle, Nature, 230: 42--43. Mussett, A.E. and Barker, P.F., 1983.4°Ar/39Ar age spectra of basalts, D S D P Site 516F. In: P.F. Barker, R.L. Carlson et al.,InitialReports of the Deep Sea DrillingProject, v. 72. U.S. Govt. Printing Office, Washington, D.C., pp. 467--470. Mutter, J.C., Talwani, M. and Stoffa, P.L., 1982. Origin of seaward-dipping reflectorsin oceanic crust off the Norwegian margin by "subaerial sea-floor spreading". Geology, 10: 353--357. Parsons, B. and Sclater, J.C., 1977. A n analysis of the variation of ocean floor bathymetry and heat flow with age. J. Geophys. Res., 82: 803--827. Rabinowitz, P.D. and LaBrecque, J., 1979. The Mesozoic South Atlantic and evolution of its continental margins. J. Geophys. Res., 84 : 5973--6002.
58 Rabinowitz, P.D., Delach, M., Truchan, M. and Lonardi, A., 1978. Bathymetry of the Argentine continental margin and the adjacent areas, Am. Assoc. Pet. Geol., Offshore Argentine Map Ser., Catalog No. 828, Tulsa, Okla. Roberts, D.G., Schnitker, D. et al., 1982. Leg 81 drill margin, Rockall Plateau. Geotimes, 27 : 21--23. Supko, P.R., Perch-Nielsen, K. e t al., 1977. Initial reports of the Deep Sea Drilling Project, v. 39. U.S. Govt. Printing Office, Washington, D.C., p.1139. Talwani, M. and Udintsev, G., 1976. Tectonic synthesis. In: M. Talwani and G. Udintsev et al., Initial Reports of the Deep Sea Drilling Project, v. 38. U.S. Govt. Printing Office, Washington, D.C., pp.1213--1242. Taner, M.T. and Koehler, F., 1969. Velocity spectra digital computer deviation and applications of velocity functions. Geophysics, 34: 859--881. Thiede, J., 1977. Subsidence of &seismic ridges: Evidence from sediments on Rio Grande Rise (Southwest Atlantic Ocean). Bull. Am. Assoc. Pet. Geol., 61: 929--940. Thompson, G., Humphris, S. and Shilling, J.G., 1983. Petrology and geochemistry of basaltic rocks from the Rio Grande Rise, South Atlantic, Deep Sea Drilling Project Leg 72, Hole 516F. In: P.F. Barker, R.L. Carlson et al., Initial Reports of the Deep Sea Drilling Project, v. 72, U.S. Govt. Printing Office, Washington, D.C. Tucholke, B.E. and Vogt, P.R., 1979. Western North Atlantic: Sedimentary evolution and aspects of tectonic history. In: B.E. Tucholke, P.R. Vogt et al., Initial Reports of the Deep Sea Drilling Project, v. 43. U.S. Govt. Printing Office, Washington, D.C., pp.791-825. Weaver, B.L., Marsh, N.G. and Tarney, J., 1983. Trace element geochemistry at Site 516F, Rio Grande Rise, DSDP Leg 72. In: P.F. Barker, R.L. Carlson et al., Initial Reports of the Deep Sea Drilling Project, v. 72. U.S. Govt. Printing Office, Washington, D.C., pp. 451--456. Wilson, J.T., 1963. Hypothesis of earth's behavior. Nature, 198: 925--929. Wilson, J.T., 1965. Submarine Fracture Zones, aseismic ridges and the International Council of Scientific Unions Line: Proposed Western Margin of the East Pacific Ridge. Nature, 207 : 907--911. Zimmerman, H.B., 1977. Clay mineral stratigraphy and distribution in the South Atlantic Ocean. In: P.R. Supko, K. Perch-Nielsen et al., Initial Reports of the Deep Sea Drilling Project, v. 39. U.S. Govt. Printing Office, Washington D.C., pp.395--406.
35
Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands
T H E E V O L U T I O N O F T H E RIO G R A N D E ATLANTIC OCEAN
LUIZ ANTONIO
PIERANTONI
CAMBOA
RISE IN T H E S O U T H W E S T
I'2 and PHILIP D. R A B I N O W I T Z ' . 2
ILamont-Doherty Geological Observatory of Columbia University,Palisades,N Y 10964 (U.S.A.) 2Department of Oceanography, Texas A&M University, College Station, TX 77843 (U.S.A.) (Accepted for publication November 7, 1983)
ABSTRACT Gamboa, L.A.P. and Rabinowitz, P.D., 1984. The evolution of the Rio Grande Rise in the southwest Atlantic Ocean. Mar. Geol., 58: 35--58. The Rio Grande Rise, a major aseismic rise in the western South Atlantic Ocean, is composed of two portions with distinct morphologies and geological histories. The western portion of the Rise (WRGR) is a large elliptical bulge with its crest at a mean depth of about 2000 m. Numerous guyots protrude from its main platform. The crust underlying this portion of the Rise is older than 80 m.y.B.P, and was formed at a spreading center probably situated close to the sea level. A widespread volcanic event affected this region during the Eocene and generated several volcanic islands located at the center of the present WRGR. This Eocene volcanism is responsible for the unusual high elevation of this region and for the generation of the observed guyots. Much leas is known about the eastern portion of the Rio Grande Rise (ERGR). It has a north--south trend, parallel to the present South Atlantic spreading center, is bounded by fracture zones, is in a conjugate position to part of the Walvis Ridge in the eastern South Atlantic and may represent an abandoned spreading center. INTRODUCTION
The broad structural highs observed within the oceanic basins and not associated with the occurrence of earthquakes are called aseismic rises. The anomalous topography of these rises and ridges (term used for linear features) suggests that they have a different geological history from the crust adjacent to them. Some of these features are pieces of continental crust (microcontinents) separated from the main continental masses during rifting and subsequent generation of oceanic basins, and thus have a fairly distinct structure from the adjacent oceanic crust (as for example the Orphan Knoll and Rockall Plateau; Laughton, Berggren et al., 1972a, b). Most of the aseismic rises and ridges, however, are composed primarily of basaltic rocks formed
0025-3227/84/$03.00
© 1984 Elsevier Science Publishers B.V.
36 within the oceanic realm as, for example, the Rio Grande Rise. In such cases the knowledge of their genesis is still not clear and several different theories about their origin have been proposed. The Rio Grande Rise, one of the major aseismic rises in the South Atlantic Ocean, extends between 28 ° and 34°S lat. and 28 ° and 40°W long. and separates the Brazil Basin to the north from the Argentine Basin to the south (Figs.1 and 2). Another major aseismic ridge, the Walvis Ridge, is located in the eastern South Atlantic at an approximate conjugate position at the opposite side of the Mid-Atlantic Ridge in relation to the Rio Grande Rise in the western South Atlantic (Fig.l). The early ideas on the origin of the Rio Grande Rise were proposed by Wilson (1963, 1965), Dietz and Holden (1970) and Morgan (1971), who suggested that the Rio Grande Rise and Walvis Ridge were formed as the South American and African plates moved away from a stationary mantle " h o t s p o t " located under the South Atlantic spreading center. Le Pichon and Hayes (1971) suggested that the east--west part of the Rise represents a transverse ridge formed along a fracture zone trend and that the north--south part is related to a rearrangement in the pattern of opening of the South Atlantic. Kumar (1979) suggested a common origin for the Rio Grande Rise and Walvis Ridge as products of an excess of volcanism between 100 and 80 m,y. ago in a segment of the Mid-Atlantic Ridge bounded by fracture zones. A synthesis of the tectonic evolution of the Rio Grande Rise is also presented by Barker (1983). We show in Fig.2 a generalized tectonic map for the western central South Atlantic Ocean. Note that the western portion of the Rio Grande Rise has an approximate elliptical shape; the eastern portion is approximately north-south trending. This later segment may be bounded by fracture zones. A northern fracture zone is well defined as a continuous series of east--west trending basement highs, that can be traced westward to form the southern boundary of the prominent Silo Paulo salt plateau before continuing to the coastline as the Florianopolis High (Gamboa and Rabinowitz, 1981). This northern fracture zone lies close to the same synthetic flow line as does the northern portion of the Walvis Ridge (Fig.l). The possible boundary fracture zone to the south is n o t well defined; its position is based primarily on offsets in magnetic lineations (Ladd, 1974;Cande and Rabinowitz, 1978; Rabinowitz and LaBrecque, 1979) and changes in basement elevation as observed on a few seismic profiles. In this paper we will discuss the geologic history of the Rio Grande Rise based primarily on interpretation of lines of multi- and single-channel seismic reflection data, and deep-sea drilling sites (Fig.3). Our studies suggest that the Rio Grande Rise is a complex feature composed of two major morphological units of different geologic history. The western portion of the Rise (WRGR) has resulted from widespread Eocene volcanism which uplifted the oceanic crust and created numerous oceanic islands. The eastern segment of the Rise (ERGR) may be the manifestation of a ridge-crest migration.
60=w.
5o °
40 = I .....
15%-~-
RIO
":
35=
._.
GRANDE
30=
2o =
I
I
RISE
~ . , . . . . ~
~III~
..----
4 5"S' -~r--
,,o'w.
/
4"
-
4"
~r
I 2O°
Fig. 1. General bathymetry of the central South Atlantic showing the relative position betw thetic flow lines from Ladd (1974) and Rabinowitz and LaBrecque (1979). Note that the es~ part of the Walvis Ridge and approximately parallel to the mid-ocean ridge axis, whereas the flow lines. The inset shows all geophysical track lines that were available for this study. They ( tory, R/V "Fred Moore" of the University of Texas Marine Science Institute and D/V " G l o text are shown in Fig.3. Note paucity of data across the ERGR. Solid triangles denote location
pp. 3 7 - - 3 8
I0°
,~
I
__
0"
EO°
I
t
..,"
I ~
,++
2()°E.
~ if:
"
.15os.
+++i
++"
~'''" J
A&
I
"•
i~)o
(~o
I~
~5°S. 2< 'E.
~n the Rio Grande Rise and the Walvis Ridge (after Heezen and Tharp, 1978). Synern part of the Rio Grande Rise (ERGR) is in an approximate conjugate position to ,estern part of the Rio Grande Rise ( W R G R ) is more closely parallel to the synthetic ,nsistof R / V "Vema"; "Robert D. Conrad" of Lamont-Doherty Geological ObservaLar Challenger" of the Deep Sea Drilling Project. Selected track lines discussed in the of earthquake epicenters at the mid-ocean ridge.
pp. 39--40
_60 °
-55 °
_ 50 °
_45 °
BASALTS (
]
PRECAMBRIAN
*"
ROCKS
[~
i
,,
•
,{.
.
-¢-=~'~~'~@.,,o ~-~ t~" . #
,
~
, ~,,
~ ~ ~--
~"0 ' ' "
--
~t'?~
- -
SAO PAUl PLATEAI
/'~
Oiapir
._,~
\~..~~~
Argentine
Basin
-,,o-
-60 o
~
'~
F]or i a n o p o l i ~ ~ = ~
•
-3°°
* ''''''"*%**'*'"*"*
-55 o
.50 o
_45o
Fig. 2. Structural map of the Southern Brazilian Continental Margin and adjacent ocm geology from the geologic map of South America (1971) and Folha Asuncion and Folh to the west (WRGR) and a north--south trending unit to the east (ERGR). Several gm the northern limit of the eastern segment of the Rise. The Rio Grande Fracture Zone, t~
_ 55
40 °
~:/
°
-30
°
_ 25 °
_20
°
Brazil Basin ~ '
:~
515
G~. •
~°
RIO GRANDE ;
.-
_30 °
o
~g _mE
•
. . . . . ,,H,,.s,,
g /
\
.
.
.
.
.
.
.
.
.
""~'T'°°"°Y°" I
~,.../.
• •
,
,o.o..,.~..,ooo..,.,..oo.o..,ool i 40
I °
I
I
I
~ -35
I °
I
I
I
J _30
I o
I
I
I
] -25
I o
I
I
I -20
o
dc basin showing the basement expressions o f the major tectonic features in this area. Land Curitiba (1974). Note that the Rio Grande Rise comprises two main units, an elliptical one ~ts and seamounts are present at the western end o f the Rise. An axial trough is observed at the north and northwest o f the ERGR, consists o f linear ridges and troughs.
41
59 °
272°8o
"I
'
'
'
'
'
'
I
22
-~c~Wsi6 _ ~ 2 ~ •
r'ws~-''
\
"< ~
/
E'
B
guyot
0
seamount
\
\
"
/
~
vv
•
/ ~,
t
S
"
J ....
.....
56~
~
_
~ .....
_
36 °
27 °
39 °
Fig.3. Structural map of the Rio Grande Rise showing the location of the seismic lines discussed in the text. Small crosses mark the location of DSDP Sites 21, 22, 357 and 516F. Seismic lines WSA-10, -11 and -13 are multi-channel lines, and are shown in Figs.5 and 6. All other seismic lines shown consist of single channel analog seismic data. Lines A - - A ' to O-'G' are given in Fig.4; Lines 1--1', 11--11' and II1--111' are given in Fig.8.
MORPHOLOGY RISE (WRGR)
AND BASEMENT
STRUCTURE
OF THE WESTERN
RIO G R A N D E
T h e w e s t e r n p o r t i o n o f t h e R i o G r a n d e Rise ( W R G R ) is an e l e v a t e d a r e a o f elliptical s h a p e (Fig.2). T h e c r e s t o f this p a r t o f t h e Rise has a m e a n d e p t h o f a b o u t 2 0 0 0 m. G u y o t s a n d s e a m o u n t s rise f r o m t h e c r e s t o f this f e a t u r e w i t h t h e i r t o p s e x t e n d i n g t o d e p t h s less t h a n 700 m b e l o w sea level. T h e s e g u y o t s are p a r t i c u l a r l y c o m m o n in t h e c e n t r a l region of the W R G R (Fig.2) w i t h a large isolated g u y o t o b s e r v e d a t t h e s o u t h e a s t b o u n d a r y o f t h e Rise. T h e W R G R is also incised b y several c a n y o n s t h a t s t a r t in t h e c e n t r a l high a r e a a n d t r e n d n o r t h t o w a r d s t h e Brazil Basin (Fig.2). We do n o t h a v e t h e d a t a c o n t r o l t o d e t e r m i n e if similar c a n y o n s t r e n d t o the s o u t h t o w a r d s t h e A r g e n t i n e Basin. T h e o b s e r v e d c a n y o n s c u t several h u n d r e d m e t e r s i n t o t h e s e d i m e n t s a n d m a y a t t a i n 5 - - 2 0 k m in w i d t h (McDoweU et al., 1977). T h e origin o f such c a n y o n s was discussed b y J o h n s o n a n d Peters ( 1 9 7 9 ) , w h o suggested t h a t t h e s e f e a t u r e s w e r e first f o r m e d d u r i n g t h e late C r e t a c e o u s or early T e r t i a r y w h e n p o r t i o n s o f t h e Rise w e r e a t or a b o v e sea level.
42 Basement in the WRGR shows two basic characteristics: it is broken by guyots and seamounts protruding above the main platform and by seamountlike structures buried unde r the sedimentary sequence above the Rise; and is fairly s mo o th in areas away f r om the volcanic structures, being tilted away only from the higher central parts of the Rise (Figs.3 and 4). Volcanic structures, such as seamounts and guyots, are the most c o m m o n features defining the b o u n d a r y of the western Rio Grande Rise (see profiles E--E', B--B' and C--C' in Fig.4). At other places, a major scarp defines the b o u n d a r y o f the Rise (see profiles G--G' and F--F', Fig.4). SEISMIC STRATIGRAPHY Three lines of multi-channel seismic (MCS) data were obtained over the central high area of the western part of the Rise (WSA-10, -11 and -13, Fig.3 and Fig.5, inset). These seismic measurements were collected aboard the University of Texas Marine Science Institute R/V " F r e d Moore". The MCS lines were recorded 12-fold and the interval velocities discussed below were obtained f r o m the velocity analyses using the semblance technique of Taner and Koehler (1969). Two of the lines (WSA-11 and WSA-13) were shot over the areas o f relatively s m o o t h basement; line WSA-10 was n o t only shot over an area where basement is highly faulted but also crossed one of the largest guyots o f the Rise. We have identified several seismic sequences on these lines which demonstrate that the guyots are younger features than the basem e n t th at forms the WRGR. In the areas o f s m o o t h basement over the main platform of the Rise, five seismic sequences can be defined (Figs.5 and 6). Sequence I corresponds to acoustic basement and is characterized by a high-amplitude reflector at its top. This sequence has a strong impedance contrast between it and the sequence above and will be shown to be representative of basaltic basement rock. The few interval-velocity values calculated for the basement sequence yield an average value of 4.3 km s-1. Barker et al. {1983} pointed out the resemblance between the reflectors within this sequence and the "dipping reflectors" believed to be intercalations of basaltic lavas with shallow marine of subaerial sediments and considered to represent an initial phase of volcanism associated with a subaerial spreading center (Talwani and Udintsev, 1976; Hinz, 1981; Mutter et al., 1982; Roberts, Schnitker et al., 1982). Sequence II is, in general, weakly stratified in the frequency range of recording (acoustically transparent) and is t runcat ed at its top by a band of high-amplitude reflectors. This sequence has an average interval velocity of 3.2 km s -~.
Fig.4. Seismic reflection profiles across the western portion of the Rio Grande Rise. Arrows locate the structural boundaries of the western portion of the Rise as seen in Figs.2 and 3. Note that steep scarps and seamounts are common features defining this boundary.
43 A (s)
c~c
_
c./c _
3
i ,-,30 Km
B(NW)
(SE)B °
A' (N) -
C (w)
C'(E) ii
I
I
=3
V.E.= 22:1
.
.
.
•
i . i , , , i 5~o - R e ,6 i o . , . . . . . . . . . •
.
.,3_
i
'"t
,
D (s)
:,
D'(w~
I-
I , i '
3-
~
i
I E (N) --Iv31-oq
E ~(s) ~
~
P05
~
'~
-r-
F (w)
3 5
FZ(E)
G
(sw)
3 _ ~ ~
G~
(NE)
44 Sequence III has many m o u n d e d reflection patterns with a more weakly stratified material in the upper portions of the sequence. Sequence III thins away from the guyots, where the mounds are most pronounced, towards the deeper parts of the Rise where no mounds are observed. This sequence has an average interval velocity of 2.35 km s -~. The upper boundary of this sequence is a high-amplitude reflector which seems to truncate the underlying reflectors, thus suggesting a n unconformity. Sequence IV has continuous, high-amplitude reflectors. Sequences IV and V cannot be separated on the basis of measured interval velocities that vary from 2.08 to 1.59 km s -~. Line WSA-10 joins line WSA-11 at the base of the slope of one of the largest guyots on the Rio Grande Rise (Figs.3, 5 and 6). The basal sequences observed over the main platform of the Rise (on lines WSA-11 and -13) exist at line WSA-10 over the guyot. The acoustically transparent sequence observed above basement on Lines WSA-11 and WSA-13 (sequence II) can be continuously followed upslope of the guyot, based on seismic characteristics and interval velocity. Above sequence II, a thick sequence of steeply dipping reflectors is observed at the northeast side of the guyot that merges with the mounded sequence present over the platform of the Rise. A pronounced surface of unconformity truncates the dipping reflectors. LITHOLOGIES Four DSDP sites were drilled on the Rio Grande Rise (Maxwell, Von Herzen et al., 1970; Supko, Perch-Nielsen et ah, 1977; Barker, Carlson et al., 1981; Fig. 3). Only Site 516F was continuously cored. We have correlated the results of Site 516F with the seismic reflection data (Figs.5 and 6). The sedimentary columns indicate that after the formation of the crust of the western part of the Rio Grande Rise, pelagic sedimentation prevailed. Limestones and marly limestones of pelagic nature were deposited above basement from Coniacian/Santonian up to the Middle Miocene. By the beginning of the Middle Eocene a drastic change in the sedimentation pattern occurred and large amounts of terrigenous material deposited. Ash layers, turbidites with volcanic fragments and volcanic breccia were deposited above the previously deposited sediments. By the Late Eocene the terrigenous influx ceased and pelagic sedimentation prevailed again in the region. Basement rocks drilled at Site 516F over the main platform of the western Rio Grande Rise consist of tholeiitic basalts, mildly LREE-enriched, presenting a trace-element chemistry characteristic of transitional mid-ocean ridge basalts {Thompson et al., 1983; Weaver et al., 1983). However, rocks which were dredged earlier from the escarpments of the guyots, towering over the platform of the Rise, yielded an alkaline basalt suite characteristic of oceanic islands and seamounts and distinct from ocean-ridge tholeiites (Fodor et al., 1977).
1 . 0 ~
A
-
~ ' ~ ' ~ - ~ -
wsa
~.-.~~
guyot
go
~
WSA 13 : wsa
AI
Main platform of the WRGR
~./c II
,
,
,
,
- 1.0
/
/
"*--
30 00
-
~
13
~
'
.
,
~
~
1.0_
~
_
,
_
t~
~
~
~
~
~
~,
~
' '
,
Mltt~|
,,oo ~
~
...................
" ~ " ° ~ " ' ~ = " " *"~
u.,,
',,,,, E ....
o, L.,. o,,..o..
¢hQlk Qnd
11
....
,',',', , , ,
,~o . . .
400
~oo/, . : . j . , ~ : , , ~ , : : . . . . . I ] ....... ~. . . . . . . .
' ~oo
',
,,
, ,
'-~ ,
.
~ ........... ~" ~ • :~.t •
~)~'~ .. ~
~'~'-"~
~
~
c~.~:~
516
" ~'::,~. ~
SITE
~
~2~--~.~
P
I
11
'
WSA
cglcg,.v, o., of Holoceal to
Fig. 5. Multi-channel lines W S A - 1 0 , W S A - 1 1 and W S A - 1 3 shot across the s u m m i t and slope o f o n e o f the largest g u y o t s o f the R i o Grande Rise. L o c a t i o n s o f lines in Fig. 3. N o t e that the buried m o u n d s observed along the platform o f the Rio Grande Rise (line WSA11) can be traced c o n t i n u o u s l y t o w a r d s the g u y o t where t h e y merge w i t h the layers dipping a w a y f r o m the volcanic core. The m o u n d s f o r m a w e d g e that is thicker close to t h e g u y o t and thins t o w a r d s the deeper parts o f the Rise. This o c c u r r e n c e and distribution o f the m o u n d s suggest that these features represent s l u m p e d material derived f r o m the v o l c a n i c structures. Line A--A' is a single-channel ref l e c t i o n profile. T h e m o u n d s o n line A--A' were t h o u g h t to be volcanic b a s e m e n t prior t o the a c q u i s i t i o n o f t h e m u l t i c h a n n e l seismic data. N o t e that the unit n u m b e r s in D S D P Core 5 1 6 F are n o t the same as the seismic s e q u e n c e numbers given in Fig.6. The d o t s o n W S A - 1 3 d e l i m i t t h e insert to the right.
--
w s a
$1te516
0 f313m
I 0o
ol
49 WSA, 13 -10
W S A
guyot
~, -10 k m
IV& V
:~.~-
_. ~ -
~
mounds
" --~:'--
--~--~~
WSA-11 Main platform of the WRGR
~
WSA-10
'*
-
--3.0
¢~
WSA-11
_ _~-~_~, ~--
~, ;,,---~,_ -~-~:-
~=
~
~ -
-'~ '
velocities
~
s
_
.~ :
_
~
20
c
s -~
l
-13
~
=
_
,~
SITE 516
S
_
'
in km
WSA 11 i '
mounds
- ~._~.~.~....
-
~
N
-~.~
~
.
,
- 1 0 k m
.
-2.o
.
.
~ - - - - - - ~
-30
¢
-15 196
-
-
2-13
t59 '~
- ~ _4 ~ - 4 ~ -~., ze2 ~---~.~-
4.00 -
k69
~
--JJ~--~eF.--;---'~5
:-
2~50
2.as 3.30
t93 ___
-
205
-;--:__:_
187 204
1.67 ~----l~-~----..~t=o
I
~ - - ~ - - - ~ ~ - : ~ - : ~ _ ~ t " o 6 q_~w
;
velocities
~§8 ¢.41
in km s-1
3SS -
--~9~ 42~
2S4 -26
262 -
-~3 ;
~?e~
: ,,2#
ZOO
45S
-
"
--30 "
~)
~:
Fig.6. Tracings s h o w i n g the interp]:etation and v e l o c i t y analyses o f lines W S A - 1 0 , WSA-11 a n d W S A - 1 3 . L o c a t i o n o f lines in :~Fig.3. N o t e t h a t the same sequences observed over the m a i n p l a t f o r m o f the Rise can be c e n t i n u o u s l y f o l l o w e d ups]ope o f the g u y o t . T h e s t e e p l y d i p p i n g layers observed at line W S A - 1 0 merge w i t h the m o u n d s observed over the platf o r m of the Rise, indicating t h a t ! t h e mounds are probably slump deposits of material
originated at the volcanic structure. Note also that the mounds at the base of sequence III disappear towards the north away ~ o m the guyots on line WSA-13. The velocity structure derived from the multi-channel seismic semblance plots agrees, in general, with the sequences defined by the seismic reflection character.
50 EVIDENCE FOR EOCENE VOLCANISM The cores from Site 516F (Barker, Carlson et al., 1981) and 357 (Supko, Perch-Nielson et al., 1977; Zimmerman, 1977) show that during the Middle Eocene m a n y turbidite layers containing volcanic debris were deposited over the western part of the Rio Grande Rise. No such turbidites were observed elsewhere in the sedimentary sequence at Sites 516F and 357. Ash layers indicating subaerial volcanism are common throughout the Middle Eocene section at Site 516F. These ash layers contain fresh euhedrat biotites which yielded a K-Ar date of 47 m.y.B.P. (Bryan and Duncan, 1983). At Site 516F a large slump deposit (~ 15 m thick) of Cretaceous sediments occurs at the base of the Middle Eocene sequence. Below this slump layer, no turbidite or ash layers occur; however, they are abundant above this allocthonous block. This slump deposit was probably displaced from its original position by the uplift of the crust related to the formation of the volcanic structures that yielded the turbidite and ash. At Site 357, located 36 km from Site 516F, a layer of fresh volcanic breccia occurs within the Eocene calcareous sequence. This layer of volcanic breccia shows coarse fragments at the b o t t o m fining upwards gradationally. The layer also contains red algae, gastropod shells and large foraminifera. The granulometric evidence found in this deposit suggests that it was deposited as a single event, probably a slump (Supko, Perch-Nielsen et al., 1977). The organic fragments indicate shallow water components coming from the adjacent high peaks (Fodor and Thiede, 1977; Supko, Perch-Nielsen et al., 1977). Thin turbidite layers containing volcanic material occur above and below the breccia layer. This breccia layer probably represents one of the major volcanic events during the Middle Eocene. Additional evidence for the Eocene volcanism at the WRGR comes from the study of the seismic lines and their correlation with the sequence drilled at Sites 516F and 357. The multi-channel lines on the western part of the Rio Grande Rise show mound-like features which indicate a change in the depositional processes over the WRGR (sequence III, Fig.5). The velocity analyses of MCS data have shown that the mound-like sequence III correlates well with the Eocene turbidites and associated ash layers drilled at Site 516F (DSDP Unit IV, Fig.5). This sequence, as noted earlier, is quite pronounced near the volcanic peaks and can be continuously followed from the location of Site 516F towards the slopes of the volcanic structures. The mound-like features gradually disappear towards the deeper water. These mounds are interpreted to be slump and turbidity current accumulations generated at the slopes of the volcanic islands. In the original interpretation of the single channel seismic data, the top of this sequence in areas close to the guyots was considered to be the top of the oceanic basement. The new MCS data show, however, that a thick sedimentary sequence exists below these mounds. The observations above indicate that the guyots are the source for the volcanic debris mixed with the turbidites. The occurrence of these layers with the turbidites and slump deposits suggest that volcanism triggered the
51 gravity flows down the slopes of the volcanoes. Layers containing reefal debris with calcareous algae, bryozoans and large foraminifera also occur within the Middle Eocene sequence containing the turbidites and ash layers and thus indicating a nearby shallow-water environment. ORIGIN AND SUBSIDENCE HISTORY OF THE WESTERN PORTION OF THE RIO GRANDE RISE Site 516F is situated at about 22 km to the northeast of a large g u y o t (Fig.3), This site is in water depths of 1328 m and basement was drilled at 1250 m below the seafloor. Thiede (1977), under the assumption that aseismic ridges similar to the Rio Grande Rise subsided at the same rate as the surrounding normal oceanic basement, used DSDP sites to examine the subsidence history of the Rio Grande Rise. He concluded by backtracking paleodepth information from Sites 21 and 357 and from dredge samples, that the Rise was a large island towering 2 km above sea level during Santonian--Campanian times and t h a t it has slowly submerged since that time. The basement at Site 516F is of Santonian/Coniacian age (~85 m.y.B.P.), as determined from the sediments overlying it, and by radiometric dates (Musset and Barker, 1983). These dates are in agreement with the age predicted for the oceanic crust of this region based on the identified magnetic anomalies and rates of opening of the South Atlantic (Ladd, 1974; Cande and Rabinowitz, 1978; Rabinowitz and LaBrecque, 1979). Basement is now at 2578 m below sea level at Site 516F. If we assume that these rocks were formed at ~ 8 5 m.y. ago and use the procedure for backtracking from present basement elevations, as described by Parsons and Sclater (1977), and correct for sediment loading, we find the basement should have been formed at ~ 2 km above sea level, in agreem e n t with Thiede's (1977) results. However, the seismic data and drilling results strongly suggest t h a t the observed guyots and seamounts are younger features (Eocene) in relation to the basement (Santonian/Coniacian). This Eocene volcanism must have influenced the subsidence of the Rise and thus prevented its subsidence history from following the normal subsidence curve expected for oceanic crust since the time of the crust's formation. We suggest here that although the original ~ 8 5 m.y. ocean crust was formed at a high elevation (close to sea level), it was n o t formed at the ~ 2 km above sea level, as suggested by Thiede (1977), but was formed below sea level and was rejuvenated in the Eocene. Milliman (1983) reported that the sediments directly overlying the basalts at Site 516F indicate shallow-water environmerits, close to sea level (possibly less than 20 m water depth). Further, there i s n o evidence at either of the deep sites (357 and 516F) for the basem e n t having been exposed subaerially. Based on the seismic and drilling data, an evolutionary model for the central portions of the WRGR can be developed as shown in Fig.7. The western portion of the Rio Grande Rise was formed at a spreading center at unusually high elevations (close to sea level) during Santonian/Coniacian
52
5 - Present
S.I. mentation
Evolutionary model for
~
the guyots and adjacent
~
a r e a s of the central platform of the WRGR
4-
J
<
Late Eocene/Early Oligocene wave erosion leveledoff the island
3-
Middle Eocene
2 -
¢
S.J.
osh ~ wave erosion ,~ " ~ _ _ S I .// "-.~.~..~,,~ I ur bi d i t y currents P/ ~ " ~ ' X ~ I Um~P-s turbidit e5
r - ~ "k--/'~
Western part of the Ri0(WRGR)Gronde Rise
~
•
/
~
__
J
"/
7/
-._dumps - -
~-
~ " ' - , ~
~1, I
I - Early Middle Eocene --~ ~ ~-
S. I
depositionof
~
-
==
s,omped material
S.I.
-]
T pelagic sedimentation
Upp--~r Cretaceous to Lower Middle'Eocenepelogi~sed iments. • . ,, , ,: , , , = Upper Cretaceous crust begins to be heated end to bulge
Fig.7. Evo|utionazy mode] for the guyots and adjacent areas of the centra] platform of the WRGR based on seismic and drilling data. Not drawn to scale. times (Barker, Carlson et El., 1981). Pelagic sedimentation prevailed over this cooiLng and subsiding crust. During the Eazly Midd]e Eocene, a thermal anomaly affected the crest of the western Rio Grande Rise creating a bulge and the commencement of the present morphology of the western Rio Grande Rise. SeverEl volcanic islands were formed mainly at the center of thLs thermal anomaly and numerous smaller volcanic structures (seamounts) pierced through the pre-existing crest in many places (stages 1 and 2 Ln Fig.?). A t stage 3 the crest at the summit of this broad rise was further tilted and faulted during the formation of large volcanic structuxes. These crustal movements also affected the pelvic sediments previously deposited. In the areas between major volcanic structures, basement was affected by intense faulting and the pre-exist£ng sediments were tilted as a result of the differentia] movements of the basement blocks. The pre-existing sediments were cfisplaced by slumps and deposited over the Rise's main platform (stages 2 and
53 3 in Fig.7). After the volcanic structures reached subaerial conditions and formed volcanic islands, they were eroded by the sea. Active volcanism in these islands yielded large amounts of pyroclastic material. The finer ash expelled by the eruptions settled over the main platform of the Rise and the coarser material was deposited closer to the volcanic pillar, prograding outward and forming a platform under the existing erosional level (wave base?), as seen at stage 3 (Fig.7). Earthquakes associated with the volcanic activity generated turbidity flows whose sediments were deposited over the main platform of the Rise in close association with the ash deposits. The volcanic activity eventually ceased, erosion by wave action proceeded to level off the island, and the area began to subside (stage 4, Fig.7). After the subsidence of the volcanoes, pelagic sedimentation prevailed again over the entire region (stage 5, Fig.7). Similar evolution is proposed by Barker (1983) who presents a detailed subsidence curve for this portion of the Rio Grande Rise, based on the results of DSDP Leg 72. THE EASTERN
SEGMENT
O F T H E RIO G R A N D E
RISE (ERGR)
The E R G R is a b o u t 600 km long and has a north--south trend, parallel to the trend of the Mid-Atlantic Ridge axis. It is b o u n d e d to the north by a fracture zone; it may also be b o u n d e d by a fracture zone to the south. Transverse profiles across this segment of the Rio Grande Rise show a triangular cross-section (Fig.8). At the northern portion of this segment a 10 km wide axial trough is observed. Although only a few seismic profiles have been obtained across this part of the Rise, this axial trough can be mapped for at least 30 kin. The trough is filled by at least 800 m of sediments and the basement relief from trough to crest is on the order of 1 km (profile I--I', Fig.8). The trough is n o t observed in the southern extent of this segment (profile II--II', Fig.8). The E R G R is separated from the WRGR by a narrow and constrained abyssal plain in which depths exceed 4400 m. A few seamounts protrude from the plain. The seismic records (Fig.8) across the plain show flat-lying well-stratified ponded sediments suggesting that gravity-displaced deposition floors it. The E R G R and W R G R may, perhaps, be morphologically continuous at their northern extremities following the strike of the Rio Grande Fracture Zone (Figs.1 and 2). The E R G R is bounded by the same synthetic flow lines and is of similar distance from the present mid-ocean ridge axis as a major part of the Walvis Ridge in the eastern South Atlantic (Fig.l). This strongly suggests that the eastern Rio Grande Rise and its conjugate portion on the Walvis Ridge were formed at the same time by the same processes. There is no feature conjugate to the W R G R in the eastern South Atlantic. We believe that these observations, together with the major morphological differences between the W R G R and the E R G R , clearly demonstrate that these two segments are distinct features with independent origins.
> o
0
0
¢-
~
o--
•
5~'~:i
~ .... '
II , - wRGR
e
WRGR
III
"'
~.iil
F
,,r,~ ~~--"~IL __
IIr
I1'
on
55 DISCUSSION The Rio Grande Rise is a complex feature, composed basically of two distinct units of different geological histories. The WRGR is an area of elevated basement, elliptical in shape, from which numerous guyots and seamounts protrude. The exact depth at which the original Cretaceous crust of this area was formed is still n o t known. However, there is no conclusive geological evidence to indicate that it was above sea level during its early formation. The present morphology of the WRGR was greatly influenced by widespread volcanism during the Eocene. This is well d o c u m e n t e d by seismic and deepsea drilling data. It is suggested here that during the Eocene a thermal anomaly developed underneath the crustal area corresponding to the WRGR and that this thermal anomaly was responsible for the generation of several oceanic islands and seamounts and for the increased elevation of this part of the Rio Grande Rise, We also propose that the canyons observed on the main platform of the WRGR were formed during the Eocene uplift of the Rise. Thus far, the origin of these canyons has been enigmatic (McDoweU et al., 1977; Johnson and Peters, 1979). In contrast to the WRGR, the ERGR has a north--south trend, parallel to the trend of the present Mid-Atlantic Ridge. The E R G R is believed to be bounded to the north and south by fracture zones, and is in an approximate conjugate position to part of the Walvis Ridge in the eastern South Atlantic Ocean. The two structural elements of the Rio Grande Rise are separated by a deep abyssal plain. The ERGR ascends gradually from this plain, whereas the contact between the plain and the WRGR is marked by steep fault scarps. Intraplate Eocene volcanism, as postulated here for the western portion of the Rio Grande Rise, also occurred extensively in the eastern South Atlantic. During the Eocene, numerous volcanic islands were built at the southwestern portion of the Walvis Ridge. These islands were later eroded, then subsided and now form the G u y o t Province of the Walvis Ridge (Connary, 1972). DSDP Site 359 was drilled over one of these guyots and confirmed its Middle Eocene age (Supko, Perch-Nielsen et al, 1977). However, the Eocene volcanism at the Rio Grande Rise and Walvis Ridge occurred at independent crust areas. The WRGR and the G u y o t Province of the Walvis Ridge are n o t contiguous features in paleoreconstructions. The G u y o t Province of the Walvis Ridge, as mapped by Connary (1972), is centered on crust varying in age from approximately 40 to 68 m.y.B.P., whereas the oceanic islands and seamounts formed by intraplate volcanism during the Middle Eocene at the WRGR is on crust older than 80 m . y . B . P . Uplift of pieces of oceanic crust related to off-axis volcanism has also been documented in other regions. For example, the Bermuda Rise in the Fig.8. Seismic profiles across the abyssal plain that separates the two portions of the Rio Grande Rise. Arrows locate the boundaries of the Rise (see locations in Fig.3). Note that in profiles I and H the sediments in the western portion of the Rise are ponded against a seamount in the abyssal plain. Note also the triangular cross-section and axial valley observed in ERGR in profile I--I'.
56 w e s t e r n central A t l a n t i c O c e a n was similarly f o r m e d during the E o c e n e b y i n t r a p l a t e v o l c a n i s m w i t h i n C r e t a c e o u s c r u s t ( T u c h o l k e a n d V o g t , 1 9 7 9 ) ; the Cape Verde Rise has b e e n s h o w n b y L a n c e l o t a n d S e i b o l d ( 1 9 7 7 ) t o consist o f Jurassic o c e a n i c c r u s t u p l i f t e d b y t h e events associated w i t h t h e f o r m a t i o n o f the Cape V e r d e Islands during the Miocene. T h e s e o c c u r r e n c e s suggest t h a t i n t r a p l a t e v o l c a n i s m m a y be a c o m m o n p h e n o m e n a t h r o u g h geologic time. ACKNOWLEDGEMENTS T h e d a t a s h o w n in this p a p e r w e r e c o l l e c t e d on cruises of L a m o n t - D o h e r t y Geological O b s e r v a t o r y research vessels " V e m a " and " R o b e r t D. C o n r a d " , t h e U n i v e r s i t y o f T e x a s Marine Science I n s t i t u t e ( U T M S I ) , research vessel " F r e d M o o r e " a n d the D e e p Sea Drilling P r o j e c t D / V " G l o m a r C h a l l e n g e r " . We wish t o t h a n k the officers, c r e w a n d scientists a b o a r d t h e s e vessels f o r their assistance in g a t h e r i n g t h e data. We are p a r t i c u l a r l y g r a t e f u l t o Drs. J o h n L a d d , R i c h a r d Buffler a n d T o m Shipley, the c h i e f scientists a b o a r d U T M S I ' S R / V " F r e d M o o r e " . A r v o B u c k , J o h n Diebold and R o b e r t H o u t z r e v i e w e d t h e m a n u s c r i p t and m a d e valuable suggestions f o r its i m p r o v e m e n t . Financial s u p p o r t f o r this s t u d y was p r o v i d e d b y N a t i o n a l Science F o u n d a tion G r a n t s OCE 7 2 - 6 0 4 2 6 A 0 4 (Office of I D O E ) , D E S 7 1 - 0 0 2 1 4 , OCE 772 5 9 9 2 a n d OCE 7 9 - 1 9 3 8 9 (Office of O c e a n o g r a p h y ) , a n d b y Office o f N a v a l R e s e a r c h C o n t r a c t N 0 0 0 1 4 - 7 5 - C - 0 2 1 0 . Financial s u p p o r t f o r L.A.P. G a m b o a was p r o v i d e d in p a r t b y C o o r d e n a n c ~ o do A p e r f e i c o a m e n t o de Pessoal de Nivel S u p e r i o r (CAPES). This is L a m o n t - D o h e r t y G e o l o g i c a l O b s e r v a t o r y C o n t r i b u t i o n No. 3 6 0 2 . REFERENCES Barker, P.F., 1983. Tectonic evolution and subsidence history of the Rio Grande Rise. In: P.F. Barker, R.L. Carlson et al., Initial Reports of the Deep Sea Drilling Project, v. 72. U.S. Govt. Printing Office, Washington, D.C., pp. 953--976. Barker, P.F., Carlson, R.L. et al., 1981. Deep Sea Drilling Project Leg 72: Southwest Atlantic paleocirculation and Rio Grande Rise tectonics. Geol. Soc. Am. Bull., 92: 294--309. Barker, P.F., Buffer, R.T. and Gambos, L.A.P., 1983. A seismic reflection study of the Rio Grande Rise. In: P.F. Barker, R.L. Carlson et al., Initial Reports of the Deep Sea Drilling Project, v. 72. U.S. Govt. Printing Office, Washington, D.C., pp. 499--517. Bryan, W.B. and Duncan, R.A., 1983. Age and provenance of clastic horizons from Hole 516F. In: P.F. Barker, R.L. Carlson et al., Initial Reports of the Deep Sea Drilling Project, v. 72. U.S. Govt. Printing Office, Washington, D.C., pp. 475--477. Cande, S.C. and Rabinowitz, P.D., 1978. Magnetic anomalies of the continental margin of Brazil. Am. Assoc. Pet. Geol., Offshore Brazil Map Set. Connary, S.D., 1972. Investigations of the Walvis Ridge and environs, Ph.D. thesis, Columbia University, Palisades, N.Y., 228 pp. Dietz, R.S. and Holden, J.C., 1970. Reconstruction of Pangea: breakup and dispersion of the continents, Permian to present. J. Geophys. Res., 75: 4939--4956. Fodor, R.V. and Thiede, J., 1977. Volcanic breccia from DSDP Site 357: Implications for the composition and origin of the Rio Grande Rise. In: P.R. Supko, K. Perch-Nielsen et al., Initial Reports of the Deep Sea Drilling Project, v. 39, U.S. Govt. Printing Office, Washington, D.C., pp.537--543.
57 Fodor, R.V., Husler, J.W. and Keil, K., 1977. Petrology of basalt recovered during D S D P Leg 39B. In: P.R. Supko, K. Perch-Nielsen et al.,InitialReports of the Deep Sea Drilling Project, v. 39. U.S. Govt. Printing Office, Washington, D.C., pp.513--523. Folha Asuncion (SG-21) and Folha Curitiba (SG-22), 1974. Ministry of Mines and Energy, Brazil. Gamboa, L.A.P. and Rabinowitz, P.D., 1981. The Rio Grande Fracture Zone in the western South Atlantic and its tectonic implications. Earth Planet. Sci. Lett., 52: 410--418. Heezen, B.C. and Tharp, M., 1978. The b a t h y m e t r y of the South Atlantic Ocean. In: General Bathymetric Charts of the Oceans (GEBCO) (5th ed.), Can. Hydrogr. Service, Ottawa, Ont. Hinz, K., 1981. A hypothesis on terrestrial catastrophes: wedges of very thick oceanward dipping layers beneath passive continental margins. Geol. Jahrb., Reihe E, 22: 3--28. Johnson, D.A. and Peters, C.S., 1979. Late Cenozoic sedimentation and erosion of the Rio Grande Rise. J. Geol., 87: 371--392. Kumar, N., 1979. Origin of "paired" aseismic rises, Ceara and Sierra Leone Rises in the Equatorial, and the Rio Grande Rise and Walvis Ridge in the South Atlantic. Mar. Geol., 30: 175--191. Ladd, J.W., 1974. South Atlantic sea floor spreading and Caribbean tectonics. Ph.D. thesis, Columbia University, Palisades, N.Y., 200 pp. Lancelot, Y. and Seibold, E., 1977. The evolution of the Central Northeastern Atlantic - Summary of results of DSDP Leg 41. In: Y. Lancelot, E. Seibold et al., Initial Reports of the Deep Sea Drilling Project, v. 41. U.S. Govt. Printing Office, Washington, D.C., pp.1215--1245. Laughton, A.S., Berggren, W.A. et al., 1972a. Site 111. In: A.S. Laughton, W.A. Berggren et al., Initial Reports of the Deep Sea Drilling Project, v. 12. U.S. Govt. Printing Office, Washington, D.C., pp.33--159. Laughton, A.S., Berggren, W.A. et al., 1972b. Sites 116 and 117. In: A.S. Laughton, W.A. Berggren et al., Initial Reports of the Deep Sea Drilling Project, v. 12. U.S. Govt. Printing Office, Washington, D.C., pp.395----671. LePichon, X. and Hayes, D.E., 1971. Marginal offsets, fracture zones, and the early opening of the South Atlantic. J. Geophys. Res., 76: 6283--6293. Mapa Geologico da America do Sul, 1964. Ministry of Mines and Energy, Brasilia, Brazil. Maxwell, A.E., Von Herzen, R. et al., 1970. Initial reports of the Deep Sea Drilling Project, v. 3. U.S. Govt. Printing Office, Washington, D.C. McDowell, S., Kumar, N., Jacobi, R.D., Johnson, D.A. and Bunce, E.T., 1977. Regional setting of Site 357, North flank of Rio Grande Rise. In: P.R. Supko, K. Perch-Nielsen et al., Initial Reports of the Deep Sea Drilling Project, v. 39. U.S. Govt. Printing Office, Washington, D.C., Pp.955--969. Milliman, J.D., 1983. Coniacian/Santonian depositional environments on the Rio Grande Rise as evidenced from carbonate sediments at Hole 516F. In: P.F. Barker, R.L. Carlson et al., Initial Reports of the Deep Sea Drilling Project, v. 72. U.S. Govt. Printing Office, Washington, D.C., pp. 395--397. Moody, R., Hayes, D.E. and Connary, S., 1979. Bathymetry of the continental margin of Brazil. Am. Assoc. Pet. Geol., Offshore Brazil Map Ser., Catalog No. 832, Tulsa, Okla. Morgan, W.J., 1971. Convection plumes in the lower mantle, Nature, 230: 42--43. Mussett, A.E. and Barker, P.F., 1983.4°Ar/39Ar age spectra of basalts, D S D P Site 516F. In: P.F. Barker, R.L. Carlson et al.,InitialReports of the Deep Sea DrillingProject, v. 72. U.S. Govt. Printing Office, Washington, D.C., pp. 467--470. Mutter, J.C., Talwani, M. and Stoffa, P.L., 1982. Origin of seaward-dipping reflectorsin oceanic crust off the Norwegian margin by "subaerial sea-floor spreading". Geology, 10: 353--357. Parsons, B. and Sclater, J.C., 1977. A n analysis of the variation of ocean floor bathymetry and heat flow with age. J. Geophys. Res., 82: 803--827. Rabinowitz, P.D. and LaBrecque, J., 1979. The Mesozoic South Atlantic and evolution of its continental margins. J. Geophys. Res., 84 : 5973--6002.
58 Rabinowitz, P.D., Delach, M., Truchan, M. and Lonardi, A., 1978. Bathymetry of the Argentine continental margin and the adjacent areas, Am. Assoc. Pet. Geol., Offshore Argentine Map Ser., Catalog No. 828, Tulsa, Okla. Roberts, D.G., Schnitker, D. et al., 1982. Leg 81 drill margin, Rockall Plateau. Geotimes, 27 : 21--23. Supko, P.R., Perch-Nielsen, K. e t al., 1977. Initial reports of the Deep Sea Drilling Project, v. 39. U.S. Govt. Printing Office, Washington, D.C., p.1139. Talwani, M. and Udintsev, G., 1976. Tectonic synthesis. In: M. Talwani and G. Udintsev et al., Initial Reports of the Deep Sea Drilling Project, v. 38. U.S. Govt. Printing Office, Washington, D.C., pp.1213--1242. Taner, M.T. and Koehler, F., 1969. Velocity spectra digital computer deviation and applications of velocity functions. Geophysics, 34: 859--881. Thiede, J., 1977. Subsidence of &seismic ridges: Evidence from sediments on Rio Grande Rise (Southwest Atlantic Ocean). Bull. Am. Assoc. Pet. Geol., 61: 929--940. Thompson, G., Humphris, S. and Shilling, J.G., 1983. Petrology and geochemistry of basaltic rocks from the Rio Grande Rise, South Atlantic, Deep Sea Drilling Project Leg 72, Hole 516F. In: P.F. Barker, R.L. Carlson et al., Initial Reports of the Deep Sea Drilling Project, v. 72, U.S. Govt. Printing Office, Washington, D.C. Tucholke, B.E. and Vogt, P.R., 1979. Western North Atlantic: Sedimentary evolution and aspects of tectonic history. In: B.E. Tucholke, P.R. Vogt et al., Initial Reports of the Deep Sea Drilling Project, v. 43. U.S. Govt. Printing Office, Washington, D.C., pp.791-825. Weaver, B.L., Marsh, N.G. and Tarney, J., 1983. Trace element geochemistry at Site 516F, Rio Grande Rise, DSDP Leg 72. In: P.F. Barker, R.L. Carlson et al., Initial Reports of the Deep Sea Drilling Project, v. 72. U.S. Govt. Printing Office, Washington, D.C., pp. 451--456. Wilson, J.T., 1963. Hypothesis of earth's behavior. Nature, 198: 925--929. Wilson, J.T., 1965. Submarine Fracture Zones, aseismic ridges and the International Council of Scientific Unions Line: Proposed Western Margin of the East Pacific Ridge. Nature, 207 : 907--911. Zimmerman, H.B., 1977. Clay mineral stratigraphy and distribution in the South Atlantic Ocean. In: P.R. Supko, K. Perch-Nielsen et al., Initial Reports of the Deep Sea Drilling Project, v. 39. U.S. Govt. Printing Office, Washington D.C., pp.395--406.