Goy, J.L.

Machare, J.

Ortlieb, Luc

Zazo, C.

Horizon / Pleins textes

Neotectonics and Plio-Pleistocene sea level 
records in southern Peru 
~7.1~. - Goy GJniv. Complutense, Madrid), J.  Machare' (Inst. Geofisico del Peru, Lima), 
L. Ort lkb  (ORSTOM-Peru, Lima), & c. Zazo (Consejo Sup. Invest. Cient., Madrid) 
The relationships between the subduction of the 
Nazca p l a t e  unde r  the  South  American 
subcontinent and the recent vertical movements, 
registered along the Peruvian coast, are  not yet 
well understood. During the Quaternary, a 
northern segment of the Peru coast has  been 
emerging, a central segment has  been apparently 
subsident, and the southern coast has  been 
emerging, locally a t  very high rates. In  this activity 
report on a cooperative work, associating several 
"old" members of the INQUA Neotectonics 
Commission, we shall focus on the southern 
segment of the Peruvian coast. 
From ]at 14"s southwards, the coastal region 
experienced Plio-Pleistocene uplift motions which 
are evidenced by staircased sequences of manne  
terraces. The spatial andgeometrical distribution 
of these remnants oflate Cenozoic high sea stands 
reflects altogetherregional crustal movementsand 
local deformations. Current correlation studies 
between well-developed series of terraces a t  110 
(17"30'S), Chala (16OS) and San J u a n  Marcona 
!15"30'Sjare aimed to characterize these regional 
and local tectonic controls. 
I 
i n  San J u a n  Marcona area, exccptional sequences 
of marine terraces are  preserved. The terraces are  
abraded platforms, separated by steep seacliffs 
(meanheight oftheorderof20m), locallyveneered 
by thin coastal deposits and generally covered by 
detlated eolian sands. More than thirty steps may 
be distinguished. The highest terraces reach 
elevations ofabout+780m and containlatePliocene 
fauna (Devries in Macharé, 1987). 
In  spite of various intents of age determination 
on marine shells, the chronostratigraphy of the 
lower terraces is still debated. Isotopic stage 5e 
age assignments have been succt:ssively proposed 
for: the +148 m terrace (IIsu & Blom, 1985; 
Osmond, 19871, the +110 m terrace (Hsu & 
Wehmiller, 19871, the +90 m terrace (Macharé, 
19871, and the +65 m terrace (Ilsu, 1988;Hsu et  al., 
1989). Accordingly, themean upliftrate estimates 
tc 
for a particular transect vary between1140 and,  
470 mn~/ lO-~y.  Additional dating of the lower 
terraces is presently under way (at Geotop lab., 
UQAM, Montreal) and will hopefully yield 
determinant results. 
The strong, and apparently continuous, vertical 
motions registered a t  San  J u a n  Marcona are  closely 
related t o  the subduction of the aseismic Nazca' 
Ridge (Machar6 et  al., 1986; Mnchar6, 1987; Hsu, 
1988; Machar6 & Ortlieb, 1990). Deformations of 
the terraces around San  J u a n  Marcona depict 
some recent faulting activity, reactivation of old 
faults, and block tilting. A detailed tectonicmap of 
this key area, registering the most rapid and 
complete sequence of Plio-Pleistocene bigh sea 
stands of South America, .is presently being 
prepared. 
At Chala, a favorable embayment, controlled by 
several faults, also recorded a long sequence of 
high sea stands (up to 28 steps have been 
distinguished). In  this case, the Pleistocene (and 
latest Pliocene ?) interglacial sea stands a re  
registered as  wave-cut surfaces on which lay marine 
deposits that  were partially covered by alluvial fan 
units. The alluvial sediments, spread over almost 
each marine terrace of the sequence, appear to 
have been covered with the intervening glacial 
stages and cooler interstages (within interglacial 
episodes). The marine deposits vary in composition 
and thickness, two units consisting in several- 
metre-thick coastal sands (correspondjpgshoreline 
angles a t  +64 m and +180 m). 
From geometrical considerations and strati- 
graphic relationships, i t  is interpreted tha t  about 
ten transgres'sive cycles can be distinguished. For 
instance, the youngest pre-Holocene complete cycle 
shows three well-defined marine episodes (sen- 
stand maxima a t  +64 m, +44 m and +31 m), 
separated by two events of alluvial fan deposition; 
locally, at the southeastern boundary of the 
embayment, two additional, lower, marine bench- 
es were formed, probably a s  a result of local 
~~~~~~ Fonds Documentaire 
deformations. If the major cycles correspond to 
interglacial periods, and the smaller sea level 
not interpreted tha t  a true subsident regime was 
installed a t  the end of the Middle Pleistocene (as 
fluctuations to interstages, i t  may be inferred that  
the isotopic stage 5e high sea stand would be 
registered at +64 m in the Chala embayment. 
The highest par t  of the sequence (above +250 m> 
is obliterated by thick sheets of alluvium. The 
highest observed manne  sediments, lying on 
abrasion surfaces in Chala region (at +240 m), 
might be of'earliest Pleistocene age, since they 
overlie Pliocene foreshored beds outcropping (at 
about +200 m) immediately north of the bay of 
Chala. 
I n  spite of a strong tectonic control of the 
morphology of the Chala embayment, there is no 
evidence of major displacements of the marine 
terraces after their formation. The main local 
tectonic activity, that is attributed to slight 
readjustments of former fracture zones, is limited 
to landslides on the walls of the principal canyon 
crossing the embayment (Quebrada Chala). 
Though, immediately southeast of Chala, the 
coastal region experienced stronger deformations, 
evidenced by faulted Pleistocene terraces and by 
upwarped Holocene beach deposits. 
The region of110 registered less spectacular series 
of marine terraces but offers probably the best 
developed sequence of Pleistocene high sea stands 
available i n  southernmost Peru. East  of 110, 
staircased terraces are  preserved at  up to about 
'* +350 m, but  those, which may be assigned a 
Pleistocene age, a re  lying a t  less than about +200 
m. Alonga 50km-long coastal segment, north and 
south of 110, Middle and Late Pleistocene marine 
deposits, often associated to alluvial units, are  
generally observed at elevations increasing with 
their relative age. Aminostratigraphic and U- 
series analyses indicate tha t  the isotopic stage 5e 
shoreline locally reaches a +25 m elevation, and 
suggest t ha t  several Middle (and Early?) Plei- 
stocene high sea stands were pl;,served at about 
+40, -1-50, -1-80, -1-120 and +160 m (Ortlieb e t  al., in 
South of 110, the marine terrace record is 
complicated by the existence of relatively thick 
coastal sediments and by some overlap of these 
Pleistocene units. "Stratigraphic observations, 
supported by some preliminary aminostratigraphic 
. data (Hsu, 1988; Hsu e t  al., 1989; Ortlieb e t  al., in 
prep.), show tha t  marine and lagoonal beds, 
probably coeval with the las t  two (or three) 
interglacials, a re  locally superposed, in normal 
stratigraphic order. Some local tectonic motions 
and particularly paleogeographic conditions have 
been involved in the overlap disposilion, but i t  is 
prep.) 
' 
. 
suggested by Hsu, 1988). At  the mouth of Rio 
hcumba,  south ofIlo, a relativelyimportantMiddle 
and Late Quaternary tectonic activity is evidenced 
by deformations of manne  terraces and alluvial 
fans. 
Referen ces 
Hsu, J.T., 1988. Emerged Quaternary marine 
terraces of southern Peru: Sea level changes 
and continental margin tectonics over the 
subducting Nazca-Ridge. Ph.D. Thesis, Cornell 
Univ., 310 pp. 
Hsu, J.T. & ßloom, AL. ,  1985. Quaternary marine 
terraces and maximum tectonic uplift rate of 
the Peruvian coast a t  15.5" S latitude. Geol. 
Soc. Amer. Abstr. Progr., v. 17, p. 614. 
Hsu, J.T. & Wehmiller, J.F., 1987. Quaternary 
tectonism over the subducting Nazca Ridge, 
south-central Peru. X I I  INQUA Congr. 
(Ottawa, 19871, Abstr. vol:, p. 235. 
Hsu, J.T., Leonard, E.M. & Wehmiller, J.F., 1989. 
Aminostratigraphy of Peruvian and Chilean 
, Quaternary marine terraces. Quatern. Sci. 
Rev., v. 8, p. 255-262. 
hfacharé, J., 1987. La marge continentale du  
Pérou: Régimes tectoniques e t  sédimentaires 
cénozoiques de l'avant-arc des Andes centrales. 
Thhse Soct. Sc., Univ. Paris XI, 391 pp. 
Macharé, J . ,  Sebrier, M ., Huaman, D. & Mercier, 
J.L., 11986. Tectdnica cenozoica de la margen 
continental perixina. Bol. Soc. Geol. Peru, v. 
Macharé, J .  & Ortlieb, L., 1990. Recent vertical 
motions and the subduction of the Nazca Ridge, 
central coast ofPeru. Intern. Symp. on Andean 
Geodytianzics (Grenoble, May 1990). Abstr. 
vol. (in press). 
Ortlieb, L., Chaleb, B., Hillaire-Marcel, C., Pichet, 
P. & Macharé, J. (in prep.). Th/U disequilibria 
and allo/isoleucine ratios in fossil shells from 
raised marine terraces of southern Peru: 
methodological problems and datingpotential. 
Osmond,  J .K. ,  1987. T h o r i u m h r a n i u m  
disequilibrium age of a suite of mollusc shells 
from a Peruvian terrace. XII INQUA Congr. 
(Ottawa, 1987), Abstr. vol., p. 235. 
76, p. 45-77. 


Neotectonics and plio-pleistocene sea level records in Southern Peru

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