SURFING THE TECTONIC SHOCKWAVE -THE NAZCA AFTERSHOCK the higher see level reached in the Neogene is 15,5 Ma-old and is only 150 m above the present see level (Ha rd e nbo l et al., 1998). In our study area, in contrast, the oJder abrasion surface is higher th an 500 m asl, evidencing the uplift of that part of the Chilean coast. The observed uplift cannot result from the subduction of the Juan Fernandez ridge since the north of the study area, far from the ridge, is more uplifted than the southern part. Moreover, we most1y observe N-$ striking normal faults, whereas the subduction of the Juan Fernandez ridge should be result in the formation of E- W faults in this part of the ChiJean coast. We also note that abrasion surfaces are also present in other parts of the Chilean coast, such as in the Mejillones Peninsula (about 23° 15' S), where 400 m-high Quaternary surfaces have been described (Ort1ieb et al., 1996a). This shows that the subduction plane geometry is not responsible for the coastal uplift since the Mejillones Peninsula is located above an inclined subduction segment, contrary to our study area. We observe that the region studied here is situated at a particularly small distancefrom the trench and that this distance is smaUer in the northern part of our study area. In fact, the trench closest area is also the most uplifted. The same phenomenon is observed by Ortlieb et al (1996a) for the Mejillones Peninsula. Thus, we argue that the phenomenon that could explain the tectonics affecting the Chilean coast is the underplating beJow the Coastal Cordillera ----long-term uplift is positively correlated with the coseismic uplift, i.e. that the post seismic and interseismic subsidence of the coast does not completely compensate the uplift that occurs during earthquakes. The coseismic uplift being larger when the coast approaches the trench Neogene tectonic and geomorphologic evolution of the chilean coast between 30°S and 32°S : study of marine abrasion surfaces above a fiat subduction segment that could result from the subduction of the Juan Fernandez ridge at around 33°S since 10 My (Yafiez et al, 2001). Along that part of the coastal zone, we observe discontinuous, parai leI to the coast, gently seaward-sloping marine abrasion surfaces. They are lirnited by a scarp both their base and their top. In the following, we characterise these surfaces by the altitude of their upper internaI Iimit, at the foot of the top scarp, called shoreline angle (Fig.l) ; we recognize up to four surface levels corresponding to terraces reaching more than 500 m of elevation (above present-day MSL): To, Tl'T2 ,T3 ; T, being the younger and T3 the older surface. According to their geographical location, their width is more or less important and their characteristic elevation varies (Fig.2-3). COASTALMORPHOLOGY Distribution ofmarine abrasion surfaces: Tois the less-developed abrasion surface. It corresponds to the active abrasion surface which is still submitted to marine action in its lower part; it contains relicts of the previous surface which are not completely levelled yet.

THE TECTONICS OF NOT YET ....

Salvador Sóares Das Silvas PREFIRA O ARAMAICO QUE HÁ EM SI ....BEMOL...THE MEDICAL FAULTS faults cutting sediment or displacing abrasion surfaces correspond to normal faults striking more or less parallel to the coast (between about N340° and N400), with a vertical slip, without any strike-slip component (Fig.2). These faults do not influence markedly the morphology of surfaces; they just influence their width and elevation. Ali the faults dori'tmove the different surfaces of the same amount of displacement. Within a same surface, the vertical movement of a fault can also vary; for instance, from 9 m (Punta Limari Sur) to 54 m (Punta Limari Norte) for T2. More generally, on the one hand,we observe that the T3 surface is more deformed by faults than the three other surfaces: it is eut by a greater number of faults, and their vertical offset is also larger; and on the other hand, we observe that the northwestern part of the study area (Altos e Talinay) is more deformed than the southern (CaletaManso-Los Vilos) and the eastern (Bahia Tongoy )zones (Fig.2). N-S variations of marine abrasion surfaces elevation: in the north, at Altos de Talinay ; we note the four surfaces whereas in the south, at Caleta Derumbe for instance, only the T3 surface is visible (Fig.2-3). South 30°58', the T, surface disappears; the T3 surface becomes wider and its shoreline angle elevation decreases from north to south (Fig.2-3). Generally, we notice that surface elevations are smaller in the south than in the north (Fig.3). This shows that the northern part of the study area was further uplifted than its southern part.

number

of

faults,

and

their

vertical

offset

is also

larger;

and on the

other

hand, we

observe

that the

northwestern

part

of

the

study

area

(Altos