Volcanic gaps and subaerial records of palaeo!sea!levels ... · Volcanic gaps and subaerial records of palaeo!sea!levels on ... the balance between constructive processes "e}usive

\PERGAMON Geodynamics 17 "0888# 006Ð018

9153!2696:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[PII] S 9 1 5 3 ! 2 6 9 6 " 8 7 # 9 9 9 2 1 ! 4

Volcanic gaps and subaerial records of palaeo!sea!levels onFlores Island "Azores#] tectonic and morphological

implicationsJ[M[M[ Azevedo�\ M[R[ Portugal Ferreira

Dep[ de Cie¼ncias da Terra\ Fac[ Cie¼ncias e Tecnologia[ da Universidade de Coimbra\ 2938 Coimbra\ Codex Portugal

Received 19 May 0886^ received in revised form 20 July 0886^ accepted 29 June 0887

Abstract

The morphological evolution of Flores Island\ as commonly observed for volcanic islands\ suggests "0#the balance between constructive processes "e}usive and moderately explosive volcanic activities andtectonic uplifting movements# and destructive processes "marine abrasion\ stream erosion\ crater!formingvolcanic explosions\ caldera collapses and tectonic subsidence# and "1# the recurrent ~uctuations of thesea!level[

Records of "a# gaps in the volcanic activity and "b# erosional and depositional marine activity are shownas]

* epiclastic deposits of marine origin* erosional morphologies\ such as abrasion platforms\ terraces\ cli}s and caves* intensive palagonitization of the volcanic rocks* vertical changes of the structures in the hydroclastic submarine formations[

Taking into account "0# the vertical crustal movements "uplift and subsidence# which may occur involcanic domains and "1# the sequence of regressiveÐtransgressive trends in the relative sea!level asexpressed by indicators of pale!sea!levels\ it is assumed that the morphological evolution of Flores Islandcomprehends three main stages[

The existence of important di}erences between the present!day altitudes of correlated marine recordsnoted in Flores\ in Santa Marie Island "Azores Archipelago# and Porto Santo Island "Madeira Archipelago#is related with their crustal behaviour and di}erent volcanic and tectonic evolution[ Þ 0888 ElsevierScience Ltd[ All rights reserved[

� Corresponding author[ Tel[] 99240 28 12911^ fax] 99240 28 26600

J[M[M[ Azevedo\ M[R[ Portugal Ferreira : Geodynamics 17 "0888# 006Ð018007

0[ Introduction

The present!day morphology of Flores Island\ as of many other oceanic islands\ is the result ofa continuous unbalanced equilibrium between the constructive and the destructive processes"Azevedo and Ferreira\ 0884b# "Fig[ 0#[ The constructive processes are prevalent during theproto!insular and young insular stages^ the destructive processes may be episodic during theconstructive stages\ but become predominant as the island gets older and older[

Besides the constructiveÐdestructive equilibrium\ the emerged area in each time of an oceanicisland evolution depends largely on the eustatic sea!level ~uctuations[ The persistence of aregressive tendency or an island uprise may allow the preservation of records of the marinepresence\ such as uplifted terraces or notches[

In Flores Island\ geomorphological and depositional records of past sea!levels are expressedin its subaerial domains[ Their recognition and subsequent interpretation were supported andintegrated on the wider frame of the geological and structural evolution of the Island\ aimed todraw up of the volcanological map at the 0]04[999 scale[

1[ Geographic\ geotectonic and geochronologic setting

Flores is one of the nine islands of the Azores Archipelago[ Together with Corvo they constitutethe Western Group of this Portuguese archipelago\ which is placed in the middle of NorthAtlantic Ocean at about 26> to 39>N\ 13> to 20>W ðFig[ 1"a#Ł[

The azorean islands extend through a 599 Km belt aligned along WNW direction and belongto a major group of Atlantic Islands*Macaronesia Islands*that also includes Madeira\ Canaryand Cape Verde Archipelago Islands ðFig[ 1"a#Ł

Fig[ 0[ Major processes "constructive and destructive# responsible for the geomorphological evolution of Flores Island[

J[M[M[ Azevedo\ M[R[ Portugal Ferreira : Geodynamics 17 "0888# 006Ð018 008

Fig[ 1[ A* Geographic setting of Azores and other Macaronesian Archipelagos "Madeira\ Canaries and Cape Verde#and B* Geotectonic setting of Azores Archipelago "adapted from Forjaz\ 0877# and older radiometric ages "Ma Bp#for each island "data collected from Abdel!Monem et al[\ 0857^ Abdel!Monem et al[\ 0864^ Fe�raud et al[\ 0879^ Fe�raudet al[\ 0873^ Ferreira and Martins\ 0872^ Forjaz\ 0877^ Azevedo et al[\ 0880#[


All of the islands and seamounts from Azores Archipelago are the result of a volcanic processrelated to the con~uence of three lithospheric plates*American\ Eurasian and African*and arestrictly associated to the Mid Atlantic Rift "MAR# and particularly to its transform faults[

Seven of these islands*Faial\ Pico\ S[ Jorge\ Terceira and Graciosa "Central Group# and S[Miguel and Santa Maria "Eastern Group# are rooted in the Eurasian Plate "Laughton andWhitmarsh\ 0863#\ in a subdomain called Azorean Micro!plate "after Forjaz\ 0877#\ whichis limited westwards by the MAR\ southwards by the East Azores Fault Zone and northÐnortheastwards by GraciosaÐTerceiraÐS[ Miguel Fracture "or Terceira Rift\ after Machado\0848#[ Contrasting with these seven islands\ Flores and Corvo are rooted westwards of the MAR\in the North American Plate ðFig[ 1"b#Ł[

Several models for the tectonic regime of the Azorean Micro!plate were proposed on the basisof neotectonic\ seismotectonic and paleomagnetic data interpretation "see for example] Machadoet al[\ 0861\ 0872^ Hirn et al[\ 0879^ Buforn et al[\ 0876^ Madeira and Ribeiro\ 0889\ 0881#\ butthey don|t really include or give a detailed explanation for the tectonic regime of the westerndomain of Flores and Corvo[ This lack of explanation is a consequence of] "0# the speci_city ofthis azorean domain\ particularly "0[0# the MAR westwards localization\ "0[1# the NS directionof the volcanic building of Flores and Corvo\ parallel to the MAR and approximately per!pendicular to that observed for the other azorean islands "NWN#\ "0[2# the very moderate seismicactivity "only two historical references\ both from the sec[ XVIII# and "1# a great scarcity ofgeophysical data for this Atlantic area[ The main exceptions are the works of Serughetti andRocha "0857#\ Krause and Watkins "0869# and Blakely "0863#[ Meanwhile\ with the support ofthe 0]14 999 geologic map of Zbyszewsky et al[ "0857#\ the recent lithological\ structural andmorphological evidences "Azevedo\ 0886^ Azevedo et al[\ 0880 and Azevedo and Ferreira\ 0884a#and the new GPS data "Bastos et al[\ 0882# it is possible to advance the following aspects]

*Flores and Corvo Islands are two subaerial pits of the same volcanic building^*The tectonic setting and volcanic processes of these two islands are\ like those presented by

the other islands\ undoubtedly associated to the MOR "this is partially con_rmed by thesimilarities found between geochemistry and isotopic data from Flores and Corvo and thosefrom all the other azorean islands "Torre de Assuncža½o et al[\ 0863#\ but the structural controlinduced by the transform faults\ very clear in the eastern islands\ is less evident here[ However\the rectilinear nature of both the north and south Flores coastlines Fig[ 3 may be the expressionof this control[

*The main tectonic structures on Flores are sinistral N29Ð39>W and up!and!down N19Ð29>Efaults "Azevedo\ 0886#^ the NS major structure suggested in Forjaz "0877# has a secondaryexpression in subaerial domains[

*The occurrence of two important periods of vertical crustal movements which were strictlyassociated to the volcanic activity "Azevedo\ 0886#] "0# an uplifting\ that was extensive to allthe insular building and was related to the very intensive volcanism developed during theevolution from the proto!island to the island stage "emergent volcanism#\ "1# a structuralcollapse of to two or three big subaerial craters located in the central area of the island\ whichled to the build!up of large and shallow calderas "Azevedo\ 0886#[

The older isotopic ages obtained for each azorean island are given on Fig[ 1"b#[ Flores has thefourth oldest subaerial formations\ presenting radiometric ages of about 1[1 Ma Bp "Azevedo et


al[\ 0880#[ Only Santa Maria\ S[ Miguel and Terceira show older ages[ This fact _gures out thesigni_cative correlation between the ages of the oldest rocks found in each island and its increasingdistance from the MOR\ which con_rms the progressive and symmetric divergence of the litho!spheric plate[

The last volcanic activity of Flores occurred 9[992 Ma Bp "Morrisseau and Traineau\ 0874#and according to Blakely "0863# and Needham and Francheteau "0863# the volcanic building ofFlores and Corvo rest on a 8[9 Ma Bp oceanic crust "anomaly 4\ after Krause and Watkins\0869#[

2[ Build!up processes and gaps in volcanic activity

Flores\ as all the other islands of the Azores Archipelago\ is a typical volcanic build!up islandwhose volcanism is associated to MAR transform faults[ There is a balance between the e}usivityand the explosivity with predomination of the basaltic lavas and pyroclasts\ but the di}erentiatedproducts\ such as hawaiites\ mugearites\ benmoreites\ traquites and very seldom riolites as beenalready reported "Cas and Wright\ 0876#[

According to the geological and structural mapping "Azevedo\ 0886 and Azevedo et al[\ 0880#\the volcanic formations were organised in two major groups "called Complexes after Azevedo etal[ "0875# to mean an assemblage of volcanic rocks\ which present genetic and spatial a.nities\ buta wide variety of petrographic and geochemical characteristics\ primary structures\ morphologies\degrees and types of alteration#]

0[ The Base Complex "BC# that comprehends two major units] Base Complex 0 "BC0# and BaseComplex 1 "BC1#[ These units "BC0 and BC1#\ which are the result of submarine and emergentproto!insular volcanism of PlioÐPleistocenic age\ are mainly composed of volcanoclastic withsome minor lava!~ows at the top "Azevedo\ 0877#[

1[ The Upper Complex "UC#] includes all the subaerial volcanic formations and may be sub!divided into three volcanic units "Azevedo and Ferreira\ 0884#] Upper Complex 0 "UC0#\Upper Complex 1 "UC1# and Upper Complex 2 "UC2#[ The lower subgroup "UC0# wasformed between 9[54 and 9[4 Ma Bp and is made up of extensive and sometimes very thicklava!~ows alternating with subordinate pyroclast^ its geochemical composition di}erentiatedfrom basaltic into traquitic nature[ The middle subgroup "UC1# whose age ranges from 9[3to 9[1 Ma Bp\ is mostly composed of basaltic and hawaiitic lava!~ows[ The upper subgroup"UC2# was build!up between 9[993 and 9[992 Ma Bp "Morrisseau\ 0874# and includes basicpyroclast deposits\ ashes with lithic fragments\ which are the result of an intensive phre!atovolcanism[

K:Ar radiometric ages "Azevedo et al[\ 0875\ 0880^ Ferraud et al[\ 0879# and 03C data "Mor!risseau\ 0874# helped to de_ne with some detail the evolution of the volcanic activity on FloresIsland[ The major periods of volcanism are shown on Fig[ 2[ They alternate with three mainstages of quiescence\ respectively located between 0[4 and 0[0 Ma\ 9[4Ð9[3 Ma and 9[1Ð9[993 MaBp[


Fig[ 2[ Radiometric ages "K:Ar determinations in circles "Azevedo et al[\ 0875^ Azevedo et al[\ 0880^ Ferraud et al[\0879# and 03C determinations in small squares "Morrisseau\ 0874#\ periods of volcanic activity "white bars# andvolcanic gaps "grey bars# _gured out in Flores Island[

3[ Subaerial records of paleo!sea!level and paleo!coastlines

Depositional and geomorphological records indicative of paleo!sea!levels have been observedalong coastal sectors of Flores "Zbyszewsky\ 0865 and Azevedo and Ferreira\ 0884#\ locatedbetween the present!day coastline and the 299 m a[s[l[ "Fig[ 3 and Table 0#\ as]

0[ Epiclastic marine deposits\ such as beach deposits and conglomeratic levels[1[ Erosional morphologies\ such as abrasion platforms and wave cut!terraces "with the local

name {{faja½s||#[2[ Intensive palagonitization of some volcanic rocks\ particularly visible in the volcanic breccias\

which mean a long!period of interaction with the sea!water[3[ Vertical changes in the compositional facies and depositional structures of the submarine

rocks\ such as a sudden shift from lava!~ows to volcanic breccias and tu}s\ or from autoclasticto pyroclastic deposits that could mean a quick change in the environmental conditions ofvolcanism\ for example a quick reduction in the dimension of water column and consequently\in the hydrostatic pressure[ These situations are particularly common on the volcanicsequences built!up during the transition from the submarine to subaerial volcanism or vice!versa[

Fossilised abrasion cli}s\ stream terraces with eustuarine conglomerates and wave!cut cavesmay also be used to infer sea!level ~uctuations[

These lithological and morphological markers have been classi_ed into _ve groups on the basisof their Table 0]

0[ Chrono and lithostratigraphic setting\ in particular their relationship with their incasingvolcanic rocks^

1[ Relationship with the quiescent periods of volcanism^2[ Present!day altitudes\ discarding the amount of the vertical crustal movements^


Fig[ 3[ Localization of the depositional and geomorphological records of marine erosional and depositional activity"see Table 0 for description#[

3[ The available data for the North!Atlantic climatic temperatures and sea!level ~uctuationswith particular attention to the warm peaks occurred during the inter!glacial stages[

4[ geomorphological characters[

The present!day beaches and scarps are called Ro and the other groups are named as R0\ R1\R2 and R3 according to their increasing comparative ages[


Table 0Main characters of paleo!sea!level records "see Fig[3 for records localization#

Group Depositional!geomorphological Altitude Chronostratigraphic settings Epochcharacters and their basis "see Fig[2# "m[a[s[l[#

Ro Present!day beaches and abrasion 9 Present!day Holocenecli}s

R0 = Santa Cruz platform 24 ! 34 Younger than UC1 "9[1 Ma Bp# The PLEISTO!platforms and deposits are over! CENEimposed on the UC1 youngest units

= Platform and epiclastic deposits of]! Ponta Delgada! Ponta da Faja½! Faja½ Grande! Faja½ de Lopo Vaz

R1 = Conglomerates of] 04 ! 19 Between 9[12 and 9[16 Ma Bp[ Theconglomerates are intercalatedbetween two lava!~ows with 9[12 and9\16 Ma Bp "K:Ar ages fromAzevedo\ 0886#

! Fazenda de Santa Cruz! Ponta da Granada "Zbyszewsky\0865#

R2 = Cli}s and wave!cut terraces of] c[a[ 099 Younger than UC1 "3 9[24 Ma Bp#The platforms are over!imposed onthe UC1 oldest units

! Faja½ do Conde! Faja½ de Pedro Vieira! Faja½ da Ponta Ruiva= Platform and epiclastic deposits ofPonta do Albernaz

R3 = Platforms\ cli}s and a very few caves c[a[ 149 Younger than UC0 "9[4 Ma Bp# Theand coarser epiclastic marine records are over!imposed on the UC0deposits of] units! Faja½ Grande!Faja½zinha! Costa do Lagedo! Ribeira da Cruz Older than UC1 "3 9\14 Ma Bp# The

platforms are partially _lled withUC1 formations

4[ Volcanic\ tectonic and morphological evolution

Taking into account "0# the volcanic sequences Fig[ 2\ "1# the vertical crustal movements"uprises and subsidences# of neo or volcano!tectonic origin "refered in par[1#\ "2# the sequenceof subaerial marine records "refered in par[3#\ "3# the general eustatic sea!level and palaeo!temperature ~uctuations "based on Emiliani oxygen!isotope stages\ in Nilsson\ 0872# and "4#palaeoclimate data obtained on North Atlantic cores "Ruddiman\ 0866 and Ruddiman and


McIntyre\ 0870^ Ruddiman and McIntyre\ 0873#\ it is possible to divide the subaerial mor!phological evolution of Flores Island into three main stages[ Herein for each stage or sub!stagethe volcanism\ crustal movements and sea!level ~uctuations are refered[

4[0[ Stage 0 "9[64 to 9[3 ma bp#[ During this major period two sub!stages might be considered]

4[0[0[ Sub!stage 0[0 "9[64 to 9[44 Ma Bp#4[0[0[0[ Volcanic activity[ It was very intensive and of the emergent type\ i[e[\ predominantlyexplosive and corresponding to the evolution from the proto!island to the island stage "Azevedoet al[\ 0880#[ The BC0 to lower UC0 units record this activity[

4[0[0[1[ Crustal vertical movements It is here a.liated an important tectono!volcanic uprise\whose amplitude might have exceed 099 m[ This uplift is interpreted as being connected to theintensive volcanism of this period\ but could be a consequence of an isostatic compensationbetween two small adjacent blocks of the lithosfer "Azevedo et al[\ 0880#[ As a matter of fact\ itshould be noticed that the subsided island "a seamount with a present!day depth of 349 m#\ whichis located 49 kilometres westwards of Flores Island\ shows a tectonic evolution "Ryall et al[\0872# opposed to that _gured out for the Flores Island] while an uprise process dominated inFlores Island\ the seamount subsided at about the same rate\ thus supporting the concept of anisostatic compensation between the two adjacent domains of the oceanic crust[

4[0[0[2[ Sea!level ~uctuation In spite of the uncertain global eustatic sea!level and palaeo!temperature ~uctuations for the North Atlantic during this time lapse "Nilsson\ 0872#\ thedevelopment of a warm period*correlative to Mindel\ Elster or Anglian Glacial Stage*and theconsequent regression tendency of the sea!level\ accelerated the emergency of the island[ Basedon the present!day data it is impossible to _gure out the accurate amplitude of this regression\but taking into account the association between the volcano!tectonic uprise and the regressivetendency of the sea!level\ the upsurge of the island during this sub!stage should have been of theorder of a few hundreds of meters\ probably c[a[ 399 m[ This is con_rmed by the present!dayoccurrence of BC0 volcanic formations at 399 m a[s[l[ It should be recalled that the BC0 unitswere built!up during proto!island emergent volcanism "Azevedo et al[\ 0875^ Azevedo et al[\0880#[

4[0[1[ Sub!stage 0[1 "9[44 to 9[3 Ma Bp#4[0[1[0[ Volcanic activity[ It comprehends the last period of the UC0 volcanism\ which wascharacterised by the extrusion of large amounts of lavas from two or three large volcanic centres[It was followed by a long lasting period of volcanic quiescence "9[4 to 9[3 Ma Bp#[

4[0[1[1[ Crustal vertical movements During this sub!stage there happened an inversion in theuprising tendency[ The very large amounts of lava extruded during the UC0 last phases ofvolcanism might explain the subsidence of the whole island and particularly the collapse of itscentral zone\ which led to the formation of volcanic calderas[


4[0[1[2[ Sea!level ~uctuation The development of a general transgressive period!correlative tothe Interglacial Stage that preceded a cold period that might be correlated to the Mindel GlacialStage "Nilsson\ 0872# was responsible for an important erosion of the subaerial domain of theisland[

The group of records R3 should have been formed during one of the peaks of this transgressivetendency[ The 149 m a[s[l[ of R3 records _gures out that the amplitude to this transgressionshould have been less than 199 m[ The neo and tectonic subsidence of the island ampli_ed thetrangressive amplitude in this speci_c domain\ as to reach 199 m a[s[l[

4[1[ Stage 1 "9[3 to 9[1 Ma Bp#

4[1[0[4[1[0[0[ Volcanic activity[ It comprehends the last major volcanic episode of this island\ the UC1volcanism Fig[ 2[

4[1[0[1[ Crustal movements No important vertical tectonic movements was registered throughoutthis stage[

4[1[0[2[ Sea!level ~uctuations A regressive tendency related to a climatic cold period!correlativeto the Emiliani Stage 7 or to the Mindel\ Anglian or Elster Glacial Stages "Nilsson\ 0872#*wasfollowed by a transgression during a warm period correlative to the Emiliani Stage 6 or toInterglacial M:R\ Holstein and Hoxnian Stages "Nilsson\ 0872#[

The chrono and lithostratigraphic settings of the R2 and R1 records "see Table 0# suggest thatthey were probable constructed during two short pauses in the overall UC1 volcanic activity\ thuscorresponding to two intermediate stabilization episodes on the general transgressive tendency[

4[2[ Stage 2 "9[1 Ma until the present#

4[2[0[4[2[0[0[ Volcanic activity[ After a long volcanic quiescent period "9[1 to 9[993 Ma Bp#\ thereoccurred a short period of volcanism\ the UC2 volcanism "9[993 to 9[992 Ma Bp#[

4[2[0[1[ Crustal movements It is probable that the slow rate subsidence of a normal oceanic crust\which is the result of the thermal contraction "Sclater et al[\ 0860# was extensible to the oceaniccrust in Flores area\ at least after the ending of the major steps of insular volcanism*UC2 andUC1 volcanism[

4[2[0[2[ Sea!level ~uctuations According to the data available after the study of some NorthAtlantic oceanic drilled cores which represent this period "see for example Ruddiman\ 0866^Ruddiman and McIntyre\ 0870^ Ruddiman and McIntyre\ 0873^ Heinrich\ 0877^ Rodrigues andDias\ 0878 and Baas et al[\ 0886# the palaeotemperatures and the eustatic sea!level went through!out an oscillatory evolution that includes two major cold periods "the _rst is correlative to Riss\Saale and Wolstonian Glacial Stage and the last one corresponds to the Last!Glacial Stage#


intercalated by two warm periods "equivalent to the Last!Inter!Glacial Stage and to the Post!Glacial Stage#[

The group of records R0 might register the peak of one intermediate transgressive period[According to the its chronostratigraphic setting "see Table 0# this transgression will correspondto the Emiliani Stage 4 or to the Last Interglacial "Eem# period "Nilsson\ 0872#\ which have beendated in North Atlantic cores between 9[016 Ma and 9[962 Ma Bp "Ruddiman and McIntyre\0873#[

5[ An attempt for the correlation with Santa Maria and Porto Santo Islands

In spite of the fairly well known volcano!tectonic setting and evolution for the MacaronesianIslands "Azores\ Madeira\ Canaries and Cape Verde Archipelago# ðFig[ 1"a#Ł\ which is expressedby a good number of papers and other scienti_c works\ the scarcity of investigation on eustaticsea!level and palaeo!climate "palaeotemperature# ~uctuations is evident[ Meanwhile\ similarrecords of paleo!sea!level have been identi_ed in Santa Maria Island "Serralheiro and Madeira\0889#\ the oldest and the eastern island of the Azores Archipelago ðFig[ 1"b#Ł\ and in Porto SantoIsland "Ferreira\ 0886#\ the oldest island of Madeira Archipelago ðFig[ 1"a#Ł[ In both situations\the sequence of records con_rmed the general regressiveÐtransgressive evolution "eustatic sea!level and palaeotemperature ~uctuations# described for Flores "see par[ 4#[ However\ the present!day altitudes of correlated records expresses great di}erences between Flores and the other twoIslands\ where they lie at lower heights[ This setting may be explained taking into account thedi}erences in the crustal behaviour and tectonic evolution of the three volcanic islands[ As amatter of fact\ contrasting with the other two islands\ Flores can be considered as being a veryyoung island\ whose last volcanic activity*UC2 volcanism*is dated of 2\999 years Bp\ while inSanta Maria the volcanism stopped during the Middle Pliocene "Serralheiro and Madeira\ 0889#and in Porto Santo it ceased in the Middle to Upper Miocene "c[a[ 8 Ma Bp\ Ferreira\ 0886#[ Asin these two older islands there is no evidence of important neo!tectonic uprises\ either the lowerheights of the correlative abrasion platforms should be explained as a consequence of the normalslow rate subsidence of the oceanic crust "Sclater et al[\ 0860#[

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Volcanic gaps and subaerial records of palaeo!sea!levels ... · Volcanic gaps and subaerial records of palaeo!sea!levels on ... the balance between constructive processes "e}usive