M. Pipan, P. Diviacco, R. Geletti, A. Del Ben, I. Finetti
Dipartimento di Scienze Geologiche, Ambientali e Marine, Universita' di Trieste
SEISMIC STUDY OF THE CRUSTAL STRUCTURE IN THE MARGUERITE BAY-ADELAIDE
ISLAND AREA (ANTARCTIC PENINSULA)
ABSTRACT
The integrated interpretation of WARR (Wide Angle Reflection and Refraction) and NVR (Near Vertical
Reflection) seismic profiles obtained in the Marguerite Bay-Adelaide Island area (Antarctic Peninsula), by the
British Antarctic Survey and the University of Trieste in the framework of a cooperative research, allows the
reconstruction of a structural model characterized, in the northern area (offshore Southern Graham Land), by a
stratified deep crust and by the effects of post-collisional extensional tectonics. The Moho depth in the northern
part ranges between 11 km in the oceanic sector and 25 km beneath Adelaide Island. Large normal faults affect
the whole crust between the continental rise and the outer continental shelf. The P-waves velocity in the mantle
is rather high (8.1 km/sec) in this part of the area of study. A maximum crustal thickness of about 30 km is
observed at the eastern end of the southern profile. The estimated P-wave velocity in the mantle is here relatively
low (7.6-7.8 km/sec). A set of faults aligned along the trend of the Adelaide Fracture Zone and its projection
across the continental margin were identified from the interpretation of the NVR seismic data, integrated by
profiles from previous surveys. The inferred location of the Adelaide Fracture Zone projection on to the
continental margin correlates with the anomalies observed in recent aeromagnetic data from the area and it
probably represents the contact between two sectors characterized by different crustal structure. This hypothesis
is consistent with the seismic data in the area of study.
STUDIO SISMICO DELLA STRUTTURA CROSTALE NELL’AREA DELLA MARGUERITE BAYADELAIDE ISLAND (PENISOLA ANTARTICA)
RIASSUNTO
L’interpretazione integrata di profili WARR (Wide Angle Reflection and Refraction) e NVR (Near Vertical
Reflection) registrati nell’area della Marguerite Bay-Adelaide Island (Penisola Antartica), nell’ambito di un
programma di cooperazione scientifica tra il British Antarctic Survey e l’Universita’ di Trieste, consente la
ricostruzione di un modello strutturale caratterizzato da una crosta profonda stratificata nell’area settentrionale
(Southern Graham Land) con evidenze di tettonica distensiva postcollisionale. La profondita’ della Moho varia
tra 11 km nel settore oceanico fino a 25 km in corrispondenza della Adelaide Island. Ampie faglie normali
interessano l’intero spessore crostale nella fascia compresa tra il rialzo continentale ed il margine della
piattaforma. La velocita’ delle onde P nel mantello e’ relativamente alta (8.1 km/sec) in questo settore dell’area
indagata. Lo spessore crostale massimo si registra all’estremita’ orientale del profilo sismico meridionale con
valori di almeno 30 km e velocita’ delle onde P relativamente basse (7.6-7.8 km/sec). L’interpretazione di dati
NVR, selezionati tra i rilievi effettuati nell’ambito di precedenti spedizioni ed integrati dai profili del presente
progetto, ha consentito di individuare un set di faglie allineato nell’area della Adelaide Fracture Zone e della sua
proiezione sul margine continentale. La posizione di tale proiezione e' correlata con le anomalie messe in luce da
recenti rilievi aeromagnetici e rappresenta probabilmente il contatto tra due settori caratterizzati da differente
struttura crostale. Questa ipotesi e' in accordo con i risultati dell'analisi dei dati sismici nell'area in esame.
INTRODUCTION
A joint British-Italian seismic exploration project in the area of Marguerite Bay and Adelaide Island (Fig.1) was
completed during the 1994/95 austral summer as part of a scientific cooperation between the British Antarctic
Survey (BAS) and the Institute of Geodesy and Geophysics of the University of Trieste (IGG). The project aims
at reconstructing the structure and the geodynamical evolution of the Pacific margin of the Antarctic Peninsula in
the area of the Adelaide Fracture Zone (AFZ) and its projection across the continental margin. The tectonic
setting and evolution of the Pacific margin of the Antarctic Peninsula is well documented by Barker (1982) and
Larter and Barker (1991). A series of ridge-crest trench collisions occurred in this area due to the arrival at the
trench of different segments of the Antarctic-Phoenix ridge (Fig.2). Since the Late Cretaceous, the direction of
subduction paralleled that of the fracture zones which tectonically segmented the subducting Phoenix plate.
During convergence, the ridge crest was parallel to the trench in most sectors of the margin, thus resulting in a
synchronous collision along each section of the ridge. Collisions occurred progressively from SW to NE starting
from 50 m.y. ago in the southern part (southern Alexander Island) to 4 m.y. ago to the south of the Hero Fracture
Zone. Magmatic activity stopped along each segment of the arc as the corresponding segment of the ridge
approached the trench. Uplift and erosion of the arc and forearc sectors took place soon after (Larter et al.,
1989). After the collision of each ridge segment with the trench, seafloor spreading and subduction stopped and
the corresponding segment of the margin became tectonically passive.
The analysis of the magnetic anomalies at the margin provides dates for the end of subduction within each
segment. The collision at the margin adjacent to Marguerite Bay took place at 19.8 Ma and that off Adelaide
Island at 16.5 Ma (Larter and Barker, 1991). A detailed aeromagnetic grid recently collected and processed by
the British Antarctic Survey (Johnson, 1997) shows abrupt offsets in the trend of the Pacific Margin Anomaly
and a strong NW-SE fabric of the magnetic anomalies in the Marguerite Bay and Adelaide Island area (Fig.8).
Such features are aligned with the trend of the Adelaide Fracture Zones in the Pacific Plate.
The BAS-IGG joint onshore/offshore seismic project was designed to study the different crustal structures on
both sides of the projection of the AFZ.
The seismic dataset obtained during the austral summer 1994/95 encompasses marine multichannel seismic
(MCS) reflection data and onshore wide angle reflection and refraction (WARR) seismic data. Parts of the
marine seismic dataset exhibit large signal to noise ratios and successfully image the deep crustal reflectors. The
processed MCS data show crustal structure and velocity layering across the oceanic basin and continental rise
and slope offshore Adelaide Island. MCS data are used to model and interpret the WARR data (Reading et al.,
1997). In particular, three deep crustal reflectors were identified and interpreted in the oceanic basin:
'A' The reflection from the top of the basalts
'C' A discontinuous intrabasement reflector observed at an average depth of 7.2 sec TWT
'Mo' A high amplitude reflection very likely associated with the Moho discontinuity
Deep crustal horizons and overlying sedimentary sequence are partially deformed near the continental rise and
slope. Reflector 'Mo' is interpreted across the continental margin and the mid shelf area in the northern sector of
the area of study.
MARINE MCS SEISMIC DATA
Field data acquisition
During the programme, marine near vertical reflection (NVR) seismic sections and onshore wide-angle
reflection and refraction (WARR) profiles were recorded simultaneously in the area of Adelaide Island and
Marguerite Bay (Fig.3). Magnetic and gravity recordings along the marine seismic sections were planned to
integrate the seismic dataset. The acquisition of the offshore near vertical seismic sections was carried out by the
IGG with the R/V OGS-Explora. The BAS obtained the WARR profiles by recording the shots fired by the R/V
OGS-Explora with six portable seismic recorders (Reading et al., 1997). The offshore near vertical acquisition
parameters were designed for the exploration of deep crustal objectives. The source was a high energy airgun
array characterized by a stable spectrum of the near-field signature up to 100 Hz.
Two lines approximately perpendicular to the trend of the Pacific Margin Anomaly were completed (Fig.3):
LINE I95131 crossed the area to the north of the AFZ projection, from the abyssal plain to the limit of the
charted waters off Adelaide Island in proximity of the front of Shambles Glacier. It was approximately
aligned with the northern shore seismic stations.
LINE I95133 was aligned with the southern shore stations. The easternmost segment is located in an area
unexplored by seismic parties, along a deep water channel which enters Marguerite Bay and is
characterized by a limited presence of floating ice.
The interconnecting line I95132 was approximately perpendicular to the projection of the AFZ on the
continental margin and was entirely recorded on the continental shelf in the area off Adelaide Island and
Marguerite Bay.
Data Processing
The MCS and WARR data were processed with the sequences reported in Tab.2. Velocity spectra, Constant
Velocity Gathers and Stacks were used for velocity analysis of the MCS data. A preliminary dereverberation was
eventually performed by means of FK pie-slice filters after testing alternative techniques [TX edge detection
based techniques (Finetti et al., 1995), KL transform based techniques].
PROCESSING SEQUENCE - BAS/IGG PROJECT - MCS DATA
1
2
3
4
5
6
7
8
9
REFORMAT (FROM SEGD DEMUX TO SEGY)
SPHERICAL DIVERGENCE CORRECTION AND TRACE EDITING
SORT
DECONVOLUTION
VELOCITY ANALYSIS (PHASE 1: MULTIPLE)
DEREVERBERATION: (FK FILTERING OR KARHUNEN-LOEVE TRANSFORM
BASED FILTERING OR TX EDGE DETECTION BASED FILTERING)
VELOCITY ANALYSIS (PHASE 2: PRIMARIES)
NMO CORRECTION, MUTE AND STACK
TIME VARIANT FILTERING (TVF)
PROCESSING SEQUENCE - BAS/IGG PROJECT - WARR DATA
1
2
3
4
5
REFORMAT (FROM RAW DATA TO SEGY)
BAND PASS FILTER AND MUTE
LINEAR NMO CORRECTION AND STACK OF NEAR-COMMON OFFSETS
(FOLD=400%)
DECONVOLUTION
NMO REVERSAL AND REDUCED TRAVELTIME DISPLAY (VEL=8 KM/SEC)
Tab.2
RESULTS
The results of the processing and interpretation of the MCS reflection data successfully image the deep crustal
reflectors to the Moho discontinuity in the northern sector of the area of study. Extensive velocity analyses
indicate interval velocities ranging between 2.7 and 3.0 km/secfor the sedimentary cover overlying the basaltic
basement in the oceanic basin. The average interval velocity for the basement in this area is 6.3 km/sec, as
results from the velocity analysis for the reflector 'Mo'. Such velocity structure corresponds to an average crustal
thickness of 12 km in the abyssal plain. A low amplitude reflector is locally observed in this area at an average
depth of 7.2 s. It marks the bottom of an intrabasement layer with average interval velocity between 5.8 and 6.0
km/sec. Large listric faults show up in the continental rise and slope area which apparently postdate the end of
the collision process in this sector of the Pacific margin of the Antarctic Peninsula (16.5 m.y. ago, after Larter
and Barker, 1991). The crustal structure of the passive margin is characterized by faulting of the deep crustal
horizons and partial deformation of the overlying Mid-Late Miocenic sedimentary sequence.
Figs. 4 shows the results of the processing (a) and interpretation (b) of the part of line I95131 obtained in the
abyssal plain. A high amplitude continuous reflection from the Moho discontinuity (Mo) can be interpreted at an
average depth of 8.3 s. The intrabasement reflector C can be traced at an average depth of 7.4 s and it is
particularly evident in the rightmost part of the figure, where diffractions originate from possible discontinuities
of such reflector. The top of the basaltic basement (S) lies subhorizontal at an average depth of 6.4 s. In this area,
the average thicknesses of the basement and of the overlying sedimentary sequence are 6.3 km and 2.2 km
respectively.
Fig. 5 shows a segment of line I95131 sampling the transition between the abyssal plain and the continental rise.
The extensional regime associated with the formation of a passive margin of Miocenic age is here interpreted.
Part of the listric faults deform and locally disrupt the deeper part of the sedimentary sequence, with an
unconformity (P) marking the separation between an apparently undisturbed sedimentary interval of possible
Plio-Quaternary age. No reflections from the intrabasement reflector can be interpreted in this sector. The
apparent dip of the Moho is related to the decreasing thickness of the lower velocity sedimentary wedge.
The structure of the continental rise and slope is shown by Fig.6. A differential displacement at the hanging wall
of the master fault in Fig.6 generates a horst-like feature bordered to the SE by a small antithetic fault. The
reflection from the Moho is partly obscured by the complex shallow structures.
Data and phase interpretations of the WARR profiles are shown in Fig. 7 (a,b). Arrivals refracted at the mantle
are consistent with seismic velocities of 8.1 km/secand 7.6-7.8 km/secfor the northern and southern profile
respectively. The mantle arrivals are also consistent with an eastward increasing Moho depth. The maximum
crustal thickness in the area of study is observed at the eastern end of the southern profile (approximately 30
km).
A set of faults aligned along the trend of the Adelaide Fracture Zone and its projection across the continental
margin were identified from the interpretation of the NVR seismic data, integrated by profiles from previous
surveys. The inferred location of the Adelaide Fracture Zone projection across the continental margin correlates
with the anomalies observed in recent aeromagnetic data from the area and it probably represents the contact
between two sectors characterized by different crustal structure (Fig.8).
DISCUSSION
The tectonic segmentation of the Antarctic Peninsula in the Marguerite Bay-Adelaide Island area is apparently
linked with differences in crustal structure and deep lithospheric evolution of the sectors separated by the
projection of the AFZ across the continental margin. The areas situated to the north and south of the interpreted
alignment of fractures are characterized by the following differences:
1. Crustal structure: the Crust is apparently not stratified and thicker in the southern sector.
2. Seismic velocities: the southern sector exhibits lower Mantle velocity and higher upper Crust velocity.
The crustal thicknesses observed in the northern and southern sectors would be consistent with different
extension rates. The reduced crustal thickness in the northern sector may result from the continuation of
extension and crustal thinning to the north of AFZ after the collision of the ridge in the Tula Fracture Zone-AFZ
segment. The lower seismic velocity of the Mantle may be related to partial melting of the subducting slab or to
intrusions across the slab of high temperature astenospheric magma.
The seismic records obtained in the area in the framework of this experiment represent a sample of the complex
response characteristic of tectonically segmented magmatic arcs. The integrated NVR and WARR seismic
investigation seems an effective strategy to study areas of such complexity as it allows the reconstruction of a
constrained crustal model and seismic velocity field. Possible improvements in the experiment for similar
investigations may encompass the recording of reverse WARR profiles and the recording of the source signature
to process both the NVR and WARR seismic data.
ACKNOWLEDGMENTS
This work was supported by PNRA grant n.040067.
Testo controllato dalla dr. Martina Busetti, Osservatorio Geofisico Sperimentale, Trieste
REFERENCES
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Finetti I., Galuppo P., Geletti R. and Pipan M., 1995: Seismic dereverberation in the xt domain by means of an edge
detection algorithm; 57th Conference, European Association of Geoscientists & Engineers, paper A053
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on a Young Subducting Plate; Journal of Geophysical Research, v.96, n. B12, p.19583-19607
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Processes, C.A. Ricci Ed., Terra Antartica Publ., Siena (Italy), p.691-696
Fig.1: Location map of the study area. The area is
centred about the projection of the Adelaide Fracture
Zone on the continental shelf.
Fig.3 - Seismic lines and onshore seismic stations location map
(onshore stations shown by dots).
Fig.2:
Tectonic setting of the Antarctic
Peninsula 20 m.y. ago (after Larter and
Barker, 1991).
Fig.4 - Seismic Line I95131, S.P. 588-720 (a)
Stack section (b) Interpreted stack. The following
reflectors are indicated: (Mo) Moho, (C)
Intrabasement discontinuity, (A) Top basalts, (B)
Deep sedimentary unconformity, (P) Shallow
sedimentary unconformity
Fig.5 - Seismic Line I95131, S.P. 1000-1146 (a) Stack
section (b) Interpreted stack The following reflectors are
indicated: (Mo) Moho, (A) Top basalts, (B) Deep
sedimentary unconformity, (P) Shallow sedimentary
unconformity.
Fig.6 - Seismic Line I95131, S.P. 1250-1426 (a) Stack
section (b) Interpreted stack The following reflectors are
indicated: (Mo) Moho, (A) Top basalts, (B) Deep
sedimentary unconformity, (P) Shallow sedimentary
unconformity.
Fig.8: Map showing the alignment of the crustal
fractures interpreted in the study area (aeromagnetic
data from Johnson, 1997).
Fig.7: WARR data and phase interpretation of the (a)
northern and (b) southern WARR profile.