Transcript GEOPHYSICAL STUDY OF SALDANHA MASSIF (MID

TECTONIC SETTING OF THE
SALDANHA MASSIF
(MID-ATLANTIC RIDGE, 36º 33’ 50’’ N)
Nuno Lourenço (1) J M Miranda (2) P. F. Silva (,2,3), R. Costa(4),
(1) Centro de Investigação Marinha e Ambiental
(2) Centro de Geofísica da Universidade de Lisboa
(3) Instituto Superior de Engenharia de Lisboa
(4) CREMINER
Objectives:
•
To study the morpho-tectonic features and geology of the
Saldanha Massif (Mid-Atlantic Ridge) using sonar, bathymetry
and seafloor observations.
•
To use degrees of oxidation in sampled basalts, materialised as
pseudo-ages (e.g. ZHOU et al, 2001) and confront them to their
structural position in the massif.
Define a interpretative model
for the generation of the massif
Data Sources:
Swath bathymetry :
SIGMA1 and SUD-AZORES (100m grids)
FLORES (20m grids)
TOBI sonar:
HEAT cruise
Nautile Submersible Observations:
FLORES cruise
SALDANHA cruise
Location
-MAR Southwest of the Azores
(36º 33’ 54’’N, 33º 26’ W)
- Non-transform offset (NTO)
between AMAR and FAMOUS
segments
Perspective View Of The New FLORES Cruise Bathymetry (VE = 3x)
Saldanha Massive
Famous S.
Morpho-tectonic Features
Characteristics:
32km2 of distinct smooth topography
Slope between 10º and 20º
Corrugations along Nuvel 1 flow lines
Some degree of masking by latter NTO/Famous tectonics
Surface Limits:
 West- Sharp NNW-SSE
depression is a structural limit.
 East- Increase of relief roughness
expresses volcanic terrain .
 South – Abyssal hill fabric
 North – left en-echelon grabens
(base of the northern NTO wall)
TOBI Interpretation
Image Characteristics:
Moderate to high backscatter
in the massif.
Mottled textures
Mass wasting and fault scarp
talus
Depression A displays a doll
tone suggesting high
sedimentation
NE-SW Linear strong acoustic
echoes interpreted as faults
(dives S10, S9, S7)
Top of Massive
Structural Meaning of the Corrugations
-Ground truth by dives S9, S7, S11, F12Small throw normal faulting on the Southern flank
Ridges (tens of meters high) with the same orientation
SEAFLOOR GEOLOGY
Highly heterogeneous and
strongly sedimented
Littered with blocks of different
sizes and natures
No Layering
Sampled lithotypes range from:
Serpentinites, Steatites, Spilites,
Gabbros, Basaltic Breccias and
Pillow fragments
TECTONIC MELANGE
Photo by IFREMER
REPRESENTATIVE GEOLOGICAL CROSS-SECTIONS FROM DIVES
DIVE S11
DIVE S9
Photos by IFREMER
Photos by IFREMER
Is Saldanha Massive a tectonic window resulting from
detachment tectonics?
Morphology, tectonics, lithological characteristics and their spatial distribution
leeds to the interpretation of Detachment Tectonics as the main
mechanism involved in the genesis of Saldanha
When comparing to other detachment
areas in the MAR, Saldanha has:
More complex tectonic environment
Lesser defined topography
with similar corrugations
Smaller time span and area
(Bathymetry from Blackman et al, 1998)
Spatial Distributiuon of Pseudo-Ages on the Massive
Indirect determination of Basalts
oxidation degree by
thermomagnetic means – Curie
temperature.
Application of the relation
between oxidation degree (z) and
pseudo-age (t) z = p + qlog(t),
with, p0.38 and q0.38, from
Zhou et al., (2001).
Confronting with Nuvel1
age estimations:
Pseudo-ages are generally higher inside (red) and lower outside (green) the detachment surface
Detachment activity develops pathways for fluid circulation enhancing oxidation
Green samples are isolated from this process and/or can also result from more
recent off-axis volcanic activity
Conclusions
-Based on morpho-tectonic and seafloor observations, we interpret the mantle section
exposed on the Saldanha Massive as a tectonic window promoted by detachment
fault activity. Surface corrugations represent small throw scarps or otherwise ridges.
Volcanic elements of the tectonic melange are regarded as remnants of delaminated
crustal blocks from the hanging wall.
-Nuvel 1 age estimation suggest that nucleation started about 0.9 My and developed
diachronically at about 1cm/yr southwards. Approximately 0.3My fault activity
was locked due to beginning of a volcanic stage on Famous Segment.
-Pseudo-age estimations agree with the detachment model:
Samples lying inside the detachment area generally have higher oxidation rates,
interpreted as a result of enhanced fluid circulation promoted by more pervasive
fracturation. West of the detachment, lower then expected pseudo-ages are interpreted
as possibly resulting from younger off-axis volcanism.