RE-EXAMINATION OF THE MAGNETIC LINEATIONS OF THE GASCOYNE
AND CUVIER ABYSSAL PLAINS, OFF NW AUSTRALIA
A THESIS SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI'I IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
IN
GEOLOGY & GEOPHYSICS
AUGUST 2004
By Masako S. Robb
Thesis Committee:
Brian Taylor, Chairperson Gregory F. Moore Garrett T. Ito Acknowledgments
We thank our co-investigators on the NW Australia study (Neal Driscoll, Garry Karner,
Andrew GoodHffe, Damien Ryan, Fiona Sutherland, Uri ten Brink and Michael Tischer) as well as the captain and crew of R/V Maurice Ewing cruise EWOl13. Archived geophysical data were provided by Geoscience Australia and Will Sager.
111 Abstract
The Exmouth and Cuvier margins of NW Australia and the adjacent Gascoyne and
Cuvier Abyssal Plains formed when Greater India rifted and separated from Australia during the Late Jurassic and Early Cretaceous. Using new and existing data, we re examine the magnetic lineations of the Gascoyne and Cuvier Abyssal Plains in order to better determine the timing of final breakup, the early seafloor spreading history, and the origin of several bathymetric features. The time of final continental break-up is similar along the middle Exmouth (at MlON or MIl) and Cuvier (at MlON) margins, and can not be the cause of their different (wide vs. narrow) rift architecture. In contrast, the intervening southern Exmouth margin did not completely separate until M7 time. In the
Cuvier Abyssal Plain, Sonja Ridge was a spreading axis that propagated northward from
M6 to M4 time at the expense of Sonne Ridge. Both ridges became extinct during M4 to
M3 time when another spreading center propagated south along the Greater India ocean continent boundary. This eliminated the 300 krn-Iong-offset Cape Range Fracture Zone and left formerly Indian lithosphere on the Australian plate. An extinct ridge and pseudofault indicate M2 to MO southward propagation within oceanic crust of the
Gascoyne Abyssal Plain as part of the same early spreading reorganization.
IV Table ofcontents
Acknowledgments "' .. '" "' .... "' ... "' ... '" '" '" '" "' .. '" "'. "'. '" "' .. '" '" '" "' .."'. '" '" "' ... '" "' .. '" '" "' .. '" '" "'. '" "'. "'. '" "' .."'... iii illJstract •••••.•••••••••••.••••••••••••••••••••.•••••••••••••••••••••••••••••••••••••••••••••• iv
List of tables '" '" "' '" '" "' .. "' '" "' .. '" "'. "'. '" "' "' "' .. "' "' '" '" "' '" "'.. vi
List offigures .. '" '" '" . '" '" '" '" '" . '" '" '" . "' '" . '" '" '" '" .. '" '" '" '" . '" . '" '" '" '" '" '" '" '" "'. "'. '" . "' .."'.. vii
1. Introduction "' '" "' "' '" '" "' .. '" "'. "' .. "' "'. "' .. "' '" '" "' '" "'. "'.... 1
2. Study area . '" .. '" "' .. "' "'. "' '" "' '" '" '" "' '" '" '" '" '" '" '" '" '" '" '" '" '" '" '" '" '" '" '" '" '" '" "' .. '" '" "'. '" 4
3. Previous seafloor magnetic studies 7
4. Data", "' .. "' .. "' .. '" '" 10
5. Results and interpretation 12
euvier Abyssal Plain...... 12
Middle Gascoyne Abyssal Plain 15
Southern Gascoyne Abyssal Plain...... 17
6. Discussion 20
Rifting to spreading transition and the evolution of early seafloor 20
Implications for the different deformational styles...... 21
Causes of the multiple propagations...... 23
7. Conclusion 25
References 38
v List oftables
1. Cruise name and the availability of bathymetric data for the selected tracks in Figure 4 27
VI List offigures
Figure
1. Bathymetric map of the NW Australian margins showing the major physiographic features and ODP sites 28
2. Previous identifications ofmagnetic lineations 29
3. Magnetic anomalies plotted perpendicular to tracks on a bathymetric base map...... 30
4. Selected tracks used for magnetic forward modeling 31
5. Projected profiles for magnetic and bathymetric data with synthetic model anomalies...... 32-34
6. Identifications of magnetic lineations (from this study) in map view 35
7. Age and geometry of the Mesozoic seafloor derived from this study...... 36
8. Evolution of the early seafloor at four different times...... 37
vii 1. Introduction
Different deformation styles are observed in the world's rifted margins. The differences include the width of deformed continent (wide vs. narrow rift) and the characteristics of the transition zone between deformed continent and oceanic crust
(abrupt or broad transition, existence of magmatism, high and/or low angle faults). How particular parameters control the style of rifting is not well understood. Identifying the seafloor spreading magnetic lineations of the adjacent abyssal plain can constrain the time and location at which a margin broke up, the duration of rifting, the rifting to spreading transition, and the tectonic evolution of the early seafloor. In many rifted margins, however, such processes are poorly determined magnetically due to the common existence of Magnetic Quiet Zones in the world ocean basins [Owen, 1983].
Some of the quiet zones may be controlled by the existence of transitional crust (that has neither continental nor oceanic characteristics) near the continent-ocean boundary (e.g., the eastern Gulf of Aden [Cochran, 1982] and south of Australia [Talwani et al., 1978]), and others are simply bordered by a long interval of earth's magnetic polarity without reversal, such as the Cretaceous Quiet Zone (e.g., West Iberia [Whitmarsh and Miles,
1995] and NW Africa [Hayes and Rabinowitz, 1975]).
The rifted margins of NW Australia exhibit major along-strike differences in crustal architecture and inferred deformational styles. For example, the extensional structures occur over a much wider region on the Exmouth margin (-250 km) than on the adjacent Cuvier margin (-65 km) [Hopper et al., 1992]. Given that the initial lithospheric conditions are expected to be similar on the two contiguous margins, what caused the two margins to be so different? Besides the major difference between the Exmouth and
1 Cuvier margins, there are differences in margin structures along the Exmouth margin. For example, the southwest corner of the Exmouth Plateau forms a bathymetric high whose origin is controversial [Ludden and Dionne, 1992]. These along-strike variations in margin structures make the NW Australian margins an excellent place to compare and contrast the different styles of margin formation. The adjacent abyssal plains (the
Gascoyne and Cuvier Abyssal Plains) preserve a Mesozoic series of magnetic anomalies juxtaposed to the margins. The details of these seafloor spreading magnetic lineations help us to understand the formation of the NW Australian rifted margins, as well as the regional tectonic evolution of the early Indian Ocean right after the continental breakup.
Despite the numerous magnetic studies in the past decades, some conflicting interpretations of the origin of the magnetic and bathymetric features reveal several problems in understanding the evolution of the region. Although recent studies ([Veevers et ai., 1985; Fullerton et ai., 1989; Mihut and Maller, 1998]) all explain the complexity of the observed magnetic anomaly patterns by ridge jumps ancIJor propagations, the direction and location of the propagators are not consistent. In some areas the correlation of anomalies is poor ancIJor the data coverage is insufficient, leaving questions as to the identification of the magnetic lineations. The complexity of magnetic lineations in this region is often associated with bathymetric features, and thus identification of magnetic lineations in their relation to bathymetry is necessary.
Using additional data, we re-examine the magnetic lineations of the Gascoyne and
Cuvier Abyssal Plains. The purposes of this study are to better determine (1) the timing of final breakup, (2) the early seafloor spreading history, and (3) the origin of various bathymetric highs. We present the results of our magnetic modeling with bathymetric
2 profiles and show a revised model for the evolution of seafloor that followed the rifting of the Exmouth and Cuvier margins.
3 2. Study area
The area of study off NW Australia includes stretched and subsided continental platforms (Exmouth Plateau and Northern Carnarvon Terrace) and two ocean basins
(Gascoyne and Cuvier Abyssal Plains) (Figure 1). We refer to the continental slope and shelf (Exmouth Plateau) adjacent to the Gascoyne Abyssal Plain (GAP) as the Exmouth margin. Similarly, we refer to the slope and shelf (Northern Carnarvon Terrace) adjacent to the Cuvier Abyssal Plain (CAP) as the Cuvier margin. At least two separate tectonic events formed the NW Australian rifted margins. The first culminated in spreading of the
Argo Abyssal Plain (AAP) in the Late Jurassic with a seafloor spreading fabric oriented
-N70oE, and the second, which began prior to MlO, created the Exmouth and Cuvier margins and separated Greater India and Australia with a seafloor spreading fabric trending -N30oE [Fullerton et ai., 1989]. The Exmouth and Cuvier margins are offset
(-300 km) along strike by the Cape Range Fracture Zone (CRFZ).
The bathymetry (Figure 1) shows the first order difference in the width of the undersea plateaus across the CRFZ, the area shallower than 2000 m being much wider to the north. Seismic reflection data also confirm that the thinned and extended continental crust covers a much wider region on the Exmouth margin (250 km or more) than on the
Cuvier margin (no more than 65 km) [Hopper et ai., 1992]. This may result from the
Exmouth margin being stretched more than the Cuvier margin before the final breakup.
That is, the wide Exmouth margin would be expected to have a longer duration of rifting.
In this case, if the timing of initial rifting is the same, then the timing of final breakup of the Exmouth margin would be later - as proposed by Larson [1977], Larson et al. [1979], and Powell and Luyendyk [1982].
4 However, recent magnetic studies suggest that the timing of final breakup is contemporaneous for the two margins [Fullerton et ai., 1989]. Hopper et al. [1992] suggested that there was a difference in the duration of rifting but that it resulted from a different time of initial rifting. They proposed that the deformation of the Exmouth margin was affected by the earlier (Argo) north-south rifting/spreading event and that, as a result, the Exmouth margin experienced a longer duration of extension even though the final breakup was contemporaneous with the Cuvier margin.
Another explanation for the difference in margin widths is the asymmetric breakup of multiple rift systems [Veevers and Cotterill, 1978]: the amount of stretching was similar but the continent simply broke up at different locations. Veevers and Cotterill
[1978] suggest that the broad marginal plateau of the Exmouth margin developed from a region initially between two localized rift systems on the outer and inner edges of the plateau.
Although previous work has highlighted the differences between the Exmouth and
Cuvier margins, there are also significant differences in margin structure along the
Exmouth margin. Bathymetry (Figure 1) shows that the southwest corner of the Exmouth
Plateau is broad, with bathymetric highs extending further offshore. The middle part of
Exmouth Plateau is indented by a deep abyssal plain. The northwest corner of Exmouth
Plateau has another bathymetric high (Joey Rise) separating the GAP and AAP. This difference in the margin bathymetry along the Exmouth margin is correlative with differences in the magnetic characteristics. In this study, we sub-divide the GAP into the middle (without bathymetric highs) and the southern and the northern (with bathymetric highs) parts. We make detailed magnetic studies of three areas; CAP, middle GAP, and
5 southern GAP. We do not include the northern GAP that relates to the formation of the
AAP. The setting of the three study areas may be described as follows:
euvier Abyssal Plain: Bounded to the north by the CRFZ and to the south by the
Wallaby Plateau. Wallaby Plateau is an area of bathymetric highs considered to be formed by post breakup magmatism [Colwell et al., 1994]. Two ridge features extend from the Wallaby Plateau into the CAP in an approximately NlO-15°E direction. Sonne
Ridge has been interpreted as an abandoned spreading center [Veevers et ai., 1985;
Fullerton et ai., 1989; Mihut and Miiller, 1998], and Sonja Ridge has been interpreted as a pseudofault [Mihut and Miiller, 1998]. The narrow continental shelf and slope of the
Northern Carnarvon Terrace descends within 50 km into the 5000-m-deep CAP.
Southern Gascoyne Abyssal Plain: The area north of the CRFZ that is characterized by a bathymetric high (3000-4000 mbsl) between the Exmouth Plateau and the GAP. Within the bathymetric high, severallineated troughs and highs (approximately
SlOoW-NlOoE sub-parallel to adjacent seafloor fabric) are seen. A continental [Larson,
1977; Larson et al., 1979; Powell and Luyendyk, 1982], transitional [Ludden and Dionne,
1992], and oceanic [Veevers et al., 1985; Fullerton et al., 1989] origin of the bathymetric high has been suggested. The transition from the southern part of the Exmouth Plateau to the 5000-m-deep abyssal plain is wide and gradual.
Middle Gascoyne Abyssal Plain: The area north of the bathymetric high and south of Joey Rise. The transition from the middle part of Exmouth Plateau to the abyssal plain is abrupt - even sharper than that of the Cuvier margin.
6 3. Previous seafloor magnetic studies
All previous studies agree with the identification of Mesozoic anomalies MO-M3, but the interpretation of anomalies older than M3 varies. Larson [1977] identified MO-M3 on the southern GAP and left the landward area (the bathymetric high) as continental crust, and also identified MO-M4 and a possible M7-MIO sequence on the CAP. Larson et al. [1979] identified MO-M4 on the middle GAP, and extended the previously interpreted M7-MlO sequence on the CAP to M5-MlO. The significant space between
M4 and M5 on the CAP was explained by a ridge jump. The ridge jump scenario was supported by the rough basement structures (seen in the seismic reflection data), including small faults, peaks, and ridges, between M4 and M5. However, no conjugate sequence (that would indicate a ridge jump) was found. Powell and Luyendyk [1982] identified the conjugate sequence MlO-M5-MlO in the CAP. They also extended anomalies up to M6 on the middle GAP. These early works explained the bathymetric high on the southern GAP as continental.
Subsequently, Veevers et al. [1985] identified M5-MlO anomalies symmetric about Sonne Ridge, which they inferred was a fossil spreading center. They tentatively identified anomalies over the southern Exmouth bathymetric high as seafloor spreading anomalies M5-M9, and suggested that this area is oceanic. They also identified a conjugate anomaly sequence MlO-M5-MlO (similar to the CAP) on the middle GAP, although their correlations between the observed and model anomalies are weak.
Fullerton et al. [1989] supported Veevers et aI's interpretation with the addition of more data. They determined that (1) the timing of final breakup for both the Exmouth and
Cuvier margins is contemporaneous (at MlO), (2) the middle GAP and CAP have
7 conjugate anomaly sequences (MIO-M5-MIO) explained by two (1-2 m.y.) ridge jumps around the time of M4, and (3) the southern GAP has an anomaly sequence MO through
MIO (Figure 2(a)). They concluded that the two ridge jumps occurred simultaneously in the Cuvier and Gascoyne basins with a 10 degree reorientation of the spreading ridge system. They inferred from the narrowing of the distance between M4 and MIO northward on the middle GAP and southward on the CAP that a bidirectional propagation occurred (northward propagation in the middle GAP and southward propagation in the
CAP).
Mihut and MUller [1998] extended the oldest seafloor spreading anomaly of the
CAP back to MIIN (Figure 2(b)). They proposed a model with a different (northward) direction of propagation for the CAP. They explained the origins of the two ridge features, Sonne and Sonja Ridge, as an extinct spreading ridge and a pseudofault respectively.
In a significant departure from previous work, Heine et al. [2002] proposed that the anomalies on the continental side of MIO in the GAP represent much older crust
(M22A-M25A) associated with the formation of the AAP (Figure 2(c)).
The conflicting interpretations of the origin of the magnetic and bathymetric features reveal several problems remaining in understanding the evolution of the region:
• The correlations among the observed anomalies and with model anomalies are
often poor in (1) the southern Exmouth bathymetric high, (2) the middle
Gascoyne area inferred to be MIO-M5-MIO, and (3) the area west of the Sonne
Ridge and east of M2 in the Cuvier basin. Previous works lack sufficient data
coverage in these areas to make convincing interpretations.
8 • The magnetic anomalies of the middle GAP close to the continent need to be re
examined in light of Heine et al.'s proposal for their completely different
orientation and age.
• The inferred directions and locations of propagators in the CAP disagree and their
proposed geometries do not match the characteristics of classical V-shaped
propagators [Hey et aI., 1989].
• The proposed pseudofault origin of Sonja Ridge is questionable. Pseudofaults are
characterized by bathymetric scarps and age offsets, not by symmetric ridges
(e.g., [Hey et aI., 1989]). Colwell et al. [1994] noted that Sonja Ridge is similar
to, and at least as significant as, Sonne Ridge.
9 4. Data
Magnetic anomalies are plotted perpendicular to track in Figure 3. In addition to the data used in previous work, we have used additional data from two sources: (1)
Geoscience Australia (GA) and (2) data collected during R/V Maurice Ewing cruise
EWOI13. Pre-existing data includes marine and aeromagnetic data from the National
Geophysical Data Center (NGDC), Naval Ocean Research and Development Activity
(NORDA), and the U.S. Naval Oceanographic Office. Marine data from the Australian
Navy (H.M.A.S. Moresby) [Fullerton et aI., 1989] were digitized. A band-pass filter was applied to all magnetic data to remove long (> 200km) and short « 2 km) wavelength components. A shift was applied to two of the aeromagnetic lines to correct for poor navigation.
We have selected representative tracks for each area (Figure 4) on which we perform forward magnetic modeling. The azimuths of these selected tracks are close to the fracture zone azimuth (l25± 50°). The cruise name and availability of bathymetric data for the selected tracks are shown in Table 1. The track numbers [1]-[17] (west part) are from the middle GAP, [17] (east part)-[29] are from the southern GAP, and [30]-[42] are from the CAP. In Figure 5(a)-(c), each track is projected onto a trend of 125° (the azimuth of fracture zones) and plotted with bathymetry. In cases where no along track bathymetry was available, satellite derived bathymetry was used.
The projected profiles for each area are compared with synthetic model anomalies. The model anomalies were calculated by 2-D forward modeling [Talwani and
Heirtzler, 1964] using an appropriate magnetic time scale (for MO and older [Channell et aI., 1995] and Mlr [Gradstein et aI., 1994]). The model assumes juxtaposed rectangular
10 blocks of uniform magnetization. No topography is included in the models presented here although other forward models and 3-D inversions were run to determine the significance of topographic effects. Variable parameters are the inclination, declination, and intensity of present magnetic field, the inclination, declination, and intensity of remanent (crustal) magnetization, the azimuth of the structure, the depth to the source block, the block thickness, and the intensity of present magnetic field.
We used the following magnetic parameters for the synthetic model anomalies to match the observed skewness; an inclination of -55° and declination of 2° for the present field, an inclination of -55° and declination of 320° for remanent magnetization [Fullerton et al., 1989] with the structure oriented N35°E. The depth to the source block is 6 km, and the block thickness is 0.5 km. The intensity for the present field is 51736 nT. The half spreading rate (in the range of 2-4.5 cm/year) and the intensity of crustal magnetization
(in the range of 6-18 AIm) are adjusted to fit the data. The half-spreading rates are consistent for the three areas.
11 5. Results and interpretation
The results of magnetic forward modeling are shown in Figure S(a)-(c). The interpreted profiles are also plotted on a map view (Figure 6).
Cuvier Abyssal Plain
Magnetic anomalies of the CAP are the best correlated and lineated of all (Figure
Sea)). The positive and negative anomalies east of Sonne Ridge (identified as MS-MlON) are strikingly welllineated, and they match the model anomalies with a constant half spreading rate of 4.5 cm/year. The location of M4 coincides with the location of Sonne
Ridge axis, supporting the previous interpretation that Sonne Ridge is an extinct ridge
([Veevers et aI., 1985; Fullerton et aI., 1989; Mihut and Miiller, 1998]). However, anomalies west of Sonne Ridge (previously identified as the conjugate sequence of MS
MlO) are poorly correlated. We do not see the conjugate sequence MS-MlO to the west of Sonne Ridge, except M5-M7 and possibly M8-MlO identified on the northern lines
([3S] and [30]). Instead, we see symmetric anomalies about Sonja Ridge. The widths of the negative anomalies both sides of the axial positive anomaly become wider southward on profiles [42], [33], and [31]. We interpret those negative anomalies as M3, and Sonja
Ridge as the axis of a northward propagator (possibly with later intrusions associated with the Wallaby Plateau magmatism). As Sonja Ridge propagated northward into the older oceanic crust, the southern tip of the Sonne Ridge died (Figure 6). Spreading ceased on Sonne Ridge at about the middle of M4.
In the western half of the Cuvier profiles, the MO-M2 sequence is welllineated and matches the model anomalies with a constant spreading rate of 3.8 cm/year.
However, the M3-M4 sequence previously identified to the east of M2 ([Fullerton et aI.,
12 1989; Mihut and Muller, 1998]) is only seen in the northern lines ([30] and [32]) and they are truncated to the south against another propagation boundary. At about the same time as Sonja Ridge replaced Sonne Ridge, the other propagator approached from the north and replaced Sonja Ridge. As a result, a remnant micro plate (that was originally part of the Indian plate) is left on both sides of the northward propagating Sonja Ridge.
Bathymetry shows another ridge feature (at -200 S, 109°E, Figure 6) between the outer pseudofault of the northward propagator and the inner pseudofault of the southward propagator (seen on profiles [30] and [32]); the origin of this ridge is unknown. Based on our interpretation of the pseudofault azimuths and half-spreading rate, propagation rates are calculated as about 10.4 crn/year and 16.5 crn/year for the northward and southward propagators, respectively.
Several lines of evidence support our interpretation of these Cuvier propagation events. Hey et al. [1989] show that the morphology of a failed rift (extinct ridge) resembles an active ridge crest, and that the morphology of a pseudofault is characterized by a seafloor scarp that corresponds to a sharp age transition. The seismic reflection data from R/V Vema (profile [33]) (shown in the bottom section of Figure 2 in Larson et al.
[1979]) show that Sonja Ridge is a symmetric ridge not a bathymetric scarp. Thus the origin of Sonja Ridge is better explained as an extinct ridge than by a pseudofault.
Furthermore, an age discontinuity in the oceanic crust occurs (at the location just east of their identified M3 in the same seismic section) as an offset of basement (higher basement to the west) and a reduced sediment thickness. M4 disappears coincident with this age discontinuity, which is where we locate the pseudofault boundary. The magnetic lineations on both sides of Sonne and Sonja Ridge (Figure 6) match well the
13 characteristic V-shaped geometry of the continuous transform propagating/failing rift model [Hey et aI., 1989]. The higher intensity anomalies (10 AIm as opposed to 6 AIm) observed inside the Sonja Ridge propagator might be evidence of the propagating rift crust that is often characterized by high-amplitude anomalies [Hey et ai., 1989]. One difference is the observed depth inside of the V-shaped wedge. Bathymetry shows that the depth inside of the pseudofaults is shallower than for the older crust (as predicted by age-depth data), not deeper as is often observed for propagating rifts [Hey et ai., 1989]
(Figure 5(a)).
The changes in spreading rate, direction, and intensity determined from our interpretation are summarized as follows. The half-spreading rate starts at 4.5 em/year from the final continental breakup (MlON) to M5, and it slows down during the propagating events (to 3.9 em/year at M4 and 3.0 em/year at M3). After the multiple propagation events cease and normal seafloor spreading resumes at M2, the rate recovers to 3.8 em/year until MO, and it decreases to 2.0 em/year after MO. A slight change in direction (about 10° ccw) of spreading is recorded in the CAP between the west and the east sides of the southward pseudofault (Figure 6). Significant changes in the intensity of magnetic anomalies are observed in the Cuvier profiles (6 AIm during MlON-M5, 10
AIm inside the northward propagator, and 18 AIm after M4). Note that very high amplitude anomalies (pairs of positive and negative) are observed along the Cuvier margin east of MlON. Their location suggests that they are associated with basement beneath the continental slope of the Northern Carnarvon Terrace. Such high amplitude anomalies are not seen on the slope of Exmouth Plateau adjacent to the middle and
14 southern GAPs (Figure 6), but similar high amplitude anomalies exist on the northernmost part of Exmouth Plateau (Joey Rise).
Middle Gascoyne Abyssal Plain
In the middle GAP, correlations among observed anomaly profiles are weak in several broad areas but reasonable matches can be made in a few key places (Figure
5(b)). The most distinctive feature is a NlO-20oE lineation identified as M4 (previously identified as MIO [Fullerton et aI., 1989]) in the middle ofthe profiles. M5 Gust east of
M4) is also welllineated. The anomalies of profile [16] reasonably match the model anomaly sequence M4-Mll (model 1), and the characteristic shapes of MIO-Mll are seen in the eastern part. Thus it is reasonable to identify the anomalies of profile [16] as single-sided anomalies up to MIl. This interpretation implies that the breakup time of the middle Exmouth margin (Mll, 131.7 Ma) is slightly earlier than that of the Cuvier margin (MlON, 130.8 Ma).
However, to the north of profile [16], the lineations are often poor between the identified M5 and MlO, implying that the area east of M5 may have not experienced such a simple spreading as MIl through M6. There is another possible interpretation for this area. Because the shape of the MlO-M11 sequence is almost identical to the symmetric anomaly sequence MlON-MlO-MlON as shown in the bottom model (model 2, Figure
5(b)), the anomalies of profile [16] can be interpreted as M4 through M9 then MlON
MlO-MlON. This interpretation proposes that the identified MlO-MlON on model 1 is a portion of Indian Plate transferred to the Australian plate, implying that a ridge propagation/jump occurred. In that case, pseudofault boundaries are expected somewhere west of the conjugate side of MION although we cannot identify their locations. We
15 prefer this latter interpretation because this scenario may help explain the observed unusually poor lineations between M6-M9, and suggests synchronous breakup (MlON) between the middle Exmouth and Cuvier margins. It is very difficult to identify the details of such propagation events in this area because of the low resolution of bathymetric data in the northern part and the magnetic data gaps in the southern part. We note that the spreading history of this area could be very complicated, and that the addition ofnew data does not favor the previous conjugate anomaly sequence MlO-M5
MIO interpretation [Fullerton et aI., 1989]. Symmetric anomaly patterns are not observed. Also, there is no magnetic evidence for older N700E trending lineations as suggested by Heine et al. [2002]. Drilling results do not support the existence of older oceanic crust (M22A-M25) in the GAP as the oldest sediments drilled in the Exmouth
Plateau are Berriasian (around M12-MI6) at site 762 and 763 and are Valanginian
(around MlO-MI2) at site 766 [Exon et aI., 1992] (Figure 1).
To the west of M4, we identify M3 and M2 on the southern profiles [6], [7], [8], and [17]. A ridge exists to the west of M2. Symmetric anomalies about the ridge suggest that it is the location of an extinct spreading center. North of profile [9], anomalies west of M4 are unidentified due to their lack of correlation. Further to the west, well defined
MO, Ml, and M2 are seen. The width of M2 becomes narrower to the south where we locate an inner pseudofault boundary from a southward propagator that gradually replaced the interpreted extinct ridge in the period between M2 and MO. The anomalies west of the pseudofault and north of profile [4] are unidentified because they are associated with seamounts. We observe a gentle curvature of MO from Middle GAP
16 through CAP (Figure 6), although the details of how the two segments link off the southern GAP are unresolved.
The relation of the timing between the pseudofault and the extinct ridge support our propagating/failing rift model in the middle GAP west of M4. However, the geometry of this propagation model is not a simple V-shaped geometry like in the CAP: anomaly
M4 between the pseudofault and failed rift appears to be segmented en echelon (Figure
6). The size of the extinct ridge is much smaller than that of Sonne and Sonja Ridges, and unlike CAP, the pseudofault boundary is not well defined by the low resolution bathymetric data.
There is a slight bend in the orientation of M4 between the northern half and southern half of the middle GAP (Figure 6). It may be related to the regional change in the spreading direction around the time of M4. The M6-MlON lineations are oriented
-200 ccw from those of the CAP (which are approximately orthogonal to the CRFZ) indicating a significant obliquity to the early opening of the middle GAP. Overall, the intensity of magnetic anomalies are more consistent (in the range of 10-14 Aim) in this area than in CAP.
Southern Gascoyne Abyssal Plain
The identification of magnetic lineations in the southern GAP is the most questionable of all (Fig. 5(c)) partly because the area contains significant data gaps in the western part and numerous bathymetric features in the eastern part. Like the middle GAP, however, a well-lineated, high intensity M4 is readily identified in the middle of the profiles.
17 To the west of M4, we locate a fracture zone between profiles [28] and [18] where there is an offset in M4 and a change in the width of M3. This fracture zone continues to the west where it crosses profile [18] and produces an apparent repetition of MO. North of this fracture zone, anomalies M2-M4 are welllineated continuations of the middle GAP sequence. Another fracture zone offsets and replicates MO on profile [19]. This fracture zone separates the southern GAP and CAP as shown in Figure 6. We can not define the details ofthe spreading geometry in this region because of the low track density. The width ofM3 is unusually wide between the well defined M4 and M2 on profile [18], and it cannot be explained by the regional half-spreading rate. Asymmetric spreading is one way to explain such an unusually wide M3 anomaly. More probably, a ridge jump occurred during the time of M3. A seamount located at a reasonable distance from M3 west ofM4 on profile [18], and it could be the location ofthe extinct ridge.
To the east of M4, anomalieslie on the southern Exmouth area of bathymetric highs where the origin of the crust is controversial. Although it is possible to interpret lineated anomalies M5-MlO with a constant spreading rate of 4.5 em/year, the bathymetric features (for example, the ridges seen near "M8" on profiles [26], [24] and
[28]) indicate that seafloor there is not created by normal seafloor spreading processes.
The shape ofthe possible M8-MlO anomalies is highly skewed, suggesting that those anomalies are likely created by bathymetric/basement structures. The multi-channel seismic reflection data (profile [24]) from Geosciences Australia (GA-162/02) shows those ridges contain well defined layers that are remarkably similar to the sediment layers observed in the adjacent continental section [Sugimoto et ai., 2002]. Also, refraction data
[Tischer et ai., 2003] show that the crust in this area is much thicker than normal oceanic
18 crust. For these reasons, we interpret the area younger than M4 as normal seafloor, M5
M7 as created by seafloor spreading with excess magmatism, and "M8-MlO" as created by a mixture of continental extension, magmatism, and continental fragments. Apparent magnetic lineations are a common feature in rifted margins where magmas have intruded fault-bounded continental blocks [Manighetti et al. 1997; Taylor et ai., 1995].
19 6. Discussion
Rifting to spreading transition and the evolution ofearly seafloor
The age and simplified magnetic lineation pattern of the Mesozoic seafloor is
plotted in Figure 7. An evolutionary model of the rifting and early seafloor spreading is reconstructed for four different times in Figure 8 using our preferred interpretation for the middle GAP (model 2 on Figure 5(b)).
In Figure 8, the gray shaded areas are deformed continental crust. Prior to MlON, the entire gray area experienced extensional strain distributed over a wide region. At
MlON, the northern and the southern parts began to break up along zones probably localized by pre-existing structures, creating the middle Exmouth and Cuvier margins.
Seafloor spreading was established in the middle GAP and CAP whereas rifting still continued in the middle region, creating a much thinner and more deformed continental crust (Figure 8(a)). Magmatism (both intrusive and extrusive) is expected in such an
extended area. In the middle GAP, a possible ridge propagation/jump started sometime
after MlO and completed before M6. At the time ofM6 (Figure 8(b)) the spreading center
of the middle GAP propagated southward into the region of the more extended continental crust. In the CAP, a spreading ridge propagated northward into the older
oceanic crust ofthe Indian plate. By the time ofM4 (Figure 8(c)), the continents had been
completely separated by seafloor spreading. A southern GAP spreading center
propagated continuously into the CAP, not into the oceanic crust but into the ocean continent boundary of the Indian plate. This is probably due to preferential rifting into the
weaker lithosphere [Vink et al., 1984]. At this time, three spreading centers were active in the CAP (Figure 8(c)). The southward propagator in the CAP eventually replaced both
20 the original spreading center (Sonne Ridge) and the northward propagating Sonja Ridge
(Figure 8(d)). In the middle GAP, a new propagator started in the north around the time of M2, continuing in an overall southward propagation in the region.
Implications for the different deformational styles
A fundamental difference between the Exmouth Plateau and the Northern
Carnarvon Terrace is the much greater width of extended continental crust in the former.
As our results confirm that the middle part of the Exmouth margin and Cuvier margins broke up at approximately the same time (MION or MIl, and MlON, respectively), it seems unlikely that the difference in width is due to a difference in rift duration across the margin. Ifthe Exmouth Plateau experienced a longer rift duration, perhaps from the earlier Argo (north-south) spreading event [Hopper et ai., 1992], faulting parallel to that direction may be expected in the Exmouth Plateau. The crustal Bouguer gravity map
(Figure 3 in Karner and Driscoll [1999]), however, shows that major structural features in the Exmouth Plateau trend N20-30°E.
Alternatively, the difference in margin width may be due to the location of final breakup [Veevers and Cotterill, 1978]. As in East Africa, a series of rift valleys can form in a very wide (could be up to -400 km) region during the rifting stage, and final breakup may occur at one of the boundary faults in this broad region. Depending on the location of breakup, broad marginal plateaus (such as Exmouth Plateau) form as a region between localized rift systems. This arbitrary location of breakup could be where pre-existing weaknesses have localized strain, perhaps caused by small variations in Moho topography [Corti et ai. 2003]. This model is difficult to test without access to the conjugate margin now subducted north of India. The east coast of Australia, however,
21 provides examples of narrow (southeast Australia) versus wide (Queensland and Marion
Plateaux) margins along strike where the extant conjugate margins (Lord Howe Rise and
Louisiade Plateau) confirm this model [Jongsma and Mutter, 1978].
Another difference is the along-strike variation in margin structures within the
Exmouth margin. Our results show that normal seafloor spreading started in the southern
Exmouth margin at a much later time (M4) than that in the middle Exmouth margin
(MlON or MIl). The southern Exmouth bathymetric high is interpreted to be formed by seafloor spreading with excess magmatism (the area interpreted as M5-M7), and the mixture of continental extension, magmatism, and continental fragments (the area east of
M7). The wider transition, longer rift duration, and larger amounts of magmatism in the southern Exmouth margin can explain the poor correlation of magnetic lineations and the transitional origin of the crust as suggested by the characteristics of the acoustic basement
[Ludden and Dionne, 1992]. Our interpretation implies that the area was still rifting when the regions to the north and south were already spreading (Figure 8). The observed geometry is similar to that predicted for the "locked zone" model of Cortillot et al.
[1982]. A similar configuration is found in the central Woodlark Basin where spreading centers are offset about 50 km by the north-south Moresby transform fault [Taylor et al.,
1999]. There, the continental margins east of the transform fault continued stretching for about 1 m.y. after the transform linking the offset spreading centers was established.
What is unusual about the offset between the Exmouth and Cuvier margins is the much greater length (-300 km) of the CRFZ.
22 Causes of the multiple propagations
We identify three propagation events in the GAP and CAP. The first event is northward propagation of Sonja Ridge in the CAP. The second and third ones are southward propagations in the CAP and GAP that resulted in the transfer of Indian oceanic lithosphere to the Australian plate and overall southward propagation in the region. We suggest that the simplification of the Indian-Australian spreading system by the elimination of the large-offset CRFZ and the reduction in the obliquity of GAP spreading may be the regional tectonic causes of these propagation events.
Reductions of large offsets between spreading centers have been observed as the consequence of ridge jumps (e.g. [Shih and Molnar, 1975]). It is an efficient way to shorten spreading center offsets in response to extensional stresses. At M4 time, the northward propagation of Sonja Ridge, replacing Sonne Ridge, was in the process of approximately halving the offset between the GAP and CAP spreading centers across the
Cape Range transform fault (Figure 8).
Alternatively, some ridge propagation/jumps are often explained by ridge-plume interactions [Hill, 1991; Schilling, 1991], but this is probably not the case for the northward propagation event in the CAP. Even though there must exist a source of excess magma that formed the Wallaby Plateau, the age of the Wallaby Plateau volcanism postdates (120-110 Ma [Colwell et al., 1994]) the timing of the Sonja Ridge propagation
(-128-125 Ma). That is, the Sonne and Sonja Ridges already existed before Wallaby
Plateau volcanism and therefore this source of excess magma is unlikely to be the cause of the ridge propagation. Why then are both Sonne and Sonja Ridges extending from
Wallaby Plateau, and what is the relation between these bathymetric features? One
23 scenario is that the source of excess magma that was responsible for the formation of
Wallaby Plateau also channeled magma into the already extinct ridges where the lithosphere was thinnest [Sleep, 1997], thereby creating the significant ridge features at a later time than their cessation of spreading.
At M4 time the oblique trends of the spreading directions between the CAP
(-125°) and GAP (-110°) still existed and Sonja Ridge had not fully replaced Sonne
Ridge by propagating all the way to the CRFZ. After the second and third propagation events, however, the overall orientation of the spreading systems became more consistent
(-115°) and the offset of the CRFZ was eliminated (Figure 8). Ridge propagation is known to be one of the mechanisms by which spreading centers adjust their reorientation
[Hey et aI., 1988]. Thus the initial offset between, and different orientation of, the GAP and CAP spreading segments were removed by ridge propagation events in the first few million years after complete continental break-up.
24 7. Conclusion
New findings from this study are summarized as follows:
(1) The timing of final break-up is determined as MlON (130.8 Ma) for the Cuvier
margin, M7 (127.8 Ma) for the southern Exmouth margin, and MlON (130.8 Ma)
or Mll (131.7 Ma) for the middle Exmouth margin. Given similar times of rift
onset and margin-parallel rift structures, the wide Exmouth margin versus narrow
Cuvier margin can not be the result of more protracted rifting and is more likely
associated with varying break-up locations within broad rift systems that formerly
boarded both margins.
(2) Magnetic anomalies on the southern Exmouth bathymetric high are not created by
normal seafloor spreading processes. The area interpreted as M5-M7 is created by
seafloor spreading with excess magmatism, and the area east of M7 (previously
interpreted as M8-MI0) formed from a mixture of continental extension,
magmatism, and continental fragments.
(3) Magnetic anomalies adjacent to the middle Exmouth margin are identified as M4
Mll, or alternatively, M4-M9 then MlON-NlO-MlON, although the correlation is
weak between M6-M9. Neither a symmetric anomaly sequence (MlO-M5-MlO,
similar to the Cuvier Abyssal Plain), nor anomalies of different orientation and
age close to the continent (similar to the Argo Abyssal Plain), were not found in
the middle Gascoyne Abyssal Plain.
(4) In the Cuvier Abyssal Plain, Sonja Ridge was a spreading axis that propagated
northward from M6 to M4 time at the expense of Sonne Ridge. Both ridges
became extinct during M4 to M3 time when another spreading center propagated
25 south along the Greater India ocean-continent boundary. This eliminated the 300 kIn-long-offset Cape Range Fracture Zone. In the Gascoyne Abyssal Plain, an extinct ridge and pseudofault indicate M2 to MO southward propagation within oceanic crust as part of the same early spreading reorganization which also reduced the obliquity of the early seafloor spreading.
26 Table 1: Cruise name and the availability of bathymetric data for the selected tracks in Figure 4.
Track # Cruise Name Aeromag Bathymetry [1] 81I05MAG x no [2] 81I05MAG x no [3] 81I05MAG x no [4] 81I05MAG x no [5] 81I05MAG x no [6] BA71FMAG no [7] BA7lFMAG no [8] BA7lFMAG no [9] DOD005AR [10] V2209 [11] pm519 * x no [12] bmr17 [13] bmr18 [14] GA128/08 [15] GA162/08 [16] GA162/01 [17] 81I05MAG x no [18] V3308 [19] V3308 [20] ODP123JR [21] bmr17 [22] bmr17 [23] bmr17 [24] GA162/02 [25] GA162/03 [26] EW0113-2 [27] GA135/01 [28] EWOl13-3 [29] GA135/02 [30] V3308 [31] pm552 * x no [32] V3405 [33] V3308 [34] V3305 [35] bmr18 [36] bmr17 [37] bmr17 [38] V3405 [39] EWOl13-5 [40] V3405 [41] EW0113-6 [42] V3405 * navigation corrected
27 m -8000 -6000 ·4000 -2000 0 2000 4000 BATHYMETRY
104' 10S' lOS' 110' 112' 114' 11S' 11S' 120' -10 ' -10'
-12' -12'
-14' -14'
-lS' -16'
·18' Southern -1S' GAP Exmouth ,~ Plateau
-20' Cuv r "~766 • 762 -20' byaaal Plain ~~ .763 (C P) " '-''''e~ Iii Oil -22' it ere -22' !t ~~ § I Northern 9 CI) 6 Carnarvon NW ".:~~=.:allaby -24' CI) Terrace AUSTRALIA -24' Plateau
-26' +----.---r--..,--r--.....,:.-...--....---r--r--..----,---r--...... ----r-_-+ -26' 104" lOS' lOS' 110' 112' 114' 116' 118' 120'
Figure 1: Bathymetric map ofthe NW Australian margins showing the major physiographic features and ODP sites (black dots and white dot show the locations ofODP sites, leg 122 and 123, respectively), The study area is located inside ofthe white box. The dotted lines show the approximate location ofthe boundaries for the Gascoyne Abyssal Plain sub-division.
28 Figure 2: PreVlOUS. ident·ft1 cation. f 89], (b) Mihut and Mullerso magnetic[1998],lina::tt~S. b~ (a) Fullerton et 1 [19 c Heme et al. [2002]. a.
29 ! yoWWn' 1m -8000 -6000 -4000 -2000 0 2000 4000 BATHYMETRY 106' 108' 110' 112' 114' -14" -14'
-16" ·16'
-18'
-20" ·20"
·22" -22"
-24' --\.!:::;;=::::!..-----.- ~~--ILi..-L:'-~~""~~~~_.-l!I~_--+_24' 106" 108' 110' 112" 114'
Figure 3: Magnetic anomalies plotted perpendicular to tracks on a bathymetry base map. Red and blue colors represent positive and negative anomalies, respectively. The digitized and aeromagnetic data are shown in lighter colors.
30 106- 108' 110' 112- 114- -14" ;--~---:'::~r---"""'--~~-'----~"":'7']II-.J...------.J...--- ...... --+ -14- [~f~
-16" [31J [181
~ -18- • -18- ~ [21 ~ ~ ~ -A , [22] \'~..... 4 ~.~, ,~~1Il -20' -20- ., [39] (l31]) [34]) 41] -24" -+----.---...... ---...----....----'"r-'--....----..----....----+ -24- 106" 108' 110' 112" 114' Figure 4: Selected tracks used for magnetic forward modeling. The cruise name and the availability ofbathymetric data for each track are shown in Table 1. 31 (a) Cuvier Abyssal Plain -~>~12S' half-spreading rale ...... pseudofault M1 r 2.0 cmtyear fracture zone Mo-M2 3.8 cmlyear M3 3.0 cmlyear extinct ridge M4 3.9 cmJyear MS-M10N4.S c ear MODEL Sonne Ridge Figure 5: Projected profiles for magnetic and bathymetric data with synthetic model anomalies for (a) CAP, (b) Middle GAP, and (c) Southern GAP. Magnetic and bathymetric data are projected into 125°. Blue numbers with square brackets are the track numbers. The position ofeach profile shows the relative geographic position but not necessarily distance between individual profiles. Black numbers label the Mesozoic magnetic anomalies. The different heights of the block models represent the differences in intensity ofmagnetization. Interpretations ofpseudofauits, fracture zones, and ridge axes are shown. 32 (b) Middle Gascoyne Abyssal Plain -->~ 125' pseudofault fracture zone extinct ridge tOOkm 500nTJ (11) -500nTl 33 (c) Southern Gascoyne Abyssal Plain -->~12S' pseudofault fracture zone extinct ridge MODEL 100km half-spreading rate M1r 2.0 cmlyear MO-M2 3.8 em/year M3 3.0 em/year M4 3.9 em/year M5-M10 4.5 em/year [Z4) soanTJ om -50an~L [28] ? J-6OOOm po -- (18) [19] 34 106' 108' 110' 112' 114' -14' +-...... ----+.a...-:::::-----'--."....----I-..:-....."....-"------:::c-r-....L.------;:,..-'--.- .....---.:..., ~,.....,...... _t_ -14' -16" -16" -18" -20' -20' -22" -22' -24' 106' 108' 110' 112' 114' Figure 6: Identifications ofmagnetic lineations (from this study) in map view. Areas shallower than 4600 m are shaded in light green. Numbers identify the Mesozoic magnetic anomalies, Anomalies are plotted in the direction of 35°. The locations for M22A-M24 are from Fullerton et aI., [1989]. 35 106· lOS· 110· 112· 114· -14· r,::====:::::::::::::;--...... ------"'------""------'---...... -, -14· ...... pseudofault - fracture zone 2 --- extinct ridge 23 24 -16· -16· -1S· -1S· Magnetic timescale (Mal 120 "10 121 122 -20· -20· 123 "11 "12 124 125 "14 126 .... "16 127 -22' . 128 -22· "19 129 130 131 "111 132 -24' +---~--...... ----.------...... -----.------+ -24· 106· 10S' 110· 112· 114· Figure 7: Age and geometry ofthe Mesozoic seafloor derived from this study. 36 (a) M10 (c) M4 - spreading ridge -- extinct ridge ---- fracture zone pseudofault (b) M6 Magnetic time scale (Mal 120 MO 121 122 123 Ml M2 124 125 M4 126 M6 127 128 M9 129 130 131 Mil 132 Figure 8: Evolution ofthe early seafloor at four different times: (a) MID, (b) M6, (c) M4, and (d) M2. Gray shaded areas are the deformed continental crust. 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Miles, Models of the development of the West Iberia rifted continental margin at 400 30'N deduced from surface and deep-tow magnetic anomalies, Journal o/Geophysical Research, 100 (B3), 3,789-3,806, 1995. 41