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The axial topographic high at intermediate and fast spreading ridges
EPSL ELSEVIER
Earth and Planetary Science Letters 128 (1994) 85-97
The axial topographic high at intermediate and fast spreading ridges Suzanne M. Carbotte a, Ken C. Macdonald b Lamont-Doherty Earth Obsert~atory, Columbia Unil~ersity, Palisades, N Y 10964, USA b Department of Geological Sciences and Marine Science Institute, Unicersity of California, Santa Barbara, CA 93106, USA
Received 25 February 1994; accepted 12 August 1994
Abstract
An axial topographic high is commonly observed at both fast spreading ridges and some segments of intermcdiate spreading ridges. At fast rates the axial high is primarily created by the buoyancy of hot rock and magma beneath the rise. As newly formed crust is transported off axis, little vestige of an axial high is observed on the ridge flanks. In contrast, at intermediate rates, a significant component of the positive topography may be a volcanic construction, preserved on the ridge flanks as abyssal hills, which are split axial volcanoes. We suggest this difference in the nature of the axial high reflects a lithosphere strong enough to support construction of a volcanic crestal ridge at intermediate spreading rates, but only rarely at fast rates. Relict overlap ridges, found within the discordant zones left by overlapping spreading centers, is one class of ridge-flank topography which appears to have a significant volcanic constructional component even at fast spreading ridges. Unlike topography away from these discontinuities, the relief and shape of overlapping spreading centers is preserved as relict ridge tips are rafted onto the ridge flanks. Reduced magma supply at these discontinuities may give rise to an axial lithosphere strong enough to support volcanic construction of overlap ridges. Low axial lithospheric strength may also account for the lack of normal faults within the innermost 1-2 km of fast, and some intermediate, spreading ridges. With a thin/weak brittle layer at the ridge crest, tensile failure will predominate and few normal faults will form. Depths to the axial magma chamber reflector observed in multi-channel seismic data limit the thickness of the brittle layer on axis to less than 1-2 km for much of the EPR. This depth is comparable to depths over which tensile failure within the oceanic crust will predominate, estimated from the Griffith criteria for fracture initiation ( ~ 0.5-1.5 km). As the brittle layer thickens/strengthens away from the ridge, shear failure begins and the large-scale normal faults associated with abyssal hill relief develop.
I. Introduction
T h e axial m o r p h o l o g y o f fast s p r e a d i n g ridges ( > 90 m m / y r ) is c h a r a c t e r i z e d by a b r o a d t o p o g r a p h i c high (Fig. 1) with relief u p to several h u n d r e d m e t e r s in excess of t h a t p r e d i c t e d from t h e r m a l s u b s i d e n c e m o d e l s of cooling o c e a n i c l i t h o s p h e r e [1]. A similar axial high is also obElsevier Science B.V. SSDI 0012-821X(94)00172-3
served along some i n t e r m e d i a t e rate ridges (Fig. 1), including p o r t i o n s o f the J u a n de F u c a Ridge, notably t h e Cleft s e g m e n t [2,3], the E a s t Pacific Rise ( E P R ) at 21°N [4], the G a l a p a g o s s p r e a d i n g c e n t e r (e.g., at 86°W [5]) a n d the P a c i f i c - A n t a r c t i c R i d g e [6]. In early w o r k on the G a l a p a g o s s p r e a d ing center, K l i t g o r d a n d M u d i e [7] s u g g e s t e d that the crestal high at this i n t e r m e d i a t e r a t e ridge
S.M. Carbotte, K.C. Macdonald~Earth and Planetary Science Letters 128 (1994) 85-97
86
EPR 21~1
(59 mm/yr)
-2.5
]
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A
~ __
O.,a,.°o. SC , ~ ' W
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= -3.0 -3.5 (110mm/yr)
B
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;
-20
10
0 distance (km)
E~. ~ao's--)((155mm/yr)
10
20
Fig. 1. Bathymetric profiles showing axial and ridge flank relief within several intermediate and fast spreading ridges (spreading rates given in brackets are calculated from Nuvell poles [65]). Arrow shows location of ridge axis. Profiles are constructed from high-resolution deep-tow bathymetry (indicated with stars), Seabeam and SASS data. Note how abyssal hills on the fast spreading ridges have much lower relief than the axial topography, whereas on the intermediate rate ridges off-axis relief is comparable to that of the crestal ridge. Deep-tow profiles for E P R 21°N taken from Normark [4]; E P R 8°55'N from Lonsdale and Spiess [66]; and E P R 19°30'S from Bicknell et al. [38]. Profile for the Galapagos Spreading Center is derived from bathymetric data published in Malahoff [5] and reproduced in Fig. 2b; for the Juan de Fuca Ridge from Baker and H a m m o n d [3]; and for E P R 9°27'N from Wilcock et al. [67]. Location of this E P R profile is shown in Fig. 5. Vertical exaggeration ~ × 10.
was built by constructional volcanism. Based on near bottom studies of the Juan de Fuca Ridge, Kappel and Ryan [2] and Barone and Ryan [8] described an alternating cycle of magmatic and tectonic activity at the ridge crest, which results in construction of an axial volcano during robust magmatic phases. This, with waning volcanism and the onset of a subsequent tectonic phase, is split in two and rafted off the axis. In this model, vestiges of the axial volcano form abyssal hills with a distinctive volcanic 'bow form' morphology on the ridge flanks.
In this paper we consider the origin of the axial topographic high developed at fast and some intermediate spreading ridges. In contrast to intermediate rate ridges, the axial high at the fast spreading rates is not primarily a volcanic construction but rather is a relief created by the buoyancy of hot rock and magma which upwell beneath the rise [9-11]. This thermal bulge decays with cooling, abyssal hills are not remnant crestal highs, and we see little vestige of an 'axial volcano' on the ridge flanks (Fig. 1). Hence, although it may be appropriate to refer to the axial high at intermediate ridges as an elongate axial volcano, such a description is misleading at fast spreading ridges. We suggest that the difference in the nature of the axial topographic high at intermediate and fast ridges may reflect the greater strength of newly formed lithosphere within the axial region at intermediate spreading rates. Low axial lithospheric strength may also explain the absence of normal faults within the innermost axial region of fast and some intermediate spreading ridges.
2. Axial topography and abyssal hills at intermediate and fast spreading ridges
Kappel and Ryan [2] describe observations near the bottom which lead them to conclude that the axial high developed along portions of the Juan de Fuca Ridge has a volcanic construction. The summit of the crestal ridge slopes gently and is convex, whereas near the ridge foot the ridge slope is concave and is steeper. Downslope draping of basaltic flows occurs on the flanks of the crestal ridge, accompanied by a lack of visible fracturing [2]. Abyssal hills in this area have a similar morphology to the crestal ridge (Fig. 1, Juan de Fuca profile and Fig. 2a), with steep slopes, presumably fault scarps, bounding the hills on their flanks facing inward. In addition, abyssal hills are symmetric about the ridge and roughly equidistance from the axis. These observations lead Kappel and Ryan [2] to conclude that abyssal hills are split axial volcanoes. As Kappel and Ryan point out, along some intermediate ridges, abyssal hills are observed which stand as high or
S.M. Carbotte, 1<2C. Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97
higher than the axis itself (Fig. 1, profile at 21°N EPR), indicating this relief is constructional rather than thermal in origin. Abyssal hills and a crestal ridge which are volcanic in origin implies local crustal thickening associated with this topography. From multi-channel seismic studies, Rohr et al. [12] find evidence for thickening of seismic Layer 2A beneath abyssal hills on the Juan de Fuca Ridge and a thinner Layer 2A beneath the intervening troughs. These Layer 2A thickness variations are consistent with largely volcanic abyssal hills, if the base of Layer 2A corresponds with the transition from pillow basalts to dikes, as is commonly assumed [13,14]. In contrast to the Juan de Fuca Ridge, on the fast spreading EPR abyssal hills of a size and relief comparable to the axial high are rarely found on the ridge flanks, and faults are commonly located on slopes of off-axis highs dipping both inward and outward [15] (Fig. 3). In these areas, abyssal hills comprise a fault-bounded horst and graben terrain, formed during extension of the lithosphere within the plate boundary zone. Recent Alvin dives on the EPR indicate that volcanic draping of outward-facing slopes may commonly occur at fast spreading ridges. This produces slopes with a volcanic surficial morphology although the underlying architecture is fault controlled [16]. Analysis of multichannel seismic data from the E P R indicates that the base of seismic Layer 2A, in general, follows the undulations of the seafloor, as expected for faulted rather than volcanic constructional relief [13,17]. In addition, seismic and gravity studies indicate the axial high is not a volcanic construction. Assuming reflection Moho corresponds with the base of the magmatic crust, multichannel seismic data indicate that crustal thickening toward the axial high does not occur [18,19]. These data also show that Layer 2A is actually thinnest right on axis where the topography is shallowest [13,14,20] which, assuming a lithologic interpretation for the Layer 2 A / 2 B boundary, supports a non-volcanic origin for this topography. Recent studies of gravity data indicate that the depth of compensation for E P R axial topography is well below the crustal levels expected if the topography was compensated by crustal thickening associated with a vol-
87
canic construction [21,22]. Instead, the source of isostatic support for the axial high within these regions is the broad region of hot rock at the axis, as defined in seismic refraction data [23], together with a low density region in the upper mantle [10,24].
3. Lithospheric strength within axial region The presence of a constructional crestal ridge at intermediate spreading rates but not fast rates may reflect a contrast in axial lithospheric strength (thickness) such that only at the intermediate to slow spreading rates is the axial region sufficiently strong to support a volcanic load. Thermal models of lithospheric cooling, including those which model effects of hydrothermal circulation, predict a thicker/stronger brittle layer within the plate boundary zone as spreading rates decrease [25-27]. These predictions are substantiated by seismic reflection/refraction studies and studies of axial seismicity. Large differences in the depth to the axial low velocity zone are observed with spreading rate, such that the crustal lid above the zone of elevated temperatures is thicker at slower rates (e.g., 1-2 km at fast rates compared with 3 - 4 km at intermediate to slow, from Purdy et al. [28]). The maximum depth of ridge earthquakes increases with decreasing spreading rate [29]. Focal depths of 3-8 km have been measured during studies of microseismicity within the axial valley on the MAR [30,31]. In comparison, much shallower focal depths of ~ 2 km have been measured during microseismicity studies on the EPR [32]. Finally, ridge-axis earthquakes large enough to be recorded teleseismically (m b > 4) have been measured from slow spreading ridges [29], whereas only microseismicity of magnitude m b ~ 1-3 [32,33] is observed at fast spreading ridges, reflecting the lower strength of the brittle layer in these areas. Bell and Buck [34] suggest that the low amplitude, along-axis variations in gravity and topography observed on the Reykjanes ridge may reflect a lower crust which is sufficiently w e a k / h o t that it undergoes rapid ductile flow. Similarly, the lack of volcanic axial highs at fast spreading ridges
S.M. Carbotte, K.C. Macdonald / Earth and Planetary Science Letters 128 (1994) 85-97
88
a 130" 30' 44 ° 50'
130" 25'
130" 20'
130 ° 15'
44 ° 45'
4 4 ° 40'
b
A ~
~ Q - - , _ ---~ ~
,
,
0
~
!
1 , ~
~
"~
~_>
lo5o,
-~~-.,~-~
1°45'
~
~
~
lO40 '
86°00'
85°55'
A
85°5°'
85°45'
Fig. 2. Bathymetry of abyssal hills on (a) the Juan de Fuca Ridge and (b) the Oalapagos Spreading Center. Abyssal hills are shaded with light stipple. Contour interval for (a) is 20 m and for (b) is 20 fathoms. Note how abyssal hills at these intermediate rate ridges have a 'bow form' morphology with steep inward facing slopes and gentle, irregular outward facing slopes. In comparison, abyssal hills at the fast spreading EPR (Fig. 3) are typically fault-bounded on both slopes. (a) is derived from Seabeam bathymetry, provided courtesy of C. Fox, gridded at 100 m and machine contoured. (b) is from SASS data published in Malahoff [5].
S.M. Carbotte, I~ C. Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97
3000
3800~
I
5km
I
2800
~ 3000 38240000I ~
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I
m
I
Fig. 3. Enlargement of portions of Deeptow profiles shown in Fig. 1 for (a) E P R at 8°55'N from Lonsdale and Speiss [66], (VE ~ × 5 ) and (b) E P R at 19°30'S from Bicknell et al. [38] (VE ~ ×5). Locations of faults identified in deep-tow sidescan sonar data are indicated. In contrast to intermediate rate ridges, abyssal hills at these fast spreading ridges are fault bounded on both inward and outward facing slopes, forming an undulating horst and graben terrain.
may result from a w e a k / h o t lower crust which flows laterally, homogenizing crustal thickness and removing topographic relief. In this scenario, the strength of the axial lithosphere as a whole will depend most importantly on the strength of the lower crust.
4. Tectonic extension within axial z o n e
Axial lithospheric strength may also contribute to the distribution of faults and fissures within the plate boundary zone at different spreading rates. Deep-tow, submersible, and S e a M A R C I studies on the fast spreading E P R indicate that fissures are the dominant tectonic features within the innermost 1-2 km of the plate boundary zone and the only fractures with significant vertical offset are the walls of the axial summit collapse zone [35-40]. Normal faults, and possibly tensile fractures with large vertical offsets ( > 5m), develop beyond this innermost region (Fig. 4). Edwards [39] has measured the distance to the first off-axis fault imaged in S e a M A R C I and II sidescan sonar data between 10o20 ' and 15°N and has found few faults within 1 - 2 km of the axial summit caldera along this entire length of ridge crest. As she points out, the ubiquitous lack of faults in the innermost axial region, for more than 400 km of surveyed ridge, indicates it is unlikely that
89
burial by lava flows accounts for the absence of faults. In contrast, at intermediate and slow spreading ridges evidence for normal faulting can be found throughout the plate boundary zone. Camera and side-scan sonar studies on the Endeavour segment of the Juan de Fuca Ridge show fractures with significant vertical offset ( > 5m) throughout the axial region [8]. At the slow spreading Mid Atlantic Ridge, ocean bottom seism o m e t e r studies have located normal fault earthquake focal mechanisms well within the inner rift valley [30,31]. In addition, normal faults are identified in submersible and other near-bottom studies throughout the rift valley floor within some ridge segments [41-43]. Other segments along intermediate spreading ridges have axial regions dominated by volcanic morphology and few faults or fissures are seen (e.g., E P R at 21°N [4,44,45]), similar to the tectonic zonation observed at fast ridges. These areas are presumably experiencing the robust magmatic phase associated with development of the volcanic crestal ridge. The lack of normal faults within the innermost axial region along fast spreading ridges could reflect a brittle layer on the axis which is too weak, in a time-averaged sense, to support shear failure. The strength of rock under tension is much less than under shear and rock subject to an applied horizontal tensional stress near the earth's surface or in areas of high pore fluid pressure will fail along tension fractures oriented perpendicular to the least compressive stress, ~r3. However, with depth and increasing overburden pressure, tensile failure is inhibited and failure occurs by shear. An estimate of the depth at which tensile failure gives way to shear can be obtained from failure criteria for initiation of fracturing (e.g. Jaeger and Cook [46]). Assuming the Griffith criteria for fracture initiation in intact rock, tension fractures will form along planes perpendicular to or3 when: s~ = - T
Provided cr[-cr~ < 4T
(1)
where the prime denotes effective stress; ~r'= ~r - P f ; Pf is pore fluid pressure; and T is rock tensile strength. From (1) the maximum value of ~r'1 for tensional failure is: ~r~ = 3T
(2)
S.M. Carbotte, K.C. Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97
90 -104°30 '
-104"20'
-104°10 ,
-104°00 '
o-I is vertical and equal to the overburden of rock and water (o r and % ) minus the pore fluid pressure:
9" 40' =
+
-- P f
(3)
If the pore space is interconnected then: Pf = % + PwgY 9 ° 30'
(4)
where Pw is water density; and y is the depth below sea floor. Substituting (4) and (3) into (2) gives the maximum depth at which tensional failure occurs, Yo: Yo = 3 T / g ( P r
-
Pw)
(5)
9" 2 0 '
9°10 '
9 ° 00'
8 ° 50'
8 ° 40'
Fig. 4. Tectonic chart of the EPR 8°40-9°45'N showing locations of faults imaged in SeaMARC II side-scan sonar data (from Carbotte and Macdonald [54]). Ticks indicate fault facing direction. The 2700 m contour enclosing the axial region is shown in heavy stipple. The width of the axial magma chamber reflector observed in multi-channel seismic data [53] is superimposed in light stipple for comparison. As observed elsewhere on fast spreading ridges, normal faults with significant throw are only found beyond ~ 1-2 km of the axis (Table 1). With models which call upon processes associated with the axial magma chamber to account for the onset of normal faulting we expect to observe a correlation between fault patterns and axial magma chamber width/presence along axis. In this region, the axial magma chamber reflector varies considerably in width yet a corresponding change in fault patterns is not observed (see text and Table 1).
Estimates of the depth at which tensile failure gives way to shear obtained using (5) depend on values chosen for rock tensile strength, which is not well known. Typical strengths measured in laboratory experiments on crystalline rocks are ~ 100 bar. Haimson and Rummel [47] have measured lower tensile strengths of 10-60 bar (mean 30 bar) for the uppermost ~ 600 m of the basaltic pile in Iceland. Using tensile strengths of 30 and 100 bar, and Or = 2900 k g / m 3, we obtain a maximum depth to which tension failure can occur within subaqueous oceanic crust of 0.48 and 1.6 kin. Using the low mean tensile strengths obtained by Haimson and Rummel [47], Gudmundsson [48] has estimated that the depth range over which normal faults will begin to develop within subaerial crust in Iceland is 0.5-1.5 km. The depth to the axial m a g m a chamber (AMC) reflector ( ~ 1200°C isotherm) observed in multichannel seismic data constrains the thickness of the brittle crust on axis ( ~ 600°C isotherm) to less than 1.6 km on average for the fast and super fast spreading E P R [18,49]. This thickness is comparable to the maximum depths over which tensile failure predominates, as estimated above, suggesting the brittle layer on axis is typically insufficiently thick to support shear failure at fast spreading rates. In addition to insufficient strength, other factors may contribute to the lack of normal faults within the innermost axial region of fast spreading ridges. Magmatic overpressure and dike injection may result in suppression of normal faulting [50]. M a g m a pressure, associated with intrusion
S.M. Carbotte, IL C. Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97
of vertical dikes, will act to increase the least compressive stress and, hence, reduce the deviatoric stress in the vicinity of the dike. If the deviatoric stress is reduced to below the shear strength of rock, shear failure will be suppressed. G o f f [51] has p r o p o s e d that the zone of partial melt b e n e a t h the axis may act as a thermal buffer to cooling, reducing the thermal contraction stress, which may govern onset of normal faulting and thereby inhibit faulting within the innermost region. F u r t h e r studies of axial microseismicity, crustal structure and n e a r - b o t t o m morphology will be n e e d e d to assess the importance of these various effects on tectonic zonation within the plate b o u n d a r y zone. Several authors have suggested that the onset of normal faulting at fast spreading ridges is associated with the A M C ; resulting from, for example, descent of newly f o r m e d crust off the A M C [15,52] or m a g m a c h a m b e r inflation/deflation [39]. With these models we would expect to observe a correlation between fault patterns and A M C w i d t h / p r e s e n c e along axis, but no clear correlation is found. T h e width of the A M C reflector observed in multi-channel seismic data between 8o50 ' and 9°40'N varies from < 0.7 km to > 4 km [53]. However, using S e a M A R C II side-scan sonar data for this same area [54], we
Table 1 Distance to first off-axis fault compared with axial magma chamber width Location Average distance to Axialmagma chamber first fault ~ (km) width b (kin) 9°45'-9°35'N 9°35'-9°17'N 9~17'-9°03'N 9°03'-8°50'N
3.3 (_+ 1.9) 4.3 (_+3.1) 2.7 (_+ 1.0) 3.2 (_+ 1.8)
< 0.7 1-1.2 3.35-4.15 0.7-1.3
a Distance to first fault outside of axial summit caldera is measured from tectonic chart in Fig. 4 every 1 min of latitude and averaged for each ridge segment (standard deviation in brackets). b From Kent et al. [53]. The axial magma chamber reflector varies in width more in this region than anywhere else where measurements are available [17]. However, we see no correlation between width of this reflector and distance to first fault, as would be expected were onset of normal faulting associated with magma chamber processes (see text).
91
find no evidence for faults forming closer to the axis where a narrower A M C is observed (Table 1, Fig. 4). Similarly, faults are not observed any closer to the axis within a region directly north of the Clipperton transform fault, where no A M C reflector is observed, than further north, where a bright continuous reflector is detected [39]. Using S e a M A R C II data from 13°-15°N, Edwards [39] m e a s u r e d an average distance of 1.04 km to the first off-axis fault; considerably lower than we measure from 8°50 to 9°40'N (Table 1). Spreading rates are only slightly slower (10%) in the northern area; however, axial m o r p h o l o g y indicates an overall lower magmatic budget, which could give rise to a stronger axial lithosphere within which faulting develops closer to the axis. F r o m 14 ° to 15°N, the ridge axis b e c o m e s ill defined and in places a small rift valley is developed [55], whereas in the 9°N region an axial high is observed throughout, presumably reflecting more robust m a g m a supply. In addition, abyssal hills in the n o r t h e r n region have greater relief and are more closely spaced, indicating greater amagmatic extension of the crust [51].
5. Exceptions In the preceding discussion we emphasized the difference in the nature of the axial high develo p e d at intermediate c o m p a r e d with fast spreading ridges. In spite of similar first-order morphology, axial t o p o g r a p h y at fast spreading ridges is primarily an elevation created by the buoyancy of hot rock and melt b e n e a t h the rise axis, whereas at intermediate rates it may have a larger contribution from volcanic constructional processes. In some cases, however, both of these mechanisms a p p e a r to contribute to sea floor relief irrespective of spreading rate. Axial t o p o g r a p h y varies dramatically along the Juan de Fuca Ridge: from a p r o n o u n c e d axial high with relief on the order of 3 0 0 - 4 0 0 m developed along the Cleft segment, to a subdued axial bulge along the Cobb Segment to the north [3]. These differences presumably reflect variations in m a g m a supply along the ridge. Wilson [21] models gravity and t o p o g r a p h y along a profile through the Cleft Segment at 45°N and
92
S.M. Carbotte, K. C. Macdonald~Earth and Planetary Science Letters 128 (1994) 85-97
Table 2 Comparison of seafloor relief at OSC's and adjacent seafloor OSC
Relict OSC relief ~ (m)
Current OSC relief b (m)
Abyssal hill relief of adjacent seafloor (m)
16°N 11°45'N 9°03'N 5°30'N 3°25'N
250, 350 c 100, 125, 225 d 300, 100, 350, 300, 200 c 460, 220 d 440, 300 d
400/400 550/375 380/360 600/620 220/280
100-200 c 50-150 c 50-150 d ~ 200 d ~ 100 ~
c d d d ~
Values given correspond with maximum peak to trough relief developed along current and relict OSC ridge/basin pairs and along abyssal hills outside of discordant zone. ~' Measurements are listed beginning with relict ridge/basin pair closest to axis in available data. b Measurements for w e s t / e a s t ridge of OSC are given. ~ Measurements made from SeaMARC II bathymetry [68]. d Measurements made from Seabeam data [57,67,69,70]. Note that relict OSC's have relief which is comparable to that observed at current OSC's and 2-3 times greater than the average relief developed elsewhere on the ridge flanks.
the E P R at 9°N. In both regions axial topography is isostatically compensated at subcrustal depths (15-30 kin), well below compensation depths expected if the crestal ridge was primarily an axial volcano. These results indicate that, in both areas, most of the axial topography appears to be due to the buoyancy of upwelling mantle, although at the Cleft segment, a small ~ 1 - 2 mgal lack of fit between Wilson's models and the observed gravity within ~ 5 km of the axis may reflect a volcanic constructional component to the axial topography. On the fast spreading EPR, remnants of overlapping spreading center (OSC) basins and ridges are found which have been rafted onto the ridge flanks (essentially intact) during the evolution of these discontinuities [56-58]. Relict overlap ridges may be one group of features on the fast spreading ridges which are predominantly volcanic constructional highs. Maximum peak to trough relief developed at overlap r i d g e / b a s i n pairs are com-
piled in Table 2 for a number of OSC's and their discordant zones and are compared with the typical abyssal hill relief of adjacent sea floor. With the exception of OSC scars within the 11°45'N discordant zone, seafloor relief developed at relict r i d g e / b a s i n pairs is 2 - 3 times greater than that of surrounding abyssal hill fabric and is comparable to that observed at current OSC's (Fig. 5). The lengths of relict overlap ridges are very similar to the length of overlap observed at current OSCs (Table 3), as are the shape and orientation of these features (Fig. 5). These observations indicate OSC relief and morphology is largely preserved as these features are transported onto the ridge flanks. Studies of sea floor fabric show a predominance of volcanic features within OSCs and their discordant zones. Deep-tow observations indicate that the slopes bounding overlap basins are largely volcanic, with the basins themselves dominated by hummocky volcanic fabric and few fault lineations [59]. In side-scan sonar
Fig. 5. (a) Bathymetry of the OSC located at 9°03'N and its off-axis discordant zone [58]. Contour interval is 50 m. The stippled areas highlight the axial region defined by the 2700 m contour and relict OSC ridges found on the west flank of the ridge. Locations of bathymetric profiles shown in (b) are indicated. Note the similarity between the lengths and orientations of the relict ridges and that of the overlap ridge tips at the current OSC. (b) Bathymetric profiles through discordant zones left by OSC's located at 9°03'N and 11° 45'N (heavy line profiles), compared with profiles through adjacent seafloor (light line profiles). Note how relief associated with relict OSC ridges is 2-3 times greater than normal abyssal hill relief (Table 2). Bathymetry is derived from the compilation of Macdonald et al. [68].
S.M. Carbotte, K.C. Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97
a
-104"40 '
-104"30'
-104"20'
-104"10'
-104°00 '
-103"50'
9 ° 30'
9" 20'
9"10'
9" 00'
8 ° 50'
9N Discordant Zone
b
profile north of discordant zo°e
-2500
C
~~-B" C '
~
E
r.- -3000 -(3
11 45N Discordant Zone
~
t
-3500
i
i
i
0
20
40
distance (km)
93
S.M. Carbotte, K.C Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97
94
Table 3 Length of overlap ridges OSC
Length of overlap at current OSC (km)
Length of relict overlap ridges ~ (km)
16°N
20 b
24, 22 c
11 °45' N
24 b
20, 13, 11 c
9°03'N 5°30'N 3°25'N
25 b 27 b I9 b
24, 19, 24, 18, 22 c 25, 15 b 25, 16 b
a Estimates are derived from extent of oblique trending bathymetry and faults [e.g. 58] defining relict and current OSC ridges. Measurements are listed beginning with relict ridge closest to axis in available data. b Measurements made from Seabeam data [57,69]. c Measurements made from SeaMARC II data [68].
data, OSC discordant zones are characterized by a paucity of faults and a predominance of volcanic textures [54,57]. A large number of morphological, petrologic, and magnetic observations indicate that OSC's are located at regions along the ridge comparatively starved of m a g m a [40,56,60,61]. More restricted m a g m a supply to these areas should be associated with a cooler, thicker lithosphere, which may be strong enough to support construction of volcanic relief at OSC ridges. Analysis of gravity data supports this hypothesis. Madsen et al. [24] and Cormier and Macdonald [62] infer higher densities associated with OSC's on the northern and southern E P R than with the shallower mid-segment areas. Madsen et al. [24] note that the along-axis variations in anomalous mass, inferred from gravity data from 9-10°N on the E P R are sufficiently large that they are unlikely to reflect changes in the size of an axial m a g m a chamber
alone. Instead,
the differences observed
require variations in the width of zone of hot rock flanking the axis and of low densities in the underlying mantle such that a narrower low density region is present at OSC's. Analysis of seismic data in the vicinity of the OSC at 9°N has lead Kent et al. [53] to the contrasting view that OSC's may, in fact, be places of enhanced m a g m a supply and crustal accretion. In this area, two-way travel times to reflection Moho reach an alongaxis maximum at 9°10'N, just north of the OSC, possibly reflecting thicker crust [63], and the A M C
reflector is wider directly north of the OSC than elsewhere (the greater width here may reflect a melt supply which is decoupled from the upper crust rather than more melt, [53]). An alternate interpretation of the seismic Moho travel-time maximum is that crustal velocities are slower within this region. Argo and deep-tow studies show more extensive fracturing on axis in this area than further north [40,59], which could account for the increase in travel times. In addition, we note that, although travel times to Moho do reach a maximum just north of the OSC, they are locally reduced within relict OSC basins [63], possibly reflecting thinner crust as expected from more restricted m a g m a supply to these discontinuities. Finally, linear volcanic edifices may also be built at the crest of magmatically robust fast spreading centers which experience long periods of volcanic quiescence for some reason. For example, the high standing, near-axis shoulders near 18-19°S [57,64] may be caused by episodes of significant volcanic construction. In these areas we would predict the crust and Layer 2A to be thicker, which can be tested with multi-channel seismic data.
6. Conclusions
Morphologic, seismic and gravity data suggest that the physical processes which contribute to development of an axial topographic high at fast, and some intermediate, spreading ridges differ. At intermediate spreading ridges a significant component of the axial topography may be built by volcanic construction, whereas at fast spreading ridges, the axial high is primarily an elevation created by the buoyancy of hot rock and magma beneath the rise. At either a fast or intermediate spreading ridge both processes may contribute to axial topography, depending on local m a g m a supply. For example, during robust magmatic phases on an intermediate spreading ridge the buoyancy of upwelling m a g m a is likely to account for a significant component of the positive relief. At fast spreading ridges, overlapping ridge tips may be primarily volcanic edifices. We suggest that
S.M. Carbotte, K.C. Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97 axial l i t h o s p h e r i c s t r e n g t h m a y p l a y a s i g n i f i c a n t r o l e in t h e n a t u r e o f t h e axial t o p o g r a p h y s u c h t h a t o n l y at i n t e r m e d i a t e s p r e a d i n g rates, o r in a r e a s o f r e d u c e d m a g m a s u p p l y o n t h e fast r i d g e s (e.g., at O S C ' s ) , is t h e l i t h o s p h e r e sufficiently s t r o n g to s u p p o r t c o n s t r u c t i o n o f a v o l c a n i c load. F u r t h e r d e t a i l e d gravity a n d s e i s m i c studies, esp e c i a l l y at i n t e r m e d i a t e r a t e ridges, a r e n e e d e d to e s t a b l i s h t h e r o l e o f v o l c a n i c c o n s t r u c t i o n in axial t o p o g r a p h y clearly. A x i a l l i t h o s p h e r i c s t r e n g t h m a y also a c c o u n t for t h e t e c t o n i c z o n a t i o n o b s e r v e d at fast, a n d s o m e i n t e r m e d i a t e , s p r e a d i n g ridges. F i s s u r e s d o m i n a t e w i t h i n t h e i n n e r m o s t axial r e g i o n a n d l a r g e - s c a l e n o r m a l f a u l t i n g d o e s n o t a p p e a r to i n i t i a t e u n t i l 1 - 2 k m o f f axis. T h i s p a t t e r n o f f r a c t u r i n g m a y r e f l e c t a l i t h o s p h e r e initially t o o w e a k a n d t h i n to s u p p o r t s h e a r f a i l u r e . I n d e e d , estimates of the depths over which tensile failure will p r e d o m i n a t e in o c e a n i c c r u s t ( 0 . 5 - 1 . 5 kin) a r e c o m p a r a b l e to t h e t h i c k n e s s o f t h e b r i t t l e p l u s d u c t i l e c r u s t a l lid o v e r l y i n g t h e axial m a g m a c h a m b e r (1.6 km). A l t h o u g h o t h e r f a c t o r s m a y also b e i m p o r t a n t , p r o c e s s e s d i r e c t l y a s s o c i a t e d w i t h t h e axial m a g m a c h a m b e r (e.g. size, o r inflat i o n / d e f l a t i o n ) a r e u n l i k e l y to b e p r i m a r y f a c t o r s g o v e r n i n g t h e initial s u p p r e s s i o n o f n o r m a l faulting.
Acknowledgments W e t h a n k C h r i s F o x for p r o v i d i n g us w i t h J u a n de Fuca bathymetry. Roger Buck and Jim Cochran provided useful reviews during preparation of the manuscript. We thank Jeffrey Karson, J o h n M a d s e n a n d an a n o n y m o u s r e v i e w e r for t h e i r t h o u g h t f u l c o m m e n t s . T h i s w o r k was supp o r t e d by g r a n t s N S F O C E 8 9 - 1 1 5 8 7 a n d O N R N 0 0 0 1 4 - 9 0 - J 1 6 4 5 to K C M . L a m o n t c o n t r i b u t i o n n u m b e r 5229. [CL]
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Earth and Planetary Science Letters 128 (1994) 85-97
The axial topographic high at intermediate and fast spreading ridges Suzanne M. Carbotte a, Ken C. Macdonald b Lamont-Doherty Earth Obsert~atory, Columbia Unil~ersity, Palisades, N Y 10964, USA b Department of Geological Sciences and Marine Science Institute, Unicersity of California, Santa Barbara, CA 93106, USA
Received 25 February 1994; accepted 12 August 1994
Abstract
An axial topographic high is commonly observed at both fast spreading ridges and some segments of intermcdiate spreading ridges. At fast rates the axial high is primarily created by the buoyancy of hot rock and magma beneath the rise. As newly formed crust is transported off axis, little vestige of an axial high is observed on the ridge flanks. In contrast, at intermediate rates, a significant component of the positive topography may be a volcanic construction, preserved on the ridge flanks as abyssal hills, which are split axial volcanoes. We suggest this difference in the nature of the axial high reflects a lithosphere strong enough to support construction of a volcanic crestal ridge at intermediate spreading rates, but only rarely at fast rates. Relict overlap ridges, found within the discordant zones left by overlapping spreading centers, is one class of ridge-flank topography which appears to have a significant volcanic constructional component even at fast spreading ridges. Unlike topography away from these discontinuities, the relief and shape of overlapping spreading centers is preserved as relict ridge tips are rafted onto the ridge flanks. Reduced magma supply at these discontinuities may give rise to an axial lithosphere strong enough to support volcanic construction of overlap ridges. Low axial lithospheric strength may also account for the lack of normal faults within the innermost 1-2 km of fast, and some intermediate, spreading ridges. With a thin/weak brittle layer at the ridge crest, tensile failure will predominate and few normal faults will form. Depths to the axial magma chamber reflector observed in multi-channel seismic data limit the thickness of the brittle layer on axis to less than 1-2 km for much of the EPR. This depth is comparable to depths over which tensile failure within the oceanic crust will predominate, estimated from the Griffith criteria for fracture initiation ( ~ 0.5-1.5 km). As the brittle layer thickens/strengthens away from the ridge, shear failure begins and the large-scale normal faults associated with abyssal hill relief develop.
I. Introduction
T h e axial m o r p h o l o g y o f fast s p r e a d i n g ridges ( > 90 m m / y r ) is c h a r a c t e r i z e d by a b r o a d t o p o g r a p h i c high (Fig. 1) with relief u p to several h u n d r e d m e t e r s in excess of t h a t p r e d i c t e d from t h e r m a l s u b s i d e n c e m o d e l s of cooling o c e a n i c l i t h o s p h e r e [1]. A similar axial high is also obElsevier Science B.V. SSDI 0012-821X(94)00172-3
served along some i n t e r m e d i a t e rate ridges (Fig. 1), including p o r t i o n s o f the J u a n de F u c a Ridge, notably t h e Cleft s e g m e n t [2,3], the E a s t Pacific Rise ( E P R ) at 21°N [4], the G a l a p a g o s s p r e a d i n g c e n t e r (e.g., at 86°W [5]) a n d the P a c i f i c - A n t a r c t i c R i d g e [6]. In early w o r k on the G a l a p a g o s s p r e a d ing center, K l i t g o r d a n d M u d i e [7] s u g g e s t e d that the crestal high at this i n t e r m e d i a t e r a t e ridge
S.M. Carbotte, K.C. Macdonald~Earth and Planetary Science Letters 128 (1994) 85-97
86
EPR 21~1
(59 mm/yr)
-2.5
]
j , )
A
~ __
O.,a,.°o. SC , ~ ' W
w ~/~) ~_j-~_~ A!ssmm/yr)
= -3.0 -3.5 (110mm/yr)
B
"~"~@'~
B'
~ ~ m m / y , )
;
-20
10
0 distance (km)
E~. ~ao's--)((155mm/yr)
10
20
Fig. 1. Bathymetric profiles showing axial and ridge flank relief within several intermediate and fast spreading ridges (spreading rates given in brackets are calculated from Nuvell poles [65]). Arrow shows location of ridge axis. Profiles are constructed from high-resolution deep-tow bathymetry (indicated with stars), Seabeam and SASS data. Note how abyssal hills on the fast spreading ridges have much lower relief than the axial topography, whereas on the intermediate rate ridges off-axis relief is comparable to that of the crestal ridge. Deep-tow profiles for E P R 21°N taken from Normark [4]; E P R 8°55'N from Lonsdale and Spiess [66]; and E P R 19°30'S from Bicknell et al. [38]. Profile for the Galapagos Spreading Center is derived from bathymetric data published in Malahoff [5] and reproduced in Fig. 2b; for the Juan de Fuca Ridge from Baker and H a m m o n d [3]; and for E P R 9°27'N from Wilcock et al. [67]. Location of this E P R profile is shown in Fig. 5. Vertical exaggeration ~ × 10.
was built by constructional volcanism. Based on near bottom studies of the Juan de Fuca Ridge, Kappel and Ryan [2] and Barone and Ryan [8] described an alternating cycle of magmatic and tectonic activity at the ridge crest, which results in construction of an axial volcano during robust magmatic phases. This, with waning volcanism and the onset of a subsequent tectonic phase, is split in two and rafted off the axis. In this model, vestiges of the axial volcano form abyssal hills with a distinctive volcanic 'bow form' morphology on the ridge flanks.
In this paper we consider the origin of the axial topographic high developed at fast and some intermediate spreading ridges. In contrast to intermediate rate ridges, the axial high at the fast spreading rates is not primarily a volcanic construction but rather is a relief created by the buoyancy of hot rock and magma which upwell beneath the rise [9-11]. This thermal bulge decays with cooling, abyssal hills are not remnant crestal highs, and we see little vestige of an 'axial volcano' on the ridge flanks (Fig. 1). Hence, although it may be appropriate to refer to the axial high at intermediate ridges as an elongate axial volcano, such a description is misleading at fast spreading ridges. We suggest that the difference in the nature of the axial topographic high at intermediate and fast ridges may reflect the greater strength of newly formed lithosphere within the axial region at intermediate spreading rates. Low axial lithospheric strength may also explain the absence of normal faults within the innermost axial region of fast and some intermediate spreading ridges.
2. Axial topography and abyssal hills at intermediate and fast spreading ridges
Kappel and Ryan [2] describe observations near the bottom which lead them to conclude that the axial high developed along portions of the Juan de Fuca Ridge has a volcanic construction. The summit of the crestal ridge slopes gently and is convex, whereas near the ridge foot the ridge slope is concave and is steeper. Downslope draping of basaltic flows occurs on the flanks of the crestal ridge, accompanied by a lack of visible fracturing [2]. Abyssal hills in this area have a similar morphology to the crestal ridge (Fig. 1, Juan de Fuca profile and Fig. 2a), with steep slopes, presumably fault scarps, bounding the hills on their flanks facing inward. In addition, abyssal hills are symmetric about the ridge and roughly equidistance from the axis. These observations lead Kappel and Ryan [2] to conclude that abyssal hills are split axial volcanoes. As Kappel and Ryan point out, along some intermediate ridges, abyssal hills are observed which stand as high or
S.M. Carbotte, 1<2C. Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97
higher than the axis itself (Fig. 1, profile at 21°N EPR), indicating this relief is constructional rather than thermal in origin. Abyssal hills and a crestal ridge which are volcanic in origin implies local crustal thickening associated with this topography. From multi-channel seismic studies, Rohr et al. [12] find evidence for thickening of seismic Layer 2A beneath abyssal hills on the Juan de Fuca Ridge and a thinner Layer 2A beneath the intervening troughs. These Layer 2A thickness variations are consistent with largely volcanic abyssal hills, if the base of Layer 2A corresponds with the transition from pillow basalts to dikes, as is commonly assumed [13,14]. In contrast to the Juan de Fuca Ridge, on the fast spreading EPR abyssal hills of a size and relief comparable to the axial high are rarely found on the ridge flanks, and faults are commonly located on slopes of off-axis highs dipping both inward and outward [15] (Fig. 3). In these areas, abyssal hills comprise a fault-bounded horst and graben terrain, formed during extension of the lithosphere within the plate boundary zone. Recent Alvin dives on the EPR indicate that volcanic draping of outward-facing slopes may commonly occur at fast spreading ridges. This produces slopes with a volcanic surficial morphology although the underlying architecture is fault controlled [16]. Analysis of multichannel seismic data from the E P R indicates that the base of seismic Layer 2A, in general, follows the undulations of the seafloor, as expected for faulted rather than volcanic constructional relief [13,17]. In addition, seismic and gravity studies indicate the axial high is not a volcanic construction. Assuming reflection Moho corresponds with the base of the magmatic crust, multichannel seismic data indicate that crustal thickening toward the axial high does not occur [18,19]. These data also show that Layer 2A is actually thinnest right on axis where the topography is shallowest [13,14,20] which, assuming a lithologic interpretation for the Layer 2 A / 2 B boundary, supports a non-volcanic origin for this topography. Recent studies of gravity data indicate that the depth of compensation for E P R axial topography is well below the crustal levels expected if the topography was compensated by crustal thickening associated with a vol-
87
canic construction [21,22]. Instead, the source of isostatic support for the axial high within these regions is the broad region of hot rock at the axis, as defined in seismic refraction data [23], together with a low density region in the upper mantle [10,24].
3. Lithospheric strength within axial region The presence of a constructional crestal ridge at intermediate spreading rates but not fast rates may reflect a contrast in axial lithospheric strength (thickness) such that only at the intermediate to slow spreading rates is the axial region sufficiently strong to support a volcanic load. Thermal models of lithospheric cooling, including those which model effects of hydrothermal circulation, predict a thicker/stronger brittle layer within the plate boundary zone as spreading rates decrease [25-27]. These predictions are substantiated by seismic reflection/refraction studies and studies of axial seismicity. Large differences in the depth to the axial low velocity zone are observed with spreading rate, such that the crustal lid above the zone of elevated temperatures is thicker at slower rates (e.g., 1-2 km at fast rates compared with 3 - 4 km at intermediate to slow, from Purdy et al. [28]). The maximum depth of ridge earthquakes increases with decreasing spreading rate [29]. Focal depths of 3-8 km have been measured during studies of microseismicity within the axial valley on the MAR [30,31]. In comparison, much shallower focal depths of ~ 2 km have been measured during microseismicity studies on the EPR [32]. Finally, ridge-axis earthquakes large enough to be recorded teleseismically (m b > 4) have been measured from slow spreading ridges [29], whereas only microseismicity of magnitude m b ~ 1-3 [32,33] is observed at fast spreading ridges, reflecting the lower strength of the brittle layer in these areas. Bell and Buck [34] suggest that the low amplitude, along-axis variations in gravity and topography observed on the Reykjanes ridge may reflect a lower crust which is sufficiently w e a k / h o t that it undergoes rapid ductile flow. Similarly, the lack of volcanic axial highs at fast spreading ridges
S.M. Carbotte, K.C. Macdonald / Earth and Planetary Science Letters 128 (1994) 85-97
88
a 130" 30' 44 ° 50'
130" 25'
130" 20'
130 ° 15'
44 ° 45'
4 4 ° 40'
b
A ~
~ Q - - , _ ---~ ~
,
,
0
~
!
1 , ~
~
"~
~_>
lo5o,
-~~-.,~-~
1°45'
~
~
~
lO40 '
86°00'
85°55'
A
85°5°'
85°45'
Fig. 2. Bathymetry of abyssal hills on (a) the Juan de Fuca Ridge and (b) the Oalapagos Spreading Center. Abyssal hills are shaded with light stipple. Contour interval for (a) is 20 m and for (b) is 20 fathoms. Note how abyssal hills at these intermediate rate ridges have a 'bow form' morphology with steep inward facing slopes and gentle, irregular outward facing slopes. In comparison, abyssal hills at the fast spreading EPR (Fig. 3) are typically fault-bounded on both slopes. (a) is derived from Seabeam bathymetry, provided courtesy of C. Fox, gridded at 100 m and machine contoured. (b) is from SASS data published in Malahoff [5].
S.M. Carbotte, I~ C. Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97
3000
3800~
I
5km
I
2800
~ 3000 38240000I ~
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I
m
I
Fig. 3. Enlargement of portions of Deeptow profiles shown in Fig. 1 for (a) E P R at 8°55'N from Lonsdale and Speiss [66], (VE ~ × 5 ) and (b) E P R at 19°30'S from Bicknell et al. [38] (VE ~ ×5). Locations of faults identified in deep-tow sidescan sonar data are indicated. In contrast to intermediate rate ridges, abyssal hills at these fast spreading ridges are fault bounded on both inward and outward facing slopes, forming an undulating horst and graben terrain.
may result from a w e a k / h o t lower crust which flows laterally, homogenizing crustal thickness and removing topographic relief. In this scenario, the strength of the axial lithosphere as a whole will depend most importantly on the strength of the lower crust.
4. Tectonic extension within axial z o n e
Axial lithospheric strength may also contribute to the distribution of faults and fissures within the plate boundary zone at different spreading rates. Deep-tow, submersible, and S e a M A R C I studies on the fast spreading E P R indicate that fissures are the dominant tectonic features within the innermost 1-2 km of the plate boundary zone and the only fractures with significant vertical offset are the walls of the axial summit collapse zone [35-40]. Normal faults, and possibly tensile fractures with large vertical offsets ( > 5m), develop beyond this innermost region (Fig. 4). Edwards [39] has measured the distance to the first off-axis fault imaged in S e a M A R C I and II sidescan sonar data between 10o20 ' and 15°N and has found few faults within 1 - 2 km of the axial summit caldera along this entire length of ridge crest. As she points out, the ubiquitous lack of faults in the innermost axial region, for more than 400 km of surveyed ridge, indicates it is unlikely that
89
burial by lava flows accounts for the absence of faults. In contrast, at intermediate and slow spreading ridges evidence for normal faulting can be found throughout the plate boundary zone. Camera and side-scan sonar studies on the Endeavour segment of the Juan de Fuca Ridge show fractures with significant vertical offset ( > 5m) throughout the axial region [8]. At the slow spreading Mid Atlantic Ridge, ocean bottom seism o m e t e r studies have located normal fault earthquake focal mechanisms well within the inner rift valley [30,31]. In addition, normal faults are identified in submersible and other near-bottom studies throughout the rift valley floor within some ridge segments [41-43]. Other segments along intermediate spreading ridges have axial regions dominated by volcanic morphology and few faults or fissures are seen (e.g., E P R at 21°N [4,44,45]), similar to the tectonic zonation observed at fast ridges. These areas are presumably experiencing the robust magmatic phase associated with development of the volcanic crestal ridge. The lack of normal faults within the innermost axial region along fast spreading ridges could reflect a brittle layer on the axis which is too weak, in a time-averaged sense, to support shear failure. The strength of rock under tension is much less than under shear and rock subject to an applied horizontal tensional stress near the earth's surface or in areas of high pore fluid pressure will fail along tension fractures oriented perpendicular to the least compressive stress, ~r3. However, with depth and increasing overburden pressure, tensile failure is inhibited and failure occurs by shear. An estimate of the depth at which tensile failure gives way to shear can be obtained from failure criteria for initiation of fracturing (e.g. Jaeger and Cook [46]). Assuming the Griffith criteria for fracture initiation in intact rock, tension fractures will form along planes perpendicular to or3 when: s~ = - T
Provided cr[-cr~ < 4T
(1)
where the prime denotes effective stress; ~r'= ~r - P f ; Pf is pore fluid pressure; and T is rock tensile strength. From (1) the maximum value of ~r'1 for tensional failure is: ~r~ = 3T
(2)
S.M. Carbotte, K.C. Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97
90 -104°30 '
-104"20'
-104°10 ,
-104°00 '
o-I is vertical and equal to the overburden of rock and water (o r and % ) minus the pore fluid pressure:
9" 40' =
+
-- P f
(3)
If the pore space is interconnected then: Pf = % + PwgY 9 ° 30'
(4)
where Pw is water density; and y is the depth below sea floor. Substituting (4) and (3) into (2) gives the maximum depth at which tensional failure occurs, Yo: Yo = 3 T / g ( P r
-
Pw)
(5)
9" 2 0 '
9°10 '
9 ° 00'
8 ° 50'
8 ° 40'
Fig. 4. Tectonic chart of the EPR 8°40-9°45'N showing locations of faults imaged in SeaMARC II side-scan sonar data (from Carbotte and Macdonald [54]). Ticks indicate fault facing direction. The 2700 m contour enclosing the axial region is shown in heavy stipple. The width of the axial magma chamber reflector observed in multi-channel seismic data [53] is superimposed in light stipple for comparison. As observed elsewhere on fast spreading ridges, normal faults with significant throw are only found beyond ~ 1-2 km of the axis (Table 1). With models which call upon processes associated with the axial magma chamber to account for the onset of normal faulting we expect to observe a correlation between fault patterns and axial magma chamber width/presence along axis. In this region, the axial magma chamber reflector varies considerably in width yet a corresponding change in fault patterns is not observed (see text and Table 1).
Estimates of the depth at which tensile failure gives way to shear obtained using (5) depend on values chosen for rock tensile strength, which is not well known. Typical strengths measured in laboratory experiments on crystalline rocks are ~ 100 bar. Haimson and Rummel [47] have measured lower tensile strengths of 10-60 bar (mean 30 bar) for the uppermost ~ 600 m of the basaltic pile in Iceland. Using tensile strengths of 30 and 100 bar, and Or = 2900 k g / m 3, we obtain a maximum depth to which tension failure can occur within subaqueous oceanic crust of 0.48 and 1.6 kin. Using the low mean tensile strengths obtained by Haimson and Rummel [47], Gudmundsson [48] has estimated that the depth range over which normal faults will begin to develop within subaerial crust in Iceland is 0.5-1.5 km. The depth to the axial m a g m a chamber (AMC) reflector ( ~ 1200°C isotherm) observed in multichannel seismic data constrains the thickness of the brittle crust on axis ( ~ 600°C isotherm) to less than 1.6 km on average for the fast and super fast spreading E P R [18,49]. This thickness is comparable to the maximum depths over which tensile failure predominates, as estimated above, suggesting the brittle layer on axis is typically insufficiently thick to support shear failure at fast spreading rates. In addition to insufficient strength, other factors may contribute to the lack of normal faults within the innermost axial region of fast spreading ridges. Magmatic overpressure and dike injection may result in suppression of normal faulting [50]. M a g m a pressure, associated with intrusion
S.M. Carbotte, IL C. Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97
of vertical dikes, will act to increase the least compressive stress and, hence, reduce the deviatoric stress in the vicinity of the dike. If the deviatoric stress is reduced to below the shear strength of rock, shear failure will be suppressed. G o f f [51] has p r o p o s e d that the zone of partial melt b e n e a t h the axis may act as a thermal buffer to cooling, reducing the thermal contraction stress, which may govern onset of normal faulting and thereby inhibit faulting within the innermost region. F u r t h e r studies of axial microseismicity, crustal structure and n e a r - b o t t o m morphology will be n e e d e d to assess the importance of these various effects on tectonic zonation within the plate b o u n d a r y zone. Several authors have suggested that the onset of normal faulting at fast spreading ridges is associated with the A M C ; resulting from, for example, descent of newly f o r m e d crust off the A M C [15,52] or m a g m a c h a m b e r inflation/deflation [39]. With these models we would expect to observe a correlation between fault patterns and A M C w i d t h / p r e s e n c e along axis, but no clear correlation is found. T h e width of the A M C reflector observed in multi-channel seismic data between 8o50 ' and 9°40'N varies from < 0.7 km to > 4 km [53]. However, using S e a M A R C II side-scan sonar data for this same area [54], we
Table 1 Distance to first off-axis fault compared with axial magma chamber width Location Average distance to Axialmagma chamber first fault ~ (km) width b (kin) 9°45'-9°35'N 9°35'-9°17'N 9~17'-9°03'N 9°03'-8°50'N
3.3 (_+ 1.9) 4.3 (_+3.1) 2.7 (_+ 1.0) 3.2 (_+ 1.8)
< 0.7 1-1.2 3.35-4.15 0.7-1.3
a Distance to first fault outside of axial summit caldera is measured from tectonic chart in Fig. 4 every 1 min of latitude and averaged for each ridge segment (standard deviation in brackets). b From Kent et al. [53]. The axial magma chamber reflector varies in width more in this region than anywhere else where measurements are available [17]. However, we see no correlation between width of this reflector and distance to first fault, as would be expected were onset of normal faulting associated with magma chamber processes (see text).
91
find no evidence for faults forming closer to the axis where a narrower A M C is observed (Table 1, Fig. 4). Similarly, faults are not observed any closer to the axis within a region directly north of the Clipperton transform fault, where no A M C reflector is observed, than further north, where a bright continuous reflector is detected [39]. Using S e a M A R C II data from 13°-15°N, Edwards [39] m e a s u r e d an average distance of 1.04 km to the first off-axis fault; considerably lower than we measure from 8°50 to 9°40'N (Table 1). Spreading rates are only slightly slower (10%) in the northern area; however, axial m o r p h o l o g y indicates an overall lower magmatic budget, which could give rise to a stronger axial lithosphere within which faulting develops closer to the axis. F r o m 14 ° to 15°N, the ridge axis b e c o m e s ill defined and in places a small rift valley is developed [55], whereas in the 9°N region an axial high is observed throughout, presumably reflecting more robust m a g m a supply. In addition, abyssal hills in the n o r t h e r n region have greater relief and are more closely spaced, indicating greater amagmatic extension of the crust [51].
5. Exceptions In the preceding discussion we emphasized the difference in the nature of the axial high develo p e d at intermediate c o m p a r e d with fast spreading ridges. In spite of similar first-order morphology, axial t o p o g r a p h y at fast spreading ridges is primarily an elevation created by the buoyancy of hot rock and melt b e n e a t h the rise axis, whereas at intermediate rates it may have a larger contribution from volcanic constructional processes. In some cases, however, both of these mechanisms a p p e a r to contribute to sea floor relief irrespective of spreading rate. Axial t o p o g r a p h y varies dramatically along the Juan de Fuca Ridge: from a p r o n o u n c e d axial high with relief on the order of 3 0 0 - 4 0 0 m developed along the Cleft segment, to a subdued axial bulge along the Cobb Segment to the north [3]. These differences presumably reflect variations in m a g m a supply along the ridge. Wilson [21] models gravity and t o p o g r a p h y along a profile through the Cleft Segment at 45°N and
92
S.M. Carbotte, K. C. Macdonald~Earth and Planetary Science Letters 128 (1994) 85-97
Table 2 Comparison of seafloor relief at OSC's and adjacent seafloor OSC
Relict OSC relief ~ (m)
Current OSC relief b (m)
Abyssal hill relief of adjacent seafloor (m)
16°N 11°45'N 9°03'N 5°30'N 3°25'N
250, 350 c 100, 125, 225 d 300, 100, 350, 300, 200 c 460, 220 d 440, 300 d
400/400 550/375 380/360 600/620 220/280
100-200 c 50-150 c 50-150 d ~ 200 d ~ 100 ~
c d d d ~
Values given correspond with maximum peak to trough relief developed along current and relict OSC ridge/basin pairs and along abyssal hills outside of discordant zone. ~' Measurements are listed beginning with relict ridge/basin pair closest to axis in available data. b Measurements for w e s t / e a s t ridge of OSC are given. ~ Measurements made from SeaMARC II bathymetry [68]. d Measurements made from Seabeam data [57,67,69,70]. Note that relict OSC's have relief which is comparable to that observed at current OSC's and 2-3 times greater than the average relief developed elsewhere on the ridge flanks.
the E P R at 9°N. In both regions axial topography is isostatically compensated at subcrustal depths (15-30 kin), well below compensation depths expected if the crestal ridge was primarily an axial volcano. These results indicate that, in both areas, most of the axial topography appears to be due to the buoyancy of upwelling mantle, although at the Cleft segment, a small ~ 1 - 2 mgal lack of fit between Wilson's models and the observed gravity within ~ 5 km of the axis may reflect a volcanic constructional component to the axial topography. On the fast spreading EPR, remnants of overlapping spreading center (OSC) basins and ridges are found which have been rafted onto the ridge flanks (essentially intact) during the evolution of these discontinuities [56-58]. Relict overlap ridges may be one group of features on the fast spreading ridges which are predominantly volcanic constructional highs. Maximum peak to trough relief developed at overlap r i d g e / b a s i n pairs are com-
piled in Table 2 for a number of OSC's and their discordant zones and are compared with the typical abyssal hill relief of adjacent sea floor. With the exception of OSC scars within the 11°45'N discordant zone, seafloor relief developed at relict r i d g e / b a s i n pairs is 2 - 3 times greater than that of surrounding abyssal hill fabric and is comparable to that observed at current OSC's (Fig. 5). The lengths of relict overlap ridges are very similar to the length of overlap observed at current OSCs (Table 3), as are the shape and orientation of these features (Fig. 5). These observations indicate OSC relief and morphology is largely preserved as these features are transported onto the ridge flanks. Studies of sea floor fabric show a predominance of volcanic features within OSCs and their discordant zones. Deep-tow observations indicate that the slopes bounding overlap basins are largely volcanic, with the basins themselves dominated by hummocky volcanic fabric and few fault lineations [59]. In side-scan sonar
Fig. 5. (a) Bathymetry of the OSC located at 9°03'N and its off-axis discordant zone [58]. Contour interval is 50 m. The stippled areas highlight the axial region defined by the 2700 m contour and relict OSC ridges found on the west flank of the ridge. Locations of bathymetric profiles shown in (b) are indicated. Note the similarity between the lengths and orientations of the relict ridges and that of the overlap ridge tips at the current OSC. (b) Bathymetric profiles through discordant zones left by OSC's located at 9°03'N and 11° 45'N (heavy line profiles), compared with profiles through adjacent seafloor (light line profiles). Note how relief associated with relict OSC ridges is 2-3 times greater than normal abyssal hill relief (Table 2). Bathymetry is derived from the compilation of Macdonald et al. [68].
S.M. Carbotte, K.C. Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97
a
-104"40 '
-104"30'
-104"20'
-104"10'
-104°00 '
-103"50'
9 ° 30'
9" 20'
9"10'
9" 00'
8 ° 50'
9N Discordant Zone
b
profile north of discordant zo°e
-2500
C
~~-B" C '
~
E
r.- -3000 -(3
11 45N Discordant Zone
~
t
-3500
i
i
i
0
20
40
distance (km)
93
S.M. Carbotte, K.C Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97
94
Table 3 Length of overlap ridges OSC
Length of overlap at current OSC (km)
Length of relict overlap ridges ~ (km)
16°N
20 b
24, 22 c
11 °45' N
24 b
20, 13, 11 c
9°03'N 5°30'N 3°25'N
25 b 27 b I9 b
24, 19, 24, 18, 22 c 25, 15 b 25, 16 b
a Estimates are derived from extent of oblique trending bathymetry and faults [e.g. 58] defining relict and current OSC ridges. Measurements are listed beginning with relict ridge closest to axis in available data. b Measurements made from Seabeam data [57,69]. c Measurements made from SeaMARC II data [68].
data, OSC discordant zones are characterized by a paucity of faults and a predominance of volcanic textures [54,57]. A large number of morphological, petrologic, and magnetic observations indicate that OSC's are located at regions along the ridge comparatively starved of m a g m a [40,56,60,61]. More restricted m a g m a supply to these areas should be associated with a cooler, thicker lithosphere, which may be strong enough to support construction of volcanic relief at OSC ridges. Analysis of gravity data supports this hypothesis. Madsen et al. [24] and Cormier and Macdonald [62] infer higher densities associated with OSC's on the northern and southern E P R than with the shallower mid-segment areas. Madsen et al. [24] note that the along-axis variations in anomalous mass, inferred from gravity data from 9-10°N on the E P R are sufficiently large that they are unlikely to reflect changes in the size of an axial m a g m a chamber
alone. Instead,
the differences observed
require variations in the width of zone of hot rock flanking the axis and of low densities in the underlying mantle such that a narrower low density region is present at OSC's. Analysis of seismic data in the vicinity of the OSC at 9°N has lead Kent et al. [53] to the contrasting view that OSC's may, in fact, be places of enhanced m a g m a supply and crustal accretion. In this area, two-way travel times to reflection Moho reach an alongaxis maximum at 9°10'N, just north of the OSC, possibly reflecting thicker crust [63], and the A M C
reflector is wider directly north of the OSC than elsewhere (the greater width here may reflect a melt supply which is decoupled from the upper crust rather than more melt, [53]). An alternate interpretation of the seismic Moho travel-time maximum is that crustal velocities are slower within this region. Argo and deep-tow studies show more extensive fracturing on axis in this area than further north [40,59], which could account for the increase in travel times. In addition, we note that, although travel times to Moho do reach a maximum just north of the OSC, they are locally reduced within relict OSC basins [63], possibly reflecting thinner crust as expected from more restricted m a g m a supply to these discontinuities. Finally, linear volcanic edifices may also be built at the crest of magmatically robust fast spreading centers which experience long periods of volcanic quiescence for some reason. For example, the high standing, near-axis shoulders near 18-19°S [57,64] may be caused by episodes of significant volcanic construction. In these areas we would predict the crust and Layer 2A to be thicker, which can be tested with multi-channel seismic data.
6. Conclusions
Morphologic, seismic and gravity data suggest that the physical processes which contribute to development of an axial topographic high at fast, and some intermediate, spreading ridges differ. At intermediate spreading ridges a significant component of the axial topography may be built by volcanic construction, whereas at fast spreading ridges, the axial high is primarily an elevation created by the buoyancy of hot rock and magma beneath the rise. At either a fast or intermediate spreading ridge both processes may contribute to axial topography, depending on local m a g m a supply. For example, during robust magmatic phases on an intermediate spreading ridge the buoyancy of upwelling m a g m a is likely to account for a significant component of the positive relief. At fast spreading ridges, overlapping ridge tips may be primarily volcanic edifices. We suggest that
S.M. Carbotte, K.C. Macdonald/Earth and Planetary Science Letters 128 (1994) 85-97 axial l i t h o s p h e r i c s t r e n g t h m a y p l a y a s i g n i f i c a n t r o l e in t h e n a t u r e o f t h e axial t o p o g r a p h y s u c h t h a t o n l y at i n t e r m e d i a t e s p r e a d i n g rates, o r in a r e a s o f r e d u c e d m a g m a s u p p l y o n t h e fast r i d g e s (e.g., at O S C ' s ) , is t h e l i t h o s p h e r e sufficiently s t r o n g to s u p p o r t c o n s t r u c t i o n o f a v o l c a n i c load. F u r t h e r d e t a i l e d gravity a n d s e i s m i c studies, esp e c i a l l y at i n t e r m e d i a t e r a t e ridges, a r e n e e d e d to e s t a b l i s h t h e r o l e o f v o l c a n i c c o n s t r u c t i o n in axial t o p o g r a p h y clearly. A x i a l l i t h o s p h e r i c s t r e n g t h m a y also a c c o u n t for t h e t e c t o n i c z o n a t i o n o b s e r v e d at fast, a n d s o m e i n t e r m e d i a t e , s p r e a d i n g ridges. F i s s u r e s d o m i n a t e w i t h i n t h e i n n e r m o s t axial r e g i o n a n d l a r g e - s c a l e n o r m a l f a u l t i n g d o e s n o t a p p e a r to i n i t i a t e u n t i l 1 - 2 k m o f f axis. T h i s p a t t e r n o f f r a c t u r i n g m a y r e f l e c t a l i t h o s p h e r e initially t o o w e a k a n d t h i n to s u p p o r t s h e a r f a i l u r e . I n d e e d , estimates of the depths over which tensile failure will p r e d o m i n a t e in o c e a n i c c r u s t ( 0 . 5 - 1 . 5 kin) a r e c o m p a r a b l e to t h e t h i c k n e s s o f t h e b r i t t l e p l u s d u c t i l e c r u s t a l lid o v e r l y i n g t h e axial m a g m a c h a m b e r (1.6 km). A l t h o u g h o t h e r f a c t o r s m a y also b e i m p o r t a n t , p r o c e s s e s d i r e c t l y a s s o c i a t e d w i t h t h e axial m a g m a c h a m b e r (e.g. size, o r inflat i o n / d e f l a t i o n ) a r e u n l i k e l y to b e p r i m a r y f a c t o r s g o v e r n i n g t h e initial s u p p r e s s i o n o f n o r m a l faulting.
Acknowledgments W e t h a n k C h r i s F o x for p r o v i d i n g us w i t h J u a n de Fuca bathymetry. Roger Buck and Jim Cochran provided useful reviews during preparation of the manuscript. We thank Jeffrey Karson, J o h n M a d s e n a n d an a n o n y m o u s r e v i e w e r for t h e i r t h o u g h t f u l c o m m e n t s . T h i s w o r k was supp o r t e d by g r a n t s N S F O C E 8 9 - 1 1 5 8 7 a n d O N R N 0 0 0 1 4 - 9 0 - J 1 6 4 5 to K C M . L a m o n t c o n t r i b u t i o n n u m b e r 5229. [CL]
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