Earth and Planetary Science Letters 252 (2006) 490 – 494
www.elsevier.com/locate/epsl
Discussion
A comment on “Bathymetry gradients of lineated abyssal hills:
Inferring seafloor spreading vectors and a new model for hills formed
at ultra-fast spreading rates” by K.A. Kriner et al.
[Earth Planet. Sci. Lett. 242 (2006) 98–110]
DelWayne R. Bohnenstiehl ⁎
Lamont-Doherty Earth Observatory, Palisades, NY 10964, United States
Received 13 March 2006; received in revised form 3 May 2006; accepted 5 October 2006
Available online 15 November 2006
Editor: R.D. van der Hilst
In their recent letter, Kriner et al. [1] present new data
on abyssal hill slope direction (aspect) derived from
multi-beam datasets collected along the global midocean system. Importantly, they note that the relative
frequency of inward- and outward-facing slopes decreases as a function spreading rate, with the sense of
asymmetry reversing at ultra-fast rates. It also is shown
that outward-facing hills have more variable aspects
than inward-facing hills at all spreading rates. They
conclude correctly that such empirical information can
be used to assess the paleo-spreading rate and the direction to the paleo-spreading center. Yet, their model
for ultra-fast hill formation shows many inconsistencies
with multi-resolution observations of faulting in this
setting [2–4]. As I show here, it is also not supported
upon closer inspection of the multi-beam data, suggesting their interpretation of aspect histograms in terms of
abyssal hill steepness should be reevaluated.
In summarizing their model for abyssal hill formation
at ultra-fast spreading rates Kriner et al. [1] state: “Our
DOI of original article: 10.1016/j.epsl.2005.05.046.
⁎ Now at: Department of Marine, Earth and Atmospheric Sciences,
North Carolina State University, Raleigh, North Carolina 27695,
United States. Tel.: +1 919 515 7449.
E-mail address: [email protected].
0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2006.10.012
preferred scenario assumes the extensional stress at
ultra-fast spreading rates is partially accommodated by
a large number of small-throw normal faults on the
inward-facing hillsides, and a lesser number of larger
throw faults on the outward-facing side. […]. On the
outward side, larger faults form through the thicker
brittle layer, which can accommodate the same amount
of strain over a fewer number of faults. The large
amount of extensional stress created by such high
spreading rates may not be fully accommodated by
increased magma upwelling at the ridge axis, so greater
amounts off-axis faulting are required to accommodate
the increased extension. […] The larger number of small
faults on the inward side creates a longer running, more
gently sloped surface than that created on the outward
side by the fewer, larger throw faults."
Kriner et al. [1] make use of multi-beam bathymetric
datasets from ultra-fast spreading portions East Pacific
Rise (EPR), chiefly between latitudes 16–20° S. Full
spreading rates here are ∼150–160 mm/yr [1]. Along
eight across-axis (E–W) transects near 19°20′ S and
19°45′ S, higher-resolution bathymetric data and sidescan
sonar imagery also exist and have been analyzed previously [2,5]. These data were collected using the 110 kHz
Deep-Tow sonar system, which maintains a footprint of
b2 m2 on the seafloor and collects imagery over a swath
D.R. Bohnenstiehl / Earth and Planetary Science Letters 252 (2006) 490–494
491
Fig. 1. a) Histogram of scarp height data from the 19°20′ and 19°45′ S regions of the ultra-fast spreading EPR, as observed in Deep-Tow imagery [2].
Inset shows a vertically expanded subset of the data. Note the greater abundance of outward-dipping faults (red circles) at small scales and the greater
abundance of inward-dipping faults (blue squares) at larger scales. b) Deep-Tow sidescan sonar example and structural interpretation from a
representative portion of the ultra-fast spreading seafloor, eastern flank ∼ 19° 45′ S latitude. Note the clustering of small, closely spaced outwarddipping faults within the hanging walls of larger, inward-dipping master faults (M).
width of ∼1.5 km. Bathymetric data are generated with a
narrow beam profiler, providing depth information accurate to within ∼1 m [6].
Faulting analysis based on the Deep-Tow data shows
a pattern opposite to that described by Kriner et al. [1].
Namely, inward-dipping faults are less abundant (35–
45%) but accommodate slightly more (50–55%) of the
total brittle strain due to their systematically larger offsets (Fig. 1a) [2]. The pattern observed is one in which
small outward-dipping faults are clustered within the
hanging walls of larger inward-dipping master faults,
forming asymmetric graben structures (Fig. 1b). This is
inconsistent with Kriner et al.'s proposed pattern of fault
development. Moreover, strain estimates along these
Deep-Tow transects are on the order of 2–4% [2], consistent with those reported along the fast-spreading EPR
(using similar methodology) [4]. Therefore, their assertion that strains are larger in the ultra-fast spreading
environment also is not well supported by observations.
Along other sections of the ultra-fast spreading EPR,
lower-resolution (surface-towed) GLORIA [3] and SeaMARC II [4] sonar imagery suggest that the pattern
observed in the Deep-Tow data is widespread. Carbotte and
Macdonald [4] report that clusters of short, closely spaced
antithetic faults are common within the hanging walls of
longer inward-dipping structures in the 18–19° S region of
the EPR, with inward-dipping faults accounting for only
40% of the faults observed. Similarly, in the 3° S area,
Searle [3] reports that inward-dipping faults are longer and
having dip-slopes that backscatter sound more extensively
throughout his ridge-flank survey—indicating that they are
steeper and more continuous than outward-dipping scarps.
This pattern also is evident in the 16° S region of the EPR
(see Bohnenstiehl and Carbotte's [2] Fig. 6).
Kriner et al.'s [1] inference that outward-facing hills are
steeper than inward-facing hills in the ultra-fast spreading
environment was based on their interpretation of the
aspect histograms, and it appears to have relied heavily on
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D.R. Bohnenstiehl / Earth and Planetary Science Letters 252 (2006) 490–494
Fig. 2. a) Histograms of slope aspect derived from (b) multi-beam bathymetric data on the eastern flank of the Mid-Atlantic Ridge (Grid #3, [1]). The
ridge axis is to the left of the image. Top panel shows a histogram of all slope aspects within this region; lower panels have been filtered to include
only steeper portions of the seafloor, as labeled. The ∼98° aspect peak represents outward-facing slopes, and the ∼ 278° aspect peak represents
inward-facing slopes. The sense of asymmetry in peak height reverses as the data are filtered to exclude low slope values, indicating inward-facing
hills are typically steeper. c) Topographic profile across a single slow-spreading abyssal hill, A–A′. The histogram pattern at slow-spreading rates
reflects the existence of more gently-sloped and longer-running outward-facing hillsides, relative to the shorter-running and steeper inward-facing
hillsides. As proposed in [1,7], this results in a greater number of outward-directed aspect values (a taller peak).
previous interpretations made within the slow-spreading
environment [7]. In Fig. 2, I show the bathymetry of a
region on the eastern flank of the slow-spreading (25 mm/
yr full-rate) Mid-Atlantic Ridge. These data, which were
retrieved as a ∼110 m grid from the RIDGE Multibeam
Synthesis Data Portal (http://www.marine-geo.org/rmbs/),
correspond to Kriner et al.'s grid number 3. Slope and
aspect data were calculated from finite differences of the
bathymetric values, using a Matlab routine equivalent to
the procedure implemented in [1].
The upper aspect histogram (Fig. 2a), which includes
all slopes within the region, shows two peaks corresponding to outward- and inward-facing hills. Outwardfacing slopes are more numerous and have more variable orientation, as evident by the height and width of
the outward peak, respectively. These results are consistent with a model in which normal faulting is the
dominant process forming steeper inward-facing hills,
with the more gently-dipping and longer-running outward-facing hills representing back-tilted blocks
(Fig. 2c) [1,7]. The relative steepness of the inwardand outward-facing hills can be further visualized by
filtering the aspect histograms for particular values of
slope (Fig. 2a). If low-slope values are progressively
excluded, the sense of asymmetry between peaks can be
seen to reverse—indicating that a greater percentage of
the steeper sloped regions are inward-facing.
In the ultra-fast spreading environment, Kriner et al.
[1] show that roughly ∼ 65% of the regions examined
have a sense of asymmetry opposite to that observed in
the slow-spreading environment (Fig. 3a, b), with inward-facing peaks being taller than the outward peaks
within the aspect histograms. They state that the greater
peak frequency of inward-facing slopes implies the
presence of steeper outward-facing hills, apparently
following the slow-spreading model outlined above.
If this were true, then a similar reversal in asymmetry,
now with the outward-facing histogram peak becoming
taller, should be observed as the data are filtered to
include only steeper portions of the seafloor. Contrary to
D.R. Bohnenstiehl / Earth and Planetary Science Letters 252 (2006) 490–494
493
Fig. 3. a) Histograms of slope aspect derived from (b) multi-beam bathymetric data on the eastern flank of the ultra-fast spreading EPR (Grid #63,
[1]). The ridge axis is to the left of the image. Top panel shows a histogram of all slope aspects within this region; lower panels have been filtered to
include only steeper portions of the seafloor, as labeled. The ∼ 102° aspect peak represents outward-facing slopes, and the ∼ 282° aspect peak
represents inward-facing slopes. As less steep areas of the seafloor are excluded, the difference in the peak height increases, without reversing its sign.
Steeper slopes also have less variable aspects, as indicated by the decreasing width of the peak. Enlarged views of the (c) bathymetry, (d) aspect and
(e) slope show the pattern of abyssal hill development. Right- and left-facing arrows indicate the dominant inward- and outward-facing hills,
respectively. Note that the four steepest hills are inward-facing.
this prediction, inward-facing peaks remain slightly
taller or equal in height, with the largest difference observed among the steepest slopes.
This implies that inward-facing, not outward-facing,
hills are steeper typically in the ultra-fast spreading
environment—a result that can be confirmed through the
direct inspection of the multi-beam datasets used in Kriner
et al.'s study [1]. In Fig. 3, I show an enlarged view of the
eastern flank bathymetry (c) near 20° S latitude, along
with the derived data products of aspect (d) and slope (e).
The aspect histogram for this subset of the data mimics
that of the entire region shown in Fig. 3a. In the slope map,
the four steepest hill faces are clearly directed inward.
Given previous sonar observations [2–4], these slopes can
be interpreted as inward-facing fault scarps. The three next
steepest hills face outward and may represent lower-throw
normal faults. Again, this pattern of abyssal hill development is inconsistent with Kriner et al.'s [1] model.
Outward-directed hills in the slow-spreading rate
environment are largely un-faulted volcanic surfaces
that separate steeper, inward-facing fault scarps (Fig. 2c).
In contrast, the fault-bounded seafloor blocks at ultra-
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fast spreading rates are further modified by a series of
structures that lie antithetic to larger, more widely spaced
inward-dipping master faults. Slip along these antithetic
(outward-dipping) structures creates a rough stair–step
appearance that can be observed within the highresolution Deep-Tow bathymetry (Fig. 1b). In the gridded multi-beam datasets, these smaller outward-facing
fault scarps are manifested as intermediate-sloped surfaces that exhibit more variable aspects than the master
faults (Fig. 3a iii). Minor bookshelf rotation along the
antithetic hanging-wall structures likely generates an
additional component of inward-facing seafloor with
low slope values (Fig. 1b, and [2]).
Unlike the predictions of the slow-spreading abyssal
hill model (Fig. 2c) [1,7], the existence of a taller
inward-facing histogram peak in the ultra-fast environment does not imply that inward-facing portions of the
seafloor are more gently sloped and longer running than
outward-facing regions. Rather, the outward-facing
histogram lobe remains wider, with the integrated frequency of outward-directed seafloor balancing the taller
inward-facing peak height at intermediate slope values
(Fig. 3a iii). The long-wavelength slope of the abyssal
flanks is nearly flat at ultra-fast spreading rates, with
only a slight outward tilt (b 0.5° in Fig. 3b) reflecting
thermal subsidence.
Finally, I would like to emphasize that regardless of
the scenario envisioned for ultra-fast hill formation, the
statistical observations made by Kriner et al. [1] provide
an effective method for assessing paleo-spreading rate
and paleo-spreading direction. This will no doubt serve
as an important tool in future studies.
Acknowledgements
Data were obtained from the RIDGE Multi-Beam
Synthesis Project, Available Online: http://www.marinegeo.org/rmbs/. GeoMapApp and other tools developed
by W. Haxby were used in exploring these datasets.
Helpful reviews by D. Smith and J. Goff improved the
quality and clarity of this comment.
References
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