1. Subsidence Analysis
1.1 Methods
The subsidence of a sedimentary basin is the combined result of loading of the crust (isostatically
or flexurally) by variations in the mass of the overlying sediment and water column, and tectonic
processes such as changes in crustal thickness or structure [e.g., Sclater and Christie, 1980; Allen
and Allen, 2005]. In this study, subsidence analyses were performed using the program OSX
Backstrip [Cardozo, 2012]. Sediments were decompacted assuming an exponential decrease in
porosity with depth [Allen and Allen, 2005]:
æf ö
æf ö
y 2 '- y1 ' = y2 - y1 - ç o ÷ *(exp(-cy1 ) - exp(-cy2 ))+ ç o ÷ *(exp(-cy1 ') - exp(-cy 2 '))
ècø
ècø
(1)
where y1’ and y2’ are the base and top of a sedimentary column before compaction, and y1 and y2
are the base and top after compaction, and 0 is the initial uncompacted porosity. Values for o
and c of Sclatter and Christie, [1980] representative of mudstone and sandstone lithologies were
used for appropriate intervals in analyzed cores (Tables S-1 & S-2). Basement depths were
corrected for sediment loading under the initial assumption of 1D Airy isostasy given by Allen
and Allen, [2005]:
ær -r ö
æ rw ö
Y = S ç m b ÷ - Dsl ç
÷ + (Wd - Dsl)
è rm - rw ø
è rm - rw ø
(2)
where Y is the depth to basement correcting for the sediment load, S is the total sediment column
thickness corrected for compaction, m, w, andb are the densities of the mantle, water, and
mean overlying sediment column, sl is the eustatic change in sea level and Wd is the paleo water
depth. For this study, we assume the effects of eustatic sea level are negligible compared to
variations in paleo water depth, which are an order of magnitude greater, and therefore set sl
equal to zero. Backstripping was performed using minimum and maximum paleo water depths
for each horizon based on published benthic foraminifera assemblages and interpretations of
volcanic and sedimentary depositional environment (Tables S-1 & S-2).
Given that forearc basin is in part flexurally supported by the rigidity of the upper and lower
plates, we additionally estimate the subsidence due to sediment loading by 1) scaling the isostatic
sediment loading by a flexural compensation factor, C, for a periodic load and 2) assuming an
infinitely rigid lower plate. For the flexurally supported case, we calculate a flexural
compensation factor following Turcotte and Schubert [2002]:
C=
rm - r c
4
D æ 2p ö
rm - rc + ç ÷
gè l ø
(3)
where m and c are the densities of the mantle and crust, D is the flexural rigidity, g is the
acceleration of gravity, and  is the wavelength of the basin. We calculate a flexural rigidity from
an estimate of the mean forearc effective elastic thickness of ~30 km. This value is based on the
depth to the 450 isotherm in the Japan arc and forearc, which increases in depth from ~20 km
depth near the arc to >40 km near the trench [Okubo and Matsunaga, 1994; Peacock, 2003]. An
effective elastic thickness of 30 km represents an upper estimate of the flexural strength of the
paleo-arc, given that the Miocene forearc was likely characterized by widespread volcanism and
higher heat flow than today [e.g., Moore and Fujioka, 1980; Sato and Amano, 1991; Yoshida,
2001]. We calculate a C of 0.2 for a 175 km wide forearc basin, and scale the basement
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subsidence calculated assuming Airy isostasy by this value. For the case of an infinitely rigid
lower plate, we assume that the loading compensation is zero.
2. Synthesis of upper plate fault kinematics
We present a synthesis of the timing of initiation and cessation of slip along extensional faults,
and the initiation of slip along contractional faults in northern Honshu Japan. We compile both
published constraints for the timing of slip as well as new data from field analyses and syntheses
of published material. Data synthesized for faults in table “ts03.docx” correspond to numbered
faults in Figure 5 in the main body of the text.
3. Convergence Rates
3.1 Methods
Stage poles are calculated following:
(4)
where Ω (latitude, longitude, and angular rotation) are finite reconstruction poles describing the
motion of North America relative to the Pacific plate from ti to 0 Ma (right hand side of equation
4) or a stage pole describing the motion of the Pacific plate relative to North America from t1 to t2
Ma (left hand side of equation 4).
3.2 Data for Tonga, Izu-Bonin and Mariana margins
Pacific-Australia motions at the Tonga trench were calculated using the Euler finite
reconstruction poles for the past ~50 Ma of Cande and Stock [2004] and Keller [2004].
Reconstruction poles were used to determine stage poles describing the angular rotation of the
Pacific plate relative to a fixed Australian plate during specific time intervals. From these stage
poles, linear velocities at a point in the modern day forearc near the location of ODP Site 841
(23S, 175W) were determined. Local convergence rates at the Tonga Trench were calculated as
the trench-orthogonal component of velocity relative to the modern trench strike of ~15,
assuming less than ~10 of rotation of the trench since 50 Ma [after, Schellart et al., 2006] (Fig.
S-1). The trench-orthogonal absolute velocity of the Pacific plate is also calculated at 23S,
175W using the stage poles of Torsvik et al., [2010], which utilizes a global moving hotspot
reference frame (Fig. S-1).
Backarc spreading rates during the opening of the South Fiji Basin and the Lau Basin were
calculated from the Euler finite reconstruction poles of Schellart et al., [2006] describing the
relative motion of the Tonga arc with respect to the Lau ridge, and the eastern South Fiji Basin to
the western South Fiji Basin. Backarc spreading rates were deconvolved to the component of
linear velocity orthogonal to the strike of the Tonga Trench. The maximum total convergence of
the Pacific Plate with respect to the Tonga arc is presented as the sum of the linear, trenchorthogonal velocities of the Pacific with respect to the Australian plate, and the Tonga arc with
respect to Australia (see Fig. 10 in text).
Relative Pacific-Philippine Sea plate convergence rates are not presented, because the
convergence rate between the Philippine Sea and Pacific plates at the Izu-Bonin and Mariana
trenches is poorly constrained. We calculate only the absolute velocity of the Pacific plate in the
global fixed hotspot reference frame of Torsvik [2010] at points in the forearcs of the Izu-Bonin
and Mariana margins, from ~40 Ma to present. Philippine Sea plate reconstructions indicate a
northward translation of the Marianna and Izu Bonin arcs by at least ~10 [Pubellier et al., 2003;
Sdrolias et al., 2004; Müller et al., 2008] with a potential ~35 clockwise rotation of the arc
[Sdrolias et al., 2004; Müller et al., 2008]. We therefore calculate absolute Pacific plate trenchorthogonal linear convergence velocities at both the modern latitude of each forearc core, as well
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as a potential paleo position 10 to the south (Fig. fs01.pdf). These data are compared to the
timing of backarc spreading rates in the Parece Vela Basin, Shikoku Basin, and Mariana Trough
(see Fig. 10 in text) [Stern et al., 1984; Sdrolias et al., 2004].
Caption for Auxiliary Figure fs01.pdf:
Rose diagram showing the linear velocity of the Pacific Plate (Pac) with respect to the
Australian Plate (Aus), and with respect to the global moving hotspot reference frame (GMH)
of Torsvik et al., (2010) at the Tonga and Izu-Bonin-Mariana (IBM) trenches from 50-0Ma. a.
Linear velocities calculated for the Tonga trench at 23°S, 175°W. b. Linear velocities
calculated for the IBM trenches at 30N, 141°E and 20°N,145°E, given the modern position of
the trench, and at 20°N,141°E and 10°N,145°E assuming a 10° northward translation of the
Philippine Sea plate. Plate convergence velocities are calculated relative to the average
modern strike of the IBM trench and to a potential trench strike rotated counterclockwise by
~35° relative to modern.
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