Paleomagnetic results from Pleistocene sedimentary deposits in the central Puget Lowland indicate that the region has experienced widespread deformation within the last 780 kyr. Three oriented samples were collected from unaltered fine-grained sediments mostly at sea level to determine the magnetostratigraphy at 83 sites. Of these, 47 have normal, 18 have reversed, and 18 have transitional (8 localities) polarities. Records of reversed- to normal-polarity transitions of the geomagnetic field were found in thick sections of silt near the eastern end of the Tacoma Narrows Bridge, and again at Wingehaven Park near the northern tip of Vashon Island. The transitional horizons, probably related to the Bruhnes-Matuyama reversal, apparently fall between previously dated Pleistocene sediments at the Puyallup Valley type section (all reversed-polarity) to the south and the Whidbey Island type section (all normal-polarity) to the north. The samples in general are of sufficient quality to record paleosecular variation (PSV) of the geomagnetic field, and a statistical technique is used to correlate horizons with significant agreement in their paleomagnetic directions. Our data are consistent with the broad structures of the Seattle uplift inferred at depth from seismic-reflection, gravity, and aeromagnetic profiles, but the magnitude of vertical adjustments is greatly subdued in the Pleistocene deposits.
|Figure 1. Map of central Puget Lowland showing paleomagnetic sampling sites (circles). Filled circles indicate sites with normal-polarity remanent magnetizations, open circles indicate sites with reversed-polarity magnetizations, and half-filled circles indicate sites with transitional-polarity directions (Table 1). Inset at the lower right shows the location of the central Puget Lowland in Washington State. Click to enlarge image
The heavily populated Puget Lowland region (Fig. 1) has been the subject of an increasing number of scientific investigations designed to clarify the nature of its seismic hazards. The Lowland has had a large number of historical earthquakes relative to its surrounding areas, and most of the larger events have been within the subducting Juan de Fuca plate [Ludwin et al., 1991; Rogers et al., 1996]. Recent geologic investigations, however, have documented major prehistoric earthquakes in the overriding North American plate, in particular along the Seattle fault [Bucknam et al., 1992; Nelson et al., 1999]. Considering the potential consequences of a large earthquake in the modern Puget Lowland, relatively little information is available concerning the region's basic structural setting and paleoseismicity. Understanding the stratigraphy, structure, and deformation of Quaternary sedimentary deposits within the Puget Lowland is important for an accurate assessment of the region's seismic hazards.
Geologic mapping in the mostly unconsolidated sedimentary
cover of the Puget Lowland is difficult primarily due to similar appearances of various Pleistocene glacial
and nonglacial deposits mantling the area, and to poor exposure caused by abundant landslide deposits, dense
vegetation, and urban development. We report here on a magnetostratigraphic study that was initially undertaken
to provide a rudimentary understanding of the regional Pleistocene stratigraphy: reversed(R)-polarity paleomagnetic
directions are assumed to indicate an age greater than ~780 ka, and normal(N)-polarity directions an age less
than ~780 ka, the Bruhnes-Matuyama boundary.
The paleomagnetic data are apparently of sufficient
quality, however, that paleosecular variation (PSV) of the geomagnetic field was recorded. Significant parts
of a polarity reversal, probably the Bruhnes-Matuyama transition, are recorded at localities near the eastern
end of The Tacoma Narrows Bridge and at Wingehaven Park near the northeastern end of Vashon Island (Fig. 1).
Transitional directions are found at a number of other sites and potentially provide a high-precision stratigraphic
marker horizon. Furthermore, similar PSV directions for sites of N, transitional, and R polarity have been correlated
using a statistical technique developed by Bogue and Coe . Finally, we compare our paleomagnetic results
with structural models developed using seismic reflection profiles beneath Puget Sound [Pratt et al., 1997]
and tomographic data from the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS; Brocher et al. ).
The oceanic Juan de Fuca plate is the northernmost
remnant of the Farallon plate subducting beneath North America, and its oblique convergence with the continental
margin is the source of great subduction-zone earthquakes [Atwater and Hemphill-Haley, 1997], complex upper-plate
folding and faulting [Johnson et al., 1999], and Cascade-arc volcanism [Smith, 1993]. Based on Neogene deformation,
paleomagnetic rotations, and geodetic data, the Cascadia forearc appears to be migrating northward and breaking
up into large rotating blocks associated with dextral transpression [Wells et al., 1998]. The Puget Lowland
lies within this transpressional zone in northwestern Washington, overlying a major crustal boundary between
Eocene basement rocks of the western Coast Range province and pre-Tertiary rocks of the eastern Cascade province
[Wells and Coe, 1985; Johnson et al., 1996].
Seismic reflection profiles beneath Puget Sound
indicate subhorizontal Paleogene and Neogene sedimentary rocks deformed by west- to northwest-trending faults
and folds [Pratt et al., 1997]. The stratigraphy in the Lowland consists of the Crescent Formation basalts overlain
in the Seattle basin by upper Eocene marine sedimentary strata, shallow-marine turbidites of the upper Eocene
to Oligocene Blakeley Formation, non-marine Miocene sediments of the Blakely Harbor Formation, and by Quaternary
deposits. Asymmetric subsidence of the Seattle basin indicates continued contractional deformation since Eocene
time [Johnson et al., 1994].
A primary subsurface feature is the Seattle uplift,
with south dipping (~20°) bedrock strata on its south flank, and steeply north dipping (50°-90°) strata and
the east-trending Seattle fault on its north flank. Pratt et al.  interpreted this and other uplifts as
fault-bend and fault-propagation folds, and suggested that the Puget Lowland is underlain by a large north-directed
thrust sheet (thin-skinned deformation) bounded by faults along the Cascade and Olympic Ranges to the east and
west [Johnson et al., 1994], respectively. Brocher et al. , on the other hand, interpret the Seattle uplift
as a pop-up structure [Wells and Weaver, 1993] bounded by the steeply dipping Seattle fault to the north and
Tacoma fault to the south. The Tacoma fault is inferred from gravity and magnetic data and a seismic-velocity
gradient similar to that associated with the Seattle fault. In addition, the Tacoma fault is inferred from the
magnitude of structural relief along this zone, particularly to the west. Conversely, structural relief along
the Seattle fault zone decreases to the west, and Brocher et al.  suggest that this relationship likely
results from the transfer of strain between the Seattle and Tacoma faults.
||Figure 2. Conceptual Quaternary stratigraphic framework for the Puget Lowland showing the younger Whidbey Island type section [Easterbrook et al., 1967] with the addition of the Olympia beds [Troost, 1999] and the older Puyallap Valley type section [Crandell et al., 1958]. At present, an age data gap exists between the older reversed-polarity section (780 ka BP) and the younger normal-polarity section (250 ka BP). Modified from Troost . Click to enlarge image
The glacial and interglacial deposits overlying
Tertiary basement rocks in the southern Puget Lowland (Fig. 2) indicate that the area was glaciated at least
six times during the Pleistocene Epoch [Easterbrook, 1994]. Near Tacoma, these deposits are approximately 400
m thick [Jones, 1996]. Correlations between units have been difficult to make due to the lack of distinctive
lithologic or textural features and to problems in dating materials of this composition and age. More recently,
laser-argon, fission-track, thermolumines-cence, amino acid, and paleomagnetic techniques have been employed
to establish a chronology for these Pleistocene sediments, particularly north and east of the Seattle-Tacoma
area [Blunt et al., 1987; Easterbrook, 1994; Troost, 1999; Troost and Booth, 1999].
The last glacial advance in the Puget Lowland was
the Puget lobe of the Cordilleran Ice Sheet, culminating ~15 ka ago during the Vashon stade of the Fraser glaciation
[Booth, 1987]. In the Puyallup River valley, one of the type sections of older, pre-Vashon glacial and nonglacial
deposits is exposed (Fig. 2). It consists of drift and outwash of the Orting, Stuck, and Salmon Springs glaciations
[Crandell et al., 1958]. All of these glacial materials and their interbedded nonglacial deposits have R-polarity
remanent magnetizations and ages greater than ~780 ka [Easterbrook et al., 1988; Easterbrook, 1994]. To the
north on Whidbey Island (~50 km north of Seattle), the upper type section of Pleistocene deposits (Fig. 2) consists
of the Double Bluff, Possession, and Vashon glacial deposits and intervening glacial and nonglacial deposits
[Easterbrook et al., 1967]. Here, the entire section has N-polarity magnetizations and ages less than ~780 ka
[Easterbrook, 1994]. No deposits between the Double Bluff and Salmon Springs glaciations (Fig. 2) have yet been
identified in the Puget Lowland [Richmond and Fullerton, 1986]. Mapping efforts in the Tacoma area have identified
the Vashon glacial drift and at least two older drifts of N polarity (this study) which may fall between the
Double Bluff and Salmon Springs deposits based on preliminary luminescence dates [Troost, 1999; Mahan et al.,
Generally, three oriented samples were collected
from unweathered (dark gray), fine-grained (silt) glacial and interglacial lake deposits (Fig. 2) at each of
86 sites in the central Puget Lowland. Horizontal benches were excavated and leveled with a circular bubble
level, and then vertical pedestals were carved over which plastic sample boxes (volume = 6 cm3) were fitted.
Before removal, one top edge of each box was marked and oriented using a magnetic compass. Local bedding was
also recorded to correct for deformation since original deposition. The sites are mostly at sea level in beach
exposures around Puget Sound, but some were also collected in road-cut (N3-N6, N16, N17, R12, N27, N28, N37,
N38, N40), stream-bank (N1, N2, R16, N39, N44), and hillside (R3, T1, T3-T6, T8, N35, N46) exposures (Fig. 1; Table 1).
| Figure 3. (a) Orthogonal projection of alternating-field (AF) demagnetization vector endpoints for a sample from site N10 showing a univectorial normal-polarity remanent magnetization. (b) Vector plot of AF demagnetization data for a sample from site T9 showing a univectorial transitional-polarity magnetization. Filled symbols in both plots indicate projections onto the horizontal plane, and open symbols, onto the vertical plane. (c) Equal-area stereoplot showing intersecting remagnetization circles fitted to demagnetization data for samples from site R10. Dots indicate poles to the great circles projected from the lower hemisphere, and the open circle indicates the mean direction (reversed-polarity) for this site projected from the upper hemisphere. The intersection is tightly constrained, and the 95% confidence ellipse is correspondingly small. Click to enlarge image
Step-wise alternating field demagnetization indicates
that these sediments carry stable characteristic remanent magnetizations (Fig. 3a, 3b). Three sites with unstable
magnetizations were discarded from further analysis. The normal-polarity characteristic magnetization vectors
were usually isolated between 20 and 100 mT. Least-squares lines are fitted to the sample demagnetization data
[Kirschvink, 1980], and Fisher  statistics are calculated for single-component site-mean directions and
for the overall mean directions. For R-polarity samples, N-polarity components were first removed or, more often,
the N-polarity overprint was removed simultaneously with the characteristic R-polarity component. Converging
demagnetization planes and Bingham statistics [Onstott, 1980], therefore, were used to determine the characteristic
magnetization direction and error limits, respectively, for most of the R-polarity sites Fig. 3c.
|Figure 4. Equal-area stereoplots of normal-polarity, reversed-polarity, and transitional-polarity site-mean directions with their 95% confidence limits. Numbers are keyed to site data listed in Table 1. Filled circles are projected from the lower hemisphere, and open circles are projected from the upper hemisphere. Small triangles indicate site-mean directions with too few samples to calculate meaningful statistics (Table 1). Click to enlarge image
In this procedure, great circles were fitted to the demagnetization end-point data that are curved rather than linear in vector plots due to overlapping coercivities for the two components of remanent magnetization. The great circles are expected to intersect at the component's direction having the higher coercivity range. The technique works best if the lower-coercivity components have random directions, making the statistical certainty of the characteristic direction greater and its 95% confidence limits correspondingly smaller and more circular. If both components are non-random, however, the intersection point of the great circles might be nearer the higher-coercivity direction or the antipode of the lower-coercivity direction, depending on how well the directions are represented by the sample population. In such cases, the error ellipses are more elongate (Fig. 4) and the mean directions are less reliable. Samples from 47 sites have N-polarity mean directions, 18 sites have R-polarity directions, and 18 sites (8 localities) have transitional directions (Fig. 4, Table 1). Two R-polarity sites (R3, R4) have an associated fission-track age of ~1.1 Ma
[Booth et al., in press]. An optically-stimulated luminescence (OSL) date for sediments just east of site R4
indicate an age of ~250 ka [Mahan et al., 2000]. OSL and thermal luminescence (TL) dates for sands and silts
at Point Defiance (N11, N12, N19) and at the intersection of I-5 and Atlantic Street (N27, N28) indicate ages
between 200 and 300 ka and of ~70 ka, respectively. OSL dates at Garfield Park in Tacoma (N17) and near Dash
Point (N25) indicate ages ›107 ka and of ~180 ka, respectively [Mahan et al., 2000].
| Figure 5. Equal-area stereoplot of site-mean directions for (a) a thick silt section just north of the eastern abutment of The Tacoma Narrows Bridge, and (b) the section at Wingehaven Park (Fig. 1). Sites R12 and R18 include the stratigraphically lowest samples at each site and have reversed-polarity mean directions. Site N37 is the stratigraphically highest site at Wingehaven Park and has a normal-polarity mean direction. Stratigraphic distances are indicated between sites as the paleomagnetic direction swings between reversed polarity and normal polarity; the path of the transition at Tacoma Narrows is shown by the dashed line. For comparison, that same path is superimposed on the Wingehaven Park data (dotted line), along with the apparent continuation of the transition to fully normal polaity (solid line). Inverted triangles indicate the normal and reversed directions of the present-day geomagnetic field in the central Puget Lowland. Click to enlarge image
A 20 m-thick silt section near the eastern end
of The Tacoma Narrows Bridge preserves part of a R-to-N transition. Similarly, at Wingehaven Park on Vashon
Island samples were collected above, below, and within a R-to-N polarity transition (Fig. 5). Directions inferred
as transitional were also found along the western shore of Tacoma Narrows (T13), along Puget Sound's eastern
shore (T8, T11), and at the northern tip (T7) and along the western shore of Vashon Island (T2, T18; Fig. 1).
The site-mean statistics are often remarkably good
for only three sample directions, although the overall dispersion of VGPs for both polarity groups (SN+R = 30°)
is significantly greater than a model value for the full range of secular variation at this latitude (SF = 17°
± 1°; McFadden and McElhinny ). Although the higher observed dispersion could have been caused by inaccuracies
in the sediment recording process (e.g., bottom paleocurrents), differential vertical-axis rotations, unrecognized
or incorrect stratal tilts, unrecognized transitional directions, and/or unrecognized overprinting, the excess
dispersion can also be attributed to higher within-site dispersion due to the low number of samples per site.
The near-antipodal mean directions of the N- and R-polarity groups, however, indicate that PSV and other sources
of error have been averaged out.
Assuming that the silt deposits at sites with well-grouped
sample directions (a95 15°) reliably record PSV, the observed paleomagnetic directions can be statistically
compared with one another to estimate the relative likelihood of sites having similar directions due to coincidence
or to significant agreement. Bogue and Coe  initially developed a statistical method to correlate paleomagnetic
directions from individual Columbia River basalt flows. Their method is based on the observation that the geomagnetic
field direction at any given locality tends to be near the expected dipole field direction. Thus, two similar
but unusual directions away from the expected field direction are more likely acquired simultaneously than two
similar directions near the expected direction. This technique is most accurate when applied over short periods
of time relative to the frequency of PSV. In this study, the period of time over which the sampled sediments
were deposited is relatively long, so correlations indicated by the statistical comparison are less certain
and therefore only the strongest correlations are considered. In addition, the overall mean direction (Table 1) was substituted for the expected dipole field direction because the observed mean has a shallower inclination
and a slightly more counterclockwise declination.
In Bogue and Coe's  method, two hypotheses
are tested. The ‘random' hypothesis (Hr) holds that similar paleomagnetic directions are random samplings of
the geomagnetic field, and the ‘simultaneous' hypothesis (Hs) holds that the directions were acquired under
the same geomagnetic field. The calculated probabilities (P) that similar paleomagnetic data (D) have arisen
from either hypothesis (P(D:Hr) or P(D:HS)) are given in Table 2. P(D:Hr) is calculated using a spherical distribution
model based on Fisher's  probability function, and P(D:HS) is the significance level from McFadden and
Lowes's  test of the null hypothesis stating that the two sample means are from populations having the
same mean but different k (concentration) values. The ratios of P(D:HS)/P(D:Hr), also given in Table 2, indicate
the relative likelihood of Hs versus Hr.
The fine-grained deposits of the Puget Lowland
apparently record polarity and PSV of the geomagnetic field. As previously mentioned, however, a number of error
sources might have affected the accuracy with which the fine-graineddeposits recorded the ambient geomagnetic
field. Paleocurrent alignment of magnetic grains is unlikely in lake bottom environments, and lake sediments
have provided consistent and reproducible records of PSV in North America [Lund, 1996]. Vertical-axis rotations
are also unlikely in deposits this young, and although unrecognized stratal tilts could contribute to the error
they too would be relatively minor. The division between transitional directions and extreme PSV directions
is arbitrary, and some of the highly dispersed N-polarity directions in Figure 4 might actually be transitional
directions (see below). Unrecognized overprinted N-polarity directions are also unlikely because uniform unaltered
sediments were sampled in which R-polarity and transitional directions were also found. Increased dispersion
due to the low number of samples collected could not be avoided without greatly increasing the sampling time.
The overall average inclinations for the fine-graineddeposits
(Table 1; Fig. 4) are also too shallow compared to the expected dipole-field direction. The shallowing of inclinations
is most likely due to compaction of the fine-grained lake sediments [Anson and Kodama, 1987], particularly in
the older Quaternary deposits due to loading by ice sheets during the multiple Pleistocene glaciations. Because
the degree of compaction is probably crudely similar at equivalent stratigraphic levels, comparisons of paleomagnetic
directions to determine equivalent horizons are most likely valid. The transitional paleomagnetic directions
for sites collected at Tacoma Narrows and at Wingehaven Park are plotted in Figure 5. Both of the stratigraphically
lowest sites at Tacoma Narrows and Wingehaven Park have R polarity (R12, R18), and the highest site at Wingehaven
Park has N polarity (N37). Intermediate directions at both localities are transitional, and clearly a R-to-N
reversal is recorded in the fine-grainedsediments. Directions at other sites with northerly declinations were
considered transitional if their mean inclinations were 25°. The cut off value is arbitrary, however,
and shallow-inclination N-polarity directions, such as those for sites N9, N15, and N27, might also be transitional
Although the transitional directions could be associated
with a number of subchrons within the Bruhnes polarity chron [Champion et al., 1988], it is most likely related
to the Bruhnes-Matuyama transition at ~780 ka. A fission-track date on an interbedded tephra layer near site
R4 indicates that Matuyama-aged sediments have been sampled there. A finite 14C date of 44,880 ± 3050 ka near
and stratigraphically above the eastern Tacoma Narrows locality [Troost, 1999] preclude these transitional directions
from being related to the Laschamp event (40 ka). Furthermore, the OSL and TL dates nearby and at Point Defiance
(N11, N12, N19) between 200 and 300 ka indicate that the N-polarity sediments are below either the well-defined
Jamaica (~180 ka) or Blake (~110 ka) events. Older subchrons have been proposed within the Bruhnes chon, but
are not as well established.
Paleomagnetic correlations of the transitional
directions also indicate that parts of the same geomagnetic reversal were sampled across the study area. The
calculated probabilities and relative probabilities of the random (Hr) or simultaneous (Hs) hypotheses [Bogue
and Coe, 1981] are given in Table 2. The first normal-polarity comparison in Table 2 is between site N1 (Fig.
4) and 9 other sites with similar directions. All of these directions are near the overall mean direction (d
< 20°), and their relative probability values are correspondingly low (40) and mixed between favoring
Hr or Hs. In contrast, a correlation test between sites N9 and N15 indicates that Hs is more likely than Hr
by a factor 9999, and so these two sites are probably within the same magnetostratigraphic horizon. Transitional
directions at the east shore of Tacoma Narrows (T4, T5) are identical to those at Wingehaven Park (T14, T15),
and that at Seahurst Park (T11) is widely correlated with directions at Redondo (T8), Peter Point on Vashon
Island (T10), and on the west shore of Tacoma Narrows (T13). In addition, some of the northernmost N-polarity
directions (N31, N33) are correlated with some of the southernmost sites (N16, N38).
| Figure 6. Map of the central Puget Lowland showing sampling sites and magnetic polarities (as in Fig. 1) superimposed on an aeromagnetic map for the region [Blakely et al., 1999]. Bold dashed lines (B and C) indicate the margins of the Seattle uplift and the limit (A) of the southward dipping ramp at the southern edge of the uplift (after Pratt et al. ). The dotted line north of the Seattle uplift marks the trace of the Seattle fault. Fine dashed lines connect sites that have been statistically correlated based on their atypical, but similar, paleomagnetic directions (Table 2). Click to enlarge image
In Figure 6, the distribution of N-, transitional-, and R-polarity sites are shown superimposed on an aeromagnetic map of the central Puget Lowland [Blakely et al., 1999]. Also shown are most of the calculated correlations between sites having relative probability factors for Hs of several thousand or more (Table 2). The aeromagnetic data show the local influence of human activity, such as The Tacoma Narrows Bridge (near T1 and N46) and at the Tacoma harbor docks (near R4). In addition, the pattern of N- and R-polarity sites near the southern end of Vashon Island appear to sharply define the southern limit of the Seattle uplift at its boundary with the adjacent Tacoma Basin (NW to SE line B). This boundary is also well defined by gravity and seismic tomography data [Brocher et al., 2001].
The pattern of paleomagnetic polarity Fig. 6 appears unrelated to the pattern of aeromagnetic anomalies indicating that the anomalies are not due to differential uplift of strongly magnetized basement rocks such as the volcanic Crescent Formation. The polarity pattern is the result of the elevation of sites, past erosion, and tectonic movements. An estimate of the elevation of each site is given in Table 1, and clearly plays a role in polarity at the thick sections with transitional directions sampled at The Tacoma Narrows Bridge and Wingehaven Park. The N-polarity sites at Christiansen Road (N3-N6) on the western coast of Vashon Island, just across from R-polarity sites on beaches of the eastern Kitsap Peninsula (Fig. 1), have much higher elevations (20-50 m). Moreover, N-polarity sites are always found above R-polarity sites indicating that R-polarity subchrons within the Bruhnes chron have not been sampled.
Deposition of fine-grainedsediments over a surface with erosional relief might also account for the close proximity of N- and R-polarity sites, but is difficult to evaluate because of limited exposures in the region. Tectonic uplift can cause the exposure of R-polarity sediments, and the majority of sites in such sediments over the Seattle uplift Fig. 6 indicate that uplift has continued since ~780 ka. The abrupt change in polarity across the southern boundary of the Seattle uplift might also indicate that the Tacoma fault intersects the land surface. In addition, a dip-slip fault along the Tacoma Narrows is indicated by the down dropping of sites T13 and N46 on the west relative to sites R12, T1, and T3-T6 on the east.
| Figure 7. North to south magnetic [Blakely et al., 1999] and gravity [Brocher et al., 2001] profiles (line P-P' in Fig. 6) across the south-central Puget Lowland are shown in (a) and (b), respectively. A model is shown in (c) in which seismic tomography data (depth to 4.5 km/sec velocity rocks) is used to constrain the gravity fit, and reversed-polarity slabs of Crescent Formation are used to fit the magnetic profile. The model is interpreted in (d) with the Seattle fault as a south-dipping reverse fault and the southern margin of the Seattle uplift as possibly a south-dipping ramp. Thickness of the thin layer of Quaternary sediments in (c) and (d) is based on data from Jones . Click to enlarge image
To determine the source of the aeromagnetic anomalies,
gravity and magnetic profiles along line P to P' (Fig. 1) were fitted by a simple subsurface model shown
in Figure 7. The fit of the gravity profile is constrained by SHIPS tomographic data indicating the depth to
rocks having 4.5 km/sec velocities [Brocher et al., 2001], which is presumably the top of the Crescent Formation (Fig. 7c). Not surprisingly, the fit to the gravity profile is excellent, but the concurrent
fit to the magnetic profile (not shown), assuming a uniformly magnetized Crescent Formation, is poor. Slabs
of reversely magnetized rock must be added to the model so that the calculated profile matches the observed
magnetic profile (Fig. 7a, 7c). Although the Crescent Formation is reversely magnetized
at the surface in some places [Beck and Engebretson, 1982; Globerman et al., 1982; Wells and Coe, 1985], locations
of reversely magnetized rocks in Figure 7c are constrained only by the shape of the aeromagnetic data. In this
model, the Seattle fault is interpreted as a south-dipping reverse fault and the southern margin of the Seattle
uplift as a south-dipping ramp (Pratt et al. ; Fig. 7d). The reversely magnetized layers within the Crescent
rocks also dip southward and might reflect stratigraphy within the formation.
In this study paleomagnetic directions from fine-grained unaltered glacial and interglacial deposits have been used to define the area's magnetostratigraphy (Figs. 1, 6). Remarkably, the horizontal plane of sampled exposures (~sea level) intersects a R-to-N polarity transition that is most likely related to the Bruhnes-Matuyama geomagnetic reversal (Fig. 5). Vertical adjustments on the order of 10 m could determine whether a sediment with N, R, or transitional directions was sampled. The transitional horizon is at least 2 m thick and serves well as a stratigraphic marker horizon within the previously undated sediments between the Salmon Springs and Double Bluff glacial deposits (Fig. 2). Seismic-reflection data analyzed by Pratt et al.  image subsurface structure to depths of several km, whereas seismic tomography [Brocher et al., 2001] does so to depths of 25 to 30 km.
|Figure 8. North-to-south cross sections beneath the central Puget Lowland based on (a) seismic reflection profiles analyzed by Pratt et al.  and on (b) the 3-dimensional seismic velocity model of Brocher et al. . Dots indicate hypocenters of local earthquakes projected E-W onto the cross sections. In (a), a thin-skinned model is shown in which the Seattle fault is a thrust fault that shallows with depth and merges with a mid-crustal decollement. Light gray areas indicate Miocene and younger deposits, and darker gray areas indicated Eocene and Oligocene deposits (after Pratt et al. ). In (b), a thick-skinned deformational model is shown in which the steeply dipping Seattle and Tacoma faults bound the Seattle uplift to the north and south, respectively. The steeply dipping faults connect at high angles with a lower crustal decollement at the base of the Crescent Formation. Focal mechanisms for the 1995 M = 5 Point Robinson and the 1997 M = 5 Bremerton earthquakes are also shown and are interpreted as having occurred on the Tacoma and Seattle faults, respectively (after Brocher et al. ). Click to enlarge image
In Figure 8, cross-sections of the Puget Lowland are shown depicting the thin- and thick-skinned structural models for the region based on the reflection and tomographic techniques, respectively. Overall, the paleomagnetic data conform to the Seattle uplift: R-polarity sites are mostly found above this structural feature (Fig. 7). The sharp boundary between the N- and R-polarity data along the Seattle uplift's southern edge (line B; Fig. 6) is consistent with a fault structure (Tacoma fault?) that apparently reaches the surface. The paleomagnetic correlation of transitional and N-polarity sites in the southern part of the study area with sites in the northern part, however, implies less deformation of the Pleistocene sediments than of the underlying Tertiary deposits.
Regional deformation of the Puget Lowland is a result of the ongoing convergence of the Juan de Fuca and North American plates expressed through both faulting and folding. Active faults in the Puget Lowland have been inferred to offset Quaternary deposits, and although only the Seattle fault has had a clear history of late Holocene surface rupture [Nelson et al., 1999], future major earthquakes are certain to occur within the region. The location and nature of active faults, as well as the overall structure beneath the Puget Lowland, are at present still open questions and more work is needed to decipher the region's complex structural setting and paleoseismicity.
Acknowledgements. We thank S. Bogue, T. Brocher, P. Haeussler, S. Johnson, T. Walsh, and particularly R. Wells for helpful discussions; P. Haeussler for collecting samples at several localities; and T. Brocher for providing a preprint of their manuscript on the SHIPS data. We also acknowledge S. Bogue, P. Haeussler and S. Johnson for constructive reviews of the manuscript, and B. Graham and D.B. Bridges for assistance in the field and laboratory.
Anson, G.L., and K.P. Kodama, Compaction-induced inclination shallowing of the post-depositional remanent magnetization in a synthetic sediment, R. Astron. Soc. Geophys. J., 88, 673-692, 1987.
Atwater, B.F., and E. Hemphill-Haley, Recurrence intervals for great earthquakes of the past 3,500 years at northeastern Willapa Bay, Washington, U.S. Geol. Surv. Prof. Pap., 1576, 108 p., 1997.
Beck, M.E., Jr., and D.C. Engebretson, Paleomagnetism of small basalt exposures in the west Puget Sound area, Washington, and speculations on the accretionary origin of the Olympic Mountains, J. Geophys. Res., 87, 3755-3760, 1982.
Blakely, R.J., R.E. Wells, and C.S. Weaver, Puget
Sound aeromagnetic maps and data, U.S. Geological Survey Open-File Report 99-514, 1999. [http://geopubs.wr.usgs.gov/open-file/of99-514]
Blunt, D.J., D.J. Easterbrook, and N.W. Rutter, Chronology of Pleistocene sediments in the Puget Lowland, Washington, Wash. Div. Geol. Earth Resour. Bull., 77, 321-353, 1987.
Bogue, S.W., and R.S. Coe, Paleomagnetic correlation of Columbia River Basalt flows using secular variation, J. Geophys. Res., 86, 11,883-11,897, 1981.
Booth, D.B., Timing and processes of deglaciation along the southern margin of the Cordilleran Ice Sheet, in North America and Adjacent Oceans During the Last Deglaciation, Riddiman, W.F., and H.E. Wright, Jr., eds., The Geology of North America, K-3, Geological Society of America, Boulder, Colorado, 71-90, 1987.
Booth, D.B., H.H. Waldron, and K.G. Troost, Geologic map of the Poverty Bay 7.5-minute quadrangle, U.S. Geological Survey Open-File Report, in press.
Brocher, T.M., T. Parsons, R.E. Blakely, N.I. Christensen, M.A. Fisher, R.E. Wells, and the SHIPS Working Group, Upper crustal structure in Puget Lowland, Washington: Results from the 1998 seismic hazards investigation in Puget Sound, J. Geophys. Res., 106, 13,541-13,564, 2001.
Bucknam, R.C., E. Hemphill-Haley, and E.B. Leopold, Abrupt uplift within the past 1700 years at southern Puget Sound, Washington, Science, 258, 1611-1614, 1992.
Champion, D.E., M.A. Lanphere, and M.A. Kuntz, Evidence for a new geomagnetic reversal from lava flows in Idaho: Discussion of short polarity reversals in the Bruhnes and late Matuyama polarity chrons, J. Geophys. Res., 93, 11,667-11,680, 1988.
Crandell, D.R., D.R. Mullineaux, and H.H. Waldron, Pleistocene sequence in the southeastern part of the Puget Sound lowland, Washington, Am. J. Sci., 256, 384-397, 1958.
Easterbrook, D.J., Chronology of pre-Late Wisconsin Pleistocene sediments in the Puget Lowland, Washington, Wash. Div. Geol. And Earth Res. Bull., 80, 191-206, 1994.
Easterbrook, D.J., D.R. Crandell, and E.B. Leopold, Pre-Olympia Pleistocene stratigraphy and chronology in the central Puget Lowland, Washington, Geol. Soc. Am. Bull., 78, 13-20, 1967.
Easterbrook, D.J., J.L. Roland, R.J. Carson, and N.D. Naeser, Application of paleomagnetism, fission-track dating, and tephra correlation to Lower Pleistocene sediments in the Puget Lowland, Washington, in Dating Quaternary Sediments, Easterbrook, D.J., ed., Geol. Soc. Spec. Pap. 227, 139-165, 1988.
Fisher, R.A., Dispersion on a sphere, Proc. R. Soc. London Ser. A, 217, 295-305, 1953.
Globerman, B.R., M.E. Beck, Jr., and R.A. Duncan, Paleomagnetism and tectonic significance of Eocene basalts from the Black Hills, Washington Coast Range, Geol. Soc. Am. Bull., 93, 1151-1159, 1982.
Johnson, S.Y., C.J. Potter, and J.M. Armentrout, Origin and evolution of the Seattle basin and Seattle fault, Geology, 22, 71-74, 1994.
Johnson, S.Y., C.J. Potter, J.M. Armentrout, J.J. Miller, C. Finn, and C.S. Weaver, The southern Whidbey Island fault, an active structure in the Puget Lowland, Washington, Geol. Soc. Am. Bull., 108, 334-354, 1996.
Johnson, S.Y., S.V. Dadisman, J.R. Childs, and W.D. Stanley, Active tectonics of the Seattle fault and central Puget Sound, Washington—Implications for earthquake hazards, Geol. Soc. Am. Bull., 111, 1042-1053, 1999.
Jones, M.A., Thickness of unconsolidated deposits in the Puget Sound Lowland, Washington and British Columbia, U.S. Geol. Surv. Water Res. Invest. Rep. 94-4133, 1996.
Kirschvink, J.L., The least-squares line and plane and analysis of palaeomagnetic data, Geophys. J. R. Astr. Soc., 62, 699-718, 1980.
Ludwin, R.S., C.S. Weaver, and R.S. Crosson, Seismicity of Washington and Oregon, in The Geology of North America, vol. 1, Neotectonics of North America, edited by D.B. Slemmons et al., Geol. Soc. Amer., 77-98, 1991.
Lund, S.P., A comparison of Holocene paleomagnetic secular variation records from North America, J. Geophys. Res., 101, 8007-8024, 1996.
Mahan, S.A., D.B. Booth, and K.G. Troost, Luminescence dating of glacially derived sediments: A case study for the Seattle Mapping Project [abstr.], Geol. Soc. Amer. Abst. with Programs, Cordilleran Section, 32, A-27, 2000.
McFadden, P.L., and M.W. McElhinny, A physical model for palaeosecular variation, Geophys. J. R. astr. Soc., 78, 809-830, 1984.
McFadden, P.L., and F.J. Lowes, The discrimination of mean directions drawn from Fisher distributions, Geophys. J. R. Astr. Soc., 67, 19-33, 1981.
Nelson, A.R., S.K. Pezzopane, R.C. Bucknam, R.D. Koehler, C.F. Narwold, H.M. Kelsey, W.T. Laprade, R.E. Wells, and S.Y. Johnson, Holocene surface faulting in the Seattle fault zone, Bainbridge Island, Washington [abst.], Seis. Res. Lett., 70, 223, 1999.
Onstott, T.C., Application of the Bingham distribution function in paleomagnetic studies, J. Geophys. Res., 85, 1500-1510, 1980.
Pratt, T.L., S. Johnson, C. Potter, W. Stepenson, and C. Finn, Seismic reflection images beneath Puget Sound, western Washington State: The Puget Lowland thrust sheet hypothesis, J. Geophys. Res., 102, 27,469-27,489, 1997.
Richmond, G.M., and D.S. Fullerton, Introduction to Quaternary glaciations of the United States of America, Quat. Sci. Rev., 5, 3-10, 1986.
Rogers, A.M., T.J. Walsh, W.J. Kockelman, and G.R. Priest, Earthquake hazards in the Pacific Northwest—An overview, in Assessing Earthquake Hazards and Reducing Risk in the Pacific Northwest, edited by A.M. Rogers et al., U.S. Geol. Surv. Prof. Pap., 1560, 1-54, 1996.
Smith, J.G., Geologic map of upper Eocene to Holocene volcanic and related rocks in the Cascade Range, Washington, U.S. Geol. Surv. Misc. Invest. Map I-2005, scale 1:500,000, 1993.
Troost, K.G., The Olympia nonglacial interval in the South-central Puget Lowland, M.S. thesis, Univ. of Washington, 123 pp., 1999.
Troost, K.G., and D.B. Booth, The Seattle geologic mapping project [abstr.], Geol. Soc. Am., Abstracts with Programs, 31, A-79, 1999.
Wells, R.E., and R.S. Coe, Paleomagnetism and geology of Eocene volcanic rocks of southwest Washington, implications for mechanisms of tectonic rotation, J. Geophys. Res., 90, 1925-1947, 1985.
Wells, R.E., and C.S. Weaver, Block deformation in Puget Lowland, in V.E. Frizzel, ed., Proceedings of the National Earthquake Prediction Evaluation Council, U.S. Geol. Surv. Open-File Report, 93-333, 14-16, 1993.
Wells, R.E., C.S. Weaver, and R.J. Blakely, Fore arc migration in Cascadia and its neotectonic significance, Geology, 26, 759-762, 1998.