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Pacific Northwest Geologic Mapping and Urban Hazards

The Canby-Molalla Fault, Oregon

R. Blakely, I. Madin, W. Stephenson, and T. Popowski

The Oregon Coast Range accommodates north-south compression, in part by strike-slip deformation on northwest-striking faults. Understanding these faults is important for assessing regional earthquake hazards, but that understanding is difficult to come by because of concealing cover of young geologic deposits.

 

Map of The Portland basin and northern Willamette Valley

The Portland basin and northern Willamette Valley (Click on any image to enlarge) are underlain by volcanic rocks of four diverse types: Eocene rocks of oceanic affinity, Western Cascades volcanic rocks, Miocene Columbia River basalt, and Pleistocene Boring Lava. Much of the region is covered by young sediments. The contrasting magnetic, density, and acoustical properties of these rocks are ideal for geophysical studies.

 

Fault map

The region is dominated by northwest- and northeast-striking faults. These faults have generated earthquakes in historic times up to M5.6, notably the 1997 Scotts Mills earthquake (M 5.6) and the 1962 Vancouver earthquake (M 5.2). Portland has experienced five earthquakes greater than M 5 in its 150-yr history. Important faults include the Gales Creek fault, Mt. Angel fault, and those within the Portland Hills fault zone. Beeson and others postulated two northwest-striking structural zones: Portland Hills-Clackamas River and Mt. Angel-Gales Creek. Note the numerous earthquakes that fall between the two zones.

 

Aeromagnetic compilation

This aeromagnetic compilation is based on five recent, high-resolution surveys. Flight lines spaced 1/4 mile apart and flown at 1000 ft above terrain. Aeromagnetic anomalies reflect volcanic lithologies, largely concealed beneath nonmagnetic sediments. Note in particular the Sylvan, Mt. Angel, and Gales Creek faults, all with northwest strike

Taken as a whole, the aeromagnetic data reflect the two northwest-striking structural zones. Now we will focus on a specific magnetic lineament between the two structural zones. The lineament is 60 km long, has NNW strike, and runs through the towns of Canby and Molalla.

 

 

Aeromagnetic anomaly

The anomaly is extraordinarily abrupt and linear, suggestive of a shallow, abrupt contact. Note approximately 4 km of right-lateral offset of anomalies north of the Willamette River.

 

Another view of the aeromagnetic anomaly

Another view of the lineament, emphasizing its linear character.

 

Map view of the area of the aeromagnetic anomaly

At its northern end (north of the river), the anomaly is associated with mapped faults of uncertain age and obvious topography. But there is no topographic expression along most of the anomaly's 60-km length (except as noted later).

 

 

Location of the subsurface magnetic contact

The dotted line shown here is the location of the subsurface magnetic contact, as determined automatically from the maximum horizontal gradient of the pseudogravity anomaly. This line indicates the location of the contact, assuming nearly vertical dip, and will appear in subsequent slides for reference.

 

Four ground-magnetic traverses

To investigate the anomaly up close and personal, we conducted four ground-magnetic traverses along farm roads south of Oregon City and east of Canby. Instrumentation consisted of a cesium-vapor magnetometer and differential GPS. Note extraordinarily steep gradients on each profile corresponding with the aeromagnetic fault trace. An offset in highly magnetic volcanic rock is the likely explanation. The next slide will focus on Profile C.

 

 

Map showing logs from over 70 water wells in the area

We have compiled logs from over 70 water wells in the area. The next slide will show the logs from six sample wells, those colored green on this map. Note that these six wells constitute a transect across the fault.

 

Well log

Wells encountered red silt, sand, clay, and gravels in the upper part of the section. Two wells immediately east of the fault penetrated CRB at about 50 m depth and bottomed therein. No CRB was encountered in any wells west of the fault (maximum depths 150 m).

 

 

spatial association between CRB wells and the aeromagnetic lineament

This map shows the spatial association between CRB wells and the aeromagnetic lineament. Thus, CRB is at approximately 50 m depth east of the lineament, and more than 150 m immediately to the west.

 

modeled Profile C

Using the water wells as a constraint, we modeled Profile C as a steeply dipping reverse fault. Such models are non-unique, but this particular anomaly is difficult to fit with anything but a steeply dipping contact. Vertical offset 150 m. Conclusion: The aeromagnetic anomaly reflects a reverse fault. The fault probably has a significant strike-slip component too, as demonstrated by the offset of anomalies north of the Willamette River noted earlier.

 

 

image shows the uninterpreted seismic section

To further nail down the fault and potentially identify youngest deformed sediments, we conducted a high-resolution seismic transect across the magnetic lineament. The transect was 600 m in length and located very near magnetic Profile C. The line was acquired with 2 m shot and receiver spacings. The source was a 10-lb (4.55 kg) sledgehammer, 60 channels per shot. We stacked four sledghammer records together for each shotpoint. Nominal 30-fold data coverage was attained. We used three 28-Hz resonant frequency geophones per receiver group in a 'cluster' array (3 phones planted within a few inches of each other around the station).

This image shows the uninterpreted section. Note the sharp reflectors at 50 m depth, truncation of reflectors at x=441, quiet zone beneath x=541, and subtle deformation in uppermost section.

 

A preliminary interpretation

A preliminary interpretation is shown here, with CRB shown in orange, Pleistocene mudstone and silts in green. Note minimum offset of 50 m, very similar to interpretation based on magnetic profile. Also note deformed Quaternary sediments and a hint of deformation in youngest overlying sediments.

 

 

a 10-m DEM just south of the Willamette River

Finally, let's take a closer look at topography west of Canby.

This map shows a 10-m DEM just south of the Willamette River. Note topographic lineament just north of seismic and magnetic profiles (which are located at southern edge of map). On the ground, this berm appears to dam a flood-cut swale creating a small depression. The berm is a surficial Missoula flood feature, and is probably about 12 to 13 ka or less.

 

 

regional-scale picture

Returning to the regional-scale picture, the Canby Molalla lineament lies between and slightly askew two NW-striking structural zones. The structural zones include right-lateral faults, and it appears that the Canby-Molalla fault does as well.

 

An aeromagnetic lineament reflects the Canby Molalla fault. Conclusions:

 

  • An aeromagnetic lineament reflects the Canby Molalla fault.Well logs and models based on ground-magnetic and seismic-reflection data indicate a nearly vertical offset in CRB, perhaps 150 m of vertical throw.Offset anomalies indicate about 4 km of right-lateral slip.We believe the fault is a synthetic shear caught between the Portland Hills–Clackamas River and Gales Creek–Mt. Angel structural zones.A topographic feature and seismic-reflection data hint that the fault is young.The proximity of the fault to Portland and Salem make it a potential hazard.
  • We are missing the critical element, namely the age of latest deformation. In the future, we hope to conduct trench excavations and LIDAR measurements.

 


The information provided on this page was originally presented at the 2001 Fall Meeting of the AGU

Authors: Richard J. Blakely (blakely@usgs.gov), U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, I. Madin, DOGAMI, Oregon Dept. of Geology and Mineral Industries, 88 NE Oregon St., Portland, OR 97232, W. Stephenson, U.S. Geological Survey, Denver, CO, and T. Popowski, Northwest Geophysical Associates, INC., 1600 SW Western Bl, Suite 200, Corvallis, OR 97333

 

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