2005

• Northern California Geological Society, Orinda, California, "Searching for Subduction Zone Seismogenesis in the Rock Record", September 28, 2005 (C. Rowe)
• AGU Fall 2005 (Invited Talk) "Friction Constitutive Properties of Fault Zone Materials" (Marone, Saffer, McKiernan, Rowe, Samuelson)

2004

• Salem State College Geology Department, Salem, Massachusetts, "What Gives?: Geologic Controls on the Onset of Seismogenesis in Subduction Thrust Systems", February 9, 2004 (C. Rowe)
• Smith College Geology Department, Northampton Massachusetts, "What Gives?: Geologic Controls on the Onset of Seismogenesis in Subduction Thrust Systems", February 12, 2004 (C. Rowe)
• USGS Earthquake Hazards Group, Menlo Park, "The Geology of Subduction Thrusts", October 2004 (C. Rowe)

The Kodiak Accretionary Complex is comprised of thrust-bounded sediment packages which were added to the Alaskan wedge from early Mesozoic through Eocene time. Some thrust faults preserved within the Complex represent paleo-decollements along which footwall units were subducted, fossilized when the sedimentary units were basally accreted to the wedge. Additionally, wedge thickening necessitated by the basal accretion of significant sediment volume, was accommodated in the forearc by motion along out-of-sequence thrusts cutting the wedge at a higher angle.

The Uganik Thrust, boundary between the mid-early Cretaceous Uyak Complex and the latest Cretaceous Kodiak Formation, has been construed by previous authors as the decollement fault along which the Kodiak Formation was subducted beneath the late Cretaceous accretionary prism. Field observations of the thrust interface suggest that the Uganik Thrust has a complex history including multiple episodes of reverse motion and more recent strike-slip motion. We suggest that the Uganik Thrust may have been active in latest Cretaceous to early Paleocene time as an out-of-sequence thrust, and this motion was accompanied by the development of high fluid flux surfaces, parallel to and within the footwall of the thrust (Kodiak Fm). Deformation in the hanging wall (Uyak Melange) is distinguished by the development of a discontinuous narrow zone of mylonitic fabric but no significant vein or cement precipitation in the hanging wall. These surfaces are characterized by a high degree of shear-related deformation and quartz vein and cement volumes several times greater than the surrounding footwall rocks.

These high-shear, fluid-focusing surfaces rapidly decline in frequency, magnitude, and thickness below the Uganik thrust. This fossilized fluid-flow system along a significant out-of-sequence thrust may elucidate the relationships between fault offset and fluid flow during wedge-thickening as well as mechanisms of sediment dehydration during rapid growth of accretionary wedges.

Citation:

Rowe, C.D., Moore, J.C., Meneghini, F., McKiernan, A. (2004) Fluid flow along an ancient out-of-sequence thrust system, Kodiak Accretionary Complex, Alaska,

2003

The up-dip portions of accretionary subduction zone decollements slide stably and are therefore aseismic, but become seismogenetic at a depth of 5-15 km. Thermal models of modern subduction zones and accretionary wedges predict that the aseismic-seismic transition occurs at 100-150C. This correlation between temperature and the onset of seismogenesis suggests that fault behavioral properties are modified by a diagenetic-metamorphic reaction affecting the fault zone mineralogy.

The Kodiak Accretionary Complex, Alaska, is a well-exposed sediment wedge associated with Mesozoic through recent Aleutian subduction. We compare two tectonic units that were subducted, one to just above the seismogenic transition, and one to within the seismogenic zone. The Eocene rocks were subducted to approximately 2.4-3.9 km and experienced temperatures of 100-125C before accreting into the wedge. The Paleocene rocks subducted to 10-14 km (280-320 MPa) and reached 215-290C. Both formations host disrupted zones interpreted as paleo thrust faults by previous authors, which were probably associated with paleodecollement systems.

Quartz cementation is rare in the paleo-thrust faults of the Eocene rocks but ubiquitous and extensive in the paleo-thrust faults of the Paleocene rocks, in fault-parallel and fault-crossing geometries. We suggest that the formation of a quartz network along and across fault zones may cause the onset of seismogenesis. The frictional behavior of sheet silicates is generally velocity strengthening, resulting in stable sliding behavior, while quartz exhibits velocity weakening, or stick-slip frictional behavior. Thus, the aseismic-seismic transition may be controlled by quartz mobility and the appearance of volumetrically significant quartz +/- calcite precipitates filling, coating, and cementing fault surfaces, creating "deadbolts" across slip surfaces, establishing frictional control over surfaces whose properties were previously controlled by sheet silicates.

Citation:
Rowe, C.D., and Moore, J. Casey, 2003, The upper aseismic to seismic transition: A silica mobility threshold,Eos Trans. AGU, 84(46), Fall Meet. Suppl., Abstract T41E-02, 2003

GSA Annual Meeting:
The upper aseismic to seismic transition: A silica mobility threshold
(Outstanding Student Paper, Structure and Tectonics Division, GSA)

The up-dip portions of accretionary subduction zone decollements slid stably and are therefore aseismic, but become seismogenic at a depth of 5-15 km. Thermal models of modern subduction zones and accretionary wedges predict that the aseismic-seismic transition occurs at 100-150 degrees C. This correlation between temperature and the onset of seismogenesis suggests that fault behavioral properties are modified by a diagenetic-metamorphic reaction affecting the fault zone mineralogy.

The Kodiak Accretionary Complex, Alaska, is a well-exposed, unaltered sediment wedge associated with Mesozoic through Eocene Aleutian subduction. We compare two tectonic units that were subducted, one to just above the seismogenic transition, and one to within the seismogenic zone. The Eocene Sitkalidak Fm. was subducted to approximately 2.4 - 3.9 km and experienced temperatures of 100-125 degrees C before accreting into the wedge. The Paleocene Ghost Rocks Fm. subducted to 10-14 Km (280-320 MPa) and reached 215-290 degrees C. Both formations host black, shiny shear surfaces associated with disrupted zones interpreted as paleodecollements by previous authors.

X-ray diffraction and microprobe results suggest that the mineralogy of the shear surfaces in both formations contain chlorite and variable amounts of other sheet silicates, i.e. illite/muscovite, smectite, and kaolinite, which may be detrital and/or diagenetic/metamorphic. Quartz cementation is rare in the Sitkalidak Fm. but ubiquitous and extensive in the Ghost Rocks Formation, in fault-parallel and fault-crossing geometries. We suggest that the formation of a quartz network across fault zones may trigger the onset of seismogenesis. The frictional behavior of sheet silicates is generally velocity strengthening, resulting in stable sliding behavior, while quartz exhibits velocity weakening, or stick-slip frictional behavior. Thus, the aseismic-seismic transition may be controlled by quartz mobility and the appearance of volumetrically significant quartz +/- calcite precipitates filling, coating, and cementing fault surfaces, creating "deadbolts" across slip surfaces previously dominated by sheet silicates.

Citation:
Rowe, C.D., and Moore, J. Casey, 2003, The upper aseismic to seismic transition: a silica cementation threshold, GSA Abstracts with Programs Vol. 35, No. 6, September 2003

2002

Subduction thrust systems produce the world's largest earthquakes. The transition from aseismic to seismogenic faulting occurs at approximately ~4-8 km depth. The chemical and physical controls on this transition are not well understood, but previous research indicates that phase transformations, fluid pressure changes, and formation of authigenic minerals and cements may produce changes in cohesion and coefficient of friction which control fault behavior.

We have described and sampled areas of paleo faulting and fluid flow in an early Tertiary subduction thrust system, Kodiak Archipelago, Alaska. We compare two formations: the upper Paleocene Ghost Rocks Fm., which previous work has shown to have been exposed to ~250degC and 12 km depth (well within the seismogenic zone) and the Eocene Sitkalidak Fm., which has been exposed to 100-125degC at 2.4-3.9 km depth, (accreted before it crossed the aseismic-seismogenic boundary.) Our goal is to define the explicit differences in structural style and vein/cement development in these two formations that were emplaced above and below the aseismic to seismic transition.

The Ghost Rocks Fm. is characterized by discrete heavily veined zones meters to tens of meters thick. Individual veins in these zones commonly reach thickness of up to several centimeters and are primarily composed of clean calcite and quartz. In contrast, the Sitkalidak Fm. is characterized by a small volume of web-like networks of very fine veins rarely exceeding a few mm in thickness. These veins are composed of laumontite and "dirty" calcite. In the Sitkalidak Fm., stratal disruption is characterized by conjugate shear fracturing, leaving lustrous black residues on shear surfaces, followed by extensional fractures with veining, indicating rising fluid pressures. In the Ghost Rocks Fm., there is little evidence for conjugate shear fracturing. Stratal disruption is accomplished by extensive extensional fracturing and veining as well as ductile deformation and rotation of sediments under non-coaxial strain. The Ghost Rocks Fm. contains "expansion breccias" of wallrock material suspended in vein material. These and other observed textures which are absent from the Sitkalidak Fm. suggest very high fluid pressures and rapid cement precipitation following or associated with cataclastic brecciation of the fault zone.

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Citation:
Rowe, C.D., Thompson, Eric, and Moore, J. Casey, 2002, Contrasts in Faulting and Veining Across the Aseismic to Seismic Transition, Kodiak Accretionary Complex, Alaska, Eos Trans. AGU 83(47),Fall Meeting Supplement, Abstract T21A-1060, 2002