Tectonically induced deformation of deep-sea sediments off Washington and northern Oregon: Mechanical consolidation

Abstract

Convergent motion of the North American and Juan de Fuca plates has resulted in deformation of Cascadia Basin sediments and accretion of these deposits to the North American continental margin. The accreted deposits, which occur as anticlinal ridges and thrust blocks, constitute the lower continental slope or borderland off Washington and northern Oregon. Over the past 2.0 m.y., approximately 30 km of this deformed material has been added to the lower slope, removing undeformed deposits from Cascadia Basin at a rate of 2.3–2.9 cm/yr. Near-surface sediments involved in this accretionary process are mechanically consolidated: muds̀tones dredged from the lower slope exhibit physical properties (water contents, 20–47%; void ratios, 0.4–1.2; preconsolidation pressures, 0.8–8.2 MPa) which differ significantly from properties of similar, but undeformed sediments (water contents, 50–250%; void ratios, 1.1–1.9). While some consolidation may be attributable to prior burial ( < 2.0 MPa ) or carbonate precipitation, neither mechanism can wholly account for the values observed. It appears that most of the consolidation has occurred in response to tectonically induced overpressures. Initial consolidation occurs rapidly across a narrow ( < 3 km ) front, defined by the base of the continental slope. Further consolidation and dewatering appears to take place, at a much reduced rate, over the entire width of the lower slope. Development of foliation is nearly ubiquitous in the deformed mudstones. This property limits the strength of the deposits (shear strengths, 90–416 kPa) and movement along these planes probably accommodates much of the strain after initial consolidation. The physical properties characteristic of Washington—Oregon deformed sediments may represent limiting values for mechanical consolidation of near-surface terrigenous sediments under horizontal stress.