AN ABSTRACT OF THE THESIS OF Annette V. deCharon for the degree of Master of Science

approved: Robert W. Embley Seismic-reflection profiles and SeaBeam bathymetry are used to examine the structural development of the Cascadia Segment, a region of back-tilted blocks flanking a central depression within the Blanco transform fault zone. The depressed position of the Cascadia Segment has caused this area to act as a sediment trap for bottom transported material moving through Cascadia Channel (Duncan, 1968; Griggs & Kulm, 1970). The existence of thick turbidite sequences atop tectonic blocks this an ideal place for seismic-reflection imaging. Escarpment slopes and fault dip angles in Cascadia Segment range between 110 and 31°, which are, on average, shallower than values reported from slow-to-intermediate-rate spreading centers. The perched turbidite sequences are assumed to have originated within a central rifting basin. The lack of unconformable sequences within the back-tilted turbidites indicates that the tectonic blocks of Cascadia Segment have been uplifted in a continuous manner. Thus, a quantitative analysis of tilt of turbidite sequences versus distance from the present central rifting basin, Cascadia Depression, is used to understand the structural development of the Cascadia Segment. The high degree of tilt seen in turbidite sequences proximal to the present central rifting basin is consistent with a model in which turbidite flows are deposited within a basin that is being uplifted and spreading away from a narrow extensional zone. Individual tectonic blocks rotate at least 6° as they move through the terraces of Cascaclia Segment. Extension of the eastern terrace has been accommodated by rotation of three large, discrete blocks whereas the western terrace has extended through block rotation and a large number of normal faults with smaller throws. Irregular morphology of the flanking blocks of this spreading center may be attributed to short-term (on the order of 106 years) asymmetric spreading. Turbidite tilt data and fault orientations demonstrate conclusively that the muting of topography from the rift valley to the rift mountains greater than 30 km off the central spreading axis is accomplished by: (1) movement along outward-dipping normal faults in the rift mountains, and (2) back-tilting of tectonic blocks within the terraces. The possibility of reverse motion along pre-existing Redacted for Privacy inward-dipping faults in the rift mountains is not refuted by tilt data from Cascadia Segment. Sediment thickness on the floor of Cascadia Depression varies between less than 50 m and greater than 850 m. Basement doming, evident in the center of the depression, may be caused by the intrusion of sills into the sediments. There is a great disparity in basement slope between the eastern half (average slope = 11.5°) and western half (average slope = 4.2°) of Cascadia Depression. Asymmetry of basement slope seems to be established in the central rifling basin and is propagated outward onto the terraces until isostatic equilibrium is reached. The local slope of Cascadia Depression's floor is the result of interaction between two components: rate of movement along faults within the basin and the rate at which turbidite-derived sediments can bury the faults. At present, movement along low-angle fault surfaces within Cascadia Depression is occurring at a higher rate than sediments are being delivered to the basin. Duncan (1968) has estimated that the rate of turbidite sediment accumulation in this area decreased rapidly about 12,000 years ago. Fault angles and offset values were used to calculate extension rates averaged over this time period. A progressive decrease in opening rate from 5.8 cm/yr to 4.1 cm/yr is evident from the northern to central parts of Cascadia Depression, respectively. This discrepancy in estimated opening rates is probably the result of: (1) "true" short-term asymmetric opening from north-to-south; (2) the masking of surface offset along faults towards the southern part of Cascadia Depression by ponding of turbidites; or (3) a combination of both. The calculated rates of opening support a model in which Cascadia Segment is a "trapped" spreading segment from an episode of rift propagation (Embley et al., in press). The calculated extension rate of Cascadia Depression coupled with the morphologic similarity between Cascadia Segment and slow-to-intermediate mid-ocean ridges support the idea that this is an area of active seafloor spreading. The tectonic evolution of Cascadia Segment is compared to existing models of rift valley evolution and crustal generation processes at mid-ocean ridges, especially in the context of steady state versus non-steady state theories of rift valley development (i.e., Davis & Lister, 1977; Kappel & Ryan, 1986). The high subsidence rate of the rift valley floor (1.8 cm/yr averaged over 6,660 years) coupled the calculated horizontal extension rate (about 5 cm/yr averaged over the past 12,000 years) may indicate that the current magma supply has waned, thus allowing extension to become the dominant mechanism at Cascadia Depression. Therefore, recent tectonics may support the idea that the balance between extension and volcanism at Cascadia Segment may be episodic in nature (after Kappel &