Annotated record of the detailed examination of Mn deposits from CNAV Endeavour and CSS Parizeau cruise stations from 1969 to 1971 over the Juan de Fuca Ridge

Bathymetry, seismic reflection profiles, magnetic data, and dredged basalt indicate that two centres of spreading have existed near the northern end of Juan de Fuca Ridge in the last 0.7 m.y. (Brunhes Geomagnetic Polarity Epoch). A short-lived eastern branch functioned only in the early part of the Brunhes Epoch. More prolonged and recent activity has occurred on the western branch, and has involved extrusion of basalt, rifting, and uplift. The anomalous relief of the northern end of Juan de Fuca Ridge compared to the ridge further south is the result of a tectonic event which occurred 100,000-350,000 years ago. Prior to this, turbidites were deposited over most of the northern part of the ridge, which consisted of a series of basement ridges and valleys of subdued relief trending NNE parallel to the magnetic anomalies. During the event, the igneous basement together with the overlying turbidites was faulted, tilted, and uplifted along the primary structural trends. The basement relief was increased in places to more than 2 km. Subsequent turbidite deposition has been confined to valleys. The Sovanco Fracture Zone between Juan de Fuca and Explorer Ridges also suffered uplift. The tectonic disturbance may have resulted from an increase in spreading rate or the change in location of the axis of spreading on Juan de Fuca Ridge, or compressional interaction between Explorer sub-plate to the north and Sovanco Fracture Zone and the northern end of Juan de Fuca Ridge. Sovanco Fracture Zone, consisting of a WNW-trending basement ridge (Sovanco Ridge) with complex associated magnetic anomalies, may have originated as a strike-slip fault or fault-zone trending east-west about 5 m.y. ago. Location of present transform movement within the fracture zone is equivocal, but apparently trends NW. A specimen of basalt from the eastern end of Sovanco Ridge yielded a fission track age of <309,000 years. Basalts dredged from 13 locations on Juan de Fuca Ridge, Sovanco Ridge, and Heck and Heckle Seamount Chains on the western flank of Juan de Fuca Ridge are low-potassium tholeiites of the ocean ridge type. Twenty-seven representative specimens of fresh basalt, as well as 15 samples of glass selvages and altered basalt, were analyzed for 9 major elements by an atomic absorption technique set up by the author. The analyses are closely similar, showing no well-developed differentiation trend. They straddle the boundary between the olivine-normative and quartz-normative fields, and the basalts may have been derived by segregation from the mantle at a depth of about 15 km. A specimen from NE of the eastern spreading centre yielded a fission track age of 0.95 ± 0.53 m.y. which is in reasonable agreement with the age of 0.7-0.9 m.y. predicted by the magnetic anomaly time scale. Increasing thicknesses of Fe-Mn coatings on dredged basalts indicate that the two NW-trending seamount chains increase in age away from the crest of Juan de Fuca Ridge. The seamounts originated and grew to virtually their present sizes while still at or near the ridge crest. Reflection profiles across the continental slope indicate that it has formed by folding, reverse faulting, and uplift of strata of Cascadia Basin. Internal deformation has caused these sediments to behave as acoustical basement in the structures beneath the slope but their sedimentary nature has been confirmed by dredging. The presence of magnetic anomalies 5-8 m.y. old beneath the lower and middle continental slope indicates subduction. The magnetic basement dips below the slope at an angle of more than 10°.

海底测深 (bathymetry)、地震反射剖面、磁测数据与拖网采获的玄武岩表明，在过去0.7百万年（即布容地磁极性世 (Brunhes Geomagnetic Polarity Epoch)）期间，胡安德富卡海岭 (Juan de Fuca Ridge) 北端附近存在两个扩张中心。其中短暂存在的东扩张支仅在布容世早期活动，而西扩张支的活动持续时间更长且更晚，伴随玄武岩喷溢、裂谷作用与隆升过程。胡安德富卡海岭北端相较于其南段的异常地形，源于10万至35万年前发生的一次构造事件。在此事件之前，浊积岩沉积覆盖了海岭北部大部分区域；彼时该区域由一系列北北东 (NNE) 向、与磁异常走向平行的舒缓起伏基底脊与谷地组成。构造事件期间，火成基底及其上覆的浊积岩沿主要构造走向发生断裂、倾斜与隆升，部分区域的基底起伏被抬升至超过2千米。后续的浊积岩沉积仅局限于谷地之中。胡安德富卡海岭与探险家海岭 (Explorer Ridges) 之间的索万科断裂带 (Sovanco Fracture Zone) 同样遭受了隆升。此次构造扰动可能源于胡安德富卡海岭扩张速率的提升、扩张轴位置的变化，或是北部探险家亚板块与索万科断裂带及胡安德富卡海岭北端之间的挤压相互作用。索万科断裂带由一条北西西 (WNW) 走向的基底脊 (索万科脊, Sovanco Ridge) 及其伴生的复杂磁异常组成，其可能起源于约5百万年前形成的东西走向走滑断层 (strike-slip fault) 或断裂带。目前该断裂带内的转换运动位置尚不明确，但整体走向似乎为北西 (NW)。采自索万科脊东端的玄武岩样品的裂变径迹年龄 (fission track age) 小于30.9万年。在胡安德富卡海岭、索万科脊以及胡安德富卡海岭西翼的赫克与赫克尔海山链 (Heck and Heckle Seamount Chains) 共13个站位拖网采集的玄武岩，均属于洋脊型低钾拉斑玄武岩。作者通过自建的原子吸收技术 (atomic absorption technique)，对27件新鲜玄武岩代表性样品、15件玻璃边与蚀变玄武岩样品开展了9种主量元素分析。分析结果高度相似，未呈现显著的分异趋势；样品落在橄榄石标准矿物组合与石英标准矿物组合的分界区间，其玄武岩可能源自约15千米深度的地幔物质分异作用。采自东扩张中心东北部的样品，其裂变径迹年龄为0.95±0.53百万年，与基于磁异常时间尺度预测的0.7~0.9百万年年龄范围基本吻合。拖网采得的玄武岩表面铁锰涂层厚度随远离胡安德富卡海岭峰顶而递增，表明两条北西向海山链的年龄随远离海岭峰顶而增大。这些海山形成并生长至现今规模时，仍位于或紧邻海岭峰顶。横穿大陆坡的反射剖面显示，大陆坡由卡斯卡迪亚盆地 (Cascadia Basin) 地层的褶皱、逆冲断层作用与隆升形成。内部变形使得这些沉积物在坡下构造中表现为声学基底 (acoustical basement)，但拖网采样已证实其沉积岩本质。下陆坡与中陆坡下方存在5~8百万年历史的磁异常，表明此处存在俯冲作用。磁基底以超过10°的角度俯冲入大陆坡下方。