Comparison of seafloor tectonic fabric at intermediate, fast, and super fast spreading ridges: Influence of spreading rate, plate motions, and ridge segmentation on fault patterns
Comparison of seafloor tectonic fabric at intermediate, fast, and super fast spreading ridges: Influence of spreading rate, plate motions, and ridge segmentation on fault patterns
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中等、快速和超快速扩张脊的海底构造结构比较：扩张速率、板块运动和脊分段对断层模式的影响
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发表时间：
1994
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通讯作者：
K. Macdonald
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作者：
S. Carbotte;K. Macdonald
利用SeaMARC II侧扫声纳对中扩张的厄瓜多尔裂谷、快速扩张的东太平洋隆起（EPR）（8°30′-10°N）和超快速扩张的EPR（18°19°S）进行了年轻海底构造形态的对比研究。我们发现断层群的特征不仅是扩展速率的函数，而且在个别脊段内沿轴线变化（即靠近大偏移和短偏移的不连续面）。我们还发现，与单独使用磁数据相比，断层方位角可以在更精细的尺度上用于检查板块运动学。断层数量随扩展速率的大部分变化可以用扩展速率与脆性层厚度的反比关系来解释。例如，在超快速扩张区域，短断层数量最多，平均断层间距和断层间距最小，断层密度最高。此外，在超快速扩张区内，长主内倾断层的附属短而密距的对偶断层群也很常见，这可能是较薄、较弱的脆性层的结果。远离脊轴的断层数量随着扩展速度的增加而增加，因此很少有向外的断层以缓慢到中等的速度被发现，而以最快的速度观察到的向内和向外的断层数量大致相等。随着离山脊的距离的增加，脆性层迅速增厚，这可能是向内断层以较慢的扩展速度占主导地位的原因。在所有扩展速率下，向外断层的平均长度较短，垂直偏移量较低。这些差异可能反映出，由于岩石圈的强度随着离脊的距离增加而增加，向外断层的活动时间较短。所有区域的断层长度和间距近似指数分布。根据断层的位移和长度分布计算断层群表示的拉伸应变，每个区域的应变估计为~ 4%。假设断层间距反映了断层开始时的断裂深度，我们推断断层开始时的脆性层厚度为~ 1 km。在岩浆伸展过程中，研究了断层群的脊段尺度变化。我们看到了更大岩浆扩张的证据，这与厄瓜多尔裂谷东部三分之一的岩浆供应长期减少有关。在转换断层的15公里范围内也发现了局部脆性扩展增加的证据。重叠扩张中心（OSCs）留下的不协调带以低断层丰度为特征。在osc，脊尖传播的离散事件可能适应由正断层沿脊的其他地方吸收的扩展。断层方位角似乎确实是板块运动的有用指标。在EPR 8°30′-10°N区域内，断层趋势记录了太平洋-科科斯板块运动的近期变化（3°-6°，约1 m.y），与EPR北部其他地方的磁异常和断层线化数据一致。在厄瓜多尔裂谷内，断层方位角在预测趋势的3°范围内分散，并与过去1.5英里内不断扩散的一个极点相一致。
We have conducted a comparative study of the tectonic morphology of young seafloor using SeaMARC II side scan sonar surveys of the intermediate spreading Ecuador Rift, the fast spreading East Pacific Rise (EPR) (8°30′–10°N), and the super fast spreading EPR (18°–19°S). We find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments (i.e., with proximity to large- and short-offset discontinuities). We also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone. Most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer. For example, regions of super fast spreading are characterized by the largest numbers of short faults, the smallest average fault spacing and throw, and the highest fault density. In addition, clusters of short, closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area, presumably the result of a thinner, weaker brittle layer. Faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates. Rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates. Outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets. These differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge. Fault lengths and spacings in all areas approximate exponential distributions. The extensional strain represented by fault populations is calculated from the displacement and length distributions of faults, and strain estimates of ∼4% are obtained for each area. Assuming that fault spacing reflects fracture depth extent where faults initiate, we infer a brittle layer thickness of ∼1 km when faulting begins. Fault populations are examined for ridge segment scale variations in amagmatic extension. We see evidence for greater amagmatic extension associated with long-term reduced magma supply along the eastern third of the Ecuador Rift. Evidence for local increased brittle extension is also found within 15 km of transform faults. Discordant zones left by overlapping spreading centers (OSCs) are characterized by low fault abundances. At OSCs, discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting. Fault azimuths do appear to be useful indicators of plate motion. Within the EPR 8°30′–10°N area, fault trends record a recent change in Pacific-Cocos plate motion (3°–6° at ∼1 m.y.) consistent with magnetic anomaly and fault lineation data from elsewhere along the northern EPR. Within the Ecuador Rift, fault azimuths scatter within 3° of predicted trends and are consistent with constant spreading about one pole for the past 1.5 m.y.