Transition of Ridge Axis Gravity with Spreading Rate: The Effect of a Weak Lower Crust

首席研究员对Geosat测高观测结果的初步分析显示，洋中脊轴线重力的突然变化及其扩散速度比地形变化更为明显。在60-70毫米/年的扩展速率下，脊轴重力信号具有高振幅并且是可变的，而在此速率以上，重力信号的突变必须表明均衡补偿机制的突变，而地形没有突变。首席研究员的解释是，海底扩张有两种不同的动态模式。本工作将从模拟和观测的角度解决脊轴形态及其随扩展速率、地壳厚度和地幔温度变化的问题。目标是建立一个山脊-轴动力学的定量模型，该模型结合了现实的非线性流动规律以及流变学上独特的地壳和地幔，以表征山脊出口/侧翼重力和山脊轴/侧翼地形的扩展速率、地壳厚度和推断的地幔温度，并通过将模型重力/地形与观测结果进行比较，建立弱下地壳层在扩张类型转换中的作用。

The principal investigator's preliminary analysis of Geosat altimetry observations reveal an abrupt change in mid-ocean ridge axis gravity with spreading rate that is more pronouced than the topographic variations. below a spreading rate of 60-70 mm/yr the ridge-axis gravity signature has high amplitudes and is variable where as above this rate the gravity depends upon both the topography and the compensation mechanism, the abrupt change in the gravity signature must signify an abrupt change in the isostatic compensation mechanism, there being no abrupt change in the topography. The principal investigator's interpretation is that there are two dynamically distinct modes of seafloor spreading. This work will address the problem of ridge-axis morphology and its variation with spreading rate, crustal thinckness, and mantle temperature from both a modeling and observational stand point. The objectives are to develop a quantitative model of ridge-axis dynamics which in corporates a realistic non-linear flowlaw as well as a rheologically distinct crust and mantle to characterize ridge exis/flank gravity and ridge axis/flanktopography in terms of spreading rate, crustal thickness and inferred mantle temperature, and, by comparing model gravity/topography with observations, to establish the role of the weak lower crustal layer in the transition between spreading types.