The study on the mechanism of imbricate fan using numerical methods
|Keywords:||增積楔狀體;褶皺衝斷帶;覆瓦狀構造;離散元素法;滑脫面;accretionary wedge;fold-and-thrust belt;imbricate fault system;distinct element method;decollements.||Issue Date:||2008||Abstract:||
臺灣島位於板塊邊界上，是一造山作用仍然活躍的活動帶，故地質活動相當頻繁，間接地構成了台灣豐富的地質景觀。此一活動造山帶，由一系列的褶皺及斷層所形成的增積楔狀體(accretionary wedge)組成。豐富而多樣的褶皺及斷層相互疊置，形成了造山帶前緣典型的一系列相互疊置褶皺及斷層所組成的褶皺衝斷帶(fold-and-thrust belt)。然而此規則排列的覆瓦狀構造(imbricate structure)，其形成的條件、原因等力學機制至今仍尚未完全被了解。此一機制的了解，也有助於探究造山帶發育的過程。研究採用離散元素法PFC2D以及有限元素法ABAQUS模擬覆瓦狀構造的形成，希望藉由數值方法模擬岩層受到推擠、擾動後產生覆瓦狀構造的過程，了解覆瓦狀構造其生成的機制，包括其生成的力學機制以及推估自然界主應力方向的改變以及應力場對其它構造發育之影響。前的研究成果顯示，在一覆土深度較深，並允許顆粒與顆粒間具有互相傳相傳遞力矩的膠結作用之環境下能夠適當地模擬出一連續之斷層系統(imbricate fault systems)，即覆瓦狀構造。岩層受到推擠作用時，其速度量值可區分成三個部分：(1) 等速區、(2)斷層錯動區、(3) 速度下降區，代表的意義為岩層受到推擠時，其會有應變能累積的情況產生，當能量累積至一定程度之後造成斷層錯動，受到圍壓束制的影響，斷層錯動不會在一錯動面持續發展，而是將能量持續傳遞，形成一系列的斷層系統，並且與介質交界之層面具有一相對滑移之滑脫面(decollement)產生。周圍環境受到影響時也會改變其岩層之行為模式，由本研究可以了解，圍、摩擦係數、鍵結力、以及阻抗比等等皆會影響覆瓦狀構造的演化。
Taiwan is an active orogenic belt formed by oblique convergent between the Philippine Sea Plate and the Eurasian Plate. This active orogeny consists of serials of folds and faults of typical accretionary wedge. Because of high activity, the Island has abundant geological structures. Serials of overlapped fold and fault structures in the foreland regime have been recognized as an imbricated fold-and-thrust belt. Despite of numerous regular distributed imbricate structure has been identified, the conditions and the mechanics still not been fully resolved. By the way, the mechanism of imbricate structure is also a key to discover the orogenic processes. his research uses distinct element method, PFC2D, as well as the finite element method, ABAQUS, to simulate the formation of imbricated thrust system. By means of numerical simulation, we would be able to understand the mechanism of imbricate fan, the change of principal stress direction, the stress field and stress state, as well as the material proprieties etc. that might affecting the structure evolution. he primary result demonstrates that the so-called “imbricate fault system” could be achieved under certain conditions, including sufficient overburden and the moment transmission between particles.imulating the imbricate trust system, the deformation of competent layer may be divided into three parts: the constant speed domain, the faulted domain, and the speed drop domain. These regions of velocity discontinuity illustrate the phenomena of strain or energy accumulation during the squeezing experiments. Since the threshold has been achieved, the faulting process may occur in order to reduce the strain energy. As a single fault has been developed and confined by the confining pressure, the fault system could not prorogate infinitely. However, a series of thrust system will be formed, and the stick-slip phenomena could be identified in a single fault plane. A serial of faults finally linked together in order to accommodate the total deformation. In this case the decollements have been generated between the interface of competent layer and the matrix.his research demonstrate the influence of confining pressure, the friction coefficient, the bonding types and strengths, as well as ratio of stiffness of rock materials that strongly affect the development of imbricate thrust system.
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