It has recently been shown that among the subduction zones initiated in the Cenozoic era, the subducting oceanic lithosphere could have any age, and was as often older than the overriding plate as younger than it. To try to explain this observation, we consider the simple setup of an oceanic transform fault and perform 2D thermomechanical experiments by applying the same convergence velocity on the two adjacent plates. We combine a non-Newtonian viscous rheology and a pseudo-brittle behavior. We focus on the influence of the fault structure at convergence onset, by testing the effects of the fault gouge depth, $z_{wz}$, and of the width of the thermal transition between the lithospheres, which allows for mimicking the settings of a transform fault as well as of an old fracture zone. We investigate large ranges of plate age and of plate age offset. In most of our experiments, the mode of convergence accommodation is basically a function of the gouge depth, $z_{wz}$, that we compare to the mean of the 2 plate thicknesses. We observe three main behaviors: (1) failure of subduction initiation if $z_{wz}$ is shallow, i.e., $\lesssim 15\text{\%}$ of the mean depth of the 2 plate bottoms if the age offset scaled by the younger age is very low (≲1) or $\lesssim 30\text{\%}$ otherwise, (2) old plate subduction, that is best favored for ratios $z_{wz}/z_{TT}\sim 0.25$ and for low offsets in relative plate ages, and (3) young plate subduction if $z_{wz}$ is quite deep ($\gtrsim 40\text{\%}$), whatever the age offset. The thermal transition width only has a second-order effect, that facilitates subduction initiation when the transition is wide. Compressive forces are overshooting reasonable thresholds ($\gtrsim {10}^{13}$ N/m) if at least one of the two plates is older than ∼30 Myr. The success of a young plate subduction and, to a lesser extent, of an old plate subduction, can be predicted by comparing $z_{wz}$ at convergence onset to the depth of the brittle-ductile transition inside the future upper plate. The subduction success and polarity depend on the possibility to form a low shear stress plane. We predict that only the young plate subduction can initiate at an active transform fault, while the old plate subduction is restricted to the setup of an old fracture zone separating two lithospheres with a small age offset scaled by the younger age. The significant predominance in our modeling of the thin plate underthrusting results from the strong influence of the plate strength contrast when lithospheres are stiff. Our experimental results are consistent with the recent records of subduction initiation at the Mussau and Hjort Trenches, as well as at the Gagua ridge (W Pacific).

最近的研究表明，在新生代开始的俯冲带中，俯冲海洋岩石圈可以有任何年龄，并且通常比上覆板块更古老，也比它更年轻。为了尝试解释这一观察结果，我们考虑海洋转换断层的简单设置，并通过在两个相邻板块上应用相同的收敛速度来执行二维热机械实验。我们将非牛顿粘性流变学和伪脆性行为结合起来。我们通过测试断层泥深度的影响，重点关注断层结构在收敛开始时的影响，$z_{wz}$，以及岩石圈之间热转变的宽度，这允许模拟转换断层以及旧断裂带的设置。我们研究了大范围的板龄和板龄偏移。在我们的大多数实验中，收敛调节模式基本上是凿痕深度的函数，$z_{wz}$，我们将其与 2 个板厚度的平均值进行比较。我们观察到三个主要行为：（1）俯冲起始失败，如果$z_{wz}$是浅的，即$\lesssim 15\text{\%}$2 个板块底部的平均深度，如果按较年轻年龄缩放的年龄偏移非常低 (≲1) 或$\lesssim 30\text{\%}$否则，（2）旧板块俯冲，这是最有利于比率的$z_{wz}/z_{\mathrm{时间}\mathrm{时间}}～0.25$对于相对板块年龄的低偏移，以及（3）年轻板块俯冲，如果$z_{wz}$相当深（$\gtrsim 40\text{\%}$），无论年龄偏移如何。热转变宽度仅具有二阶效应，当转变较宽时有利于俯冲起始。压缩力超过合理阈值（$\gtrsim {10}^{13}$N/m)，如果两块板中至少有一块的历史超过 ∼30 Myr。年轻板块俯冲的成功以及在较小程度上旧板块俯冲的成功，可以通过比较来预测$z_{wz}$在收敛开始时到达未来上板内脆性-延性转变的深度。俯冲的成功和极性取决于形成低剪切应力平面的可能性。我们预测，只有年轻板块俯冲才能在活动转换断层处开始，而老板块俯冲仅限于分隔两个岩石圈的老断裂带的设置，其年龄偏移按较年轻的年龄缩放。我们的薄板逆冲模型中的显着优势是由于岩石圈刚性时板块强度对比的强烈影响造成的。我们的实验结果与 Mussau 和 Hjort 海沟以及 Gagua 海脊（西太平洋）俯冲起始的最新记录一致。