Their car was caught in a traffic jam, thus causing the delay.
then 表示时间（在当时）或顺序（然后），跟这两个表示因果的副词，没有可比性，不可用 then 替换。
该句也不可用 hence 替换。hence 表示“鉴于前者”，推出后者”，之间不是必然的因果关系，前者只是为 hence 所作之事提供一种理由。
但是，thus 表示前面叙述的事件导致已有的结果。
我们可以这样理解：
1. 使用 thus时，前面可以转化成 because 从句。
Their car was caught in a traffic jam, thus causing the delay.
= Because their car was caught in a traffic jam, it caused the delay.
2. 使用 hence 时，前面可以转化成 since 从句。
We suspect they are trying to hide something, hence the need for an independent inquiry. 
= Since we suspect they are trying to hide something, we need an independent inquiry.