What mountains did westbound silk road travelers have to cross before reaching marakanda?

They would have to cross the Himalayas, Karakorum, and Kunlun mountain ranges. 

Himalayas, Karakorum, and Kunlun ranges.

Explanation:

The silk route travelers connected the east Asia with southeast Asia.With Persia and the Arabian peninsula.  And it began with the Han dynasty in China. And it expanded to central Asia and thus they extended the great wall of china to protect the trade route. The northern route included many mountainous ranges like the china, Kazakhstan and Afghanistan. The Himalayas in the India and the southern route was of the Karakorum ranges and Ganges delta.