From BiblePortal Wikipedia

King James Dictionary [1]

Thereupon', adv. there and upon. Upon that or this.

The remnant of the house of Judah, they shall feed thereupon.  Zephaniah 2

1. In consequence of that.

He hopes to find you forward,

And thereupon he sends you this good news.

2. Immediately.

Webster's Dictionary [2]

(1): ( adv.) Upon that or this; thereon.

(2): ( adv.) On account, or in consequence, of that; therefore.

(3): ( adv.) Immediately; at once; without delay.