From BiblePortal Wikipedia
King James Dictionary 
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.
Webster's Dictionary 
(1): ( adv.) Upon that or this; thereon.
(2): ( adv.) On account, or in consequence, of that; therefore.
(3): ( adv.) Immediately; at once; without delay.